The semitendinosus (ST) tendon is commonly used as a replacement graft during anterior cruciate ligament (ACL) reconstruction. There are many advantages of using the ST tendon, including ease of harvest during ACL reconstructive surgery, suitable morphology for use as an ACL graft, and lower donor-site morbidity (24). Despite these advantages, a considerable decrease in knee-flexion torque during deep knee flexion has been reported after harvesting the ST tendon for ACL reconstruction (18,21,27,28), although the peak knee-flexion torque of the ST tendon-harvested limb could be recovered to at least 90% of normal (1-6,10,11,13-17,25,26,30). Torque deficits in deep knee flexion can influence athletic performance in sports activities that require more strength at deep knee flexion. For example, judo athletes cannot perform effectively standing techniques using leg, and gymnasts and ballet dancers cannot pose in maximum knee flexion if they have strength deficits in deep knee flexion. However, the cause of this weakness remains unclear.
In most cases, the ST tendon can regenerate with a morphology similar to the native tendon after being harvested for use as an ACL graft (6-10,12,19,20,22,23,26,27,29). More recently, morphological changes including atrophy (7,8,13,29) and shortening (12,19,29) of the ST muscle belly have been confirmed in patients with ACL reconstruction using the ST tendon. These morphological changes in the ST as a muscle-tendon complex could be reasonably assumed to be a factor causing torque deficits in deep knee flexion. However, little information is available on the relationship between the morphology of the ST muscle-tendon complex and deep knee flexion torque in the ST tendon-harvested limb. The present study aimed to examine knee-flexion torque and morphology of the ST muscle-tendon complex after harvesting the ST tendon for ACL reconstruction and to clarify the relationship between deficits in knee-flexion torque and morphological changes in the ST muscle-tendon complex.
MATERIALS AND METHODS
Twenty-three patients (10 males, 13 females, mean age ± SD: 22 ± 4 yr) with isolated unilateral ACL ruptures 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 an orthopedic surgeon (TF), according to the technique described by Rosenberg et al. (24). All procedures were performed in accordance with the ethical standards of the committee on human experimentation at the University of Tokyo. The patients were fully informed of the procedures and the purpose of the present study and gave their written informed consent.
The same rehabilitation program was used for each patient. The knee was placed in a hinged knee brace for the first week, with isometric quadriceps and hamstring exercises encouraged on the third day after surgery. Then, active range-of-motion exercises and isotonic knee-extension and -flexion exercises were permitted. Partial weight bearing was allowed at the second week after surgery and progressed to full weight bearing at the third week, followed by half-squat exercises. Bicycling and resistance exercises including leg presses and leg curls were also introduced at the third week. Jogging and full-speed running were permitted at the third and fourth months, respectively. Agility drills including jumping exercises were initiated at the fifth month, and the patients were permitted to return to sports activity at the sixth to eighth months after surgery.
At the time of evaluation (average: 23 months after surgery; range: 12-43 months), all patients had returned to their previous sport activities without any pain or restriction. However, the level of performance was not always fully recovered compared with their preinjury levels.
Isometric knee-flexion torque was measured using a dynamometer (Biodex System III, Biodex Medical Systems, NY). Each patient was seated in a prone position with 0° of hip flexion and the lower body tightly secured to the seat (Fig. 1). The axis of the patient's knee was adjusted to coincide with the rotating axis of the dynamometer. Before evaluation, compensation was performed to exclude the effects of gravity on the measurement of torque. Initially, the patients were supervised while performing the movement, and then practice trials were allowed. Two trials of isometric knee flexion with maximum voluntary effort were performed for 3 s at 45 and 90°, representing shallow and deep angles of knee flexion, respectively. The contralateral knee was tested before the reconstructed knee. For the ACL-reconstructed and the contralateral knees, the mean torque value of the two trials was calculated and expressed as a percentage relative to the patient's body weight (%BW).
Evaluation of the ST morphology.
