Overview
The patella is a sesamoid bone, that is, a bone that develops inside a tendon. It lies within the tendon of insertion of the quadriceps femoris muscle and rests on the trochlea of the femur, with which it forms the femoropatellar joint, part of the stifle joint (articulatio genus). It works as a pulley: it lifts the tendon away from the axis of rotation of the stifle and so increases the lever arm of the quadriceps, while protecting the tendon from friction against the femur.
Shape and surfaces
Ovoid, elongated proximodistally and compressed craniocaudally.
- Base (basis patellae): broad proximal end where the quadriceps tendon arrives.
- Apex (apex patellae): pointed distal end from which the patellar ligament (ligamentum patellae) runs to the tibial tuberosity. This ligament is in fact the continuation of the quadriceps tendon.
- Cranial surface (facies cranialis): rough and convex, subcutaneous and easy to palpate.
- Articular surface (facies articularis): smooth and cartilage-covered, slightly convex in both directions; it fits the groove of the femoral trochlea.
Parapatellar fibrocartilages
On either side of the patella the quadriceps tendon contains the medial and lateral parapatellar fibrocartilages (fibrocartilagines parapatellares), which widen the contact area with the trochlear ridges and anchor the femoropatellar ligaments that stabilise the patella medially and laterally.
Extensor mechanism
Quadriceps, patella, patellar ligament and tibial tuberosity form a functional unit: the extensor mechanism of the stifle. For the patella to stay centred in the trochlea during flexion and extension, these structures must be aligned in the same plane as the femoral shaft. Any deviation (femoral varus, medially displaced tibial tuberosity, tibial rotation) tends to pull the patella out of the groove.
Clinical relevance
- Patellar luxation: the most common disease of this bone. In small breeds it is almost always medial; in large breeds it may be lateral. When the patella luxates chronically, the cartilage of its articular surface erodes.
- Patellar fractures: uncommon; they result from direct trauma and the fragments are pulled apart by the quadriceps.
- Radiology: on the lateral stifle view the patella should sit over the trochlea; a position that is too proximal (patella alta) is associated with luxation in large breeds. The tangential craniocaudal ("skyline") view shows the depth of the trochlea.