S Fuchs-Winkelmann, T Heyse (Marburg, Allemagne)
A biomechanical study was carried out in order to compare pressure as well as quadriceps power for knee extension of different inlays for total knee replacements, depending on the design of the prosthesis as well as the state of the posterior cruciate ligament.
Method and material
Eight knee joints of human cadavers were used. A genesis ii total knee was implanted and varying inlays were applied: cruciate retaining (conventional cr and highly conforming dd) and posterior stabilized (ps) (smith & nephew, memphis, tn, usa). The pressure measurement was carried out with the k-scan 4000 (tekscan, boston, ma, usa). The testing was conducted with the hannover knee joint simulator. A simulation of an isokinetic extension experiment of 120° of knee flexion until full extension at a constant extension moment of 31 nm was carried out. Mean and peak retropatellar contact pressures as well as the quadriceps power for knee extension were documented in relation to the design of the prosthesis and the state of the posterior cruciate ligament (pcl). Quad power was measured by a low load cell
Results: lowest retropatellar contact pressures were shown for the ps inlays. The lowest quadriceps power required for knee extension was found for the ps implant, especially in the pcl deficient situation.
In this biomechanical study, femoropatellar mean and peak pressure after implantation of a total knee replacement showed lower results for the ps prosthesis in comparison with the cruciate retaining designs. It still needs to be clinically proven, if this leads to a lower incidence of postoperative patellar pain with the ps design. Also the needed quadriceps power was lower for ps in comparison with both cruciate retaining designs. For daily clinical practice can therefore be concluded that a ps-inlay is preferable at least for patients with an insufficient pcl. A highly conforming inlay like the dd showed no advantages at an insufficient pcl.
Whether these results will influence the decision in favour of a ps-inlay in the daily clinical practice will have to be further examined, as for such a decision various different factors have to be considered.