Basic Scleral Lens Fitting and Design
SCLERAL LENS BASICS
Basic Scleral Lens Fitting and Design
Scleral contact lenses are an important tool when fitting challenging patients.
By Christine W. Sindt, OD, FAAO
Dr. Sindt is a clinical associate professor of ophthalmology and director of the contact lens service at the University of Iowa Department of Ophthalmology and Visual Sciences. She is also an AOA Contact Lens and Cornea Section Council Member.
The concept of scleral contact lenses has been around for nearly 120 years. Yet, despite their excellent comfort, they fell out of favor post World War II because of hypoxia complications. With new high-oxygen-transmission lens materials, sophisticated lathe software and a growing demand for superior performance, scleral contact lenses are experiencing a resurgence among contact lens fitters.
Scleral lenses are successful for keratoconus, highly toric corneas, post-trauma, dry eye and neurotrophic keratitis, including persistent epithelial defects and penetrating keratoplasties. Frequently, scleral lenses postpone or eliminate the need for further surgeries. Scleral lenses are contraindicated in grafts with endothelial cell counts of less than 800 since edema may develop.
Practitioners trained in fitting corneal GP contact lenses may initially find the idea of large-diameter lenses challenging. While fluorescein evaluations are similar, tear dynamics, lens selection and product performance are markedly different. However, in many ways scleral lenses are much easier to fit and offer greater initial patient acceptance and reduced chair time. This article discusses the basic skills for scleral lens fitting.
Scleral lenses are categorized by lens diameter (Table 1). The size of the lens will determine the amount of acceptable bearing and clearance as well as the fluid dynamics. Corneo-scleral lenses exhibit corneal bearing — similar to a corneal GP design — and a thin rim of scleral touch. They range in size from 12.9mm to 13.5mm. Semi-scleral lenses, although larger than corneo-scleral lenses, have a similar corneal bearing pattern with a slightly broader scleral landing area. Semi-scleral lenses range in size from 13.6mm to 14.9mm.
Because most contact lens fitters are comfortable with corneal GP lenses, there is a temptation to "go just a little bit larger" when first fitting scleral lenses. However, corneoscleral and semi-scleral lenses land on or near the limbus and can be the most difficult size to fit (Figures 1a, b, c). A lens that places excessive pressure on the limbus may eventually cause inflammation and discomfort. Therefore, I recommend starting with a mini-scleral or full scleral design.
Figure 1. Lens diameter affects the lens landing location and distribution of scleral bearing. Smaller lenses are often more difficult to fit because of excessive pressure near the limbus. This is demonstrated with the 13.6mm lens on the left (a), followed by the 15.0mm lens (b) and the larger 18.2mm lens on the right (c).
Figure 2. A lens should not bend with pressure 2(a) (top left). If bending occurs (2b) (bottom left), limbal swelling can result (2c) (right).
Mini-scleral lenses are probably the most popular design among United States fitters, while European fitters prefer the larger full scleral designs. Mini-scleral designs range in size from 15.0mm to 18.0mm. They exhibit minimal corneal clearance with moderate areas of scleral landing. A full scleral design, 18.1mm to 24+mm, will have complete corneal clearance and a broader area of scleral alignment. Full scleral lenses are easier to fit than mini-scleral lenses on extremely complex corneas because the cornea is completely vaulted.
The lens should be adequately rigid regardless of the size or design to prevent lens flexure. When gripping the lens between thumb and forefinger, the lens should not bend with moderate pressure (Figure 2a). If flexure occurs (Figure 2b), there is risk of creating negative pressure under the lens unless the lens is fit significantly flat to prevent a sealed system. Negative pressure results in limbal swelling (Figure 2c).
Fitting Concepts and Lens Selection
A scleral lens fit is based on the sagittal depth under the lens compared to the sagittal depth of the cornea. Factors which affect the sagittal depth for the lens and cornea, respectively, include base curve radius/keratometry (K) values, peripheral curves/peripheral corneal asphericity and lens diameter/corneal diameter. While keratometry values might be the same between patients, a smaller eye will result in significantly less sagittal depth. Corneo-scleral and semi-scleral designs require less sagittal depth compared to the cornea, similar to the way a corneal GP lens is fit slightly flatter than K, to avoid lens adherence. For a mini-scleral lens, near alignment of sagittal depth is desired, while for full scleral lenses a large sagittal depth and maximal corneal clearance is preferred.
When selecting an initial lens, keep in mind the desired corneal clearance, based on the lens diameter. It's best to consult the individual manufacturer's fitting guide for product-specific tips. With this in mind, however, the topography reference sphere —the curve which aligns with the most number of points on the cornea — is often a good place to start. Subsequent lens selection is made simply by observing the lens fit.
Fluorescein Patterns and Fit Evaluations
Corneo-scleral lenses and semi-scleral lenses should demonstrate significant fluorescein dye uptake behind the lens, similar to when fluorescein is instilled into the eye with a corneal GP lens, albeit slightly slower. Mini-scleral lenses will have a very slow dye uptake and may require nudging of the inferior or superior lens edge to accelerate the process.
Full scleral lenses will not demonstrate dye uptake during a reasonable examination period, and it's recommended to put the dye in the bowl when applying the lens. The use of a yellow Wratten filter is essential when evaluating scleral lens fluorescein patterns.
