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Dynamic finite element analysis simulates post-TKA kinematics
Researchers designed a three-dimensional model using the Meniscal Bearing Knee prosthesis to predict motion and internal stresses.
Italian researchers used finite-element analysis to simulate kinematics and internal stresses following TKA and applied a three-dimensional approach using the Meniscal Bearing Knee prosthesis to predict motion and stresses under various loading conditions.
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The researchers reported favorable results for patients who received the Meniscal Bearing Knee (MBK) prosthesis (Zimmer) in TKA, and modifying the MBK would likely improve flexion and kinematics in some patients, said Andrea Baldini, MD, at the 7th European Federation of National Associations of Orthopaedics and Traumatology Congress in Lisbon, Portugal.
“The classic, so-called ‘implicit finite element (FE) analysis’ consumes a relative position of the femoral component to the polyethelene insert,” said Baldini, an orthopedic surgeon at the First Orthopaedic Clinic, University of Florence, Italy. “For explicit finite element analysis, we mean that it uses force-driven models. It can predict simultaneously the kinematics and the internal stresses. For this reason, it was called ‘dynamic.’”
Flexible body
The tibial insert was modeled as a flexible body with 82,212 noded solid tetrahedral elements, with a Poisson ratio of 0.46. The femoral and tibial components were simulated as rigid structures, Baldini noted.
Measurements included linear soft tissue constraints of 30 N/mm AP and 0.6 N-m/degree rotational displacements. Axial load was 4.9 mm, medially displaced to attain a medially biased (60-40) condylar load allocation, according to the researchers’ abstract.
To simulate gait, stair climbing and deep knee bending with FE models, the researchers obtained waveform information from the proposed International Standards Organization and the literature, Baldini noted.
Overall, the MBK showed “pretty good clinical trial results compared to a gold standard that is the LPS (Legacy Posterior Stabilized, Zimmer) prosthesis,” Baldini said. However, some design changes might improve flexion and guide kinematics. The MBK femoral component was modified using high-flex femoral condyles, which were thicker and with a smaller radius of curvature after 100° of flexion. A minor meniscal modification (reducing the posterior lip of 1.5 mm) yielded almost 13° to 14° of more flexion from the MBK.
A stair descent simulation showed loads of up to more than 40,000 N, with contact stresses of less than 14 Mpa in the meniscus, Baldini reported.
Peak contact stresses
Peak contact stresses for each activity evaluated were less than 14 Mpa for both the original and modified versions of the MBK. A kinematic analysis showed a similar number of displacements for the various designs of the standard MBK during gait, with an average rotation of 3.7° and an average AP glide of 2.5 mm. In simulated stair climbing and deep knee bending, the PS model of the MBK showed a more reproducible pattern of posterior rollback in flexion without increasing contact stresses, he noted.
“But if you see, each patient’s kinemetic is not so reproducible and not so consistent, so our purpose was to reach a consistent and reproducible pattern for application,” Baldini said.
The researchers created a finite element mesh. They attached the femur with a tibial post, rods for hip flexion and a meniscal insert driven by the femur.
Again, the results were positive. The simulation showed stresses of less than the 12 Mpa considered acceptable. The stair simulation again showed less than 14 Mpa for the modified implant. The deep-knee-bend simulation again showed less than 10 Mpa. The stresses were mainly concentrated around the post, depending on the position of the polyethylene, Baldini said.
“So, the developmental value of guided mobile total knee arthroplasty is very attractive although technically difficult,” Baldini said. “Designing a feature of a guiding mechanism was highly informative and improved flexion. New medical research using computational analysis with finite elements an in vitro simulations may offer an earlier insight.”
For more information:
- Baldini A, Aglietti P, Carfagni M, et al. Explicit finite element analysis of Meniscal Bearing Knee under various simulated conditions. #F739. Presented at the 7th European Federation of National Associations of Orthopaedics and Traumatology Congress. June 4-7, 2005. Lisbon, Portugal.