A quantitative three-dimensional templating method for shoulder arthroplasty: biomechanical validation in cadavers

Background Press-fit humeral components for total shoulder arthroplasty have notable potential complications that may be minimized by preoperative templating and improvements in stem design. The purpose of this study was to develop a 3-dimensional templating technique for the humeral stem and to validate this templating in cadaveric specimens. Materials and methods A cylindrical stem and a stem with a rectangular cross-section were selected for templating and force measurements. Templating was carried out for 15 clinical patients and 16 cadaveric shoulders, including calculation of the cortical-implant volume ratio (CIVR). Insertion forces for stem broaching and impaction were measured for 15 patients and 8 paired cadaveric shoulders. Hoop strain and periprosthetic fractures were monitored in cadaveric shoulders with strain gauges. Results A significant difference in the CIVR was noted between rectangular and cylindrical stems. No difference was observed in impact forces for ideally sized rectangular or cylindrical stems. A difference in insertion forces was found between oversized cylindrical and oversized rectangular implant stems and also between ideal and oversized cylindrical implant stems. The difference in maximal hoop strain between ideally sized rectangular and cylindrical stems was also statistically significant. Conclusions CIVR is useful to predict an ideal humeral stem size. Cylindrical stems have a different design rationale for fixation than rectangular stems. Surgeon awareness of the fixation rationale for a particular stem design is important because different stem types have different effects on the insertion force. More anatomic humeral stem designs may help to minimize the risk of complications and optimize stem fixation.