Asymmetric vs symmetric polyethylene inserts in mobile bearing TKA, a clinical and biomechanical study

Objectives This study aims at comparing the effects of symmetric and asymmetric designs for the polyethylene insert currently available for mobile bearing total knee arthroplasty (TKA). The investigation was performed both clinically and biomechanically through finite element analysis and experimental cadaveric test.

Methods The clinical study involved the retrospective analysis of 303 patients with a mobile bearing TKA. All patients received the same femoral and tibial components; concerning the insert, 151 patients received a symmetric design (SD) and 152 an asymmetric design (AD).

The biomechanical study consisted in a 3D finite element model and an experimental cadaveric study.

For the finite element study, a lower leg model was developed, implanted with the same TKAs and analyzed during gait and squat activities; the implant kinematics and bone-stresses were investigated for the two insert solutions.

Five fresh-frozen cadaveric knee specimens, with no history of musculoskeletal problems, were then investigated. The femoral bone of each specimen was clamped to a specific designed frame and optical marker sets were fixed on the tibial and femoral bones and also on the polyethylene mobile bearing insert. Each leg was analyzed during 10 passive flexion-extension cycles from 0 to 120° of flexion and the tibio-femoral and insert kinematics were recorded during the motion. The tests were performed in native and replaced configurations, using both symmetric and asymmetric designs of the insert.

Results and Conclusion The clinical follow-up returned that, 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.

The finite element analysis results showed that the SD induces higher tibial-bone stresses than the AD; both designs led to similar kinematics, comparable to literature, but however SD rotates less on the tray, increasing the relative motion between femoral and insert components, while AD permits greater insert rotation.

Experimental tests results, accordingly, showed that the AD allows a better compliance in intra-extra rotation: the rotation angle between tibia and insert indeed matches the one found in the tibio-femoral while the SD insert returned instead lower results, implying a greater relative motion between femoral and insert components.

The biomechanical analysis thus 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.

Stichwörter biomechanics, clinical, TKA, mobile bearing

Publication History

Article published online:
15 October 2020