Structure
The patella is a flattened, roughly triangular bone with its base at the top and its apex pointing down towards the tibia.
Anterior surface
The front of the patella is convex and rough, marked by vertical striations and perforated by small vascular foramina. Fibres of the quadriceps tendon sweep over it to continue into the patellar ligament. It is separated from the skin by the prepatellar bursa.
Posterior surface
The upper part of the back of the patella is articular and covered by thick hyaline cartilage, among the thickest in the body. A vertical ridge divides it into a larger lateral facet and a smaller medial facet, matching the shape of the femoral trochlea. A narrow strip along the medial border, the odd facet, contacts the medial femoral condyle only in deep flexion. The lower part of the posterior surface is non-articular, roughened for the patellar ligament and related to the infrapatellar fat pad.
Borders
The base is thick and receives the quadriceps tendon. The medial and lateral borders thin towards the apex and give attachment to the vasti and the patellar retinacula.
Attachments
The patella is suspended between the quadriceps tendon above and the patellar ligament below, and steadied from each side by retinacula.
| Site | Structure attached |
|---|---|
| Base | Rectus femoris (anterior) and vastus intermedius (posterior) via the quadriceps tendon |
| Lateral border | Vastus lateralis and the lateral patellar retinaculum |
| Medial border | Vastus medialis, including its low oblique fibres, and the medial patellar retinaculum |
| Upper medial border | Medial patellofemoral ligament |
| Apex and lower posterior surface | Patellar ligament, running to the tibial tuberosity |
The patellar ligament is functionally the continuation of the quadriceps tendon, which is why the patella is classed as a sesamoid bone. The medial patellofemoral ligament runs from the femur, near the adductor tubercle and medial epicondyle, to the upper medial patella. It is the main soft-tissue restraint against lateral displacement of the patella in early knee flexion.
Articulation and function
The patella articulates only with the femur, gliding in the trochlear groove as part of the knee joint cavity.
- Tracking: in full extension the patella lies above the trochlea and is mobile. As the knee flexes it engages the groove and the contact area moves from its lower to its upper posterior surface.
- Stability: the lateral lip of the trochlea is higher and more prominent than the medial lip, and the oblique fibres of vastus medialis pull medially. Both resist the natural tendency of the patella to slide laterally.
- Q-angle: the angle between a line from the anterior superior iliac spine to the centre of the patella and a line from the centre of the patella to the tibial tuberosity. It reflects the lateral pull of quadriceps and is larger in women on average.
- Leverage: by holding the quadriceps tendon away from the axis of the knee, the patella increases the moment arm of quadriceps and so the force it can generate in extension.
- Protection: it shields the front of the femoral condyles and reduces friction on the quadriceps tendon.
Blood supply
The patella is supplied by the peripatellar anastomosis, formed by the genicular branches of the popliteal artery, the descending genicular artery and the anterior tibial recurrent artery. Vessels enter mainly through the middle of the anterior surface and near the apex, leaving the upper pole relatively poorly supplied.
Clinical relevance
The patella is commonly injured because it is subcutaneous, loaded by powerful quadriceps and prone to slip laterally.
Fracture
A direct blow, such as a fall onto the knee or a dashboard injury, causes comminuted fractures. A sudden quadriceps contraction against a flexing knee causes a transverse fracture. If the fragments separate and the retinacula tear, the extensor mechanism fails and the patient cannot perform a straight leg raise. Displaced fractures need fixation. Avascular necrosis can affect the upper fragment after transverse fractures.
Dislocation
The patella almost always dislocates laterally, typically in adolescents twisting on a flexed knee. The medial patellofemoral ligament usually tears. Risk factors include patella alta (a high-riding patella), trochlear dysplasia, a large Q-angle and ligamentous laxity.
Bipartite patella
Failure of an accessory ossification centre to fuse leaves a separate fragment, usually at the upper lateral corner. It is often bilateral and has smooth, corticated margins on X-ray, which distinguishes it from an acute fracture.
Patellar height and soft tissue
The Insall–Salvati ratio (patellar ligament length divided by patellar length) is about 1 in normal knees; values above 1.2 suggest patella alta and below 0.8 patella baja. Prepatellar bursitis (housemaid's knee) follows repeated kneeling. Sinding-Larsen–Johansson disease is a traction injury of the lower pole in adolescents.