Defining the relationship between the hip, pelvis, and lumbar spine.

Introduction: While the hip, pelvis, and lumbar spine are known to move synchronously, their exact relationship has yet to be determined. The aim of this study was to define the biomechanical relationship between the hip, pelvis, and lumbar spine. Methods: A cohort of fifty healthy subjects between 18- and 35-years-old were recruited to participate, including 25 males and 25 females. Exclusion criteria included presence of back or hip pain, inability to stand upright or sit with hips flexed to 90°, history of ankylosing spondylitis, femoroacetabular impingement, or prior hip or spine surgery. Three lateral radiographs were obtained for each patient, including standing, relaxed seated, and the flexed-forward seated views. Radiographic variables measured included sacral slope (SS), pelvic-femoral angle (PFA), and lumbar lordosis (LL). Calculations included the change in SS (ΔSS), the change in PFA (ΔPFA), the change in LL (ΔLL). Pearson correlations were used to determine the relationship between ΔSS, ΔPFA, and ΔLL with significance set to p<.05. Results: The average age of participants was 25.70±2.34 years with a body mass index (BMI) of 24.10±3.02 kg/m Conclusions: Motion between the hip, pelvis, and lumbar spine is highly correlated. For every degree increase of hip flexion, the pelvis tilts posteriorly 0.71, causing the lumbar spine to flex 1.58 degrees to maintain upright posture. For every degree increase of hip flexion, the lumbar spine decreases flexion 0.86 degrees. Given the increased interest and clinical applicability of spinopelvic biomechanics in the spine community, these definitions are essential to moving forward with research in this topic.