Diurnal variations in articular cartilage thickness and strain in the human knee

Abstract

Due to the biphasic viscoelastic nature of cartilage, joint loading may result in deformations that require times on the order of hours to fully recover. Thus, cartilaginous tissues may exhibit cumulative strain over the course of each day. The goal of this study was to assess the magnitude and spatial distribution of strain in the articular cartilage of the knee with daily activity. Magnetic resonance (MR) images of 10 asymptomatic subjects (six males and four females) with mean age of 29 years were obtained at 8:00 AM and 4:00 PM on the same day using a 3T magnet. These images were used to create 3D models of the femur, tibia, and patella from which cartilage thickness distributions were quantified. Cartilage thickness generally decreased from AM to PM in all areas except the patellofemoral groove and was associated with significant compressive strains in the medial condyle and tibial plateau. From AM to PM, cartilage of the medial tibial plateau exhibited a compressive strain of −5.1±1.0% (mean±SEM) averaged over all locations, while strains in the lateral plateau were slightly lower (−3.1±0.6%). Femoral cartilage showed an average strain of −1.9±0.6%. The findings of this study show that human knee cartilage undergoes diurnal changes in strain that vary with site in the joint. Since abnormal joint loading can be detrimental to cartilage homeostasis, these data provide a baseline for future studies investigating the effects of altered biomechanics on diurnal cartilage strains and cartilage physiology.

Introduction

Articular cartilage is an avascular and aneural connective tissue that provides a nearly frictionless surface to distribute loads across diarthrodial joints (Mow et al., 1992). During activities of daily living, articular cartilage experiences numerous cycles of relatively high levels of load. For example, the knee joint transfers forces of several times body weight during activities such as gait and stair climbing (Kutzner et al., 2010). Due to the viscoelastic properties of cartilage (Mow et al., 1980), the resulting deformation may not completely recover following each cycle of loading (Eckstein et al., 2006). Thus, cartilaginous tissues may exhibit cumulative strain throughout the day that recovers with prolonged periods of unloading (Paajanen et al., 1994, Sitoci et al., 2012).

In vivo studies have demonstrated that repeated joint loading has been shown to cause reversible decreases in the thickness and volume of articular cartilage (Eckstein et al., 2006, Van Ginckel et al., 2011b). For example, Eckstein et al. reported decreases in patellar cartilage volumes after performing activities such as knee bends, running, squatting, and walking (Eckstein et al., 2005b). Other studies have reported decreases in ankle cartilage volumes after landing from a jump (Van Ginckel et al., 2011a, Van Ginckel et al., 2011b). Furthermore, activities of daily living have been shown to decrease femoral cartilage thickness by up to 0.6 mm between morning and evening MR imaging scans (Waterton et al., 2000). Similarly, decreases in height of the intervertebral discs of the lumbar spine have been observed between morning and night, with changes in disc height of approximately 1 mm (Paajanen et al., 1994).

These studies suggest that cartilage might experience significant strains due to diurnal changes in cartilage thickness. However, there is limited data on the diurnal strains induced by the thickness changes experienced in the femoral, patellar, and tibial cartilage. These diurnal strains potentially play an important role in understanding normal cartilage function, as mechanical strains affect the osmotic environment of chondrocytes (Guilak et al., 1995, Wang et al., 2002). Osmotic stresses, secondary to mechanical loading of the cartilage extracellular matrix, are believed to play an important role in regulating the metabolic activity of chondrocytes (Browning et al., 2004, Phan et al., 2009), and thus, in maintaining normal joint physiology (Guilak, 2011). Furthermore, quantifying diurnal strains in healthy subjects may provide baseline data for future studies evaluating potential alterations in cartilage loading in populations at high risk for the development of OA. For example, altered cartilage loading has been thought to play a role in the development of osteoarthritis in both obese patients and patients with knee ligament injuries (Andriacchi et al., 2004, Griffin and Guilak, 2005, Van de Velde et al., 2009). Thus, the objective of this study was to assess the magnitude and distribution of diurnal strains in the articular cartilage of the knee with daily activity using magnetic resonance (MR) imaging and 3D modeling techniques. Our hypothesis was that activities of daily living would result in significant diurnal compressive strains in the cartilage of all three compartments of the knee. We also examined how these parameters were correlated to body mass index (BMI) and daily activity level.