Computer Modelling of Wrist Biomechanics: Translation into Specific Tasks and Injuries.

Area of Science:

• Biomechanics
• Orthopedic Surgery
• Medical Imaging

Background:

• The carpus is a complex mechanical system with no standard configuration, hindering biomechanical understanding.
• Advancements require models that account for individual carpal variations.
• This study aims to develop a kinetic model for reverse analysis of patient-specific carpal biomechanics.

Purpose of the Study:

• To present the rules-based motion (RBM) concept for wrist biomechanics, defining motion by morphology, constraint, interaction, and load.
• To apply the stable central column theory (SCCT) to the carpus, identifying a "two-gear four-bar" linkage through reverse engineering.
• To assess carpal motion using a 3D dynamic visualization model and test the hypothesis of consistent motion patterns.

Main Methods:

• Created 3D models from CT scans of five normal wrists in various positions (flexion, extension, radial/ulnar deviation).
• Animated proximal and distal carpal rows in a virtual environment with the radius/proximal bones immobilized.
• Analyzed and compared rotational axes and bone positions in sagittal and coronal planes.

Main Results:

• The carpus demonstrated two degrees of freedom (pitch and yaw) with inherent stability.
• The proximal row moved in a singular arc with variable extent; the distal row moved on a pivot/saddle joint axis.
• Consistent isometric constraints were observed, with distinct row motion patterns identified during different wrist movements, though the extent of motion varied.

Conclusions:

• The study supports the SCCT and RBM concepts for understanding wrist biomechanics and functional tasks.
• Advanced 3D modeling and reverse engineering of individual wrist rules can inform mathematical models for "what if" surgical scenario testing.
• Quantitative 3D CT analysis, virtual surgical planning, and intervention testing offer a pathway for improved prognosis of wrist injuries.