RESUMEN
Patient-specific flanged acetabular components are utilized to treat failed total hip arthroplasties with large acetabular defects. Previous clinical studies from our institution showed that these implants tend to lateralize the acetabular center of rotation. However, the clinical impact of lateralization on implant survivorship is debated. Our goal was to develop a finite element model to quantify how lateralization of the native hip center affects periprosthetic strain and implant-bone micromotion distributions in a static level gait loading condition. To build the model, we computationally created a superomedial acetabular defect in a computed tomography 3D reconstruction of a native pelvis and designed a flanged acetabular implant to address this simulated bone defect. We modeled two implants, one with ~1 cm and a second with ~2 cm of hip center lateralization. We applied the maximum hip contact force and corresponding abductor force observed during level gait. The resulting strains were compared to bone fatigue strength (0.3% strain) and the micromotions were compared to the threshold for bone ingrowth (20 µm). Overall, the model demonstrated that the additional lateralization only slightly increased the area of bone at risk of failure and decreased the areas compatible with bone ingrowth. This computational study of patient-specific acetabular implants establishes the utility of our modeling approach. Further refinement will yield a model that can explore a multitude of variables and could be used to develop a biomechanically-based acetabular bone loss classification system to guide the development of patient-specific implants in the treatment of large acetabular bone defects.
RESUMEN
Aims: When performing revision total hip arthroplasty using diaphyseal-engaging titanium tapered stems (TTS), the recommended 3 to 4 cm of stem-cortical diaphyseal contact may not be available. In challenging cases such as these with only 2 cm of contact, can sufficient axial stability be achieved and what is the benefit of a prophylactic cable? This study sought to determine, first, whether a prophylactic cable allows for sufficient axial stability when the contact length is 2 cm, and second, if differing TTS taper angles (2° vs 3.5°) impact these results. Methods: A biomechanical matched-pair cadaveric study was designed using six matched pairs of human fresh cadaveric femora prepared so that 2 cm of diaphyseal bone engaged with 2° (right femora) or 3.5° (left femora) TTS. Before impaction, three matched pairs received a single 100 lb-tensioned prophylactic beaded cable; the remaining three matched pairs received no cable adjuncts. Specimens underwent stepwise axial loading to 2600 N or until failure, defined as stem subsidence > 5 mm. Results: All specimens without cable adjuncts (6/6 femora) failed during axial testing, while all specimens with a prophylactic cable (6/6) successfully resisted axial load, regardless of taper angle. In total, four of the failed specimens experienced proximal longitudinal fractures, three of which occurred with the higher 3.5° TTS. One fracture occurred in a 3.5° TTS with a prophylactic cable yet passed axial testing, subsiding < 5 mm. Among specimens with a prophylactic cable, the 3.5° TTS resulted in lower mean subsidence (0.5 mm (SD 0.8)) compared with the 2° TTS (2.4 mm (SD 1.8)). Conclusion: A single prophylactic beaded cable dramatically improved initial axial stability when stem-cortex contact length was 2 cm. All implants failed secondary to fracture or subsidence > 5 mm when a prophylactic cable was not used. A higher taper angle appears to decrease the magnitude of subsidence but increased the fracture risk. The fracture risk was mitigated by the use of a prophylactic cable.
