The most important finding of this study is that femoral version is significantly different between study group and control group in growing rabbits after patella dislocation.
Femoral version shows the relative position of the femoral neck axis and the transcondylar axis or coronal plane of the distal femur. Femoral anteversion refers to anterior rotation of the femoral head from the coronal plane. While Femoral retroversion is defined as the condition that the femoral neck axis locates posterior to the transcondylar axis or the coronal plane of the femur [17].
For human beings, there is 30°to 40°of femoral anteversion at birth, then it decreases to 10°to 15°when skeletally mature. Most of the alteration occurs before the age of 8[18, 19]. For rabbits, there were 10°of anteversion in the femur at first. The anteversion disappeared by the eighth week and by the time the rabbits was skeletally mature, 10°to 15°degrees of femoral retroversion has been observed[20]. The decreasing trends of the femoral version development between human-beings and rabbits are same. For adults, femoral retroversion is not as common as femoral anteversion. In the study by Hartel, a total of 1070 thin-slice CT scans of left femurs were analyzed and 77 subjects (7.8%) were found with retroverted femur (range − 23.6°–0.2°)[21].
Femoral version relates to the stability and function of the hip and knee joints and is an important clinical factor for many disease, including torsional syndromes, femoral fractures, hip dysplasia, Legg-Calve-Perthes disease, slipped capital femoral epiphysis and anterior cruciate ligament (ACL) rupture[22, 23, 24, 25, 26, 27, 28, 29, 30, 31]. Femoral version also affects patellar stability. The increased femoral anteversion has been regarded as a risk factor for patellar instability, as it produces a lateralizing force on the patella. The lateralizing force exists even after medial patellofemoral ligament reconstruction, contributes to the inferior clinical outcomes, even reconstruction failure[6, 32, 33, 34].
In this study, the femoral retroversion decreased after patella dislocation in growing rabbits. Patella dislocation may cause the alteration of the strength direction of rectus fomoris muscle. Also, we found knee or ankle lateral rotation in activities of rabbits after patella dislocation. The alteration of strength direction and bone position may be the reason for the femoral version difference during growth. The version of the femur changed significantly after patella dislocation, but the tibia torsion did not change significantly in the growing rabbits after patella dislocation. Similar with humans, in the lower extremity, the femur may be abnormal obviously but the tibia and fifibula are well formed or only slightly hypoplastic. And the foot may be normal despite the severe anomalies in the proximal part of the lower extremity[35].
Just like the previous animal experiments[7, 8, 9, 10, 11], the outcomes of the studies indicates that the aberrant version of the femur may not only be the risk factor for patella dislocation, but also be the consequence of patella dislocation. After patella dislocation in growing rabbits, the morphology of patella, femoral trochlea and the version of the femur were found changed significantly. These findings may develop pathology and etiology of patella instability. This emphasize the importance of the early effective treatments for patella instability in children, considering the possibility of pathological conditions caused by femur version deformity.
The first limitation of the study is the different structure of animal model. Although rabbits have been widely used for the orthopedic studies, the structure of the hind limbs are different from human beings’. So the conclusion of this study may not be right for humans. Second the knee rotation measurements were not involved in this study because of the extreme flexion in the knee joints in the rabbits. The third limitation is the sample size of the rabbits. And it could be more reliable if a higher number of experimental animals were used. Also, the reason for the alteration of femoral version in biomechanical and molecular biological level should be researched.