eMedicine Specialties > Orthopedic Surgery > Knee

Patellofemoral Arthritis

Author: Kathleen A Hogan, MD, Fellow in Arthroplasty, Department of Orthopedics, Brigham and Women's Hospital
Coauthor(s): H Del Schutte, Jr, MD, Associate Professor of Orthopedic Surgery, Department of Orthopedic Surgery, Medical University of South Carolina
Contributor Information and Disclosures

Updated: Sep 12, 2008

Introduction

Patellofemoral arthritis usually affects 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.

History of the Procedure

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 sheer and compressive forces, abnormal patellar tracking, and patella subluxation and tilting.

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 Surgical therapy).

Problem

Arthritis of the patella refers to the presence of degenerative changes underneath the kneecap (the patella). This form of arthritis can have no symptoms, it may cause only vague anterior knee pain, or it may result in 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. This article focuses primarily on the problem of osteoarthritis of the patellofemoral joint.

Frequency

• Chondromalacia: In 1962, Outerbridge observed that one 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.

Etiology

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.

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 later problems of 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 a result of 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.³ 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 of Finland found an association between prior ACL reconstructions with bone-patella-bone allografts and the subsequent development of patellofemoral arthritis.⁴ 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.

Relationship of osteoarthritis of the patellofemoral joint to 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.

In 1995, Kujula and colleagues conducted a longitudinal study to address the mechanical factors leading to arthritis of the patellofemoral joint.⁵ A total of 292 patients with osteoarthritic knees were evaluated radiographically. 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. 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.⁶ However, whether this finding represents a cause or a result of arthritic damage to the joint was not determined.

Pathophysiology

Articular cartilage in the patella differs from that of other joints in several ways. The 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 a 23% increase in thickness.⁷ The proteoglycan content was 19% higher in the femoral compared with that of the patellar cartilage. The significance of these differences to 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.^8,9 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.¹⁰

Huberti and colleagues concluded that, at normal Q angles, pressure is evenly distributed across the patella.¹¹ 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.¹² 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 predominately 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.

Presentation

History taking

The patient should be asked the following questions regarding a history of trauma or instability:

• How long has the pain been present?
• What makes it worse?
• Is the condition aggravated by prolonged squatting, stair climbing, or other activities?
• Is the pain dull and achy or is it sharp?
• Have you sought treatment for this condition in the past?
• Why are you seeking treatment now?
• Has there been a recent change in activity?
• What type of work doe you do?
• Have you had prior knee surgeries?

Arthritis and conditions of malalignment typically result in pain that is worse with activity. Constant pain that does not vary with activity suggests a referred or nonmechanical origin.

Isolated patellofemoral arthritis may cause anterior knee pain that worsens with stair climbing or when rising from a seated position and is not present with other activities, such as walking or running on level surfaces.

Patella instability is associated with intermittent sharp pain at the kneecap. A feeling of "giving way" may be related to muscle weakness or to instability. Recurrent patellar subluxation or dislocation may cause an acute osteochondral fracture or chronic cartilage damage as a result of repeated microtrauma.

Observation

Astute observations of gait and of lower-extremity alignment are an essential component of the physical examination. Leg-length discrepancies, Q angle, and torsional deformities of the femur, tibia, and foot should be noted. Flexion contractures of the limb should be noted. Gait should be observed carefully, with the patient not wearing shoes. Excessive pronation of the feet, patella tracking, and rotation of the lower limb should be observed. Muscle tone and atrophy of the quadriceps and hamstrings should be assessed. Patella tracking with passive flexion and extension and with squatting should be determined.

Physical findings must be evaluated in the context of patient complaints. A study of 210 adults with asymptomatic knees revealed abnormal radiographic or physical examination findings in 95% of women and 79% of men.¹³ Patellar crepitus, a hypermobile patella, and lateral position of the patella on the axial radiographs were common in this group of patients. Unfortunately, no long-term follow-up was performed to determine whether subjective complaints developed; however, these findings emphasize the point that a diagnosis cannot be based on physical findings alone.

