HME is diagnosed radiographically by identifying two or more benign lesions that typically occur around the metaphysis of long bones [17]. Osteochondromas around the hip can cause growth disturbance, hip dysplasia, coxa valga, and hip joint subluxation [4-7,18]. HME has a significant negative impact on the activities of daily life [19,20]. Therefore, in our study we focused on those with an immature skeletal system to investigate (1) changes in proximal femur radiographic parameters during growth, (2) the relationship between the HEA and the NSA, and (3) the effect of patient’s age and sex on the progression of coxa valga, acetabular dysplasia, and hip joint subluxation.
Before discussing the findings of our study, some limitations should be addressed. First, there were a small number of the patients is in each group, especially in group 2. However, this is a longitudinal study with a wide age range of patients and we obtained sufficient parameter data from yearly follow-up radiographs. Moreover, statistical analyses were done using a linear mixed model. It is particularly useful when multiple correlated measurements are made on the same statistical units, in settings like the present study [16]. Second, there was a significant difference in the mean age at the initial presentation between group 1 and 2 due to the small number of patients. However, there was a significantly different pattern between the two groups as shown in Figure 2 (A). In normal development of the hip, the difference of the mean value of NSA and HEA should be less than 5°in the age of 5 and 9 years. In our study, the initial differences of NSA and HEA were more than 10°, and those differences were maintained during follow-up. So we presumed that the differences of the HEA was not age related, or minimally related with age.Third, the study was based on AP pelvic radiographs; therefore, it is difficult to determine the accurate location and size of the masses around the hip. Poter et al. [4] quantified the number of osteochondromas and bony area on AP radiographs at three anatomical sites. However, it is unreliable to assess the location and size of lesions without three-dimensional computed tomography images [21]. Additionally, even though there were no masses around the proximal femur the majority of the patients still had coxa valga deformity.
In the present study, the development of the hip was observed in 30 patients (57 hips) with HME. There was a significant difference in the HEA between groups and the HEA had a significant effect on the progression of coxa valga. However, even though acetabular dysplasia and hip subluxation are not related to the HEA, a significant decreasing trend in AI and a significant increasing trend in MP were found in both groups as age increased.
At the final follow-up, 48.3% of the hips had an abnormal MP with 42.1% classified as borderline and 5.3% classified as subluxated. Moreover, there was a significant increasing trend in the MP in both groups as age increased. The prognosis of a borderline subluxated hip is unclear and may develop a labral tear, sarcomatous changes, or progressive subluxation of the hip joint [4,7]. The present study demonstrates that patients with HME are at risk for eventual subluxation with further longitudinal growth (p < 0.001, Table 2), and it seems to be a faster process in patients with a lower HEA. We should emphasize that it is important to monitor the development of the MP and determine the need for surgery at the right time to prevent the progression of hip subluxation.
The AI and CEA are reliable measurements to evaluate the radiographic features of acetabular dysplasia [22,23]. In our study, the incidence of coxa valga (87.7% of hips had a NSA >135°) and acetabular dysplasia (19.4% of hips had an abnormal AI and CEA at the final follow-up) was lower than that reported by El-Fiky [5]. The finding suggests that coxa valga is common in patients with HME, but acetabular dysplasia is less common.
A previous study found that the femoral head is prevented from growing from a horizontal to a more vertical direction due to the presence of lesions at the medial femoral neck which leads to coxa valga deformity [9]. Others consider coxa valga the initial deformity which accentuates acetabular dysplasia with further longitudinal growth [10]. The incidence of coxa valga was 87.7% in our study, and the patients with a lower HEA tend to have a lower NSA (Figure 2). We should emphasize that a more horizontal proximal femur physis affected the development of coxa valga deformity. Especially in those who were more severely affected it appears that the proximal femoral physis was positioned in a more horizontal direction compared to normal. Even if there were no lesions around the proximal femur, some of the patients still had a more horizontal physis leading to coxa valga when the skeletal system matures. The current study demonstrated that a lower HEA is associated with a higher NSA as growth continues [24] (Figures 3 and 4).
The present study was the first to assess the effects of the HEA on the development of coxa valga, acetabular dysplasia, and hip subluxation. It would be better to evaluate patients who have a HEA far below normal with serial radiographs to ensure that they will develop severe coxa valga over time. For patients who developed coxa valga and who need a varus osteotomy to better position the proximal femur, it may be reasonable to re-establish a normal HEA rather than just the NSA. Further research is needed to examine the influence of hip development on the HEA with a larger number of patients. Furthermore, future research should focus on alterations of the proximal femur by guided growth techniques that aim to prevent patients from developing severe coxa valga and eventually hip joint subluxation.