Unlike corneal GP lenses, you should evaluate scleral lens fits from the edge to the center. First evaluate the edge and landing area, then the limbal region and, finally, the central fluorescein pattern (Figures 3a, b, c).
The Edge The edge of the contact lens should not exhibit vascular impingement, conjunctival blanching or scleral indentation. Scleral indentation is common in smaller scleral designs that are fit with excessive sagittal depth, while it's rare in mini-scleral and full scleral designs because of the relatively large landing area. What may be common in larger designs is focal areas of vascular impingement or blanching. Locations for focal impingement include pinguecula elevations, 3 o'clock to 9 o'clock on a highly toric sclera, or under the upper lid when a tight lid exerts compressive forces.
There are several ways to overcome these focal areas of excessive bearing. In the case of pinguecula, a focal notch can be cut into the posterior surface of the lens to vault the elevation. Use a permanent or surgical marker to mark the area to be removed. Either the laboratory can remove the plastic or you can do it in-house if diamond/polishing tools are available.
Figure 3. 3a (left) shows low sagittal depth with excessive limbal clearance and an apical bearing central pattern. 3b (center) shows an aligned sagittal depth with good limbal clearance and aligned central pattern. 3c (right) shows excessive sagittal depth with minimal limbal clearance and a large central bubble.
If you observe 3 o'clock-to-9 o'clock blanching in the absence of pinguecula, order a toric periphery lens or decrease the overall sagittal depth of the lens — either by flattening the base curve or adjusting the peripheral curves — provided it does not result in edge lift off from 12 o'clock to 6 o'clock.
Figure 4. Excessive midperipheral clearance will result in bubble formation (4a). Good midperipheral alignment is shown in 4b.
Edge lift-off will result in foreign body sensation and possible lens loss. It most commonly occurs with oval cones, pellucid marginal degeneration or highly irregular grafts.
The Limbus To maintain healthy stem cells, the limbus should always be bathed in fluid. Excessive limbal clearance will exhibit bubbles (Figure 4a). Fluorescein should extend from just outside the limbal region onto the cornea, covering the limbus completely (Figure 4b).
If a fenestration hole is desired, place it in this limbal clearance area. Fenestration holes assist in lens removal by ameliorating the suction under the lens. They should be approximately 1mm in diameter with polished edges to prevent debris buildup. A hole placed directly on the cornea will have no effect. A hole is not necessary for proper functioning of the lens, nor will it prevent adherence, and it can often cause frustration by allowing bubbles to be sucked up under the lens.
Central Clearance If the sagittal depth is too high, central bubbles will form under the lens (Figure 5a). Conversely, if the sagittal depth is too low, the lens will exhibit central touch (Figure 5b).
Figure 5. Too high sagittal depth (5a) versus too low sagittal depth (5b).
Application and Removal While the lens may look quite large at first, scleral lenses are not difficult to apply. The larger diameter is frequently easier to see and handle.
To apply the lens:
1. Fill the bowl with saline solution. Use preservative-free saline to prevent a solution-induced toxicity from trapped biocide under the lens. You can add fluorescein at this point, if applying during an exam.
2. Work over a clean cloth placed on a table or the patient's lap to prevent fluorescein from splashing on the patient's clothing.
3. Have the patient point his nose at the floor. This position will allow the bowl to remain filled with fluid and will prevent application bubbles from being trapped under the lens.
4. Hold the lens either with the tripod finger method or by using a plunger.
5. Slide the lens under the upper lid, then position under the lower lid. If using a plunger, pinch the end of the plunger to remove it from the lens.
6. Inspect the lens for trapped bubbles.
To remove the lens:
1. Gently massage the sclera to break the suction.
2. Place the plunger on the lower third of the contact lens.
3. Using an out and upward motion, lift the lens out of the eye.
4. Remove smaller lenses by pushing the lids under the lens or by pulling at the lateral canthus.
There are five primary problems when troubleshooting scleral contact lenses, all of which you can manage with contact lens changes or changes in contact lens care.
Mucous buildup under the lens. When this occurs, consider decreasing the sagittal depth or changing to a less viscous solution. Post-surgical eyes tend to make mucus. In some cases, the patient may need to remove the lens part way through the day to clean the lens.
Lens hurts upon removal with subsequent difficulty wearing it the next day. Scleral compression will cause rebound hyperemia and inflammation. Try changing diameter or peripheral curves.
Lens hurts upon application but otherwise the eye feels fine. Mucus may adhere to the back of the lens and cause a plaque-like formation. It's important to ensure that the inside of the bowl is cleaned daily. Progent (Menicon) will remove this plaque.
Bubbles under lens. When this problem occurs, fill the bowl to the brim with fluid. Remove the fenestration hole or, if the bubble is central, reduce the sagittal depth of the lens. If the bubble is peripheral, increase the diameter or flatten the peripheral curve radii.
Lens fogging. This can result from non-wetting lenses; therefore, changing solutions may be beneficial. You can also polish the lenses to remove biofilm and makeup. It's also important to avoid lotions with a lanolin base.
When fitting challenging patients, scleral lenses represent one of the most important tools available to fitters today. Their use should continue to increase because they offer excellent vision, good initial comfort, and they eliminate centration-related problems that can occur with smaller diameter lenses. CLS
Contact Lens Spectrum, Issue: October 2008