No abstract

High-flexion knee replacements have been developed to accommodate a large range of motion (RoM > 1208). Knee implants that allow for higher flexion may be more sensitive to femoral loosening as the knee load is relatively high during deep knee flexion, which could result in an increased failure potential at the implant-cement interface of the femoral component. A 3D finite element knee model was developed including a posterior-stabilized high-flexion knee replacement to analyze the stress state at the femoral implant-cement interface during a full squatting movement (RoM 1558). During deep flexion (RoM > 1208), tensile and shear stress concentrations were found at the implant-cement interface beneath the proximal part of the anterior flange. Particularly, the shear stresses at this interface location increased during high flexion, from a peak stress of 4.03 MPa at 908 to 6.89 MPa at 1408 of flexion. Tensile stresses were substantially lower, having a peak stress of 0.72 MPa at 1008 of flexion. Using data from earlier interface strength experiments, none of the interface beneath the anterior flange was predicted to fail in the normal flexion range (RoM 1208), whereas the prediction increased to 2.2% of the interface during deeper knee flexion. Thigh-calf contact reduced the knee forces, interface load, and failure risk beyond 140-1458 of flexion. Based on the more critical stresses at the femoral fixation site between 1208 and 1458 of flexion, we conclude that the femoral component has a higher risk of loosening at high-flexion angles. Keywords: total knee arthroplasty; high flexion; femoral loosening; finite element analysis; implant-cement interfaceThe traditional goals of total knee arthroplasty (TKA) are pain relief and restoration of normal knee function. Several clinical studies demonstrated that TKA patients receiving a standard knee replacement in general achieve maximal flexion angles limited to roughly 1208 of flexion.1 Hence, active knee patients experience limitations during activities such as squatting and kneeling.2 High-flexion TKAs have been developed to facilitate a larger post-operative range of motion (RoM > 1208). High-flexion implants are mostly based on successful standard designs with the posterior condylar geometry adapted to accommodate increased joint load occurring during deep knee flexion.In a recent follow-up study, Han et al. 3 reported a disturbingly high incidence of early femoral loosening for high-flexion TKA. They observed aseptic femoral loosening in 38% of the operated cases at a mean follow-up of 23 months. Furthermore, the occurrence of loosening was closely related to the maximal flexion angle achieved after TKA. In nearly all cases of loosening, the femoral implant-cement interface debonded, particularly beneath the anterior flange, with radiolucent lines visible on lateral radiographs. Due to this debonding process, the femoral component migrated into a position of increased flexion during deep knee bends. A similar mode of failure was earlier described by King ...

No abstract

No abstract

a b s t r a c tAseptic loosening at the implant-cement interface is a well-documented cause of failure in joint arthroplasty. Traditionally, the strength of the implant-cement interface is determined using uni-axial normal and shear loading tests. However, during functional loading, the implant fixation sites are loaded under more complex stress conditions. For this purpose, the strength of the implant-cement interface under mixed-mode tensile and shear loading conditions was determined in this study using interface specimens with varying interface roughness. For the lowest roughness value analyzed (R a ¼ 0.89 mm), the interface strength was 0.40-1.95 MPa at loading angles varying between pure tension and shear, whereas this was 4.90-9.90 MPa for the highest roughness value (R a ¼ 2.76 mm). The interface strength during pure shear (1.95-9.90 MPa) was substantially higher than during pure tension (0.58-6.67 MPa). Polynomial regression was used to fit a second-order interpolation function through the experimental interface strength data (R 2 ¼0.85; po 0.001), relating the interface strength (S [MPa]) to the interface loading angle (a [degrees]) and interface roughness (R a [mm]): Sða,R a Þ ¼ 0:891R 2 a þ 0:001a 2 À0:189R a À0:064aÀ0:060. Finally, an interface failure criterion was derived from the interface strength measurements, describing the risk of failure at the implant-cement interface when subjected to a certain tensile and shear stress using only the interface strength in pure tensile and shear direction. The findings presented in this paper can be used in numerical models to simulate loosening at the implant-cement interface.