OCULAR SURFACE DISEASE: What’s Different About Fitting for OSD?
Julie Song OD, FAAO, FSLS
Ocular Surface Disease: Normal Cornea
While having one corneal disease does not preclude the same patient from having another one, often patients who are obtaining scleral lenses fit solely for therapeutic treatment of ocular surface disease have “normal corneas.” That being said, generally, in patients who have complex ocular surface disease and need as much oxygen transmission as possible, fitting scleral lenses that have a shallow vault could prove beneficial to the patient’s oxygen transmissibility.1 For patients with typical scleral lens fitting philosophies, practitioners often aim for an initial clearance of 200-300 microns over the central cornea. An estimated ~100 microns of post lens tear film can be expected to be absorbed by the cornea or leaked out as the scleral lens settles over several hours.1-2 Over vaulting of the scleral lenses can potentially induce corneal compromise through hypoxia, which can result in neovascularization and edema. Another consideration to make for patients with therapeutic lens fits is to ensure that there are efforts made to reduce suction of the scleral lens. Aside from edge/landing zone adjustments to ensure that the lens edge is not too tight or too flat, certain scleral lens manufacturing companies have optional additions to the design of the lens, such as channels or fenestrations, that can reduce the amount of suction the lens has on the patient’s cornea. Additionally, the presence of conjunctival prolapse, which occurs when there is excessive negative pressure underneath the scleral lens and the conjunctiva (often redundant conjunctiva) pulls towards the center of the cornea and covers the limbal area in that region of the lens to attach to the cornea. This can also put the patient at risk for vascularization and corneal scarring if the limbal region in that area and peripheral cornea show evidence of hypoxia. In these cases, reducing the limbal vault may be indicated.
- Kim YH, Tan B, Lin MC, Radke CJ. Central Corneal Edema with Scleral-Lens Wear. Curr Eye Res. 2018;43(11):1305-1315. doi:10.1080/02713683.2018.1500610
- Craig JP, Nichols KK, Akpek EK, et al. TFOS DEWS II Definition and Classification Report. Ocul Surf. 2017;15(3):276-283. doi:10.1016/j.jtos.2017.05.008.
Ocular Surface Disease: Large Diameter
When fitting scleral lenses, the first consideration made is often choosing an appropriate diameter for the scleral lenses. Often, scleral lens diameters are selected based on the horizontal visible diameter of the patient’s eyes. However, depending on the etiology and severity of the patient’s ocular surface disease, larger diameter scleral lenses may be indicated to offer additional protection for the patient’s cornea.1 Traditionally, a scleral lens has been categorized by diameter into corneo-scleral, semi-scleral, mini-scleral, and full scleral groups depending on diameter size.2 A full scleral by that definition would be a scleral lens whose diameter was 18.1 mm to 24.0+ mm.2 While scleral lenses are not quite defined in the same manner as it was traditionally categorized, it is still important to remember what constitutes a large diameter scleral lens at 18.0 mm+. Some examples of patients who may benefit from significantly larger diameter scleral lenses are patients who have graft versus host disease, neurotrophic keratitis, and more.3
- Fadel D. Modern scleral lenses: Mini versus large. Cont Lens Anterior Eye. 2017;40(4):200-207. doi:10.1016/j.clae.2017.04.003
- Scleral Contact Lens Fitting Guide. PentaVision. https://clspectrum.com/issues/2010/october/scleral-contact-lens-fitting-guide/
- Craig JP, Nichols KK, Akpek EK, et al. TFOS DEWS II Definition and Classification Report. Ocul Surf. 2017;15(3):276-283. doi:10.1016/j.jtos.2017.05.008.
Ocular Surface Disease: Surface Wettability
Surface wettability assesses the ability of a liquid to spread across a surface. Due to the complexities of the anterior segment in patients who have ocular surface disease, often the tear film is insufficient to adequately wet the surface of the scleral lenses and allow for sufficient visibility. When fitting therapeutic scleral lenses, it is important to evaluate and devote close attention to the appropriate material when ordering these lenses. Two such parameters that are important to evaluate are the oxygen permeability (Dk) of the lens to optimize the amount of oxygen that reaches the corneal surface, as well as the wetting angle to aid in the patients’ visual clarity with complex ocular surface disease. Tyler’s Quarterly and the GP Lens Institute (www.gpli.info) are recommended resources that provide a list of the different GP lens materials including their Dk and wetting angle. It is important to choose lens materials with high Dk and low wetting angles for the most optimized therapeutic lens material for your patient. If the patient is still experiencing difficulty with surface wettability after accounting for the Dk and the wetting angle, Tangible Hydra-PEG (Tangible Sciences) can be added to encapsulate the lens to further improve wettability, provided that the patient is cognizant of the corresponding approved solutions that can be used. It is recommended to follow the manufacturer’s and eye care practitioner’s instructions regarding contact lens disinfecting solutions for each lens material and coating.