The importance of appropriate muscle strength balance around the knee has been emphasized for many reasons. Abnormal muscle strengths may cause muscle imbalance and joint instability. Because many knee diseases result in muscle atrophy11,15 and sometimes in muscle strength imbalance,5 rehabilitation of the muscle is always recommended. Proper cocontraction patterns of the knee are associated with stability of the knee joint. Because the major knee motion is flexion extension, the muscle strength ratios of hamstring to quadriceps were used to understand the muscle strength changes of the knee.5 Followup of relative muscle strength of hamstring to quadriceps after total knee replacement was not reported until Berman et al in 1991,5 who reported that there was no complete recovery of the quadriceps even after 2 years postoperatively. The muscle strength ratio of hamstring to quadriceps of a replaced knee after long term functional adaptation of the muscles remains unknown.
Management of the anterior cruciate ligament and posterior cruciate ligament during total knee replacement surgery influences gait pattern or stairclimbing efficiency.2,3,8 Andriacchi and Birac3 reported that patients with cruciate retaining total knee replacement had more normal gait during stairclimbing than patients with cruciate sacrificing replacement. Although the retained or resected posterior cruciate ligament during total knee replacement has been discussed by many researchers in many ways, different changes of muscle strength with posterior cruciate retained or cut total knee arthroplasty are unknown. Clinical studies also showed that rupture of the posterior cruciate ligament causes a change in muscle strength; there are relatively lower levels of quadriceps' strength compared with hamstring strength.7,26 Different designs of total knee prostheses may cause different effects on muscle strength change. Thus, the relative muscle strengths of knees with cut posterior cruciate ligaments would be different from those with retained posterior cruciate ligaments after total knee replacement.
The purpose of this study was to investigate the differences in hamstring to quadriceps ratios of different prosthetic design factors, including cut or retained posterior cruciate ligaments for total knee arthroplasty. The study also examined the change of muscle strengths around the knee after greater than a 6-year adaptation to determine whether the hamstring to quadriceps ratio returned to the level of that of the healthy group.
MATERIALS AND METHODS
Between 1981 and 1988, 50 knees of 36 patients who received total knee replacement surgery were included for muscle testing. Twenty-two patients were unilaterally operated on and 14 patients bilaterally. All operations were performed by the first author (CHH), who was experienced in performing total knee replacement surgery.17 Three prostheses designs were used: (1) total condylar (Howmedica, Rutherford, NJ); (2) low contact stress meniscal bearing (Depuy, Warsaw, IN); and (3) low contact stress rotating platform (Depuy, Warsaw, IN). The total condylar and low contact stress rotating platform designs were intended for surgery, in which the posterior cruciate ligament was cut. With the low contact stress meniscal bearing design, the posterior cruciate ligament was retained. The patients were evaluated in 1994. Sixteen knees of 9 healthy subjects with no knee disease also were evaluated for comparison. Informed consent was obtained from every subject before being included in the study.
The patients' reasons for surgery included 1 knee with traumatic arthritis and 49 knees with osteoarthritis. All patients were evaluated using the Hospital for Special Surgery Knee Rating Score system.
The basic data of the subjects are listed in Table 1. The postoperative time for all patients ranged from 6 to 13 years. The mean Hospital for Special Surgery Knee Rating Score was 91.8 points at the most recent followup. The mean patient weight was 67.9 kg (range, 42-90 kg) and the mean age was 68 years (range, 51-78 years). Mean followup times for total condylar, low contact stress rotating platform, and low contact stress meniscal bearing prostheses were 10.4 years (range, 8-13 years), 6.3 years (range, 5-9 years), and 6.7 years (range, 5-9 years), respectively. The Student's t-tests on the number of followup years among the 3 patient groups were not statistically different, except for total condylar prosthesis (p > 0.05). Mean patient weights were 64.5 kg (range, 51-80 kg) in the total condylar group, 72.0 kg (range, 50-90 kg) in the low contact stress rotating platform group, and 67.5 kg (range, 42-90 kg) in the low contact stress meniscal bearing group. The mean Hospital for Special Surgery Knee Rating Scores for all patient groups were greater than 90 points at the most recent followup. Mean weight and mean age of healthy subjects were 59.1 kg (range, 46-86 kg) and 68.0 years (range, 63-75 years), respectively. The weight of the healthy group was less than that of the other groups (p < 0.05). The age among 4 groups was not statistically different (p > 0.05).
