Implications of humeral short-stem diametral sizing on implant stability

BACKGROUND: Shoulder arthroplasty humeral stem design has evolved to include various shapes, coatings, lengths, sizes, and fixation methods. While necessary to accommodate patient anatomy characteristics, this creates a surgical paradox of choice. The relationship between the surgeon’s selection of short-stem implant size and construct stiffness, resistance to subsidence and micromotion has not been assessed. METHODS: Eight paired cadaveric humeri were reconstructed with surgeon-selected (SS) and 2-mm diametrically larger (SS+2) short-stemmed press-fit implants. Each reconstruction was subjected to 2000 cycles of 90° forward flexion loading, and stem subsidence and micromotion were measured using optical tracking. Compressive stiffness of the stem-bone reconstruction was then assessed by applying a load in-line with the stem axis that resulted in 5 mm of stem subsidence. RESULTS: Increasing stem size by 2 mm resulted in the construct stiffness more than doubling compared to SS stems (−741 ± 243 N/mm vs. −334 ± 120 N/mm; P = .003; power = 0.971). These larger stems also subsided significantly less than their SS counterparts (SS: 1.2 ± 0.6 mm; SS+2: 0.5 ± 0.5 mm; P = .029; power = 0.66), though there were no significant changes in micromotion (SS: 169 ± 59 μm; SS+2: 187 ± 52 μm; P = .506; power = 0.094). CONCLUSIONS: The results of this study highlight the importance of proper short-stem sizing, as a relatively small 2 mm increase in diametral size was observed to significantly impact construct stiffness, which could increase the risk of stress shielding and implant loosening. Future work should focus on developing tools that objectively quantify bone quality and aid surgeons in selecting the appropriate size short-stem humeral implants for a particular patient.