Inclusion of the Acetabular Labrum Reduces Simulated Range of Motion of the Hip Compared With Bone Contact Models

Abstract

Purpose: To determine whether inclusion of the acetabular labrum affects the maximum range of motion (ROM) during simulation of the flexion-adduction-internal rotation impingement examination.

Methods: Three-dimensional surface reconstructions of the femur, hemi-pelvis, and labrum from computed tomography arthrography images of 19 participants were used to simulate maximum ROM during the flexion-adduction-internal rotation examination. Simulations were conducted for positions between 70° and 110° flexion and 0° and 20° adduction at 10° increments to measure maximum internal rotation and the position of contact between the femur and acetabular rim (bone-to-bone) or the femur and labrum (bone-to-labrum). Internal rotation angles and clock-face position values were compared between the 2 contact scenarios for each position.

Results: The ROM in the bone-to-labrum contact model was significantly less than that of the bone-to-bone contact model for all evaluated positions (P ≤ .001, except at 110° flexion and 20° adduction, P = .114). The inclusion of the labrum reduced internal rotation by a median [interquartile range] of 18 [15, 25]° while altering the position of contact on the acetabular clock-face by -0:01 [-0:27, 0:16]. The variability in contact location for the bone-to-labrum contact scenario was nearly double that of the bone-to-bone contact scenario, as indicated by the interquartile range.

Conclusions: Inclusion of the anatomy of the acetabular labrum in collision models used to simulate impingement examinations reduced the internal rotation ROM by approximately 20° and increased variability in the location of contact relative to the acetabular rim.

Clinical relevance: While standard bone-to-bone contact ROM simulations may be informative with respect to the relative change in ROM based on a surgical intervention (e.g., pre- and post-osteochondroplasty for cam-type femoroacetabular impingement), they may not accurately represent the clinical ROM of the joint or the kinematic position at which damage may occur due to shape mismatch between the femur and acetabulum.

Fig 1

CT arthrogram of the left hip of a representative participant. (A) Radio-opaque contrast agent was injected into one hip of each participant to visualize the cartilage and labrum. (B) Segmentation of the femur (gray), pelvis (tan), and acetabular cartilage (dotted magenta) and labrum (teal) on a single coronal slice from CT. The acetabular cartilage was used to define the center of the acetabulum, but only the surfaces of the femur, pelvis, and labrum were used in the simulations. (CT, computed tomography.)

Fig 2

A representative pelvis (tan), left femur (semi-transparent gray), and labrum (semi-transparent teal) in neutral orientation displaying the landmarks and reference coordinate systems from an (A) inferior, (B) anterior, and (C) left view. Coordinate system axis definitions are shown with dotted black lines. The pelvic reference was defined from both ASIS and the midpoint of the pubic tubercle landmarks (red). The femoral reference was defined from the femoral head center, the knee center, and the posterior aspect of the femoral condyles landmarks (red). (ASIS, anterior superior iliac spine.)

Fig 3

Anterior view of left hemipelvis and proximal femur models for a representative participant during simulated 90° flexion, 20° adduction, and internal rotation to contact. Semitransparent femurs represent the initial position, whereas the opaque femur represents the transformed position. (A) From a neutral orientation the femur is flexed to 90°, (B) adducted to 20°, (C) and internally rotated until the femur contacts either the bony surface of the acetabulum or soft-tissue surface of the labrum. The region of contact between the femur and the (D) acetabulum and (E) labrum is shown with an asterisk (∗).

Fig 4

Maximum internal rotation angle for range of motion simulations for the femur-acetabulum and femur-labrum scenarios. Each plot represents a specific angle of adduction and each of the associated flexion angles. Boxplot represents the median and interquartile range, whereas the vertical lines indicate 1.5 times the interquartile range. Semitransparent violin plot displays the distribution of the internal rotation values for each simulation.

Fig 5

Maximum internal rotation angle for range of motion simulations for the femur-acetabulum and femur-labrum scenarios. Each plot represents a specific angle of adduction and each of the associated flexion angles. Boxplot represents the median and interquartile range, whereas the vertical lines indicate 1.5 times the interquartile range.