FEMORAL HEAD TRANSLATION IS INCREASED IN PAINFUL HIPS.

INTRODUCTION: Hip pain and osteoarthritis are thought to be largely related to biomechanical disruptions including abnormal stress distributions and altered kinematics. Many of these disruptions are associated with anatomical deformities such as those associated with femoroacetabular impingement (FAI). The normal femoral head has long been assumed to be constrained by the ball-and-socket configuration of the hip joint and therefore not translate during hip rotation. Recent work has measured femoral head translations for a range of postures and explored their relationships to hip morphology and symptoms. These studies have been limited by the postures that can be imaged in conventional MR and CT scanners. We have recently developed and validated an approach using upright open MRI to measure femoral head translation for postures representing a large range of hip motions. OBJECTIVE: To determine whether femoral head translation is different between hips with and without pain, and between hips with different FAI deformities, for four postures. METHODS: This study used 79 participants (25 males, 54 females, average age 45.6±6.5) enrolled in the Investigations of Mobility, Physical Activity, and Knowledge Translation in Hip Pain project, a population-based study of participants aged 20-49 with and without hip pain. Cases with hip pain were defined as self-reported pain in the groin or upper thigh in the past 12 months that lasted longer than 6 weeks or occurred on three or more occasions. Each participant was assessed for FAI morphology. Pincer morphology was defined as a lateral center edge (LCE) angle > 40° or a positive crossover sign on an anteroposterior view weight-bearing radiograph. Cam morphology was assessed as alpha angle > 55° in a T1-weighted hip scan on an oblique axial plane using a 3T MRI scanner (Philips Healthcare, Best, The Netherlands). Mixed deformity was defined as positive results for both cam and pincer assessments. Hip translation was measured in 4 postures using an Upright Open MRI scanner (0.5T MROpen, ASG Italy). We scanned the participants in supine FADIR, sitting FADIR, sitting neutral, and sitting cross legged postures (Figure 1), as well as in supine as a reference posture. We used a T1-weighted GFE sequence in the sagittal plane of the hip (Table 1). We scanned the pelvis in the axial plane with the participant in the supine position to establish pelvic coordinates. Femoral head positions were defined as the location of the femoral head center relative to the acetabular cup center in pelvis coordinates. The relative femoral head translation for each tested posture was defined as the relative movement of the femoral head from the supine posture to the tested posture. We tested the hypotheses that hip translation is affected by pain and FAI deformity type using two-way ANOVAs. RESULTS: We found increased anterior femoral head translation in hips with pain for the sitting FADIR (p=0.002), sitting neutral (p=0.038) and sitting cross-legged (p=0.035) positions, increased lateral translation in hips with pain for the sitting cross legged position (p=0.017) and increased superior translation in hips with pain for the sitting FADIR position (p=0.045). For the number of specimens tested we found no differences in hip translation in other directions for hips with pain. We found no differences in any component of femoral head translation between the four FAI deformity types (cam, pincer, mixed, no deformity) for any posture. CONCLUSION: Femoral head translation is increased in painful hips in some postures. This may be due to increased hip instability, but does not appear to be directly related to FAI deformity.