USE IT OR LOSE IT: THE RELATIONSHIP BETWEEN KINEMATIC JOINT LOADING AND DEPTH-SPECIFIC SUBCHONDRAL BONE MINERAL DENSITY

According to Wolff's law, bone tissue is influenced by its mechanical environment and adapts in response to the mechanical load that acts on it. This law has been supported in studies analyzing bone adaptation in the knee and hip, and have also shown that changes to the bone have been found to precede cartilage degeneration in diseases such as osteoarthritis. However, the relationship between bone adaptations and joint loading has not been established in the wrist. Our primary objective is to evaluate the correlation between kinematic joint contact area (JCa) and subchondral volumetric bone mineral density (vBMD), as it relates to depth from the subchondral surface. Our secondary objective is to determine the amount of variance in subchondral vBMD that can be explained by kinematic joint contact. We hypothesize that an increase in joint loading, therefore an increase in joint contact area, will result in an increase in vBMD within all depth-specific layers from the subchondral surface. We analyzed 20 otherwise healthy participants (n = 11 females and n = 9 males). Following recruitment, all participants underwent a series of CT scans; a static CT scan accompanied by a calibration phantom to obtain measures of vBMD in 3 normalized depths; 0 – 2.5, 2.5 – 5, and 5 – 7.5 mm. Participants also underwent a four-dimensional CT scan (4DCT) where the participant was instructed to perform maximum wrist extension to maximum wrist flexion. We created 3D bone models of the distal radius, scaphoid, and lunate, and analyzed vBMD and JCa in the radiolunate (RL) and radioscaphoid (RS) joints separately. To determine the relationship between vBMD and kinematic JCa in the RL and RS, several Pearson product-moment correlation coefficients were calculated where vBMD was the dependent variable, and kinematic JCa throughout every 10 degrees of motion (extension to flexion) were the independent variables. Independent variables that were statistically significantly associated with vBMD were assessed using a regression model. The independent variables were entered into the model in steps; (1) significant correlations, (2) sex, and (3) age. Descriptive statistics demonstrated more JCa and higher vBMD in the RS joint for all degrees of motion and all depths from the subchondral surface. The correlation analysis demonstrated significant positive correlations between vBMD in region 3 of the RL (5 – 7.5mm) and extension at 40, 30, 20, and 10 degrees of motion (p < 0.05), and flexion at 30 and 10 degrees of motion (p < 0.05). There was also a significant correlation between vBMD in region 2 of the RS (2.5 – 5mm) and extension at 40, 30 and 10 degrees of motion (p < 0.01), and flexion at 30 and 20 degrees of motion (p < 0.05), as well as a significant correlation between vBMD in region 3 of the RS (5 – 7.5 mm) and extension at 40, 30, and 10 degrees, and flexion at 30 degrees. The regression analysis demonstrated that 38% of the variance in vBMD in region 2 of the RS can be explained by the significant degrees of motion in the wrist (p = 0.04), while the sex of the participant contributed an additional 13% (p = 0.02) of the variance. In the RS region 3, 33% of the variance can be explained by the significant degrees of motion in the wrist (p = 0.04), while the sex of the participant contributed an additional 7% (p = 0.07) of variance. Age did not contribute to the variance in the relationship between vBMD and kinematic JCa. Overall, our results supported our hypothesis wherein an increase in vBMD was significantly, positively associated with an increase in JCa. Most notably, this relationship was demonstrated in the deeper regions of the distal radius that is primarily composed of trabecular bone; a region that is anatomically more adaptive to change and metabolically active. In the RS joint, 38% and 33% of the variance in vBMD in region 2 and 3 (respectively), could be explained by kinematic JCa alone. These findings are consistent with previous literature indicating that the RS joint is the main site of force transmission in the wrist, further supporting our hypothesis that an increased JCa is positively associated with a higher vBMD. This relationship is paramount to better understanding joint remodeling following injury, early osteoarthritic signs in the joint, and the influence of vBMD and JCa adaptations on pain.