Patella

The patella, commonly known as the kneecap, is a triangular sesamoid bone located at the front of the knee joint. It plays a crucial role in knee mechanics, acting as a fulcrum for the quadriceps muscle and protecting the anterior aspect of the knee. Understanding its anatomy, development, and function is essential for diagnosing and managing knee disorders.

Anatomy of the Patella

Gross Anatomy

The patella is roughly triangular in shape, with a base at the superior end and an apex at the inferior end. It is the largest sesamoid bone in the human body and lies within the tendon of the quadriceps femoris muscle.

• Anterior Surface: Convex and rough, providing attachment for tendinous structures.
• Posterior Surface: Smooth and concave, divided into medial and lateral facets that articulate with the femoral condyles.
• Borders:
  • Superior border: Forms the base of the patella
  • Inferior border: Forms the apex, giving attachment to the patellar ligament
  • Medial border: Provides attachment for the medial retinaculum
  • Lateral border: Provides attachment for the lateral retinaculum

Internal Structure

The patella consists of an outer shell of cortical bone surrounding a core of cancellous bone. This structure allows it to withstand compressive forces while maintaining lightweight characteristics.

• Bone Composition: Cortical bone on the surface with spongy cancellous bone internally.
• Ossification Centers: Develop during childhood, typically appearing between 3 and 6 years of age.
• Patellar Facets: Medial and lateral facets articulate with the femoral condyles and are covered by articular cartilage.

Embryology and Development

The patella begins as a cartilaginous structure during fetal life and undergoes ossification postnatally. Its development is crucial for normal knee mechanics and patellofemoral articulation.

• Formation: The patella forms from a cartilaginous model within the quadriceps tendon during fetal development.
• Ossification Timeline: Primary ossification centers appear between ages 3 and 6, and secondary centers may appear later, fusing in adolescence.
• Variations: Some individuals may have bipartite or multipartite patella due to incomplete fusion of ossification centers, usually asymptomatic.

Blood Supply

The patella receives its blood supply from multiple arteries that form an anastomotic network around the knee, ensuring adequate perfusion for bone and cartilage maintenance. Vascularization is crucial for healing after injury or surgery.

• Superior Genicular Arteries: Branches of the femoral artery supplying the upper portion of the patella.
• Inferior Genicular Arteries: Branches of the popliteal artery supplying the lower portion of the patella.
• Anterior Tibial Recurrent Artery: Contributes to the patellar anastomosis.
• Patellar Anastomotic Network: A network of vessels around the patella ensures collateral circulation in case of vascular compromise.

Innervation

The patella is innervated primarily by branches of the femoral and tibial nerves. Sensory innervation plays a significant role in proprioception and pain perception, which is important in both normal function and pathology.

• Femoral Nerve: Provides sensory fibers to the anterior aspect of the patella and surrounding structures.
• Tibial Nerve: Supplies posterior structures contributing to proprioceptive feedback.
• Function in Pain Perception: Nerve endings in the periosteum and articular cartilage help detect injury or stress to the patella.
• Proprioception: Sensory input from the patella contributes to knee joint position sense, aiding in coordinated movement.

Articulations and Ligamentous Attachments

Patellofemoral Joint

The patella articulates with the femur at the patellofemoral joint, which allows smooth gliding during knee flexion and extension. This joint reduces friction and distributes forces across the anterior knee.

• Articular Surfaces: The posterior surface of the patella has medial and lateral facets that articulate with the femoral trochlea.
• Function: Facilitates knee extension by enhancing the leverage of the quadriceps muscle.

Ligament Attachments

The patella serves as a central anchoring point for multiple tendons and ligaments, stabilizing the knee and transmitting forces from the quadriceps to the tibia.

• Quadriceps Tendon: Attaches to the superior border of the patella and continues the extensor mechanism of the knee.
• Patellar Ligament: Extends from the apex of the patella to the tibial tuberosity, transmitting quadriceps force to the tibia.
• Medial Retinaculum: Provides medial stability and prevents lateral displacement of the patella.
• Lateral Retinaculum: Supports lateral stability and assists in patellar tracking.

