Contact Lens Case Reports
Calculating Scleral Lens Sag
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Our patient was a 64-year-old female with a longstanding history of keratoconus. She underwent a penetrating keratoplasty (PKP) OS only at age 43. She has since struggled with a wide range of corneal lens designs. Due to her bulging graft and asymmetric topography (Figure 1), a decision was made to refit her with a scleral lens.
Figure 1. Axial, elevation, and OCT displays of the patient following PKP for keratoconus.
Estimating Sagittal Height
At Pacific University, we have developed a technique to estimate the sagittal depth of both normal and irregular corneas. The system is based on the knowledge that in both normal and abnormal eyes, the height of the cornea and sclera at a chord from 10.0mm to 15.0mm is approximately 2,000µm (Figure 2) (Copilevitz et al, 2012).
Figure 2. Sagittal height of normal and keratoconus eyes (approximately 2,000µm) at a chord of 10mm to 15mm.
We use the Medmont corneal topographer to calculate the sagittal height (sag) of the cornea from 10.0mm to the corneal apex. In this case, the Medmont value was 2,332µm. The ocular coherence tomography (OCT) value was almost identical at 2,330µm (Figure 3). Adding the Medmont value (2332µm) to the corneal/scleral sag at 10mm to 15.0mm (2,000µm), the estimated total sag of the cornea/sclera at 15.0mm was 4,332µm. To this, we added 350µm (300µm to 400µm for apical clearance) (4,682µm) to calculate the initial diagnostic lens sag.
Figure 3. Medmont (2,332µm) and OCT (2,330µm) sagittal height from 10.0mm to the apex.
Testing the Formula
A 16.5mm diagnostic lens with a sagittal height of 4,700µm fit well, demonstrating 300µm to 400µm of apical clearance, limbal clearance, and scleral landing (Figure 4). Scleral lens designs frequently position temporally and slightly inferiorly. The greater elevation of the nasal quadrant of the sclera pushes the lens temporally. The vertical decentration is secondary to gravitational and upper lid forces. The result is an area of fluorescein thinning always in the superior nasal quadrant.
Figure 4. The 4,700µm sag diagnostic lens on the patient’s left eye.
This technique can help estimate the sagittal height of most eyes, thereby helping to improve accuracy with initial diagnostic lens selection. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #220.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.