Abstract
Aims
The aim of this study was to optimize screw hole placement in an acetabulum cup implant to improve secondary initial fixation by identifying the region of thickest acetabulum bone. The “scratch fit” of modern acetabular cup implants with highly porous coatings is often adequate for initial fixation in primary total hip arthroplasty. Initial fixation must limit micromotion to acceptable levels to facilitate osseointegration and long term cup stability. Secondary initial fixation can be required in cases with poor bone quality or bone loss and is commonly achieved with bone screws and a cup implant with multiple screw holes. To provide maximum secondary initial fixation, the cup screw holes should be positioned to allow access to the limited region of thick pelvic bone.
Patients and Methods
Through a partnership with Materialise, a statistical shape model of the pelvis was created utilizing 80 CT scans (36 female, 44 male). To limit the effect of variation outside the area of cup implant fixation, the shape model includes only the inferior pelvis (cut off at the greater sciatic notch and above the anterior inferior iliac spine).
A virtual implantation protocol was developed which creates instances of the pelvis shape model that accurately simulate the intraoperative reaming of the acetabulum to accept the cup implant. First a sphere is best fit to the native acetabulum and the diameter is rounded to the nearest whole millimeter. The diameter of the best fit sphere is increased by 1mm to simulate bone removal during the spherical reaming procedure. The sphere is translated medially and superiorly such that it is tangent to the teardrop and removes 2mm of superior acetabulum. The sphere is used to perform a Boolean subtraction from the shape model to create a virtually reamed pelvis shape model.
The Materialise 3-Matic software was used to perform a thickness analysis of the prepared shape models. The output of the thickness analysis is displayed as a color “heat map” where green represents thin bone and red is thick bone. The region of thickest bone was identified and used to drive ideal screw hole placement in the cup implant to access this region.
Results
The analysis finds there is a limited arc of thick bone in the acetabulum that begins superiorly and extends posterior-inferior that accounts for only about 15% of total reamed surface area. Maximum screw purchase is provided when screw holes in the cup implant are placed over this limited region of thick bone. The thickest bone, located superiorly, facilitates the placement of a long bone screw up the iliac column and the posterior-inferior region of thick bone facilitates the placement of additional posterior screws.
Conclusion
The shape model development, virtual implantation protocol, and heat map thickness analysis allowed the placement of bone screw holes directly over the limited region of thick pelvic bone. This allows maximum screw purchase which is important in achieving adequate secondary initial fixation with bone screws.
Disclaimer
Author is an engineer employed by DJO Surgical who funded this study.
