In Vitro Simulation of Shoulder Motion Driven by Three-Dimensional Scapular and Humeral Kinematics.

Area of Science:

• Biomechanics
• Robotics
• Orthopedics

Background:

• Investigating clinical shoulder conditions requires simulating three-dimensional (3D) shoulder motion using in vivo kinematics within physiologically relevant biomechanics.
• Existing methods often lack the ability to replicate subject-specific, complex 3D scapular and humeral movements in a controlled laboratory setting.

Purpose of the Study:

• To develop and validate a robotic framework for simulating subject-specific 3D scapulothoracic and glenohumeral kinematics in cadavers.
• To assess the system's performance in terms of kinematic accuracy, repeatability, and muscle force repeatability.
• To analyze the influence of input kinematics and cadaver variability on simulated shoulder biomechanics.

Main Methods:

• Utilized an industrial robot to control scapular range of motion (ROM) and a custom robot with a trajectory prediction algorithm for humeral motion.
• Simulated seven healthy subject-specific 3D kinematic trajectories in six cadavers.
• Quantified system performance using orientation accuracy, repeatability metrics (±0.1 mm, <0.5 deg), and muscle force repeatability (<33% variability).

Main Results:

• Achieved high accuracy and repeatability for scapular (±0.1 mm, <0.5 deg) and glenohumeral motion (<3 deg mean absolute error for elevation, plane, and rotation).
• Kinematic accuracy remained consistent regardless of input kinematics or cadaver specimen variations.
• Observed significant effects of subject-specific motion and cadaver variability on muscle forces, particularly in the deltoid muscles.

Conclusions:

• The developed robotic system accurately simulates subject-specific 3D shoulder kinematics in cadavers, providing a valuable platform for biomechanical research.
• The system's ability to differentiate muscle force variations due to input conditions highlights its utility for studying shoulder biomechanics.
• This framework allows for future investigations into shoulder pathologies considering diverse populations and their unique movement patterns.