As impingement syndrome in the hip joint can be caused by an abnormal contact between bony and soft tissue structures, this study postulated that the risk of hip impingement syndrome is higher in patients with MHE around the hip joints.
FAI symptoms have been reported to be more common than IFI symptoms in general patients who do not have MHE, however, IFI symptoms have been reported more than FAI symptoms in patients with MHE of the hip. The characteristic deformities of MHE hips with coexisting exostoses around the proximal femur may have a crucial role. Therefore, this study was conducted to investigate the deformities of the hip in patients with MHE and their relationship to hip impingement syndrome.
In general, the main characteristic deformities of MHE hips have been reported to be coxa valga in the proximal femur and occasional features of acetabular dysplasia . Moreover, the presence of coxa valga has also been reported as a developmental consequence associated with acetabular dysplasia . Consistent with previous studies [18, 20], the main characteristic deformities of MHE hips in this study were also identified to be coxa valga and acetabular dysplasia. Furthermore, as shown in Table 2, each deformity measure was confirmed to have a significant correlation.
In the comparison of right and left MHE hips, coxa valga and acetabular dysplasia were found as in the overall MHE hips; however, there was no significant difference between the right and left hips. In the classification according to sex, similar to common deformities of MHE hips, coxa valga and acetabular dysplasia were found, although there was no significant difference. However, the sex comparison according to the alpha angle value showed a significantly higher increase in the male hip joint. This study can be assumed to be similar to a report  showing that MHE as a genetic disease is more deformed in men. However, in this study, as only the alpha angle showed a difference, more detailed studies according to sex are needed.
This study also investigated the relationship between impingement syndrome in MHE hips and the development of deformities. Considering the two measured values of Sharp’s angle and the CE angle, which indicate acetabular dysplasia, the coverage of the femoral head would be reduced. Therefore, pincer-type FAI caused by over coverage of the femoral head seems less likely in patients with MHE. In contrast, FAI in patients with MHE is likely to appear as a cam-type owing to the exostoses around the femoral head-neck junction. According to reports on cam-type impingement syndrome in general patients, the possibility of FAI is high when the alpha-angle value is > 55° . Although 62 MHE hips with an alpha angle of ≥ 55° were observed in this study (Fig. 7), only 14 of 62 MHE hips had FAI symptoms. The reason why only a few hips presented with FAI in patients with MHE is that the increased alpha-angle value had been compensated by other characteristic deformities of the MHE hip. Coxa valga and acetabular dysplasia, for example, could increase the working distance between the proximal femur and the acetabulum. The schematic illustration in Fig. 8 proposes an explanation for this possibility. FAI symptoms in MHE hips occur as a result of the interaction between these compensating values and conflicting values.
Some studies [9, 12, 13, 20] have reported that IFI syndrome may result from the abnormal contact due to the reduced distance around the ischium and lesser trochanter area. Similarly, IFI in MHE hips has been reported to be caused by the shortening of the ischio-femoral distance due to exostoses around the lesser trochanter area and coxa valga deformities. As shown in Table 6, the IFI symptom group showed a much higher incidence of coxa valga than the non IFI symptom group in this study. The deformities observed in the IFI symptom group showed significant differences from those in the non IFI symptom group. Although significant increases in coxa valga deformity and acetabular dysplasia were observed, they were conflicting factors in the FAI symptom group. However, in the IFI symptom group, only coxa valga deformity was a risk factor for IFI and acetabular dysplasia does not have an association with IFI. Rather, it seemed that the risk of developing IFI symptoms increased when exostoses occured around the lesser trochanter. In a study on MHE deformities, Porter et al.  reported that the occurrence of exostoses was more prevalent in the medial side of the proximal femur. In the IFI symptom group in this study, the minimum ischio-femoral distance measured with hip CT was significantly decreased (P < 0.001) as described in Table 7 and coxa valga was significantly increased (P < 0.001) as described in Table 6. As a result, these characteristic deformities in MHE hips may explain why more IFI symptoms appear in patients with MHE.
To compare the symptomatic hips between FAI and IFI, isolated FAI and IFI symptom hips were additionally sorted, however, no significant difference was found between the two groups (Table 8).
Although this study performed a CT study in patients strongly suspected to have IFI symptoms, there have been limitations in comparative analyses in that CT studies were not performed in all asymptomatic patients. In addition, there may be limitations in distinguishing soft tissue impingement or bony impingement owing to the use of CT rather than magnetic resonance imaging.
However, this study is meaningful in that it investigated the association between the characteristic deformities of MHE hip joints and impingement syndrome. Furthermore, this study also showed that the most characteristic deformity in MHE hips was coxa valga regardless of the impingement type, often accompanied by acetabular dysplasia. Further, this study confirmed that the characteristics of the deformities act as differently in each type of impingement syndrome.