Jack Allen Contact Lenses

27, Redburn Industrial Estate, Woodall Rd, Ponders End,
Enfield, Middlesex, EN3 4LQ

Lenses

    RGP beyond the limbus: Profile Versatile Scleral                       (PVS) Introduction

 Intra-limbal RGP corneal lens principles are well understood, needing a balance between corneal contact, clearance and adequate mobility. By contrast, sclerals need a static peripheral bearing surface with clearance extending beyond the limbus. There is no clear demarcation between the cornea and the sclera. The corneal curvature flattens towards its periphery, and forms a continuous curve with the paralimbal sclera, which has been shown to be close to conic section. Given a corneal diameter of 12mm, a minimum bearing annulus and limbal clearance both 1mm, the minimum diameter for a scleral is in the region of 16mm. Lenses 12 to 16mm have a mix of peripheral corneal and scleral bearing, and may or may not have an effective edge clearance.

Mid range projection The main body of the fitting set is the mid range, suitable for steep normal, keratoconic and post surgical topography. As this is where the variation in corneal topography is the greatest, oblate and prolate designs are available in fitting set for the normal to moderately steep designs.

 Low projection (flat) shapes Low projection (flat) shapes are as oblate a design are possible to allow a sufficiently broad bearing surface within 16.50mm diameter. A constant BOZR has been used with appropriate adjustments to the midperiphery to effect apical profile increments. The positive power can be reduced by using a less flat BOZR. A prolate option can be made as a special request.

 High projection A clinically highly significant but less frequently encountered requirement is for the globic cornea. The diameter is progressively increased with each apical profile increment. The BOZR is retained at an steeper limit: the apical profile increment is achieved by changing the effective optic zone diameter and the angle of the tangent to the BOZR. This feature of the PVS system is to address the problem of tight transition junctions just beyond the limbus, and to reduce minus power.