In compression plating, anatomical reduction and compression across the fracture site are the basic principles necessary to achieve primary bone healing. However, varying amounts of gap at the fracture site frequently occur due to technical pitfalls, such as overbending of the plate and inaccurate reduction, and due to the fracture configuration itself. Little is known as to how fracture gap affects stability of the bone-plate construct. We analyzed the effects of fracture gap size (1 and 4 mm) and bone defect (25%, 50%, 75%, 100%) on the biomechanical stability of the compression plate-bone construct through validated finite element analysis. The stiffnesses of eight different models were compared with the stiffness of an ideally compressed model (0 mm/0%). Stress concentration in form of peak von Mises stress (PVMS) was also evaluated. The decrease in stiffness depended mainly on the depth of bone defect. The decrease in stiffness was similar in models with the same defect and different gap size. Considerably more stress was concentrated around the central hole of the plate in gap models with the depth of bone defects of 75% and 100% than with smaller defects. We concluded that even a thin fracture gap (1 mm) with no contact between the fracture after plating decreases stiffness exponentially; contact at the fracture surfaces of > or =50% was necessary to avoid undue stress concentration in the plate.