The key findings of the current study were that breech presentation could caused trochlear dysplasia in neonatal rats and that the incidence of trochlear dysplasia increased with the Swaddling time.
Trochlear dysplasia is characterized by a shallow, flattened
trochlear groove. They are the common knee disorder in children and adolescents with an incidence of 29 to 43 per 100 000 . Currently, the exact aetiology of trochlear dysplasia and the related risk factors are not consensus. However, the numerous evidence in the previous studies had revealed the correlation of trochlear dysplasia and the risk factors . Breech presentation has long been reported to be a potential risk factor of trochlear dysplasia [10, 11, 22]. It is well known that the cartilaginous trochlear develops early on in gestation, between 9 and 16 weeks . Given that trochlea appears well established in the prenatal period, so, it is conceivable that an intrauterine developmental process maybe the aetiology of trochlear dysplasia .
The relationship between breech presentation and trochlear dysplasia had been demonstrated through epidemiological methods [11, 24]. DeVries CA et al. recently reported that the incidence of trochlear dysplasia in breech presentation was 13.5% . Breech patients were found to have a shallower and flatter trochlear groove than those those with not breech . The study by Øye CR reported that knees with extended position could bring about a 45-fold increased risk of dysplasia compared to knees with free to flex . Although the relationship between breech presentation and trochlear dysplasia had been well demonstrated [11, 24], there have been no experimental studies indicating the pathological process of breech position related trochlear dysplasia. Therefore, the study aimed at disclosing the pathogenesis of breech-related trochlear dysplasia based on a neonatal rat model.
Immature animal models had been frequently used to study the risk factors of trochlear dysplasia [21, 25, 26]. In the study, we fixed the rats in hip flexion and knee extension with medical tape to simulate the intrauterine breech posture that was most associated with trochlear dysplasia [11, 12]. Compared to rigid fixation, the elasticity of medical tape could allow for minor movement which was closer to the natural circumstance of intrauterine breech position . It is well known that breech presentation was fixed by the time of 32 weeks of gestation . In general, account for 85–90% of children born in the breech position had the knees extended . So we used swaddling right similar to that in human breech presentation after birth in order that the developmental potential and morphology of the knee was the closest to intrauterine status.
In our study, a neonatal rat model was established to simulate the intrauterine breech posture. At 5 days after birth, the average degree of trochlear sulcus angle in control group was 130.2 ± 3.4°and the average degree of the trochlear sulcus angle in the short Swaddling group was 136.5 ± 4.9°. The mean trochlear sulcus depth showed the greater difference: 0.23 ± 0.04 mm in the short Swaddling group versus 0.31 ± 0.03 mm in the control group. There were significant difference between the short Swaddling group and control group at 5 days after birth (P༜0.01). The mean trochlear sulcus width in control group was 1.27 ± 0.02 mm and the mean trochlear sulcus width in the short Swaddling group was 1.28 ± 0.02 mm. No significant differences were seen in well-known measurements between the short Swaddling group and control group at 5 days after birth (P = 0.56). Compared with the average degree, there were 42 trochleas from 21 rats (female 14/30,male 7/30) appeared trochlear dysplasia among the total 30 rats of the short Swaddling group. At 10 days after birth, the average degree of trochlear sulcus angle in control group was 128.5 ± 3.4°, the average degree of trochlear sulcus angle in the prolonged Swaddling group was 135.8 ± 4.6°. The mean trochlear sulcus depth of prolonged Swaddling group and control group was 0.32 ± 0.03 mm and 0.42 ± 0.02 mm. Significant differences were seen in well-known measurement between the prolonged Swaddling group and control group at 10 days after birth (P༜0.01). However, the mean trochlear sulcus width in control group was 1.52 ± 0.02 mm and the mean trochlear sulcus width in the short Swaddling group was 1.53 ± 0.02 mm. No significant differences were seen in well-known measurements between the short Swaddling group and control group at 10 days after birth (P = 0.53). There were 52 trochleas from 26 rats (female 14/30༌male 12/30) appeared trochlear dysplasia. This indicated that breech presentation was associated with trochlear dysplasia, and the pathogenesis of breech-related trochlear dysplasia was a chronic process. In the study, prolonged swaddling caused a 16.7% increase in the prevalence of trochlear dysplasia (from 70% after short swaddling for 5 days to 86.7% after prolonged swaddling for 10 days). This finding supported that the severity of the pathological changes increases as the knees remain under this condition for a longer period. How to explain this phenomenon?
We know the unique matching relation of the patella and femoral trochlea, which is the basis of its biomechanical function . Stress stimulation plays an important role in articular cartilage and subchondral bone development . Researches have showed that mechanical stress is one of the most important factors affecting of cartilage and bone development [30, 31]. During knee bending, the patella enters the femoral trochlear groove track and loads produce between the femoral trochlea and the patella. The mechanical loading from the patella is transmitted from the trochlear cartilage to subchondral bone. This mechanical stress stimulates the growth and remodelling of the femoral trochlea and the patella . DeVries CA et al. found that breech positioning was associated with trochlear dysplasia , which is consisted with our finding. It is well known that the patella does not engage the trochlear groove until 30° of knee flexion. So we believe that the main reason is that the mechanical stress of patella, which would result in the trochlear dysplasia. In the study, it was no significant difference of the mean trochlear sulcus width between the control group and each experimental group. The finding indicate that mechanical stress does not stimulate the growth of trochlea sulcus width in a short period after breech presentation. The female had been recognized as a risk factor of trochlear dysplasia. There was a female: male ratio of greater than 3:1 [32, 33]. In our study, we found that there was a greater number of female rats than male rats appeared trochlear dysplasia at different stage of the experimental group. However, it was no significant difference in the incidence of trochlear dysplasia between the two genders. The application of swaddling might have a overwhelmed effect than that of sexual differences. The overwhelming effect of swaddling were also observed in previous studies [12, 34].
There were several limitations of our study. First, we used neonatal rats to simulate breech presentation rather than establishing a real intrauterine animal model, which made it difficult to completely reflect the anatomical situation of the human. Ren et al. had endeavored to establish such a simulate breech presentation model by connecting the hindlimb skin with the chest wall through intrauterine operation. However, none of the operated pups survived the delivery . Second, a continuous observation of the knee joint in animal model would have been more conclusive. However, radiological methods such as X-ray or computerized tomography (CT) could not function well in such tiny knees in neonatal rats. Third, the study included the investigation of femoral groove development; However, further studies to include the morphological changes of patella could also be performed in order to get more information about breech presentation.