Magnetic resonance imaging (MRI) scans were used to calculate the muscle volume and the muscle length of the ST and to confirm the presence of the regenerated ST tendon after harvesting for ACL reconstruction. MRI scans were obtained with a 0.5-T scanner (FLEXART, TOSHIBA Medical Systems, Tokyo, Japan). The patients were in a supine position with the knee in full extension. T1-weighted, spin-echo, transaxial sequences were performed over the thigh perpendicular to the femoral shaft with the following parameters; a slice thickness of 10 mm, an interslice gap of 2 mm, a repetition time of 1650 ms, an echo time of 20 ms, a field of view of 250 mm × 250 mm, and a matrix size of 160 × 256 pixels. The images were taken from the ischial tuberosity to 50 mm below the knee-joint space. Both the ACL-reconstructed and contralateral limbs were separately examined in all subjects.
To obtain the muscle volume of the ST, the anatomical cross-sectional area of the ST from each image was calculated using Scion Image (Scion Corporation, Frederick, MD). Muscle volume was determined by summing the anatomical cross-sectional area of each image times 12 mm, which is the slice thickness plus the interslice gap. Muscle length of the ST was defined as the length from the ischial tuberosity to the distal musculotendinous junction of the ST.
All values are reported as mean ± SD. As an index of recovery, each value of knee-flexion torque, muscle volume, and muscle length 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 in torque, muscle volume, and muscle length. Before application of the paired t-test, a multivariate analysis of variance was performed to examine the effect of the limb side (i.e., contralateral limb vs ACL-reconstructed limb) on a group of dependent variables (i.e., torque, muscle volume, and muscle length). The analysis showed that the main effect of the limb side was significant (P = 0.004). Pearson's product-moment correlation coefficient (r) was used to examine the relationship between percent contralateral value for every measured variable and the time since surgery. The threshold for statistical significance was set at P < 0.05 for all tests.
The isometric knee-flexion torque of the ACL-reconstructed limb tended to be lower at 45° (P = 0.06) and 90° (P < 0.0001) compared with the contralateral limb (Fig. 2). The percentage of the isometric knee-flexion torque of the ACL-reconstructed limb compared with to that of the contralateral was apparently lower at 90° (74.0 ± 26.8%) than at 45° (94.1 ± 16.7%).
Evaluation of the ST morphology.
The volume of the ST in the ACL-reconstructed limb (111.4 ± 55.7 cm3) was significantly smaller than in the contralateral limb (152.5 ± 65.5 cm3; P < 0.0001) (Table 1). Thus, atrophy of the ST in the reconstructed limb was confirmed.
The muscle length of the ST in the reconstructed limb (24.3 ± 4.3 cm) was significantly shorter than in the contralateral limb (28.1 ± 3.1 cm; P < 0.0001) (Table 1). In 19 of the 23 patients, the shorter muscle length of the ST was attributable to a proximal shift of the distal musculotendinous junction in the ACL-reconstructed limb (Fig. 3A). In the remaining four patients, the length of the ST in the reconstructed limb was the same as in the contralateral limb (Fig. 3B).
In 21 of the 23 patients, the regeneration of the ST tendon-like structure was confirmed (Fig. 4A). In these patients, the entire regenerated tendon-like structure passed the knee joint and was inserted into the distal structures. In the remaining two patients, a tendon-like structure was not identified (Fig. 4B).
Correlation between muscular recovery and time since surgery.
There was no significant correlation between time since surgery and percent contralateral value of each of knee-flexion torque at 45° (r = 0.30, P = 0.17), knee-flexion torque at 90° (r = 0.14, P = 0.54), muscle volume (r = −0.06, P = 0.77), or muscle length (r = 0.16, P = 0.46).
Relationship between torque and morphology.
Based on the morphological changes in the ST muscle-tendon complex after harvesting its tendon, patients could be divided into three groups. In four patients, the regeneration of the ST tendon-like structure was confirmed, and the ST muscle length of the ACL-reconstructed limb was the same as that of the contralateral limb (group I). In 17 patients, the regeneration of the ST tendon-like structure was also confirmed; however, the ST muscle of the ACL-reconstructed limb was shorter than that of the contralateral limb (group II). In the remaining two patients, the ST tendon-like structure was not identified, and the length of the ST muscle of the ACL-reconstructed limb was shorter than that of the contralateral limb (group III).
