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Objectives and questions: The introduction of the locking plate allowed overcoming some problems of internal fixation with conventional plates. However, fixation failure is still observed. One issue may be a peri-implant fracture at the end of the plate after consolidation of the initial fracture. The aim of the study was to investigate the influence of screw type and length at the end of the construct regarding stress concentration as a surrogate for the risk of peri-implant fractures.

Material and methods: A finite element analysis was conducted. The diaphyseal cortical bone was simulated with a hollow cylinder. Osteoporotic bone was mimicked with reduced elasticity modulus and cortical thickness. A titanium 4.5/5.0 mm LCP 7-hole plate was applied to the bone, with screws in holes 1, 3, 5 and 7 or screws in holes 1, 2, 4, 6 and 7. The screw in hole 1 was locking, either monocortical (MLS) or bicortical (BLS), respectively non-locking bicortical and oriented orthogonally (NLS) or in an oblique manner (ONLS). A bending moment and an axial force were applied, while the strain in the bone-screw interface area was evaluated.

Results: For normal bone, the lowest strain in the screw-bone interface was observed with NLS or ONLS in the first screw hole and the second hole left empty. This corresponds to a 51% reduction in strain compared to the BLS. For osteoporotic bone, BLS in holes 1 and 2 resulted in the lowest strain.

Discussion and conclusions: For locking plate fixation with normal bone quality, a NLS or ONLS should be used at the end of the construct and the second to last hole should be left empty to minimize the risk of later peri-implant fractures. In osteoporotic bone, however, it would be advantageous to fill the last two screw holes of the construct with BLS.