Primary stability of fixation methods for periprosthetic fractures of the humerus: a biomechanical investigation

Background: The incidence of periprosthetic fractures of the proximal humerus is gradually increasing, following an increase in reverse shoulder arthroplasties in recent years. Locking plate fixation and revision arthroplasty are both valuable treatment methods. However, the primary stability of fixation methods for periprosthetic fractures has not been investigated in detail. The aim of this study was to analyze and compare the primary stability of the common treatment measures. Materials and methods: Cemented reverse total shoulder arthroplasty (Delta Xtend; DePuy Synthes, Warsaw, IN, USA) was performed in 5 shoulders, and a distal, mid-diaphysis humeral fracture (Wright and Cofield type B) was induced. The implant was left in place, and 3 distinct fixation scenarios were tested: osteosynthesis using 4.5-mm locking plate fixation (subgroup A), 4.5-mm locking plate fixation with an additional 3.5-mm locking plate (subgroup B), and 4.5-mm locking plate fixation with an additional K-wire cerclage (subgroup C). The specimens were tested in a biomechanical setup simulating activities of daily living including rotation. Strain gauges (4-wire strain at 120 U; Vishay Measurements Group, Chartres, France) mounted on the 4.5-mm locking plates were used to evaluate the strain of the fixation and to give an estimate of 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 subgroups A, B, and C. A maximum measured strain of 216.85 mm/m in subgroup A resulted in bending of the locking plate (length, 134 mm) of 0.03 mm. In subgroup B (277.01 mm/m), the plate strained 0.04 mm compared with a strain measurement of 0.01 mm in subgroup C (75.93 mm/m). Conclusion: Additional K-wire cerclages or additional 3.5-mm 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. Level of evidence: Basic Science Study; Biomechanics (c) 2021 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.