Computer tomographically determined design parameters for optimized fit of an acetabular reconstruction cage

Reinforcement rings are widely used for treatment of large acetabular defects but significant migration due to a lack of implant integration into the bone is a common problem. Although insufficient congruence between implant and acetabular bone has been identified main factor in early implant loosening, there are no reconstruction ring design parameters based upon anatomical data of the pelvis available.

In this study such parameters were calculated on the basis of standardized computer tomograms of the hip region of 10 male and 10 female patients (40 data sets).

The center of rotation, the acetabular entrance plane and the geometry of a simulated cranial and caudal flange of a reconstruction ring were determined. The flanges geometry was defined by the angles between the flanges and the entrance plane, the angle between the flange projections onto the entrance plane, the torsion and the width of each flange depending on the distance to the center of rotation, and the flanges torsion and width at their origin at the acetabular circumference.

An optimal alignment between reconstruction ring and the periacetabular bone could be achieved with a medial angulation of 19.9 degrees +/- 19.4 degrees of the cranial and 14.7 degrees +/- 7.1 degrees of the caudal flange against the entrance plane. The angle between the flanges projections onto the entrance plane was calculated to be 162.2 degrees +/- 6.4 degrees . At its origin from the acetabular circumference the cranial flange is twisted 28.5 degrees , the caudal flange -0.8 degrees against the entrance plane. The maximal flanges widths were calculated to 21-54 mm for the cranial and 22-25 mm for the caudal flange depending on the distance from the acetabular circumference.

The given design parameters may result in optimized implants respecting the pelvic anatomy and possibly providing improved fixation.