Abstract
Introduction
Reverse total shoulder arthroplasty continues to have a high complication rate, specifically with component instability and scapular notching. Therefore, the purpose of this study was to quantify the effects of humeral component neck angle and version on impingement free range of motion.
Methods
A total of 13 cadaveric shoulders (4 males and 9 females, average age = 69 years, range 46 to 96 years) were randomly assigned to two studies. Study 1 investigated the effects of humeral component neck angle (n=6) and Study 2 investigated the effects of humeral component version (n=7). For all shoulders, Tornier Aequalis® Reversed Shoulder implants (Edina, MN) were used. For study 1, the implants were modified to 135, 145 and 155 degree humeral neck shaft angles and for Study 2 a custom implant that allowed control of humeral head version were used.
For biomechanical testing, a custom shoulder testing system that permits independent loading of all shoulder muscles with six degree of freedom positioning was used. (Figure 1) Internal control experimental design was used where all conditions were tested on the same specimen.
Study 1
The adduction angle and internal/external humeral rotation angle at which impingement occurred were measured. Glenohumeral abduction moment was measured at 0 and 30 degrees of abduction, and anterior dislocation forces were measured at 30 degrees of internal rotation, 0 and 30 degrees of external rotation with and without subscapularis loading.
Study 2
The degree of internal and external rotation when impingement occurred was measured at 0, 30 and 60 degrees of glenohumeral abduction in the scapular plane with the humeral component placed in 20 degrees of anteversion, neutral version, 20 degrees of retroversion, and 40 degrees of retroversion.
Statistical analysis was performed with a repeated measures analysis of variance with a Tukey post-hoc test with a significance level of 0.05.
Results
Study 1
Adduction deficit angles for 155, 145, and 135 degree neck-shaft angle were 2 ± 5 degrees of abduction, 7 ± 4 degrees of adduction, and 12 ± 2 degrees of adduction (P <0.05), respectively. Impingement-free angles of humeral rotation and abduction moments were not statistically different between the neck-shaft angles. The anterior dislocation force was significantly higher for the 135degree neck-shaft angle at 30 degrees of external rotation and significantly higher for the 155 degree neck shaft angle at 30 degrees of internal rotation (P<.01). The anterior dislocation forces were significantly higher when the subscapularis was loaded (P <0.01).
Study 2
Maximum external rotation was the limiting position for impingement particularly at 0 degrees of abduction. Maximum external rotation before impingement occurred increased significantly with increasing humeral retroversion (p < 0.05) (Figure 2). No impingement or subluxation occurred at any humeral version in 60 degrees of glenohumeral abduction.
Conclusion
In reverse shoulder arthroplasty, 155 degree neck-shaft angle was more prone to impingement with adduction but had the advantage of being more stable. In addition, 40 degrees of retroversion has the largest range of humeral rotation without impingement.
