(A-24) Evaluating Force Propagation Across the Thumb Joints in Individuals with Early- and End-Stage Carpometacarpal Osteoarthritis

Introduction:: Carpometacarpal osteoarthritis (CMC OA) is a degenerative joint disease that affects the base of the thumb and is characterized by weakness, pain, and reduced mobility [1]. Biomechanical loading has been identified as a contributing factor to OA progression at large joints, like the hip and knee [2]. However, little is known about the role of abnormal loading in CMC OA. To elucidate mechanical factors influencing disease progression, this study aims to evaluate how internal forces propagate across the thumb joints in individuals with early- and end-stage CMC OA during a key pinch task. We hypothesized participants with end-stage CMC OA will demonstrate decreased force production and abnormal loading patterns compared to those with early-stage CMC OA.

Materials and Methods:: This is a sub-analysis of an ongoing IRB-approved study of women with CMC OA. For this sub-analysis, four women with early-stage CMC OA (age: 74.3 ± 10.7 years) and four with end-stage CMC OA (age: 63.5 ± 17.2 years) were studied. Disease severity was determined from hand radiographs using the Eaton-Littler criteria (early-stage = Stage I or II; end-stage = Stage II or IV). Thumb kinematics and kinetics were collected using standard methods. Briefly, marker-based motion capture data were collected at 100 Hz using a 12-camera system and a custom marker set with at least three markers on each thumb segment. External thumb-tip forces were collected at 3,000 Hz using a multi-axis force sensor. Each participant completed three trials of key pinch by exerting their maximum thumb-tip force onto the force sensor. For analysis, anthropometrically scaled models [3] were generated in OpenSim v4.4. and inverse kinematics was used to calculate thumb joint angles. Force propagation was then analyzed by inputting these joint angles and the measured external force into inverse dynamics and the joint reaction analyzer (Fig. 1A). These processes output internal joint reaction forces at the carpometacarpal (CMC), metacarpophalangeal (MCP), and interphalangeal (IP) joints. To evaluate differences in force magnitude, the average maximum external thumb-tip and internal joint force from each participant was calculated. To evaluate differences in loading patterns, the proportion of force exerted in each anatomical direction was calculated. The angles between the average force vectors from each cohort were also calculated.

Results, Conclusions, and Discussions:: The early-stage CMC OA cohort generated larger pinch forces than the end-stage CMC cohort. Specifically, the average pinch force magnitude was 42.9 ± 13.0 N for the early-stage cohort and 26.6 ± 17.3 N for the end-stage cohort. Similar differences in average force magnitude were also observed in the internal forces at all three thumb joints (Fig. 1B). Interestingly, the proportion of force exerted in each anatomical direction differed across the thumb joints and between cohorts. For example, the internal forces at the MCP and IP joints had nearly equal components in the early-stage cohort but were biased toward the x-direction in the end-stage cohort (Fig. 1C, blue and gray bars). In contrast, at the CMC joint, the largest component of the internal force was in the y-direction for both cohorts. These differences in force direction are further illustrated by the angles between the average force vectors for each cohort at the CMC, MCP, and IP joints, which were 10.2°, 15.1°, and 26.5° respectively. Overall, the detected differences in force magnitude and direction may partially explain known clinical differences in joint stability between early- and end-stage CMC OA.



Our findings suggest differences in force production and joint stability may be related to disease severity in CMC OA. However, our results should be interpreted with caution due to the small sample size. Immediate next steps will expand these analyses to a larger cohort. Understanding the role of biomechanical loading on thumb function and CMC OA disease severity is crucial for creating effective interventions and improving patient outcomes.






Acknowledgements (Optional): : Funding from National Institutes of Health (R01 AR078817, KL2 TR001429, F31 AG074645) and Florida Graduate Student Preeminence Award.

References (Optional): : [1] Neumann, D. et al. (2003) J Orthop Sports Phys Ther; 33:386-399. [2] Stief, F. et al. (2011) Gait & Posture, 33:490–495. [3] McFarland, D.C., et al. (2022) IEEE Trans Biomed Eng; 1-12.