A computer-aided system for longer transacetabular screw trajectory research.

Abstract

Objective To overcome the inherent limitations of traditional technique, we promoted that long transacetabular screws could be safely implanted into ilium, pubis and ischium. Methods Twenty 3D hemi-pelvis models were constructed using CT data, and each was divided into four parts: posterior ilium, anterior ilium, pubis and ischium. Each part was undertaken parallel osteotomy and the contour coordinates of each section were recorded. The acetabular cup in each model was defined as a hemisphere with 52 mm diameter. An uniform lattice at equal intervals of 2° in longitude-latitude on the acetabular cup were regard-ed as entry points. A computer aided program was developed to simulate screw trajectories in all directions through every entry point, then select out all the useful entry points and corresponding trajectories. Then the optimum direction of screw trajectory of each entry point was also calculated. Topographic maps of entry zone were delineated in the acetabular quadrant system to present the results. Results According the bone anatomy, the posterior ilium was the most favorable area for the implantation of long screws, secondly the an-terior ilium, thirdly the pubis and minimally the ischium. The entry zone in posterior ilium has the longitude from -148.0° to 29.2°, latitude from 7.5° to 87.3°, and α angle for trajectory direction was from -85.3° to -53.8°,β angle from 46.3° to 60.2°. In anterior ilium has the longitude from -29.0° to 31.4°, latitude from 7.6° to 54.2°, and α angle was from -36.2° to -49.8° ,β angle from 19.7° to 33.8°. In pubis has the longi-tude from 56.4° to 100.6°, latitude from 6.8° to 23.2°, and α angle was from 91.3° to 117.2°, β angle from 10.5° to 17.9°. And the anatomy of ischium was not permitted transacetabular screws to reach the distant 1/3 part of the isehium ramus. Conclusion The long transacetabular screws were permitted to deeper insertion when the trajectories were not perpendicular to the cup wall. The computer-aided system developed in the study was useful for screw trajectory simulation and selection. Key words: Acetabulum; Tomography,X-ray computed; Bone screws