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
