Reduction of geometric misfit in straight antegrade humeral nailing by evaluating the effect of entry point angulation using a three-dimensional computational analysis.

BACKGROUND:Straight antegrade intramedullary nails are generally inserted utilising the apex as the surgical entry point in accordance with the mechanical axis of the bone. Our objective is to optimise the bone-nail fit in intramedullary nailing by subjecting the surgical entry point to varying angulations in both the mediolateral and anterior-posterior directions via a quantitative fit assessment in each configuration to identify the optimal angulation, defined as the angulation with the lowest occurrence of thin-out to improve nail fitting within the humerus. METHODS:Computed tomography (CT) scans from 10 cadaveric humeri models were used to generate three-dimensional bone models. The centreline profile of each humerus model was determined by dividing the humerus into multiple slices and identifying its respective centroid. The guidewire and nail models were then established and inserted into the humerus using the apex as the standard entry point. The bone-nail fit was measured utilising three fit quantification parameters: thin-out distance, nail protrusion volume into the cortical shell and deviation distance (top, middle, bottom) between the nail's longitudinal axis and medullary cavity centroid. FINDINGS:Results revealed a statistically significant association between angulation and occurrence of thin-out (p < .001) and showed that the optimally angulated entry point resulted in decreased cortical breach across the nail insertion depth compared to the standard entry point. INTERPRETATION:Our findings suggested that the current straight nail design may require further modifications to optimise the nail trajectory within the medullary canal by decreasing the bone-nail geometric mismatch to potentially maximise its working length.