Introduction: This study was initiated to evaluate cortical and cancellous bone density (BD) changes of the acetabulum after cemented and uncemented total hip arthroplasty (THA) using computer tomography (CT)-assisted osteodensitometry in-vivo.

Materials and Methods: 15 cemented ZCA Pfannen cups (Zimmer, USA) (age 78 years) and 21 press-fit Trilogy cups (Zimmer, USA) (age 72 years) were implanted by one surgeon. All hips were investigated by a standardized CT-mode (slice thickness 2 mm, table feed 5 mm, extended CT-scale). 6 CT-scans at the level of the cup and 4 scans above the dome of the cup were analyzed 2 weeks and 2 years after surgery. Cancellous and cortical bone mineral density (BMD) (CaHA mg/ml) were elaborated with a special software tool (CAPPA postOP, CAS Innovations AG, Erlangen).

Results: 2 years after index operation cemented cups showed mild cancellous BD loss (−8% to −20%) and no significant cortical BD changes cranial, significant cortical (−4% to −20%) and cancellous (−16% to −44%) BD loss ventral and no significant BD changes dorsal to the cup.

For press-fit cups we observed highly significant (p< 0,01) cancellous BD loss in all sectors (−17% to −53%), cortical BD loss ventral and dorsal to the cup (−12% to −23%) and very limited BD loss cranial (−4% to −13%) to the cup.

Conclusions CT-assisted osteodensitometry allows a thorough assessment of the actabular bone in-vivo. Different patterns of stress shielding were observed for cemented and press-fit cups. For the press-fit cup high BD loss for both cortical and cancellous bone was observed in all areas adjacent to the pelvic implant, except for cortical BD at the acetabular dome, suggesting fixation of the cup in the cranial cortical bone. Comparetively less BD loss was seen for cemented cups in all sectors, especially for cancellous bone, suggesting a more physiological stress transfer to both cortical and cancellous pelvic bone.