Thickening of subchondral bone is regionalised by loading of damaged articular cartilage and contra-lateral changes are linked to gait modification

Abstract

Introduction: Thickening of subchondral bone (SCB) is an important hallmark of osteoarthritis (OA). The changes in subchondral bone that occur during OA have been investigated in animal models, including the mouse, however, their relationship to articular cartilage degeneration and the resultant modification in mechanical joint loading have yet to be fully determined. We have therefore used a non-surgical mouse model of mechanical knee joint trauma in which localised cartilage lesions develop in the lateral femur. In this study we defined changes in the SCB and epiphyseal trabecular bone in mouse knees loaded for different periods of time. In addition, changes in contra-lateral nonloaded joints were also analysed to determine whether they represent appropriate controls for such studies. Methods: Eight week-old male CBA mice were loaded non-invasively with different regimes: controls (n1⁄44), once loaded with 2weeks of habitual use (n1⁄47), 2weeks loaded (n1⁄48) or 5weeks loaded (n1⁄43). Joints were analysed with micro-CT (Skyscan, Belgium) and analysed using Dataviewer (for appropriate orientation) and CTAn (Skyscan). Hand-drawn regions of interest (ROIs) were defined including the lateral and medial compartments of femur and tibia, with further SCB plate ROIs central regions of the femoral condyles and cranial and caudal ROIs in the lateral femur. Percentage bone volume, trabecular thickness and SMI measurements were analysed. In addition, gait analysis was performed using Digigait system, consisting of videocapture of the paws of the mouse running on a transparent treadmill. Results: No significant differences between left and right limbs were seen in control non-loaded mice. SCB thickness was increased only after 5 weeks of loading when compared to control non-loaded mice, with greatest changes apparent in the lateral femur (control: 0.098 0.005mm, 5weeks: 0.129 0.006mm, p<0.0001). These increases were also significant when contra-lateral joints were used for comparison. Increases in SCB plate thickness were also evident in the lateral compartment of both the femur and tibia of the contralateral limb (left) in response to applied joint loading of the right limb in all loaded groups (vs control). Increases were also evident in the medial femur central regions of the contra-lateral left knee in all loaded groups. Gait analysis revealed that changes from normal gait were induced in the contra-lateral limb by joint loading from the second week after initiation of loading in both mice loaded once or repetitively for 2 weeks. Gait changes were not, however, found in the loaded limb. Discussion: Together these data indicate that prolonged applied joint loading is required to produce increases in the SCB thickness and that these are most marked in the lateral femur, sites of articular cartilage lesions. We also showed that SCB is sensitive to changes in gait particularly in the contralateral limb. These data support the hypotheses that concomitant articular cartilage damage aggravates focal SCB thickening induced by applied loading and, that changes in gait produce corresponding generalised, non-focal, increases in SCB thickness in contralateral joints. It is also tempting to conclude that contra-lateral joints do not necessarily provide a useful control in studies into bone changes in mechanicallyinduced models of OA.