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chapter 1: normal anatomy

Patellar Morphology

Dye (6) has noted that amphibians and some reptiles do not have osseous patellae. However, lizards, birds, and mammals do. One must speculate, then, based on this observation, that a bony patella is important in terrestrial existence. Several standard anatomy texts contain significant omissions regarding the complex patellar form, the details of which are important to a full understanding of the function and pathology of the patella. The peripheral borders of the patella form a vague triangle, slightly wider than high, with the apex pointing distally (Fig. 1.2, A and B). DeVriese (7), in his anthropologic studies, failed to find any notable racial differences, with limits for lengths varying from 47 to 58 mm, and for width, from 51 to 57 mm. Vallois (8) developed his patellar index [I = (width X 100/length), which nearly always exceeds 100. The variations are slight, from 100 (American Indian) to 106.2 (Madagascar native).

Whereas width and height are remarkably constant, thickness is quite variable, ranging from 2 to 3 cm as measured in the equatorial plane between the median ridge and the superficial cortex. This measurement on average of 2.5 cm does not include the articular cartilage, which also attains its maximum height at the same level. Variations in both bone and cartilage thicknesses within a given patella determine its particular surface contour that can only be fully appreciated on review of serial sections.

Grelsamer et al (9) studied 564 patients and noted three different patellar shape patterns when analyzing overall patellar length compared with length of the articular surface. They described the "Cyrano" long nosed patella, in which the distal nonarticular portion of the patella is particularly long (see Fig. 1.3).

Anterior Surface

Slightly convex in all directions, the anterior surface is divided into three parts. The rough superior third, the base of the triangle, receives the insertion of the quadriceps tendon. The superficial portion of this tendon continues over the anterior surface to form the deep fascia, which is densely adherent to the bone. The middle third reveals nu¬merous vascular orifices and is crossed by numerous vertical striations giving a fuzzy or bristled appearance as seen on the axial radiograph. The inferior third terminates in a V shaped point, which is enveloped by the patellar tendon.

Posterior Surface

This side of the patella can be divided into two parts. The inferior portion, which is nonarticulating, represents a full 25% of the patellar height. This inferior surface, form¬ing the apex of the rough triangle of the patella, is dotted with vascular orifices whose vessels pass through the densely adherent infrapatellar fat pad. The superior, or artic¬ular, portion of the posterior surface is completely covered by hyaline cartilage (Fig. 1.4) and makes up approximately 75% of the height of the patella. This articular car¬tilage, reaching a 4 to 5 mm thickness in its central portion, is the thickest in the body.

Articular Surface

Roughly oval in shape, the articulating portion of the patella is divided into lateral and medial facets by a vertical ridge (Fig. 1.5). The median ridge is oriented in the longi¬tudinal axis of the patella and has roughly the same degree of prominence throughout. The two facets it separates may be roughly equal in size, but, in general, the lateral facet predominates. Wiberg (10) has described the different facet configurations of the patella, which range from medial/lateral facet equality to extreme lateral facet prominence, sometimes referred to as the Wiberg "hunter's cap" patella.

Medial Facet

This portion of the articular surface shows the greatest anatomical variation. It is subdivided into the medial facet proper and a much smaller "odd" facet along the medial border of the patella (Figs. 1.4, 1.5). This odd facet is separated from the remainder of the medial facet by a small vertical ridge. We have labeled this the "secondary ridge" in that it is less prominent than the median ridge and may develop after birth in response to functional loads applied to the knee. The secondary ridge runs obliquely in a generally longitudinal sense, being closer to the median ridge proximally than distally. It is also more prominent distally than proximally on most specimens (see Fig. 1.5, A to C). This ridge conforms to the curve of the lateral border of the medial condyle with the knee in full flexion, whereas the median ridge conforms to the straight medial border of the lateral condyle. This feature has often been overlooked in anatomical textbooks and articles on the patella. One possible reason for this is the fact that this secondary ridge is often purely cartilaginous (Fig. 1.5) and, not always being reflected in subchondral bone, may not be apparent on tangential radiographs of the patellofemoral joint (Fig. 1.6, A and B). There is considerable individual variation in the prominence of the secondary ridge. Also, the odd facet may be in nearly the same plane as the remainder of the medial facet or assume as much as a 60° angle to it. The odd facet may be slightly concave or flat. The remainder of the medial facet also shows great variation, but is usually flat or slightly convex. The surface configuration of the articular surface is determined not only by the underlying subchondral bone, but also by variations in thickness of the patellar cartilage itself. This makes determination of the patellofemoral articular cartilage condition difficult on any patellofemoral radiographs or imaging studies that do not also employ contrast media, computerized tomography, or magnetic resonance imaging.

Lateral Facet

Both longer and wider, the lateral portion of the articular surface is concave in both vertical and transverse planes. Some authors (11, 12) have described three transverse segments on the articular surface, which are delineated in the adult by the presence on both medial and lateral facets of two transverse ridges at the junction of each third. These ridges supposedly isolate three segments of different functional significance as the lower, middle, and upper thirds of the patella are progressively brought into contact (in this order) with the femur during flexion. Emery and Meachim (13) and Ficat (14) have drawn attention to a subtle, relatively constant ridge separating the middle and lower thirds. It is more often present on the lateral facet (Fig. 1.7).

Base of the Patella

The proximal margin of the patella forms a triangle with its apex directly posteriorly. It is inclined distally from posterior to anterior, merging with the anterior surface of the patella. Anteriorly, the surface is very irregular and receives the insertion of the quadriceps tendon with the rectus femoris anteriorly, the vastus medialis and lateralis in the midportion, and the intermedius posteriorly. Posteriorly, there is a small free section between the tendon insertion and the insertion of the synovium at the posterior margin. Often there is a small peripatellar fat pad that fills this space, although at the level of the quadriceps insertion, this fat pad is often nonexistent.

Apex of the Patella

The distal pole forms a rounded projection that receives the attachment of the patellar tendon.

Paramedian Borders

These are roughly vertical at the level of the articular surface of the patella, but then become thinner and run obliquely distally and toward the midline to converge at the apex of the patella. The medial border is considerably thicker than the lateral border while both sides receive the attachment, from posterior to anterior, of the synovium, the joint capsule, the patellofemoral retinaculum, and the quadriceps expansion (the vastus medialis descending more distally than the lateralis). The lateral border receives a fibrous retinaculum, which is comprised of two major layers. The superficial oblique lateral retinaculum reflects anterior to the patella and blends with the expansion, in whereas the deep transverse lateral retinaculum inserts directly into the lateral patella.

        

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