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Manufacture of Epikeratophakia Lens

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Manufacture of Epikeratophakia Lens Eye (1988) 2,395-399 Manufacture of Epikeratophakia Lens B. L. HALLIDAY London Summary Epikeratophakia lenses may be manufactured from donor cornea either with a lathe or using devices that make flat cuts on to previously deformed cornea. Details are given of a manufac­ turing technique using a cryolathe, including the storage and preparation of donor corneas and derivation of the formulae used to determine the radius of lathe cut. Epikeratophakia has become established as a donor cornea may either have a curved cut relatively safe surgical alternative for the cor­ made on it, as with a lathe, (Fig. 1) or it may rection of aphakia. Non surgical methods of be cut flat across its surface, as with a guil­ correction with spectacles or contact lenses lotine (Fig. 2). In the latter case it is neces­ are generally preferable. If these methods sary to deform the cornea before cutting so are impractical and if intraocular lens implan­ that the flat cut results in the desired curved tation is contraindicated, then surface. epikeratophakia may be appropriate. The The advantages of lathing are that the technique has also been used in myopia and required curvature is cut directly and there­ in keratoconus. fore predictably, and that the high speed of The surgery is relatively easy. Although rotation guarantees symmetry in the finished techniques are still evolving, all involve the product. Existing small lathes, such as those removal of epithelium from host cornea and used in contact lens manufacture may be cutting a peripheral groove to receive the modified quite easily to make them suitable. epikeratophakia lens. The skills involved in It is necessary to freeze the cornea to allow it conventional corneal grafting can be easily to be cut with a steel or diamond tipped tool. transferred to epikeratophakia and the post­ Freezing causes fractures in Bowmans operative management is similar to that of membrane and probably kills all the kerato­ lamellar grafts. cytes.1 Prior treatment of the cornea with a In contrast, manufacture of lenses from cryopreservative such as dimethylsulphoxide donor cornea appears to be relatively dif­ and using a controlled rate of freezing may ficult. Special equipment is needed to make allow keratocyte survivaJ.2 the lenses, and formulae are required to cal­ The flat cut techniques are performed on culate the effective refractive power of a cornea fresh from the eye bank so eliminate manufactured lens. Some of the techniques freezing damage. The keratocytes remain of manufacture are discussed, lathing for­ viable and this may result in quicker recovery mulae are derived, and a detailed description of vision postoperatively. The presence of of cryo lathing given. viable keratocytes may however make the epikeratophakia lens liable to rejection, Techniques of Manufacture which never occurs with cryolathed lenses. These fall into two broad categories. The The flat cut may be made either with a mic- Correspondence to: B. L. Halliday FRCS, Moorficlds Eye Hospital, City Road, London, ECIV 2PD. Presented at the Annual Congress of the Ophthalmological Society of the United Kingdom, April 1988. 396 B. L. HALLIDAY rokeratome using a high speed oscillating have been described for keratomileusis and, steel blade or may be made with an excimer using different geometric principles, for laser. Commercial lens manfuacturing equip­ epikeratophakia.3,4 ment using a micro keratome is available (BKS-lOOO, Allergan Medical Optics), but so far excimer laser manufacture is experimen­ tal. Calculation of Lathed Radius Interactions between the manufactured lens and host cornea will modify the final effective optical power. These unpredictable interac­ tions, including relative dehydration by host endothelial cells and repopulation of the lens by host keratocytes, limit the value of purely theoretical calculations of lens power. Such calculations do however provide a base from Fig. 2. Donor cornea (left) is sucked on to a die which modifications can be made in the light (right) to deform it. A flat tangential cut is made. On releasing suction a lens of the desired power of clinical experience. remains. There is a fundamental difference between lathing a contact lens and lathing an epikeratophakia lens. With a contact lens the back surface is lathed to fit on to the patient's cornea and the front surface radius is chosen to give the required refractive power. In con­ trast an epikeratophakia lens is cut only from behind; anteriorly Bowmans layer remains intact. This single cut therefore has to take account both of host keratome try readings and of the refractive power required. Figure 3 shows how the lathed lens changes shape when applied to host cornea. It is possible to calculate this change of shape and so calcu­ late a theoretical power for a given Fig. 3. An epikeratophakia lens (left) is sewn on to epikeratophakia lens. Similar calculations a cornea (centre). This deforms the lens (right) by a predictable amount. o c Fig. 4. The geometry of an epikeratophakia lens Fig. 1. Donor cornea (left) is supported on the may be expressed by the radii of curvature of the lathe base (right) then the posterior lamellae are two surfaces rl and r2 and by the overall diameter lathed away leaving a lens of the desired power. 