AUGMENTED FOUR-SCREW VERSUS NON-AUGMENTED SIX-SCREW PHILOS CONFIGURATION FOR PLATING OF OSTEOPOROTIC PROXIMAL HUMERUS FRACTURES: A BIOMECHANICAL STUDY

Proximal humerus fractures are common in patients with low bone mineral density. Locking plate osteosynthesis using either Minimal Invasive Plate Osteosynthesis (MIPO) or Open Reduction and Internal Fixation (ORIF) technique is widely performed. However, it has not yet been biomechanically investigated whether augmented MIPO achieves stabilization comparable to ORIF screw configuration for instable proximal humerus fracture fixation in osteoporotic bones. Fourteen paired human cadaveric humeri with simulated unstable three-part proximal humerus fractures (AO 11-B1) were stabilized using PHILOS plates with four proximal head screws in both groups (row A and B). In the ORIF group, two additional calcar screws were used (row E) while in the MIPO group, the four screw tips were augmented with bone cement. Cyclic axial loading tests were conducted until failure and interfragmentary movements were monitored using optical motion tracking and anteroposterior x-ray imaging. Initial axial construct stiffness and cycles to failure showed no significant differences between groups (p=0.171, p=0.397). Under increasing cyclic loading, the ORIF group exhibited a significant increase in varus deformation (p=0.029), head displacement (p=0.038), and screw bending in row A (p=0.003), whereas no significant increase in these parameters was observed in the MIPO group. Although the overall values were higher in the MIPO group, there was no significant difference in absolute values between the groups (p=0.071). From a biomechanical perspective PHILOS plates with four augmented screws as used in the MIPO technique demonstrated comparable initial construct stability and cycles to failure as compared to the commonly used PHILOS plates with additional calcar screws as used in the ORIF technique. Moreover, the ORIF group exhibited significantly greater increases in varus deformation and humeral head displacement under cyclic loading, while the MIPO group, despite higher overall values, showed no significant increase. This suggests that the MIPO technique may be a valid alternative to the ORIF technique, particularly in the context of low bone mineral density.