Quantifying the effect of tibial stem extension length for primary TKA in obese patients: A biomechanical study.

Background: Obesity can cause higher failure rates and inferior clinical outcomes after Total Knee Arthroplasty (TKA). This study aimed to analyze the biomechanical consequences of a short or long-extension stem on primary TKAs in a BMI-dependent manner.

Methods: Several finite element models (FEM) were developed to assess the biomechanical effects of adding stem extension in primary TKA on stress distribution, resistance to sinking, motion, and shear forces at the bone/implant interface as a function of different BMIs.

Results: Adding an extension stem improves stress distribution and resistance to sinking. It also reduces deformations and displacements at the bone/implant interface, avoiding implant tilting.

Conclusion: The FEM showed that a long extension would give better results if maximum unloading and stabilization of the implant were sought. However, a greater risk of stress shielding must be taken into account. This is why a short extension seems to be the most suitable since it homogenizes the mechanical loading and reduces the effects of implant tilting compared to the case without extensions. The addition of an extension stem could be beneficial in the case of patients with high BMIs.

Level of evidence: IV; biomechanical computational study.