Isokinetic and Isometric Testing
The Cybex 350 Dynamometer (Lumex, Ronkonkoma, NY) was used to measure muscle performance in this study. Isokinetic and isometric contractions of the quadriceps and hamstring muscles were tested sequentially in each subject. The warmup exercise before the tests consisted of walking for 5 minutes. Before testing, subjects were stabilized in the exercise chair at the pelvis and ipsilateral thigh to prevent excessive movement. They were not allowed to grasp the handles during the tests. Two submaximal trials and 1 maximal trial were done before each test session. The shaft of the dynamometer (Lumex) was aligned with the knee joint axis while the knee was flexed 90 °, and the range of angular movement of the knee joint was from 0 ° to 100 °. A padded strap at the distal end of the dynamometer (Lumex) lever arm was secured to the lower leg just above the ankle. A rest period of 30 seconds was set between each test.
In isokinetic testing, 3 maximal flexion extension cycles for 120 ° per second and 180 ° per second were done. The speeds were chosen so as to exert no pain on the patients and to be done by the patients. In isometric testing, each session consisted of 3 attempts of maximal contraction for 3 seconds at 30 ° and 60 ° knee flexion, respectively. Maximal torque values of the quadriceps and hamstring muscles were recorded and used to calculate the hamstring to quadriceps ratios.
Throughout the test, subjects were asked to do their maximal muscle strength. Vocal encouragement was standardized for each subject.
Radiographic and Hospital for Special Surgery Knee Rating evaluations were done on each patient. The radiographs were examined by the authors. The Hospital for Special Surgery Knee Rating is based on a point score with a maximum of 100 points. To exclude other factors that might influence the testing results, such as pain during testing, only patients with scores of greater than 85 points were recruited into the study.
The Student's t-test and paired t-test were used to analyze the data.
The peak torque values of the muscle strengths of the hamstrings are listed in Table 2. In each group, the muscle strengths at isometric 30 ° were greater than those at isometric 60 ° (p < 0.05). The muscle strengths of the hamstrings at isokinetic 120 ° per second were greater than those at isokinetic 180 ° per second (p < 0.05).
The peak torque values of the quadriceps' strengths are listed in Table 3. In isometric testings, muscle strengths at 60 ° were greater than those at 30 ° (p < 0.05). In isokinetic testings, muscle strengths at 120 ° per second were greater than those at 180 ° per second (p < 0.05).
The ratios of muscle strengths with 3 prostheses design groups and 1 healthy group are listed in Table 4. The ratios of the isokinetic testings in 180 ° per second were greater than those in 120 ° per second, and the ratios of isometric testings at 30 ° were greater than those at 60 °. The hamstring to quadriceps ratios increased as isokinetic testing speeds increased and decreased as isometric testing angles increased.15 The 3 designs of prostheses had this same tendency as shown in Figure 1. In isometric 30 °, the hamstring to quadriceps ratio of the healthy group was less than those of the others, except low contact stress rotating platform. In the other 3 testings, the hamstring to quadriceps ratios among 3 designs of prostheses were not significantly different (p > 0.05); however, the ratios of the healthy group were all less than those of the 3 types of prostheses design groups (p < 0.05).
All basic data among the 3 patient groups for Hospital for Special Surgery Knee Rating Scores, body weights, and ages were similar and there was no statistically significant difference, except that the postoperative periods of the patients with total condylar prostheses were longer than those of the other 2 groups. The knee scores, body weight, and ages did not affect the results among the 3 patient groups (Table 1). The weight and age of the healthy group were compared with those of the 3 patient groups. The mean age of the healthy group was the same as that of the total number of patients, but the mean weight of the healthy group was less than the mean weight of the other 3 groups.
Patients with total knee replacement often showed muscle atrophy due to disuse before or early in the postoperative period.20 The recovery of muscle strength was expected after total knee replacement. However, different designs of total knee prostheses might cause different effects on muscle strength recovery. The authors hypothesized that the muscle strength of the knee in different designs of total knee replacements, with sacrificed or retained posterior cruciate ligament, would result in different hamstring to quadriceps ratios. However, the results did not support this hypothesis. The hamstring to quadriceps ratios were similar among the 3 patient groups with different types of total knee arthroplasties and were higher than those of the healthy group.
