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CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post corneal swelling to a level comparable to normal overnight swelling, thus reintroducing the option of fitting ScCLs without the complicating impact of incorporating a fenestration 3,4,5 . Advantages and disadvantages of ScCLs Advantages The large size creates a scleral bearing surface and retains a pre-corneal fluid reservoir providing optical neutralisation of an astigmatic or irregular corneal surface and corneal hydration. Very high powers, up to +/- 40.00D, are possible because there are minimal lid traction and centre of gravity effects. They cannot be dislodged by the lids and are often relatively comfortable even for the unadapted eye because the lid margin is in contact with the surface of the lens rather than the edge. If RGP non-ventilated ScCLs are used, corneal contact can be minimal. ScCLs, including RGP lenses, can be kept dry when not being worn. There is no risk of contaminated storage solutions since none is used, so the risk of infections is reduced. They are robust, the surface can be polished or resurfaced periodically, and they are not easily lost. Less dextrous patients may find them easier to handle because they do not need to be precariously balanced on a fingertip. Disadvantages While not producing any lid sensation, there is a feeling and an appearance of bulk, and some people are intimidated by their large size. They cover a large area of the anterior globe, considerably reducing the oxygen available to the cornea, although the introduction of RGP materials has led to a major improvement. Because they are fitted with corneal clearance, the VA and the ability of the lens to reduce distortions in ectatic corneal conditions may be reduced compared to rigid corneal lenses. However, if an RGP ScCL can be worn when a corneal lens cannot, the comparison is not valid. Indications ScCLs are indicated simply when the unique advantages of ScCLs can be applied. They can be considered to manage any eye condition if there is a strong enough reason for contact lens correction, and in the management of some ocular surface disease. There have been a number of publications in recent years describing ScCL applications. 6,7,8,9,10,11,12,13,14,15,16,17,18 Irregular or abnormal corneal topography Corneal ectasia Keratoconus or other primary corneal ectasias (PCE) form the majority of current indications for ScCLs. RGP corneal lenses Figure 2 Irregular corneal topography as a consequence of a transplant that has unexpectedly sprung forward. There is a clear depression in the superior mid-periphery which would be a major problem for corneal lens fitting. A nonventilated RGP ScCL is unaffected by a localised irregularity like this provided it is sealed on the sclera may be simply impossible to fit in very advanced cases, for example, as shown in Figure 1, but there is also an application for ScCLs when corneal lenses persistently dislodge, or are not well tolerated due to excessive mobility or the formation of epithelial erosions. Corneal transplant The second largest group for which ScCLs are indicated are post-corneal transplants if there is a residual refractive error, or if the surface remains irregular, illustrated in Figure 2. Even if the astigmatism is very high, which still happens sometimes even with the most expert surgery, non-ventilated RGP ScCL fitting post-transplant is usually a comparatively straightforward exercise. The lenses are fitted with full corneal clearance irrespective of corneal topography, therefore the appearance is much the same whether the cylinder is 2.00D or in excess of 20.00D. Figure 1 Downwardly displaced apex in an advanced keratoconus. Many corneal lenses were tried, but all dislodged almost immediately. ScCL fitting was a straightforward process with only two lenses necessary to establish corneal clearance Corneal trauma or postsurgery The vision in traumatised eyes with corneal scarring may be improved with ScCLs if the injury has caused irregularities, but has left some areas of reasonably transparent cornea in the pupillary region. Some cases of unsuccessful refractive surgery may benefit if supplementary correction with rigid contact lenses is helpful in reducing visual disturbances such as starbursting and 27 | October 20 | 2006 | OT
CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post ghosting. Grossly abnormal topography would not be expected after refractive surgery, but some patients remain intolerant of corneal lenses, in which cases ScCLs could be tried. Normal topography with high refractive errors ScCLs may be indicated when high power rigid corneal lenses cause intractable problems of excessive mobility or poor centration. They may be used whatever the ametropia if there is intolerance to corneal or hydrogel lens wear in high myopia, or hypermetropia or significant non-pathological corneal astigmatism. Therapeutic or protective applications ScCLs uniquely retain a pre-corneal fluid reservoir providing corneal hydration in serious dry eye conditions, such as Stevens Johnson Syndrome or ocular cicatricising pemphigoid. A ScCL in situ may give an improved environment for corneal healing where newly formed epithelium is continually sloughed away by the action of the lids, or may prevent tear film evaporation with poor lid closure or lid absence. The lens retains a fluid reservoir which maintains some degree of corneal hydration and protects the cornea from trichiasis and lid margin keratinisation, and the front surface provides hugely improved refracting surface, giving