A Novel Approach of Customized Pelvic Implant Design Based on Symmetrical Analysis and 3D Printing

Abstract

In pelvic trauma patients, the mismatch of complex geometries between the pelvis and fixation implant is a fundamental cause of unstable and displaced pelvic ring disruption, in which secondary intervention is strongly considered. The geometrical matching in the current customized implant design and clinical practice is through the nonfractured hemi-pelvis for the fractured pelvis. This design philosophy overlooks the anatomical difference between the hemipelves, and further, the geometrical asymmetry at local area still remains unknown. This study analyzed the anatomical asymmetry of a patient's 3D pelvic models from 13 patients. The hemipelves of each patient were registered by using an iterative closet algorithm to an optimum position with minimum deviations. The high deviation regions were summarized between the hemipelves in each case, and a color map was drawn on a hemipelvis model that identified the areas that had a high possibility to be symmetrically different. A severe pelvic trauma case was used to comprehend the approach by designing a 3D printed implant. Each fracture was then registered to the mirrored uninjured hemipelvis by using the same algorithm, and customized fixation implants were designed with reference to the fractured model. The customized fixation plates showed that the implants had lower geometrical deviation when attached onto the re-stitched fracture side than onto the mirrored nonfractured bone. These results indicate that the symmetrical analysis of bone anatomy and the deviation color map can assist with implant selection and customized implant design given the geometrical difference between symmetrical bones. The novel approach provides a scientific reference that improves the accuracy and overall standard of 3D printed implants.

Keywords: 3D printing; anatomical design; customized implant; geometrical fit; pelvic fracture; symmetrical analysis.

FIG. 1.

(A–E) A morphometric symmetrical comparison of hemipelves (A) originally converted 3D pelvis model; (B) separated area of interest; (C) hemipelvis registration referring to the mirrored side; (D) deviation color map, where the green stands for deviations below 1 mm, and the dark blue stands for higher deviation area; (E) high deviation areas (>4 mm), the circled areas stand for deviations higher than 4 mm, the green stands for deviation below 4 mm, and the ignored areas are shaded in pink (looks gray in picture, because the transparent pink is mixed with green). Color images are available online.

FIG. 2.

(A–H) Diagram of the fracture registration technique (A) originally converted 3D pelvis model; (B) separated area of interest; (C) fracture segmentation; (D) fracture registration referring to the mirrored uninjured hemipelvis; (E) registered fractures hold their optimal position and are then merged; (F) customized fixation plate design referring to the restitched fracture pelvis; (G) geometry validation of the designed fixation plate; (H) deviation analysis of customized fixation plate. Color images are available online.

FIG. 3.

Color map of case quantities that have high deviation at the same region. a to l refer to areas labeled in Table 1. The darkest area stands for the highest probability that the deviation between hemipelves is greater than 4 mm, and no case exhibited deviation greater than 4 mm at the gray areas among the 13 cases. Color images are available online.

FIG. 4.

(A, B) Customized fixation plates (A) computer-aid-design fixation plates model; (B) 3D printed fixation plates in Ti6Al4V.