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

Purpose To assess the influence of neck-shaft angle (NSA) (145° vs. 155°) on abduction and adduction using three-dimensional (3D) simulations, to find the best configuration and preserve sufficient abduction, while improving adduction, and therefore reducing impingement risk. Methods One hundred and four shoulders that underwent reverse shoulder arthroplasty (RSA) were imported into the image processing software TruMatch ® to create virtual shoulder models. Semi-constrained and long-stemmed implant design for RSA was used and four configurations were simulated: (i) 155°NSA and 42 mm glenosphere (ii) 145°NSA and 42 mm glenosphere, (iii)155°NSA, 38 mm glenosphere, and bony increased offset-RSA (BIO-RSA) and (iv) 145°NSA, 38 mm glenosphere, and BIO-RSA. Results Whether or without BIO-RSA, configurations with 145° NSA (compared to the 155°) had significantly smaller abduction, and distalisation shoulder angle. Configurations with 145° NSA also had significantly greater adduction, lateralisation shoulder angle, flexion, extension, internal (IR) and external (ER) rotations. Regarding changes in muscle length between native anatomy and simulated RSA, whether with or without BIO-RSA, both NSA configurations would lengthen the deltoid, but shorten the subscapularis, infraspinatus and teres minor. Conclusion Whether with or without BIO-RSA, decreasing the NSA could increase adduction and therefore reduce risks of scapular notching, but decrease abduction. Furthermore, decreasing the NSA could increase IR, ER, flexion and extension. Level of Evidence : Comparative simulation study: Level IV.