Introduction: Recent knee prosthesis designs postulate to allow more flexion of the knee without increasing the contact stress on the polyethylene inlay. The study’s purpose was to compare the tibiofemoral contact stress of four different inlay types of a new “high flexion knee prosthesis system under dynamic, physiologic loading conditions in an in vitro test. Thereby, the cruciate retaining and posterior stabilizing designs were compared.
Material and Methods: The TriathlonTM system was implanted into 5 knees of fresh frozen human specimens. The prosthesis system is available with a mobile bearing (MCR) or fixed bearing inlay (FCR). In addition, the posterior stabilized design was although tested in the same manner (MPS=mobile bearing and FPS=fixed bearing inlay). After implantation, specimens were mounted for biomechanical testing in a knee simulator. The test mimicked both the speed and resulting moment exerted during a lower limb isokinetic extension test in a range of tibial motion from 120 degrees flexion to full knee extension. The quadriceps force cylinder was force-controlled provided a constant torque on the lower limb of 31 Nm whereby forces of up to 1500 N were recorded. Inlay motion was measured using an ultrasonic tracking system (CMS 100TM, Zebris GmbH; Isny, Germany). The tibiofemoral peak contact stress was measured using resistive ink, 0.1-mm-thin pressure sensors (Tekscan, Boston), measuring 572 points per compartment at 10 Hz.
Results: Average maximum peak contact stress was 18.8 (+/−6.6 MPa) at 11.5° flexion for the medial inlay (MCR) and 23 MPa (+/−11.7 MPa) at 119.5 deg for the lateral inlay (MCR). For the FCR we measured 23 MPa (+/−9,6 MPa, medial) at 67.5° and 19.7 MPa (+/−8.2 MPa, lateral) at 119.9° knee flexion. The average maximum contact stress on the MPS inlay was 19 MPa (+/− 12.6 MPa, medial) at 7.5° and 20 MPa (+/− 13.4 MPa) at 120°. For the FPS we measured 20.8 MPa (+/−13.1 MPa, medial) at 40.6° and 19 MPa (+/−11.6 MPa, lateral) at 120° knee flexion.
The maximal rotation recorded for the inlay centers was 1.6 ± 1.2 mm at 1.2° knee flexion and 4.3 ± 3.3 mm at 1.3° for the MPS design.
Discussion: This in vitro study characterizes the movement of the mobile bearing inlays of the TriathlonTM system on the tibial baseplate under simulated physiologic loading. With the dynamic ultrasonic tracking device the range of motion and internal/external rotation movement can be accurately determined. The results of this study correlated qualitatively to radiographic measurements. Contact pressures seem not to exceed the maximum contact stress of UHMWPE significantly even at high flexion angles up to 120°. The posterior stabilized design showed no significant lower maximum contact stresses than the cruciate retaining design.
