Patellofemoral Arthritis 

Arthritis of the patella refers to the presence of degenerative changes underneath the kneecap (the patella). Manifestations of this form of arthritis range from no symptoms to vague anterior knee pain to severe difficulties with stair climbing and ambulation. The term chondromalacia is used to describe early alterations in the articular cartilage of the patella that may eventually lead to patellofemoral arthritis. (See Clinical Presentation.)

Patellofemoral arthritis usually affects patients who have patellofemoral laxity, subluxation, malalignment, and a high-riding patella, as well as the same patient population affected by arthritis of other joints. Increasing age, obesity, overuse, chronic joint instability, prior interarticular fractures, and systemic inflammatory conditions are risk factors for the development of arthritis in the hip, knee, patella, and other weight-bearing joints. Patellofemoral arthritis can also occur in younger patients as a result of malalignment or trauma. (See Epidemiology.)

Ludloff, Axhausen, and Budinger reported damage to the articular cartilage of the patella after trauma in the early 1900s. In 1924, Koening first used the term chondromalacia to describe this condition. In the 1960s, Outerbridge described damaged articular cartilage of the patella in young patients undergoing open meniscectomy.^{[1, 2]}

Subsequent research has focused on anatomic and biomechanical causes of damage to the patellofemoral joint, such as shear and compressive forces, abnormal patellar tracking, and patella subluxation and tilting. (See Etiology.)

Patellectomy was one of the first surgical procedures performed for patellofemoral arthritis. However, realization of the importance of the biomechanical role of the patella has led to the development of alternative surgical procedures. (See Treatment and Management.)

Go to Rheumatoid Arthritis for more complete information on this topic.

See the images of patellofemoral arthritis below.

The patella is a sesamoid bone, the largest in the body, embedded in the quadriceps and patella tendons. A subcutaneous bursa separates the patella from the overlying skin. The articular surface of the patella consists of 4 facets: inferior, middle, superior, and medial vertical.

During flexion, the patella moves within a groove in the femur, the trochlea. The patella is thought to travel in a J-shaped pattern, moving laterally with knee extension.^[3]

The lateral aspect of the trochlea, the vastus medialis, and the medial patella-femoral ligament prevent excessive lateral translation. The anatomy of the distal femur and the vastus lateralis and lateral patella-femoral ligaments provides restraints against medial subluxation.

The patellar plexus provides the blood supply to the patella. The plexus is an anastomosis of the superior and inferior genicular arteries, which are branches of the popliteal artery,

Articular cartilage in the patella differs from that of other joints in several ways. The patellar cartilage is not necessarily congruent with the contours of the underlying subchondral bone. In 60% of patellae, the thickest area of articular cartilage is located lateral to the thickest area of underlying bone.

Some biomechanical studies have also indicated that this cartilage is less stiff and, thus, more compressible than that of other joints. One cadaveric comparison of femoral and patellar articular cartilage showed that patellar cartilage had a 66% higher permeability, a 30% lower compressive aggregate modulus, and was 23% thicker.^[4] The proteoglycan content was 19% higher in the femoral cartilage than in the patellar cartilage. Whether these differences help prevent or promote arthritic changes in the patellofemoral joint is not yet clear.

Using MRI, Herberhold and colleagues studied the relationship of static loading to cartilage deformation in fresh-frozen cadaveric knees.^{[5, 6]} After 214 minutes of static loading with 150% body weight, the thickness of the articular cartilage was reduced by 44% in the patella and 30% in the femur. The changes in the thickness of articular cartilage of the patella were greatest at the lateral facet, the area of thickest cartilage.

Of note, 7% of the final deformation occurred during the first minute, and 25% occurred in the first 8 minutes. In the initial response to loading, the cartilage appeared to be stiffer than it was in response to prolonged loading. In normal loading, fluid in the cartilage is thought to support the applied load and to prevent cartilage deformation from occurring.

The main function of the patella is thought to be improving the mechanical advantage of the quadriceps extensor mechanism by increasing the lever arm of the muscle. The patella also acts to dissipate the forces generated in the patella tendon during knee flexion and extension.

The angle of force of the quadriceps muscle group (ie, the Q angle) is thought to be a factor in the development of knee injuries and arthritis. However, no findings conclusively support this assertion. The Q angle is measured as the angle between a line connecting the patella to the tibial tubercle and a second line between the anterior superior iliac spine and the center of the patella. A larger Q angle is thought to increase the lateral tracking of the patella mechanism.^[7]

Huberti and colleagues concluded that, at normal Q angles, pressure is evenly distributed across the patella.^[8] Increases in this angle, however, result either in a shifting of pressure to the lateral facet or a change in the distribution of force. Cadaveric studies demonstrate that with an increasing Q angle, the patella shifts more laterally and rotates medially as the knee is flexed. This change is thought to increase lateral contact at the patellofemoral joint and, possibly, to increase the incidence of patella subluxation and dislocation.

