vue Points de Personalization Personalizacíon THEME TEMA INTERNATIONAL REVIEW OF OPHTHALMIC OPTICS REVISTA INTERCIONAL DE ÓPTICA OFTÁLMICA

Transcripción

1 vue INTERNATIONAL REVIEW OF OPHTHALMIC OPTICS Points de REVISTA INTERCIONAL DE ÓPTICA OFTÁLMICA THEME Personalization TEMA Personalizacíon 69 AUTUMN / OTOÑO 2013 BI-ANNUAL / SEMESTRAL 2013 ESSILOR INTERNATIONAL

2 SUMMARY / SUMARIO 69 AUTUMN / OTOÑO 2013 BI-ANNUAL / SEMESTRAL 2013 ESSILOR INTERNATIONAL SCIENTIFIC & MEDICAL The Neuro Ocular Plane (NOP) - Emmanuel Alain Cabanis The role of the eye s centre of rotation in lens design - Mo Jalie Does the eye rotation center play a role in the choice of lens type? - Hans Bleshøy Study of vergence movement dynamics Bérangère Granger, Tara Alvarez, John Semmlow Personalization: increasing lens efficiency Cécile Pétignaud CIENTÍFICO & MÉDICO El Plano Neuro Ocular ( PNO) - Emmanuel Alain Cabanis El papel del centro de rotación de los ojos en el diseño de las lentes - Mo Jalie Juega el centro de rotación del ojo un papel en la selección de las lentes? - Hans Bleshøy Estudio de la dinámica de las vergencias Bérangère Granger, Tara Alvarez, John Semmlow La personalización: un vector de eficacia de las lentes Cécile Pétignaud 25 BEST PRACTICE The ideal in practice client journey Andy Hepworth 58 PRÁCTICAS ÓPTIMAS El recorrido ideal del cliente «en la tienda» Andy Hepworth 28 PRODUCT Crizal Prevencia : the first preventive non-tinted lenses for everyday wear with protection from UV rays and harmful blue light Coralie Barrau, Amélie Kudla, Eva Lazuka-Nicoulaud, Claire Le Covec 60 PRODUCTO Crizal Prevencia : las primeras lentes preventivas de uso diario no tintadas, que protegen de los UV y de la luz azul perjudicial. Coralie Barrau, Amélie Kudla, Eva Lazuka-Nicoulaud, Claire Le Covec TO READ ON PARA LEER EN The reliability of eye-head coordination Guillaume Giraudet, Jocelyn Faubert La solidez de la coordinación cabeza-ojo Guillaume Giraudet, Jocelyn Faubert

3 EDITO JEAN-PIERRE CHAUVEAU Director of Publication Dear Readers, In this issue 69, we address the theme of eyewear lens personalization. Although the production of corrective lenses has always been based on a prescription for the necessary power and prisms, which is itself personalized for each of our two eyes, the possibilities of personalizing eyewear lenses have evolved massively over the past 10 years or more. Lenses can be personalized to suit the position they will occupy in front of both eyes, thus giving additional freedom in the choice of frames and their adjustment to the customer s face. Measurements of the precise position of the eyes in terms of the frames chosen and adjusted can therefore enable lens manufacturers to optimise compliance with the power and prism prescription for the corrective lenses mounted in the frames. Research into the visual system as a whole, both static and dynamic, has resulted in the discovery of new and relevant personalization parameters to direct optimisation of corrective lens geometry. The visual cortex, associated with both eyes, interacts with our inner ear, our balance and then our posture, depending on the use we make of our vision. Professor Emmanuel Alain Cabanis presents the importance of the N.O.P. (Neuro Ocular Plane) for the position of the head, depending on the direction of the gaze. This is a reference article on the biometrics of the visual system, which passes through the two centres of the eyeballs and is, to a certain extent, our visual gyroscope in all the static visual tasks that we perform. Professor Mo Jalie reminds us of the key role played by the eyes centres of optical rotation in the optical engineering of corrective lenses. This article shows the importance of taking care with the parameters for mounting the lenses in their frames, and the adjustment of the frames on the customer s face. Control of the position of corrective lenses in terms of each of the centres of rotation of the eyes means better oculomotor comfort and maximised vision correction performance. A study in Denmark carried out by Dr Hans Bleshoy and comparing two types of lenses from the same family, one of which is calculated using actual rotation centre position measurements, shows the importance of this type of personalization. It is also important to take account of the head s posture, because people who move their head more than their eyes can generate a conflict of vision with the inner ear when wearing progressive lenses. Guillaume Giraudet, researcher at the Montreal School of Optometry, tells us about the study carried out on the individual strength of eye/head coordination strategy (to read on Bérangère Granger et al. sets out the recent discoveries made with the inter-individual study of the dynamic of eye vergence movements. This vergence and accommodation behaviour in transit mode translates the visual system s ability to adapt to the object environment observed through corrective lenses. Two other ensuing articles will be available to consult on our website. Also available on the website is a video interview with Professor Mo Jalie on personalized lenses in general. Cécile Pétignaud sets out the main types of personalization parameters already well known and used by the various ophthalmic lens manufacturers, and Andy Hepworth takes us over the various stages in a customer s visit to a sales outlet, underlining the importance of taking their personalized profile into account. Coralie Barrau et al. presents the new Crizal Prevencia product which reduces the damaging and cumulative effects of harmful light (Blue-Violet and UV). And finally, ever loyal to our Art & Vision section, this time we offer you an article by Christophe Birades on the history of spectacles in Korea, based on objects taken from the Hanbit Museum of Old Spectacles, created in Seoul by Mr Lee Cheong Su. Happy reading Director of Publication

4 SCIENTIFIC & MEDICAL THE NEURO OCULAR PLANE (NOP) A double natural cephalic reference, that of the head posture in Homo sapiens, standing, looking straight ahead, it is the neuroanatomy of visual pathways, from the cornea to the calcarine fissure. EMMANUEL ALAIN CABANIS Member of the National Academy of Medicine, Univ. Paris 6, MD, PhD. France The French Ophthalmology Society (SFO) elected as its annual reporter E. Hartmann on Radiography in ophthalmology. A clinical atlas (1936), then H. Fischgold et coll. (1966) for Neuroradiological exploration in ophthalmology and, for 1996, the author of this article, for Imaging in ophthalmology, the 3 rd phase in this 30 year cycle, due to chance and the need for the development of X-rays into digital neuroimaging (X-ray and magnetic scanner, MRI) [1, 2], Fig.1. Focussing on digital technology, which came into being in 1972 (X-ray scanner), this report summarises 40 years of progress to date, that of the new digital anatomy (2008, MRI, Fig. 1), both normal and pathological, of visual pathways in Homo Sapiens. An axial section (horizontal) of the head containing the optic nerve, from its papilla through to the optic canal, performed by my friend Professor Ugo Salvolini (Universita di Ancona, 1 st X-ray scanner in Italy), spreads out as far as possible the intra-orbital segment of the two optic nerves, in primary gaze position, excluding the partial volume effect (Fig.2). The transversal diameter of the optic nerve in vivo became measurable. The first NOP section, thick axial (6 mm), 1 year after the presentation of the X-ray scanner invention by Godfrey Newbold Hounsfield (1972) in London (Nobel Prize for Medicine in 2003), provided the first maximum axial vision of the eyeball (increased in myopics). The NOP was born. Five years later, as head of the neuroimaging department at the Quinze- Vingts National Ophthalmology Hospital I confirmed this section on the new ND 8000 scanner (Thomson CGR) which had been evaluated for 4 years in the factory (Fig. 2). After the first NOP publication by the Société Anatomique de Paris (1978) Professor A. Delmas was kindly informed Dear Friend, your work is reminiscent of Broca s visual plane, I ve checked. Both delighted with this first scientific validation and furious at having missed his first centenary reference, before the author became professor of neuroimaging and radiology at the Pierre et Marie Curie Paris 6 University (and associate Anatomy Professor), he was to contribute actively to the book entitled Paul Broca géant du 19 e siècle (Paul Broca, giant of the 19th century) [3]. An anatomist and anthropologist, Broca wrote in 1873 ( ) The head is horizontal when a person is standing and looking towards the horizon. That is the natural direction of the gaze ( ). The 1976 annual report of the French Ophthalmology Society (SFO), 762 p. and 257 co-authors, devotes 83 p. ( ) to chapter 2 The twelve anatomies of in vivo visual pathways for 4 reasons. 1. Anatomy is plural, from microscopic anatomy to surgical anatomy. 2. The power of digital tools (X-ray scanner then MRI, image processing and nuclear imaging), in terms of both sensitivity and spatial resolution, leads to a proliferation of in vivo results leading to chemistry and therefore molecular anatomy and genomics. 3. MRI has provided the fourth dimension, sagittal, frontal and oblique (3D) to horizontal bidimensional exploration (2D) of the head. 4. Logically ordered, normal results make their mark, validated by the perspective of half a century and hundreds of thousands of clinical observations. The notion of space and cephalic references in digital anatomy, in vivo, is therefore the first of the 12 approaches to the head, a spherical shape with two orthogonal diameters, one, horizontal, of the sensory relays containing NEURO OCULAR PLANE + 1* 2* 3* FIG. 1 The control panels in a 3Tesla MRI room, in front of the Faraday cage (dappled). 4 FIG. 2 Initial observation of the NOP in an adult using an X-ray brain scanner (1973). In primary gaze position, the axial section and thick (6mm) cephalic transverse section contains, from front to back, the relative hyperdensities of the 2 crystalline lenses, of the heads of the two optic nerves and of the 2 optic canals.

