The patella is the largest sesamoid bone in the human body and is located anterior to knee joint within the tendon of the quadriceps femoris muscle, providing an attachment point for both the quadriceps tendon and the patellar ligament. The patella primarily functions to improve the effective extension capacity of the quadriceps muscle by increasing the moment arm of the patellar ligament. Additionally, the patella protects the quadriceps tendon from frictional forces by minimizing tendon contact with the femur and acts as a bony shield for deeper structures in the knee joint.

The patella is located deep to the fascia lata and fibers of the rectus femoris tendon anterior to the knee joint. The superior third of patella acts as the attachment point for the tendinous fibers of the rectus femoris and vastus intermedius of the quadriceps, while the vastus medialis and lateralis attach on the medial and lateral borders of the patella respectively. The individual tendons of the muscles that make up the quadriceps femoris coalesce at their attachment points and continue superficially over the anterior surface of the patella to form the deep fascia, which adheres to the bone. The patellar ligament envelopes the inferior third of the patella and attaches the bone to the tibial tuberosity.

The patella primarily functions to improve quadriceps efficiency by acting as a fulcrum to increase the moment arm of the extensor mechanism of the knee. In physics, a moment refers to the tendency of a force to cause rotation of an object around a specific point or axis; it is equal to the product of a force and its moment arm, the perpendicular distance from the line of action of that force to the axis of rotation. The force required for knee extension (torque) is directly dependent on the moment arm of the knee joint, the perpendicular distance between the patellar ligament and the axis of rotation at the knee. [1]

During extension from a fully flexed position, the patella initially serves primarily as a link between the quadriceps tendon and the patellar ligament, allowing the quadriceps to generate torque on the tibia. However, twice as much torque is needed for the final 15 degrees of extension compared to that which is required to get to that point from full flexion, and the patella helps achieve this by increasing the moment arm during extension. By displacing the quadriceps tendon-patellar ligament linkage away from the axis of knee rotation, the effective moment arm is increased, which contributes an additional 60% of torque that is needed for the last 15 degrees of knee extension. [1]

Static and dynamic alignment of the patella is clinically important for understanding the etiologies of patellofemoral pain. The static alignment of the patella depends on the depth of the femoral sulcus, the height of the lateral femoral condyle wall, and the shape of the patella. Gross alignment of the patella is often assessed in the supine position with the knee in full extension since there is minimal contact between the femur and patella, and the patella is the most mobile in this position. With the knee in full extension, the patella usually lies superior to the trochlea and in the middle of the two condyles, although there may be a slight lateral deviation. When the knee is in slight flexion, the patella should lie at or slightly proximal to the joint line. In this position when the knee is bent to 30 degrees, the ratio of patellar ligament length compared to the patellar height should be around 1.0. A ratio significantly lower or greater than 1.0 may be indicative of patella baja or patella alta respectively. Individuals with patella alta are at greater risk for patellar subluxation. Each border of the patella should also be equidistant from the femur. Anterior or posterior tilt is described by the position of the inferior pole of the patella in the sagittal plane. Depression of the inferior pole is referred to as an inferior tilt, while the elevation of the inferior pole is referred to as superior tilt. Inferior tilt may pinch or irritate the patellar fat pad deep in the patellar ligament and cause pain. In the transverse plane, lateral or medial tilt refers to depression of either the lateral or medial border of the patella, respectively. Lateral tilt can lead to patellofemoral compression syndrome. Rotation of the patella is described by the direction of rotation of the inferior pole. Lateral or medial rotation of the patella may suggest underlying torsion of the tibia. [2]

Dynamic movement, or patellar tracking, is dependent upon active quadriceps contraction, the extensibility of the connective tissue around the patella, and the geometry of the patella and trochlear groove. During tibiofemoral motion, the patella acts as a gliding joint and has movement in multiple planes. Superior glide occurs during extension of the knee as the quadriceps contracts and pulls the patella superiorly. Conversely, inferior glide occurs during flexion at the knee. Lateral and medial glide refer to the tracking of the patella to the lateral or medial side. In normal tracking of the patella, there should be a little medial or lateral glide, although in full knee extension the patella does sit slightly lateral due to external rotation of the tibia. The articulating surface of the patella changes as the knee passes through its range of motion. As the knee flexes, the contact point of the patella moves inferiorly and posteriorly along the femoral condyles, and more proximally on the patella itself. Initially, during flexion, the lateral facet of the patella is the first aspect to contact the uppermost part of the lateral femoral condyle, but, by 30 degrees of flexion, the contact area is equally distributed on either side of the patella and femoral condyles. The contact area of the patella also expands with knee flexion, increasing from approximately 2.0 cm at 30 degrees flexion to about 6.0 cm at 90 degrees. This distributes the joint forces over a greater surface area and helps prevent the potentially damaging effects of repetitive high compressive loads on the joint. By 90 degrees of knee flexion, the superior aspect of the patella comes into contact with an area of the femoral groove just above the femoral notch. In deep flexion, the patella bridges the intercondylar notch with contact occurring only at the outermost medial and lateral borders of the patella. When the knee is in full flexion, the only contact exists between the odd facet of the patella and the lateral surface of the medial femoral condyle. [2]

In utero, the patella develops from a continuous band of fibrous connective tissue of the mesenchymal interzone along the anterior surface of the knee joint at the distal end of the femur. Around week 9 of gestation, chondrification of this fibrous connective tissue begins to separate the previously continuous band into the quadriceps tendon superiorly, and the patellar ligament inferiorly, and the patella becomes completely cartilaginous by week 14. The medial and lateral patellar facets are initially equal in size, but the lateral facet usually becomes larger than the medial facet by week 23 of gestation. Primary ossification of the patella does not typically occur until age 5 or 6, but radiographic evidence of ossification may be present by age 2 or 3. Initially, in this process, there are multiple small foci of ossification, but these quickly coalesce and spread to the margins of the what will eventually become the adult bone. Periosteum quickly forms on the anterior surface of the patella, but the other margins of the bone retain a chondro osseous interface that persists through adolescence, leaving these areas susceptible to avulsion fractures until skeletal maturation. [1]

The patella is supplied by a vast vascular network that can be separated into extraosseous and intraosseous divisions. With contributions from the anterior tibial recurrent arteries, the supreme, medial superior and inferior, and lateral superior and inferior geniculate arteries form an extraosseous anastomotic ring around the patella. The intraosseous vascular supply consists of the mid patellar vessels, which enter vascular foramina on the anterior surface of the middle third of the patella, and the polar vessels, which enter between the attachment of the ligamentum patellae and the articular surface on the deep surface of the patella. [1]

The anterior cutaneous innervation of the knee is derived from nerve roots L2 through L5. The anteromedial innervation of the knee comes from the genitofemoral, femoral, obturator, and saphenous nerves. The lateral femoral and lateral sural cutaneous nerves supply anterolateral innervation.[1] The intraosseous innervation of the patella is subject to some debate. Several studies have concluded that the primary intraosseous innervation is derived from a medially located neurovascular bundle, but others have found that both superomedial and superolateral nerves were important for patellar innervation. [3],[4]