Review Question - QID 218124

Early designs that include the use of patch porous coating, such as that exhibited in implant "A," instead of continuous porous coating, as shown in implant "B," have been associated with an increased incidence of metaphyseal osteolysis.

Although cemented total knee arthroplasty (TKA) continues to be the gold standard in the United States, there are patient populations that see higher failure rates with cemented TKAs, including the obese, morbidly obese, and younger active males. Thus, cementless TKA is on the rise because of the postulated potential benefits of long-term biologic fixation similar to that seen in cementless total hip arthroplasty. The design of some implants, however, includes transition points between the highly porous metal and smooth implant surfaces (implant "A"). It has been shown that fibrous bridges form along these transition points, conducting wear debris via "effective joint spaces," thereby contributing to increased metaphyseal and diaphyseal osteolysis. The benefits may still outweigh the risks of use in certain patient populations, but longer-term data is still needed and it is recommended that implants with circumferential, fully porous coated designs (implant "B") are used to effectively seal off the tibial metaphysis from particulate debris and reduce the risk of subsequent osteolysis.

Menighini, et al. provide a review of cementless fixation in total knee arthroplasty. The authors note the configuration of porous coating on the tibial tray has been shown to affect the incidence of tibial osteolysis, with those containing patch-porous coating or smooth metal tracks separating pads of porous coating on the undersurface of the tibial tray allowing a path of minimal resistance for the egress of particulate wear debris and the subsequent development of osteolysis in the proximal tibia. They cite Whiteside, et al. who reported a high rate of osteolysis in the first-generation Ortholoc Modular tibial component (Wright Medical Technology, Arlington, Tenn), which contained patch-porous coating separated by smooth metal bridges on the tibial undersurface. However, when compared with the results of the next-generation Ortholoc II tibial component (Wright Medical Technology) with a continuous porous coating, there were no demonstrated cases of osteolysis in 675 TKAs without cement.

Helm, et al. published on the preliminary results of using an uncemented trabecular metal tibial component in total knee arthroplasty. The authors prospectively reviewed a cohort of 105 consecutive primary total knee arthroplasties using an uncemented highly porous metal tibial component at a minimum 3-year follow-up and found that there was a significant improvement in Oxford Knee scores and Short Form-12 scores postoperatively with no radiolucency seen at the implant-bone interface on any postoperative radiograph. They concluded that the 3-year results using this prosthesis are as good as those published for the commonly used cemented prostheses but that longer follow-up is required to see whether these results are maintained over time.

Miller, et al. published on the results of cemented versus cementless total knee arthroplasty in a retrospectively reviewed cohort of 400 patients. The authors found that there was no statistical difference in age, BMI, or preoperative Knee Society Scores between the 2 groups, that patients in both groups had a similar incidence of postoperative complications, and that the cementless tibial baseplates did demonstrate areas of radiographically increased bone density at the pegs of the tibial baseplate. They concluded that the use of a highly porous cementless tibial baseplate may be beneficial in providing long-term durable biologic fixation similar to the success of cementless total hip arthroplasty.

Figure A is a standing, AP radiograph of the bilateral knees showing varus pattern, bone-on-bone arthritis. Figure B shows implant "A" which is the first-generation Ortholoc Modular tibial component (Wright Medical Technology, Arlington, Tenn) mentioned in Whiteside, et al. above. It contains a patch porous coating separated by smooth metal bridges (red arrow). Implant "B" is the second-generation Ortholoc iteration that contains a fully porous-coated baseplate.

Incorrect Answers:
Answer 1: Implants with patch porous coating have not been shown to loosen earlier than those with continuous porous coating despite the increased risk of osteolysis.
Answer 2: Highly porous metal has not shown an increased risk of metal hypersensitivity compared to standard or revision cemented components.
Answer 4: Though an increased risk of intra-operative fracture exists with cementless components, especially if bone quality is poor, the postoperative risk of periprosthetic fracture is not increased specifically due to the transition points in the implant design.
Answer 5: Tibiofemoral instability has not been shown to result from this subset of cementless component design features.