5 SCIENTIFIC & MEDICAL FIG. 3 Top left the anatomical diagrams describing the human body, Homo sapiens standing, looking towards the horizon. Top middle, skull without a mandible (removed) placed on a board with 2 needles stuck into the 2 optic canals at the back and the 2 centres of the orbital surfaces at the front, virtual diagram of vision parallel to the horizontal board. Top right, Paul Broca. Bottom, model of the optic pathways, in white, orthogonal to the cervical spine and the arterial axes. FIG. 4 Display of several cephalic orientation planes on a median cephalic section of the head (MRI), with the NOP defining the horizontal. Top left, NOP with CA-CP (white anterior and posterior commissure mammillary body), CP-MB chiasmatic point-mammillary body), OM (orbitomeatal). Top right, bicommissural verticals (ACV and PCV). Bottom left, the NOP horizontal. Bottom right, Orbitomeatal plane (+ 20 over the previous one). the neuronal vision pathway and the other perpendicular to the previous one, containing the oculomotor pathways, from the cortex to the cerebral trunk. Since the nineteen fifties, stereotactic neurosurgery teaches rigorous spatial identification for the cerebrum and the diencephalon. On his model of the optic pathways, Henry Hamard modelled in white the horizontal optic pathways, orthogonal to the arterial vascular and cervico-encephalic axes and to the direction of the cervical spine (Fig. 3). Added to this is the oculomotor organisation, orthogonal to the optical pathways, axial and transversal (like the horizontal section obtained by X-ray scanner, if the head is correctly placed in the machine). 1. Historically, orientation planes of the head were firstly those of its skeleton, the skull, at the origins of anthropology and human and compared animal palaeontology, from Daubenton (1764) to Virchow- Hoelder (1850), and then from A. Delmas and B. Pertuiset in orbitomeatal planes (1959) [3] or the bicommissural CA-CP planes of Talairach and Szikla ( ) [4], to the vestibular plane of Dr Perez dissecting the semi-circular canals of the inner ear (1982) [5], and the various orientations of the dry skull (and then in vivo using standard X-ray and vascular and ventricular neuroradiology) (Fig. 4). biometric, orbital and maxillofacial works, with 3 contributions made by MRI in 1984: 1. confirmation in vivo of axial and transversal layout of the visual pathways, 2. increased justification of a cephalic spatial reference within a poly-dimensional anatomic technique, 3. imagination of a new plane, vertical this time, the Transhemispheric Oblique Neuro- Ocular Plane, complementary since it is an oblique vertical of the head (see above). This Fig.3 shows on a sagittal MRI section of the head, strictly oriented in the NOP, the NOP, OM and AC-PC. This horizontal aspect of the visual pathways shows, like corporal anatomy overall, very slight individual variability due to age first (angulation of the chiasma in children) and the ethnicity (brachycephaly v. dolichocephaly). From the cornea to the calcarine fissure the NOP therefore contains the sensorial pathways of vision. This axial and transversal layout of optical pathways, shown clearly in descriptive neuroanatomy and everyday in vivo MRI, as well as in functional MRI and neurotractography, is particularly well suited to exploration by X-ray scanner and MRI. 2. The axial plane of NOP visual pathways from the X-ray scanner (1973) to Paul Broca (1873), meets the orbital definition (X-Ray scanner, MRI, other axial photonic imaging of the head awaited): Plane of horizontal section of the head, of millimetre thickness (5 to 1) which, in any position of the gaze, includes, symmetrically sectioned from front to back, the 2 crystalline lenses according to their longest axis, the 2 optic nerve heads and the 2 optic canals [1] (Fig. 2). The NOP therefore includes the 3h-9h horizontal meridian of the emmetropic eyeball, it is the horizontal meridian plane of the orbital pyramid whose apex is at the orbit orifice of the optic canal. This plane leads to axial exploration of the optic nerves using the X-ray scanner and MRI, avoiding the partial volume effect which hinders exploration of the canalicular and intra-orbital segment of the 2 optic nerves. 120 years earlier P. Broca wrote ( ) The head in the direction it is during life, when it is balanced on the spine and the patient is looking straight ahead on the dry skull ( ) The direction of this horizontal visual axis ( ) a line which, starting from the optic aperture, will pass through the orbital opening, a skull positioned on the craniostat is fitted with two orbital needles (Fig. 3,4) [2]. This intuition on skeleton (the skull) confirmed, 113 years later, by X-ray scanner and MRI of the head (the contents, brain), is therefore confirmed as the new plane of vision and visual pathways, by multiple FIG. 5 In vivo and in morte, NOP of visual pathways 3D referencing of the head) (X-ray scan and MRI) shown here by a red line on the face of the bald headed man with a moustache. The so-called Francfort planes (+ 7, below, in black) OM (and AC-PC) used in traditional radiology and stereotactic neurosurgery (in red + 20 below). In MRI recognition of grey matter (cortex, nuclei) and white matter, left, right, confirms the anatomic correlation of the visual pathways, from the cornea to the calcarine fissure. 5

