Bone resorption triggered by high radial stress: The mechanism of screw loosening in plate fixation of long bone fractures

Screw loosening is a common complication in plate fixation. However, the underlying mechanism is unclear. This study investigated screw loosening mechanisms by finite element analysis (FEA) simulation and clinical X-ray feature analysis. Two FEA models incorporated bone heterogeneity and orthotropy, representing fracture fixation using dynamic compression plate (DCP) and locking compression plate (LCP), were developed. These models were used to examine the volume of bone exceeding a certain stress value around each screw under physiologically-relevant loading conditions. These damaged bone was then separated and compared by the axial stress and radial stress of each screw. In addition, features of patients' X-ray images showing screw loosening were analyzed to validate the loosening features simulated by the models. The FEA study showed that more damaged bone was found at the central two screws which gradually decreased toward the two end screws in all groups. More bone was damaged by the radial stress of each screw than by the axial stress. The radiological analysis of screw loosening showed that bone loss occurred at the screw closest to the fracture line first then subsequent bone loss at the screws further away from the fracture line occurred. This study found that the two screws nearest to the fracture line are more vulnerable to loosening. The radial stress of the screw plays a larger role in screw loosening than the axial stress. Bone resorption triggered by the high radial stress of screws is indicated as the mechanism of screw loosening in the diaphyseal plate fixation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1498-1507, 2019.