Effects of the Extraocular Muscles on the Response of the Human Eye Under Static and Dynamic Loading Conditions

The purpose of this study is to determine the effect of the extraocular muscles on the response of the eye under static and dynamic loading conditions. Two different types of tests have been performed. First, static displacement tests were conducted utilizing computed tomography to visualize the displacement of the eye within the orbit. Six eyes were tested in situ in matched pairs, one eye with the extraocular muscles transected, and one eye with the extraocular muscles left intact. A rigid impactor was used to displace the eye in 5 mm increments from 0 mm to 30 mm and the load required to displace the eye was measured. Second, dynamic impact tests are planned to elucidate differences in response at high rates. Again, matched pairs of eyes will be tested with the extraocular muscles left intact on one eye and transected on the other. The impactor is accelerated to approximately 10 m/s prior to striking the cornea and the load and displacement response of the eye is measured under this dynamic impact. Force-displacement responses of eyes with intact and with transected muscles will be analyzed. At present, only one dynamic impact test has been performed. Also, a comparison of the injury outcomes of eyes with and without intact musculature is planned. For static tests, the extraocular muscles increase the force required to produce a given deflection by thirteen percent, though this varies with the specific indenter geometry. For the first dynamic impact test, the peak forces are greatest with extraocular muscles intact (295 N for intact vs. 250 N for transected), while the translation of the eye is greater with the extraocular muscles transected (16 mm for transected vs. 12 mm for intact). Data from both static and dynamic test results suggest that extraocular muscles affect the in situ response of the eye to impact, especially when relatively large translations of the eye are observed. Since enucleated eyes are frequently used for impact testing, it is important that additional testing be performed to elucidate any differences in force-deflection and injury response. Injury Biomechanics Research