Biomechanical Comparison of the Roof Step Cut Technique with the Bulk Bone Graft Technique During Total Hip Arthroplasty for Hip Dysplasia: a Finite Element Analysis.

PURPOSE OF THE STUDY Different techniques have been reported to reconstruct the defi cient acetabulum during total hip arthroplasty (THA). The purpose of this study was to compare the biomechanical features of the bulk bone graft (BBG) technique (Harris acetabular plasty) and the Roof Step Cut (RSC) technique using fi nite element analysis. MATERIAL AND METHODS Based on a female patient's dysplastic hip CT scan, 3D models were assembled according to the two techniques. For the Harris technique, an irregular BBG was sculpted from a solid sphere, while for the RSC technique, the graft was sculpted into a step-cut shape with a similar size. Each graft was fi xed with two compression screws at two different angles (0° and 45°). Four fi nite element models were used to compare the von Mises stress distribution and total deformation of the grafts and the screws. The pressure and sliding distances of the contacts between the bone graft, metal cup and acetabular host bone were also analyzed. RESULTS For both of the bone grafts and the screws, compared to the Harris models, the maximum stress of the RSC models was signifi cantly lower (16.56 MPa, 25.50 MPa vs 97.13 MPa, 112.72 MPa) and the total deformation was signifi cantly smaller (0.0096 mm, 0.0089 mm vs 0.022 mm, 0.018 mm). 45° inserted screws generated higher stress at the end of the screws and on the outside of the bone graft. In case of 0° inserted screws, the maximum value was mainly located in the middle of the screws, inside the screw channel as well as at the contact area between the graft and the host bone. At all analyzed contacts, the RSC technique shows signifi cantly lower pressure and sliding distances, irrelevant to the screw's insertion angle. In comparison, the model of BBG with 45° screws showed a signifi cant sliding effect and higher contact pressure. CONCLUSIONS Compared with the BBG technique, the step-shaped graft of the RSC technique could signifi cantly reduce the maximum stress and deformation of the graft and the screws, and decrease the pressure and sliding distance between the bone graft, metal cup and the acetabular host bone. The angle of screw placement affects the location of stress and deformation. Key words: developmental dysplasia of the hip, total hip arthroplasty, acetabular reconstruction, bone graft, fi nite element analysis.