Micromotion secondary to axial, torsional, and shear loads in two models of cementless tibial components.

Tibial component loosening and lack of bone ingrowth remain serious problems in uncemented knee arthroplasty. Initial implant stability is the most important factor in achieving bone ingrowth. Eccentric axial loading causing liftoff has been identified as a potential problem, but the role of torsion and shear stability is unclear. This study assesses the micromotion in two uncemented components subjected to eccentric axial (lateral, medial, posterior, anterior), shear, and torsional loads. Four configurations were tested: Tricon M (pegs) (Smith and Nephew Richards, Memphis, TN), Genesis (stem only) (Smith and Nephew Richards), Genesis with screws, and Genesis with pegs. Nine pairs of cadaver tibias were implanted, and cyclic loads were applied. Micromotion was measured with two linear variable differential transformers. Micromotion values for each mode of loading were compared using a one-way analysis of variance. Genesis with screws had the least micromotion for all applied loads (14-33 micrometers), whereas Genesis with stem only has the maximum value of micromotion (27-212 micrometers). Genesis wit and Tricon had intermediate ranges of micromotion (32-121 and 107-140 micrometers, respectively). It is concluded that peg fixation and stem fixation are inadequate and result in significant micromotion. Stem plus screws provides the most stable fixation and may be compatible with bone ingrowth.