Abstract
Background
The acetabular labrum is an essential stabilizer of the hip joint, imparting its greatest effect in extreme joint positions where the femoral head is disposed to subluxation and dislocation. However, its stabilizing value has proved difficult to quantify. The objective of the present study was to assess the contribution of the entire acetabular labrum to mechanical joint stability. We introduce a novel “dislocation potential test” that utilizes a dynamic, cadaveric, robotic model that functions in real-time under load-control parameters to map the joint space for low-displacement determination of stability, and quantify using the “stability index”.
Methods
Five fresh-frozen human cadaveric hips without labral tears were mounted to a six-degree-of-freedom robotic manipulator and studied in 2 distinct joint positions provocative for either anterior or posterior dislocation. Dislocation potential tests were run in 15° intervals, or sweep planes, about the face of the acetabulum. For each interval, a 100 N force vector was applied medially and swept laterally until dislocation occurred. Three-dimensional kinematic data from conditions with and without labrum were quantified using the stability index, which is the percentage of all directions a constant force can be applied within a given sweep plane while maintaining a stable joint.
Results
Global stability indices, considering all sweep planes, were significantly greater with labrum intact than after total labrectomy for both anterior (Figure 1A) (p = 0.02) and posterior (Figure 1B) (p<0.001) provocative positions. Regional stability indices, based upon the expected range of dislocation for each provocative position, were also significantly greater and of slightly larger magnitude for the intact condition than after total labrectomy (p<0.001).
Conclusions
This is the first known application of a six-degree-of-freedom robot to recreate mechanical hip impingement and dislocation to elucidate the role of the labrum in hip stability. Our results suggest that at least in extreme positions, the labrum imparts significant overall mechanical resistance to hip dislocation compared to the condition without the labrum. Regional contributions of the labrum are greatest in the direction of dislocation as foretold by joint position as indicated by region-based stability indices. Future studies involving more clinically relevant injury patterns with greater soft tissue preservation in a younger cadaveric population would better reflect the in vivo effects of labral injury so that treatment strategies can be developed accordingly.
