The Effect of the Coronal Alignment of Extension Stem on Component Displacement and Stress During Total Knee Arthroplasty: A Finite Element Study

OBJECTIVE: The biomechanical interaction between coronal alignment and short extension stem use in total knee arthroplasty (TKA) remains insufficiently understood, particularly regarding the balance between tibial component stability and stress shielding. This study aims to investigate the effects of varying bone cut angles on the tibial side and the use of a short extension stem on tibial component displacement and stress distribution. Additionally, it will examine the stresses experienced by both cortical and cancellous bone during primary TKA, employing three-dimensional finite element analysis and elucidating the biomechanical effects of combining kinematic alignment (KA) with a short extension stem in TKA. METHODS: A healthy adult female volunteer was selected to create a tibial model of the left knee joint utilizing three-dimensional reconstruction and finite element analysis software. Six finite element models were developed based on a standard tibial plateau prosthesis and a tibial prosthesis with a 30 mm short extension stem. These models were categorized according to mechanical alignment (MA), KA, and a 3° valgus alignment. The simulations were designed to replicate the loads and constraints experienced during weight-bearing activities of the lower limb. Subsequently, we analyzed the displacement and stress distribution of the tibial prosthesis and examined the stress distribution within both the tibial cortical bone and cancellous bone. RESULT: In the KA model, there was a notable increase in displacement, an elevation in mean cortical bone Von Mises stress, and a reduction in mean component Von Mises stress when compared to both the MA and the 3° valgus alignment model. The design of the short extension stem effectively decreased the displacement of the prosthesis but led to increased stress on the prosthesis itself. Additionally, this design led to a reduction in stress on the cortical bone, suggesting a more pronounced stress-shielding effect. CONCLUSION: The application of KA in TKA will increase prosthetic displacement and reduce the stress-shielding effect, while the application of short extension stem will reduce prosthetic displacement and increase the stress-shielding effect. The combination of the KA and short extension stem may play a complementary role, but this needs further research to confirm.