Decreasing the neck shaft angle from 155° to 145° improves adduction, but reduces abduction: A computed tomography...

Area of Science:

• Orthopedic surgery
• Biomechanical engineering
• Medical simulation

Background:

• Reverse shoulder arthroplasty (RSA) is a common procedure for complex shoulder pathologies.
• Optimizing implant design, including neck-shaft angle (NSA), is crucial for improving functional outcomes and minimizing complications like impingement.

Purpose of the Study:

• To evaluate the impact of two neck-shaft angles (145° vs. 155°) on shoulder abduction and adduction using 3D simulations.
• To determine the optimal NSA for preserving abduction while enhancing adduction and reducing impingement risk in RSA.

Main Methods:

• Virtual shoulder models were created from 104 patient datasets undergoing RSA.
• Four simulated configurations were analyzed: varying NSA (145°/155°) and glenosphere size (38mm/42mm), with and without bony increased offset-RSA (BIO-RSA).

Main Results:

• Lower NSA (145°) significantly reduced abduction and distalization but increased adduction, lateralization, flexion, extension, internal rotation (IR), and external rotation (ER).
• Both NSA configurations led to deltoid lengthening and shortening of key rotator cuff muscles (subscapularis, infraspinatus, teres minor).

Conclusions:

• Decreasing NSA in RSA can enhance adduction, potentially reducing scapular notching, and improve overall rotational and flexion/extension range of motion.
• However, a lower NSA may compromise abduction; careful consideration of NSA is necessary to balance these effects for optimal RSA outcomes.