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

Abstract

PurposeWhen 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.MethodsA 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.ResultsCompared 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.ConclusionFlexing 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.Electronic supplementary materialThe online version of this article (10.1007/s00167-018-4900-z) contains supplementary material, which is available to authorized users.

Influence of Variation in Sagittal Placement of the Femoral Component after Cruciate-Retaining Total Knee Arthroplasty

“…On the other hand, as femoral component flexed more, it increased the quadriceps tendon-to-femur force in mid-flexion. 59 Furthermore, the mean flexion contracture in the flexed component was comparable to that observed in the control group. Although it was slightly greater than 5 degree, which is normal for postknee arthroplasty, it was not concerning in previous studies.…”

“…55 In addition, a more flexed implant increased the patellar tendon moment arm at knee extension and, to a lesser extent, in mid-flexion, which may be relevant for those activities involving large quadriceps action in the first arc of the knee range of motion. 59 This mechanism can be The Journal of Knee Surgery explained by the trochlear groove more anteriorly positioned and distally with more femoral-component flexion. On the other hand, as femoral component flexed more, it increased the quadriceps tendon-to-femur force in mid-flexion.…”

Flexible versus standard intramedullary rod in posterior stabilized primary total knee arthroplasty: protocol for a randomized controlled trial

“…This would lead to more natural femorotibial kinematics through increasing the patellar tendon moment arm, and thus the possibility for larger knee power output without influencing muscle strength. This effect has been found in a modeling study [24] and could potentially lead to better functional outcome in vivo. Indeed, patient-reported functional outcome is higher up till 2 years in patients operated using the flex rod when compared the standard rod [17].…”

“…It is expected that due to less oversizing in the TKA procedure using the flex rod activities of daily living (ADL) such as stairclimbing, and rising from a chair can be performed more optimally [6]. Marra et al have also shown in their modeling study that less oversizing resulted in better patellofemoral kinematics during rising from a chair [24]. It is also expected that with the use of the flex rod, the proper fit of the TKA will result in an overall smaller size chosen by the surgeon, as has been found in a retrospectively analyzed cohort and a modeling study [8,17].…”

Optimal Sagittal Insertion Depth and Direction of Femoral Intramedullary Rod in Total Knee Arthroplasty in Chinese Osteoarthritis Patients

“…Sagittal plane positioning and alignment of the femoral component are associated with long-term anterior knee pain, with femoral component extension being a major risk factor 6 . Therefore, placement of the femoral component in slight flexion on the sagittal plane has been recommended to avoid notching 7 . However, excessive flexion may cause patellofemoral overstuffing or anterior impingement between the tibial post and the intercondylar box of the femoral component in posterior-stabilized TKA, resulting in polyethylene wear and cam-post engagement 8 .…”