The influence of tibial component intramedullary stems and implant-cortex contact on the strain distribution of the proximal tibia following total knee arthroplasty. An in vitro study.

High implant-bone stress levels, particularly on rather weak cancellous bone, have been implicated as the predominant cause of aseptic tibial component loosening, the most frequent cause of total knee arthroplasty (TKA) failure. This strain gauge study on six fresh cadaver specimens has revealed that loss of tibial component-cortex contact resulted in a 33%-60% decrease in principal strain values in the proximal tibia. The implication of this finding is that in the absence of implant-cortex contact, the cancellous bone of the proximal tibia must assume the increased load, with the risk of implant loosening and/or sinkage. This study has also demonstrated that the use of tibial components with intramedullary stems was accompanied by marked stress shielding of the proximal tibial cortex over the length of the given stem. This observation refutes the claim that such intramedullary stems are non-weight-bearing and points out the potential risks of cortical atrophy and tibial fracture at the tip of the stem when such implants are used. Although caution must be exercised in extrapolating in vitro laboratory data to clinical practice, this study favors the use of TKA tibial components that rest on the cortical bone and have either no stems or only short intramedullary stems.