Neck shaft angle (NSA) in Reverse Shoulder Arthroplasty (RSA): 135 versus 145 degrees at minimum 2 years follow-up.

Abstract

The most common complication with Reverse Shoulder Arthroplasty (RSA) Grammont based design with a 155° neck shaft angle (NSA) is scapular notching. Scapular notching has been associated with both reduced clinical outcomes. Reducing the humeral neck shaft angle from 155 degrees has been shown to reduce the incidence of scapular notching however it is unknown whether there is a difference in scapular notching between a 145 and 135° NSA. The purpose of this study was to evaluate the effect of decreasing the NSA on scapular notching rate and postoperative range of motion comparing 145° and 135° NSA stems at minimum two year of follow-up. Consecutive patients undergoing primary RSA with a neck shaft angle of either 145° or 135° between January 2014 and February 2019 were retrospectively reviewed. Patients were included if they were over the age of 18, had minimum clinical follow-up of 24 months with true postoperative anteroposterior radiographic view. One hundred and three patients were included for the final analysis: 73 with a 145° NSA and 30 with a 135° NSA stem. The mean age and mean follow-up were respectively 70.9 years (range, 52.0-89.0) and 32.1 months. The overall incidence of scapular notching was 46.6 %. There was a statistically significant difference in scapular notching between the 145° (53.4%) and 135° (30%) NSA groups (p=.028). There was no difference in terms of postoperative Constant-Murley Score (mean, 66.1 Vs 68.2; p =.395) , Subjective Shoulder Value (mean, 76,5 Vs 83.1%, p= .167), forward flexion (mean, 140° Vs 142°, p= .704), abduction (mean, 123.2° Vs 121.5° , p= .771), external rotation with the arm at the side (mean, 34.1° vs 37.3°, p= .341) and internal rotation (mean, 5.3 vs 5.4 pts p=.336) between the two groups. This is the first study to compare the effect of a 145° vs 135° neck shaft angle on scapular notching rates. The key finding of this study is that scapular notching rate was significantly reduced from 53% to 30% in 135° NSA compared to 145° NSA, after at least 24 months of follow-up. Our data also shows suggests that glenoid lateralization and inferiorization has an influence on scapular notching. We are unable to state that the reduced scapular notching rate was due to a reduction in NSA alone. Despite a lower rate of scapular notching, the 135°NSA group has not shown any significant better clinical and functional outcomes.