The effect of torsion on the appearance of the rupture surface of the ACL of rabbits.

Abstract

This study investigates how torsion affects the mechanical properties and appearance of the rupture surfaces of the anterior cruciate ligament (ACL). Rupture of the ACL is frequent among athletes and twisting of the knee during rupture is often implicated as a major factor in the injury. The problem is compounded by the ACL having limited ability to heal when ruptured. Thirty-two specimens were stretched to failure at tension--torsion rates of 10 mm/min and 1 degree/s, 10 mm/min and 40 degree/s, 500 mm/min and 1 degree/s and 500 mm/min and 40 degree/s. Results from these experiments were compared with those from pure tension tests performed at displacement rates of 10 and 500 mm/min. The changes that occurred during the rupture process were monitored by video recordings and the appearance of the rupture surfaces studied by scanning electron microscopy (SEM). We found significant differences in stiffness between tests carried out with and without torsion. The macro and microscopic appearance of the ruptures from specimens tested with torsion at different displacement rates were similar although, for the slow displacement rate, the rupture surfaces were different from those from tests carried out without torsion. The failure modes for all tests carried with torsion were similar, with all the specimens failing in the ligament substance by fibre pullout. The modes of failure for tests carried at 500 mm/min with torsion were different from those tested without torsion. Half of the specimens tested without torsion failed by tibial avulsion, whereas all specimens tested with torsion failed in the ligament substance. These results suggest that torsion affects both mechanical and structural properties of the ACL and should, therefore, be considered when designing or selecting replacements for the ACL.