Because major motions of the knee are in the sagittal plane (flexion and extension), the Cybex dynamometer (Lumex) was used to analyze the muscle strengths during flexion and extension of the knee in this study. The ratios of peak torque values of hamstring and quadriceps muscles were used for comparison so that differences caused by the different muscle strength in different subjects could be eliminated and the cocontraction effect on the joint stability could be evaluated. The gravity effect on the leg was not corrected because in daily activities, the muscles of humans behave under the gravity effect.
In extension isometric testing, muscle strength at 60 ° was greater than at 30 ° (Table 3). This finding was similar to the results reported by Murray et al.22 In isokinetic testing, quadriceps torque values were significantly greater than hamstrings at each test speed, the peak torque decreased as the testing speed increased (Tables 2, 3), and the hamstring to quadriceps ratio increased as the speed increased (Fig 1). These results also were the same as those reported in previous investigations.4,14,16,21,27
Kannus and Jarvinen19 reported that the optimum isokinetic hamstring to quadriceps ratios recommended by most investigators were between 0.5 and 0.8. In the results of the healthy group, the isokinetic values were in this range. However, most of the patient groups' values were not. They were much higher than those of the healthy group (p < 0.05). This means that the quadriceps of the patients were weaker than those of the healthy subjects in the same age.
Disuse atrophy may be the reason for higher hamstring to quadriceps ratios and lower levels of muscle strengths.20 Usually the subjects in this study were advised to reduce their activities after total knee surgery to increase the survival period of the surgery. This may be the reason why all hamstring to quadriceps ratios of the patient groups were higher than those of the healthy group. Another possible reason may have been the difficulty of obtaining the true performance of the highest torque value; some patients were afraid of failure of their total knee arthroplasty during testing.
One more possible reason for higher hamstring to quadriceps ratios may have been the design of the cut anterior cruciate ligament in these 3 prostheses, because this can affect knee joint stability. The major function of the anterior cruciate ligament is to prevent anterior tibial translation.13 Patients with anterior cruciate ligament deficient knees, even with compensation of the meniscus,23 always show relatively lower levels of muscle strengths of the quadriceps and no change of the strengths of hamstrings.10,18 These 3 prosthesis designs could not effectively compensate for the function of anteroposterior stability of the anterior cruciate ligament. This resulted in the different types of muscle cocontraction compared with those of the healthy subjects. As in anterior cruciate ligament deficient knees,1,18 there were higher levels of relative strengths of hamstring to quadriceps. Therefore, the hamstring to quadriceps ratios increased.
The surface curvatures of the low contact stress meniscal bearing and low contact stress rotating platform prostheses were the same, although the low contact stress rotating platform prosthesis was designed for use with a cut posterior cruciate ligament and the other for use with the posterior cruciate ligament retained.6 The results of the 2 groups were almost the same in every test (p > 0.05). This implied that whether the posterior cruciate ligament was cut did not affect the relative muscle strengths.24 Although the posterior cruciate ligament was cut, its functions were compensated, possibly by the posterolateral complex or popliteus muscle. The anterior curvature of the tibia component might provide some constraint to prevent relative anterior translation of the femur to the tibia, just as the meniscus assisted in patients in whom the posterior cruciate ligament was cut.7,9,12 All these factors could contribute to compensation for the function of the posterior cruciate ligament, resulting in the same hamstring to quadriceps ratios found in patients in whom the 2 types of prostheses were used.
Both the total condylar and low contact stress rotating platform prostheses were designed for use with a cut posterior cruciate ligament. However, the tibia curvatures of these 2 types were totally different. The tibia insert of the total condylar prosthesis was more concave than that of the low contact stress rotating platform prosthesis. These differences did not appear to influence the cocontraction types of the hamstring and the quadriceps in this study.
Patients with a ruptured anterior cruciate ligament showed a greater decrease in quadriceps muscle strengths than hamstring strength.25 Because the 3 types of prostheses were designed for use with a cut anterior cruciate ligament, the authors thought that the design might be the reason for higher hamstring to quadriceps ratios. However, most prostheses currently in use were designed for use with a cut cruciate ligament, either anterior cruciate or anterior and posterior cruciate ligaments, and knee muscles with these prostheses cannot reach the level of those of healthy subjects.
Even after total knee replacement surgery with successful results for 6 to 13 years, the hamstring to quadriceps ratios of the patients were still higher than those of the healthy subjects, and the cut or retained posterior cruciate ligament during total knee replacement surgery would not influence the relative muscle strengths. The muscle imbalance of these patients still existed.
The authors thank Professors Yi-Shiong Hang and Tang-Kue Liu for their kind assistance in conducting this experiment.
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