Patella

The patella, commonly known as the kneecap, is a flat, triangular sesamoid bone embedded within the tendon of the quadriceps femoris muscle. It plays a critical role in the biomechanics of the knee joint, aiding in force transmission and protecting underlying structures. As the largest sesamoid bone in the human body, the patella has a unique anatomical form and function, essential for proper locomotion and knee stability.

Structure

The patella has a broad superior end called the base and a pointed inferior end known as the apex. The anterior surface is convex, roughened for ligament attachment, and subcutaneous, making it easily palpable. The posterior surface is smooth and divided into medial and lateral facets, which articulate with the femoral condyles.

Surfaces

• Anterior surface: Convex and rough, provides attachment for tendinous fibers and subcutaneous tissue.
• Posterior surface: Smooth, covered with articular cartilage and divided by a vertical ridge into medial and lateral facets.

Borders and Angles

• Superior border (base): Wide and flat, gives attachment to the quadriceps tendon.
• Inferior angle (apex): Pointed and gives attachment to the patellar ligament.
• Medial and lateral borders: Provide attachment to retinacular fibers and act as guiding tracks within the trochlear groove of the femur.

Location

The patella is situated anterior to the distal femur and embedded within the quadriceps tendon. It lies within the anterior compartment of the knee joint and is subcutaneous, making it easily accessible for clinical palpation and examination. Its posterior surface articulates with the trochlear surface of the femur, forming the patellofemoral joint.

Ossification

The patella begins to ossify between the ages of 3 to 6 years. Ossification starts from multiple centers that gradually fuse into a single bony structure. Incomplete fusion of these centers may result in a bipartite patella, often discovered incidentally in imaging studies.

Function

• Force transmission: The patella increases the leverage of the quadriceps tendon by elevating its angle of insertion onto the tibia, which enhances knee extension strength.
• Protection: Shields the anterior surface of the knee joint from trauma.
• Articular surface guidance: Helps maintain alignment of the quadriceps tendon during knee movement, ensuring smooth articulation with the femoral condyles.

Articulations

The patella forms a synovial joint with the femur called the patellofemoral joint. This articulation is part of the knee complex and is critical for flexion and extension. During movement, the patella glides along the trochlear groove of the femur.

Articulating Bone	Joint Type	Movement
Femur	Synovial (plane-type)	Gliding during knee flexion and extension

Ligament and Tendon Attachments

• Quadriceps tendon: Attaches to the superior pole (base) of the patella and contains the patella within its fibers.
• Patellar ligament: Extends from the apex of the patella to the tibial tuberosity.
• Medial and lateral patellar retinacula: Reinforce the joint capsule and help stabilize patellar tracking.

Relations

• Anterior: Subcutaneous tissue and skin.
• Posterior: Femoral condyles and the synovial membrane of the knee joint.
• Superior: Quadriceps femoris tendon.
• Inferior: Patellar ligament leading to the tibial tuberosity.

Blood Supply

The patella receives its vascular supply primarily from the genicular branches of the femoral, popliteal, and anterior tibial arteries. These vessels form a periarticular anastomosis around the knee, ensuring consistent blood flow even during movement.

Nerve Supply

Innervation of the patella and surrounding structures comes from the femoral nerve (via branches to the quadriceps) and contributions from the saphenous nerve. These nerves carry both proprioceptive and pain-related fibers.

Developmental and Anatomical Variations

• Bipartite/tripartite patella: A congenital condition where the ossification centers fail to fuse completely. Usually asymptomatic but can be mistaken for fractures.
• Patella alta: Abnormally high patella positioning.
• Patella baja: Abnormally low patella positioning.

Clinical Significance

• Patellar dislocation: Most often lateral, particularly in young athletes and individuals with shallow trochlear grooves or weak medial stabilizers.
• Chondromalacia patellae: Softening and degeneration of the articular cartilage on the posterior patellar surface, often leading to anterior knee pain.
• Patellar tendon rupture: A complete tear of the patellar ligament, usually associated with trauma or degeneration.
• Patellar fractures: Often result from direct trauma and can disrupt the extensor mechanism of the knee.

Imaging

The patella is clearly visible on standard anteroposterior (AP) and lateral knee X-rays. Special views like the sunrise (skyline) view are used to evaluate patellofemoral alignment and cartilage surface. CT and MRI provide detailed information about tracking, soft tissue injury, or cartilage integrity.

Palpation

Due to its superficial location, the patella is easily palpated. The base can be felt superiorly, while the apex lies inferiorly near the tibial tuberosity. Medial and lateral movement can be appreciated during physical examination, especially during flexion and extension of the knee.