SUBFAILURE INJURY OF THE RABBIT ANTERIOR CRUCIATE LIGAMENT

Ligamentous injuries range in severity from a simple sprain to a compl ete rupture. Although sprains occur more frequently than complete fail ures. only a few studies have investigated the phenomena of these subf ailure injuries. The purpose of our study was to document the changes in the load-deformation curve until the failure point, after the ligam ent has been subjected to an 80% subfailure stretch. Thirteen paired f resh rabbit bone-anterior cruciate ligament-bent preparations were use d. One of the pairs (control) was stretched until failure; the other ( experimental) was first stretched to 80% of the failure deformation of the control and then stretched to failure. Comparisons were made betw een the load-deformation curves of the experimental and control specim ens. The nonlinear load-deformation curves were characterized by eight parameters: failure load (F-fail), failure deformation (D-fail), ener gy until failure (E(fail), deformations measured at 5, 10, 25, and 50% of the failure load (D-5, D-10, D-25, and D-50, respectively), and st iffness measured at 50% of the failure force (K-50)). There were no si gnificant differences in the values for F-fail, D-fail, and E(fail) be tween the experimental and control ligaments (p > 0.33). In contrast, the deformation values were all larger for the experimental than the c ontrol ligaments (p < 0.01), The deformations D-5, D-10, D-25, and D-5 0 (mean +/- SD) for the control were 0.36 +/- 0.13, 0.49 +/- 0.23, 0.8 1 +/- 0.35, and 1.23 +/- 0.41 mm. The corresponding deformations for t he experimental ligaments were, respectively, 209, 186, 153, and 130% of the control values. K-50 was also greater for the experimental liga ment (125.0 +/- 41.7 N/mm compared with 108.7 +/- 31.4 N/mm, p < 0.03) . These findings indicate that even though the strength of the ligamen t did not change due to a subfailure injury, the shape of the load-dis placement curve, especially at low loads, was significantly altered. U nder the dynamic in vivo loading conditions of daily living, this may result in increased joint laxity, additional loads being applied to ot her joint structures, and, with time, to joint problems.