Patella

Introduction

Introduction to the Patella

The patella (kneecap) is the largest sesamoid bone in the human body. It is a flat, triangular bone situated within the tendon of the quadriceps femoris muscle, anterior to the distal femur. The patella protects the knee joint, increases the mechanical advantage of the quadriceps by acting as a pulley, and improves the efficiency of knee extension. Its deep surface articulates with the femoral trochlea, forming the patellofemoral joint. Despite its small size, the patella is essential for normal gait, stair climbing, and activities requiring powerful knee extension. Its superficial location makes it vulnerable to direct trauma and fractures, while its sesamoid nature predisposes it to tracking disorders and degenerative changes.

Study

1. General Features and Shape

The patella is a flat, sesamoid bone with a triangular shape. It has a broad superior base, an inferior apex, two surfaces (anterior and posterior), and three borders (superior, medial, and lateral). The bone lies within the quadriceps tendon, and its apex gives attachment to the patellar ligament (ligamentum patellae), which connects it to the tibial tuberosity. The patella is widest at its base and tapers to a blunt point inferiorly. The entire bone develops within the tendon of the quadriceps femoris, and its articular surface varies in morphology.

2. Surfaces of the Patella

Anterior surface: The anterior surface is convex and roughened by the attachment of the quadriceps tendon. The superficial fibres of the rectus femoris and the vastus intermedius attach to the base; the vastus medialis and lateralis insert into the corresponding medial and lateral borders. The anterior surface is covered by a prepatellar bursa and then by skin, making it subcutaneous and palpable.

Posterior (articular) surface: The posterior surface is divided into articular and non‑articular regions. The upper three‑quarters is articular and covered by thick hyaline cartilage—the thickest in the body (up to 5–7 mm). A broad vertical ridge separates the articular surface into a larger lateral facet and a smaller medial facet. A second, faint ridge may further subdivide the medial facet into a small 'odd' facet along the extreme medial border. The facets articulate with the corresponding surfaces of the femoral trochlea during knee movements. The lower quarter of the posterior surface is rough and non‑articular, related to the infrapatellar fat pad (Hoffa’s fat pad).

3. Base, Apex, and Borders

Base (superior border): The base is the broadest part of the patella, facing superiorly and posteriorly. It receives the insertion of the rectus femoris and the vastus intermedius tendons of the quadriceps muscle.

Apex (inferior pole): The apex is the pointed inferior end, directed downwards. It gives attachment to the patellar ligament (tendon), which runs to the tibial tuberosity. The infrapatellar fat pad lies behind the apex.

Medial border: Provides attachment to the vastus medialis muscle and the medial patellar retinaculum.

Lateral border: Gives attachment to the vastus lateralis muscle and the lateral patellar retinaculum. The lateral retinaculum receives contributions from the iliotibial tract.

4. Ossification and Development

The patella ossifies from a single primary centre (occasionally multiple) that appears at 3–6 years of age. Ossification begins in the mid‑portion and expands radially. The entire process is usually complete by puberty. In about 2% of the population, a secondary ossification centre at the superolateral corner fails to fuse, giving rise to a bipartite patella, which is usually asymptomatic but may be mistaken for a fracture. The patella is cartilaginous at birth, and its ossification centre can be visualized radiographically from about age 3–4. The bone reaches adult morphology by the late teens.

5. Functions of the Patella

The patella serves several biomechanical roles. It acts as a pulley, increasing the lever arm of the quadriceps tendon and thus the efficiency of knee extension by up to 50%. It centralises the divergent forces of the quadriceps components into a single line of pull. The thick articular cartilage reduces friction and distributes compressive forces across the femoral trochlea. The patella also protects the anterior aspect of the knee joint from direct trauma.

6. Clinical Considerations

Patellar fracture: Commonly caused by direct trauma (e.g., dashboard injury) or violent quadriceps contraction. Transverse fractures with displacement disrupt the extensor mechanism and require surgical fixation. Comminuted fractures may require partial or total patellectomy.

Bipartite patella: An anatomical variant where the superolateral corner is separated by a fibrocartilaginous synchondrosis. It is usually asymptomatic but can be mistaken for a fracture. Painful bipartite patella may require excision of the fragment.

Patellar dislocation: Usually lateral, due to the more prominent lateral trochlear lip and the Q‑angle. Associated with medial patellofemoral ligament (MPFL) rupture, medial retinacular strain, and osteochondral fractures. Recurrent instability may require MPFL reconstruction or tibial tubercle transfer.

Patella alta and baja: Patella alta (high‑riding patella) is associated with recurrent dislocation and chondromalacia; patella baja (low‑riding patella) can cause restriction of knee flexion. The Insall‑Salvati ratio is used to assess patellar height.

Chondromalacia patellae: Softening and fibrillation of the patellar articular cartilage, causing anterior knee pain. It often affects the medial and odd facets. Patellofemoral osteoarthritis is a common degenerative condition.

Summary

Summary of the Patella

The patella is a large, triangular sesamoid bone within the quadriceps tendon. Its anterior surface is subcutaneous; its posterior surface bears articular facets for the femoral trochlea, covered by the thickest articular cartilage in the body. It ossifies at 3–6 years and may form a bipartite variant. The patella increases quadriceps efficiency and protects the knee joint. Clinical conditions include fractures, dislocations, patellar height abnormalities, and chondromalacia.