PatellaEdit
The patella, commonly known as the kneecap, is a flat, triangular bone situated at the front of the knee joint. It is the largest sesamoid bone in the human body, developing within the tendon of the quadriceps muscle group as part of the mechanism that powers knee extension. The patella articulates with the femur in the patellofemoral joint, riding within the groove of the distal femur called the femoral trochlea. By acting as a fulcrum, the patella increases the leverage of the quadriceps and improves the efficiency of knee extension, while also helping to protect the knee joint from friction and impact during movement.
In life, the patella participates in complex motion as the knee bends and straightens. Its posterior surface is covered with articular cartilage to minimize wear as the patella glides over the femoral trochlea. The shape and tracking of the patella are influenced by the surrounding soft tissues, including the quadriceps tendon, the patellar tendon, and the retinacular fibers that stabilize the kneecap within the patellofemoral joint. The patella also serves as a protective shield for the front of the knee, distributing some load from the quadriceps across the knee as it flexes.
Anatomy
Structure and surfaces: The patella is a roughly triangular bone with a broad superior base and a pointed inferior apex. Its posterior aspect bears a smooth articular surface that interacts with the femoral trochlea. The articular surface typically comprises medial and lateral facets, whose contact patterns change with knee flexion and extension.
Attachments and biomechanics: The patella is embedded within the quadriceps tendon proximally, with the distal aspect connected to the tibial tuberosity via the patellar tendon. This arrangement transforms the strong, vertical pull of the quadriceps into knee extension torque. The patellofemoral joint is a component of the larger knee joint, and its mechanics are influenced by the alignment of the lower limb, the depth of the femoral trochlea, and the tone and balance of surrounding muscles.
Blood supply and innervation: The patella receives arterial input from branches of the genicular arteries, part of the vascular network that nourishes the knee joint, and its sensory innervation is derived from branches of the femoral nerve and related nerves that convey information about joint position and pain.
Development and variation: The patella forms by ossification during childhood and adolescence. In most people, a single ossification center fuses over time, but some individuals have bipartite or tripartite patellae, where parts of the bone remain separated or only partially fused. These variations are usually incidental and may be discovered on imaging after trauma or during evaluation for knee pain.
Function
The patella enhances the mechanical advantage of the quadriceps by increasing the moment arm of the knee extensor mechanism. As the knee extends, the patella travels within the patellofemoral groove, altering its contact area with the femur. This arrangement reduces the stress on the quadriceps tendon by distributing forces more broadly and can lower the risk of tendon injury. The patella also contributes to protecting the knee joint from anterior impacts and helps convert muscle force into efficient, controlled leg extension.
Development and variation
Ossification of the patella typically occurs in childhood, with fusion of the ossification centers progressing through adolescence. The timing and pattern of development can vary between individuals. Bipartite or tripartite patellae represent common anatomical variants, present in a minority of people, and most often become clinically relevant only if there is trauma or anomalous tracking of the patella within the femoral groove.
Clinical significance
The patella is involved in several common knee conditions, ranging from overuse injuries to acute trauma.
Patellofemoral pain syndrome: A frequent cause of anterior knee pain, especially in athletes and active individuals. Contributing factors include maltracking of the patella, imbalances in hip and thigh musculature, overuse, and anatomical variations. Management emphasizes targeted physical therapy to strengthen the quadriceps and hip abductors, improve conditioning, and optimize patellar tracking, sometimes accompanied by bracing or patellar taping.
Patellar dislocation and subluxation: The kneecap can temporarily or persistently move out of its groove, typically laterally, often following trauma or sudden directional changes. Treatment may involve reduction, immobilization, and rehabilitation to restore stability and strength, with surgical options considered in recurrent cases.
Patellar fracture: Fractures can occur from direct trauma to the front of the knee or from indirect mechanisms during movement. Management depends on fracture pattern and displacement and can range from immobilization to surgical fixation.
Patellofemoral osteoarthritis: Degenerative changes at the patellofemoral joint may contribute to knee pain and functional limitation, particularly in older adults. Management includes activity modification, physical therapy, pain control, and, in some cases, surgical intervention.
Other considerations: Anatomical variants such as bipartite patella are typically incidental findings; their clinical significance is tied to symptoms and the history of trauma. Imaging and evaluation should distinguish these variants from fractures or persistent patellar maltracking.
History and evolution of understanding
Historically, descriptions of knee anatomy and function evolved from early anatomical studies to modern biomechanical analyses. Advances in imaging and modeling have clarified how patellar tracking is governed by bone geometry, soft-tissue constraints, and neuromuscular control, informing contemporary approaches to prevention and rehabilitation of patellofemoral disorders.