Background

Achieving adequate fixation and avoiding fractures are fundamental prerequisites in total knee arthroplasty; consequently, surgeons need to determine the correct set-up for each patient, choosing from a wide array of stem solutions for both femoral and tibial components; these different possibilities are characterized by various lengths, shapes, bone interfaces and features. Several designs are currently available on the market, yet no evidence-based biomechanical guideline is currently available to quantitatively lead the selection among the different options.To help the surgeon in the decision-making process, this study consists in an in-silico analysis of different combinations of stem features, in order to provide a more in depth insight of their influence on the implant outcomes.

Materials And Methods

Relying on currently available products, different stems were designed and applied to a knee prosthesis, then virtually implanted and analyzed using a previously validated finite element model. the following parameters were analyzed:
- length/diameter (120mm/22mm, 160mm/20mm, 220mm/15mm and 280mm/15mm);
- shape (straight and anatomical, i.e. bowed stems),
- cross-section (absence or presence of flutes);
- stem-end (full or presence of slots);
- bone-prosthesis interface (press-fit or cemented stems).
Von Mises bone stress,Risk of Fracture (RF) and implant micromotions were analyzed in different regions of interest during a squat activity up to 120° flexion, in order to address one of the worst-case scenarios.

Results

The results of this study demonstrated that the presence of a femoral stem always induces an increase in bone stress. Proportionally to stem length increasing, the magnitude of the stress peak and the RF rose, the latter going up to 91% for the 280mm stems; the position of this peak is usually situated at the stem tip, introducing a possible location of failure. Long stems furthermore implied the presence of stress-shielding in the distal femur. Bowed stems lead to a reduction of the stress peak at the tip as well as in the overall stress, while in the straight stems a stress concentration can be observed approximatively at the middle of its length.
Regarding the tibial stem, similar outcomes can be found together with the fact that cemented stems showed lower micromotions both at the bone-tibial tray interface and at the stem tip compared to press-fit stems, reducing the risk of implant loosening.
The simulations performed showed that the flutes act as stress risers, increasing the RF in each model incorporating them; opposite behavior is instead referred to the presence of slots.

Conclusions

The results of this study demonstrated that anatomical shapes and slots reduce the bone stress and the risk of fracture, while flutes have an opposite effect acting as stress risers; 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.