Reducing the Risk and Management of Patellofemoral Instability After Calipered Kinematically Aligned Total Knee Arthroplasty

Overview

The discussion and video links in this chapter assist the surgeon in reducing the risk of patellofemoral instability and in managing it when it occurs after calipered kinematically aligned (KA) total knee arthroplasty (TKA). The first section reports the incidence, time of onset, and causes of patellofemoral instability using currently available femoral components designed for mechanical alignment (MA). The second section uses results from in vivo and in vitro kinematic studies to show that the use of femoral components designed for MA with KA is an unlikely cause of patellofemoral instability. The third describes intraoperative verification checks that minimize excessive flexion of the femoral component and the use of an anatomically designed patella component to reduce the risk of patellofemoral instability. The fourth proposes a redesign of the prosthetic trochlea to increase the lateral coverage of the anterior femur and to closely coalign the orientations of the prosthetic and native trochlear groove. The goal of these design changes is to promote the capture and containment of the patellar implant during knee flexion. The case studies in the final section showcase options for managing patellofemoral instability, including observation, arthroscopic lateral release, and open lateral release and medial reefing. The objective is to encourage those surgeons that perform calipered KA TKA to minimize flexion of the femoral component, use an anatomic patella component, and choose a femoral component designed explicitly for KA when they become available.

Incidence, Time of Onset, and Causes of Patellofemoral Instability after Calipered Kinematically Aligned Total Knee Arthroplasty

Video 19.1 shows a patient actively demonstrating the lateral subluxation of the patella and a discussion of the incidence, time of onset, and causes of patellofemoral instability.

To determine the incidence of patellofemoral instability, a review was done of consecutive primary TKAs performed between 2006 and 2015 using KA and a femoral component designed for MA. Thirteen out of 3212 calipered KA TKAs in the cohort presented with patellofemoral instability for an incidence of 0.4% during a 2- to 9-year follow-up. In 9 of the 13 patients (70%), the presentation was lateral subluxation of the patella that occurred without trauma during typical daily activities, with a mean onset of 5 months. Active quadriceps contraction caused lateral subluxation of the patella in extension during the initiation of knee flexion, and the patella spontaneously reduced into the prosthetic trochlear groove at 15–30 degrees of flexion (see Video 19.1 ). When an examiner passively moved the knee, the patella tracked normally.

A case-match study design identified two causes of patellofemoral instability. One cause was excessive flexion of the femoral component with respect to the sagittal anatomic axis of the distal femur. The mean flexion of the femoral components in those patients with patellofemoral instability was 11 degrees, and 6 degrees in the controls ( Fig. 19.1 ). Each 5 degrees of flexion of the femoral component moves the proximal edge of the prosthetic trochlea 5 mm distally and requires a one-size reduction in the size of the femoral component. These effects reduce the coverage and cross-sectional area of the prosthetic trochlea, compromising the capture of the patella component. The second cause was the use of a dome-shaped patella component, as patellofemoral instability did not occur with an anatomic-shaped patella component and an unresurfaced patella. Soft tissue overgrowth of the small dome-shaped patella component during 5 months of implantation caused a gradual loss of congruency, which explains the atraumatic and late onset of patellofemoral instability.

Use of Femoral Components Designed for Mechanical Alignment Does Not Explain Patellofemoral Instability after Kinematic Alignment

Video 19.2 discusses studies showing that KA of the femoral component restores patellofemoral kinematics closer to the native knee than MA.

Results from in vivo and in vitro kinematic studies indicate that the use of femoral components designed for MA with KA was an unlikely cause of patellofemoral instability. ^, An in vivo study of patients doing a deep knee bend showed that KA with a posterior cruciate ligament retaining (CR) component placed without flexion largely restored the proximal-distal patellar contact locations of the native contralateral knee. Two cadaveric studies showed KA better restored patellar kinematics and patellofemoral contact pressure distribution closer to those of the native knee than MA TKA during flexion. ^, A three-dimensional analysis of several brands of femoral components designed for MA aligned on normal femurs showed that KA more closely restored the native trochlea than MA. ^, The explanation for this paradox is that KA restores the native Q-angle and patellofemoral kinematics, whereas MA increases or decreases the native Q-angle in those patients with constitutional varus or valgus limbs, respectively. Hence, the penalties for the use of MA and the consequential deviation from the prearthritic Q-angle are abnormal patellofemoral kinematics and pressures, which KA mitigates even when the femoral component and prosthetic trochlea are designed for use with MA.