Einfluss der intrinsischen Stabilität des Polyethylen-Inlays auf den patellofemoralen Druck und die Quadricepszugkraft nach Implantation einer Knietotalendoprothese

Anterior knee pain after Total Knee Arthroplasty (TKA) is one of the most common patient complaints leading to numerous revision procedures. Changing the kinematic behaviour of patello-femoral joint with increased patello-femoral contact pressure and quadriceps extension force could add to its sequelae. The aim of this in vitro study was to compare the influence of different prosthesis designs on patello-femoral contact mechanics and quadriceps extension force after implantation of cruciate retaining (CR flat, DD) and posterior stabilized (PS) TKA under dynamic conditions. Furthermore, the influence of the PCL and both height and conformity of the inlay on knee kinematics were investigated. Methods: 8 fresh frozen human cadaveric knee specimens underwent testing in a kinematic device simulating an isokinetic knee extension cycle from 120° of flexion to full extension. Knee motion was driven by a hydraulic cylinder applying sufficient force to the quadriceps tendon to produce an extension moment of 31 Nm. Patello-femoral contact pressure was measured by means of a pressure sensitive film (Tekscan®, Inc., Boston, USA). Both patello-femoral contact pressure and quadriceps extension force were recorded after implantation of a TKA (Genesis II, Smith & Nephew, Schenefeld, Germany) with application of different polyethylene inlays. First, different cruciate retaining (CR) designs were tested before (CR 11, CR 9, DD) and after (CR 11, DD) resection of the PCL. The CR TKA was then removed to be replaced by a posterior stabilized (PS) design and measurements were repeated with an 11 mm PE inlay. The patella remained unresurfaced. A paired sampled t-test to compare means (significance, P ≤ 0.05) was used for statistical analysis. Results: After implantation of the posterior stabilized design (PS) patello-femoral contact kinematics were comparable to physiological knee kinematics whereas cruciate retaining TKA showed higher patello-femoral contact pressures (mean contact pressure: PS: 3,58 ± 1,25 MPa; CR 11: 4,31 ± 1,40 MPa; CR 9: 4,23 ± 1,40 MPa; DD: 4,27 ± 1,34 MPa; CR 11 without PCL: 4,18 ± 1,26 MPa; DD without PCL: 3,99 ± 1,44 MPa respectively peak pressure: PS: 6,12 ± 2,37 MPa; CR 11: 7,17 ± 2,41 MPa; CR 9: 7,05 ± 2,45 MPa; DD: 7,12 ± 2,53 MPa; CR 11 without PCL: 6,89 ± 2,30 MPa; DD without PCL: 6,63 ± 2,52 MPa). The patello-femoral contact pressure in PS TKA was mostly significantly lower compared with the cruciate retaining (CR) design (p ≤ 0,006, p ≤ 0,02, p ≤ 0,01, p ≤ 0,03 and p ≤ 0,18 respectively p ≤ 0,02, p ≤ 0,02, p ≤ 0,01, p ≤ 0,07 and p ≤ 0,39). There was a trend to a more physiological patello-femoral contact pressure area (medialized, proximalized). Quadriceps extension force decreased significantly with the PS TKA compared to the CR type after resection of the PCL (PS: 1131 ± 108 N; CR 11 without PCL: 1203 ± 92 N; DD without PCL: 1192 ± 80 N; p ≤ 0,01, p ≤ 0,01). Regarding to maximal quadriceps extension force PS TKA decreased significantly in comparison with all other TKA designs (PS: 1560 ± 145 N; CR 11: 1683 ± 156 N; CR 9: 1712 ± 157 N; DD: 1685 ± 175 N; CR 11 without PCL: 1729 ± 162 N; DD without PCL: 1689 ± 123 N; p ≤ 0,04, p ≤ 0,008, p ≤ 0,02, p ≤ 0,01 und p ≤ 0,02). Height (11 mm versus 9 mm) and conformity (CR flat versus DD) of the polyethylene inlay did not influence knee kinematics whereas integrity of the PCL affected knee kinematics. After resection of the PCL in cruciate retaining TKA (CR flat 11/9, DD) quadriceps extension force increased significantly (CR 11: 1147 ± 88 N; CR 9: 1151 ± 83 N; DD: 1150 ± 98 N; CR 11 without PCL: 1203 ± 92 N; DD without PCL: 1192 ± 80 N; CR 11 without PCL versus CR 11: p ≤ 0,009; CR 11 without PCL versus CR 9: p ≤ 0,02 respectively DD without PCL versus CR 11: p ≤ 0,05; DD without PCL versus CR 9: p ≤ 0,01; DD without PCL versus DD: p ≤ 0,03). An effect on patello-femoral kinematics could not be detected. Conclusions: The results of this in vitro study suggest that a posterior stabilized TKA design can reproduce more physiological knee kinematics in comparison with a cruciate retaining design. It is hypothesized that this could be due to a better reproducible rollback of a PS design. High conforming inlays (DD) which are supposed to improve ap-stability in case of an insufficient PCL did not show physiological knee kinematics after resection of the PCL. Height and conformity of the inlay could not influence knee mechanics whereas the PCL is important for ap-stability after implantation of a TKA. In case of insufficiency of the PCL or instability of the knee joint posterior stabilized TKA designs therefore should be prefered to cruciate retaining TKA.