Contact Lens Case Reports
Scleral Lens Settling
BY Patrick J. Caroline, FAAO, & Mark P. André, FAAO
Last month we discussed the condition referred to as conjunctival prolapse, conjunctival hooding, or conjunctival chalasis. Another unique conjunctival response to scleral lenses is a condition known as lens settling or conjunctival compression. To understand the mechanics behind this phenomenon, we must first review the goals of fitting scleral lenses that exceed 15mm in diameter. Here are some fitting rules to keep in mind.
Rule 1 Upon initial application, the center of the lens should clear the corneal apex by approximately 300 to 400 microns. Clinically, this apical clearance is best estimated by viewing the thickness of the lens, tear film, and cornea through the optic section of the slit lamp.
Rule 2 The lens should clear the limbus 360 degrees around (or as much as possible).
Rule 3 The full weight, pressure, and bearing of the lens should come to rest on the bulbar conjunctiva/Tenon's capsule (Figure 1).
Figure 1. A well-fitted scleral lens with apical clearance, limbal clearance, and bulbar conjunctiva/Tenon's capsule “landing.”
When a 9.5mm corneal GP lens is applied, the resulting lensto-cornea fitting relationship changes little with time. In other words, due to the relative rigidity of the underlying corneal tissue, there is very little “lens sinkage” that takes place. This is not the case with scleral lenses that come to rest on the bulbar conjunctiva and Tenon's capsule—both of which are extremely soft and compressible tissues. Therefore, scleral lenses sink (lose sagittal height) on the eye as the lens “settles in” and compresses the conjunctival/Tenon's tissues.
Two intriguing questions are: How much does a scleral lens sink with time? Is the settling amount consistent among patients? To look at this we measured the apical clearance of a 16.5mm scleral lens on 15 normal eyes using anterior segment OCT. The subjects then wore the lenses for eight hours, and the apical clearance measurements were repeated.
The average apical clearance at dispensing was 316 microns. The average apical clearance following eight hours of wear was 220 microns. Therefore, the scleral lenses “settled” on average 96 microns. However, in our study the amount of settling varied significantly by subject, with a range in sagittal depth loss from 70 microns to 180 microns.
In another study, John Mountford, OD, (2012) found the average lens settling following one month of lens wear to be 146 microns with a range of 106 microns to 186 microns.
Ultimately, the amount of lens settling in pathologic eyes will be governed by multiple factors that include the age of the patients, the condition of the underlying conjunctiva/Tenon's capsule, and the design and diameter of the scleral lens.
Figure 2. Lens settling secondary to conjunctival/Tenon's capsule compression.
No Long-Term Effects
As with conjunctival prolapse, the somewhat disconcerting look of conjunctival compression appears to be a benign event and does not appear to carry with it any long-term ramifications. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #198.
|Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.|