Physical examination

The physician should determine the presence of ligamentous laxity, instability, and patella maltracking, and attempt to elucidate the source of pain. One should try to elicit whether the pain arises predominately from the soft tissues or from articular degeneration.

• Patella tracking: The patella normally enters the trochlea from a lateral position and becomes centralized with increasing knee flexion, traveling in a J pattern. Abnormalities observed include excessive lateral tracking increasing the angle at which the patella enters the trochlea.
• Q angle: The Q angle is the angle between the anterior superior iliac spine to the patella and the patella through the shaft of the tibia. Normal values are less than 20°. Women typically have larger Q angles than men because of the wider position of their hips. While an association is thought to exist between varus knee alignment and the development of osteoarthritis, no study has ever definitively linked Q angles to knee pathology.
• Limb length: This is measured from anterior iliac spine to the medial malleolus.
• Palpation: The goal of palpating the structures of the anterior knee is to determine whether the patient's complaints are related to arthritic changes or to the underlying soft tissues. Attention should be focused on the lateral retinaculum, the quadriceps and patellar tendons, and the quadriceps muscle.
• Compression test: The patella is compressed as the patient flexes his knee. Pain often is elicited by this maneuver if arthritis is present. Resisted knee extension also may reproduce the patient's symptoms in arthritic conditions.
• Apprehension test: Attempt to laterally displace the patella with the knee in extension. Patients with instability contract their quadriceps muscles or complain of pain because of the feeling of subluxation
• Measurements: Measurements should be taken of femoral anteversion, knee valgus, tibial pronation, leg length discrepancy, and Q angle.
• Tests of stability and motion: A thorough knee examination of both the affected and nonaffected sides should be performed. The presence of crepitus is nonspecific. A standard knee examination should be performed. Passive and active range of motion should be recorded. Strength and tightness of hamstring and quadriceps muscle groups should be determined. Knee stability to varus and valgus stress should be assessed. Stability of the ACL can be determined by anterior drawer and Lachman tests. The stability of the patella to medial and lateral stress should be determined, as should the lateral patella tilt.

Differential diagnosis

The differential diagnosis in patients presenting with anterior knee pain includes the following conditions:

• Trauma
• Neuroma
• Multicompartment knee arthritis
• Septic arthritis
• Tendonitis of the patella tendon or quadriceps tendon
• Iliotibial band syndrome
• Prepatella bursitis
• Pes anserine bursitis
• Patella maltracking due to quadriceps weakness or structural abnormalities
• Referred pain from the hip or spine
• Neoplasms

Indications

Patients typically present with anterior knee pain. Indications for surgical management include pain, loss of functional ability, arthritis that is correlated with the symptoms, and symptoms that do not respond to physical therapy. Medical conditions such as hypertension, coronary artery disease, and diabetes should be medically managed and stabilized as much as possible prior to surgery. The patient must understand the procedure and be willing to participate in his or her postoperative rehabilitation. In patients with patellofemoral arthritis, the goal of surgical procedures should be to improve patellar biomechanics and to correct articular damage.

Relevant Anatomy

The patella is a sesamoid bone, the largest in the body, and 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.¹⁴ 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, an anastomosis of the superior and inferior genicular arteries that are branches of the popliteal artery, provides the blood supply to the patella.

Contraindications

The procedures described for the surgical treatment of patellofemoral arthritis are elective; patients should be in relatively good health prior to the operation.

Contraindications for surgery include hemodynamic instability, current respiratory infection, recent myocardial infarction, and compromised skin integrity in the surgical field. Medical issues, such as hypertension, pulmonary disease, coronary artery disease, and diabetes, should be well controlled prior to surgery. Medical clearance prior to surgery is recommended for patients with multiple medical issues. Additionally, patients should be able to comply with postoperative rehabilitation regimens.

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