The goal of this study was (1) to investigate the relationships between the bony contours of the knee and the popliteus tendon before and after TKA and (2) to analyse the influence of implant sizing. Our hypothesis was that an apparently well-sized prosthesis, will modify the position or the tracking of popliteus tendon. 4 fresh frozen cadavers were selected. The popliteus tendon was injected with contrast dye and a CT-scan was performed from full extension to full flexion with increments of 20°. Afterwards a total knee arthroplasty (TKA) was performed. Each cadaver received either a normal-sized, oversized, undersized or mobile bearing prosthesis. After TKA the limb was scanned again using the same protocol as before. 3D-reconstructions were created using Materialise Mimics software. These 3D-models were then imported into custom made Matlab software to measure and compare the deviation of the popliteus tendon before and after TKA.Background
Methods
To determine the mechanisms and extents of popliteus impingements before and after TKA and to investigate the influence of implant sizing. The hypotheses were that (i) popliteus impingements after TKA may occur at both the tibia and the femur and (ii) even with an apparently well-sized prosthesis, popliteal tracking during knee flexion is modified compared to the preoperative situation. The location of the popliteus in three cadaver knees was measured using computed tomography (CT), before and after implantation of plastic TKA replicas, by injecting the tendon with radiopaque liquid. The pre- and post-operative positions of the popliteus were compared from full extension to deep flexion using normosized, oversized and undersized implants (one size increments). At the tibia, TKA caused the popliteus to translate posteriorly, mostly in full extension: 4.1mm for normosized implants, and 15.8mm with oversized implants, but no translations were observed when using undersized implants. At the femur, TKA caused the popliteus to translate laterally at deeper flexion angles, peaking between 80º-120º: 2.0 mm for normosized implants and 2.6 mm with oversized implants. Three-dimensional analysis revealed prosthetic overhang at the postero-superior corner of normosized and oversized femoral components (respectively, up to 2.9 mm and 6.6 mm). A well-sized tibial component modifies popliteal tracking, while an undersized tibial component maintains more physiologic patterns. Oversizing shifts the popliteus considerably throughout the full arc of motion. This study suggests that both femoro- and tibio-popliteus impingements could play a role in residual pain and stiffness after TKA.
Analysis of the morphology of the distal femur, and by extension of the femoral components in total knee arthroplasty (TKA), has largely been related to the aspect ratio, which represents the width of the femur. Little is known about variations in trapezoidicity (i.e. whether the femur is more rectangular or more trapezoidal). This study aimed to quantify additional morphological characteristics of the distal femur and identify anatomical features associated with higher risks of over- or under-sizing of components in TKA. We analysed the shape of 114 arthritic knees at the time of primary TKA using the pre- operative CT scans. The aspect ratio and trapezoidicity ratio were quantified, and the post- operative prosthetic overhang was calculated. We compared the morphological characteristics with those of 12 TKA models. There was significant variation in both the aspect ratio and trapezoidicity ratio between individuals. Femoral trapezoidicity was mostly due to an inward curve of the medial cortex. Overhang was correlated with the aspect ratio (with a greater chance of overhang in narrow femurs), trapezoidicity ratio (with a greater chance in trapezoidal femurs), and the tibio- femoral angle (with a greater chance in valgus knees). This study shows that rectangular/trapezoidal variability of the distal femur cannot be ignored. Most of the femoral components which were tested appeared to be excessively rectangular when compared with the bony contours of the distal femur. These findings suggest that the design of TKA should be more concerned with matching the trapezoidal/ rectangular shape of the native femur.
Analysis of the morphology of the distal femur, and by extension of the femoral components in total knee arthroplasty (TKA), has largely been related to the aspect ratio, which represents the width of the femur. Little is known about variations in trapezoidicity (i.e. whether the femur is more rectangular or more trapezoidal). This study aimed to quantify additional morphological characteristics of the distal femur and identify anatomical features associated with higher risks of over- or under-sizing of components in TKA. We analysed the shape of 114 arthritic knees at the time of primary TKA using the pre- operative CT scans. The aspect ratio and trapezoidicity ratio were quantified, and the post- operative prosthetic overhang was calculated. We compared the morphological characteristics with those of 12 TKA models. There was significant variation in both the aspect ratio and trapezoidicity ratio between individuals. Femoral trapezoidicity was mostly due to an inward curve of the medial cortex. Overhang was correlated with the aspect ratio (with a greater chance of overhang in narrow femurs), trapezoidicity ratio (with a greater chance in trapezoidal femurs), and the tibio- femoral angle (with a greater chance in valgus knees). This study shows that rectangular/trapezoidal variability of the distal femur cannot be ignored. Most of the femoral components which were tested appeared to be excessively rectangular when compared with the bony contours of the distal femur. These findings suggest that the design of TKA should be more concerned with matching the trapezoidal/ rectangular shape of the native femur.
