Asymmetric polyethylene inserts promote favorable kinematics and better clinical outcome compared to symmetric inserts in a mobile bearing total knee arthroplasty
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This study aims at comparing the effects of symmetric and asymmetric designs for the polyethylene insert currently available and also for mobile bearing total knee arthroplasty (TKA). The investigation was performed both clinically and biomechanically through finite element analysis.
303 patients, with a mobile bearing TKA, were analyzed retrospectively. All patients received the same femoral and tibial components; for the insert, 151 patients received a symmetric design (SD) and 152 an asymmetric design (AD). Additionally, a 3D finite element model of a lower leg was developed, resurfaced with the same TKAs and analysed during gait and squat activities. TKA kinematics, and bone-stresses were investigated for the two insert solutions.
After surgery, patients’ average flexion improved from 105°, with 5° of preoperative extension deficit, to 120° (AD-group) and 115° (SD-group) at the latest follow-up. There was no postoperative extension deficit. No pain affected the AD-group, while an antero-lateral pain was reported in some patients of the SD-group. Patients of the AD-group presented a better ability to perform certain physical routines. Biomechanically, the SD induced higher tibial-bone stresses than the AD. Both designs replicated similar kinematics, comparable to literature. However, SD rotates more on the tray, reducing the motion between femoral and polyethylene components, while AD permits greater insert rotation.
The biomechanical analysis justifies the clinical findings. TKA kinematics is similar for the two designs, although the asymmetric solution shows less bone stress, thus resulting as more suitable to be cemented, avoiding lift-off issues, inducing less pain. Clinically, and biomechanically, an asymmetric mobile bearing insert could be a valid alternative to symmetric mobile bearing insert.
Level of evidence
Case–control study retrospective comparative study, III.
KeywordsTKA Mobile bearing Asymmetric insert Symmetric insert Insert congruency Biomechanics Kinematics
This work was supported by FNRS (Fonds National de la Recherche Scientifique, CDR 19545501, CDR 29155446) and by FER ULB (Fonds d’Encouragement à la Recherche, FER 2014, FER 2017). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Compliance with ethical standards
Conflict of interest
All authors declare that they have no conflict of interest.
All procedures performed in this study involving human participants were in accordance with the ethical standard of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Every patient gave his/her written informed consent to have his/her clinical records later used for this prospective study.
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