LAUSR

BACKGROUND The incidence of periprosthetic fractures of the proximal humerus is gradually increasing, following an increase in reverse shoulder arthroplasties (RSA) in the last years. Locking plate fixation and revision arthroplasty are both valuable treatment methods. However, the primary stability of fixation methods for peri-prosthetic fractures has not been investigated in detail. The aim of the present study was to analyze and compare the primary stability of the common treatment measures. MATERIAL AND METHODS Cemented reverse total shoulder arthroplasty (DeltaXtend, Fa. Johnson & Johnson, Norderstedt, Germany) was performed in five shoulders and a distal, mid-diaphysis humeral fracture (Wright and Cofield type B) was induced. The implant was left in place and three distinct fixation scenarios were tested: osteosynthesis using 4.5 mm locking plate fixation (subgroup A); 4.5 mm locking plate fixation with additional 3.5 mm locking plate (subgroup B); 4.5 mm locking plate fixation with additional K-wire cerclage (subgroup C). The specimens were tested in a biomechanical setup simulating activity of daily living including rotation. Strain gauges (4-wire-120 Ohm, Fa. Vishay) mounted on the 4.5 locking plates were used to evaluate the strain of the fixation and to give an estimate for primary stability. RESULTS Regarding the simulation of activities of daily living, no statistically significant differences were found in the measured strains on the locking plate between the subgroups A, B and C. A maximum measured strain of 216.85 μm/m in subgroup A resulted in bending of the locking plate (length 134 mm) of 0.03 mm. In subgroup B (277.01 μm/m) the plate strained 0.04 mm compared to subgroup C (75.93 μm/m) with strain of 0.01 mm. CONCLUSION Additional k-wire cerclages or additional 3.5 locked plating did not increase primary stability. With a stable prosthetic implant in place, 4.5 mm locked plating is sufficient to address periprosthetic humeral shaft fractures in the present in vitro setup.