Function of the Patella

The patella plays a vital role in the biomechanics of the knee joint by improving the efficiency of the quadriceps muscle and protecting anterior knee structures. Its functions are integral to daily activities such as walking, running, and jumping.

• Mechanical Advantage: Increases the leverage of the quadriceps tendon, enhancing knee extension strength.
• Force Transmission: Distributes tensile forces from the quadriceps to the tibia effectively.
• Protection: Shields the anterior knee structures, including the joint capsule and underlying bones, from trauma.
• Stability: Contributes to proper patellar tracking within the femoral groove, maintaining joint alignment.

Biomechanics

The patella is essential in knee biomechanics, influencing the forces and motion of the extensor mechanism. Its position and movement affect patellar tracking and load distribution across the knee joint.

• Patellar Tracking: The patella glides within the femoral trochlear groove during flexion and extension, guided by the quadriceps tendon and retinacular ligaments.
• Force Distribution: Transmits compressive and tensile forces across the knee, reducing stress on the femoral condyles.
• Lever Arm Function: Increases the moment arm of the quadriceps tendon, enhancing knee extension efficiency.
• Influence on Knee Stability: Proper patellar alignment contributes to medial-lateral stability and prevents dislocation or subluxation.

Clinical Significance

Common Disorders

The patella is prone to various injuries and pathological conditions that can impair knee function and cause pain.

• Patellar Fracture: Can occur due to direct trauma or falls, often requiring surgical intervention depending on displacement.
• Patellar Dislocation: Usually lateral, often associated with ligamentous laxity or trauma.
• Patellofemoral Pain Syndrome: Pain around the patella due to maltracking or overuse.
• Chondromalacia Patellae: Softening or degeneration of articular cartilage, leading to anterior knee pain.

Congenital Variations

• Bipartite Patella: Non-fused accessory ossification centers, generally asymptomatic but may cause pain after trauma.
• Patella Alta: High-riding patella, associated with instability or predisposition to dislocation.
• Patella Baja: Low-riding patella, can limit knee flexion and alter biomechanics.

Imaging of the Patella

Imaging is essential for diagnosing patellar injuries, congenital variations, and degenerative conditions. Different modalities provide complementary information on bone, cartilage, and soft tissue structures.

• X-ray: Standard radiographs evaluate bone alignment, fractures, and patellar height.
• MRI: Provides detailed images of cartilage, ligaments, tendons, and bone marrow changes.
• CT Scan: Useful for complex fractures and preoperative planning, giving 3D visualization of bony structures.
• Ultrasound: Assesses superficial structures like the quadriceps tendon, patellar ligament, and effusions in real time.

Surgical Considerations

Patellar surgery is indicated in cases of fracture, malalignment, or degenerative joint disease. Surgical approaches aim to restore function and maintain patellofemoral stability.

• Patellar Fracture Fixation: Techniques include tension band wiring, screws, and partial patellectomy in severe cases.
• Patellar Resurfacing in Knee Arthroplasty: Used in total or partial knee replacements to improve joint mechanics and reduce pain.
• Realignment Procedures: Medial patellofemoral ligament reconstruction or tibial tubercle transfer to correct maltracking or recurrent dislocation.

Rehabilitation and Physiotherapy

Rehabilitation after patellar injury or surgery is essential to restore strength, flexibility, and normal knee function. Physiotherapy focuses on reducing pain, improving range of motion, and preventing recurrence of patellofemoral disorders.

• Post-injury Rehabilitation: Includes rest, ice, compression, elevation, and gradual mobilization to promote healing.
• Strengthening Exercises: Focus on quadriceps, hamstrings, and hip muscles to stabilize the patella and improve knee mechanics.
• Flexibility Training: Stretching of the quadriceps, hamstrings, and iliotibial band to reduce tension on the patella.
• Prevention: Emphasis on proper biomechanics, footwear, and conditioning to avoid recurrent patellofemoral pain or dislocation.

References

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