Rigid Lens Misalignments Affect Image Quality in Eyes With Keratoconus

The misalignment of a sphero-cylindrical rigid contact lens affects visual image quality more in eyes with keratoconus compared with non-keratoconic eyes, according to research published in Ophthalmic and Physiological Optics. In some eyes, a specific misalignment may lead to an improvement in visual image quality, according to the report. 

Researchers used 20 keratoconic and 20 non-keratoconic ocular biometry models, and shifted a previously determined best sphero-cylindrical rigid lens by up to ±1mm from the line of sight and rotated it up to ±15°. A total of 52,111 lens location samples were ray-traced to determine the effect on the wavefront aberration. The team calculated the logarithm of visual Strehl ratio (log₁₀[VSX]) for each aberration structure and used it to predict the associated changes in logMAR visual acuity.

According to the report, image quality variations in the misalignment space were unique to each eye. A 2-letter loss generally occurred with smaller misalignments in eyes with keratoconus (10.5±4.7° of rotation or 0.27±0.13mm of shift) compared with non-keratoconic eyes (13.4±1.8° and 0.39±0.15mm, respectively), due to larger cylindrical errors.

On average, among eyes with keratoconus, 14.4±14.9% of misalignment space saw VSX values higher than the lower normal VSX threshold. Among eyes without keratoconus, these values were 48.5±18.5%.

However, centered alignment did not always provide the best correction in eyes with keratoconus. A total of 5 eyes gained 1 letter and 4 eyes gained a 2 letter improvement with a decentered lens position. Eyes without keratoconus maintained the most optimal visual correction with a centered lens in 18 of the 20 models.  

“Based on these analyses, it is clear that even minor amounts of contact lens misalignment affect visual image quality more in keratoconic eyes than in normal eyes,” according to the researchers. “In some cases, a specific combination of lens shift and lens rotation may also lead to an improvement in visual image quality that can be translated into an acuity gain of one or more letters.”

Study limitations include the use of simulated models and failure to consider contact lens tilt or prism ballasting.