Kneecap
Patella
location_on Anterior knee, embedded within the tendon of the quadriceps femoris
The patella is the largest sesamoid bone in the body, embedded within the quadriceps tendon anterior to the knee joint. It is a triangular bone with a broad superior base and a pointed inferior apex. The patella increases the mechanical advantage of the quadriceps by displacing the patellar tendon anteriorly, increasing the moment arm for knee extension by approximately 50%. The posterior surface has medial and lateral facets that articulate with the femoral trochlea.
Key Anatomical Features
- Largest sesamoid bone in the body, triangular in shape
- Anterior surface is roughened for the quadriceps tendon attachment
- Posterior surface has medial and lateral articular facets for the femoral trochlea
- Base is the broad superior border where the quadriceps tendon inserts
- Apex is the pointed inferior tip from which the patellar ligament originates
- Increases the lever arm of the quadriceps by approximately 50%
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Rectus femoris | Base of patella via quadriceps tendon | Extends the knee and flexes the hip |
| Vastus lateralis | Base and lateral border via quadriceps tendon | Extends the knee |
| Vastus medialis (especially VMO) | Base and medial border via quadriceps tendon | Extends the knee and stabilizes the patella medially |
| Vastus intermedius | Base of patella via quadriceps tendon | Extends the knee |
| Patellar ligament | Apex to tibial tuberosity | Transmits quadriceps force to extend the knee |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Patellofemoral joint | Synovial plane | Femoral trochlea (intercondylar groove) |
Common Pathologies
Patellar fracture
Usually from a direct fall onto the knee or forceful quadriceps contraction. Transverse fractures are most common. If the extensor mechanism is disrupted, the patient cannot perform a straight leg raise.
Patellar dislocation
Almost always dislocates laterally due to the lateral pull of the quadriceps and the valgus angle of the knee. More common in adolescent females. Rupture of the MPFL (medial patellofemoral ligament) is the essential lesion.
Patellofemoral pain syndrome
Anterior knee pain exacerbated by climbing stairs, squatting, and prolonged sitting (theater sign). The most common cause of anterior knee pain in young active patients.
Chondromalacia patellae
Softening and degeneration of the articular cartilage on the posterior patellar surface, causing anterior knee pain and crepitus during knee flexion.
Clinical Relevance
The patellar reflex (knee jerk) tests the L3-L4 spinal segment by tapping the patellar ligament. A bipartite patella (unfused accessory ossification center, usually superolateral) is a normal variant present in about 2% of people and should not be confused with a fracture. The Q-angle (angle between the quadriceps pull line and the patellar tendon) averages 14 degrees in males and 17 degrees in females; increased Q-angles predispose to lateral patellar dislocation.
Development and Ossification
The patella begins as a cartilaginous nodule in the quadriceps tendon. Ossification begins from one or more centers between ages 3 and 5. A bipartite patella (unfused second center) occurs in about 2% of the population, usually at the superolateral corner, and is bilateral in about 50% of cases.
Did You Know?
- Patella means small plate or shallow dish in Latin
- The patella increases quadriceps efficiency by about 50% by increasing the moment arm for knee extension
- Humans are born without ossified patellae; they remain cartilaginous until ages 3-5
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