Main Findings
The clinical symptoms of patients with DDH are primarily related to the increase of local mechanical stress and dynamic hip instability caused by insufficient coverage of the hip joint [14]. Although reports on the causes of pain symptoms are gradually increasing, the influencing factors of pain onset age and pain degree are still not completely clear [6, 14].
In this study, we found that a large distance between the medial femoral head and the ilioischial line and the sharp angle can lead to an earlier pain onset age in patients with DDH; a small LCEA and a large distance between the medial femoral head and the ilioischial line are the risk factors for severe pain.
Influencing Factors of Pain Onset Age
The external movement of the centre of the hip joint is one of the imaging findings of patients with DDH. John C. Clohisy uses the distance between the medial femoral head and the ilioischial line on x-rays to describe the degree of external movement of the centre of the hip joint and proposes that the distance between the medial femoral head and the ilioischial line is 0–10 mm [11]. The external movement of the centre of the hip joint in patients with DDH lengthens the gravity lever arm and increases the joint reaction [15], which may lead to earlier onset of hip pain and increase the likelihood of severe pain. Sharp angle reflects acetabular development and its coverage of the femoral head and can be used to diagnose and predict the progress of DDH. The normal reference value [16] is 38–42°.
We found that the sharp angle negatively affects the pain onset age of patients with DDH, which may be due to the increase of joint contact pressure caused by insufficient coverage of the femoral head when the sharp angle is large. The subsequent static overload leads to the degeneration of articular cartilage, and the overload of soft tissue structure is the ultimate common cause of pain in patients with DDH [6].
Yusuke Kohno et al. also found that an extremely sharp angle is associated with early pain in patients with dysplastic hips [12]. In addition, they also suggested that combined anteversion is a risk factor for the early onset of pain. The combined anteversion is the sum of the femoral anteversion angle and the AAA, which represents the morphological relationship between the two on the axial position. The increase of the femoral anteversion angle and the shortening of the femoral neck led to the early development of secondary osteoarthritis [17]. We speculate that the AAA may also be one of the independent factors affecting the pain onset age. We considered this factor and conducted a single-factor analysis, but it was not statistically significant.
Previous studies have shown that acetabular retroversion is related to the decrease of the coverage area of the femoral head, and hip pain occurs earlier in patients with acetabular dysplasia with acetabular retroversion than those without acetabular retroversion [18]. Since there was no acetabular retroversion in the x-rays of 83 patients, we did not verify it.
Small LCEA is a Risk Factor for Severe Pain
Small LCEA is a risk factor for severe pain in patients with DDH, which is related to the contact area of the femoral head and the biomechanical state of the hip joint. LCEA is an independent factor affecting the contact area of the femoral head. The smaller LCEA limits the area that can be used for proper load distribution and increases the hip joint instability, which leads to articular cartilage injury [19]. In patients with DDH, the contact stress of the hip joint increases sharply to the lateral edge, while the increased LCEA can improve the lateral coverage of the femoral head, reduce the contact stress of the hip joint and change the position of the peak contact stress [20, 21]. A hip joint with a larger LCEA has a larger contact area of the femoral head and a better biomechanical state, which is the main reason why severe pain does not occur easily.
In addition, Eduardo N. Novais et al. [20] found that preoperative LCEA is an independent influencing factor of LCEA < 22° after a Bernese periacetabular osteotomy, and patients with DDH with lower preoperative LCEA are more likely to have an inadequate correction, resulting in surgical failure. Therefore, for patients with small LCEA, how to choose the appropriate operation time and achieve an effective correction of LCEA still needs to be further studied.
Limitations of the Study
This study had certain limitations, however. First, our sample size was small. Second, our study was a retrospective case study. This type of study inherently has various sources of bias, including selection bias, measurement and evaluation bias, as well as lack of follow-up. Finally, this was a short-term follow-up study, and it did not provide medium- to long-term follow-up results.