Paper 79: Semitendinosus Tendon Regeneration after ACL Reconstruction: A Morphological and Functional Study

Abstract

The semitendinosus (ST) tendon is commonly used as an autogenous graft for anterior cruciate ligament (ACL) reconstruction. Several post-operative studies reported that the ST tendon regenerates with a similar histological structure. However, shortening and atrophy of the ST was confirmed more than one year after surgery. Moreover, athletes who perform in deep knee flexion, such as dance, judo, and gymnastics complained of decreased performance ability due to ST muscle atrophy. However, the mechanism of muscle atrophy and decreased knee flexion torque has not been reported and the process of ST tendon regeneration after operation is unclear. Thus, the purpose of our study is to examine the regeneration process of the ST tendon longitudinally as well as evaluate the ST muscle function after surgery. Twenty-nine patients (13 males, 16 females, mean age ± SD: 22.3 ± 3.8 years) who had obtained isolated unilateral ACL rupture participated voluntarily in the present study. All patients were either recreational or competitive athletes belonging to a high school, college or recreational league team. Arthroscopically assisted reconstruction with an autogenous quadrupled ipsilateral ST tendon was performed by the same orthopaedic surgeon. The same rehabilitation program was used for each patient. At the time of evaluation (Average: 22.4 months post-operation, Range: 12 to 43 months), all patients had returned to their previous sport activities without any pain or restriction. Isometric knee flexion torque was measured using a dynamometer (Biodex System III, Biodex Medical Systems, NY, USA). Patients were seated in a prone position with 0 degrees of hip flexion and the lower body tightly secured to the seat. Two trials of isometric knee flexion with maximum voluntary effort were performed for three seconds at 45, 90 and 105 degrees representing shallow and deep angles of knee flexion, respectively. For each of the ACL reconstructed and the contralateral knee, the mean torque value of the two trials was calculated and expressed as a percentage relative to the patient's body weight (%BW). Magnetic resonance imaging (MRI) scans were used to calculate the muscle volume of the ST, cross-sectional area (CSA) of the ST tendon, length of the ST tendon, and to confirm the presence of the regenerated ST tendon after harvesting for ACL reconstruction. The patients were in a supine position with the knee in full extension. MRI scans were obtained with a 0.5-T scanner (FLEXART, TOSHIBA Medical Systems, Tokyo, Japan). T1-weighted spin-echo, transaxial sequences were performed over the thigh perpendicular to the femoral shaft. The images were taken from the ischial tuberosity to 50 mm below the knee joint space. To obtain the muscle volume of the ST, the anatomical CSA of the ST from each image was calculated using Scion Image (Scion Corporation, MD, USA). Muscle volume was determined by summing the anatomical CSA of each image times 12 mm which is the slice thickness plus the interslice gap. The CSA of the ST tendon was also measured using Scion Image (Scion, USA). Tendon length of the ST was defined as the length from the joint line of the knee to the distal musculotendinous junction of the ST. As an index of recovery, each value of knee flexion torque, muscle volume, tendon length and tendon CSA in the ACL reconstructed limb was expressed as a percentage relative to that in the contralateral limb (% contralateral). A paired t-test was used to test for side-to-side differences. Pearson product-moment correlation coefficient was used to examine the relationship between the values for every measured. One-way ANOVA and Fisher's LSD post-hoc test was used to test for difference between degrees of the tendon regeneration to asses the knee flexion torque and muscle volume. The threshold for statistical significance was set at p<0.05 for all tests. In 27 of the 29 patients, the regeneration of the ST tendon-like structure was confirmed. In the remaining two patients, a tendon-like structure was not identified. There was no significant correlation between time since surgery and % contralateral value of each of knee flexion torque at 45, 90, 105 degrees, muscle volume, tendon length and CSA of ST tendon. The isometric knee flexion torque of the ACL reconstructed limb was significantly lower at each of 45 (133.3 ± 28.8 vs. 120.2 ± 26.9 Nm), 90 (82.1 ± 20.9 vs. 57.5 ± 21.0 Nm) and 105 (65.9 ± 21.2 vs. 44.3 ± 16.8 Nm) degrees as compared to that of the contralateral limb. The percentage of the isometric knee flexion torque of the ACL reconstructed limb to that of the contralateral was apparently lower at 90 and 105 degrees ( 71.5 ± 25.0 %, 69.4 ± 29.1 %) than at 45 degrees ( 91.4 ± 17.2 %). The volume of the ST in the ACL reconstructed limb (120.1 ± 55.6 cm3) was significantly smaller as compared to the contralateral limb (164.7 ± 65.3 cm3). Thus, atrophy of the ST in the reconstructed limb was confirmed.The tendon length of the ST in the reconstructed limb (28.8 ± 3.1 cm) was significantly longer than that in the contralateral limb (24.8 ± 3.1 cm). Thus longer tendon length of the ST was due to a proximal shift of the distal musculotendinous junction in the ACL reconstructed limb. Furthermore, there was significant correlation between % contralateral value of muscle volume and that of tendon length (r=−0.70, r2=−0.48). The CSA of the ST tendon in the reconstructed limb (0.18 ± 0.12 cm2) was significantly grater than that in the contralateral limb (0.081 ± 0.049 cm2). In 20 of the 29 patients, the CSA of the ST tendons was greater or equal that of the contralateral limb (Group 1). In the 7 patients, the CSA of the ST tendons was smaller than that of the contralateral limb (Group 2). In the remaining two patients, tendon-like structure was not identified (Group 3). Knee flexion torque at 45 degrees was tended to be lower Group 2 than Group 1 (p=0.06), and significantly lower Group 3 than Group 1 and Group 2. Group 3 had significantly lower muscle volume than Group 2 with lower volume than Group 1. The findings obtained in this study indicate that changes in knee flexion torque are associated with the degree of morphological changes in the ST muscle-tendon complex after harvesting its tendon for ACL reconstruction. By analyzing the torque measurements together with the morphological results, we propose that the regenerated tendon-like structure is considered to have the similar function to the native ST tendon when contributing to knee flexion. As our results of this study, CSA of ST tendon in the reconstructed limb is better to be bigger than in the contralateral limb to avoid the decrease of the knee flexion torque and the muscle volume. From our previous study, we are concerned that the important point for the ST tendon regeneration is to control the excessive tension of ST tendon and at the same time avoiding the muscle atrophy during the early months after operation. To prevent deficits in knee flexion torque after ACL reconstructive surgery, therefore, further studies are needed to investigate the operative technique and rehabilitation program enable to regenerate the ST tendon-like structure while maintaining the morphology of the ST.