Are Abnormal Muscle Biomechanics and Patient-reported Outcomes Associated in Patients With Hip Dysplasia?

BACKGROUND: Developmental dysplasia of the hip (DDH) is a major risk factor for the early development of hip osteoarthritis. Recent studies have demonstrated how DDH alters hip muscle moment arms and elevates muscle-induced biomechanical variables such as joint reaction forces and acetabular edge loads. Understanding the link between abnormal biomechanics and patient-reported outcome measures (PROMs) is important for evidence-based clinical interventions that improve patient symptoms and functional outcomes. To our knowledge, there are no reports of the relationships between muscle-induced biomechanics and PROMs. QUESTIONS/PURPOSES: (1) Are there associations between PROMs and muscle-induced hip biomechanics during gait for patients with DDH and controls? (2) Are there associations among PROMs and separately among biomechanical variables? METHODS: Participants in this prospective cross-sectional comparative study included 20 female patients with DDH who had no prior surgery or osteoarthritis and 15 female individuals with no evidence of hip pathology (controls) (age: median 23 years [range 16 to 39 years]; BMI: median 22 kg/m 2 [range 17 to 27 kg/m 2 ]). Muscle-induced biomechanical variables for this cohort were reported and had been calculated from patient-specific musculoskeletal models, motion data, and MRI. Biomechanical variables included joint reaction forces, acetabular edge loads, hip center lateralization, and gluteus medius muscle moment arm lengths. PROMs included the Hip Disability and Osteoarthritis Outcome Score (HOOS), the WOMAC, International Hip Outcome Tool-12, National Institutes of Health Patient-Reported Outcome Measure Information System (PROMIS) Pain Interference and Physical Function subscales, and University of California Los Angeles activity scale. Associations between PROMs and biomechanical variables were tested using Spearman rank-order correlations and corrected for multiple comparisons using the Benjamini-Yekutieli method. For this study, associations between variables were considered to exist when correlations were statistically significant (p < 0.05) and were either strong (ρ ≥ 0.60) or moderate (ρ = 0.40 to 0.59). RESULTS: Acetabular edge load impulses (the cumulative acetabular edge load across the gait cycle), medially directed joint reaction forces, and hip center lateralization most commonly demonstrated moderate or strong associations with PROMs. The strongest associations were a negative correlation between acetabular edge load impulse on the superior acetabulum and the HOOS function in daily living subscale (ρ = -0.63; p = 0.001), followed by a negative correlation between hip center lateralization and the HOOS pain subscale (ρ = -0.6; p = 0.003), and a positive correlation between hip center lateralization and the PROMIS pain subscale (ρ = 0.62; p = 0.002). The University of California Los Angeles activity scale was the only PROM that did not demonstrate associations with any biomechanical variable. All PROMs, aside from the University of California Los Angeles activity scale, were associated with one another. Although most of the biomechanical variables were associated with one another, these relationships were not as consistent as those among PROMs. CONCLUSION: The associations with PROMs detected in the current study suggest that muscle-induced biomechanics may have wide-reaching effects not only on loads within the hip, but also on patients' perceptions of their health and function. As the treatment of DDH evolves, patient-specific joint preservation strategies may benefit from targeting the underlying causes of biomechanical outcomes associated with PROMs. LEVEL OF EVIDENCE: Level III, prognostic study.