DDH is one of the most common lower limb deformities (1, 2, 20). It is characterized by multiple pathological changes, including in the acetabulum, proximal femur, and soft tissue around the hip (4, 21). Although the DDH capsules are tightened during surgery (4, 5, 22, 23), some patients still have poor clinical function (24). In the present study, all patients underwent similar surgery and physical therapy performed by two senior paediatric orthopaedists and the same physical therapist, respectively. Some patients continued to have poor clinical function of the hip. The capsule factors may be of concern. The main components of capsular collagen I and III were detected and analysed among the different groups according to different factors.
The major types of collagen in the hip joint capsule are I and III(9, 25). Three variations in the COL1A1 gene promoter have been reported in patients with DDH, and a higher rate of total variation in the COL1A1 gene contributes to DDH (26). Skirving et al. (27) reported that the ratio of collagen I and III changes in the joint capsule in children with DDH compared with normal children. Hagiwara et al. (28) fixed the knees of rats and detected the expression of collagen I and III at different stages. They found no changes in the expression of collagen I and III or in acquired ankylosis. These studies show that collagen I is important for resistance to tension in the joint capsule. The changes in collagen may result in differing degrees of joint laxity in DDH.
Age and sex are factors that influence the clinical function of the hip joint (22, 29). Male patients often have poorer function than female patients (4, 30). One study showed that sex is an independent factor with a smaller contribution than age to passive stiffness of the hip capsule ligaments (31). In the present study, all of the patients were at a good age for surgery. There were no significant differences among the patients by age or sex. Our patients were all under 6 years of age, and their clinical function was good to excellent for their age. Collagen I and III did not change with age. Our results suggest that collagen I and III in the hip joint capsule may not be correlated with joint function among patients of different ages and sexes.
The degree of joint dislocation is an important risk factor for a poor prognosis of DDH (4, 30), and many studies have shown that a high degree of dislocation results in poor clinical outcomes (19, 30, 32). However, patients under 6 years of age with a high degree of hip dislocation have been found to achieve good results after a one-stage operation with pelvic osteotomy, femur shortening, and capsulorrhaphy (4, 30). Clinical outcomes can be improved by the implementation of surgical techniques in some patients with a high degree of hip dislocation (4, 33). However, there are still some patients who have poor clinical results (4, 34, 35). We speculated that capsule factors may play a role in clinical outcome. Our patients were divided into three groups according to Tonnis grade, and the expression of collagen I and III was analysed in these different groups. The expression of collagen III did not differ among the groups. However, a lower expression of collagen I was observed in patients with a higher degree of dislocation. The results suggest that collagen I is related to the degree of dislocation in DDH patients. A lower expression of collagen I may lead to joint laxity and a subsequently higher degree of dislocation.
The clinical function of DDH is one of the main evaluation criteria for successful treatment. DDH risk factors include age, degree of dislocation, and AVN (4, 30). Some studies have shown that joint laxity is beneficial for clinical function in DDH (14, 36). Joint laxity may be correlated with the expression of collagen in the hip joint capsule. In the present study, the patients were divided into three groups according to McKay’s classification. The expression of collagen III showed no differences among the three classification groups. However, a higher expression of collagen I was observed in the classification group with poor McKay scores. Given that collagen I is associated with DDH and capsule laxity, the higher expression of collagen I in the present study might have resulted in less capsular laxity, which might have led to poor postoperative clinical function of hip joints in patients with DDH. Our results suggest that collagen I plays an important role in the clinical function of DDH.
There were limitations in the present study. First, we need more useful clinical scoring criteria than the McKay classification to evaluate the outcomes of clinical function of the hip. All patients need further and longer follow-up, even up to skeletal maturity, to confirm the ultimate clinical and radiographic outcomes. Further research on the relation between collagen I and the clinical function of postoperative DDH patients will be conducted.