Patellofemoral arthritis is a common cause of anterior knee pain. The patellofemoral joint is composed of the bony patella, which is a sesamoid bone embedded in the quadriceps and patella tendons and the femur. A prepatellar bursa separates the patella from the overlying skin. The patella sits within the intracondylar or trochlear groove where the lateral condyle is of slightly greater diameter than the medial condyle. There are four articular facets: the inferior, superior, middle and medial vertical.
The patella is primarily stabilized medially by the medial patellofemoral ligament (MPFL). This ligament originates from the adductor tubercle and inserts onto the superomedial border of the patella, resisting lateral traction of the patella. The lateral patellofemoral ligament (LPFL) originates from the lateral femoral condyle and attaches to the superolateral border of the patella and resists medial traction of the patella. The medial and lateral patellotibial ligaments and retinaculum also provide static stability of the joint. Dynamic stability is provided by vastus medialis, which provides medial resistance to lateral translation, and the vastus lateralis, which provides lateral resistance to medial translation. The quadriceps tendon attaching at the proximal patella creates a Q angle with a valgus axis that creates a lateral force across the patellofemoral joint. Blood supply is from the geniculate arteries: superior, medial, inferior, lateral, anterior, and descending. Sensory innervation of the anterior knee is from the lateral and anterior cutaneous branches of the femoral nerve as well as the infrapatellar branch of the saphenous nerve.
In healthy knees, the articular cartilage of the patellofemoral joint can reach a thickness of up to 7 to 8 mm; thus, it is a potent shock absorber. A complex matrix of glycosaminoglycans lowers the friction coefficient and creates a nearly frictionless surface for flexion and extension of the knee. Chondrocytes produce the proteoglycans that balance synthesis and degrade the matrix based on the chemical and biomechanical demands of the joint space.
Patellofemoral stability is provided by both the static and dynamic anatomical supporting structures. During flexion, the patella moves within the groove acting as a lever-arm, extending the functional length of the femur. In addition to a proximal-distal movement within the groove, the patella is thought to have a lateral motion with knee extension moving in a J-shaped pattern. Varus and valgus alignment of the knee, as well as any rotational component of the femur or tibia, also determine biomechanical patellofemoral function.
Patellofemoral arthritis refers to the presence of degenerative changes to the joint. Chondromalacia refers to degenerative changes in the articular cartilage of the patella that frequently precipitate the development of osteoarthritis. Patellofemoral arthritis should be considered a separate disease from medial and lateral compartment femorotibial arthritis. It may be unicompartmental or associated with femorotibial arthritis in either the medial or lateral compartments, or both.
Patients with patellofemoral arthritis frequently have a history of instability, including dislocation or subluxation, laxity, malalignment which usually manifests as lateralization, muscle imbalance, or high riding patella which is known as "patella alta." Anterior cruciate ligament (ACL) reconstruction with either hamstring or patella tendon graft is another risk factor for developing patellofemoral arthritis.
Risk factors for the development of patellofemoral arthritis include age, obesity, history of interarticular or patellar fracture, prior dislocation or subluxation, overuse from sport including high-intensity running or weight training, and history of arthritis in other joints. Systemic inflammatory disorders such as rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, juvenile idiopathic arthritis, and systemic lupus erythematosus are among other risk factors.
It is estimated that 14 million people in the United States have symptomatic osteoarthritis in their knees, and half are less than 65 years of age. A systematic review found that half of the people with knee pain have patellofemoral involvement. Thus, patellofemoral arthritis is very common. Researchers found patellofemoral osteoarthritis in 79% of cadavers over the age of 65 and more than 50% of patients undergoing meniscectomy have evidence of chondromalacia.
Patellofemoral arthritis is a broad term that reflects osteochondral degeneration of the joint space. The arthritis is typically a combination of degeneration of the articular cartilage, termed chondromalacia, as well as abnormal biomechanical tracking of the patella within the trochlear groove which is most often in the lateral patellar facet.
Compared to the medial and or central facets, the lateral facet is most often overloaded. This is associated with lateral malalignment of the patella within the trochlear groove. Valgus knee alignment, dysplasia of the patella or trochlea, and malrotation of the tibia can all accelerate this phenomenon and the development of patellofemoral arthritis. The direction of the force vector of the quadriceps muscles and knee extensors can also overload the lateral patellar facet. Degeneration of the patellofemoral joint can develop secondary to abnormal stressors on the patella caused by patella alta, increased Q-angle combined with secondary myofascial mechanical disruption, atrophied vastus medialis obliquus combined with the loss of function of the lateral retinaculum, or deficiency of the medial patellofemoral ligament.
Intra-articular fractures of the patella and trochlea caused by micro- or macro-trauma is a risk factor for developing arthritis. Other contributors include patella instability, osteo- or inflammatory arthritis, overweight or obesity, and genetic quality of the cartilage. Trochlear dysplasia is identified radiographically in three out of four cases of isolated patellofemoral arthritis. Some studies suggest that the patellar cartilage is less stiff and thus more compressible compared to other areas of articular cartilage. Whether this reduces or increases the likelihood of development of patellofemoral arthritis is unclear.
The Q angle is the line measured from the ASIS to the center of the patella and from the tibial tubercle to the center of the patella. Researchers have shown that a normal Q angle evenly distributes pressure across the patella. However, increased Q angle, also termed valgus malalignment, shifts pressure to the lateral facet. This is thought to lead to increased risk of patellar subluxation or dislocation and subsequent patellofemoral arthritis. Laxity or tearing of the medial patellofemoral ligament, often in association with subluxation or dislocation, has been shown to contribute to patellofemoral instability and subsequent patellofemoral arthritis.
