Arthroscopic dynamic analysis of scapular notching in reverse shoulder arthroplasty

Despite the success of reverse shoulder arthroplasty, the rate of scapular notching phenomena is high and represents a significant risk of glenoid prosthetic component loosening over the long term. The aim of this study was to perform an arthroscopic dynamic analysis of the impingement of the humeral cup on the neck of the scapula in order to highlight the causes of the occurrence of this notch and possible ways to avoid it.

The SMR reverse shoulder prosthesis (Lima(®)) was successively implanted in 11 shoulders of seven cadavers. Three glenospheres (36 mm, 36 mm with a 4-mm lowering offset, and 44 mm) were consecutively tested on each shoulder. An arthroscopic study was then performed in each case looking for scapular notching depending on the position of the shoulder, its exact location, and any resulting prosthetic instability. Rates of notching for each glenosphere in the different shoulder positions were compared.

For each glenosphere, the highest rates of notching were recorded in the zero abduction position. Increasing abduction reduced the rates of notching and prosthetic instability for all types of glenosphere. External rotation increased both risks with 36-mm glenospheres but reduced them with the 44-mm glenosphere. The lower center of rotation, thus, reduced the risk of notching but increased the risk of instability associated with it. Increasing the prosthetic diameter reduced both risks.

Reducing the risk of scapular notching and prosthetic instability requires, in addition to a lower center of rotation, the use of glenosphere diameters greater than 36 mm. The use of a humeral cup with a posteroinferior indentation should also contribute to reducing this risk.