A guide module for placement of posterior column lag screws in anterior-posterior column acetabular plate ——design by three-dimensional models

Abstract

Objective To report our self-designed guide module for placement of posterior column lag screws in anterior-posterior column acetabular plate using CT reconstruction data. Methods The CT scan data of 50 normal adult pelves were collected from February 2012 to April 2013, involving 30 males and 20 females with an average age of 46.4 years (range, from 25 to 69 years). The data were imported into Mimics 10.01 software for reconstruction of semi-pelvic models. Virtual cylindrical implants were placed in-traosseously in both the left and the right posterior columns. The perpendicular distance (OP) from the insertion point O of the virtual cylindrical implant to the arcuate margin (P) and the distance (PI) from the point P to the point I, the crosspoint of the extension line of the ischial ramus and the arcuate margin were measured respectively. The angle (∠φ) between the direction of screws and the plane of guide module and the angle (∠θ) between the direction of screws and the long axis of guide module were also measured respectively. Results The average length of PI was 0.98±0.13 cm, with 1.08±0.22 cm in females and 0.95±0.27 cm in males. The difference between genders was not statistically significant (P>0.05). The average length of OP was 1.09±0.26 cm, with 1.06±0.29 cm in females and 1.12±0.24 cm in males. The gender difference was not statistically significant either (P>0.05). The mean value of ∠φ was 55.43°±3.64°, with 55.33°±4.00° in females and 55.50°±3.43° in males. The difference between genders was not statistically significant (P>0.05). The ∠θ value in females was 39.21°±2.45° and 35.58°±2.31° in males. The gender difference was statistically significant (P<0.05). Conclusions In design of the guide module, the nail holes should be located about 1 cm away from both the posterior edge and the medial edge, the angle between the screw direction and the module plane should be approximately 39° in females and 35° in males, and the angle between the screw direction and the long axis of the module approximately 55°. Key words: Acetabulum; Fractures, bone; Bone nails; Bone plates; Surgery, computer-assisted