The present prospective observational study is the first to develop a predictive model for hip abductor and knee extensor strength 12 months after THA with an interaction term. Our finding that the interaction term between preoperative muscle strength and age is significant suggests the possibility that the effect of preoperative knee extensor muscle strength on knee extensor muscle strength 12 months after THA depends on age. While middle-aged patients showed good recovery of postoperative muscle strength regardless of preoperative muscle strength, elderly patients with low preoperative muscle strength showed insufficient recovery.
The TUG time 12 months after THA was significantly slower (117%) than that of healthy subjects in this study. This result is similar to that reported in a systematic review 5. Both hip abductor strength and knee extensor strength 6 months after THA were significantly lower compared to those of healthy subjects (74% and 87%, respectively). Consistent with these findings, a previous study with similar background factors reported a significant reduction in hip abductor and knee extensor strength 6 months after THA (both 75% of healthy subjects) using the same measurement method, suggesting that both muscles remain weak 6 months after THA 6. In the present study, knee extensor strength, but not hip abductor strength, recovered to the level of healthy subjects 12 months after THA. These findings differ from those of a previous study which reported that hip abductor strength was comparable to that of healthy subjects, whereas knee extensor strength was significantly lower 12 months after THA 8. This discrepancy may be due to differences in muscle strength measurement methods as well as patient background factors, surgical procedures, and postoperative protocols. As only a few studies have compared lower limb muscle strength between THA patients and healthy subjects, much remains unknown regarding the recovery status of lower limb muscle strength 12 months after THA. Detailed studies, such as multicenter studies, are warranted.
We created statistical models with interaction terms to clarify factors that determine postoperative muscle strength. Preoperative muscle strength is a strong factor that determines postoperative muscle strength, but there may be interaction effects between preoperative muscle strength and other factors. For example, skeletal muscle mass and muscle strength decrease with age 25, 26, and recovery of muscle strength up to 12 months after THA is expected to be poorer in elderly patients than in middle-aged patients. In addition, disuse muscle atrophy due to long-term illness 27, 28, muscle damage due to excessive leg extension, and decreased postoperative physical activity 29 may impede postoperative muscle strength recovery. The inclusion of interaction terms between preoperative muscle strength and specific factors allows for the assessment of whether the partial regression coefficient of preoperative muscle strength is dependent on other factors. If the interaction with a particular factor is significant and increases the partial regression coefficient of preoperative muscle strength, then the expected value of postoperative muscle strength in subjects with low preoperative muscle strength may be even lower. Therefore, including interaction terms not only improves the accuracy of the predictive model, but also identifies subjects whose postoperative muscle strength is likely to decrease.
The link function of the model for knee extensor strength is an inverse function. When the regression coefficient is negative, postoperative muscle strength increases exponentially as preoperative muscle strength increases. Our analyses revealed that preoperative muscle strength, age, body weight, physical activity, and disease duration were common factors that determine postoperative muscle strength (Step 2 for hip abductor strength and Step 3 for knee extensor strength). Consistent with this, previous studies have reported that preoperative muscle strength, age, body weight, and body mass index are factors related to muscle strength 15, 25, 26, supporting the validity of our models. The interaction between the amount of physical activity and preoperative muscle strength after surgery was not significant, and was not a factor that influenced the effect of preoperative muscle strength. While a number of studies have reported on the causal relationship between muscle strength and physical activity 30, 31, it remains unclear as to whether maintaining high physical activity 2 months after THA improves muscle strength 12 months after THA.
A significant interaction between preoperative muscle strength and age was observed for knee extensor strength. Muscle strength recovered to almost normal levels in elderly patients with high preoperative muscle strength, and in middle-aged patients regardless of preoperative muscle strength. Knee extensor strength may be difficult to improve in the short term 6, but given that there is no direct effect of osteoarthritis, or intraoperative invasion with the anterior-lateral approach (i.e., there are few structural issues), some improvement is expected in the long term. However, recovery of muscle strength in elderly patients with low preoperative muscle strength was poorer compared to middle-aged patients, and muscle strength 12 months after THA did not reach normal levels. A wide variety of symptoms and conditions are associated with aging, including common disorders such as systemic muscular atrophy and weakness, as well as sarcopenia (less common). These conditions reduce not only skeletal muscle mass but also nutritional status and physical activity, making the elderly more prone to disuse syndrome 32. Elderly patients with preoperative knee extensor weakness are more likely to have generalized muscle weakness, which contributes to increased postoperative muscle weakness and low muscle strength after THA. This may explain why the effect of preoperative muscle strength on postoperative knee extensor strength depends on age. Our findings suggest the need to consider the interaction between preoperative muscle strength and age when predicting postoperative muscle strength.
Many patients with hip osteoarthritis reportedly have abductor shortening and atrophy 29,33. Hip abductor damage is also observed postoperatively, even with minimally invasive procedures 34. The abductor muscles often exhibit structural problems, which may impede postoperative muscle strength recovery. Leg extension was extracted as a factor that determines postoperative hip abductor strength, suggesting that abductor degeneration 33 associated with leg shortening and leg extension may inhibit muscle recovery. On the other hand, no significant interaction between preoperative muscle strength and leg extension was observed, indicating that recovery of muscle strength is poor if preoperative muscle strength is low. Structural problems of the abductor muscles are important issues that require further investigation.
In the present study, elderly patients with low preoperative knee extensor strength did not achieve sufficient recovery 12 months after THA, suggesting the need for active training before surgery. Knee extension training includes squatting, a low-risk easy exercise. The postoperative course is expected to improve if patients are instructed to engage in voluntary training during the waiting period before surgery.
This study has some limitations. First, the number of dropouts was high. In addition, although within the predicted range, nearly 30% of patients were excluded for not completing the long-term survey (i.e., 12 months after THA). Thus, the presence of selection bias cannot be denied. However, when preoperative hip abductor strength and knee extensor strength were compared between dropouts and analyzed patients, no significant difference was observed between the two groups, suggesting that the impact of dropouts may have been minimal. Second, since patients with orthopedic complications other than hip osteoarthritis were excluded, it is possible that only patients with a better than average course of THA were included in our analyses.