Is Apical Bearing Acceptable When Fitting Keratoconus?
By Gregory W. DeNaeyer, OD, FAAO
The three general fitting patterns of corneal GP lenses on a keratoconic eye include: 1) apical clearance; 2) apical touch; and 3) three-point touch (Korb et al, 1982). Baseline data from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study found that 88 percent of patients who wore GP lenses had been fitted with or were wearing flat lenses (Zadnik et al, 2005). Keep in mind that this was before scleral lenses became popular.
Historically, many practitioners have considered apical bearing corneal GP lenses to provide improved comfort and vision for keratoconus patients. What's more, it is generally easier to fit flatter lenses with the increasing corneal steepening in moderate to severe keratoconus.
Vision and Comfort
The literature is somewhat mixed about which fitting technique provides keratoconus patients with better acuity. Sobara et al (2000) reported that lenses fitted 0.2mm to 0.3mm flatter than the subject's habitual GP lenses had improved visual acuity. Jinabhai et al (2010) reported on a single case study that demonstrated improved visual acuity and reduced higher-order aberrations when a moderately keratoconic eye was fitted with relatively flatter lenses. CLEK study results imply that steep-fitting lenses may provide better visual acuity. However, this may be associated more with the mild condition than with the fit itself.
Patients in the CLEK study reported similar contact lens-related comfort regardless of the fitting strategy.
Scarring and Progression
Thirty years ago, Korb et al (1982) reported in a clinical study of seven patients that flat-fitting lenses cause more corneal scarring compared to steep-fitting lenses. Five-year data from the CLEK study demonstrated that any type of lens wear increased the risk of incident scarring and that, among rigid lens wearers, flatness of fit was associated with incident scarring (Barr et al, 2006). The CLEK study was observational, however, and did not determine causal proof that one fitting technique is safer than another.
Kenney et al (2003) reported that causation of keratoconus involves a “cascade hypothesis” in which keratoconic corneas have abnormal or defective enzymes in the lipid peroxidation or nitric oxide pathways that lead to oxidative damage. Their recommendation is that keratoconus patients minimize their exposure to oxidative stress, which includes mechanical trauma caused by poorly fitting contact lenses. Keep in mind that the incidence of corneal scarring is highest among those younger than 20 years of age and that keratoconus seems to level out in many patients by the fourth decade, possibly secondary to age-related cross-linking (Barr et al, 2006; Kamiya et al, 2009; Elsheikh et al, 2007).
It's reasonable to recommend that no fitting change for a keratoconus patient who has a longstanding history of successful wear (good vision and comfort with no contact lens-related complications) with a flat-fitting lens. This especially applies to keratoconus patients who are middle-aged and have shown relative stability.
For younger patients or those who show significant lens-related epithelial disruption, however, adjusting or refitting a lens that provides sufficient apical clearance is probably in their best interest. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #192.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. His primary interests include specialty contact lenses. He is also a consultant to Visionary Optics. Contact him at email@example.com.