Factors Associated with Persistent Knee Extensor Weakness in the Long-Term Postoperative Phase After Total Knee Arthroplasty

This study aimed to identify factors associated with knee extensor strength in patients during the long-term postoperative phase after TKA. 

A cross-sectional study included 71 patients undergoing primary TKA for osteoarthritis (mean age: 76.9 ± 6.6 years; 85.7% female) who were at least six months postoperative and could walk independently. Exclusion criteria included revision surgeries, fractures within the last 10 years, and severe comorbidities affecting lower limb function. All surgeries were performed using an anterior straight incision with a medial parapatellar approach, followed by a standardized rehabilitation protocol. Knee extensor strength on the operated side was measured using a hand-held dynamometer (HHD: μTas F-1, Anima Corp., Japan). Patients performed maximum isometric knee extensions for approximately 3 seconds, repeated twice. The average value was used for analysis, adjusted for leg length and body weight. Several factors were assessed for their relationship to knee extensor strength, including quadriceps muscle characteristics, knee pain, range of motion (ROM), and demographic and clinical data. Quadriceps muscle characteristics were evaluated via ultrasound (Noblus, Hitachi, Japan), measuring the combined thickness of the rectus femoris (RF) and vastus intermedius, as well as the vastus medialis (VM). Echo intensity (EI) of the VM and RF was also recorded. Knee pain was measured using a visual analog scale (VAS), and knee flexion and extension ROM were assessed. Age, gender, body mass index (BMI), postoperative duration, and surgical details were collected.Pearson’s correlation coefficient was used to analyze the relationship between knee extensor strength and variables such as muscle thickness, EI, ROM, age, BMI, and postoperative duration. Multiple regression analysis was performed to identify independent predictors of knee extensor strength.

The mean postoperative period for participants was 3.4 ± 2.9 years. Knee extensor strength was negatively correlated with VM EI (r = -0.39, p = 0.001) and positively correlated with knee extension ROM (r = 0.35, p = 0.003). Multiple regression analysis identified VM EI (β = -0.003, 95% CI: -0.006 to -0.001, p = 0.031) and knee extension ROM (β = 0.013, 95% CI: 0.004 to 0.023, p = 0.004) as significant predictors of knee extensor strength. None of the other variables demonstrated substantial associations.

In patients during the long-term postoperative phase after TKA, VM EI and knee extension ROM are key determinants of knee extensor strength. Knee extension torque decreases as knee flexion increases, a phenomenon likely more pronounced in patients with limited ROM following TKA. The VM, directly impacted by surgical intervention, is prone to degeneration of non-contractile muscle tissue, as indicated by increased EI, which may contribute to knee extensor muscle weakness.

Rehabilitation programs focusing on improving knee extension ROM and strengthening the VM may help improve knee extensor strength in patients during the long-term postoperative phase after TKA.