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CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post for RGP materials compared to PMMA. For the same reason, there may be more surface/tarsal plate interaction with RGP compared to PMMA. Furthermore, the visual performance in terms of visual acuity, contrast and reduction of distortions or ghosting is reduced when there is a visual axis contact zone. Impression ventilated PMMA proved to be a successful option in many cases before RGP materials were available. A ventilated impression PMMA lens is easier to produce than RGP due to its superior thermoplasticity, and because it can be cut and polished with minimum risk of irreparable damage. Impression ventilated ScCLs very often have central corneal contact zones after settling back, irrespective of the material. If so, PMMA is more likely to be tolerated. Impression PMMA is arguably the method of choice for elderly aphakes. Centration is optimised with the impression method of fitting as there is a near glove fit on the sclera. Thus the front optic can be sited over the visual axis to minimise aberrations. Preformed fenestrated PMMA may be a preferable option to RGP fenestrated because it may be fitted with less initial clearance so that apical contact following any settling on the globe is better tolerated than if an RGP lens is similarly fitted. However, the chances of a successful outcome with a moderately or advanced ectasia and irregular topography remain limited for the reasons discussed previously. Impression or preformed nonventilated PMMA has no place for long term wear as the onset of hypoxia is very rapid. However, it is worth remembering that advanced keratoconus causes extremely poor unaided vision and spectacles may do next to nothing by way of correction. Therefore, all contact lens options should be borne in mind. If good vision to deal with a particular task is urgently required for a few minutes, a non-ventilated PMMA lens provides just that. The fitting and production are the simplest for all scleral lens types provided the scleral zone seals adequately on the sclera, which should be the case following an impression lens if not with a preformed lens. Preformed non-ventilated RGP ScCL fitting PMMA ScCLs, whether preformed or impression of preformed, are comparatively infrequently required nowadays. The fitting Figure 3 Optimum preformed lens fitting with scleral alignment and optic zone clearance Figure 4 Steep fitting scleral zone illustrating fluorescein encroaching almost to the periphery of the lens. There is also a small amount of blanching at the edge Figure 5 Flat fitting scleral zone on the same eye as seen in Figure 4. The fluorescein is only seen just beyond the limbus, and there is obvious blanching in the mid-periphery systems for both have been well documented in previous textbooks and summarised recently 19 , so it is not appropriate to expand further on the preceding discussion which outlines their role. However, the fitting processes for RGP ScCLs have been more recently developed, hence the principles are outlined in the following text. Lens diameter The diameter is chosen according to the appearance at the assessment and fitting process, and can be between 16mm and 25mm. In fact, 18mm to 23mm is a more usual range for preformed lenses. Indeed, 16mm is very small for a lens to be defined as a ScCL, that is, having a scleral bearing surface and essentially with corneal clearance. If the corneal diameter is taken to be 12mm, and it is deemed desirable to allow an extra 1mm all round for limbal clearance, a 16mm total diameter only allows a 1mm scleral bearing annulus. The author acknowledges the valuable role for lenses with a diameter smaller than 16mm, but they do not conform to this definition of a ScCL and are fitted according to a different rationale. A small variation in the diameter may not make much difference, but if a lens were to be made progressively smaller the original bearing surface would eventually be eliminated. The new bearing surface would then be the sector of the sclera which was previously the transition zone with the larger diameter lens. The result is likely to be 29 | October 20 | 2006 | OT
CONTINUING EDUCATION AND TRAINING Gain 2 CET credits - enter online at www.otcet.co.uk or by post Figure 6 The optic zone sagitta, and consequently, the apical clearance, is increased by steepening the BOZR Figure 7 The optic zone parameters can be defined according to the projection of the optic zone from the continuation of the scleral curve measured axially at the apex and at the limbus increased settling back with reduction of the optic zone clearance. There are some comparative advantages and disadvantages of smaller and larger lenses. The author’s observation is that the scleral topography just outside the limbus is more regular and symmetrical than the more peripheral sclera. Smaller diameter ScCLs may be indicated when a tighter fitting is necessary. For example, they should be tried when a non-ventilated 23mm diameter lens fails to seal well enough on the sclera to prevent admission of an air bubble into the pre-corneal reservoir. Bearing on the most symmetrical region of the sclera leads to noticeably less decentration, which may reduce any prismatic effects. Smaller lenses can be made thinner than larger lenses because the rigidity is greater with smaller diameter lenses. The reduced mass, and less movement on the eye compared to larger lenses, may render a contact zone more tolerable. Hence they can be fitted with less corneal clearance, which may prove to be advantageous if the vision is improved with a visual axis contact zone. A final advantage of the improved centration and closer proximity to the cornea is a more even depth of the pre-corneal fluid reservoir, this increases the possibility that a fenestration can be more successful than with larger diameter lenses. However, on the downside, the increased tightness of the fitting on the eye and the reduced limbal clearance are significant drawbacks at times. Although such lenses are more likely to retain an air-free pre-corneal reservoir they are more difficult to insert without a bubble in the first place, and because they fit tighter on the eye, they are distinctly more difficult to remove than larger diameter lenses. Optimal scleral zone alignment The bearing surface should be spread as evenly as possible over the sclera, but with optic zone clearance. In fact, 13.50mm to 14.50mm is a usual back scleral radius (BSR) range for most preformed ScCLs. The bearing surface is displaced away from the limbus if the scleral zone is too steep, and may cause the lens to vault as it rests only at the periphery, giving an appearance of excessive apical clearance. A flat fitting scleral zone shifts the bearing surface nearer to the limbus, but does not appreciably affect the apical clearance. Figure 3 diagrammatically illustrates scleral zone alignment and corneal clearance. Figures 4 and 5 are fluorescein photographs showing a steep and flat fitting scleral zone respectively. A glove fit on the sclera is not possible, nor essential, but the scleral zone needs to be sufficiently sealed to prevent the introduction of air bubbles into the pre-corneal fluid reservoir. An overall view of the scleral zone with a hand-held low magnification lamp is sufficient to see the extent of the clearance beyond the limbus. Any areas of conjunctival blood vessel blanching, which are due to localised compression while the lens is in situ, can be seen simultaneously with a white light source. Corneal and limbal clearance Achieving optimal clearance at the limbus and at the apex is determined by varying the back optic zone radius (BOZR) and the back optic zone diameter (BOZD) in combination to give the optic zone sagitta (OZS). Varying the BOZR with a constant BOZD gives rise to a precisely calculable change to the central corneal clearance, as in Figure 6, but varying the BOZD with a constant BOZR does not because the curvature of the scleral bearing surface is an unknown quantity. This is the same principle as traditional style PMMA ScCL fitting, but most modern fitting systems utilising the OZS principle calculate the combined specifications to give significant variations in apical and limbal clearance, therefore it is not necessary for the practitioner to try combinations of BOZRs and BOZDs. The corneal clearance can also be varied by changing the optic zone projection (OZP) in progressive increments but without reference to the BOZR or BOZD. Figure 7 illustrates OZP diagrammatically. The optic zone Figure 8 Off-axis apical contact zone in an advanced keratoconus wearing a nonventilated RGP scleral lens. This amount of contact was tolerated by the wearer and did not lead to any corneal erosion 30 | October 20 | 2006 | OT
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