PROJECT SUMMARY / ABSTRACT Wrist osteoarthritis (OA) is a prevalent, debilitating condition; however, limited attention has focused on the scaphotrapeziotrapezoid (STT) joint. The STT is positioned on the radial side of the wrist, spanning both rows of carpal bones and bridging the thumb and wrist joints. STT OA impacts between 15% and 24% of adults over 45 years of age, with increased prevalence and incidence given advancing age and female sex. However, compared to hand joints, the STT has received limited diagnostic and therapeutic attention. The wrist is anatomically complex: the number, unique geometries, small size, and close proximities of carpal bones pose challenges for diagnostic imaging and guiding evidence-based surgical interventions. Due to intricate carpal interrelationships, a strong biomechanical foundation of normal STT function during motion and under loading is critical for tailoring interventions that remedy symptomatic STT OA without compromising the remainder of the carpus or thumb. Thus, there is a need to understand the complex interplay between carpal bones during motion in unaffected participants. Specifically, there is a need to measure bone motion dynamically to understand the relationships between carpal bones during various motions of the wrist and hand under different loading conditions. 4DCT (3DCT over time) yields a time series of image volumes captured during motion with high spatial and temporal resolution, offering the exciting capability to capture bones dynamically. Interosseous proximity distributions, a proxy for pressure, and centers of closest proximity will be used to describe the joint relationships as a function of motion and loading, conferring an in-depth understanding of the STT joint in both unaffected and pathological states. Further, the magnitude and variability of joint space and shape at each STT articulation have not been rigorously quantified. Statistical shape modeling (SSM) and machine learning of STT carpal bones collected from unaffected and pathological STT joints will allow us to elucidate the interactions between interacting geometries of articulating surfaces during motion and bone morphology. Our study aims to understand STT arthrokinematics in normal and pathological conditions in the following aims. Aim 1: Quantify STT arthrokinematics during unresisted and resisted wrist and thumb motions, separately, in participants without wrist OA and quantify normal variations in STT morphology. Aim 2: Quantify STT arthrokinematics during unresisted versus resisted activities in patients with early-stage STT OA using 4DCT and compare 3D morphologies of unaffected and pathologic STT joints. 4DCT data elucidate motions and loads that stress the joint, while morphologic data indicate sites of structural change. A parallel analysis will allow us to define structure-function interactions at the STT joint. The Aims will culminate in a comprehensive view of how STT loading impacts arthrokinematics i...