Characterisation of the Sealing Function of the Hip Labrum in Functional Activities (2013)

Introduction: The sealing function of the acetabular labrum is central to the stability of the hip and the health of the joint. Disruption of the labrum has been shown to reduce intra-articular pressure and increase the rate of cartilage consolidation during static loading. Functional activities require movement of the hip through wide ranges of joint motion which disrupt joint congruency, and thus may alter the seal. This study was performed to test the hypothesis that the sealing function of the labrum varies with the position of the hip during functional activities. Methods: Six fresh cadaveric hip joint specimens were obtained from donors of average age 45.5 ± 16.1 years (range 25–63 years). Each specimen was dissected free of soft tissue, leaving the capsule and labrum intact, potted in mounting fixtures, and placed in a loading apparatus. Catheters were inserted into the central and peripheral compartments of each hip to allow infusion of fluid and monitoring of compartment pressures via miniature transducers (OMEGA Engineering, Inc). After application of a joint load of 0.50 BW, fluid was introduced into the central compartment at a constant rate until transport was indicated by a rise in pressure within the peripheral compartment. These measurements were performed with each hip placed in 10 functional positions ranging from −5 to 105 degrees of flexion, −5 to 13 degrees of abduction, and −25 to 35 degrees of external rotation simulating the sequential stages of gait, stooping, and pivoting. Motion analysis was performed via reflective marker arrays attached to the femur and pelvis to allow computer visualization of the position of the pelvis and femur using CT reconstructions. In each hip position, we measured the peak pressure (kPA) developed within the central compartment prior to fluid transfer to the peripheral compartment. Results: The sealing capacity of the labrum varied wit hip joint position (p = 0.003). Peak pressures were greatest during pivoting (average peak central compartment pressure: 33.9 ± 5.0 kPa; p = 0.001), compared to both gait (22.5 ± 4.1 kPa; p = 0.047) and stoop (9.6 ± 3.6 kPa; p = 0.004). During the pivoting maneuver, peak pressures increased in a linear fashion with increasing hip extension and external rotation. Conversely, the sealing capacity of the labrum was reduced during stooping once hip flexion exceeded 70° and internal rotation was greater than 15°. The positions simulating gait consisted of minimal to no joint rotation, and peak pressures varied little between the four positions. Discussion: The sealing ability of the labral seal increases with external rotation of the hip joint, and decreases with increased flexion combined with internal rotation. Changes in joint congruency that occur when the hip is placed in high flexion and internal rotation may be responsible for the loss of sealing function of the labrum during stooping. Our results reveal the importance of joint position during functional activities on the ability of the labrum to seal the central compartment of the hip. Future research will further assess the relationship between these positions and joint injury.