The Clinical Unknowns of Wearing Scleral Lenses
By Gregory W. DeNaeyer, OD, FAAO
Scleral contact lenses are being fit at an unprecedented rate. The primary reason for fitting scleral lenses is to manage corneal irregularity or ocular surface disease (OSD). However, manufacturers and fitters are starting to target scleral contact lens use to correct refractive error or presbyopia for healthy patients. Unfortunately, there are significant questions that remain unanswered regarding scleral lenses and the possible consequences of wearing them.
Reservoir Exchange, Debris
A well-fit scleral contact lens semi-seals to the eye, holding a fluid reservoir between the lens and anterior surface. A trapped reservoir of fluid accumulates debris and metabolic waste that becomes concentrated over time. It’s well documented that debris buildup can negatively affect vision, but it is not known what short-term or long-term effects this debris or waste accumulation has on the ocular surface.
The corneal surface of an eye that is wearing a scleral lens has to rely on the transmissibility of the lens material for its oxygen supply. With this in mind, a key advance in scleral lens success has been the development of hyperpermeable materials with Dk values of 100 or more. However, there are other scleral lens factors that limit oxygen.
First, scleral contact lenses are routinely manufactured with increased relative center thicknesses to avoid flexure. Increasing thickness decreases the transmissibility of the lens. Secondly, oxygen must also permeate the reservoir, and its permeability is inversely related to its thickness. Scleral lenses are fit to vault the eye and may have reservoir thicknesses that range between 100 microns to 400 microns. Recently, Michaud et al (2012) reported data based upon computations that used material Dk, lens thickness, and reservoir thickness to determine what scleral contact lens parameters minimized hypoxia-induced corneal swelling. Their recommendations were to use the highest Dk available (>150) with a maximum central thickness of 250 microns and to not vault the cornea by more than 200 microns. Many scleral contact lens fits undoubtedly fall outside of these suggested parameters and may be causing hypoxic-related edema. An important next step is to determine whether minor amounts of hypoxic stress or subclinical edema that’s induced from scleral contact lens wear increases clinical complication risk.
Limbal Stem Cells
The general guideline for fitting scleral lenses includes fully vaulting the limbus as well as the cornea. The limbus contains stem cells that are vital to corneal epithelial health and integrity. The theory is that a lens that bears on the limbal area may compromise the integrity of these stem cells. However, by definition, corneal-scleral lenses share bearing between the cornea and the sclera, which means that ultimately there is some limbal bearing when these lenses are fit. Additionally, larger scleral lenses may settle or decenter, which results in limbal bearing. In these cases, is scleral lens wear predisposing the patient to limbal stem cell deficiency?
Many questions remain unanswered regarding scleral lens wear. The answers to these questions may become more critical if scleral lens fitting expands to patients who are wearing them for refractive rather than for medical needs. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #207.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio, and a consultant to Visionary Optics, B+L, and Aciont. You can contact him at email@example.com.