"A round bone at the front of the knee" is how the patella is often referred to. If only it were that simple! The patella is nearly as much a piece of cartilage as it is a bone. When viewed from the top it is indeed roundish, though the inferior pole in some patients gives it more the appearance of a plump rain drop. Whereas the dorsal (back) surface is all bone, three quarters of the ventral surface (underbelly) is cartilaginous. And what cartilage!

The articular cartilage of the patella is the thickest cartilage in the human body [cartilage is a thin, super-smooth surface - 8 times more slippery than ice - that covers the ends of our bones. It allows the bones to move about each other quietly and painlessly].

Patellar cartilage is not one smooth, shield-like surface. Rather, it features a number of facets. These facets vary tremendously in size and shape from patient to patient to the point where one could legitimately call them the fingerprints of the knee.

When one looks at a kneecap from which the cartilage has been removed - and therefore only bone is left, the undersurface appears to have just two facets. This is of course what happens on an x-ray, since x-rays show only bone, not cartilage. It is therefore common to refer to the medial facet and the lateral facet of the kneecap.

Considering the importance given to cartilage lesions in "patellar pain", it is interesting to note that one investigator has noted from a personal experience that the articular cartilage of the patella is insensate (S. Dye, personal communication).

The articulation between the patella and the trochlea in the axial plane is relatively congruent, that is as one goes from medial to lateral there is contact between the patellar cartilage and that of the underlying trochlea. On anatomic sections there is no contact at the center of the trochlea.

During every day use it is probable that this space disappears as the patellar cartilage deforms. When the patellofemoral joint is seen from the side, the situation is quite different. Most of the patellar cartilage is not in contact with the trochlea. As such it can be said that in the sagittal plane the patellofemoral joint is quite incongruent.

Viewed from the side, the patella is roughly rectangular with a triangular piece tacked on to its lower end. This piece is the non-articulating “inferior pole”. In normal patients the longest (diagonal) length of the patella is somewhere between 1.2 and 1.5 times the length of the articular surface. This becomes important when imaging the patella.

Viewed in axial cross-section, the bony patella is V-shaped, the lateral side being longer. See Anatomic Abnormalities.