Does Increased Cortical Screw Adhesion on the Far Cortex Result in Higher Resistance against Pull-Out?

Aim: Cortical screws exert compression on the fracture line by applying pressure to the surrounding cortex, while the screw moves within the bone structure through the threads as a result of cyclic movement. To achieve this compression, the cortical screw threads must adhere to the far cortex. The aim of this biomechanical study was to biomechanically evaluate the effect of varying degrees of contact with the far cortex on the resistance against pull-out and to determine the ideal amount of cortical adhesion. Material and Method: A biomechanical study was conducted on the diaphyseal portions of 12 synthetic femur bones without the formation of any fracture models. The synthetic bones were initially divided into three groups, as follows: partial contact with the far cortex, full contact with the far cortex, and passed through the far cortex. The prepared models were subjected to testing, and after the bone was affixed within the compression device, the head of the screw on the bone was grasped with the aid of a tool, and a tensile force was applied to the cortical screw head until pull-out (load to failure). Results: A significant difference was observed when the pull-out strengths were compared between groups (p=0.021). Post-hoc analyses revealed that this statistical difference was due to the group in which at least three threads passed through the far cortex. Conclusion: When choosing the cortical screw length, a stronger pull-out resistance can be expected with a longer cortical screw length and passing the distal end through the far cortex. However, this should be decided taking into account the characteristics of the anatomical region to be treated, the nearby neurovascular structures, and the risk of tendon-soft tissue irritation.