The most important finding in this study was to establish a new angle, which can be used to predict excessive femoral anteversion in patients with patellar dislocation. The medial condylar angle was smaller in patients undergiong DFO than control group, with statistical difference. Correlation analysis showed that the values of femoral anteversion were significantly correlated with medial condylar angle.
Femoral anteversion is an important parameter of hip and knee joint disorders, which affects patellofemoral force distribution18. Excessive femoral anteversion may lead to anterior knee pain and patellar subluxation19,20. For patients with femoral anteversion greater than 25° measured by 3D CT, DFO should be considered to correct rotational deformity21.
The femoral anteversion can be measured by MRI or CT. Although these two methods provide reliable measurement, they have some limitations, such as cost, moving artifacts, and measurement complexity. For example, CT scanning may bring disturbing radiation exposure risks, especially for young patients14. Botser et al. compared the measurement methods of femoral anteversion between CT and MRI. They reported that compared with MRI, the value measured by CT was larger, with an average difference of 8.9°10. However, due to the complex three-dimensional shape of the femur, the measurement of femoral anteversion can not be accurately obtained in the two-dimensional model, which leads to its limited value in a preoperative plan for DFO. Therefore, it is necessary to guide patients with patellar dislocation to undergo DFO by simple full-leg standing lower limb radiographs.
Although many studies measured femoral anteversion in different ways, no studies have evaluated this angle in full-leg standing lower limb radiographs of patients with patellar dislocation according to an easy-to-use imaging parameter11,12,15. A smaller medial condylar angle means higher possibility of DFO operation in patients with patellar dislocation, because excessive femoral anteversion was associated with smaller medial condylar angle. Therefore, medial condylar angle can be used as an alternative method to screen patients undergoing DFO surgery. The full-leg standing lower limb radiographs may be helpful for surgeons to clinically evaluate and screen patients who may meet the indications of DFO.
Because of the excessive femoral anteversion, the rotation of the femur leads to the morphological changes of distal femur. Due to the natural anatomical features of lower limbs, the position of medial femoral condyle in healthy individuals was in good alignment. Tear of ligament, deformity of bone structure, and other factors that may affect the force vector of the knee joint may lead to change of distal femur14. The disorder of internal biomechanical structure may lead to the loss of good rotational alignment of the knee joint. In patients with patellar dislocation, the rotational deformity was one of the factors, which may lead to the disorder of medial femoral condyle. In patients with mild rotational deformity, the internal ligament structure can compensate for the abnormality of bone structure, to avoid abnormal image examination and clinical symptoms.
There are some limitations to this study. Firstly, there are some errors in the measurement method of medial condylar angle. Because the transition between the distal femur and the medial femoral condyle is an arc, the medial condylar angle is only a parameter to preliminarily judge the femoral anteversion. The purpose of this study was to establish a parameter that can be easily visualized on the full-leg standing lower limb radiographs and can be used to evaluate the rotational deformity. Secondly, the parameter proposed in this study does not include all the factors that lead to the patellar dislocation, because only a few landmarks were analyzed. Further research will be conducted to analyze other factors. Third, this study is a case series study without a control group of normal knee joints without patellar instability. Although there are few patients with abnormal medial condylar angle without patellar instability, the control group may be an important assistant.