The current study found that GMC is significantly correlated with coxa valga and the increase of SFP angle. A possible explanation for this might be that the onset of GMC in childhood affects hip development. It has been demonstrated that gluteal intramuscular injection with benzyl alcohol, as a solvent for penicillin, in childhood is a significant cause of GMC [7]. As children are in the period of growth and development, the morphology of bone development changes according to physiological needs with the age increase to maintain the balance of normal coordinated development of bones and muscles. The gluteal muscle degeneration and contracture, caused by chemical and physical damage of drug injection, forms a fibrosis band that stretches the pelvis and femoral epiphysis. The skeletal growth in traction direction destroys bones and gluteal muscles’ physiological balance, secondarily affecting the pelvis and femur’s normal morphology. As shown in Fig. 3, AP pelvic radiographs of the representative cases showed marked increases in NSA and SFP angle in the GMC patients compared with the patients without GMC.
The upper outer quadrant of the buttocks, where the gluteus maximus and the anterior fibers of the gluteus medius are located, is a common intramuscular injection site in clinical practice [23]. Therefore, the gluteus maximus and gluteus medius are also the most frequently involved muscles in injection-induced gluteus contracture. A biomechanical study in China [23] showed that although the contractural gluteus muscle still has the characteristics of viscoelastic material, the contracture band’s ultimate strength and elastic modulus are significantly higher than those of the normal gluteus muscle. In contrast, the ultimate strain of the contracture band is significantly lower than that of the normal gluteus muscle. That is to say, the elasticity of the contracture gluteal muscles is greatly reduced on the one hand, and the strength is significantly increased on the other hand.
From the perspective of the local anatomical relationship, the gluteus maximus arises from the posterior gluteal line of the ilium and the rough area of bone, descending laterally and ending at the gluteal tuberosity of the femur and the iliotibial band. Gluteus medius arises between the posterior and anterior gluteal line, ending on the greater trochanter’s lateral surface. Therefore, the contracted gluteal muscles pull outward and downward on the iliac bone, disrupting the pelvis’s force balance, resulting in a forward pelvic tilt (PT). However, limited to the retrospective imaging studies, lateral pelvic radiographs were not obtained to determine the pelvic tilt’s exact degree. Previous studies have proposed SFP angle as devices predicting PT in AP pelvic radiograph [21, 22], and Hu et al. [24] verified the SFP angle’s reliability in estimating PT in Chinese Han nationality adults. In the current study, GMC with higher SFP angle means has lower PT according to the estimation equation (PT = 75 − SFP angle) [21]. A reduced PT (defined as the angle between the vertical and the line from the center of femoral head to the midpoint of the sacral plate on the lateral pelvic radiograph) means that the pelvis is tilted forward [25]. It can therefore be assumed rationally that the GMC may induce forward pelvic tilt. However, the assumption is based on a forecast, and future research should be undertaken to investigate the relationship between GMC and measured exact pelvic tilt.
On the other hand, since the attachment of the gluteus maximus is located in the gluteal tuberosity of the femur and the iliotibial band, the contracture of the gluteus will produce upward and backward pulling force on the gluteal tuberosity, thus causing the femur abnormal external rotation and extension. The hip joint is relatively stable, and normal individuals do not require gluteus medius muscle contraction to maintain an upright state. However, gluteus medius contracture bands are always under tension due to shortened muscle length and reduced elasticity. Therefore, the anterior fibers of the gluteus medius, positioned almost in the sagittal plane, have persistent traction on the femur in the direction of abduction. Bone development is characterized by reshaping in the direction of traction, as exemplified by the iliac hyperdense line sign, which is a cortical change caused by sustained traction of the gluteus maximus contracture [14].(Fig. 3A) Taken together, the contracture muscles represented by the gluteus maximus and the gluteus medius disrupt the biomechanical balance of the hip joint and generate a sustained outward posterosuperior traction force that induces osseous remodeling. In the long-term, the gluteus muscle contracture eventually leads to abnormal development of the femoral neck and the NSA’s enlargement, thus forming coxa valgus. This study found that there was no correlation between the occurrence of coxa valga and patient age. A possible explanation for this might be that most bones cease growing after reaching adult age.
