Flexing and downsizing the femoral component is not detrimental to patellofemoral biomechanics in posterior-referencing cruciate-retaining total knee arthroplasty

Abstract

Purpose: When downsizing the femoral component to prevent mediolateral overhang, notching of the anterior femoral cortex may occur, which could be solved by flexing the femoral component. In this study, we investigated the effect of flexion of the femoral component on patellar tendon moment arm, patellofemoral forces and kinematics in posterior-referencing CR-TKA. Our hypothesis was that flexion of the femoral component increases the patellar tendon moment arm, reduces the patellofemoral forces and provides stable kinematics.

Methods: A validated musculoskeletal model of CR-TKA was used. The flexion of the femoral component was increased in four steps (0°, 3°, 6°, 9°) using posterior referencing, and different alignments were analysed in combination with three implant sizes (3, 4, 5). A chair-rising trial was analysed using the model, while simultaneously estimating quadriceps muscle force, patellofemoral contact force, tibiofemoral and patellofemoral kinematics.

Results: Compared to the reference case (size 4 and 0° flexion), for every 3° of increase in flexion of the femoral component the patellar tendon moment arm increased by 1% at knee extension. The peak quadriceps muscle force and patellofemoral contact force decreased by 2%, the patella shifted 0.8 mm more anteriorly and the remaining kinematics remained stable, with knee flexion. With the smaller size, the patellar tendon moment arm decreased by 6%, the quadriceps muscle force and patellofemoral contact force increased by 8 and 12%, and the patellar shifted 5 mm more posteriorly. Opposite trends were found with the bigger size.

Conclusion: Flexing the femoral component with posterior referencing reduced the patellofemoral contact forces during a simulated chair-rising trial with a patient-specific musculoskeletal model of CR-TKA. There seems to be little risk when flexing and downsizing the femoral component, compared to when using a bigger size and neutral alignment. These findings provide relevant information to surgeons who wish to prevent anterior notching when downsizing the femoral component.

Keywords: Alignment; Biomechanics; CR; Chair; Component; Femoral; Flexion; Force; Model; Musculoskeletal; Patellofemoral; Posterior-referencing; Quadriceps; Rising; Sagittal; TKA; Total knee arthroplasty; Total knee replacement.

Fig. 1

Twelve simulated post-operative cases with three different sizes and four degrees of flexion of the femoral component. Illustration of the twelve custom post-operative cases simulated in this study. From left to right four degrees of flexion of the femoral component are shown: 0°, 3°, 6°, 9°. Three sizes of the femoral component (blue: size 3, red: size 4, yellow: size 5) plus the pre-operative bone are shown in overlay for each flexion of the femoral component (FFC) angle. Note that in every case the most distal and most posterior ends of the outlines of the femoral component are made to match tangentially, to simulate a posterior referencing and to preserve the posterior condylar offset

Fig. 2

Patellar tendon moment arm. Patellar tendon moment arm (PTMA) at varying knee flexion angle during a rising-from-a-chair simulation. From left to right the results in mm for size 3, 4 and 5 are shown. Each line series correspond to a flexion of the femoral component (FFC) angle. The flexion angle in the abscissa indicates the phases of the rising and sitting motion

Fig. 3

Peak forces on the knee extensor mechanism. Peak forces on the knee extensor mechanism during a rising-from-a-chair simulation. From left to right: patellar tendon force (PTF), quadriceps muscle force (QMF), quadriceps tendon-to-femur force (QTFF), and patellofemoral contact force (PFCF). Results are reported in body weights (BW)

Fig. 4

Tibiofemoral distraction and patellofemoral antero-posterior translation. Kinematics of a patellofemoral antero-posterior translation and b tibiofemoral distraction, at varying knee flexion angle during a rising-from-a-chair simulation. From left to right the results in mm for size 3, 4, and 5 are shown. Each line series correspond to a flexion of the femoral component (FFC) angle. Kinematics from the custom cases are plotted relatively to the intact case. The rising and sitting phases for each curve are overlapped

Fig. 5

Ligament forces. Ligament force of the a medial patellofemoral ligament (MPFL) and b posterior cruciate ligament (PCL), at varying knee flexion angle during a rising-from-a-chair simulation. From left to right the results in N for size 3, 4, and 5 are shown. Each line series correspond to a flexion of the femoral component (FFC) angle. The flexion angle in the abscissa indicates the phases of the rising and sitting motion

Fig. 6

Illustrative case for the alignment in flexion of a downsized femoral component. Illustrative case for the alignment in flexion of a downsized femoral component with preservation of the posterior condylar offset (PCO). Size 5 with 0° FFC fits the antero-posterior dimension of the femur, however, mediolateral overhang is observed, which is detrimental. Downsizing the femoral component (Size 4, 0° FFC) reduces the mediolateral overhang, but creates anterior notching of the femoral cortex, if the PCO is preserved. Flexing the smaller component by a few degrees in the sagittal plane (Size 4, 6° FFC) may concomitantly preserve the PCO, while limiting mediolateral overhang and preventing anterior notching