Biomechanics of basic medical screws: Study 1: METHODS: Cortical bone screws of 3.5 mm were implanted in five pairs of cadaveric humerus of three different bone density (normal, reduced bone mass, osteoporotic) types. The screws were implanted at maximum torque, and at 50%, 70%, and 90% of maximum torque, respectively, and the screws’ resistance to extraction strength, bone cortical thickness, and bone mineral density were measured. RESULTS: In the osteoporotic and normal bone groups, the screw with 50% of maximum torque had the highest pullout strength, but there was no significant difference compared to the screws with other torques. KEY POINT: This study demonstrates that maximum torque screwing does not result in greater resistance to extraction and increases the risk of slipped wires. Screw pull-out resistance is not a major cause of internal fixation failure, as only in rare cases do plate screws experience the vertical forces that lead to pull-out. STUDY 2: METHODS: Pre-drilled holes were made in synthetic and cadaveric bone and self-tapping screws of 3.5 mm with spacers were screwed into the same holes in one and five occasions to measure the screw’s resistance to extraction. RESULTS: In all specimens, the pull-out resistance of the screws decreased when they were inserted multiple times in the same bore. KEY POINT: Many cases require repeated screwing in the same nail tract, such as poor fracture repositioning, plate resetting, and screw size and length changes, when it is important to be alert to the fact that the screw’s resistance to extraction and fixation will decrease. Study 3: METHODS: Twenty-five 3.5 mm locking screws were implanted into the foam artificial bone with a standard locking plate and a torque of 1.7 Nm. Multiple angles were implanted to test the effect of different misaligned implantation angles on the bending strength. Results: When the implantation angle increased, the bending strength decreased. Screw implantation angles less than 3° failed by screw deformation. When the implantation angle was greater, it failed by screw pull-out. Point: Although the implantation of non-variable angle locking screws is performed by means of a guide, misclasping can still occur. The surgeon needs to be aware that the mechanical fixation strength of the locking screw is reduced after implanting the screw more than 3° off the vertical axis.