In this study, we evaluated various measures of PFJ morphology and alignment to determine which could predict the radiographic severity of lateral PFOA. The most important findings of the present study were: 1) deeper trochlea (smaller sulcus angle), patellar lateral displacement (larger congruence angle), and patellar lateral tilt (smaller lateral patellar angle) are associated with more severe lateral PFOA; 2) type III patellar morphology is associated with more severe lateral PFOA and 3) a statistically U-shaped relationship between the patellar height (Caton-Deschamps index) and severity of lateral PFOA.
Previous studies identified that most of the variability in alignment can be explained by the combination of morphology parameters, highlighting the complex interaction between patellofemoral joint morphology and alignment [17, 18]. These intricate interactions between morphology and alignment may result in pathologic alterations of the patellofemoral joint. In most patients, such a multifaceted etiology makes the identification of a single pathological factor for PFOA extremely challenging. Therefore, we evaluated PFJ morphology and alignment to determine which predicted the radiographic severity of lateral PFOA. All the cases in this study were hospitalized for total knee arthroplasty or arthroscopic surgery. Through further intraoperative observation, we did not find a case with isolated medial PFOA, possibly because the medial patellar instability is an objective iatrogenic condition . Therefore, we just analyzed the influencing factors for the severity of lateral PFOA in all cases. In this study, we identified a statistically U-shaped relationship between the patellar height (Caton-Deschamps index) and severity of lateral PFOA, which suggested that extreme values of the patellar height have a higher incidence of more severe lateral PFOA. Several previous studies suggested the patella Alta as a possible risk factor for PFOA [20, 21], while some other studies suggested that patella Baja can produce increased patellofemoral contact pressures at early knee flexion, which can potentiate arthritis progression [22, 23]. We propose several explanations for this finding. On the one hand, a higher sagittal position of the patella would lead to a reduced PFJ contact area during knee extension. These abnormal mechanical mechanics of PFJ can lead to patellar instability, PFJ malalignment, and increased shear forces and PFJ contact pressure, which may be risk factors for PFOA [24, 25]. On the other hand, a lower patella may create excessive stress on the articular surface at knee flexion, which causes degeneration of chondrocytes and contributes to the formation of PFOA.
Furthermore, our study also suggested that a smaller sulcus angle (a narrow trochlea) increased the severity of lateral PFOA, but not flatter trochlea. This is different from our speculation that greater sulcus angle increased prevalent PFOA, which was identified in some previous studies [26, 27]. Ali et al. indicated that a larger sulcus angle was associated with severe cartilage defects . Similarly, another study indicated an association between larger sulcus angle and PFJ cartilage injury . However, our study identified a different view that narrow trochlea increased prevalent PFOA. We hypothesis that a narrow trochlea may produce a smaller contact area in PFJ and subject to excessive stress on the PFJ surface. Meanwhile, shallower trochlear sulcus could increase the PFJ contact area and reduce the articular surface contact pressure, which produces a better distribution of the stress on PFJ.
The association of the patella morphology and PFOA has only been sparsely reported in the literature. In our patient population, the patella type, classified according to Wiberg, showed a significant prevalence of type II patella (74%), followed by type I patella (36%) and type III patella (14%). In this study, type III patellar morphology is associated with more severe lateral PFOA. A previous study showed that Wiberg patella type III, which presented a medial border dysplasia or a short patellar apex, are more often involved in patients with patellar dislocation , and patella lateral displacement is associated with more severe lateral PFOA . On the other hand, Wiberg type III patella presented only a small amount of contact with the medial femoral condyle due to the low surface area of the medial facet, which increased the contact pressure of the lateral patella facet on the lateral femoral condyle during knee flexion.
Besides, we also identified that congruence angle was associated with the severity of lateral PFOA positively, while lateral patellar angle was associated with the severity of lateral PFOA negatively. These results are consistent with the results of previous studies that patella lateral displacement or patella tilt could reduce the contact area of the PFJ and increased contact stress on the PFJ surface [31, 32]. Excessive stress on the lateral articular surface of the PFJ could increase the risk of lateral PFOA. This may be why previous studies recommended applying a knee taping to force the patella medially and away from the overloaded lateral compartment as a treatment for PFOA [33, 34].
Several strengths of our study are remarkable. First, the CT scan of the knee was used to define the radiographic PFOA rather than the skyline view of knee radiographs. CT image facilitates the evaluation of radiographic PFOA and measurement morphology and alignment of PFJ. Second, it is worth mentioning that there were no substantial changes in the results when adjusting for radiographic TFOA, suggesting that our findings were not confused by radiographic TFOA.
We recognize several limitations in this study. Firstly, CT was used to define the PFOA. CT is not as sensitive as MRI in the detection of structural lesions. Nevertheless, all patients in this study were hospitalized for total knee arthroplasty or arthroscopic surgery, so our findings by CT were consistent with the results of further intraoperative observation, which suggested the robustness of our findings. Secondly, the patella alignment was measured by CT scans during knee extension. Therefore, the influence of the patella and femoral movement during knee flexion cannot be considered when measuring patella alignment. In the future, it is necessary to measure and analyze the PFJ alignment at different flexion angles. Thirdly, our study only analyzed the imaging findings, which need to be studied in combination with the patient's symptoms in the future.