The labrum acts as a stabilizer of the hip joint. But in the dys-plastic hip, it becomes part of the weightbearing surface of the acetabulum. It is postulated that this causes overload of the superior and/or anterior labrum in tension and shear, leading to labral injury.19
Some studies do not support the theory that the labrum plays a crucial role in hip stability, although it has been demonstrated that the presence of the labrum per se has no significant influence on the weightbearing function of the normal joint.20 A possible role of this structure is to seal the joint, thereby providing stability by allowing atmospheric pressure to aid in keeping the joint reduced. The contribution of the labrum to joint stability is expected to be greatest at the extremes of motion, where impingement can cause traumatic dislocation.
Several studies have explored the use of MR arthrography to identify labral pathology. Hodler, et al in 1995 showed that MR arthrography is superior to plain MR in identifying labral surface lesions, but neither method could identify labral degeneration on a consistent basis.24 Likewise, Czerny, et al in 1996 also showed MR arthrography, with 90% sensitivity and 91% accuracy, to be superior to plain MR in detecting labral lesions.25 The work of Petersilge, et al in 1996 also supports the use of MR arthrography in detecting labral lesions.26 Leunig, et al in 1997 found MRA to be 63% sensitive in detecting 16 tears and 92% sensitive for 12 degenerative labra.27 McCarthy and Marchetti demonstrated a 74% sensitivity, an 83% specificity, and a 78% accuracy in diagnosing anterior labral pathology. However, the sensitivity for detecting posterior lesions (20%) and lateral tears (11%) was much poorer. Their study also emphasized the importance of off-axis tangential images to properly visualize the anterior hip quadrant.23
We performed the following anatomic study to determine the prevalence, location, and morphology of acetabular labral lesions in adult hips.
Fifty-four acetabulae were harvested from adult cadavers ranging in age from 48 to 102 years (average age was 78). A generous incision was carefully made in each hip capsule, so the femoral head could be dislocated in an atraumatic manner, to avoid creating labral lesions postmortem. Each specimen was examined by gross observation and stereomi-croscopy. All labral lesions were described in terms of both location and morphology, and classified as one of five types:
1. Complete labral separation from the acetabular rim. (Figure 12.1.) This sometimes was observed in combination with other lesions described below. The separation always occurred at the junction of the labrum and the articular cartilage, such that a probe could be passed from the articular surface through the separation to the capsule.
2. Fraying of the free edge of the labrum. In these specimens, the labrum remained firmly attached to the acetabular rim, but the free edge consisted of frayed and fibrillated tissue with little or no structural support.
3. A circumferential depression at the labral-cartilage junction was frequently observed. In this lesion the labrum remains attached to the acetabular rim and is similar in appearance and location to a complete separation of the labrum at the labral-cartilage junction. Based on the appearance and prevalence of these two lesions, we believe that the appearance of a discontinuity in the acetabular surface at the labral-cartilage junction is a precursor to complete labral separation.
4. Flat, degenerative labral lesions. (Figure 12.2.) These lesions are essentially thin extensions of the labral edge along
the inner aspect of the hip capsule. These lesions may extend as much as 2 to 3 times the height of the normal labrum. The labral-capsular sulcus between the abnormal labrum and the hip capsule remains intact. The labral tissue is always tough and scarlike in appearance; it clearly differs from the smooth, resilient consistency of the normal labrum.
5. Fraying at the labral-cartilage junction. The appearance is similar to the fraying of the labral edge that we more frequently observed, but involves only the junctional area. This lesion may be an intermediate lesion between partial separation (groove) and complete separation of the labrum from the acetabular rim.
The acetabulum is divided into eight equal regions to describe the location of various lesions: anteroinferior, anterior, an-terosuperior, superior, posterosuperior, posterior, posteroin-ferior, and medial. The inferior region is occupied by the transverse ligament. The normal weightbearing regions of the acetabulum are the anterosuperior and superior regions.
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