- Top of page
- PATIENTS AND METHODS
Osteoarthritis (OA) of the knee is a common musculoskeletal disorder (1). Patients with OA of the knee frequently report limitations in their ability to perform activities of daily living (functional ability), such as stair climbing, walking, and household chores (2–4).
Muscle strength has been shown to be a determinant of the ability to perform daily activities in patients with OA of the knee (5, 6). Available evidence from studies on the effectiveness of muscle strengthening for knee OA demonstrates consistent improvement in ability after the intervention (7–9). However, the magnitude of the effect varies considerably between patients. These differences may be attributable to factors that interfere with the relationship between muscle strength and functional ability, i.e., muscle strengthening may be more effective in some patients than in others (10, 11). Joint laxity is one factor that may contribute to this difference in efficacy.
Joint laxity is defined as the displacement or rotation of the tibia with respect to the femur in the varus-valgus direction (10). Joint laxity may affect the relationship between muscle strength and functional ability. However, 2 opposing hypotheses exist concerning how the relationship between muscle strength and functional ability is influenced. One hypothesis is that in patients with a high knee joint laxity, there is a stronger relationship between muscle strength and functional ability. This hypothesis is based on the assumption that in patients with high laxity, muscle activity around the knee compensates for the absence of ligamentous control due to impairments of the passive restraint system. Taking on this dual role increases the importance of muscle strength for adequate functioning, which is reflected in a stronger relationship between muscle strength and functioning. Studies in patients with anterior cruciate ligament (ACL) deficiency have shown that the loss of stability provided by ligaments and capsule can be compensated by increased muscle activity (11, 12). The pattern of increased muscle activation was also found in patients with OA of the knee (13). Compared with age-matched healthy adults and young adults, patients with OA of the knee had higher muscle activity during the execution of daily activities. Therefore, in lax knee joints the role of muscle strength becomes more important, resulting in a stronger relationship between muscle strength and functional ability. The other hypothesis is that in patients with high knee joint laxity, there is a weaker relationship between muscle strength and functional ability (14). This hypothesis is based on the assumption that in patients with high laxity, muscle activity can no longer stabilize the knee, resulting in inadequate control of joint motion. In these patients, functional ability will be affected regardless of the level of muscle strength, resulting in a weaker relationship between muscle strength and function. In view of these 2 opposing hypotheses, the objective of this study was to establish the influence of knee joint laxity on the strength of the relationship between muscle strength and functional ability.
- Top of page
- PATIENTS AND METHODS
Two opposing hypotheses of the influence of joint laxity on the relationship between muscle strength and functional ability were tested in patients with OA of the knee. Our results confirm the first hypothesis, i.e., high joint laxity is associated with a stronger relationship between muscle strength and functional ability.
The results of the present study may be explained by the results presented by Hortobagyi et al (13). In that study, patients with OA had a significantly higher coactivity of knee muscles than age-matched healthy adults and young adults. Patients with knee OA revealed increased muscle coactivation while executing activities of daily living. Coactivation is considered to provide active stabilization of the knee in the absence of adequate stabilization by the passive restraint system (ligaments and capsule) (11, 12). It is likely that coactivation of muscles will only succeed in stabilizing the knee joint when there is sufficient muscle strength. This means that muscle strength is a prerequisite for successful joint stabilization through muscle coactivation. Therefore, we hypothesize that coactivation will be more successful in providing joint stability and subsequently in maintaining functional ability in patients with high muscle strength than in patients with less muscle strength, indicating a close relationship between available muscle strength and successful stabilization through muscle coactivation. This would mean that in patients with high knee joint laxity, differences in muscle strength result in relatively large differences in functional ability compared with patients with low-laxity knee joints. Comparable results were found in a study by Doorenbosch and Harlaar (11), where subjects with an ACL deficiency, i.e., high anterior-posterior laxity, compensated the loss of passive stability (laxity) by developing higher coactivation levels of knee muscles, i.e., active stabilization. Similarly, the results of McNair and Marshall (12) support the hypothesis that higher levels of co-contraction of quadriceps and hamstrings during movements in patients with ACL deficiency provide an active stabilization of the knee to compensate for the loss of the passive structure.
