The most important finding of this study was that the femoral version was significantly different between the study group and control group in growing rabbits after patellar dislocation, and the tibial torsion was not significantly different between the two groups.
The 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 the anterior rotation of the femoral head from the coronal plane of the femur. And 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 [22].
For human beings, there is 30° to 40° of the femoral anteversion at birth. It decreases to 10° to 15° when skeletally mature. Most of the alteration occurs before the age of 8 years[23, 24]. 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 were skeletally mature, 10° to 15° degrees of femoral retroversion has been observed[21]. 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, 1070 left femurs were performed thin-slice CT scans, and 77 subjects (7.8%) were found with the retroverted femur (range -23.6° – 0.2°)[25].
The femoral version relates to the stability and function of the knee and hip joints. The abnormal femoral version affects many diseases, including torsional syndromes, fractures of the femur, hip dysplasia, Legg-Calve-Perthes disease, and anterior cruciate ligament (ACL) rupture[26, 27, 28, 29, 30]. The 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[31]. The lateralizing force exists even after medial patellofemoral ligament reconstruction, contributes to the inferior clinical outcomes, even reconstruction failure[32, 33].
In this study, the femoral retroversion decreased after patellar dislocation in growing rabbits. Patellar dislocation may cause the alteration of the force direction of rectus femoris muscle. Also, we found knee or ankle lateral rotation in activities of rabbits after a patellar dislocation. The alteration of strength direction and the 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 tibial torsion did not change significantly in the growing rabbits after patellar dislocation. Similar to humans, in the lower extremity, the femur may be abnormal although 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[34].
For the previous experimental studies using rabbit models[8, 9, 10, 12], patellar dislocation or instability could lead to femoral dysplasia, patellar dysplasia, and higher TT-TG. A higher sulcus angel and lower trochlear depth, which indicated femoral trochlear dysplasia, were found in the rabbit knees after early patellar dislocation or instability[8, 9]. In another study, a longer diameter and higher Wiberg-angle of the patellas were found after patellar instability, which showed patellar dysplasia could occur after early patellar instability[10]. In the study by Niu, the TT-TG in the patellar dislocation group at the last follow-up was 3.0 ± 0.7mm, while the TT-TG in the control group was 1.0 ± 0.4mm (P < 0.05).
The femoral dysplasia, patellar dysplasia, and higher TT-TG were also regarded as risk factors for patellar dislocation[1, 2, 3, 4, 5]. So femoral dysplasia, patellar dysplasia, and high TT-TG are not only risk factors for patellar dislocation but also could be the consequences of patellar dislocation. In the present study, the aberrant femoral version was observed after patellar dislocation in the growing rabbits. Similarly, the abnormal femoral 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 patellar instability, and emphasize the importance of the early effective treatments for patellar instability in children, considering the possibility of pathological conditions caused by femoral 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, a three-dimensional (3D) method for measuring the alignment of the 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 the femoral neck-shaft angle or postural deformity[4, 15, 16, 17, 18]. Considering the high accuracy of the method and the extreme flexion of the knee and hip joints in rabbits, the 3D method has been taken into 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 is different from human beings’. For example, adults often have femoral anteversion while mature rabbits often have femoral retroversion. On the other hand, although the rabbits had patellar dislocation surgery at one-month-old to imitate the early patellar dislocation of humans, it still could not imitate the first patellar dislocation of humans for they have abnormal knees presumably due to a genetic abnormality. So the conclusion of this study may not apply to humans. But the rabbit models have been widely used for patellar dislocation studies[8, 9, 10, 12]. The present study is the first research focusing on the influence of patellar dislocation to torsional alignment. It 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 tibial torsion can sufficiently reflect the torsional alignment of rabbit hindlimb. The third limitation is the sample size of the rabbits. Although the sample size is enough according to the sample size calculation, it could be more reliable if a higher number of experimental animals were used. Also, the reason for the alteration of the femoral version at the biomechanical and molecular level should be researched in the future.