In closed-chain exercises with the foot planted on the floor (eg, squatting), contact forces increase with progressive degrees of knee flexion. However, in open-chain exercise in which the foot is off the floor (eg, hamstring curls), no corresponding increase in patella contact force occurs as the knee progresses through a range of motion.

The portion of the patella that is in contact with the trochlea also changes during range of motion.^[9] With the knee extended, only the distal aspect of the patella contacts the trochlear groove. With progressive flexion, the contact shifts to the proximal aspect of the patella. At greater than 90° of flexion, the contact area is predominantly in the center of the patella, which corresponds to the thickest area of articular cartilage.

The medial facet of the patella articulates with the trochlea only during positions of full flexion. The force on the patella increases with knee flexion from 0-60°. However, no consensus exists among researchers regarding the relative amount of force generated with progressive flexion. The contact forces likely are related to the amount of force being generated by the quadriceps muscles during deep flexion exercises. Gait lab analysis has shown that walking with the foot plantar flexed, as occurs when high-heeled shoes are worn, increases the forces in the patellofemoral joint and in the medial compartment of the knee.

Patellofemoral arthritis can be a result of inflammatory conditions or mechanical abnormalities. Inflammatory conditions include rheumatoid arthritis; often, the entire knee joint is involved. Mechanical abnormalities can be a result of prior fractures, inherent malalignment, muscle imbalances, or chronic instability.

Chronic posterior cruciate ligament injury can lead to instability and pressure on the patellofemoral joint, causing arthritis and pain.

Association with anterior cruciate ligament reconstruction

Some orthopedists believe that reconstruction of the anterior cruciate ligament (ACL) with a patella tendon graft may lead to subsequent patellofemoral pain, loss of motion, and arthritis. However, whether these late complications are a result of the injury itself, the anatomic alignment that contributed to the injury, or the surgical reconstruction is not clear.

In an animal model, ACL transection caused significant changes in joint pressure and the articular cartilage after only 4 months.^[10] The thickness of the articular cartilage significantly increased in the patella, the medial and lateral femoral condyles, and in the patellar groove, with total increases in the range of 42-100%. Although the joint contact area was increased, overall peak contact pressures were reduced. The thickening of the cartilage may provide a protective modification of joint pressure, or it may represent early arthritic changes.

In a retrospective clinical study, Jarvela et al found an association between prior ACL reconstructions with bone-patella-bone allografts and the subsequent development of patellofemoral arthritis.^[11] Although evidence of arthritis along the lateral and medial tibial femoral joints was found in 15% and 18% of these patients, respectively, 47% had radiographic evidence of patellofemoral arthritis.

At follow-up at a mean of 7 years, patellofemoral arthritis was mild in 34%, moderate in 12%, and severe in 1%. Shortening of the patella tendon after the index procedure was significantly associated with the development of arthritis. However, the location of the bone tunnel was not correlated with the development of arthritis.

Patellofemoral joint osteoarthritis and arthritis in other knee compartments

Osteoarthritis of the patellofemoral joint should be considered as an entity separate from disease in the medial and lateral tibiofemoral compartments of the knee. Not all patients with patellofemoral arthritis have osteoarthritis in the other compartments; arthritis may develop at different times and with different etiologies in the different compartments of the knee.

Some studies have indicated that risk factors for the development of patellofemoral arthritis include increasing body weight, high-intensity running or weight lifting, prior knee injury, and prior patellar dislocation or subluxation.

Kujula and colleagues conducted a longitudinal study to address the mechanical factors leading to arthritis of the patellofemoral joint^[12] and found that of those with radiographic evidence of patellofemoral arthritis, 70% had changes to the lateral side of the patella. In this subset of patients, the likelihood of a varus knee alignment was significantly increased compared with those with medial patellofemoral arthritis, who were more likely to have a valgus knee. A total of 292 patients with osteoarthritic knees were evaluated radiographically.

In one third of the patients with patellofemoral arthritis, no radiographic evidence of osteoarthritis was present in other compartments of the knee. This isolated patellofemoral arthritis was seen more often in a valgus alignment than was isolated tibiofemoral arthritis.

In another study, CT of 40 knees in flexion revealed that in patients with patellofemoral arthritis, the tibial tubercle had a significantly more lateral position than it does in normal knees.^[13] However, whether this finding represents a cause or a result of arthritic damage to the joint was not determined.

Chondromalacia

In 1962, Outerbridge observed that half of his patients had evidence of irregularities in the articular cartilage of the patella at the time of meniscectomy.

Patellofemoral arthritis

Approximately 5% of patients with osteoarthritis of the knee have symptomatic patellofemoral arthritis in the absence of tibiofemoral arthritis. The etiology of the arthritis is divided equally among patellar dislocation, fracture, and primary osteoarthritis.