The most important finding of this study is that femoral version is significantly different between study group and control group in growing rabbits after patellar dislocation, while the tibia torsion was not significantly different between two groups.
Femoral version shows the relative position of the femoral neck 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 of the femur. 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 [20].
For human beings, the femoral anteversion at birth is 30° to 40°, then it decreases to 10° to 15° when skeletally mature. Most of the alteration occurs before the age of 8 years[21, 22]. 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[19]. Although the decreasing trends of the femoral version development between human-beings and rabbits may look numerically similar, the femoral version between them was different. Actually, 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°)[23].
Femoral version relates to the stability and function of the knee and hip joints and abnormal femoral version affects many disease, including torsional syndromes, fractures of femur, hip dysplasia, Legg-Calve-Perthes disease and anterior cruciate ligament (ACL) rupture[24, 25, 26, 27, 28]. 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[29]. The lateralizing force exists even after medial patellofemoral ligament reconstruction, contributes to the inferior clinical outcomes, even reconstruction failure[30, 31].
In this study, the femoral retroversion decreased after patellar dislocation in growing rabbits. Patellar 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 patellar 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 patellar dislocation, but the tibia torsion did not change significantly in the growing rabbits after patellar dislocation. Similar with humans, in the lower extremity, the femur may be abnormal obviously but the tibia and fibula 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[32].
For the previous animal experiments[8, 9, 10, 12], patellar dislocation or instability could led to femoral dysplasia, patella dysplasia and higher TT-TG. These factors were also regarded as risk factors for patellar dislocation[1, 2, 3, 4, 5]. So femoral dysplasia, patella dysplasia and high TT-TG are not only risk factor for patellar dislocation, but also could be the consequences of patellar dislocation. In the present study, the aberrant femur version was observed after patellar dislocation in the growing rabbits. Similarly, abnormal femur version may not only be a risk factor for patellar dislocation, but also be the consequence of patellar dislocation. These findings may develop pathology and etiology of patella instability, and emphasize the importance of the early effective treatments for patella instability in children, considering the possibility of pathological conditions caused by femur version deformity.
From previous studies, patellar dislocation has been successfully achieved in growing rabbits after patellar dislocation surgery[8, 9, 10, 12]. In the present studies, patellar dislocation was observed by CT scans from each rabbit immediately after surgery and at the last follow-up, which showed the patellar dislocation model was obtained successfully. It is possible that alignment values have not been measured precisely because two-dimensional (2D) measurements can be affected by the location of the radiation source and the limb position[4]. Recently, three-dimensional (3D) method for measuring the alignment of lower extremity has been widely used, which was proved to have high intra-observer and inter-observer reliability. And the method is not influenced by femoral neck-shaft angle or postural deformity[4, 14, 15, 16, 17]. Considering the high accuracy of the method and the extreme flexion of knee and hip joints in rabbits, the 3D method has been taken account in this study and achieved high intra-observer and inter-observer reliability (Table. 3).
The are several limitations of the study. First, the structure of the hind limbs of rabbits are different from human beings’. For example, adults often have femoral anteversion while mature rabbits often have femoral retroversion. So the conclusion of this study may not be applied for humans. But the rabbit models have been widely used for the patellar dislocation studies[8, 9, 10, 12]. The present study is the first research focusing the influence of patellar dislocation to torsional alignment and has high intra-observer and inter-observer reliability, which may enrich the etiology and pathology of patellar dislocation. Second the knee rotation measurements were not involved in this study because of the extreme flexion in the knee joints in the rabbits. But the femoral version and tibia torsion can sufficiently reflect the torsional alignment of rabbit hindlimb. 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. But the sample size is enough according to the sample size calculation. Also, the reason for the alteration of femoral version in biomechanical and molecular level should be researched in the future.