Does acromion anatomy affect the risk of acromion stress fracture after reverse shoulder arthroplasty?

IntroductionReverse shoulder arthroplasty (RSA) alters normal anatomic relationships and results in increased deltoid forces. Which alter physiologic stress patterns in the acromion resulting in fractures. The purpose of this study was to evaluate the effect of acromion anatomy on the stress levels and risk of acromial fracture after RSA.MethodsA lateralization onlay design of the Exactech Equinoxe RSA standard stem was used for all four different acromial sizes (ranging from −5.0 mm to +5.0 mm) and compared to standard normal shoulder model (acromial size 0). A finite element analysis (FEA) model was then constructed for each case and quasi-static analysis was carried out to determine the highest minimum principal stress (HMPS) for each case and this was used to predict fatigue life percentage (FLP) of the acromion.ResultsFor smaller acromion sizes of −5 mm and -2.5 mm, the HMPS was found to be 1.87 and 1.24 times higher than the standard, respectively. The HMPS for the +2.5 mm acromial size was 0.95 times compared to the standard and 1.04 times higher for the +5 mm acromial size. According to our model, the highest FLP was seen when the acromion size was +2.5 mm (case 4 - 178%) and the lowest FLP was seen when it was -5 mm (−0.06%).ConclusionOur results suggest that there is an optimal acromion size (+2.5 mm from normal male) that leads to the lowest fracture risk in RSA. Surgeons must be aware of acromion size as a critical factor in deltoid tensioning, acromial stress, and risk of acromial fracture when selecting optimal implant designs and sizes for RSA.Level of evidence: Basic Science Study