Tension-Torsion Characteristics of the Canine Anterior Cruciate Ligament—Part II: Experimental Observations

An experimental program was devised to determine whether the canine anterior cruciate ligament (ACL), with an apparent helical twist of its component fiber bundles, could support a torque during axial loading to failure. At the point of first significant failure, the anterior cruciate was found to develop an average maximum torque of 122 ± 26 N-mm (X¯ ± SEM) for the nine tension-torsion tests performed. A nearly linear axial force-torque curve was also observed where the average slope of all tests was 5.2 ± 0.2 mm⁻¹ (X¯ ± SEM). The experimental axial force data was then used to determine material parameters in the constitutive equation for the fascicle and underlying bone. A nonlinear ligament model based on this response was found to reproduce the axial force data adequately and to reasonably predict the observed ligament torque over the entire subfailure loading range. The presence of a ligament torque during axial loading has implications in the design and selection of a ligament substitute. Such a substitute has already been examined in the canine in another study. The results also suggest more refined experiments which could relate the mechanical properties of a ligament to its detailed macrostructure and insertion site geometries.