Sex-based difference in the morphology, tensile properties and ultrastructure of the human anterior cruciate ligament and patellar tendon

Anterior Cruciate Ligament (ACL) is an important ligament in the knee which stabilizes it during various daily activities. ACL injury is common is sports such as basketball, soccer and football. Torn ACL does not heal and needs to be reconstructed with a graft tissue to restore knee stability. It is well known that female athletes tear their ACL 2-8 times more frequently than their male counterparts. Higher rates of ACL injury in women translate to more ACL replacement surgeries, more revision surgeries, and a higher chance of developing arthritis with aging. However, there exists no sex-based disparity in the tear rate of reconstructed ACL. Therefore, it is reasonable to believe the tissue mechanics of the ACL itself might play a role in the existing sex-based disparity in the ACL injury incidence. Sex-based differences in the size and tensile properties of ACL may be critical from injury point of view. A biomechanical cadaveric study was conducted to study the possible sex-based differences in the size, tensile properties and ultrastructural variables of human anterior cruciate ligaments. Similar study was also carried out in patellar tendons which is a tissue used for ACL reconstruction. The effect of tissue fatigue on the properties and mechanical behavior of both ACL and patellar tendon were investigated. The effect of notch width on the force generated in the ACL during combined valgus and external tibial rotation was also studied. Results show that female ACLs are not only smaller, but do not grow in proportion to the body height and are of lower mechanical quality even after adjustments to the body anthropometry. No such disparity existed in case of patellar tendon. The female ACL had lower percentage area occupied by the collagen fibrils than the male ACL. The mass density of patellar tendon was found to be a good predictor of its mechanical quality. Smaller intercondylar notch was found to increase the in-situ forces in the ACL. The effect of notch impingement on the forces in the ACL could not be verified. Cyclic loading at higher load was found to affect the mechanical behavior of the ACL but not the patellar tendon. The results are significant from basic science point of view as they show fundamental difference between material make-up of the ACL that possibly is a major reason for sex-based difference in ACL injury incidence. It also appears from the results that patellar tendons have similar material make-up irrespective of sex. Further, the results are also significant from clinical point of view as it shows that mass density of patellar tendons can be used to predict the mechanical quality of graft material prior to ACL reconstruction hence minimizing the chances of a graft of lower mechanical quality being used for reconstruction.