Objectives: The aim of this study was to examine the influence of different prosthesis designs (Deep-Dish (DD) vs. posterior stabilized (PS)) on the patello-femoral pressure. The femoro-patellar pressure depends among other things on the AP stability of the knee joint. The use of DD has been described to be equally applicable with a resected or deficient PCL.

Methods: Fresh frozen human knee specimens (n = 8, 7 male, 1 female) 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. The amount of patellofemoral contact pressure and its distribution was measured by means of a pressure sensitive film (Tekscan^®, Inc., Boston, USA). Patellar contact pressure was examined first after implantation of a cruciate retaining TKA (Genesis II, Smith& Nephew, Memphis, USA). An 11 mm polyethylene (PE) DD insert was tested before and after resection of the PCL. Finally, the femoral component of the CR TKA was removed and replaced by a posterior stabilized (PS) model repeating measurements with an 11 mm PE inlay. The patella was not resurfaced throughout the whole procedure. A paired sampled t-test was applied for comparison of means and considered significant at p ≤ 0.05.

Results: There was no statistical significant difference of patello-femoral peak and mean contact pressures of the DD inlay before and after resection of the PCL. After implantation of the PS TKA peak pressure was significantly lower (Mean: 6.12 ± 2.37 MPa, Range: 10.68 – 3.30 MPa) in comparison with the DD type (7.12 ± 2.53 MPa, 11.94 – 3.55 MPa; p < 0.01) with a preserved PCL. Also the mean contact pressure turned out to be lower with the PS design (p < 0.006; PS: 3.58 ± 1.25 MPa, 5.91 – 2.08 MPa, DD: 4.27 ± 1.34 MPa, 6.66 – 2.18 MPa). The contact area was also significantly smaller with the PS design (p < 0.03, PS: 140.84 ± 40.04 mm2, 188.47 – 65.10 sq mm, DD: 175.97 ± 24.46 sq mm, 222.56 – 142.56 sq mm).

After resection of the PCL differences in contact pressures and contact area between DD and PS failed to reach statistical significance although there was an obvious tendency towards lower pressures with the PS-design.

Conclusions: The results of this study suggest that a posterior stabilized TKA design reduces the retropatellar peak and mean pressure as well as the contact area in comparison with a deep-dish design when the PCL is preserved. The better reproducible rollback with a PS model could serve as a possible explanation. However, this difference is less pronounced when a DD inlay is applied after resection of the PCL. Nevertheless, a PS rather than a DD design is recommended in the PCL deficient knee.

Objective: Decreased quadriceps strength may contribute to anterior knee pain after total knee arthroplasty (TKA). The quadriceps force necessary to establish full extension is strongly dependent on the position and the relative length of the lever arms over the knee joint. The purpose of this in vitro study was to investigate the amount of quadriceps force required to extend the knee isokinetically after TKA in dependence of different prosthesis designs and the state of the posterior cruciate ligament (PCL).

Methods: Eight fresh frozen human knee specimens were tested in a kinematic device that simulated 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. The quadriceps force was measured using a load cell attached to the quadriceps tendon after implantation of a cruciate retaining (CR) TKA (Genesis II, Smith& Nephew, Memphis, Tn, USA) applying a conventional and a highly conforming polyethylene (PE) inlay before and after resection of the PCL. Finally, the femoral component of the CR TKA was replaced by a posterior stabilized (PS) design and measurements were redone.

Results: No significant differences in the average quadriceps force were detected between the different PCL retaining inlays (CR, highly conforming) as long as the PCL was intact. However, after resection of the PCL, the required quadriceps force increased significantly for both designs (CR: 4.7%, p < 0.01, Highly conforming: 3.5%, p < 0.03). After implantation of the PS femoral component quad force decreased to its initial levels with forces significantly lower compared to the PCL deficient knees provided with a CR (−6.0%, p < 0.01) or highly conforming (−5.1%, p < 0.01) inlay. With a PS design average quadriceps extension force was not significantly different from cruciate retaining TKA inlays at an intact PCL.

Conclusions: The data of this in vitro study suggest that the quadriceps extension force is significantly higher for knees after cruciate retaining TKA with PCL deficiency, independent of the use of a CR or DD inlay. Thus, the integrity of the PCL should be secured in clinical practice when using a cruciate preserving TKA design.