The acetabular labrum seal: a poroelastic finite element model

Abstract

Objective. The aim of the study is to investigate the labrum’s ability to seal a pressurised layer of synovial fluid within the joint, and to study the influence of this sealing mechanism on cartilage deformation, interstitial fluid pressure and collagen solid matrix stresses.

Background. Cartilage degeneration has been observed in conjunction with labrum pathology. However, little is known about the function of the labrum. Experimental observations have been reported which are consistent with a sealing function of the labrum.

Methods. The model was an axisymmetric geometric approximation of the acetabular and femoral cartilage layers and the surrounding labrum. A poroelastic formulation was used to account for the solid and fluid components of these hydrated tissues. A sensitivity analysis of the labrum material properties was carried out.

Results. With a compressive load of 1200 N applied across the joint model, the labrum could seal a layer of pressurised fluid between the femur and acetabulum, thus preventing contact of the articulating surfaces. With this sealing effect, loads were transferred across the joint predominantly by uniform pressurisation of the interstitial fluid of the cartilage layers. In the absence of this sealing, strains within the solid matrix of the cartilage layers were higher (e.g. 20% vs. 3%).

Conclusions. The labrum can seal against fluid expression from the joint space. This sealing function protects the cartilage layers of the hip.

Relevance

Current treatments for labrum damage and early arthrosis may compromise the sealing function of the labrum. With continued study of the function and importance of the labrum, new surgical repair strategies can be developed to maintain the overall function of the hip joint.

Introduction

The acetabular labrum is a fibrocartilagenous rim attached to the osseous margin of the acetabulum with a tissue structure similar to the knee menisci. It deepens the acetabular socket and extends the coverage of the femoral head. Inferiorly, it joins smoothly with the transverse acetabular ligament to bridge the acetabular notch, forming a complete circle. It is triangular in cross-section, with its base attached to the acetabulum and its apex forming the free edge of the labrum, which is turned in against the femoral head.

Little is known about the acetabular labrum, its significance in normal joint function, and the possible consequences of labrum pathology. The labrum is composed of hydrated tissue, with collagen fibre bundles oriented predominantly in the circumferential direction, aligned with the acetabular rim [1]. Like the morphologically similar meniscus, it is plausible that the labrum is uniquely adapted to its function in the hip joint, and any damage to the labrum may contribute to joint degeneration. Several clinical studies have shown that labral injuries, such as tears or intra-articular impingement, lead to changes consistent with osteoarthrosis [2], [3], [4], [5], [6].

Observations have been reported which are consistent with a sealing function of the labrum. Simple experiments with cadavers were conducted to study the role of the labrum in stabilising the human hip joint [7]. In a more comprehensive study, Takechi et al. [8] measured the intra-articular pressure in the hip, using a needle pressure transducer, both inside and outside the labrum. Both groups concluded that the labrum contributed to the stability of the hip joint through a partial seal of the joint, creating a negative intra-articular pressure upon joint distraction, and by the structural resistance to dislocation provided by the labrum tissue itself. Terayama et al. [9] demonstrated that a 0.2–0.6 mm thick fluid film remained sealed between the articulating surfaces of fresh cadaveric hip joints after applying a 1000–1500 N compressive load across the joint. In these joints, the labrum remained in tight contact with the opposing femoral cartilage surface (Fig. 1). A similar sealing and stabilising phenomenon has been demonstrated for the glenohumeral joint [10], [11], [12]. Besides improving the stability of the joint, this possible sealing function of the labrum could enhance a fluid film lubrication mechanism in the hip joint and prevent direct solid-on-solid cartilage contact.

By sealing against fluid expression from the cartilage layers, loads applied to the joint are carried by fluid pressure within the cartilage, shielding the collagenous solid matrix of the cartilage from high stresses [13], [14], [15]. Failure of this seal would lead to higher loading in the solid matrix of the cartilage surfaces and increased friction, contributing to the degenerative changes of osteoarthrosis. The goal of this study is to investigate the ability of the acetabular labrum to seal the hip joint, using a finite element computer model, and also to study the influence of this sealing mechanism on cartilage deformation, fluid pressures and solid stresses.