Effect of fin length and shape of stemless humeral components in a reverse shoulder implant system: a FEA study

Abstract

The recent reverse total shoulder arthroplasty (TSA) implant designs employ shorter humeral component stems; some designs even eliminate the stem completely. The short stem provides several advantages: preserved bone, less stress shielding, ease of revision surgery, and freedom of humeral component location due to the minimal stem length and non-complex shape. However, due to the reduction in contact surface and frictional fixation, this stemless design may cause failure of the implant, especially during early stages following surgery. In this study, the effect of fin shape and fin length of stemless humeral components were investigated, and hypothesis was that stress and micromotion would decrease as the ratio of fin length to baseplate radius increases and the overall size of the humerus component increased. 15 different 3D models of stemless humeral components, 5 different lengths and 3 different shapes, within reverse TSA systems were developed. Each humerus component was analyzed using finite element analysis (FEA) to investigate the effects of stem length and fin shape on the initial stability of the component. The highest stress and micromotion were observed at the shortest fin length; lowest stress and micromotion were found at the longest fin length. Results suggest that the length of the fin affects the stress at the bone as well as the amount of micromotion; 20–30% longer fin length than baseplate radius would be ideal for the stemless humeral component design. However, the various fin shapes showed mixed effects on the results and further investigation is required.