Our study demonstrates that the gluteus medius muscle shows marked atrophy in the affected hip. This phenomenon was also shown in subjects with advanced OA hip joint pathology. Gait patterns may provide some further explanation for the gluteus medius muscle response to advanced OA hip joint pathology [26
]. The gluteus medius muscle atrophy appears inherently linked to offloading strategies used in gait during late stage joint pathology [28
]. However, the cause in patients with DDH may be related not only with these factors described above but also the femur being displaced proximally, the femur being in a position of abduction with the reduced abductor lever arm and increased gluteus medius activation angle.
In contrast to other studies which selected only one plane both in the affected side and normal side [17
],we choose three different sections. This approach can help to obtain more information about the gluteus medius muscle. We selected different planes in the contralateral healthy hip and the affected one taking into consideration the proximal migration of the femur on the dysplastic side. We selected the contralateral healthy hip as the control group maintaining consistency across patients despite the differing effects of age and height between the patient population we examined, which in itself can affect muscle quality and quantity.
The gluteus medius muscle is one of the main soft tissues that limit femur reduction and limb length discrepancy correction in patients with DDH. To overcome this problem, the release of the gluteus medius muscle is necessary in the mildly dysplastic hip [14
] and combined with femoral osteotomies in the severely dysplastic hip [12
]. Except one-step soft tissue releases, slow release by continuous iliofemoral distraction were also described [29
] in the severely dysplastic hip. Either technique will affect the overall length and function of the gluteus medius muscle. Therefore, how much the gluteus medius muscle could be released and lengthened needs to be further evaluated. The contracture extent of soft tissue around the hip was thought to be an important factor for limb elongation length [31
] in patients with DDH.
Accompanied with the contracuture of abductor musculature, the adductor musculature also does contract. Thus, except the gluteus medius muscle release, proper release of adductor muscles is vital to keep abduction-adduction balance and hip stability during hip reconstruction in THA.
Patient age at the time of the operation and the postoperative changes in the volume of the gluteus medius muscle are related to abductor muscle strength restoration in pelvic support osteotomy of the congenital hip dislocation [11
]. The preoperative muscle strength was an essential factor in postoperative muscle strength recovery in patients with DDH, for whom abductor-sparing periacetabular Osteotomy was performed [32
]. The posterolateral approach, which has a lesser disruption of abductor musculature and more anatomic dissection [33
], should be adopt in patients with DDH performed THA.
Muscle strength is proportional to not only the muscle volume or cross-sectional area (CSA) but also the muscle radiological density (RD), which may represent the actual amount of contractile muscle [34
]. Thus we used CSA combined with muscular RD [17
] to evaluate the atrophy of gluteus medius muscle. The reduced CSA and RD of gluteus medius muscle in patients with DDH implicated reduced muscle strength. Trendelenburg sign, which is indicated by gluteus medius muscle weakness, often could not be improved in some patients performed THA [12
]. The reduced muscle strength may be one of the causes of abductor dysfunction. At the same time, the abnormal femoral offset and abductor moment arm should be paid attention to in abduction function reconstruction in patients with DDH during THA, because the abductor function deficit may result from an intrinsically reduced muscular strength or may be the indirect result of biomechanical alterations induced by abnormal femoral offset and abductor moment arm.
The gluteus medius muscle has the reduced strength preoperative and length changes postoperative in patients with DDH during THA, Some specific rehabilitation exercise should be designed to strengthen the muscle and keep the stability of hip post-operative as for hip arthroscopy post-operative [37
]. Weight-bearing exercises may provide more functional benefit because this type of exercise often activates a greater number of muscle group [38
]. Muscle strength recovery and gait adaption was not complete one year after total hip arthroplasty in patients with unilateral osteoarthritis or osteonecrotic hips [39
]. Another study demonstrated a slow morphological recovery in cross-sectional area (CSA) and radiological density (RD) of hip muscles compared to the healthy limb two years after THA in patients with osteoarthritis [41
]. Considering the changes of gluteus medius muscle in patients with DDH, we think that patients need more time for muscle strength recovery and gait adaption.