Developmental dysplasia of the hip (DDH) is strongly associated with an increased risk for hip osteoarthritis. Skeletal deformities undeniably contribute to detrimental biomechanical loading in dysplastic hips, but cannot explain all types of damage and symptoms that patients with DDH experience. Characterizing the geometry and function of the muscles spanning the hip is a logical next step in our progression of knowledge about DDH pathomechanics. In this study, we compared skeletal geometry, muscle volumes, intramuscular fatty infiltration, moment arms, and isometric strength in patients with DDH (N = 20) to healthy controls (N = 15). Femoral coverage was significantly less in patients (p < 0.001, Cohen's d effect size = 2.2), femoral neck-shaft angles were larger (p = 0.001, d = 1.3), and hip joint centers (HJCs) were more lateral (p = 0.001, d = 1.3). These skeletal abnormalities were associated with smaller abductor muscle moment arms in patients with DDH (e.g., gluteus medius [GMED]: p = 0.001, d = 1.2). Patients with DDH also had larger GMED volumes (p = 0.02, d = 0.83), but no differences in fatty infiltration, compared to controls. Isometric strength of the hip abductors, extensors, and flexors was lower in patients, but not significantly different from controls. The abnormal skeletal geometry, lateralized HJC, and reduced muscle moment arms represent a chronic biomechanical disadvantage under which patients with DDH operate. This phenomenon causes increased demand on the abductor muscles and results in high medially and superiorly directed joint reaction forces, which can explain reports of superomedial femoral cartilage damage in patients. The abnormal muscle geometry and function, in context with abnormal skeletal structure, are likely strong, but underappreciated, contributors to damaging loads in DDH.
Femoral version (FV) deformities are common in patients with developmental dysplasia of the hip (DDH) and may contribute to cartilage damage due to abnormal joint loading. Derotational femoral osteotomy (DFO) surgery corrects FV deformities. However there is little consensus about the femoral transection location for DFO, and its influence on joint loads is unknown. The purpose of this study was to compare the effects of two common DFO locations on muscle forces and hip joint reaction forces (JRFs) in patients with DDH. DFO was simulated in nine patients with DDH and abnormal FV using patient‐specific musculoskeletal models. Femoral transection for DFO was separately simulated proximal and distal to the lesser trochanter and FV values were corrected to an idealized 15°. JRFs during early and late stance of gait were compared between the two simulated transection locations. Most changes to JRFs were similar between proximal and distal DFO, however, statistically significant differences were found for the medial JRF component during late stance among patients with femoral anteversion (p = 0.01). Force changes from five hip muscles were significantly different between DFO locations, however, changes were minimal. Most changes after DFO in patients with femoral retroversion were opposite of those with femoral anteversion, with anteroposterior and superior JRFs increasing after retroversion correction. After DFO correction, superior and medial JRFs in DDH patients remained elevated compared to controls. Understanding the influence of DFO location on muscle‐generated hip forces can help surgeons justify decisions and potentially standardize surgical correction of FV deformities in patients with DDH.
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