The newly developed modified PFA with inferior patellar measurement can measure the inferior patellar mobility with good intra- and inter-examiner reliability (ICC, 0.87–0.99) and is suitable for clinical application. The main findings in this study are as follows. There was a significant difference in the absolute value of IPD between young men and women, but when adjusted for body height, the significant difference disappeared. Absolute and normalized values for IPD were significantly lower in elderly women than in young women. In addition, the seiza-impossible group of the elderly had significantly lower IPD than the seiza-possible group and young women. Additionally, in the seiza-impossible group of the elderly, there were significant positive correlations between the absolute and normalized values for IPD and the knee flexion angle.
Methods for evaluating the patella include tracking26, magnetic resonance imaging (MRI), computed tomography, conventional radiography in the static position27–30, and medial and lateral mobility assessment21–24. However, our method is a non-invasive method for dynamic evaluation of inferior patella mobility. It has been shown that the decrease in inferior patellar mobility is related to limited knee joint flexion31. To date, however, the inferior patellar mobility has been measured using subjective measurements of patellar flexibility [The Cincinnati Knee Rating System and Kolowich (quadrant) method]17, 19. Hence, we are the also demonstrate its relationship with knee flexion angle using objective measurements. Additionally, using the mean ± 2 SDs (94.2% of normal distribution among men and 94.6% of that among women), it was necessary to establish the mean inferior patellar mobility in young men and women and adjust for normalized by body height, as described in the previous reports21.
The inferior patellar mobility values among both the seiza-possible and seiza-impossible groups of elderly women were significantly lower than that among young women, with both elderly groups showing half the mobility of their young counterparts. Diminished muscle flexibility and increased soft tissue stiffness due to aging have been demonstrated in humans32. In particular, decreased soft tissue and muscle flexibility near the base of patella and distal thigh may affect the inferior patella mobility11–13. The inferior patellar mobility was significantly lower in the seiza-impossible group than in the seiza-possible group. Furthermore, there was a significant correlation between inferior patellar mobility and knee flexion angle in the seiza-impossible group, suggesting that inferior patellar mobility influenced knee flexion angle.
Suprapatellar pouch lesion is considered to be one of the causes of the inferior patellar mobility. As the patella and suprapatellar pouch are connected, and adhesion of these two components may cause pain, flexion contractures, and stiffness13,33, which may reduce patellar mobility. Kapandji also reported that, when knee flexion increases from 90 degrees to more than 135 degrees, the suprapatellar pouch becomes completely ‘unpleated’ (i.e., straight without creases)34. The function of the suprapatellar pouch during knee flexion has been reported to be anatomically important; hence, it is possible that the inferior patellar mobility may be caused by diminished flexibility of soft tissues around the knee, including the suprapatellar pouch11.
The relationship between the flexibility of soft tissues around the knee and inferior patellar mobility was described as playing a role in some diseases that cause limited knee joint flexion. In the case of knee osteoarthritis, it has recently been suggested that joint inflammation is not only confined in the articular cartilage of the knee but also affects the surrounding soft tissues, leading to peripatellar lesions and knee stiffness35. In patients with knee joint fibrosis, the signs and symptoms of arthrofibrosis include reduced patellar mobility, diminished knee ROM, tenderness around the knee, pain, and atrophy of the quadriceps. Inferior patellar mobility has been shown to be a factor affecting diminished knee flexion ROM36–40. Furthermore, in patients with knee joint fibrosis, lesions around the patella such as in the infrapatellar fat pad, pretibial recess, anterior interval, and suprapatellar pouch may arise41, affecting inferior patellar mobility.
However, the causative factors of inferior patellar mobility have not been clearly determined because of the lack of methods that objectively evaluate it. Moving forward, it is necessary to clarify the relationship between inferior patellar mobility and soft tissue flexibility around the knee in patients with limited knee flexion. If the limit of inferior patellar mobility are identified, improvements in such mobility using therapeutic interventions can lead to an increased knee flexion angle.
Our study has some limitations. First, the subject was a healthy group, and patellar mobility was not compared in the patient population. In future studies, it would be necessary to examine patellar mobility in patients with knee osteoarthritis. Second, we did not compare our results with those obtained using gold-standard procedures (i.e., MRI or X-ray fluoroscopy) for validation. Ota et al. evaluated the mobility of the medial and lateral patella with MRI using conventional PFA and obtained good results22. Moreover, the position of the patella at rest needs to be specifically examined, as it may be related to the inferior mobility of the patella. Specifically, it is necessary to evaluate the Insall-Salvati ratio using radiography27. Third, we did not quantify the force that pushes down the patella; rather, we used a dynamometer that we practiced pushing at 80 N prior to measurement. However, it was unclear whether the optimal force was applied to push the patella downward in actual measurements. To solve this problem, it is necessary to consider obtaining measurements while pressing back the patella in real-time using a sheet sensor, for example.