Abstract
Introduction:
The subscapularis muscle experiences significant strain as it accommodates common movements of the shoulder. Little is known about what happens with this obligatory strain once the subscapularis insertion is disrupted and repaired in the course of shoulder arthroplasty. Subscapularis failure is a serious known complication after shoulder arthroplasty. It is not known what the effect of increasing the thickness of the shoulder head will have on subscapularis strain. It is our hypothesis that the use of large or expanded humeral heads during shoulder replacement will cause increased tension in the repaired subscapularis. The primary purpose of this study was to identify the optimal manner to perform a passive range of motion (PROM) program without invoking a significant increase in strain in the repaired subscapularis. The secondary purpose was to determine the impact of varying the thickness of the humeral head on subscapularis strain using the same PROM protocol.
Methods:
Eight fresh-frozen, forequarter cadaver (four female, four male) specimens were obtained following IRB approval. An extended deltopectoral incision was performed so that the subscapularis insertion site could be well visualized. PROM exercises with the following motions were evaluated: external rotation, abduction, flexion and scaption. An optical motion analysis system was used to measure strain in the subscapularis. The same measurement protocol was repeated after performing a subscapularis osteotomy and after placement of an anatomic hemiarthroplasty of three different thicknesses (short, tall, expanded).
Results:
A decrease in joint laxity (less strain but more tension on the subscapularis) was observed in abduction, external rotation, and forward flexion, following implantation of the shoulder arthroplasty components. For abduction and forward flexion, we observed a trend of decreasing laxity with increasing humeral head component thickness. For external rotation, all components displayed a similar reduction in joint laxity. With the short humeral head, strain was similar to native joint with passive scaption and flexion but not with external rotation or abduction.
Discussion:
The PROM that tends to minimize tension on the subscapularis is forward flexion and scaption whereas external rotation and abduction will stress the subscapularis repair. Therefore passive forward flexion or scaption do not need to be limited but clearly external rotation should have passive limits and abduction should probably be avoided.
The subscapularis muscle is under greater preload tension after shoulder joint arthroplasty. Even the short head size humeral component demonstrated decreased laxity compared to the intact joint. This suggests that even the shortest head size available may not be anatomical and perhaps a thinner humeral head size would be more representative of the normal anatomy.
