Association Between Hip Translation and Hip Rotation and Anatomy: A Pilot Quasi-static MRI Study.

Background: There is little known about translation of the hip and the relationship with hip rotation and morphology in asymptomatic patients.

Hypotheses: (1) Femoral head would exhibit significant translations in asymptomatic hips, (2) femoral head translations would correlate to femoral rotations, and (3) range of femoral head translations would correlate to hip morphology.

Study Design: Cross-sectional study; Level of evidence, 3.

Methods: A total of 11 individuals (age, 23-47 years; 64% female) with asymptomatic hips underwent hip magnetic resonance imaging (MRI) in the following postures: neutral (supine), midflexion, maximum-flexion, internal rotation, internal rotation + midflexion, internal rotation + maximum-flexion, adduction, flexion-abduction-external rotation (FABER), extension, and lateral abduction. All rotations were passive. MRI-generated 3-dimensional hip models were used to quantify femoral rotations and translations. Femoral head diameter, acetabular diameter, lateral center-edge angle, alpha angle, femoral anteversion, acetabular version and inclination, and neck-shaft angle were measured from MRI. A test was used if measured translations were statistically significant. Linear regression was used to assess the associations between translation and rotation. Pearson correlation was used to assess the relationships between hip anatomy and range of femoral head translations.

Results: In all tested positions, the femoral head translated anteriorly by 2 ± 1 mm (maximum 5 mm,  < .001), posteriorly by 1 ± 1 mm (maximum 6 mm,  < .001), superiorly by 2 ± 2 mm (maximum 7 mm,  < .001), inferiorly by 2 ± 2 mm (maximum 6 mm,  < .001), laterally by 1 ± 1 mm (maximum 4 mm,  < .001), and medially by 2 ± 1 mm (maximum 5 mm,  < .001), relative to the rested supine position. Femoral flexion was associated with posterior translation of the femoral head ( = .038). Femoral abduction was associated with medial translation of the femoral head ( = .042). Higher femoral anteversion and smaller alpha angle were associated with a higher total magnitude of femoral head translation in the anterior-posterior direction ( < .04). Smaller femoral anteversion, higher acetabular inclination, smaller lateral center-edge angle, and lower neck-shaft angle were associated with a higher total magnitude of femoral head translation in the superior-inferior direction ( .03).

Conclusion: Our study demonstrated that, during passive physiologic movement, asymptomatic hips on average translated up to 2 mm (with up to 7 mm maximum translation in some positions), which is potentially related to hip rotations and morphology. Further investigations are warranted to understand the normal and pathologic hip translations and their impact on hip function (ie, instability and impingement).