Abstract
Objectives:
Traditionally, the hip has been thought of as a particularly stable ball and socket joint with a fixed center of rotation and no translation. This view has been the basis for several simulation platforms studying hip impingement and instability for treatment planning and outcome analyses. More recently, in vivo imaging and cadaveric studies have suggested that, in addition to rotation, the femoral head and center of rotation (COR) translate within the joint throughout physiologic range of motion. A recent MRI study has shown an average translation of 2 mm, with up to 7 mm translations in certain positions, among subjects with asymptomatic hips. Importantly, substantial magnitudes of translation can alter hip contact and impingement patterns with downstream effects on the treatment planning and patient outcomes. However, there is a paucity of literature regarding how hip translation may influence hip impingement. Therefore, the purpose of this study was to use a validated simulation platform, capable of simulating hip translation during impingement analysis, to map the trajectory of the hip COR and its effects on impingement-free hip range of motion (ROM).
Methods:
Following institutional review board approval, the New Mexico Decedent Image Database (NMDID) was queried to identify computed tomography scans with full coverage of the hip and femur from subjects with no documented bone and joint pathology (n = 1222 hips, 611 patients, age: 30.4 ± 8.8; 67% male patients). A custom-developed and validated program (VirtualHip, Boston Children’s Hospital) was used to automatically segment the bones, identify landmarks (Dice 0.94 – 0.99), and define an anatomical coordinate system based on International Society of Biomechanics (ISB) recommendations. The pelvis was then fixed in 3D and the femur was moved to simulate uniplanar rotations (ie, flexion: 0° to 150°, extension: 0° to 50°, abduction/adduction: 0° to 75°, internal /external rotations: 0° to 75°). The simulations were first done assuming a fixed hip COR (no translation) and then repeated by incrementally allowing the hip COR to translate by up to 5 mm with increments of 1 mm, avoiding bone-to-bone penetration or hip dislocation (distance between the femoral head and acetabulum exceeding the joint space width measured at neutral). Mixed linear models were used to evaluate the relationships between COR translation and impingement-free rotation (maximum rotation prior to impingement). The effect of translation on ROM was characterized by calculating the proportion of hips able to achieve each ROM. Finally, the path of the hip COR was mapped throughout the tested ranges of motion.
Results:
The effect of COR translation on achieved impingement-free range of motion along with COR trajectories are shown in Figure 1. Hip flexion was associated with COR posterior (by 4.0 ± 0.9 mm), lateral (only after 100°, by 3.6 ± 1.4 mm), and superior (only after 100°, by 2.0 ± 1.8 mm) translations (P < 0.001). Hip extension was only associated with anterior COR translation (by 0.8 ± 0.7 mm, P < 0.001). Hip adduction was associated with anterior (by 2.9 ± 1.3 mm), lateral (by 4.0 ± 1.3 mm), and superior (by 4.4 ± 0.6 mm) translations (P<0.01). Hip abduction was only associated with medial (by 2.6 ± 1.5 mm) and inferior (by 2.9 ± 1.7 mm) translations (P < 0.02). There were minimal COR translations during hip internal rotation (P > 0.05). Hip external rotation was associated with anterior (by 3.7 ± 1.4 mm) and inferior (by 1.2 ± 1.3 mm) translations (P < 0.05).
Conclusions:
This is the first study to map the hip COR trajectory under common movements in a large cohort of patients. The current findings highlight the importance of hip translation (nonfixed COR) on physiologic hip range of motion. As the hip is not a perfect ball and socket joint, with an aspherical femoral head, translation is required to accommodate physiologic rotation (ie, glide mechanism) without bony impingement between the femur and pelvis. These preliminary observations highlight the need for a comprehensive assessment of normal and pathologic hip translation along with their role in biomechanics, injury risk, and response to treatment. A clear understanding of the role of hip translation in hip function and health will improve clinical care through better diagnosis and personalized treatment planning, which can ultimately lead to improved treatment outcomes, particularly lower risk of osteoarthritis.
