Retrieval Analysis and Finite Element Modeling of Orthopaedic Porous-Coated Implants

Recent Nationwide Inpatient Sample (NIS) studies have shown that implant loosening remains one of the common reasons for revision in total joint replacement. In an effort to reduce loosening caused by long-term breakdown of the cement mantle, manufacturers introduced cementless technologies to provide biological fixation by hard tissue ingrowth at the bone-implant interface. One new material, porous tantalum, has had promising clinical results with well-fixed implants and relatively few reported cases of loosening. The factors that may be causing the lower incidence of aseptic loosening remain unknown. Therefore, the goal of this dissertation was to investigate the factors affecting bone fixation in porous tantalum implants through retrieval analysis and finite element (FE) modeling. Through the Implant Research Center's retrieval program, the proportion of implants revised for aseptic loosening was compared between the porous tantalum implants and historical porous-coated implants. Retrieval analysis protocols were developed to assess and determine factors that affect bone ingrowth in porous tantalum hip and knee implants. Porous tantalum tibial trays were histologically analyzed to determine locations of fibrous tissue or bone ingrowth. The results showed that the amount of bone observed varied by implant type and location within each implant for retrieved porous tantalum acetabular shells, femoral stems, patellas and tibial trays. Retrieval results showed that modular tibial tray implants had higher bone ingrowth than the monoblock tibial trays. However, the bone ingrowth in the porous tantalum was lower than that observed in historical porous-coated implants. Histological analyses of tibial trays demonstrated bone ingrowth primarily in the superficial depth of the tibial trays, with fibrous tissue also present. The FE models showed that there was more favorable initial implant stability in the modular tibial tray compared to the monoblock implant. This study showed that the lower incidence of aseptic loosening did not appear to be associated with increased bone ingrowth for the retrieved porous tantalum hip and knee implants. Initial FE models showed that lower micromotion for the modular tibial tray compared the monoblock. This dissertation presents a multifaceted approach for analysis of highly porous biomaterials.