Introduction

High-flexion knee implants have been developed to accommodate a large range of motion (ROM > 120°) after total knee arthroplasty (TKA). In a recent follow-up study, Han et al. [1] reported a disturbingly high incidence of femoral loosening for high-flexion TKA. The femoral component loosened particularly at the implant-cement interface. Highly flexed knee implants may be more sensitive to femoral loosening as the knee load is high during deep knee flexion [2], which may result in increased tensile and/or shear stresses at the femoral implant fixation.

The objective of this study was to analyse the load-transfer mechanism at the femoral implant-cement interface during deep knee flexion (ROM = 155°). For this purpose, a three-dimensional finite element (FE) knee model was developed including high-flexion TKA components. Zero-thickness cohesive elements were used to model the femoral implant-cement interface. The research questions addressed in this study were whether high-flexion leads to an increased tensile and/or shear stress at the femoral implant-cement interface and whether this would lead to an increased risk of femoral loosening.

High-flexion knee replacements have been developed to accommodate a large range of motion (ROM > 120°) after total knee arthroplasty (TKA). Femoral rollback or posterior translation of the femoral condyles during knee flexion is essential to maximise ROM and to avoid bone-implant impingement during deep knee flexion. The posterior cruciate ligament (PCL) has been described as the main contributor to femoral rollback. In posterior-stabilised TKA designs the PCL is substituted by a post-cam mechanism. The main objective of this study was to analyse the mechanical interaction between the PCL and a highflexion cruciate-retaining knee replacement during deep knee flexion. For this purpose, the mechanical performance of the high-flexion cruciate-retaining TKA design was evaluated and compared with two control designs including a highflexion posterior-stabilised design.

Materials & Methods: Prosthetic knee kinematics and kinetics were computed using a three-dimensional dynamic finite element (FE) model of the knee joint. The FE knee model consisted of a distal femur, a proximal tibia and fibula, a quadriceps and patella tendon, a non-resurfaced patella, TKA components and a posterior cruciate ligament in case cruciate-retaining designs were evaluated. Tibio-femoral and patello-femoral contact were defined in the FE knee model and the polyethylene insert was modelled as a non-linear elastic-plastic material. Three different rotating platform TKA systems were analysed in this study: the high-flexion cruciate-retaining PFC Sigma CR150, the high-flexion posterior-stabilised PFC Sigma RP-F and the conventional cruciate-retaining PFC Sigma RP (Depuy, J& J, UK). Both the polyethylene stress characteristics and the tibio-femoral contact locations were evaluated during a squatting movement (ROM = 50° – 150°).

Results: During deep knee flexion (ROM > 120°), the high-flexion cruciate-retaining TKA design showed a lower peak contact stress (74.7 MPa) than the conventional cruciate-retaining design (96.5 MPa). The posterior-stabilized high-flexion TKA design demonstrated the lowest peak contact stress at the condylar contact interface (54.2 MPa), although the post was loaded higher (77.4 MPa). All three TKA designs produced femoral rollback in the normal flexion range (ROM ≤ 120°), whereas the cruciate-retaining designs showed a paradoxical anterior movement of the femoral condyles during high-flexion.

Discussion: PCL retention is a challenging surgical aim and affects the prosthetic knee load and kinematics as shown in this study. In addition, for adequate functioning the PCL should not be too tight or too lax after surgery. Hence, we investigated the effect of PCL laxity on the prosthetic performance and the best-balanced PCL was used in our simulations. Although PCL balancing is not an issue for posterior-stabilized TKA, we found the tibial post to be loaded relatively high for this implant type.

Introduction: Recently, high-flexion knee implants have been developed to provide for a large range of motion (ROM > 120°) after total knee arthroplasty (TKA). High-flexion knee implants are more likely subjected to large knee loads than conventional implants since knee joint forces increase with larger flexion angles. Highly conforming knee replacements are designed to minimise polyethylene peak stresses during (deep) knee flexion.

