CHANGES IN MOMENT ARM OF THE SEMITENDINOSUS AND KNEE MOMENT POST ACL RECONSTRUCTION

Abstract

The anterior cruciate ligament (ACL) is the most common ligament to rupture in the knee, especially among athletes whose sports are multidirectional (Buller et al., 2017). 1 in every 3000 Americans tears an ACL each year, which results in between 100,000 and 300,000 reconstruction surgeries every year (Macaulay et al., 2012). Commonly the reconstruction is performed using an autograft from the semitendinosus tendon (one of the hamstring muscles). After surgery, the semitendinosus tendon regenerates in the majority of patients, but it is very rare for the semitendinosus to spontaneously attach itself near the original insertion site (Dziedzic and Bogdan, 2014). It is not known how this change in insertion site affects the moment arm of the semitendinosus and the flexor and extensor moments of the knee. This project was a simulation study using OpenSim (Delp et al. 2007). A musculoskeletal model that includes tibiofemoral contact locations and forces (Lerner et al., 2015) was used to alter the insertion site of the semitendinosus and observe how this affects the moment arm of the muscle. To investigate the changes in knee flexor and extensor moments, a generic musculoskeletal model and data of a subject running at 5 m/s was used (Hamner and Delp, 2013). The semitendinosus has been found to attach to the semimembranosus and sartorius muscles (Dziedzic and Bogdan, 2014), so both models were used to change the insertion site of the semitendinosus to the same insertion sites as the semimembranosus and sartorius. The data generated in OpenSim showed a substantial decrease in the moment arm of the semitendinosus when it was moved to the same insertion site as the semimembranosus and sartorius muscles. There was also a decrease in knee flexor moment produced by the hamstrings while there was an increase in knee flexor moment produced by the gastrocnemius. This could potentially increase risk of reinjury to the ACL because the hamstrings are protective of the ACL while the gastrocnemius stresses it. The results of this simulation show that the change in insertion site of the semitendinosus affects both the moment arm of the muscle and the flexor and extensor moment of the knee, which could potentially be a reason for the increased risk of developing OA post-ACL reconstruction. Future research should focus on measuring adaptations with imaging in individuals who have undergone ACL reconstructive surgery. A wider variety of movements should also be used to evaluate changes in muscle forces and determine how ACL reconstruction affects contact stresses in the knee joint.