Thumb motion is not the same as first carpometacarpal joint motion

Area of Science:

• Orthopedics
• Biomechanics
• Medical Imaging

Background:

• Thumb motion is a critical metric for evaluating hand conditions and treatment effectiveness.
• A significant portion of recent literature inaccurately equates general thumb motion with first carpometacarpal (CMC) joint motion.
• This misrepresentation stems from using technologies that measure overall thumb movement rather than specific joint articulation.

Purpose of the Study:

• To emphasize the importance of precise terminology in reporting thumb and CMC joint motion.
• To quantify the discrepancy between measured thumb motion and actual CMC joint motion.
• To clarify the limitations of non-imaging techniques in assessing CMC joint kinematics.

Main Methods:

• Analysis of computed tomography (CT) scans from 46 healthy individuals.
• Utilized 3D markerless bone registration to calculate thumb rotation (first metacarpal relative to radius) and CMC joint rotation (first metacarpal relative to trapezium).

Main Results:

• Thumb rotation is an unreliable indicator of CMC joint rotation, with significant variability observed.
• At 20° of thumb rotation, CMC joint rotation varied widely (3°–30°).
• Thumb motion consistently over-predicted CMC joint motion by an average of 10°, with wide 95% Limits of Agreement (-11° to 30°).

Conclusions:

• CMC joint motion cannot be accurately predicted from general thumb motion measurements.
• Skeletal imaging technologies are necessary for precise 3D CMC joint motion assessment.
• Goniometers and skin markers measure thumb motion, not CMC joint motion; mislabeling these is scientifically inaccurate and critical to avoid in research and clinical reporting.