Histopathologic Analysis of the Morpho-Functional Zones of the Human Acetabular Labrum

The structural and functional adaptation of soft tissues to mechanical load controls their ability to withstand injury and influences their capacity for healing. Similar to the knee meniscus, the acetabular labrum exhibits zonal differences in mechanical load distribution, resulting in distinct regions with unique structural and functional properties. However, little is known about the effect of these zonal adaptations on the severity and distribution of labral degenerative changes. This study aims to assess the impact of labral zonal adaptations on the severity and distribution of histopathologic features. Human tissue was obtained from 9 embalmed cadavers, comprising a total of 16 hemipelves (10 males and 6 females) with an average age of 80 years (age range 66-99). Each hip was divided into 8 distinct regions, resulting in 128 regional segments. Slides were stained using Hematoxylin and Eosin (H&E) and Safranin-O (Saf O), with the incorporation of fluorescent scanning of eosin (F-Eosin). Labral histopathologic features were assessed using established modified grading criteria for the knee meniscus. These features were evaluated both globally across the anatomical quadrants of the hip joint and zonally across the inner and outer zones. The global analysis of the labrum revealed a similar distribution of histopathologic features across the superior, anterior, inferior, and posterior quadrants of the hip joint. Conversely, across 128 labral segments, pairwise zonal assessments revealed a significant increase (p < 0.05) in the severity of degenerative features, which were predominantly concentrated in the inner labral zone near the articular surface. These degenerative changes encompassed alterations in matrix proteoglycan content, cellularity, collagen organization, and labral articular surface, including the lamellar layer. The increased compactness of labral fibers in the inner zone, minimal vascular penetration, and significant degenerative changes imply that it is a vulnerable area for injury with a potentially limited capacity for healing. The delineation of these distinct zonal frameworks highlights the labrum's functional adaptation to its mechanical environment. The zonal analysis of the labrum provided a considerably more detailed perspective on the distribution dynamics of histopathologic features compared to previous global analyses, offering a more precise understanding of the anatomical factors that may explain zone-specific vulnerability to injury and degeneration.