To evaluate the relationship between knee-flexion torque and morphological changes in the ST muscle-tendon complex after harvesting the ST tendon for ACL reconstruction, the percentage of muscle volume and muscle length of the ST and knee-flexion torque in the ACL-reconstructed limb relative to those in the contralateral limb (% contralateral) were summarized in Table 2 for each of the three groups mentioned above. In the patients in group I, the isometric knee-flexion torque of the reconstructed limb was similar to that of the contralateral limb at both 45 and 90° of knee flexion. In group II, knee-flexion torque of the reconstructed limbs was similar to the contralateral limb at 45°; however, the torque value at 90° tended to be lower than that of the contralateral limb. In group III, knee-flexion torque of the reconstructed limbs tended to be considerably lower than that of the contralateral limbs at both 45 and 90°.
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. The deficits in knee-flexion torque of the ST tendon-harvested limb are partly explained by the morphological changes in the ST muscle-tendon complex.
For the patients with regeneration of the ST tendon-like structure at the ST tendon-harvested site (groups I and II), knee-flexion torque of the ST tendon-harvested limb was recovered at shallow knee-flexion angles. In all cases with regeneration, the regenerated tendon-like structure passed the knee joint and was inserted into the structures distal to the knee joint. On the other hand, patients without regeneration of the tendon-like structure (group III) demonstrated deficits of knee-flexion torque at both shallow and deep knee flexion, most likely because of the lack of a functional tendon to transmit forces from the ST to the tibia. The functionality of the regenerated tendon-like structure is supported by several studies (9,10,12). The histological study of Ferretti et al. (10) indicated that the regenerated ST tendon seemed to be very similar to normal tendon. Eriksson et al. (9) reported that adequate tension in the regenerated ST tendon was created by voluntary muscle contraction. Hioki et al. (12) used a novel MRI technique called the tagging snapshot technique and claimed that the ST with a regenerated tendon-like structure moved proximally during active knee flexion. By analyzing the torque measurements together with the morphological results, we propose that the regenerated tendon-like structure is considered to have a function similar to that of the native ST tendon when contributing to knee flexion.
Knee-flexion torque in the ACL-reconstructed limb was recovered almost completely at deep knee flexion in only those patients with ST tendon-like structure regeneration and the same length of the ST as the contralateral limb (group I). This suggests that the recovery of knee-flexion torque at deep knee flexion is attributable to not only the regeneration of the ST tendon-like structure but also to the maintenance of the ST muscle length. On the other hand, a postoperative decrease in knee-flexion torque at deep knee flexion in the patients from groups II and III might have been a result of shortening and atrophy of the ST. To prevent deficits in knee-flexion torque after ACL reconstructive surgery, further studies are needed to investigate the operative technique and rehabilitation program, enabling regeneration of the ST tendon-like structure while maintaining the morphology of the ST.
A limitation of our study is that the morphology of the regenerated ST tendon-like structure was not evaluated in detail. Investigation on the tendon morphology showed that the insertion site of the regenerated ST tendon was more proximal than the native tendon (20,22,26). The insertion sites of the tendons of knee-flexor muscles including the ST affect the knee-flexion moment arm that contributes to the production of knee-flexion torque. In patients with ACL reconstruction using the ST tendon, it would be useful to further investigate the morphology of the ST muscle-tendon complex in more detail.
In conclusion, the present study showed that changes in knee-flexion torque were associated with morphological changes in the ST muscle-tendon complex after harvesting the ST tendon for ACL reconstruction. Regeneration of the ST tendon-like structure and maintenance of the morphology of the ST are necessary to ensure postoperative recovery of knee-flexion torque at both shallow and deep knee flexion.
We are grateful to the staff at the clinic attached to the Foundation for Oriental Medicine Research for their assistance. This work was partially supported by Waseda University Grant for Special Research Projects (2004B-922).
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