The Relationship Between Quadriceps Muscle and Tendon Morphology and Physical Performance in Patellofemoral Pain Syndrome

Objective: Patellofemoral pain syndrome (PFPS) is one of the most common causes of anterior knee pain and is associated with biomechanical, muscular, and functional impairments affecting the extensor mechanism of the knee. Quadriceps muscle dysfunction, altered tendon morphology, and impaired lower extremity biomechanics have been suggested to contribute to patellofemoral joint instability and pain development. The aim of this study was to evaluate the muscle and tendon thicknesses of the extensor mechanism using ultrasonography in individuals with PFPS and to investigate the relationship of these measurements with knee pain, knee function, and physical performance, with particular emphasis on the combined assessment of muscle morphology, tendon morphology, and functional performance parameters. Methods: This cross-sectional study was conducted between 5 November 2019 and 15 December 2019, including 80 individuals aged 18–45 years who presented with anterior knee pain and were clinically diagnosed with patellofemoral pain syndrome (PFPS). Demographic characteristics of the participants were collected. Pain severity was assessed using the Visual Analog Scale (VAS), and functional status was evaluated with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Physical performance was assessed using the 6 m walk test and the five-repetition sit-to-stand test. Ultrasonographic examination was used to measure rectus femoris muscle thickness, vastus intermedius muscle thickness, quadriceps tendon thickness, and patellar tendon thickness using a high-frequency linear probe in a standardized supine position with the knee relaxed and the lower extremity muscles at rest. Results: The mean age of the participants was 32.11 ± 7.08 years, and the mean body mass index (BMI) was 25.05 ± 4.11 kg/m2. Of the participants, 42 (52.5%) were male and 38 (47.5%) were female; 46 (57.5%) were smokers and 34 (42.5%) were non-smokers. Ultrasonographic measurements showed that rectus femoris muscle thickness was 1.98 ± 0.45 cm, vastus intermedius muscle thickness was 1.75 ± 0.53 cm, quadriceps tendon thickness was 0.54 ± 0.12 cm, and patellar tendon thickness was 0.35 ± 0.08 cm. Rectus femoris, vastus intermedius, and quadriceps tendon thicknesses were significantly higher in males compared to females (p = 0.001). Individuals with BMI > 25 had greater rectus femoris (p = 0.023) and vastus intermedius (p = 0.001) muscle thicknesses. A negative correlation was found between rectus femoris muscle thickness and WOMAC total (r = −0.227, p = 0.042) and WOMAC pain scores (r = −0.233, p = 0.028). Additionally, a significant relationship was observed between quadriceps tendon thickness and the five-repetition sit-to-stand test (r = −0.247, p = 0.044). Conclusions: In patients with PFPS, quadriceps muscle and tendon thicknesses were found to be associated with certain demographic and clinical parameters. Ultrasonographic evaluation of muscle and tendon structures may be a useful, non-invasive, dynamic, and radiation-free method for better understanding the clinical characteristics of PFPS and its relationship with physical performance. Ultrasonographic assessment may also provide complementary information for rehabilitation planning and functional evaluation in individuals with PFPS, although these findings should be interpreted cautiously because of the cross-sectional design and weak correlations observed.