a considerable visual benefit. There is also an application in less severe dry eye or tears dysfunction pathology. Sometimes the patient may describe very distressing and disproportionate ocular discomfort with very little visible corneal disruption. Mucus filaments adherent to the cornea are very painful when they pull on the epithelium or finally detach, but wave freely in the precorneal fluid reservoir behind a ScCL, and appear to float off into the fluid pool with little discomfort. ScCLs may be used as a prop for some cases of ptosis. Cosmetic shells A realistic iris can be encapsulated into PMMA scleral shells to mask unsightly blind eyes, or to relieve intractable diplopia or glare in cases of aniridia. Recreational or occupational applications Particles behind rigid corneal lenses in dusty work place environments are very troublesome. ScCLs eliminate this problem almost entirely. Most recreational activities are satisfactorily managed with the use of hydrogel lenses, but ScCLs are still indicated on occasions for contact or water sports. ScCL fitting options Ventilation by some means is a prerequisite for PMMA lenses whether impression or preformed, but non-ventilated designs are the preferred option for RGP. Some limitations still remain for irregular ocular topography with RGP preformed sclerals, but considerably more irregular eyes can be fitted using preformed RGP sclerals because of the tear reservoir compared to fenestrated PMMA. Impression lenses of some kind can be used virtually irrespective of irregular topography. All the following alternatives are possible, and have different applications. RGP RGP sclerals should now be considered the first choice for the great majority of ScCL cases. Some long-term wearers have worn the lenses for over 50 years, so these should be considered a long-term option for most new referrals. As the great majority of ScCL referrals are for PCE or corneal transplant, it is crucial to minimise the risk of corneal hypoxia. A transplant may have to be a future management option for any PCE referred for ScCL fitting. If so, corneal neovascularisation, while not necessarily sight-threatening in its own right in the early stages, may increase the risk of transplant rejection. Preformed non-ventilated RGP can be used for the great majority of cases, irrespective of corneal topography. There are more potential problems with the scleral topography, but if the scleral zone is well enough sealed, a pre-corneal fluid reservoir is retained without bubbles and with minimal settling on the globe. The optimum clearance at the visual axis is approximately 0.25mm, but can be more than twice that value at the limbus in some sectors without causing any problems provided it remains air free. The principal problem with preformed non-ventilated RGP ScCLs is that they have to be inserted filled with saline. This requires keeping the lens horizontal at the moment of insertion, so it is necessary also to have the patient’s head horizontal and facing the floor. This is a more difficult task for patients, and sometimes for the practitioner as well, compared to the relatively simple procedure for inserting a fenestrated lens, which can be inserted with the head in the normal upright posture. Preformed fenestrated RGP is indicated if a fenestration is beneficial, for example, if retention of the pre-corneal fluid reservoir at the moment of insertion is impossible. However, there is a serious limitation because air bubbles are admitted into the pre-corneal fluid reservoir and cross the visual axis if the pre-corneal fluid reservoir depth is greater than 0.1mm. It is unlikely that a tear pool of uniformly shallow depth can be maintained with even just a moderately irregular corneal topography. Impression non-ventilated RGP is an option if there is an intractable problem retaining an air free pre-corneal fluid reservoir with a non-ventilated preformed design. The ultimate fitting target is a flush back surface for the scleral zone and an optic zone clearance similar to that for a non-ventilated preformed RGP. As the scleral zone is fabricated from an eye impression, a very good alignment can be expected with effective sealing on the scleral zone, hence there is scope for increasing the optic zone clearance with a good prospect of retaining a pre-corneal fluid reservoir. Impression fenestrated RGP is a major undertaking because it is a departure from mainstream manufacturing and, in addition, requires very precise fabrication of the back surface to keep a uniform depth of the pre-corneal fluid reservoir. However, there may be a better chance than with a fenestrated preformed RGP lens because the optic zone is individually fabricated from the exact shape of the eye. PMMA PMMA ScCLs still have a role to play but there is an acknowledged long-term threat of hypoxic corneal changes, so these would not normally be the first choice for new ScCL wearers. However, if a PMMA lens has been worn successfully for some years without significant sequelae, there is no reason for a change is to be made. Most impression PMMA lenses for moderate or advanced corneal ectasias were ventilated, hence the initial clearance at the visual axis could not be much more than 0.1mm. After a period of settling on the globe, an almost universal occurrence with a ventilated lens, the result was apical contact. The author’s observation is that contact is better tolerated in PMMA than with RGP materials. The probable explanation is the increased co-efficient of friction 28 | October 20 | 2006 | OT
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