6 SCIENTIFIC & MEDICAL FIG. 6 In vivo, respecting the NOP means people can see one another and speak to each other. The horizontality defined by the black line placed on the door (behind the two people in profile) positions the NOP, fixed at 7 on the Frankfort and vestibular skeletal planes (6 5). When lifting their chins to 20, the 2 men stand to attention, looking towards the horizon with an angular difference (+ 20 ) compared to the OM. As an illustration, Fig.6. shows the angular difference of functional postures (therefore anatomical sections) of the cephalic orientation of the two people. The angular difference on OM (+ 20 ), is therefore compensated horizontally, i.e. if both subjects lift their chin by 20. They are then standing to attention, looking straight forward towards the horizon. The black line on the door behind the two subjects in profile, shows this (NOP), fixedly angled at 7 on Francfort and vestibular skeletal planes (6 5). The NOP MRI (Fig. 5), with comparative anatomical control (in cadaver) checks that the NOP contains the visual pathways, from the cornea to the calcarine fissures, at the same heights as the optic canals, from the mesencephalon and even from the culmen of the vermis cerebellum, in the falcotentorial angle. Two points should be underlined here as they are essential: 1. The NOP is orthogonal in the direction of the cerebral trunk on the sagittal sections of the MRI, which contains the corticospinal or pyramidal tract. 2. The NOP is therefore perpendicular to the floor of the fourth ventricle. All this brings us back to the intuition for which Broca could have had no other proof than a skeleton and two knitting needles: The head is horizontal when a man is standing, looking straight ahead towards the horizon. This is natural direction of the gaze. The book mentioned at [1] refers to the practical application of installing the patient in the tunnel of the machine which, it would appear here, is quite unexpected for the reader. 3. The oblique trans-hemispheric neuro-ocular plane or OTNOP, the oblique vertical cephalic reference (Fig. 7). Beyond the horizontal plane of the X-ray scanner, MRI shows the 3 dimensions of the head and their digital reconstruction. The creation of oblique sections, in every spatial plane, was soon achieved. Now, this type of oblique anatomy is without reference system in the classic anatomical books. These works are restricted to the usual 3 planes, OX, OY, OZ. A reference system would therefore appear to be even more important in this circumstance of oblique vertical exploration, using the NOP. The intra-orbital optic nerve is then the reference from its intra-ocular system through to the optic canal, whatever the position of the gaze. Another reference comes in, the presence of the foramen magnum and of the atlanto-axial joint in the OTNOP section, because it follows the vertical meridian of a globe, the optic nerve, the chiasmatic decussation and the contralateral strip, down to the contralateral occipital section of the globe observed. This is an oblique vertical section plane of the head, of millimetre thickness (1 to 5) which, in any indifferent position of the gaze, includes: the crystalline lens according to its large vertical axis, the head of the homolateral optic nerve, the homolateral optic canal and the foramen magnum above the odontoid apophysis of the axis(c2) (Fig. 7) [1]. The plane is limited by the angular geometry of the direction of the optic nerve and it is difficult to obtain both the crystalline lens and the head of the optic nerve in the same plane since the latter passes, in fact, through the macula. The skeletal fixedness of the OTNOP on the cervicooccipital hinge in MRI has been shown in 41 European patients of average age, 39 of whom had the same anatomical layout of the anterior visual pathways. In the NOP, the direction of the 2 optic nerves, from the head to the optic canal, is crossed through in the middle with the superior projection of the odontoid apophysis. Electronic superimposition of the references obtained in the NOP (odontoid apophysis at the front and foramen magnum at the back) leads one to observe that the vertical projection of the direction of the 2 optic nerves occurs exactly on the vertical up from the odontoid apophysis of the axis (C2). Reference must be made here to former, known correlations existing between cervicooccipital biomechanics and the constraints of the oculocephalogyric reflex. The functional fixedness of this projection is interesting. The OTNOP acts as an oblique functional and descriptive vertical anatomical reference of the head. BIOMETRIC AND QUANTITATIVE OCULO-ORBITO-ENCEPHALIC ANATOMY Bios (life) and metron (measurement) meet once the references have been fixed. Between 1974 and 1995, from the X-ray scan to MRI, work proceeded and was verified [1, 2]. This field alone is summarised here. 1. Angular biometry of the NOP of the Francfort skeletal plane (NOP/FR) = 7 (average m = 6 49 and σ = 2 38 ) (see details of the 4 groups of measurements ). 2. Angulation of the NOP on the vestibular plane (Perez, Delattre and Fenart) and on the OM/CA-CP plane is measured on average at (σ = 5 13 ) in 52 young adults. Added to this is a notion of parallel between the NOP and Broca s alveolar-condyl plane, found in a bite (Fig.5, pencil bitten by the model). All the skeletal data confirms the fixedness of orientation of the visual plane on the skeleton of the head. The OM/CA-CP parallel agrees with the NOP/OM-CA-CP angulation of an average of 20 (and not of 15 or 10 as has been stated in literature). Visual cephalometry and its foremost practical application, oculo-orbital topometry, is therefore based on a certainty, that of the anatomical correlations established between the spatial orientation of the brain (visual pathways) and of its skeleton (the bony globe of the skull). Fig.3. resumes the fixedness of the NOP on Francfort, the vestibular plane, the ocular globe, topometric reference sphere (neuro-ocular index and dissociation of populations with papilledemas by HIC, in the middle, left and centre). The facial contour achieved based on the NOP by X-ray scan models the end appearance of the ocular vestibulography used on board the European space laboratory (Nov.-Dec. 1983). 3. Biometrics, oculo-orbital and facial topometry, exophtalmometry 3.1. Definitions of distances and indices, normal readings in an emmetropic patient, in the NOP by X-ray scan the letter o indicating the standard gap per calculated average. Fig.5 shows the oculo-orbital contours and measurements established on the axial section of the NOP by X-ray scan in an emmetropic adult ( ). The methods used are indicated in the book referred to [1]. FIG. 7 Oblique trans-hemispheric neuro-ocular plane (OTNOP): left, trajectory of the sections used and the result, right, compare with the median sagittal plane of the head with MRI. The series are normal, adults and children, pathological in dysthyroid ophthalmopathy. The contour on the console or work station of the X-Ray 6

7 SCIENTIFIC & MEDICAL scanner or MRI provides these detailed measurements (Fig. 7). First contour: line joining the anterior point of the 2 external orbital pillars in the NOP. Since this is a thick section (6mm) it is not a line but, by definition, a plane. The readings indicated below refer to figure 9. The External Bi-Canthal Distance (EBCD) measures the distance between the two external orbital pillars (m = mm, σ = 4.43). The Inter-Ocular Distance (IOD) = distance between the central point of the 2 crystalline lenses (m = mm, σ = 3.62). The Maximum Inter-Plane Distance (MIPD) measures the distance between the 2 external orbital walls in view of their possible temporal convexity (m = 28.7 mm, σ = 2.67). The External Ante-Bicanthal segment (EABC) measures the distance between the PEBC and the tangent at the anterior corneal hyperdensity (m = mm, σ = 1.96). The Retro External Bi-Canthal Segment (REBC) of the ocular globe measures the distance between the PEBC and the tangent at the posterior coroid-scleral hyperdensity, close to the head of the optic nerve. The Maximum Axial Length (MAL) of the globe measures the distance between the tangent at the anterior corneal hyperdensity and the tangent at the posterior coroid-scleral hyperdensity, close to the head (centro-ocular perpendicular to the PEBC) (m = mm, σ = 1.03). The transversal diameter of the optic nerve (DON) is measured at the mid-section of its intra-orbital segment (m = 3.5 mm, σ = 0.5). The transversal Diameter of the Right Internal Muscle (DRIM) measures the maximum interval separating its medial and lateral sides. The Cantho-Bicanthal Distance (CBCD) measures the interval separating the cutaneous surface of the internal canthus, at the front, from the external bicanthal plane at the back (measurement of the thickness of soft areas). The Apex Temporal Distance (ATD) measures the interval separating the tangency points of the Anterior Temporal Plane (ATP) with the temporal cavities. The External Bicanthal Plane Temporal Apex (BPTA) measures the interval between the External Bicanthal Plane (EBCP) and the Anterior Temporal Plane (ATP). The establishment of biometric indices according to H.V. Valois (the shortest distance related to the longest multiplied by one hundred) establishes classifications around the average and variance limits at 2 σ. Thus, it may be recalled that Retzius horizontal cranial index offers segmentation between the mesocephalic skull, the doichocephalic skull and the brachycephalic skull. The first index established is still the most important because it is in everyday, systematic usage. This is the Ocular- Orbital Index (OOI) or exophtalmometry index, which relates the AEBC segment to the MAL (m = 65.44, i.e. 65% of the length of the globe, in adults, projecting out of the PEBC (Fig. 9). The figure of 68% in one of the first series corresponded to an error of including patients with ametropia. The Neuro-Ocular Index (NOI) relates the diameter of the intra-orbital optic nerve at its mid section to that of the ocular globe (m = 14.8 mm, σ = 0.74) [6]. The histogram in figure 8 isolates the significant difference of the 2 populations, with and without papilledemae [7]. The External Bicanthal Ocular Index (EBCOI) relates the External Ante-Bicanthal segment to the External Retro- Bicanthal segment (m = 1.91). The Inter-Ocular Distance Index (IODI) relates the Inter-Ocular Distance (IOD) to the External Bicanthal Distance (EBCD) (m = 65.35). The Inter-Pupil Distance would therefore appear to correspond, on average, to two thirds of the External Inter-Canthal Distance. The Teleorbitism Index (TOI) relates the Maximum Inter-Plane Distance (MIPD) to the External Bicanthal Distance (EBCD) (m = 29.42). Synthesis work in ocular-orbital biometry [8] relates the thousands of measurements, tables and numerous inter-correlations of the characters seen earlier. Only some of these are related here. The right/left symmetry of measurements, which presents a high correlation coefficient, a reflection of binocular vision (for MAL R/L r = , for EABC R/L, r = ). Orbital Depth (Depth R/L, r = ), will be looked at later. The position of the ocular globe explains the high index correlation (for IOD/EBCD, r = , for IOD/MIPD, r = , for IOD/MIPD, r = ). These are transversal indices. In the sagittal plane a negative correlation is observed between the Ante-Bicanthal segment of the ocular globe and the Orbital Depth (r = ). The Orbital Depth related to its aperture angle shows high correlation (r = ). The nature (matching, anatomical closeness ) of significant correlations, like their multi-factor analysis completes the statistical work referred to earlier [8]. Correlation with Hertel s exophtalmometry is established [9] Maxillofacial biometry in the NOP, by X-ray scan and embedded ocular facial contouring [10]. The quality of the previous statistical correlations resulted in a request to use NOP references for the acquisition of facial contouring by X-Ray scanner as from This contouring produces a large scale ocular globe, a key factor in an ocular stimulator-recorder used on board the space shuttle (Space Lab European Research, 1983). The practical creation of the equipment was entirely satisfactory. A horizontal dento-maxillofacial biometric application for the X-Ray scan in the NOP was quickly sought [11]. A population of 76 patients was therefore studied, presumed healthy for the anatomical region under consideration, and aged between 19 and 82 years, with an average cephalic index = 78 (74/84). 7 measurements were established, 4 linear and 3 angular, on the cranial base. The Inter- Pterygoid distance (IPD) measures the gap between the anterior extremity of the 2 pterygoid apophyses (m = 36 mm (31/48). The Inter-Styloid Distance measures the gap between the base of the 2 styloid apophyses (m = 76 mm (89/63). The Inter-Condylar Distance (ICD) measures the gap between the central point of the two mandibular condyles on N Modification MODIFICATION PAPILLAIRE of the papilla Normal PAPILLE NORMALE Papilla FIG I.N.O. FIG. 9 Exophtalmometry by axial section (MRI or X-Ray scan), anterior visual pathways, from crystalline lens optic canal. 7

