Extensor tracking describes all contact of the patellar and femoral articular surfaces from full extension to maximum flexion, with or without prosthetic resurfacing. How the extensor tracks affects knee function, patient comfort, and component wear. The extensor mechanism can be conceived as a flexible linear structure under dynamic loads. Tracking describes its physical relationship with a rigid but articulated “track,” formed by the femoral trochlea in extension and then by the distal intercondylar geometry as knee flexion proceeds.

The orientation of the extensor is determined by the origin of the quadriceps muscle and its direction of pull proximally, the patella interacting with the trochlear groove, and the location of the tibial tubercle. Tensile load on a pliant extensor mechanism that is angled in valgus at the knee moves the (unrestrained) patella laterally. Straightening the pull of the extensor on the patella will enhance tracking. In most knees, maneuvers that move the proximal or distal vectors medially will align the quadriceps origin, the patella, and the tibial tubercle (Fig. 18.4a–c). These force vectors could theoretically be changed by modifications to the attachment of the extensor to the skeleton, but the anatomically complicated quadriceps origins are difficult to modify. Preferential vastus medialis strengthening [4] and medial surgical advancement are more practical interventions [5]. The tibial tubercle is probably the most significant structure in this equation. While the tubercle can be repositioned surgically, it is more commonly malpositioned inadvertently by the rotational position of the tibial component.

In contrast to the supple and dynamic extensor, which controls the patella, the trochlear groove is structural and static. Tracking will improve if the groove can be located where the patellar is driven by knee function. Because the patella moves laterally when loaded, anything that positions the prosthetic trochlear groove laterally is beneficial. In a somewhat frivolous analogy, the patella is a willful teenager, determined to go where it wants. The trochlear groove must be established in that place to contain it. In another analogy, centralizing the patella in the trochlear groove is a process of conciliation: reducing lateral excursions of the patella and simultaneously positioning the femoral trochlea in a more lateral position.

Maltracking is the converse of a matched trajectory of the extensor with the structure of the trochlea [6–8]. Maltracking can be described as incremental subluxation in millimeters of translation or degrees of tilt [9]. Complete escape from the trochlear groove is a qualitative change—dislocation. Then, the medial-patellofemoral ligament will have ruptured, and the patella can only return to the front of the knee with extension of the joint (Fig. 18.5). As the patella dislocates laterally, part of the quadriceps muscle vector creates a valgus moment on the tibia-femoral joint, exacerbating tibia-femoral instability (Fig. 18.6). If the extensor migrates behind the axis of knee flexion, it can temporarily and aberrantly function as a knee flexor (Fig. 18.6).

Clinical problems that result directly from patellar maltracking include knee instability [10–12], buckling, pain, patellar fractures, patellar component loosening (Fig. 18.7), patellar component breakage, and wear [8]. Indirect problems associated with the same mechanics include stiffness [13].

Extensor tracking might conceivably be improved in an arthroplasty by eliminating frontal plane angulation, except that some degree of anatomic tibial-femoral valgus (even in conjunction with an overall mechanical varus [14, 15]) is desirable. While ideal alignment has reemerged as a topic for discussion [16, 17], a valgus tibial-femoral angle decreases the energy consumption of human gait by decreasing medial-lateral oscillations of the human center of gravity with each step [18]. Despite current debates, most surgeons believe that some degree of valgus alignment also decreases medial load and aseptic loosening [19].

The patella-femoral joint experiences high anterior to posterior compression when the quadriceps contracts to extend the knee, to maintain a given flexion angle, or to slow the rate of flexion. Patellofemoral forces increase with flexion and diminish almost to zero in full knee extension [1]. The patella, sitting between the anterior femur and the extensor mechanism, displaces the quadriceps and patellar tendons anteriorly by a distance equal to its thickness, even in full extension, as a means to increase the moment arm in terminal extension. The extensor is never completely straight in the sagittal or frontal planes in most knees.

The magnitude of the lateral vector on the patella increases directly with the load generated by the quadriceps. The more forcefully the quadriceps contracts, the greater the force tending to straighten the extensor in both planes. Muscle action drives both desired knee extension and undesired lateral patellar subluxation. Extension power will be lost if the patella escapes laterally, out of the groove. Tension in the medial patella-femoral ligament (MPFL) resists lateral patellar dislocation, and the lateral trochlear ridge exerts compression medially against the lateral patellar facet to oppose patellar subluxation and keep the patella in front of the knee (Fig. 18.8a). Given the obliquity of the native lateral facet joint, much of this constraining force is shear. Part of the vector will be compression that forces the patella anteriorly in opposition to patella-femoral compression (Fig. 18.8b). In the words of Kapandji: “…the force of the quadriceps, directed obliquely superiorly and slightly laterally, is turned into a strictly vertical force” [3]. We fail to appreciate this interplay when extensor function is separated into extensor power and patellar tracking for analysis. A comparison of patellofemoral imaging with the extensor loaded and unloaded illustrates the potent effect of muscle contraction on a extensor tracking [20].

When the knee is fully extended, the patella sits proximal to and above the femoral trochlear groove. Even under conditions where maltracking is imminent, the problem will not be apparent in this position. As the knee flexes, the patella descends toward the groove, which is wider and flatter proximally, much like the opening of a funnel viewed in cross section. With further flexion, the patella is “captured” and descends into a conforming articulation, first in the trochlear groove and then between the distal femoral condyles, from which it cannot easily escape. In conditions of patella-femoral dysplasia, the confining lateral trochlear ridge may not easily guide and then restrain the patella, which can dislocate. Only with the patella in the groove can the quadriceps forcefully extend the knee joint.

Particular rotational phenomena aid patellar tracking at the extremes of flexion and extension. First, the “screw-home” mechanism locks the joint in full extension, diminishing quadriceps activity during standing. This involves external rotation of the tibia relative to the femur in terminal extension, displacing the patella laterally above the trochlear groove and seemingly increasing the risk of maltracking. The popliteus muscle however “unscrews” the mechanism prior to initiating flexion, rotating the tibia internally before the patella descends into the trochlear groove.

Secondly, femoral “rollback” describes the track of the femoral condyle on the tibial plateau anterior to posterior as flexion progresses. Though originally described as a symmetric path on both tibial condyles [21], more rollback occurs on the lateral tibial plateau. The tibia and its tubercle effectively rotate internally under the femoral condyles in deep flexion as the lateral femoral condyle progresses posteriorly on the tibia [22]. This draws the patella medially into the groove. Neither the asymmetric rollback [23] nor the popliteus-driven [24] screw-home mechanism is reliably replicated by arthroplasty surgery, leaving the replaced knee at an increased risk of patellar maltracking.