Biomechanical analysis of the accommodative apparatus in primates

Abstract

Background:  The restoration of natural accommodation in the presbyopic and cataract affected eye is a subject of intense research effort. A new instrument has been developed to test the viability and efficacy of procedures and methods to restore accommodation ex vivo in animal or human eyes.

Methods:  A section of the globe containing the crystalline lens, zonules, ciliary muscle and sclera is bonded into eight curved shoes. After dissecting the sclera between the shoes, even radial load is applied to stretch the zonules and capsular bag to simulate the natural accommodative process. The associated change in optical power is measured using a modified Scheiner's disk method. Changes in the diameter of the lens and ciliary processes are recorded, as well as zonular load.

Results:  No effective change in power was observed for the three presbyopic human eyes under four millimetres diameter stretching; the diameter of the ciliary aperture increased by between 1.8-mm and 2.3-mm, while the maximum increase in lens diameter was 0.19-mm. For the three younger monkey eyes, the diameter of the ciliary aperture increased by 1.4-mm with a corresponding increase in the lens diameter of between 0.50-mm and 0.65-mm. Stretching forces were generally higher for the human than for the monkey eyes, reaching maxima of 35-mN and 52-mN, respectively. The monkey eyes changed power by between 9.1 and 10.1 dioptres. An almost identical, progressive increase for lens diameter, power and stretching force versus stretch distance was found for all three monkey eyes.

Conclusion:  The better understanding of the mechanisms and forces involved in the primate accommodative apparatus will assist with the development of accommodating IOLs and other methods to restore accommodation.

Key words:

• ciliary muscle
• crystalline lens
• lens stretcher
• simulation of accommodation

ACKNOWLEDGEMENTS

We are grateful to Esdras Arrieta MD for surgical assistance, Dr Norma Kenyon and Dr Dora Berman‐Weinberg of DRI, Dr Linda Waterman of DVR at the University of Miami Miller Medical School, and to Dr Rakhi Jain at AMO Inc for scientific support. We also thank David Borja, Noel Ziebarth, Derek Nankivil, Izuru Nose and William Lee of the Ophthalmic Biophysics Centre for their technical assistance with the experimental studies on tissues performed at the Bascom Palmer Eye Institute.

Some of the experimental work was carried out at the Ophthalmic Biophysics Center, Bascom Palmer Eye Institute , University of Miami, Miami, Florida.