gms | German Medical Science

Objectives: The role of PCL management and the choice of the polyethylene insert design in total knee arthroplasty are still debated in literature. The PCL is considered one of the primary stabilizers of the joint and its retention may influence knee stability, kinematic, proprioception and it may reduce the shear forces on the tibia. Among the different insert designs, a higher congruency of the insert was developed in order to provide a more physiological motion of the implant and to correct small tibial rotational misalignment, minimizing polyethylene wear.

To check this, the purpose of this analysis was to analyze the biomechanics of a fix and a mobile bearing total knee arthroplasty with conventional and ultra-congruent insert during squat activities, using finite element analysis (FEA), and to check the performance in a patient with a healthy PCL and in a patient with a deficient PCL.

Methods: The finite element model was based on an already validated and published knee model. A GEMINI SL fixed bearing CR (FB) and a GEMINI SL mobile bearing CR (MB) (Waldemar Link GmbH & Co) were selected for this study and implanted following the proper indication provided by the manufacturer. A 10 s loaded deep squat, up to 120°, was simulated. The boundary conditions of this movement, in terms of flexion-extension angle, axial and anterior-posterior force, were taken accordingly to previously experimental activities.

Results and conclusion: Performing the squat motor task, the MB design was able to complete the task, reaching 120° with both PCL condition; on the contrary, FB design was able to accomplish the task only with a healthy PCL. In the case of a deficient PCL it dislocates at 75 degrees of flexion.

The MB insert is characterized by a higher congruency with respect the FB design, with a 4.5 times higher average contact areas. The congruency is also maintained for the deficient PCL configuration.

The MB insert is able to guide the kinematics even with a PCL deficient, with mainly effect of an increase of anterior translation but keeping almost the same internal-external femoral rotation. On the contrary, the FB insert, with a lower congruency is not able to stabilize the joint inducing an irregular kinematic pattern both in terms of antero-posterior rotation and translation.

These kinematics trend are also similar to some of the one founded by Walker et al. The intra-rotation pattern, especially for the mobile bearing design it is very similar to the one reported in the literature by Victor et al analyzing native knee specimens under similar boundary conditions used in the present activity.

In conclusion, results demonstrated how the high congruency of the MB design is able to guarantee the knee stability even when the PCL is deficient. Contrariwise, the FB insert, with a lower congruency is not able to stabilize the joint inducing an irregular kinematic pattern and enabling the dislocation of the components.