The Birmingham Knee Replacement (BKR, Jointmedica, UK) is a newly designed knee replacement which combines a high conformity during the complete ROM with the principles of rotating platform and high-flexion TKA. The main objective of this study was to analyze the mechanical performance of the BKR during its full ROM (0°–155°) and investigate whether its high conformity could be maintained during high-flexion. In addition, the BKR polyethylene loading computed in this study was compared with other mobile bearings.

Materials & methods: TKA performance was analyzed using a three-dimensional dynamic finite element (FE) model of the knee joint. The FE knee model consisted of a distal femur, a proximal tibia and fibula, a quadriceps and patella tendon, a non-resurfaced patella and TKA components. Tibio-femoral and patello-femoral contact were defined in the knee model. Three different posterior stabilised rotating platform TKAs were subsequently incorporated: the high-flexion BKR, the high-flexion PFC Sigma RP-F and the standard PFC Sigma RP (Depuy, J& J, USA). The polyethylene insert was modelled as a non-linear elastic-plastic material in each TKA system. Polyethylene loading parameters as well as the tibio-femoral contact point locations were computed during an entire flexion movement (0°–155°).

Results: In the normal flexion range (flexion ≤ 120°) the three knee implants behaved very similar except for the polyethylene loading at the post. At 120° of flexion, the contact stress at the dish was ±45 MPa for all implants whereas the maximal post-cam contact stress came down to 26.7 MPa for the BKR which was half the amount of contact stress experienced by both PFC Sigma implants. During high-flexion (flexion > 120°), the contact stress difference at the post between the BKR and the PFC Sigma RP-F became smaller and came down to 37.9 MPa and 60.7 MPa, respectively. The total amount of plastic deformation at maximal flexion (155°) was smaller for the BKR (577 mm3) in comparison with the Sigma RP-F (2256 mm3). Femoral rollback was negligible for the BKR in the high-flexion range in comparison with the Sigma RP-F (1.9 mm).

Discussion: A comparison between different geometrical models using finite element techniques is jeopardised by differences in element distribution within the various models. These differences may affect calculated parameters such as peak stress values. However, in this study the models were very similar which would indicate that the differences in stress patterns found are due to design differences rather than model artefacts.

The current study therefore indicates that the BKR benefits from its high conformity during the full ROM. Hence, the BKR demonstrated relatively low polyethylene stresses. The quadriceps efficiency during deep knee flexion may be lower in case of the BKR since the femoral rollback was negligible at these flexion angles. Whether this phenomenon is of any clinical relevance is unknown.

Recently, high-flexion knee implants have been developed to provide for a large range of motion after total knee arthroplasty. Since knee forces increase with larger flexion angles, it is commonly assumed that high-flex-ion implants are subjected to large loads in the highflexion range (flexion > 120°). However, high-flexion studies often do not consider thigh-calf contact which occurs during high-flexion activities such as squatting and kneeling. We hypothesized that thigh-calf contact is substantial and has a reducing effect on the prosthetic knee loading during deep knee flexion.

The effect of thigh-calf contact on the loading of a knee implant was evaluated using a three-dimensional dynamic finite element knee model. The knee model consisted of a distal femur, a proximal tibia and fibula, a patella, high-flexion components of the PFC Sigma RP-F (Depuy, Warsaw, USA) and a quadriceps and patella tendon. Using this knee model, a squatting movement was simulated including thigh-calf contact characteristics of a typical subject which have been described in an earlier study.

Thigh-calf contact considerably reduced the implant loading during deep knee flexion. At maximal flexion (155°), the compressive knee force decreased from 4.9 to 2.9 times bodyweight. The maximal joint forces shifted from occurring at maximal flexion angle to the flexion angle at which thigh-calf contact initiated (±130°). The maximal polyethylene contact stress at the tibial post decreased from 49.3 to 28.1 MPa at maximal flexion.

This study confirms that thigh-calf contact reduces the knee loading during high-flexion. Both the joint forces and the polyethylene stresses reduced considerably when thigh-calf contact was included.