In the verification of the characteristics differences between the GMC patients with or without coxa valga deformity, this study unexpectedly found that younger patients appeared to be more likely to develop coxa valga (P value: 0.116 left and 0.02 right). This result may be explained by the fact that patients with more severe contracture lesions, which are more likely cause coxa valga and earlier symptom, are more likely to seek therapeutic intervention early. In addition, the results of the t-test show that there are significant differences in acetabular depth (P value: 0.102 left and 0.01 right), FHCI (P value: 0.005 left and 0.02 right) and OF ratio (P value: 0.003 left and 0.03 right) between GMC patients with or without coxa valga. A possible explanation for this might be that, as noted previously, patients with more severe contractural gluteal muscles are more likely to have coxa valga while also contributing to deformities of the hip and pelvis. Another possible explanation for this is that patients with coxa valga inherently have genetic or acquired factors for the skeletal deformity. Therefore, future work based on grading the severity of GMC patients is required.
One interesting finding is the difference of risk factors in logistic regression for coxa valga between the left side and right side. There are several possible explanations for this result. Firstly, there is a biomechanical difference between the left hip and the right hip, and the difference in the dominant side can also lead to uneven development. Secondly, there is measurement inaccuracy due to the tilted position of the pelvis. Because the occurrence of GMC is related to the concentration of benzyl alcohol and injection frequency, contracture is more severe on the side with more injection times. Unilateral or bilateral cases with different contracture degrees usually cause uneven force on the pelvis in the coronal plane, resulting in a left or right obliquity of the pelvis while tilting forward. There are similarities between the attitude expressed by Zhang et al. [4], who suggested that GMC can cause leg-length discrepancy and pelvic obliquity. Henebry et al. [26] also demonstrated that pelvic tilt could alter the hip’s radiographic markers in standing AP pelvic radiograph. Thirdly, the changes mentioned above of pelvic tilt and femoral NSA caused by GMC alter the hip joint’s biological stress axis, affecting the acetabulum development and further resulting in the radiographic performance alteration. These alterations including the increase of CE angle and the decrease of acetabular depth and FHCI. The factors existing in GMC change according to the lesion’s degree, and they influence each other, leading to the current results. In future investigations, the severity of GMC should be graded to eliminate confounding factors.
The knee is the most common site of osteoarthritis (OA) in clinical practice. Several risk factors are associated with knee OA, among which skeleton misalignment is one of the inherent risk factors for knee OA [27]. Misalignment of lower extremity changes the normal biomechanical mechanism, thus producing a direct effect on OA progression or indirectly affecting the surrounding tissue [28]. Previous studies have shown that both varus and valgus malalignment of knee joint significantly increases the risk of medial and lateral knee OA, respectively [19, 29,30,31,32]. Because the knee joint cannot function independently of the rest of the lower extremity kinematic chain, the hip and ankle’s biomechanical mechanism also significantly affects the knee joint load. Chang et al. [33] showed that, during weight-bearing activity (such as walking), the abnormal gait caused by the limitation of hip abduction could lead to the center of gravity shift and forces acting on the medial compartment cartilage of the stance limb raise, thus increasing the risk of OA. Coskun Benlidayi et al. [34] conducted a study that identified that coxa valga is associated with knee OA severity. The author also set a cut-off value of NSA to predict that people with NSA more than 134.4° have an 8-fold increased risk of severe knee OA [34]. Therefore, coxa valga is a significant risk factor of OA that cannot be ignored.
Additionally, Huang et al. [3] demonstrated that GMC could cause patellofemoral instability (mainly due to iliotibial band contracture, the contracture band often involved in GMC) and knee pain, and arthroscopic release of contracture fibrosis band can significantly alleviate the symptom. It is possible to hypothesize that GMC may result in OA and knee pain by causing coxa valga and patellofemoral instability. Therefore, more attention should be paid to GMC as a significant risk factor of OA and knee pain because it causes coxa valga and induces patellofemoral disease.
A limitation of this study is that patients’ postoperative AP pelvic radiographs were not obtained due to patient wishes and the patients’ own considerations about time and cost. However, Huang et al. [3] have found that surgical release of GMC can significantly reduce the tilt and lateral shift of the patella by comparing preoperative and postoperative knee CT images. Additionally, multiple studies [3, 5, 9, 12, 35, 36], from the functional and symptomatic perspective, have shown that early surgery effectively reduces patient symptoms and improves the quality of life, with high patient follow-up satisfaction. It is reasonable to assume that early diagnosis and surgery are beneficial to patients, while it also needs to be validated by further studies based on comparison of preoperative and postoperative radiographic parameters of AP pelvic radiograph. Another limitation of the current study is that the GMC patients admitted to this department were almost all injection-induced, failing to consider other risk factors, including cerebral palsy. Cerebral palsy as a significant contributor to musculoskeletal disorders requires future work to explore its association with GMC and coxa valga.