Our results are not in agreement with conclusions presented by Sharma et al (9, 10, 14). In one of those studies (14), it was stated that high laxity was associated with a weaker relationship between muscle strength and functional ability in patients with knee OA (supporting the second hypothesis). A likely explanation of this discrepancy is the difference in analytical approach. The conclusions of Sharma et al were based on a comparison of the correlations between muscle strength and disability in a high-laxity group and low-laxity group. Between these 2 patient groups, there was a small difference in correlation between quadriceps strength and WOMAC physical function (r = −0.27, 95% confidence interval [95% CI] −0.46, −0.05 in the low-laxity group and r = −0.19, 95% CI −0.40, 0.04 in the high-laxity group) and between hamstring strength and WOMAC physical function (r = −0.30, 95% CI −0.39, 0.03 in the low-laxity group and r = −0.21, 95% CI −0.42, 0.02 in the high-laxity group). Given these 95% CIs, it is not likely that the differences in correlation reported by Sharma et al were statistically significant. Additionally, for our particular research question, the use of regression coefficients is preferable. First, using a regression model with an interaction term of muscle strength and laxity allows for one analysis using data from all patients, whereas a correlational analysis similar to the approach used by Sharma et al would require dividing the research group into patients with high and low laxity based on an arbitrary cutoff point. Second, the P value of the regression coefficient of the interaction term provides an immediate insight into the statistical significance of the impact of laxity on the relationship between muscle strength and functional ability.
It should be noted that there are some differences between the populations and measurement equipment and protocols of our study and the study by Sharma et al (14). Our patients were on average more disabled (higher WOMAC physical function score), although age, sex, body mass index, pain, and OA severity were similar. With regard to the measurement protocols and equipment, there were differences between the studies in measuring laxity and muscle strength. In our measurement of laxity, we applied a different method of leg fixation to the device; used a lower torque, which was also applied in a different manner; and used an electronic sensor to assess varus-valgus rotation rather than an analogous device. In our study, muscle strength was measured isokinetically with a lower velocity (60°/second as opposed to 120°/second). Muscle strength was also corrected for body weight and was expressed in SI units rather than feet/pound. However, although these differences may have influenced the results, we believe that the statistical analysis is the main reason for the different conclusions.
The direct relationship between laxity and functional ability was found to be weak (walking time) or absent (WOMAC physical function). Therefore, although laxity is an important factor in instability of the knee (25), the direct effect of laxity in functional ability seems to be relatively limited.
Some issues need to be addressed concerning the methods used in this study. First, the interrelationship of joint laxity between left and right knees in patients with OA of the knee was established and showed a high correlation. Consequently, joint laxity of the left and right knees of the same patients was averaged and used in subsequent analyses. Second, the interrelationship of muscle strength between the left and right knees was established, also showing a high correlation. The results of the muscle strength measurements were averaged in the same manner and were used in subsequent analyses. This indicated that both knee joint laxity and muscle strength are characteristics of a specific patient, instead of characteristics of a specific knee. This finding has been reported previously for muscle strength (5).
In considering the implications of this study for exercise therapy, it is useful to consider some limitations first. One limitation is that an adequate level of joint laxity is unknown. In the absence of a known cutoff point to separate normal angular deviation under load from abnormal deviation (laxity), the differentiation between high and low laxity is only relative. The second limitation of this study is that it was a cross-sectional study of 86 patients from 1 rehabilitation center and causal conclusions were not allowed. Nevertheless, our results support the use of exercise therapy in patients with OA with high knee joint laxity. Based on the results presented here, patients with high laxity can be predicted to benefit from interventions aimed at increasing muscle strength.
In conclusion, patients with OA with high knee joint laxity show a stronger relationship between muscle strength and functional ability than patients with OA with low knee joint laxity. Patients with OA with high knee joint laxity and low muscle strength are most at risk of being disabled.