The Trochlear Groove of a Femoral Component Designed for Kinematic Alignment Is Lateral to the Quadriceps Line of Force and Better Laterally Covers the Anterior Femoral Resection Than a Mechanical Alignment Design

Abstract

Background: A concern about kinematically aligned (KA) total knee arthroplasty (TKA) is that it relies on femoral components designed for mechanical alignment (MAd-FC) that could affect patellar tracking, in part, because of a trochlear groove orientation that is typically 6° from vertical. KA sets the femoral component coincident to the patient’s pre-arthritic distal and posterior femoral joint lines and restores the Q-angle, which varies widely. Relative to KA and the native knee, aligning the femoral component with MA changes most distal joint lines and Q-angles, and rotates the posterior joint line externally laterally covering the anterior femoral resection. Whether switching from a MAd- to a KAd-FC with a wider trochlear groove orientation of 20.5° from vertical results in radiographic measures known to promote patellar tracking is unknown. The primary aim was to determine whether a KAd-FC sets the trochlear groove lateral to the quadriceps line of force (QLF), better laterally covers the anterior femoral resection, and reduces lateral patella tilt relative to a MAd-FC. The secondary objective was to determine at six weeks whether the KAd-FC resulted in a higher complication rate, less knee extension and flexion, and lower clinical outcomes. Methods: Between April 2019 and July 2022, two surgeons performed sequential bilateral unrestricted caliper-verified KA TKA with manual instruments on thirty-six patients with a KAd- and MAd-FC in opposite knees. An observer measured the angle between a line best-fit to the deepest valley of the trochlea and a line representing the QLF that indicated the patient’s Q-angle. When the trochlear groove was lateral or medial relative to the QLF, the angle is denoted + or −, and the femoral component included or excluded the patient’s Q-angle, respectively. Software measured the lateral undercoverage of the anterior femoral resection on a Computed Tomography (CT) scan, and the patella tilt angle (PTA) on a skyline radiograph. Complications, knee extension and flexion measurements, Oxford Knee Score, KOOS Jr, and Forgotten Joint Score were recorded pre- and post-operatively (at 6 weeks). A paired Student’s T-test determined the difference between the KA TKAs with a KAd-FC and MAd-FC with a significance set at p < 0.05. Results: The final analysis included thirty-five patients. The 20.5° trochlear groove of the KAd-FC was lateral to the QLF in 100% (15 ± 3°) of TKAs, which was greater than the 69% (1 ± 3°) lateral to the QLF with the 6° trochlear groove of the MAd-FC (p < 0.001). The KAd-FC’s 2 ± 1.9 mm lateral undercoverage of the anterior femoral resection was less than the 4.4 ± 1.5 mm for the MAd-FC (p < 0.001). The PTA, complication rate, knee extension and flexion, and clinical outcome measures did not differ between component designs. Conclusions: The KA TKA with a KAd-FC resulted in a trochlear groove lateral to the QLF that included the Q-angle in all patients, and negligible lateral undercoverage of the anterior femoral resection. These newly described radiographic parameters could be helpful when investigating femoral components designed for KA with the intent of promoting patellofemoral kinematics.

Personalized alignment techniques better restore the native trochlear groove compared to systematic alignment techniques in total knee arthroplasty

“…However, variations ranging from 3° to 11° have been observed between manufacturers [4]. This has recently led to the design of femoral components specific to personalized alignment that feature a 20.5° capture in the trochlear groove [42]. The present study suggests that the KA approach would result in even more valgus being added to the TA, which would further displace the trochlea and patella from their native positions.…”

“…As this understanding has grown, alignment philosophy has also evolved to a more individualized approach [17,29]. However, doing so with standard femoral components that were initially intended for MA has led to the concern that this may be deleterious to the PFJ [26,41,42]. The average femoral component that was designed for MA has a distal trochlear sulcus angle of around 83°-85°, or 5°−7°valgus [2,32].…”

Better forgotten joint scores when the angle of the prosthetic trochlea is lateral to the quadriceps vector in kinematically aligned total knee arthroplasty

“…A second explanation is that four 3‐D analysis studies reported a difference between three femoral component designs positioned with unKA and the native trochlea and concluded that although randomized trials have not shown differences in patellofemoral complications between KA TKA and MA TKA, a femoral component explicitly designed for KA that more closely restores the native trochlear anatomy might improve patient‐reported satisfaction and function [14, 15, 21, 22]. Finally, a study of sequential bilateral unKA TKA with a PTA of 20.5° valgus explicitly designed for KA in one knee and a PTA of 6° valgus in the opposite knee reported that the 20.5° valgus PTA of the KA‐designed femoral component was lateral to the QV in all patients with negligible lateral undercoverage of the anterior femur resection surface [25]. Hence, the present study’s finding that the FJS is 17 points better when the PTA is lateral to the QV suggests that a KA‐designed femoral component with features of a lateralized prosthetic trochlea and a wider PTA that is lateral to the QV in all patients might better ‘funnel’ patella engagement and improve PROMs.…”

“…A PTA that deviates lateral relative to the QV (i.e., positive PTA-QV angle) includes the patient's Q-angle, allowing the natural line of force. However, a PTA that deviates medially excludes the patient's Q-angle, causing an unnatural line of force with potential consequences of shifting the patella medially and adversely affecting patellofemoral kinematics and clinical outcome scores [25].…”

“…One factor that can cause variability in the PTA‐QV angle is the coronal alignment target used to set the femoral component [25]. For example, in unrestricted kinematically aligned (unKA) TKA, the PTA relationship to the QV varies widely because the femoral component is set to restore the patient's pre‐arthritic distal femoral joint line, which varies from 100° valgus to 88° varus in the non‐osteoarthritic knee [11].…”