8 SCIENTIFIC & MEDICAL the temporal articular facet (m = 103 mm (93/116)). The Extreme Inter- Zygomatic Distance (EIZD) measures the longest transversal zygomatic diameter (m = 117 mm (110/120)). 3 angle measurements complete the series. The Sagittal Plane Condyle Angle (SPCA) measures the orientation of the condyle on the median sagittal plane (m = 63 5 (R), 66 8 (L)). The Sagittal Plane Ramus of the Mandible Angle (SPRMA) measures the orientation of the mandible ramus angle (m = 14 5 (R), 12 (L)). The Angle of the Posterior Wall of the Maxillary Sinus (APWMS) measures the orientation of the posterior-external wall of the maxillary sinus on the sagittal plane (m = 38 9 (R), 43 3 (L)) Exophthalmometry and dysthyroid ophthalmopathy: from I-III grading to the De Saint-Yves syndrome [12]. Dysthyroid ophthalmopathy was the first practical field of application of ophthalmometry in the NOP (Fig.10). In 1978, it was shown that the cephalic fixedness of the visual pathways plane enables quantification of ocular-orbital topographical normality in adults. The Ocular-Orbital Index (OOI) is used to establish 4 topometric classes. Beyond normality (60 < IOO < 70), a grade I axile exophthalmia is confirmed in the value: 70<IOO < 100. Grade II is defined by OOI = 100, that is to say the tangency of the posterior pole on the External Bicanthal Plane (EBCP) and grade III by a value of IOO > 100, that is to say by the projection of the posterior pole of the globe out of the External Bicanthal Plane. This is therefore, strictly speaking, an exorbitism. Figure 10 reminds us that, although exophthalmia can be stated absolutely (increase in the value of the OOI), in one of the 2 eyes, and in a relative way from one eye to the other (difference of the OOI and millimetre difference in the EABC segment), the inversion of the OOI index in newborns and the very old must be remembered (maximum enophthalmia with OOI of 30 %). An ocular dystopia moves the horizontal ocular meridian of the NOP vertically. This situation does not prevent recognition of the plane itself, with the approximation becoming firstly clinical-cutaneous (lateral markers) and then anatomical on the X-Ray or MRI scan image. The symmetry of the external orbital pillars, optic canals and lateral masses of the ethmoid enable recognition of the visual plane. Shifting of the globe is then easily measured on the succession of section planes. For the past 30 years (1983), MRI has undertaken vertical and oblique exophthalmometry, that of the OTNOP (Fig. 11). Results quantified as normal and variants are the object of research work (unfortunately now halted) that was to give an answer, by the vertical plane of the MRI, to ocular-orbit biometry in case of vertical movement of the globe (process occupying the space adjacent to a horizontal wall or a malformation syndrome, for example). Evolutive monitoring under medical treatment or after surgery requires precise biometry in a strict NOP only. Whence the obligation of using MRI for therapeutic monitoring, the repetition of the examination, in circumstances of cephalic tilting and acquisition parameters permitting anatomical comparisons. It is necessary to carry out an initial pre-therapeutic examination to act as an undisputed anatomical reference, which will become a medico-legal obligation. This truth of ophthalmometry by X-Ray or MRI scan represents the fulfilment of the following observations: reality and fixedness of the NOP, reality, fixedness and symmetry of ocular-orbit biometry in normal adults (conditions of emmetropia and binocular vision). Between 1980 and 1982, 432 observations of dysthyroid ophthalmopathy (amongst measured by X-Ray scan) were brought together after the publication of a preliminary series of 60 cases [12]. In collaboration with N. Newman, B. Illic, T. Laroche and S. Liotet, various series permitted the definite validation of exophthalmometry and a better knowledge of the mechanisms of endocrine ophthalmopathy. It was biometric and anatomic comparisons in patients followed and treated for Basedow s disease and in patients consulting primarily for isolated exophthalmia or inaugural oculomotor disorder that resulted in further knowledge. The name of De Saint Yves syndrome was suggested in view of the anatomic and biometric observation of axial, unilateral or bilateral exophthalmia, still unrecognised initially and clinically, before any biological verification. It is still awaiting a nosology framework, based on the biometric observations made by X-Ray or MRI scan. It is an isolated exophthalmia, often barely visible clinically (1 to 2mm), with a normal O.O.I. Grades of exophthalmia FIG. 10 Grades of exophthalmia (dysthyroid ophthalmopathy). FIG. 11 Clinical application of the OTNOP: the direct view of the 4 segments of the optic nerve (intra-ocular, intra-orbital, intra-canal and intra-cranial intracisternal), offers varied semiological diagrams, showing the diameter and nerve signal: atrophia, SEP plaque, vascular accident, intrinsic and extrinsic tumour pathology, dilation of spaces by HIC. 8

9 SCIENTIFIC & MEDICAL muscular volume and increased volume of the intra- and extraconic fatty compartments. Mr de Saint-Yves, the first ophthalmologist surgeon, describes in his treaty published in 1773, i.e. 67 years before Basedow and 64 years before Graves, the existence of a fatty issue when an inferior palpebral incision is made in an exophthalic and tachycardic patient. This was the object of a presentation made to the National Academy of Medicine, for which a prize was awarded [12]. It will be remembered that physiological enophthalmia observed at the extreme ages of life is caused by the low relative volume of intraorbital, retrobulbar intraconic and extraconic fatty compartments. The close hormonal dependency of the intra-orbital lipocyte explains the frequency of dysthyroid exophthalmy, as well as the first application of quantitative orbital-ocular biometry by X-Ray scan in exophthalmometry. This biometric data is used by E. Modigliani in MRI, with correlation of endocrinological therapeutic monitoring. 4. Ocular-orbital growth, strabology: orbit angles and depths by X-Ray scan Our ophthalmologist colleagues have shown that a significant ocular-orbital biometric difference can be explained by the occurrence of an acquired organic unilateral amblyopia (traumatic cataract), with converging strabism before puberty and divergent afterwards. Details of the results of this work are not included here. [1]. The growth of the normal globe is shown by ultrasound scan, as indicated in the book referred to [1]. This anterior-posterior axial measurement of the ocular globe, in utero, reproduces the exponential shape of the growth graphs of the foetus exactly, from the age of 3 months to birth and then from birth up to the age of 9 years [1]. Today these direct linear measurements are accessible by MRI of the foetus in utero with high anatomic resolution enabling on its own the recognition of the existence of a congenital malformation syndrome. The FO report by H. Mondon and P. Metge already mentioned [9] provides a table of the average of the linear, angular and index measurements in myopia. Observation of a dominant posterior expansion of the ocular globe is the main result of the study. Measurements of the orbital volume by X-Ray scan, from living to fossil skull, provide useful data on the growth of the orbital volume, from birth to the age of 20, of a factor 4 approximately, along with a constancy in orbital volume in recent paleanthropians ( Ferrassie I, Cro-Magnon, La Chapelle aux Saints I ). Dynamic muscular biometry (IRMOD) in NOP and OTNOP, in adults, is revealed in muscular and angular detail, with calculation of the globe s centre of rotation, in the aforementioned book. Reference must also be made to the work done by A. Roth and C. Speeg-Schatz in oculomotor surgery and strabology [15]. 5. Direct recognition of an optical neuropathy (tumour, vascular accident, genetic congenital atrophy of the optic nerve, ), either directly or by intracranial hypertension, is another major application of this work. In the 3 planes which they themselves therefore define, the 2 intra-orbital segments of the optic nerves become the key to encephalic exploration, to its inflammatory effects (S.E.P.), tumours, and degenerative effects (glaucoma and rarefaction of neuro-optical neuro-tractography. 6. Biometry of intracranial and encephalic visual pathways, sectional and vascular descriptive anatomy by X-Ray scan and MRI, anatomy of development (embryology) and of growth, velocimetric, then molecular and genetic circulatory anatomy are the chapter headings that conclude the study of these twelve anatomies. The density of illustrations, from sectional anatomy to 3D anatomy, explains why it is not possible to report on this material here, nor even to condense it within the space available. The reader is invited to refer to the work mentioned on numerous occasions [1], another document is in the course of preparation. Points de vue REFERENCES 1. L imagerie en ophtalmologie. Cabanis EA, Bourgeois H., Iba-Zizen M-Th et 257 collaborateurs, rapport de la Société Française d Ophtalmologie, Masson, Paris, 1996 (762 p.) 2. Imagerie de l encéphale, de la cellule à l organe. La neuro-imagerie aujourd hui. Une introduction. Cabanis EA., Iba-Zizen M-Th., Habas C., Istoc A., Stievenart J-L., Yoshida M., Nguyen TH., Goepel R., Séance commune Académie nationale de Médecine et Académie des Sciences, ANM, Paris, , Bull. Acad. Natle Méd., 2009, 193, n 4, Sur le plan horizontal de la tête et sur la méthode trigonométrique. Broca P. Bull. Soc. Anthropol., Paris, Paul Broca. Un géant du XIX e siècle. Monod-Broca P., Vuibert, Paris, 2005 (310 p.) 5. Topométrie crânio-encéphalique chez l homme. Delmas A., Pertuiset B., Masson et Cie, CC Thomas, Paris, Springfield, 1959 (515 p.) 6. Referentially oriented cerebral MRI anatomy. Talairach J., Szikla G., Tournoux P. George Thieme Verlag, Stuttgart, Le plan orbitaire chez l adulte jeune, sa position relative à d autres éléments architecturaux de la tête. Etude vestibulaire. Fenart R., Vincent H., Cabanis EA., Bull. Mém. Soc. Anthropol., Paris, 1982, 9, 13, les coupes orbitaires axiales dans le plan OM avec erreur diagnostique consécutive puisque, derrière le globe oculaire gauche, on croit voir une tumeur qui n en est pas une ; 8. Computed tomography of the optic nerve, part 2. Size and shape modifications in papilledema. Cabanis EA., Salvolini U., Rodallec A., Menichelli F., Pasquini U., Bonin P., J Comput. Assisted Tomogr., 1978, 2, Tomodensitométrie et œdème papillaire dans l hypertension intracrânienne, Rodallec A., Thèse Méd., Paris, Contribution de la tomodensitométrie au diagnostic des ophtalmopathies dysthyroïdiennes. De Hounsfield (1972) à De Saint-Yves (1722). Cabanis EA., Mémoire pour l obtention de la médaille de la Ville de Paris, Académie nationale de Médecine, Paris, 1982 (150 p., biblio) 11. Biométrie oculo-orbitaire axiale in vivo, par tomodensitométrie orientée selon le plan neurooculaire. Iba-Zizen Cabanis M-Th., Mémoire pour le DERBH mention anatomie, université Paris 5 René Descartes, 1983 (160 p.) 12. La myopie forte. Metge P., Maurin JM., rapport de la Société Française d Ophtalmologie, Masson, Paris, Stimulateur-enregistreur des mouvements oculaires. Olivier S., Pohl D., Mémoire, Ecole nationale supérieure des Arts et Métiers, Paris, 1982 (110 p.) 14. Radiologia maxillo-facciale et odontostomatologia. SIRMN, A. Chiesa, Monduzzi, Bologna, 1983, Compte rendu du SKERI Symposium des 7 et 8 novembre Roth A. et coll. Proceedings 41 e semaine strabologique internationale, Société suisse d ophtalmologie, Zermatt

