gms | German Medical Science

Objectives: Achieving adequate fixation is a fundamental prerequisite in revision total knee arthroplasty (TKA); consequently, surgeons need to determine the correct set-up for each patient, choosing from a wide array of stem solutions. Several designs among which to choose are currently available on the market, yet no evidence-based biomechanical guideline is available up to date to quantitatively lead the selection among the different options; this study therefore aims to address this lack by analyzing different configurations via finite elements analysis and comparing their relative performances in terms of bone stress distribution in different regions of interest.

Methods: A Posterior Stabilized TKA was taken into consideration and, relying on currently available products, different femoral and tibial stems were designed and virtually implanted in a previously validated finite elements knee model. The parameters addressed in the design of the different features included stem design characteristics as length/diameters and shape (straight and bowed stem) (see Figure 1 [Fig. 1]), fixation technique (press-fit or cemented stem) and eventual added features (effect of slots and flutes). For the tibial stem, instead, only two different lengths were taken into account (with the two fixation approaches). A squat activity up to 120° flexion was performed for each analyzed configuration, and bone stress and Risk of Fracture (RF) were analyzed in different regions of interest.

Results and conclusion: Considering the femoral stem, results indicated that all parameters had an influence on bone stress distribution; maximum von Mises stress and RF were always located close to the tip of the stem and were higher in case flutes were present; the model implanted with bowed stems returned instead lower stresses, same as found with slotted stems. Long stems generated stress-shielding in the distal bone. Regarding the tibial stem, cemented stems showed lower micromotions both at the bone-tibial tray interface and at the stem tip compared to press-fit stems, thus reducing the risk of implant loosening.

The results of this study demonstrated that anatomical shapes and slots reduce the bone stress and the risk of fracture, while flutes accomplish an opposite effect; no relevant differences were found in this regard when alternating between cemented and press-fit stem configurations. Cemented tibial stems reduce antero-posterior micromotions playing an important role in preventing implant loosening.