Scleral Lens Manufacturing: Special process or just another lens to make?
We at LED have been involved in setting up labs to produce or improve scleral lens production for a few years, now. Although scleral lenses have been around since at least the late 1930's, their resurgence is being touted as a boon to business for independent Contact Lens manufactures. On the plus side, more practitioners are fitting scleral lenses and with the fitting sets that accompany this practice, labs are seeing an increase in production volume over their corneal lens production. On the minus side, setting up to make these lenses, the time to process a lens, the blank cost, diamond life, and working with a qualified fitter, all make for a lengthy decision process (Should we get into this?). There is also lens design licensing to consider.
My perspective has to do with the manufacturing process and machinery/tooling involved to produce scleral lenses. I have seen a wide range of processes which are dictated by the existing machinery in the lab or the prospect of new machinery leading to new processes. Labs make a choice to either set up a separate production line for scleral lenses or integrate that production into the main corneal production.
One detail is custom adaptors we have made to locate the scleral lens blank for base blocking. We have made cups that the large diameter fits into to be used in a blocking machine. The customer orders an adaptor for, say a 17, 19, or 21 mm blank and we have them send the blanks to us. We have found that they are actually 17.22, 19.43, and 21.20. If the lab is not transfer blocking, the adaptor controls prism. This is OK as long as the adaptor is concentric and the lens blank diameters are concentric and precise. A 17.15 diameter lens blank will be loose in the 17.23 adaptor and could yield 1/4 diopter of prism, not to mention the edge thickness problems. I don't know if the lens blanks are consistent diameter from brand to brand. So using the adaptor could create some problems later in the process if the blank diameter is not monitored.
Following are examples of labs that I have worked with; all of which are making scleral lenses. As in the CL industry as a whole, there is a variety of manufacturing methods in place. As I mentioned above, these examples address the decision of separate or integrated scleral production.
Lab I: This is an older lab that has been making lenses "by hand" on single compound manual ball bearing lathes and pitch polishing. They have actually been making scleral lenses for many years. Each lens is fitted to the patient and hand beveled to fit on both scleral and corneal lenses. The curves are spherical and hand blended. This is an integrated corneal/scleral process. All lenses are made by the same technicians on the same machines. There isn't a "specials" production line or even a "specials" technician. Production: 10-20 lenses per day, total. I can imagine that rejects are common and very costly.
Lab II: This is what I would call an average lab with older 3 axis CNC lathes and pitch or soft tool polishing. The peripheral curves (PC's) may be hand beveled. The 3 axis CNC lathes are limited in their ability to turn the deep sagital depths of scleral lenses. The software may also have limitations for the lens designs. The lab owners wanted to get into the scleral lens market, but found out about the concerns that I mentioned in the introduction. They decided to train a technician to make scleral lenses. This means that the technician needed access to a base curve lathe, polisher, blocker and other devices, all during regular production. Or, they could bring out the old manual equipment from storage and set it up in a separate workspace. The manufacturing process involves turning the base curve (BC) (held in a collet), reading the center thickness (CT), and polishing it. There is the option of grinding the PC's, or waiting until the front curve (FC) is turned and de-blocked. This lab may be at a disadvantage relative to Lab I which has the patent present for the fitter to work with. It may be that the technician will "rough in" the PC's assuming the fitter will complete that job. Scleral production, 10-20 lenses per day. Rejects...?
In both of these examples, there is no capacity (or need?) to make several trial sets of twenty lenses each. However in Lab II, it would be better for the fitter if s/he had a trial set to be able to order the finished lens from the lab.
Lab III: Scleral lens production is fully integrated with the corneal lens production, with the possible addition of one piece of equipment. This is a modern lab with two axis CNC lathes, fitted with multiple diamond tools, soft tool polishing, and transfer blocking. Ideally, there would be enough diamond tools mounted to process both corneal and scleral lenses. Some would argue that transfer blocking is less necessary on scleral lenses because the size of the lens blank makes it less sensitive to collet crimp. This may be true, but when integrating scleral lens production into the corneal lens production, transfer blocking has the advantage of tooling that will compensate for the wide range in center thickness and how they are mounted on the polishing machine. In addition there is the advantage of better prism control.With the transfer blocking method, all of the lens blanks are blocked to a flat top tool (hollow or solid) and held in the same 12.7 mm diameter, 13 mm deep lathe collet. This lends it self to auto-loading. The entire back surface including the edge is turned, CT measured, if necessary, and then polished. Polishing is where the machine needs to be set up to stroke out to the edge. On a bladder polisher, stroke is typically equal side-to-side. For scleral BC's, the stroke needs to be set from just over center to the far side, say 40 degrees. This will allow the lens to traverse the bladder tool so that the edge of the lens is near the center of the bladder. This action will polish the entire back surface in a similar time it takes to polish a corneal lens. The down-side is re-setting the bladder polisher from corneal to scleral and back throughout the day. If volume warrants it, it is probably a good idea to consider having a bladder polisher dedicated to scleral production. This system allows for trial lens set production, especially if an autoloader is employed. A side concern about trial sets is individual lens identification. Markers are available for this task; both on lathe or off.This process is limited by the cycle time and availability relative to corneal lens production. With autoloading, production could be 50 scleral lenses or more per day. Rejects should be low assuming the correct parameters are provided by the practitioner.
The three examples show a circular progression: Lab I is integrated by default, dependent on the machinery and technicians on hand. Lab II is not integrated because of the limitations of the machinery. Lab III found that their machinery with some minor modifications could produce scleral lenses within the corneal process.
Taking on scleral lens production is a major decision both for production and sales. Many labs are successful because they looked at all the aspects of producing and marketing scleral lenses.