10 SCIENTIFIC & MEDICAL THE ROLE OF THE EYE S CENTRE OF ROTATION IN LENS DESIGN MO JALIE University of Ulster, UK Rotation of the eyes in their sockets is brought about by the action of the extrinsic ocular muscles. The muscles enable the eyes to scan the field of vision and in the case of the spectacle wearer, to view through extra-axial points of the spectacle lenses. This statement sums up, in a nutshell, the problem facing the designer of spectacle lenses; how to produce a lens which has the same effects when viewing off-axis, as it does when the subject views through the optical centre? When the eyes rotate behind a spectacle lens to view off-axis objects, it is assumed that they rotate about a fixed point near the middle of the eyeball. To the lens designer tracing meridional rays into the eye, the fact that the real pupil of the eye, which of course, rotates with the eyeball, can be totally ignored and an assumption made that there is a small fixed stop located at the eye s centre of rotation through which the chief ray of the oblique pencil passes on its way to the fovea (Fig. 1). Even when real rays (skew rays) are traced through the actual pupil of the eye, it is the pupil size which becomes important, rather than its position, and the chief ray of the oblique pencil is still assumed to pass through the eye s centre of rotation. The vertex sphere is an imaginary spherical surface centred at the eye s centre of rotation, the radius of which is called the centre of rotation distance, CRD, and the off-axis powers of the lens are measured at the vertex sphere. These powers are known as the oblique vertex sphere powers since they are measured along the oblique ray path from the vertex sphere. Figure 2 illustrates how the lens designer might show the variation in the oblique vertex sphere powers for a D spectacle lens made first with a D front spherical curve (Fig. 2a) and secondly with a D convex hyperboloidal front curve (Fig. 2b) whose asphericity has been chosen to eliminate the difference between the tangential and sagittal oblique vertex sphere powers to produce a point focal lens. It is seen from these field diagrams that in the case of the lens with spherical surfaces, as the eye rotates away from the optical axis, the tangential oblique vertex sphere power, F T, increases at a faster rate than the sagittal oblique vertex sphere power, F S. When the eye has rotated through 35º from the optical axis, the sagittal power, F S, whose value is D, has hardly changed from the back vertex power of D, It will be realised that when designing a spectacle lens to have a particular off-axis effect, for example, to be free from aberrational astigmatism (point focal lens), it is the position of the eye s centre of rotation with respect to the lens which must be known, rather than the vertex distance, which is simply the distance from the back vertex of the lens to the cornea. Of course, the vertex distance determines the correct back vertex power of the lens but, otherwise, is of no real interest to the lens designer. It can also be seen in Figure 1 that when the eye rotates behind the lens, away from the optical axis, the distance from the apex of the cornea to the back surface of the lens increases. Thus, in order to be able to compare the off-axis effects of different forms of lenses it is necessary to set up a reference surface at which the off-axis powers can be measured. This reference surface, which is concentric with the eye s centre of rotation, is called the vertex sphere and is shown by the dashed circular trace in figure 1 which just touches the back vertex of the lens. FIG. 1 The significance of the eye s centre of rotation. Note the imaginary stop placed at the position of the eye s centre of rotation. The dashed line which passes through the back vertex of the lens is the vertex sphere from which the oblique vertex sphere powers are computed in oblique gaze. FIG. 2 Field diagrams comparing the off-axis performance of spectacle lenses of power D. a) D lens (CR 39) of poor form, made with spherical surfaces. Front curve, +5.50, axial thickness 4.0mm, CRD 27 mm. b) D lens made with convex hyperboloidal surface, p = -1.75, Front curve +5.50, axial thickness 4.0mm, CRD 27 mm. When the eye is looking along the axis, through the optical centre of the lens (0º) the effect of the lens is D. When the eye rotates upwards (+ sign on the rotation angle) or downwards (- sign on the rotation angle) the tangential and sagittal oblique vertex sphere powers differ from the axial value.the variation in power is plotted horizontally. 10

