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Introduction: The purpose of this study was to investigate the kinematics of the thumb and index finger during in-place precision pinch (IPP) for individuals with carpal tunnel syndrome (CTS). CTS entails proprioceptive and sensory dysfunction, including pain and tingling, of the affected digits, most notably the thumb and index finger. IPP approximates a functional daily task that focally involves the thumb and index finger for grasp and manipulation of small objects. Characterizing differences in movement features during IPP for individuals with CTS versus healthy, able-bodied (ABL) controls demonstrates the functional consequences of CTS and provides insight into the related patho-mechanism.

Materials & Methods: Six CTS and six ABL individuals participated. Each subject performed 50 discrete cycles of IPP at a metronome pace of 0.5Hz. To perform a cycle of IPP, a subject placed their arm in a splint (Figure 1A), while transitioning their hand between open (all digits comfortably extended) and closed tip-pinch (thumb and index finger pads contacting) configurations. Subjects donned retro-reflective markers (Figure 1B) to motion capture full kinematic description of the thumb and index finger relative to a local hand coordinate system. Figure 1. A) Subject performing in-place pinch B) Markers utilized for motion tracking

Results: Relative to ABL controls, CTS subjects demonstrated compromised ability to open and bring the thumb into opposition during precision pinch movements. When normalized by hand-width, the average maximum opening (inter-nail distance) was 1.54±0.29 for ABL and 1.17±0.12 for CTS. CTS subjects exhibited either or both smaller range of motion and greater trajectory variability in pronating the thumb at the carpometacarpophalangeal (CMC) joint and extending/flexing at the interphalangeal (IP) and metacarpophalangeal (MCP) joints compared to ABL (Figure 2). Figure 2. Thumb joint angles notable for pinch opposition

Conclusions: These pilot results suggest that CTS individuals have compromised pinching kinematics. CTS leads to a relative inability to maximally open and bring the thumb into opposition with the index finger. This functional deficit is likely due to compression of the median nerve, specifically affecting muscles responsible for basal thumb function.