Introduction

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.

Structure and Function

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]

Embryology

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]

Blood Supply and Lymphatics

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]

Nerves

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]

Muscles

The quadriceps femoris is a large muscle group of the anterior thigh consisting of the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis, that acts as the primary extensor muscle of the knee. These 4 muscles converge into the quadriceps tendon at the superior aspects of the patella, which allows the components of the quadriceps femoris to act together to extend the leg at the knee joint. [5]

Physiologic Variants

The articular surface of the patella consists of 7 facets: 3 medial and 3 lateral facets that articulate with the femoral groove when the knee is flexed, and a facet on the medial border that only articulates with the medial femoral condyle in deep knee flexion when patellar rotation is beyond 90 degrees. Many variations in facet size and configuration have been identified. The Wiberg classification system is used to describe the shape of the patella based on the asymmetry of the medial and lateral facets. Type I, with a prevalence of about 10%, is characterized by concave, nearly symmetrical facets. A type II patella has a medial facet that is flat or slightly convex and much smaller than the lateral facet. Type II patellas are the most common with a prevalence of nearly 65%. A type III patella, which is found in 25% of people, also has a smaller medial facet like that of type II, but the medial facet in a type III patella is always convex. [1]

Many other anatomical variations of the patella have been described, including hypo and hyperplastic variants, patella parva and patella magna, respectively. A “hunter’s cap” patella is one in which the lateral facet accounts for almost the entire articular surface of the patella. Half-moon and pebble-shaped patellas have also been described. [6]

Clinical Significance

Patellar Instability

Patellar instability refers to a range of clinical manifestations from abnormal medial or lateral displacement to dislocation or subluxation of the patella. The cause of patellar instability is often multifactorial but can most commonly be attributed to anatomical and mechanical imbalances of the patellofemoral joint. These imbalances result in chronic instability and secondary flattening of the lateral aspect of the femoral trochlea. This causes the patella to slip laterally during flexion and either dislocate completely or snap back medially to its correct position as flexion progresses. After an acute injury such as dislocation or subluxation, patellar instability can be treated non-surgically with immobilization and decreased weight-bearing. Once the knee has healed, physical therapy can help correct the mechanical imbalances that led to instability in the first place. However, since tissue damage often occurs during a during a dislocation, the patella often remains less stable after the injury than it was prior and recurrence of dislocation is common. After multiple dislocations, surgery is generally recommended to correct the underlying problem. This usually involves arthroscopic reconstruction of the ligaments holding the patella in place. [1],[6],[7]

Trochlear Dysplasia

Trochlear dysplasia, a common cause of recurrent patellar instability, refers to one or more anatomical defects of the femoral trochlea that effect normal tracking of the patella. Defects seen in trochlear dysplasia include decreased height of the medial femoral condyle, decreased trochlear depth, an increased sulcus angle, and/or a decreased lateral trochlear facet that is either flat or convex. Trochlear dysplasia can be identified radiographically by the “crossing sign,” defined as a convergence of the deepest part of the femoral groove with a most prominent aspect of the lateral femoral trochlear facet. The crossing sign is seen in 96% of individuals with objective patellar dislocation and 85% of those with recurrent patellar instability. Trochlear dysplasia is treated similarly to patellar instability with surgical intervention reserved for those who suffer from recurrent dislocations. Surgical interventions include medial patellofemoral ligament reconstruction, tibial tubercle osteotomy, or tracheloplasty. [1],[6],[7]

Patella Alta

Patella alta is characterized by a high riding patella that lies superior to the trochlear groove of the femur, which leads to failure of the patella to articulate with the trochlear groove until later in flexion. This places the patella at an increased risk for dislocation. Patella alta is also associated with other patellofemoral abnormalities including chondromalacia patella, a dysplastic condyle, a dysplastic trochlea, small patella, excessive patellar tilt, joint effusion, and ligamentous laxity. The Insall-Salvati index measures the ratio of the length of the patellar ligament to the greatest diagonal length of the patella on a lateral radiograph of a flexed knee, and it is a well-validated measurement used in the diagnosis of patella alta. Generally, patella alta is diagnosed if the Insall-Salvati ratio is greater than 1.2. Conservative treatment of patella alta involves manual gliding to modify the resting height of the patella before knee extension or taping to correct the positional fault of the patella. Surgically, patella alta can be treated with tibial tuberosity osteotomy where the attachment of the patellar ligament is moved inferiorly on the tibia. [1],[8],[9]

Patella Baja

Patella baja, or a low-riding patella, is characterized by a decreased distance between the inferior pole of the patella and articular surface of the tibia when the patella is in the distal position of the femoral trochlea and/or a permanent shortening of the patellar tendon. Patella baja can cause anterior knee pain, joint stiffness, alterations in joint mechanics, a decreased lever arm, and extensor lag, and a reduction in the range of motion. Like patella alta, patella baja can be defined by the Insall-Salvati index; a ratio of 0.8 or less is diagnostic of patella baja. In normal individuals, the patella does not articulate with the trochlea when the knee is in full extension; whereas, in patella baja, the patella is always in contact with the trochlea, even in full extension. Patella baja is commonly seen after rupture and repair of the patellar ligament, patellar fracture or high tibial osteotomy due to decreased tension forces of the quadriceps muscle that allow the patella to sit more inferiorly. Treatment of symptomatic patella baja usually involves surgical intervention to proximalize the patella. A variety of surgical techniques such as transferring the tibial tubercle to restore patellar height and lengthening of the patellar tendon using autografts or allografts have been described, but there is no gold standard in the treatment patella baja. [1],[6],[10]

Patellofemoral Arthritis

Patellofemoral joint arthritis is characterized by the loss of articular cartilage on either surface of the patella and the trochlear groove. The etiology of patellofemoral arthritis is multifactorial, but in general, it is thought to occur due to abnormal forces across the patella that result in secondary degenerative changes to the joint. Micro or microtrauma, weight and activity level, and genetic quality of the cartilage may also affect the development of patellofemoral arthritis. Most cases of patellofemoral arthritis can be treated non-surgically with nonsteroidal anti-inflammatory drugs (NSAIDs), low-impact exercise, weight loss, physical therapy, cortisone injections, and viscosupplementation, which involves injecting hyaluronic acid into the joint to improve the quality of the synovial fluid. Once non-surgical management has failed, a variety of surgical procedures may be considered to help symptoms. Chondroplasty involves arthroscopically trimming and smoothing the roughened arthritic joint surface. This is usually an option considered in cases of mild to moderate cartilage wear. Cartilage grafting may also be used to fill defects in the articular cartilage, but this procedure is usually reserved for younger patients who only have small areas of cartilage damage. In older patients with refractory symptoms, a partial or total knee replacement may be recommended. During a partial or patellofemoral knee replacement, damaged bone and cartilage surfaces are removed and replaced with metal and polyethylene components secured to the bone with cement. A partial knee replacement cannot be done if there is arthritis involving parts of the knee other than the patellofemoral joint. In that case, a total knee replacement is generally performed to replace all of the cartilaginous surfaces of the knee. Metal prosthesis is placed at both the end of the femur and the top of the tibia with a plastic spacer in between to create a smooth gliding surface. [1]