11 SCIENTIFIC & MEDICAL but the tangential power, F T has become D. The difference between these two values, given by F T - F S is the oblique astigmatic error, OAE, which amounts to D. This degree of aberrational astigmatism would, of course, cause a significant amount of blur when the subject viewed through this zone of the lens. In order to eliminate this astigmatism for the 35º zone of the lens, when just spherical surfaces are employed, the lens must be bent into a more steeply curved form with a D front curve. Naturally this more steeply curved form will be thicker and heavier and more bulbous in appearance, not to mention that, from an observer s point of view, the subjects eyes appear larger when viewed through the lenses. Figure 2b illustrates the improvement in the optical performance when an aspheric design is used for this lens. The front surface is a convex hyperboloid with an asphericity, p = -1.75, whose inherent surface astigmatism neutralizes the aberrational astigmatism of oblique pencils. For the 35º zone of the lens, the tangential and sagittal oblique vertex sphere powers are each D and the OAE for this zone has been completely eliminated. Note, however that the off-axis performance is not perfect, there is a power error of D for the 35º zone. The lens designer refers to this error as mean oblique error, MOE. One of the major advantages of digital surfacing is that the software enables individual fitting characteristics to be entered upon receipt of the order by the laboratory to ensure that the original design criteria is still fulfilled by the lens. A typical situation is demonstrated by the field diagrams shown in figure 3. In figure 3a, the point focal aspheric D design whose off-axis performance is illustrated in figure 2b has now been mounted 4mm closer to the eye than the designer intended, the CRD being only 23 mm. The field diagram shows quite clearly that the lens is now afflicted with aberrational astigmatism amounting to some 0.25 D for the 35º zone of the lens. Although this is only a small amount of astigmatism, it will be appreciated that we can no longer honestly describe the lens as being point focal! However, if the input software is told that the CRD which is required for this wearer is 23 mm, it can be incorporated into the design steps, with the result that the asphericity of the convex hyperboloidal surface will change to the necessary value (p = -3.02) in order to restore the point focal property of the lens for the prescribed fitting parameter (Fig. 3b). It is important for comfortable binocular vision that any differential prismatic effect which is encountered when the eyes rotate to view extra-axial objects in the field is not excessive. This is particularly so when considering vertical differential prism because the eyes should not be called upon to exert supravergence movements. When single vision lenses are worn, it is usually only in cases of anisometropia that differential prism might present a problem. However in the case of progressive power lenses, it is important when the eyes execute version movements to ensure that both Minkwitz astigmatism and the mean power in different zones are similar to ensure that vertical differential prism is kept to a minimum. Needless to say, since the eyes rotate about their centres of rotation, accurate knowledge of its position is necessary to ensure comfortable vision. This requirement of ensuring that the vertical differential prismatic effect remains within tolerable limits is but one of the important features of the new Varilux 4D S-series progressive lenses ( Synchroneyes ). It is obvious from this discussion that when the position of the lens in front of the eye is incorporated into the design, the essential piece of information which is required is not the vertex distance but the centre of rotation distance. How can we measure the CRD in practice? The difficulties are not just practicai ones, in that we do not have access to the eye s centre of rotation! It is now understood that there is not a single point about which the eye rotates but that the position varies not only from eye to eye but also with the direction of gaze. In the past, it was usual for the designer to choose an arbitrary value based upon the best measurements available. For example, Donders (1864) [1] described a practical method which he used to investigate the position of the centre of rotation (which he referred to as the centre of motion ) and concluded that its mean distance from the pole of the cornea is about 13.5mm. Assuming an average value for the vertex distance to be about 12mm this would give an average CRD of 25.5 mm. M. von Rohr (1908) [2] when designing the original series of Punktal lenses assumed a value of 25 mm for the CRD, but recognised that in degrees of moderate to high myopia the centre of rotation was likely to move backwards with increasing axial length. One should bear in mind that the spectacle lenses of 100 years ago were of quite small diameter and that the vertex distance will increase, and therefore the CRD, not only as the power of the concave surface of a meniscus lens becomes more steeply curved, but that the sag of the back surface will also increase with diameter. These points were taken into account by Everitt [3] in the design of the Ultor series of best form lenses marketed by Stigmat Ltd. of London. Everitt chose the values: CRD = 25mm for plus lenses and CRD = 25 - F V / 6 for minus lenses where F V is the back vertex power of the lens. This rule recognised the increasing backward shift of the centre of rotation in axially myopic eyes. For example, in the design of a D lens, the CRD would have been taken to be 26mm, which was probably a reasonable value for the small lens diameters which were in use at the time. Fry and Hill (1962) [4] found that in a group of 28 of their subjects, the mean position of the eye s centre of rotation was 0.79 mm nasalwards from the visual axis and 14.8 mm behind the corneal pole. A typical mean value today which is frequently used in English speaking countries for the CRD is 27 mm (made up from a vertex distance of 12 mm, with the centre of rotation assumed to lie 15 mm behind the corneal pole). FIG. 3 Field diagrams comparing the off-axis performance of an aspheric D lens fitted at a shorter CRD (23 mm) than the designer intended. a) D aspheric lens made with convex hyperboloidal surface, p = -1.75, Front curve +5.50, axial thickness 4.0 mm, CRD 23 mm. b) Free-form D aspheric lens made with convex hyperboloidal surface, p = -3.02, Front curve +5.50, axial thickness 4.0 mm, CRD 23 mm. 11

12 SCIENTIFIC & MEDICAL The importance of the position of the eye s centre of rotation is becoming increasingly recognised in current ophthalmic practice. It is likely that the next edition of the International standard, ISO 13666, Ophthalmic Optics Spectacle Lenses Vocabulary will include the following two new definitions: - mechanical ocular centre of rotation point in the eye shifting the least during movements of the eye - optical ocular centre of rotation base point of the perpendicular drawn from the mechanical ocular centre of rotation onto the line of sight. The first of these new definitions recognises that the visual axis (line of sight) may not pass through the point about which the eyeball rotates, which must lie close to the centre of curvature of the sclera, whilst the second describes a method by which the position of the centre of rotation that is of concern to the lens designer may be located. Today, it is no longer necessary to estimate the position of the eye s centre of rotation. Its position can be measured precisely with the sophisticated fitting instrument, VisiOffice + illustrated in figure 4. Visioffice + is designed to provide not just the CRD but all the necessary fitting data required for the accurate positioning of spectacle lenses and also to determine the dominant eye for the new Varilux 4D S-series progressive lenses. Points de vue FIG. 4 Visioffice + (Essilor) For precise measurement of the position of the eye s centre of rotation and other fitting parameters for personalized lenses. REFERENCES 1. Donders F C 1864 Accommodation and Refraction of the eye. The New Sydenham Society, London. 2. Henker O 1924 Introduction to the Theory of Spectacles. Jena School of Optics, Germany 3. Everitt P F 1933 The Stigmat Guide to Authentic Best Form Lenses. Stigmat Ltd, London 4. Fry G A, Hill W W 1962 The center of rotation of the eye. American Journal of Optometry 39:

13 SCIENTIFIC & MEDICAL DOES THE EYE ROTATION CENTER PLAY A ROLE IN THE CHOICE OF LENS TYPE? HANS BLESHØY Director and optometrist, Bleshøy Optometri, Denmark INTRODUCTION The position of the optical centre and progression zones in spectacle lenses has been discussed for many decades. The effect of head- and eye movements has been investigated in situations involving everyday tasks such as distance vision, computer work and reading in combination with static and dynamic performance. The inter-person variability is big when looking at such parameters as PD, vertex distance, head shape as well as more general body related aspects such as movement, body position, head tilt etc. It is not uncommon that we observe a person with a slight head tilt to one side or the other (Fig. 1) or demonstrating a slight head turn right or left. Very frequently the muscles of the neck and upper tarsus are seen as important elements in variations in head posture, and problems in those muscles will almost certainly be a causative factor in some form for corrective counter measure in head and eye positioning. In addition to this we have to allow for the visual needs of each individual. FIG. 1 Example of head inclination. Optometrists in the clinics are all familiar with persons who function without visual problems when using a standard pair of reading glasses which has not been fitted in any particularly way. Despite not taking account of variations between right and left eye, different reading distances or general visual needs, these persons do not complain of vision related problems. On the other hand we are aware of people with high visual demands in varied situations, in which settings even small inadequate corrections will cause significant problems. FIG. 2 14mm 11mm Lense type 11 mm, Lens type 14 mm. Research into the structure and design of progressive lenses has been ongoing for many decades. All serious glass manufacturers devote significant resources to research aimed at achieving a better understanding of visual function in different behavioral patterns. The very understanding of the visual drawbacks we experience with by age, has led to a remarkable transformation in the design of spectacle lenses. The need to optimize the visual function has changed dramatically over the last 10 years and we now have work related tasks which put the visual system to the limit. This involves vision based decision making and not least effectiveness in our busy business lives today. We know that the demand for energy, by the visual function, is very high. It is estimated to demand between 25-50% (Jensen 2008) of the total energy available. By optimizing the visual function, we may be able to limit the wrong or inappropriate waste of energy, which then may be made available for other and more useful purposes. PURPOSE AND BACKGROUND FOR THE STUDY Anatomic as well as physiological circumstances are very variable from person to person. Head position plays a significant role and accordingly the centre of eye rotation (ERC) is of interest. Every optometrist has come across Listing s law during their education, which describes the eye position during saccades. Many studies have followed and Crawford & Vilis (1991) showed that during slow movements the eye position will often deviate from Listings area despite being relatively small variations. During fast eye movements these are likely to be compensated for by a head movement. In this manner a continuous communication exists between eye- neck- & shoulder muscles groups and the visual input which is dictated by the level of concentration and awareness of the person as well as the demand of the visual task in which that person is engaged. Controlling these elements is highly associated with the physical and mental status of the person, and in turn puts a high demand on the energy available at the time. In the literature we may find numerous and quite individual variations on eye behavior regarding position, and it may seem impossible to take all of these into account at all times. This, however, should not stop us being aware of possible problems, and in the individual circumstance deal with possible solutions which may solve or at least reduce the visual discomfort and inefficiency which may be encountered during everyday life. Over the last few years, Essilor has been very interested in the center of rotation of the eye (ERC). This research has led to a better understanding of what the ERC actually is, the position of the ERC and which effect there is on the use of spectacles with single vision and multifocal lenses. With this research in mind, Essilor has developed a production technique which may compensate for the individual deviations in ERC. Definition of the center of rotation of the eye (ERC): Centre of rotation of the eye When the eye rotates in its orbit, there is a point within the eyeball that is more or less fixed relative to the orbit. This is the centre of rotation of the eye. In reality, the centre of rotation is constantly shifting but by a small amount. It is considered, for convenience, that the centre of rotation of an emmetropic eye lies on the line of sight of the eye 13.5 mm behind the anterior pole of the cornea when the eye is in the straight ahead position (straightforward position), that is when the line of sight is perpendicular to both the base line and the frontal plane. Millodot: Dictionary of Optometry and Visual Science, 7 th edition Butterworth-Heinemann 13

