gms | German Medical Science

Objectives: Different Total Knee Arthroplasty (TKA) implants are currently available on the orthopedic market, and since this variety involves different levels of constraint (each with its biomechanical consequences on the joint performances) a biomechanical comparative study could represent a helpful tool for the clinical decision-making process. Among the TKA models available, significant role is covered by Cruciate Retaining (CR) prosthesis, Posterior Stabilized (PS) and Condylar Constrained Knee (CCK) ones. A further characterization is made between Mobile-Bearing (MB) and Fixed-Bearing (FB) implants, depending on the insert-tibial tray level of constraint. Among the MB, a further distinction between standard and ultra-congruent inserts is made. To compare the knee biomechanics induced by different levels of constraint, this study analyzed 4 configurations from the same family of products (to avoid the effects induced by changes in brand-related design) using as boundary conditions the typical knee joint daily activities.

Methods: Finite element analysis was used for the knee model, developed from an already validated one. The TKA family analyzed counts 4 types of designs: 2 MB CR (with a standard insert and with an ultra-congruent one), 1 FB PS and 1 FB CCK. The different designs were incorporated in the 3D finite element model of the lower leg and analyzed in 3 different configurations: gait landing and taking-off phases and chair-rising. Implant kinetics (contact areas and contact pressures, polyethylene and tibial bone stresses) were calculated at the different loading conditions for each design and compared among configurations.

Results and conclusion: Bone stress distribution in the different regions of interest of the tibia were relatively homogeneous regardless of the type of design used. The main difference in terms of contact area was observed between the MB and FB models: their result are different in all the analyzed loading conditions, with the MB CR ultra-congruent design presenting the highest contact area value (almost 3 times higher than the FB inserts). Comparing biomechanically the different level of constraint it was possible to highlight differences in terms of implant and bone stress, with the main relevant difference being between the mobile and fixed bearing models. Moreover, the different models exhibited a symmetrical medial and lateral distribution of the contact areas, which is not always common among all the currently available prosthesis (i.e. medial pivot designs). In each configuration, the distribution of the stress on the different zones of the tibia (proximal, distal, cortical, total) is relatively homogeneous between the models. Changing the prosthetic implant would therefore not induce a big variation of the stress distribution in the different regions of the tibial bone, while it would however significantly change the distribution of stress at the interface between the prosthetic components.