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chapter 1: normal anatomy

Trochlear Surface of the Femur

The articular portion of the anterior surface of the distal femur, which articulates with the patella, has been referred to variously as the patellar facets of the femur, the patellar groove, the femoral sulcus, and the trochlea. The term "trochlea" is presently chosen here because of its conciseness and lack of confusion with the articular surface of the patella. The trochlear surface of the femur is divided into two facets, medial and lateral. Proximally, they are in continuity with a shallow groove conforming to the contours of the distal patellar articular surface. Curving distally and posteriorly, this groove deepens to become the intercondylar notch.

Lateral (External) Trochlear Facet

This extends more proximally, is larger overall, and projects further anteriorly than the medial facet in most people. The cartilage covering is significantly thinner on the femur than on the patella, being approximately 2 to 3 mm. It is also thinner on the medial facet than on the lateral. The trochlea represents only one part of the femoral pulley with respect to the patella, albeit the most functional part. In full extension with the quadriceps tightened, the patella articulates with the supratrochlear fat pad of the femur and in full flexion with that portion of the medial and lateral femoral condyles that articulate with the tibial plateaus in full extension (see Chapter 2).

The lateral trochlea facet provides a buttress to lateral patellar subluxation and helps to maintain the patella centered in the trochlea during normal knee function from 15° knee flexion into full flexion. There is substantial variability in trochlear depth, however. A flat lateral trochlea is common in patients with lateral patellar instability.

Supratrochlear Fossa

This is situated on the anterior surface of the femur immediately proximal to the trochlear facet. It is slightly depressed and almost triangular in shape. Medially, it is subtly delineated by the rounded anteromedial distal femoral metaphysis. Laterally, the border is more sharply defined by the anterolateral metaphyseal ridge that runs into the superolateral border of the trochlea. At this juncture, there is often a small tubercle where the capsule attaches. This fossa, dotted with vascular channels, is covered with a prefemoral fat pad. At the lateral margin, an area marked by thickened, fibrotic, blanched synovia appears as a fibrocartilaginous extension of the lateral facet. This is the site of contact with the patella under full forced extension (quadriceps setting) (Fig. 1.10). Because the lateral trochlear facet extends both more proximally and more anteriorly than the medial, the superior trochlear border runs obliquely posteriorly and distally from lateral to medial (Fig. 1.7). The cartilaginous border of the lateral facet terminates subtly, which perhaps favors lateral displacement of the patella at the end of extension. The proximal cartilaginous border of the medial facet is, by contrast, more marked and can form an abrupt convex ridge that has the potential of causing difficulties at the moment of patellar engagement in the trochlear groove as flexion proceeds (16). However, this is not frequently the case because the patella usually enters the trochlear sulcus from a characteristic, obliquely lateral position. This pattern has been demonstrated consistently using computerized tomography.

Trochlear Condylar Junctions

The condylar surface is separated from its corresponding trochlear surface by a slight groove. This corresponds to the impression of the meniscus at full extension, which explains its secondary appearance in the adult. These indentations are delineated anteriorly by a subtle ridge. Due to the medial condyle's smaller size, greater distal projection, and greater obliquity, the trochlear condylar junctions are asymmetrical (Fig. 1.7). The medial ridge is directed convexly, anteriorly, and laterally and is less well developed than the lateral ridge.

        

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