14 SCIENTIFIC & MEDICAL In the present study we define the normal vertex distance as 12 mm, which provides us with a total distance from the center of eye rotation (ERC) to the back surface of the spectacle lens to be 13,5 mm + 12 mm = 25,5 mm. This standard will be used in the following analysis of the clinical data. STUDY DESIGN Hypothesis: Eyecode TM will provide an improvement in visual function and visual comfort for the spectacle user. Study design: patients were selected amongst existing users of Essilor lenses in the following categories: a. Varilux Comfort New Ed 4 patients Lens type A b. Physio 2,0 F360 4 patients Lens type B c. Physio 2,0 4 patients Lens type C 2. The patients were contacted from a list of patients generated from a patient database (Optik-IT - practice based database). Patients had to be using one of the lens types mentioned above, and were approached in a numerical order as generated by the Optic-IT database. In the event a patient did not which to participate, the patient next on the list was contacted. All patients were contacted by the same investigator (HB). 3. Inclusion criteria: a. Must have been issued new spectacle lenses in one of the 3 categories within the past 6 months b. Must participate on a voluntary basis c. Must be able to attend the necessary visits to the clinic d. Start of the study during week 26 where ERC measurements must be performed for lenses with Eyecode TM e. Fitting of new lenses in existing frame starting week Measurements for Eyecode TM and ordering of lenses with EyecodeTM. All measurements were performed by the same investigator (JJ Essilor) 5. Test of Eyecode TM design during approx. 2 weeks 6. Filling in questionnaire 1(Tab. 3) 7. Exchange of Eyecode TM lenses back to the original lenses without Eyecode TM 8. Filling in questionnaire 2 (Tab. 4) 9. Forced choice of preferred lenses between the two lens types with/ without Eyecode TM. Preferred lenses to be fitted and issued 10. Conclusive report RESULTS Patients were recruited into the following three categories: a. Varilux Comfort New Ed 3 patients b. Physio 2,0 F360 4 patients c. Physio 2,0 4 patients It was not possible to recruit all 4 patients in group a within the time limit. All participants accepted the inclusion criteria. Raw data for ERC are presented in table 1 & 2. Individual data are presented for right and left eyes. Most patients were hyperopes, which is not unusual for a presbyopic population. The group consisted of 7 hyperopes, 2 emmetropes and 2 myopes. Most patients showed good harmony in ERC between right and left eyes, and only patient No 6, 7 and 10 deviated in their ERC between right and left eye of up to 0.9 mm. For analysis purposes the mean between right and left ERC was used. When switching from lenses without Eyecode to lenses manufactured on basis of Eyecode, all participants answered questionnaire 1. It is noteworthy that all participants experienced the shift to something positive or unchanged compared to the original lens. None of them experienced a negative effect. TAB. 1 Clinical data for ERC. Raw data for ERC ERC (mm) Right eye Left eye Normal average (25,5 mm) Group average (23,4 mm) Client TAB. 2 Patient data for ERC. 14

15 SCIENTIFIC & MEDICAL Better Worse No change TAB. 3 Assessment of the change in lenses without Eyecode TM to the lenses with Eyecode TM design (Questionnaire 1). Easy Difficult No change TAB. 4 Assessment of the change in lenses from Eyecode TM design to lenses without Eyecode TM (Questionnaire 2). In such clinical trials, there may be a relative high risk that the test persons automatically will believe that something new means an improvement. In order counter such an effect in the best possible way (if not blind study is used) is to ask the test persons to wait several days before completing the questionnaire, but answer it within days. This reduces the immediate favorable effect of something being new, and helps in making the optical function focus of the assessment. In addition a cross-over test is applied, by switching the test lenses back to the original lenses. The results of this second phase are presented in the answers to the second questionnaire and are illustrated in Table 4. CONCLUSION The response given when changing back to lenses without Eyecode TM are almost unanimous in all areas. None of the test person experienced any advantage by changing back to the original lenses. All of them decided on using the lenses with Eyecode TM when asked which lenses they would prefer. Further all had the offer to keep the original lenses and have them fitted into a similar frame at only the cost of the frame, but none of them accepted this offer. It is noteworthy that only two test persons experienced difficulties in converting back to the original lenses, where 8 of the test persons didn t experience ant difficulties. This may appear somewhat misleading when each question is analyzed separately. The answers in general gives the impression that the original lenses performed worse or indifferent to the Eyecode TM lenses. It might be expected that the test persons demonstrating the largest deviation from the norm value of ERC would be those who showed the biggest advantage. However, this advantage was not seen to be exclusive to this group. Table 5 shows the level of positive responses in all questions of questionnaire 1. We may observe a correlation between the level of deviation from norm ERC (25.5 mm) and the level of positive responses when changing to lenses with Eyecode TM technology. 15

16 SCIENTIFIC & MEDICAL At the cross over back to lenses without Eyecode TM design none of the test persons demonstrated a positive response. Table 6 shows that all test persons experienced a poorer visual function when returning to the original lenses. However, we cannot conclude any correlation between the test persons with the largest deviation also show the largest response. Changing from EyeCode lenses back to the original lenses, none of the test persons experienced improvement in any of the sections (table 6). The section which demonstrated the least difference between the lens designs was when changing between light and dark conditions. The majority of the test persons did not feel any difference. When analyzing all questions in table 3, and weighing each question equally, it may be observed how each test person judge the advantages when changing from the original lens design to lens design with Eyecode TM (Table 7). Only 3 test persons valued the advantages to be less than 50%. In the same way we may analyze all questions in questionnaire 2, when changing from Eyecode TM lenses back to the original lenses. This compiles all data in table 4, and the result may be seen in table 8. Likewise we observe that only 3 test persons judge the disadvantages to be less than 50% when changing back to the original lenses. The conclusion is that the vast majority of the test persons judge the Eyecode TM design to be the most advantageous (Table 6). The ultimate choice between lens designs with or without Eyecode TM was decided in various ways. All test persons who answered this question declared that it was easy to change to the new design. It is of interest, though that a large group (8 out of 10) also mentioned that it was easy to revert back to the original design. When the test persons were asked to make at choice of which lens design they wanted to continue with after the test period, all of the test persons decided to use Eyecode TM designed lenses (Table 9 & 10). Positive reaction in questionnaire Deviation in ERC from norm value (mm) TAB. 5 The noted advantage in relation to the level of deviation from norm ERC value. Deviation in ERC from norm value (mm) -6 Negative response in -4 changing back to lenses -2 without Eyecode TM TAB. 6 Negative response when changing back to the original lenses without Eyecode TM. Percentile improvement for each individual persn when changing to lenses with Eyecode TM Persons who already were used to more advanced lens designs (F-360) were those who appreciated the advantages of Eyecode TM the most. The less advanced designs like Varilux Comfort, and to a certain degree also Physio 2, also appreciated the Eyecode TM design although to a slightly lesser degree. Furthermore we may see that deviations of more than 1 mm in ERC from the norm of 25.5 mm, tend to make it even more appreciated that Eyecode TM design will improve the visual performance and comfort. This pilot study may only provide an indication on the effect of Eyecode TM design. The small number of test persons limits the possibility of statistical analysis. It may, however, give an indication on the effect of using individual designs for persons who deviate more than 1 mm from the standard centre of rotation of the eye (ERC). This may be even more important and relevant to persons who have a high visual demand, and who already are very much aware of their choice of individually designed lenses such as F TAB Percentile improvement for each individual person when changing to lenses with Eyecode TM. Percentile worsening for each individual person when changing back to lenses without Eyecode TM Va Var Var Comfort Ph Physio-2 Physio-2 F-3F-360 F-360 There is in this study exclusively focused on the clinical assessment available from randomly selected test persons. There is no attempt to explain how a sophisticated lens design as Eyecode TM is designed to compensate for individual variations in the eye s rotation center. The study and the findings must be assessed based on those practical clinical conditions which optometrists encounter in their everyday lives. % Var Var Comfort Comfort Physio-2 Physio-2 F-360 F ERC (mm) TAB. 8 Percentile worsening for each individual person when changing back to lenses without Eyecode TM. 16

