Are continuous locking headless bone screws with locking plate fixation effective for bridging fracture gaps in far cortical locking constructs? An in vitro biomechanical and finite element study under simplified axial loading

Introduction Locking screws typically provide more stable fixation than conventional cortical screws. However, the rigid connection between the locking screw head and plate hole can cause stress concentrations and lead to early failure under cyclic loading. This study aimed to compare the mechanical stability of a continuous (headless) locking screw with standard locking screws and cortical screws, either in a standard (flush with the bone) or in far cortical locking constructs, using biomechanical testing and finite element analysis (FEA). Methods The Trident Distal Radial Locking Plate (A-Plus Biotechnology Co., Ltd., Taiwan) was used to repair a simulated distal radial diaphysis fracture and was secured to the bone using different screw types. The fracture gap was bridged with the plate placed flush with the bone and with the plate offset from the bone surface by 4 mm (far cortical locking (FCL)). The constructs were subjected to static compression and cyclic axial loading tests to assess construct stiffness and yield load, as well as endurance and failure mechanisms under cyclic loading, while FEA was used to evaluate the von Mises stress distribution on the plates and screws. Results The Group 3a (flush with bone, continuous locking screws) had a significantly higher yield load than Groups 2b (FCL with locking screws) and 3b (FCL with continuous locking screws) (p < 0.05). In addition, Group 1 (flush with bone, cortical screws) was found to be significantly stiffer than Groups 2b and 3b (p < 0.01). For the dynamic testing, Group 3a showed intermediate endurance and failed predominantly by screw pull-out, whereas Groups 1 and 2 primarily failed by plate deformation. The FEA results showed stress concentrations at the screw neck of the standard locking screws and around the threaded regions of the continuous locking screws. Conclusion Continuous locking screws placed flush with the bone had the greatest fixation strength of all configurations tested. However, using continuous locking screws in an FCL configuration may redirect stress to the threaded region of the screw neck, potentially increasing the risk of failure.