Articular cartilage friction increases in hip joints after the removal of acetabular labrum

Abstract

The acetabular labrum is believed to have a sealing function. However, a torn labrum may not effectively prevent joint fluid from escaping a compressed joint, resulting in impaired lubrication. We aimed to understand the role of the acetabular labrum in maintaining a low friction environment in the hip joint. We did this by measuring the resistance to rotation (RTR) of the hip, which reflects the friction of the articular cartilage surface, following focal and complete labrectomy. Five cadaveric hips without evidence of osteoarthritis and impingement were tested. We measured resistance to rotation of the hip joint during 0.5, 1, 2, and 3 times body weight (BW) cyclic loading in the intact hip, and after focal and complete labrectomy. Resistance to rotation, which reflects articular cartilage friction in an intact hip was significantly increased following focal labrectomy at 1–3 BW loading, and following complete labrectomy at all load levels. The acetabular labrum appears to maintain a low friction environment, possibly by sealing the joint from fluid exudation. Even focal labrectomy may result in increased joint friction, a condition that may be detrimental to articular cartilage and lead to osteoarthritis.

Introduction

Tears of the acetabular labrum are common in athletes and the elderly (Bharam, 2006, Binningsley, 2003, Feeley et al., 2008, Haene et al., 2007, McCarthy et al., 2003a, McCarthy et al., 2003b, Narvani et al., 2003, Seldes et al., 2001). It has been proposed that damage to the acetabular labrum can predispose to the development of osteoarthritis in the hip joint (Dorrell and Catterall, 1986, Ferguson et al., 2000, Ferguson et al., 2003, McCarthy et al., 2001, McCarthy et al., 2003a, McCarthy et al., 2003b). Abnormal biomechanics in the hip joint following labral tears has been postulated to promote cartilage degeneration, but the function of the labrum and its possible role in protecting articular cartilage is not well understood.

A damaged acetabular labrum could possibly lead to hip instability, which may alter the normal loading environment (Crawford et al., 2007, Ito et al., 2009, Safran et al., 2009). Recently it has been found that articular cartilage consolidation in a hip joint with torn labrum is greater than the cartilage consolidation in a hip with intact labrum (Hlavacek, 2002, Ferguson et al., 2000, Ferguson et al., 2003). Because compressive loads are predominantly borne by the fluid phase of the cartilage matrix, it has been hypothesized that the labrum has a sealing function in the hip joint, preventing pressurized fluid from escaping the articular cartilage in the femoro-acetabular joint to the non-articular peripheral compartment, and thus limiting cartilage consolidation (Ferguson et al., 2000, Ferguson et al., 2003).

Joint fluid is known to have a role in reducing friction in joints (Caligaris and Ateshian, 2008, Krishnan et al., 2004, Schmidt and Sah, 2007, Wong et al., 2008a, Wong et al., 2008b). Biphasic lubrication (Ateshian, 2009, Caligaris and Ateshian, 2008, Caligaris et al., 2009, Gleghorn and Bonassar, 2008, Katta et al., 2008) and boundary lubrication (Crockett et al., 2007, Hou et al., 1992, Schmidt et al., 2007, Schmidt and Sah, 2007) both depend on joint fluid. Furthermore, the fluid film layer that develops between cartilage surfaces delivers important lubricant molecules to the cartilage surface (Bell et al., 2006, Crockett et al., 2007, Forsey et al., 2006, Gleghorn et al., 2009, Kawano et al., 2003, Schaefer et al., 2004, Schmidt and Sah, 2007). Effective sealing of fluid within the joint should thus be important for maintaining biphasic and boundary lubrication. If the acetabular labrum has a role in sealing the hip joint from fluid extravasation during loading, labral tears could allow fluid extravasation and possibly impair biphasic and boundary lubrication. This would result in an elevation of hip joint friction.

Change in articular cartilage friction in whole femoro-acetabular joints following labral damage and resection has not been previously measured. Increased cartilage friction at the hip joint would manifest as increased resistance to movement between the femur and the acetabulum. The objective of this study was to determine if there is a change in resistance to rotation (RTR) of the hip joint before and after focal and complete labral resection when the joint is loaded to a physiologic magnitude of cyclic loading. In this study, we hypothesized that resistance to rotation of a mechanically loaded hip joint increases after focal and complete labrectomy, and this would reflect elevated friction on the femoro-acetabular articular cartilage surfaces.