Hamstrings and Quadriceps Muscle Recruitment Following ACL Reconstruction with an Autograft

Abstract

Anterior Cruciate Ligament reconstructions (ACLr) are one of the most common knee surgeries. The torn ligament is most often reconstructed with an ipsilateral autograft from the semitendinosus (ST), patella (PT), or quadriceps (QT) tendon. Surgical outcomes are generally positive, but around 50% of individuals develop osteoarthritis in the 15 years after their ACLr. The reasons for this are not completely understood, but reduced and altered knee joint loading have been suggested as the probable causes. It has also been shown that the ST tendon does not restore its pre-surgical properties with reduced tendon stiffness, reduced fibril organization, and increased length common. This study hypothesized that donor site tendon stiffness would also be reduced following ACLr with a BPTB or QT graft. Additionally, it hypothesized that these altered donor site tendon properties would cause altered muscle group force sharing in the hamstrings (ST graft) or quadriceps (BPTB and QT grafts). These deficits would be most pronounced at shorter muscle lengths and lower isometric torque levels and cause a shift of the torque-length curve towards longer muscle lengths.

Data were collected on 15 participants who had an ACLr (5 ACLr – ST; 6 ACLr – PT; 4 ACLr – QT) and 11 healthy matched controls. Participants completed isometric contractions (knee flexion for ACLr – ST and Healthy – ST; knee extension for ACLr – PT, ACLr – QT, and Healthy – PT) in four different positions that altered muscle length and two different torque levels while active shear wave elastography images were taken of the muscles in the hamstring or quadriceps groups. Active shear modulus was multiplied by muscle PCSA to estimate muscle forces. EMG data were also collected. Donor site tendon shear modulus was also measured. Both limbs were tested, with the unaffected limb serving as an estimate of pre-ACLr behavior. Dependent samples t-tests, repeated measures ANOVAs, and effect sizes were calculated within the ACLr and healthy groups to test for between limb and within limb differences.

The ACLr – ST group had consistently lower ST tendon stiffness on the injured side and produced less force out of the affected ST. Deficits in force production were not affected by contraction level, but they were smaller at longer muscle lengths. The degree of ST tendon stiffness recovery did not impact the size of ST force deficits and there were no consistent shifts in the torque-length curve. Donor site tendon stiffness was not consistently lower in the PT or QT following ACLr with these grafts. As such, there were no large effects on quadriceps muscle force production or load sharing. This improved healing of the PT and QT compared to the ST is likely due to the surgical technique. While the entire ST tendon is used for ACLr, only a portion of the PT and QT are sourced which leaves a scaffold for the tendon to regenerate on.

This study was limited by small sample sizes per each graft type; however, it suggested that donor site tendon recovery is more complete following ACLr with a PT or QT autograft than with a ST graft and that deficits in the ST tendon lead to reduced force production. Future research should examine the timeline of PT and QT recovery, as well as explore rehabilitation techniques that can preferentially activate the ST to improve tendon healing and consequently normalized knee joint loading due to more normal knee flexor force sharing.