The patella (figs. 467, 468), the largest of the sesamoid bones, is situated in front of the knee-joint in the tendon of the quadriceps femoris. It is flat and triangular, and has an anterior and a posterior surface, three borders, and an apex.
The anterior surface is convex, perforated by apertures for the passage of nutrient vessels, and marked by numerous rough, longitudinal striae. It is separated from the skin by a bursa and is covered, in the recent state, by an expansion from the tendon of the quadriceps femoris ; this expansion is continuous below with the superficial fibers of the ligamentum patella;. The posterior surface presents in its upper part a smooth, oval, articular area, divided into two facets by a vertical ridge ; the ridge corresponds to the groove on the patellar surface of the femur, and the facets to the medial and lateral parts of the same surface ; the lateral facet is the broader and deeper. A narrow strip is marked off from the lower and medial part of the medial facet. This area comes into contact with the medial condyle of the femur in extreme flexion of the knee-joint. Below the articular surface there is a rough, convex, non-articular area ; its lower part gives attachment to the ligamentum patella; ; its upper part is covered by the infrapatellar pad of fat.
The base or superior border is thick, and sloped from behind, downwards, and forwards : it gives attachment, except near its posterior margin, to that portion of the quadriceps femoris which is derived from the rectus femoris and vastus intermedius, The medial and lateral borders are thinner and converge below: they give attachment to those portions of the quadriceps femoris which are derived from the vastus medialis et lateralis. Near the junction of the superior and lateral borders there is a small, shallow, circular depression into which a part of the tendon of the vastus lateralis is inserted.
Structure.-The patella consists of a nearly uniform dense spongy substance, covered by a thin compact lamina. The cancelli immediately beneath the anterior surface are arranged parallel with it. In the rest of the bone they radiate from the articular surface towards the other parts of the bone.
Ossification.-The patella is ossified from a single center, which usually makes its appearance in the second or third year, but may be delayed until the sixth year. Ossification is completed about the age of puberty.
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