Experimental protocols for ex vivo lens stretching tests to investigate the biomechanics of the human accommodation apparatus.

PURPOSE: To explore alternative experimental protocols to investigate the biomechanical behavior of the crystalline lens and zonules using ex vivo stretching.

METHODS

Radial stretching tests were conducted on the anterior segment (consisting of lens, zonules, ciliary body, and sclera) of four pairs of presbyopic human donor eyes. A simple mechanical model is used to describe the behavior of the anterior segment when tested in this way. Each pair of samples was initially stretched with the ciliary body intact. One sample was retested after cutting the ciliary body radially, and the other sample was retested after removing the lens.

RESULTS

The external forces needed to stretch the sample with the ciliary body intact were significantly greater than for the tests in which the ciliary body had been cut. The forces measured with the ciliary body intact and lens in situ were comparable to the sum of the forces measured in the tests in which the ciliary body had been cut (lens in situ) and the forces measured in the tests on the intact ciliary body with the lens removed.

CONCLUSIONS

When stretching tests are conducted on the anterior segment, significant circumferential tensions develop in the ciliary body. This means that the forces applied to the lens and zonules cannot be related directly to the forces applied by the external loading system. If radial cuts are introduced in the ciliary body prior to testing, however, then this difficulty does not arise.