17 SCIENTIFIC & MEDICAL TAB. 9 & 10 The test persons final choice between lenses with or without Eyecode. The demands for an optimal visual function are greater than ever. We are measured by the effectiveness and productivity in our workplaces, and the visual function is that of our senses which delivers by far the most information in our daily lives. Although there is a risk of too detailed conclusions, this study gives us a feeling that more individualized lens designs, such as Eyecode TM, may be able to satisfy our visual needs to a higher degree. It is therefore recommendable that information on these newer individualized designs are given at least to those persons who may be considered the target group (+/- 1.0 mm deviation from std ERC). As a minimum future spectacle wearers should be informed about the new designs, in order that they may be able to make an informed decision. Points de vue Statement of independence The author of this report has a natural curiosity in trying to combine theoretical issues with clinical practice in order to provide the most advantageous vision correction to those who need them. With this in mind, Essilor Denmark asked the author to conduct a clinical evaluation on how spectacle lenses designed to compensate for deviating centers of rotation of the eye, are received by the end user. The author has no financial interest in the product, and the investigation was undertaken without any specific demands from Essilor. The conclusion and interpretation is that of the author alone. REFERENCES 1. Jensen 2008; Hjernen - før, nu og i fremtiden. Hjernens udvikling hos mennesket (S 25). Hjerneforum Crawford & Vilis 1991; J Neurophysiology (65); Millodot 2009; Dictionary of Optometry and Visual Science, 7 th edition Butterworth-Heinemann 17

18 SCIENTIFIC & MEDICAL STUDY OF VERGENCE MOVEMENT DYNAMICS BÉRANGÈRE GRANGER Optométriste O.D. R&D Optics-Vision Science Department - Essilor, Paris France TARA L. ALVAREZ Associate Professor Department of Biomedical Engineering New Jersey Institute of Technology, New Jersey, USA JOHN SEMMLOW Ph.D. Professor Rutgers University and Robert Wood Johnson Medical School New Jersey Institute of Technology, New Jersey, USA Eye movements, particularly vergence movements, are extremely important for the visual exploration of space in depth, both from a kinetic point of view for precision of fixing on the fovea and from a static point of view, for stability of fixation limited to the macular area. For a long time researchers believed that the vergence dynamic worked using a closed loop system (feedback control). The vergence oculomotor system compared the current position of eye with the desired visual stimulus and would move the eyes until the eyes were adjusted to align on the target. The input signal for this system is vergence disparity required to fix a target that activates the vergence generator, by means of sensorial processing. Effective vergence of the eyes is then subtracted from the required vergence until the difference between the two reaches nil. The vergence model or Dual Mode Theory put forward by John Semmlow in 1984 (Fig.1) now considers there to be double control of the motor command. This model includes an initial rapid vergence phase or Transient component causing the impulse needed to move each eye quickly, despite the viscosity of the eyeball. This initial phase works in an open loop also called preprogrammed control, i.e. it does not depend solely on visual information. It is then followed by a slower or Sustained component phase, which brings the 2 eyes to their final optimal position. This second, visually guided phase, operates in a closed loop. The combination of speed and precision reflects both the difficulty of the motor task and the complexity of the neuron control systems. This model has also been confirmed by neurophysiological data that shows the existence of phasic cells (Transient Component) and tonic cells (Sustained Component) in the brain areas responsible for vergence movements [3, 4]. (Fig 1) This is a very interesting approach since it translates the visual system s capacity to partially pre-programmed control in ocular vergence. We believe that this property could play a part in compensation for the optical disparities caused by a new visual environment, notably when adapting to new prescription lenses. Based on this hypothesis, we have been working in collaboration with John Semmlow s and Tara Alvarez teams at the New Jersey Institute of Technology (Newark, NJ) since 2003, to study the dynamic characteristics of vergence movements, particularly during modification of the visual task. ANALYSIS OF THE VERGENCE RESPONSE Type of movement In all our experiments, we used stimulation in the mid-sagittal plane only in order to observe pure or symmetric vergence movements FIG. 1 Dual-Mode Theory (Semmlow and Hung ). 18

19 SCIENTIFIC & MEDICAL compared to asymmetric vergence movements where the eyes move between targets positioned differently, both in terms of direction and distance, with such movements requiring an association of vergence movements and eye saccades. Experimental conditions FIG. 2 Device and experimental conditions. To observe pure vergence movements, stimulations must be used only in the mid-sagittal plane. To do this we used a haploscopic device fitted with two video screens, which project the image for the right eye and the image for the left eye (Fig. 2). Recording of ocular movements is done using an Skalar (Skalar Iris/ model) limbus tracking sensor with 0.1 resolution. Data are acquired at a frequency of 200Hz. This system can record only horizontrol or vertical movements. Last year, the system was improved by integrating a video based system ISCAN which tracks the pupil and corneal reflection at 240Hz and can measure horizontal and vertical movements simultaneously as well as pupil diameter. Eye movements are recorded and registered separately. The head movement is stabilized using a head and chin rest assembly to reduce any influence from the vestibular system. The target is a green LED to stimulate accommodative vergence and disparity vergence. The target is presented in different positions (8, 12, 16 and 20 ) based on which vergence movements are recorded in 4 stages. (Fig 2) All measurements were made on a control sample of 8 subjects aged 18 to 35 years. Identification and quantification of vergence movement components Independent component analysis (ICA) is a method of data analysis involving statistics, neuron networks and signal processing. Historically it has frequently been used as a method for separating sources which are occurring simultaneously but are independent. The classic illustration is the cocktail party problem. At a cocktail party P microphones are set out in a room where N people are talking in groups. Each microphone records the superposition of the conversations of the people around it, and the problem consists of finding the voice of each individual after removal of the other voices, seen as interference. There must be as many microphones are there are independent sources. FIG. 3 Illustration of the Validation of Independent Component Analysis (ICA) in Vergence Responses. 19

20 SCIENTIFIC & MEDICAL ICA is used to solve this problem by considering simply that people talking at a given moment in time have independent conversations [6]. Within the context of our study, this method was used to isolate and then quantify the motor components of the Transient and Sustained vergence response (Fig.3) on which the concept of the Dual mode theory vergence model relies [2]. The vergence response shown, in Figure 3 (left side), is broken down into principle components (right side). The known model sources are shown in blue while the sources from ICA are shown in red. The model simulations and known inputs are very similar validating that ICA is appropriate to dissect vergence to study transient component (TC) and sustained component (SC). To quantify the dynamic performance of vergence, we relied on a quantitative parameter calculated based on the recording of eye movements. This performance criterion or Peak Velocity is calculated based on maximum speed, according to the amplitude of the movement, for each of the components. CHARACTERISTICS OBSERVED Differences linked to type of movement An observation of pure vergence movements shows differences depending on the type of movement. Indeed, vergence dynamics are different for convergence and divergence (Fig.4). Also, the convergence dynamic would appear to be independent of the initial position of the stimulus whereas divergence movements depend on this position, that is to say, the closer the target the quicker the response. These results are important since they infer divergence is not merely relaxation of convergence. Results of neurological studies have shown, moreover, that the control system is different, due to the identification of distinct nerve cells [1]. Differences between individuals An analysis of the dynamic characteristics of vergence movements also shows differences between individuals. The speed and intensity of the movement vary from one person to another for a given disparity, as shown in Figure 5. The peak of the transient component (blue responses in Figure 5) can vary substantially between individuals. A study of the dynamic components in vergence response shows us that there are different dynamic profiles for a given task. What happens when the visual task is changed or repeated? What is the capacity of the oculomotor system to adapt to a new visual environment? ADAPTIVE MODIFICATIONS Introduction of a new phase to the initial experimental protocol With the aim of defining the impact of an adaptive modification to the dynamic characteristics of vergence, we introduced a new phase to the initial experimental protocol to study how much a person could change his or her vergence Peak Velocity during an oculomotor learning experiment. After the control stage, the baseline phase in which we recorded a only 4 steps, the individual began the modification phase. During the modification phase, the individual start saw 4 deg steps randomly intermixed with double steps (2 steps of 4 each with a 200 msec delay between for a total disparity stimulus of 8 ). There were five times as many double steps to single step. The experiment sought to determine how the double steps influenced the Peak Velocity of the single steps. Typical 4 Degree Divergence Eye Movements Position (deg ) Typical 4 Degree Convergence Eye Movements Velocity (deg /s) Velocity (deg /s) Velocity (deg /s) Subject : 01 Position (deg ) Subject : 02 Time (s) Far responses Near responses Time (s) Position (deg ) Velocity (deg /s) Subject : 01 Position (deg ) Subject : 02 Time (s) Far responses Near responses Time (s) FIG. 4 Example of a recording of dynamic responses for convergence and divergence (4 ). 20