Zone geometry, pupil size may affect accommodation in multifocal contacts
Researchers found that young adult wearers of multifocal contact lenses viewing binocularly with significant transition zones generally take advantage of the lens add powers to reduce their accommodative responses, according to a study in Optometry & Vision Science.
Accommodation and pupil behavior of eight young adults a mean age of 27 years and wearing multifocal contact lenses was measured using a high-resolution Shack-Hartmann aberrometer (AMO Wavefront Sciences, Albuquerque, MN) while the subjects fixated a 20/40 character positioned between 2 M and 20 cm in 0.25-D steps.
Subjects were directed to fixate on the letter and keep it as focused as possible, according to the study. Researchers measured refractive states while wearers viewed binocularly and monocularly with single-vision and both center-distance and center-near +2.00 D add multifocal contact lenses (MFCL).
Subjects were fit bilaterally with three contact lens designs: CooperVision Biofinity aspheric single-vision lenses with nominal power of -0.25 D, CooperVision Biofinity center-distance multifocal contact lenses with nominal distance power of -0.25 D and add power of +2.00 D, and CooperVision Biofinity center-near multifocal contact lenses with nominal distance power of -0.25 D and add power of +2.00 D. All three contact lenses had a base curve of 8.6 mm and diameter of 14.0 mm and were manufactured from 52% comfilcon and 48% water.
The refractive state was defined using three criteria: the dioptric power that would minimize the root mean square wavefront error, focus the pupil center and provide peak image quality.
Ex vivo maps showed that the single-vision lens is aspheric with a power of -1.00 D at the edge of the measured 8-mm pupil with the central power of -0.05 D. Ex vivo power maps for the center-distance or center-near multifocals confirmed central power of -0.50 D and +1.50 D, according to the study.
“The refractive state maps of distance corrected eyes over the measured pupil diameters reveal the consistent myopic drift as the accommodation levels increased from the left-to-right panels,” researchers wrote.
Refractive state varied dramatically across the pupil when fit with either the center-distance and center-near multifocals, according to the study. The aspheric single-vision lens resulted in negative spherical aberration in the eye and contact lens and affected the optics over the pupil center to be slightly more myopic than the surrounding area.
Researchers found that all three of the tested contact lenses introduced changes in the measured spherical aberration of the eye and contact lens over the natural pupil sizes. Pupil diameters were found to be smaller during binocular than monocular viewing. Data revealed little accommodative response over the most distinct range of target vergences.
When the target vergence was greater than -1.50 D, both paraxial and minimum root mean square refractive states increased to track the target vergence and the gains approached.
Researchers wrote that subjects fit with the center-distance multifocal were accommodating to achieve focus in the pupil midperiphery, taking advantage of the lens’ add power to focus near targets and leaving the pupil center hyperopically defocused.
The researchers concluded that “young accommodating subjects viewing binocularly through center-distance and center-near multifocal contact lenses with significant transitions zones generally do not focus either the pupil center or pupil margins.” – by Abigail Sutton
Disclosures: The authors reported no relevant financial disclosures.