Multibody system of the upper limb including a reverse shoulder prosthesis

Area of Science:

• Biomechanics
• Orthopedic Surgery
• Musculoskeletal Modeling

Background:

• Reverse shoulder replacement is used for cuff tear arthropathy and fractures.
• Existing musculoskeletal models rarely focus on reverse shoulder biomechanics.
• Prosthesis alters normal shoulder anatomy and function.

Purpose of the Study:

• To develop a biomechanical model of the upper limb with a reverse shoulder prosthesis.
• To evaluate the impact of joint geometry and position on shoulder biomechanics.
• To analyze changes in muscle and joint forces after reverse shoulder replacement.

Main Methods:

• A musculoskeletal upper limb model was adapted for the DELTA® reverse prosthesis.
• Two models simulated anatomical and reverse shoulder joints.
• Muscle force sharing was solved using video-acquired motion data and energy minimization.
• Changes in muscle lengths, moment arms, and joint reaction forces were evaluated.

Main Results:

• Significant changes in muscle lengths were observed, with teres major and minor shortening.
• Muscle moment arms changed substantially, with increased deltoid moment arms.
• Increased activity in deltoid, teres minor, and coracobrachialis muscles was noted.
• Reverse shoulder joint reaction forces were up to 15% lower than in normal anatomy.

Conclusions:

• Reverse shoulder prosthesis significantly alters upper limb biomechanics.
• The model supports the biomechanical advantages of reverse shoulder design.
• Findings provide data for researchers modeling reverse shoulder prostheses.