Eye (1988) 2,395-399

Manufacture of Lens

B. L. HALLIDAY

London

Summary Epikeratophakia lenses may be manufactured from donor 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 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 . 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 and required curvature is cut directly and there­ in . fore predictably, and that the high speed of The 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 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 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 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.

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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. The lens may be Fig. 5. Two designs of lens. On the lefi is a lens cut with this radius all the way to the edge, or with a wing that tucks into a pocket in the cornea. more usually a wing may be made to facilitate On the right a lens without a wing sits on top of the suturing (Fig. 5). The wing is lathed to the cornea. same radius of curvature as the front surface of the lens; usually 8 mm. Adding a wing cotton tipped stick keeping the cornea moist increases the central thickness but this does to maintain uniform hydration. Partial drying not have a significant effect on the optical could distort the cornea and so induce astig­ power. matism in the finished lens. The cornea is Typical dimensions of the lathed lens are a then punched from behind with a trephine wing thickness of 0.2mm, central thickness of about 2mm larger than the intended diameter between 0.5 and 0.75mm and an overall of the lens. After soaking the cutting tool and diameter betwen 6 and lOmm. Central thick­ the base of the lathe in a proprietary glutaral­ ness depends on the power of the lens and dehyde solution, the cornea is mounted and the overall diameter. Lathed radii of curva­ the lathe turned slowly allowing centration to ture vary between 8mm for the wings and for be checked. After freezing to approximately plano lenses (for keratoconus) and over -40° Celsius, lathing starts with initial cuts of 20mm for some aphakic powers. about O.Imm. For the final cut an advance of only 0.02mm is made. The lens is then Practical Manufacture finished to the required diameter with the The author uses a modified contact lens lathe second cutting too\. The lens is replaced in K­ to manufacture epikeratophakia lenses. The Sol and stored until use. lathe is kept in a laminar flow cabinet to reduce the chance of airbourne contamina­ Conclusions tion. A cylinder of carbon dioxide gas feeds There are no insurmountable problems in cryo elements in the cutting tool and in the setting up a cryolathe facility for the man­ headstock of the lathe. An aluminium base ufacture of epikeratophaia lenses. At current cut to a radius of 8mm supports the donor prices initial equipment costs would be cornea which is held in place initially by sur­ covered by the savings over 100 commercially face tension and then by the adhesion of ice purchased lenses. After this time the cost of when the cornea is frozen. A diamond tipped lens manufacture becomes extremely low. It cutting tool is used mounted on a double is not known, nationally, how many patients radius base of a design that allows two radii would benefit from surgery; true level of of cut, for the wing and for the optic zone, to demand may only become apparent when be preset. A second cutting tool, mounted cheap lenses are available at every major perpendicular on the lathe, may be used referral centre. where needed to give a finishing diameter to the lens. References Donor corneas, obtained from our Eye 1 Binder PS, Zavala EY, Baumgartner SD, Nayak Bank, are routinely stored in K-Sol (Cooper­ SK: Combined morphologic effects of cryola­ vision-Cilco) at 4°C for between one and thing and lyophilization on epikeratoplasty three weeks. Epithelium is removed with a lenticules. Arch Ophthalmol1986, 104: 671-9 MANUFACTURE OF EPIKERATOPHAKIA LENS 399

2 Lee TJ, Lee Wan W, Kash RL, Kratz KL, keratoplasty. Arch Soc Am Ophthal Optom Schanzlin D1: Keratocyte survival following a J 970, 8: 103-64. controlled rate freeze. Invest Ophthalmol Vis 4 Werblin PP and Klyce SD: Epikeratophalvia. Sci1985, 26: 1210-15. The surgical correction of aphakia (i) Lasing 3 Barraquer JI: Calculations for refractive of corneal tissue. Curr Eye Res 1981, 1: 123-9.