Know the Reverse
Geometry Lens Rules
BY JOHN MOUNTFORD, DIP. APP. SC, FAAO, FCLSA
All successful GP lens fitting requires knowledge of the "rules." So what are the "rules" for fitting reverse geometry lenses? We've all seen papers that say if the lens is too steep or too tight, then
you must flatten the alignment (or tangent) curve (AC) or the reverse curve (RC) and vice versa if the lens is too flat. But because the exact curves of most currently available RGLs are proprietary, how can we maintain any control over the lenses we use if we don't know their parameters or the rules that control how they fit?
The most common RGLs are four- or five-zone, and have spherical or tangent peripheral curves. The back optic zone diameter
(BOZD) is usually constant (6mm), with reverse zones between 0.6mm and 1mm wide. If you increase lens diameter, then the reverse zone width remains constant and the alignment zone increases or, alternatively, the sigmoid curve depth and width change and the tangential periphery remains constant.
You can apply some simple rules to each basic design to help maintain some control over the fitting. With four- and five-zone lenses (assuming a BOZD of 6mm and a RC of 0.6mm), the rules are:
- Steepening the RC by 0.15mm (0.75D) increases lens sag and apical clearance by 10µm
- Flattening the RC by 0.15mm (0.75D) decreases lens sag and apical clearance by 10µm
- Steepening the AC by 0.05mm (0.25D) increases the apical clearance by 5µm
- Flattening the AC by 0.05mm (0.25D) decreases the apical clearance by 5µm
In the case of a five-zone lens, alterations to AC2 have little effect on the lens fit unless it's dramatically steep.
Figure 1 shows a steep lens with excessive apical clearance. Figure 2 shows a lens in which I've flattened the RC by 0.75D and the AC by 0.50D, resulting in a total sag change of 20µm.
CRT lenses have a minimum sag difference of 25µm, which sometimes isn't fine enough to resolve a fitting difficulty. The solution is to alter the cone angle (CA). In general, the rules for CAs are:
- Changing the CA by one degree changes the sag by 10µm
- Changing the CA by one degree changes the contact point by 0.1mm
Rules for Success
Having a basic understanding of the "rules" for RGL fitting provides the same benefit as knowing the rules for normal GP fitting: A higher first-fit success rate.
Dr. Mountford is an optometrist in private practice specializing in advanced contact lenses for
keratoconus, post refractive surgery and pediatric aphakia. He is a
visiting contact lens lecturer to QUT and UNSW, Australia.
Contact Lens Spectrum, Issue: January 2005