EVAS II: Advanced Lens Stretcher to Determine Accommodation Forces vs. Power and Geometry

Abstract

Purpose: : To develop and validate a method and instrument to simulate and quantify the accommodation mechanism in the primate eye.

Methods: : The newly developed ex-vivo accommodation simulator (EVAS II) stretches the crystalline lens under controlled conditions while obtaining quantitative data on stretching load, optical power and lens diameter changes. It also includes features to determine anterior and posterior lens shape, lens thickness and axial motion and ciliary body motion, using coaxial imaging, OCT (Uhlhorn, ARVO 2007), Scheimpflug (Borja D, UM PhD proposal 2007) and 35-50MHz UBM technologies. A section of the globe containing intact crystalline lens, zonules, ciliary muscle and a segmented sclera is bonded to 8 curved shoes. The shoes are magnetically coupled to computer controlled actuators, pulling the 8 segments radially in 0.5 mm diameter increments to a maximum of 4 mm diameter change. At each step of the stretch and release cycle, measurement data and images are automatically recorded for later analysis. The optomechanical properties of 3 human (48 to 71 years old) and 3 cynomolgus monkey (4 to 8 years old) eyes have been assessed thus far.

Results: : The 3 human eyes were all presbyopic and no change in lens power and diameter was observed. The total zonular stretching force reached 30.4 to 52.0 mN (0.9 to 3.4 mN STDEV). For the 3 pre-presbyopic monkey eyes, a change in lens power of 9.1, 9.4 and 10.1 diopters was measured, which was associated with a lens diameter increase of 0.50, 0.58 and 0.65 mm and a force increase of 29.2, 31.1 and 34.7 mN respectively. There was minimal hysteresis in lens diameter change between the stretch and release cycle (mean: 0.007mm, STDEV: 0.024 mm). Although, both force and diameter increased exponentially with pulling distance, the ratios for diameter per diopter change and the force per diopter change were relatively constant over the stretch distance (0.06 mm/D and 3.33 mN/D respectively). These results compare well with published data (Manns et al, IOVS 2007).

Conclusions: : The new instrument generates reliable optical, geometrical and mechanical measurements for ex-vivo simulated accommodation. The results from human and non-human primate eyes are consistent with the Helmholtz theory of accommodation.

Keywords: accomodation • presbyopia