Biomechanical Performance of Four Cement-Augmented vs. Six Non-Augmented Screws in Low Bone Density Humerus Fractures.

Abstract

Proximal humerus fractures are frequent in patients with low bone mineral density. PHILOS plates are widely used with either Minimally Invasive Plate Osteosynthesis (MIPO) or Open Reduction and Internal Fixation (ORIF) techniques. However, it remains unclear whether four cement-augmented screws provide biomechanical stability comparable to four non-augmented screw plus two additional calcar screws in unstable fractures in osteoporotic bone. 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; Figure 1C). In the 6S group, two additional calcar (inferomedial support) screws were used, while in the 4S+ group, the four screw tips were augmented with bone cement. Cyclic axial loading tests were conducted until failure. Interfragmentary movements were compared between the groups. Initial axial construct stiffness and cycles to failure showed no significant differences between groups (p=0.171, p=0.397). Although interfragmentary motion was slightly higher in the 4S+ group, this difference was not significant (p=0.071). Under cyclic loading, the 6S group exhibited a significant progressive increase in varus deformation (p=0.029), head displacement (p=0.038), and screw bending in row A (p=0.003), while no significant increase was observed in the 4S+ group. From a biomechanical perspective, PHILOS plates with four cement-augmented screws demonstrated comparable initial construct stability and cycles to failure compared to PHILOS plates with four non-augmented head screws plus two additional calcar screws in a possible ORIF screw configuration. While absolute interfragmentary motion was slightly higher in the 4S+ group, only the 6S group showed progressive instability under repeated loading. These findings suggest that cement-augmented 4S+ is a biomechanically valid alternative to 6S, particularly in osteoporotic bone.