2AD. MANUFACTURE OF EPlKERATOPHAKIA LENS 397 Figure 4 shows a cornea that has been Therefore the area of the lens is given in: lathed. The front surface of the cornea still Area = V2rF (ABD) + 1/2AD(BC) - 1/2r22 has its original radius, r1, of about 8 mm. The (ACD) (7) back surface has been cut with a lathe set to a Now the above angles can be expressed: . radius of r2. The lathing has left the overall ABD = sin-I (AD/rl) (8) diameter of the lens equal to 2AD. ACD= sin-I (ADIr2) (9) From the diagram Substituting into (7) from (8) and (9) gives AB2 + AD2 =r12 Area = V2r12 (sin-I (AD/rl)) + 1/2AD(BC) - Therefore V2r22 (sin-I (AD/r2)) (10) AB = � (rF -AD2) (1) BC is known (3) and when this is substituted From the diagram; into (10), the area may be calculated know­ (AB+ BCF + ADZ =r22 ing only the overall diameter of the lens Therefore (2AD), the base radius of the lathe (r1) and BC= � (r22 -AD2)- AB (2) the radius of lathe cut (r2). Substituting into (2) for AB (from (1)) gives; The radius of lathe cut required for a given BC= � (r22 -AD2) -� (rF -AD2) (3) theoretical power of epikeratophakia lens follows from these calculations. An iterative Now from the diagram: computer program has been developed. r2 + EF = r1 + BC Working to a given overall diameter the com­ Therefore; puter first makes a guess at a radius of lathe EF=rl-r2+BC (4) cut. Formulae (5) and (10) are then used to Substituting in (4) for BC (from (3)) gives calculate the central thickness and cross sec­ EF = r1 - r2 + � (r22 - AD2) - � (r12 - tional area of the resulting.1ens. AD2) (5) The same formulae are then applied to work out the new shape of this lathed lens It is therefore possible to calculate the lens when applied to host cornea. Again an itera­ thickness EF having fixed the values for the tive technique is used. The central thickness lens radius AD, and for the radius of cut r2. of the lens is unchanged and the new back It is necessary to calculate how this lens ; radius of the lens equals host keratometry, changes shape when it is used clinically. As but of course the overall diameter of the lens the lens is sewn in place its back radius reduces when it is placed on the host cornea becomes equal to that of host keratometry (Fig. 3). The computer therefore guesses the (Fig. 3). Calculation of the resulting front new front radius and knowing central thick­ radius will give the effective power of the ness can calculate the new overall diameter. lens. For this calculation of change in lens It can then calculate the cross sectional area. shape it may be assumed that both the central Repeated guesses at front radius are made thickness and the volume of the lens remain until the calculated cross sectional area constant. On the diagrams constant volume is equals that of the original lens. When it does equivalent to constant cross sectional area the computer has effectively found the actual and so it is with this that the calculation pro­ shape of lens on the host and can work out its ceeds. optical power. This power is compared with From the diagram the area of the lens may the desired power and successive guesses be given in terms of the sectors EBD and made at the original radius of lathe cut until FCD and the triangle DBC: the required optical power is reached. Area= EBD+ DBC - FCD (6) The above calculation gives a theoretical Now these individual areas may be calcu­ required radius of cut. Clinical experience lated: has shown that the actual radius of cut needs EBD= V2rl2 (ABD) where ABD is in radians to be flatter than this. An explanation may be DBC= V2AD(BC) that the donor lens is relatively thick and is FCD = V2r22 (ACD) where ACD is in further thickened by freezing. When the lens radians stabilises on the host it thins and so the power 398 B. L. HALLIDAY of the lens effectively reduces. The computer program has been modified with an evolving 'fudge factor' which is a simple percentage adjustment performed on the linear dimen­ sions of the lathed lens before considering the change in shape as the lens is put on the eye. Design of Lens Once a radius of back cut has been decided lathing may be performed.
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