Biomechanical evaluation of the expansive cannulated screw for fixation of femoral neck fractures

Background: Femoral neck fracture is one of the common clinical traumas, especially amongst elder patients. This study aims to test, compare and evaluate the bone-screw interface strengths, the fatigue strengths, and the stabilities of our newly designed expansive cannulated screw (ECS) and the common cannulated compression screw (CCS) in the fixation of femoral neck fracture, which is a summary of recent research. Methods: Twenty-four pairs (48) of fresh femur specimens were randomly divided into four groups with six pairs (12) in each. To simulate one-legged standing, the maximum compressive strength and the single-screw axial pull-out force were compared between the fixed femoral necks treated with two ECSs and two CCSs, two ECSs and three CCSs or three ECSs and three CCSs, respectively. The screws were also subjected to 1 000 000 cycles of a loaded fatigue test and the results were recorded. Findings: When the same number of screws was used, the ECS showed significantly greater maximum compressive strength than the CCS (P < 0.05), but no significant difference in fixation effectiveness was detected between the two ECSs and the three CCSs groups. The maximum axial pull-out strength of the ECS was also significantly greater that of the CCS (P < 0.01); however, there was no sign of fatigue in both the ECS and CCS after 1 000 000 cycles of loaded fatigue test. Interpretation: The ECS shows better fixation performance than the currently and commonly used CCS; under certain circumstances, fixation with two ECSs can achieve the same effect as that with traditional three CCSs.