Histological evaluation of the joint reveals chondrocytes, collagen fibers, proteoglycans, and water.
Anterior knee pain is the most common clinical complaint of patients with patellofemoral arthritis. This pain is typically made worse by standing up or sitting down, walking up or down stairs or hills, and kneeling. Exercises that make it worse include lunges and squats. Essentially, any activity that increases flexion of the knee increases the force load on the patellofemoral joint and causes pain. Patients may report popping, cracking, and grinding symptoms. They may report a history of stiffness or pseudo-locking due to the friction of the patella in the trochlear groove and a sensation of instability or "giving way." It is also important to clarify if the patient has had any subluxation or dislocation events.
A gait exam should be performed assessing for foot pronation, valgus or varus knees, and rotational malalignment of the tibia or femur. Gait exam findings associated with patellofemoral osteoarthritis include increased anterior pelvic tilt through the stance phase, increased lateral pelvic tilt on the contralateral side, increased hip adduction and lower hip extension during stance. The provider may measure the Q angle. The provider may observe an effusion, quadriceps, or hamstrings atrophy. The patella may be “squinting” or tilted inward, or alternatively, “frog eyed” and tilting outward. Patella tracking and mobility during passive flexion and extension should be observed.
Palpation during passive flexion-extension of the joint may reveal crepitus and is the exam finding most suggestive of patellofemoral arthritis. In patellofemoral arthritis, there can be tenderness around the medial or lateral patellar facet, or on the medial or lateral femoral condyles. The patellar grind test, also known as the Clarke test, is performed with the patient supine with the knee in full extension. The provider places their hand on the proximal patella and asks the patient to slowly contract their quadriceps muscle while resisting proximal movement of the patella. A positive test is a pain at the patellofemoral joint. The examiner may also passively move the patella in a superior-inferior or medial-lateral direction while applying posteriorly directed pressure. The patellar apprehension test involves placing a laterally directed force over the patella with the knee in full extension and at 90 degrees of extension. A positive test is a pain or quadriceps recruitment to avoid pain. This test primarily looks for patellar laxity or mobility but can also be suggestive of patellofemoral arthritis.
Standard radiographs are typically sufficient to evaluate the patellofemoral joint. AP, lateral, and axial views are routinely obtained. These can be used to evaluate for joint space narrowing, subchondral sclerosis, osteophytes, articular degeneration, and patellar alignment. The lateral view can evaluate the alignment of the patella (alta, normal, or baja), femoral condylar dysplasia, and arthritis. The axial view, also termed "merchant" or "sunrise" view, can evaluate for patellar malalignment, trochlear groove depth, and arthritis. Both lateral and axial views can evaluate the patellofemoral joint space to assess the degree of arthritis. The Rosenberg or AP view is best used to evaluate the femorotibial compartments. Radiographic evidence of patellofemoral arthritis is as high as 34% in women and 19% in men over the age of 55.
Radiographs can be used to quantify changes in the patella in cases of suspected patellofemoral arthritis. The sulcus angle is typically about 138 degrees. The congruence angle is used to measure lateral patellar displacement and is normally negative 6 degrees. The Insall-Salvati ratio, which is used to assess for patella alta, is the ratio of the length of the patella ligament to patella bone measured on the lateral view. The patellofemoral index ratio compares the distance between the medial and lateral articular spaces.
If the diagnosis is in doubt, CT can better assess the patellofemoral joint although this is unnecessary in most patients with known or suspected arthritis. CT may be utilized to help identify lateral patellar subluxation or femoral trochlear dysplasia. In the setting of trauma where patella osteochondritis dissecans or other ligamentous injury is suspected, MRI is the imaging study of choice. Ultrasound may have a role in diagnosing patellofemoral arthritis when radiographs are unavailable; although the inability to see the articular side of the patella limits diagnostic utility. Arthroscopy can also be used for diagnosis and therapy.
Inflammatory causes of patellofemoral arthritis should be considered in the right clinical context. Serology for Lyme disease, rheumatoid arthritis, gonococcal arthritis, etc. should be considered when appropriate.
Treatment of patellofemoral arthritis has been challenging to clinicians. This is partly due to its variety of causes as well as growing but an inadequate understanding of cartilage regeneration. In most cases, management is non-operative, and conservative treatment is indicated.
Physical therapy is a mainstay for treatment and can alleviate patellofemoral pain by strengthening the quadriceps femoris complex, most often the vastus medialis, as well as stretching the lateral patellar retinaculum. This can help with maltracking and range of motion of the joint. In patients with patellofemoral pain syndrome, which includes patellofemoral arthritis, 67% reported resolution of symptoms within 6 months of initiating physical therapy, and 80% graded their knee as excellent after 7 years. Weight loss can decrease force loads on the anterior knee and alleviate pain. Activity modification may be helpful, including decreasing the frequency of squats, lunges, jumps, and other activities with prolonged flexion and increasing other activities that place less stress on the anterior knee.
Analgesics, preferably non-opiates such as NSAIDs and acetaminophen, can be used to help treat pain. Glucosamine and chondroitin sulfate may reduce or decrease the degree of knee pain. Intra-articular injections can also reduce pain symptoms. Corticosteroid injections are generally first-line therapy with the most robust evidence supporting their use. Viscosupplementation is an alternative option for some patients, but the evidence is mixed as far as efficacy. Platelet-rich plasma and other regenerative medicine modalities may have a role in treating patellofemoral osteoarthritis although the evidence is mixed with these options as well and more research is generally required.
Kinesio-taping of the patella is well supported in the literature and can provide relief. This is aimed at preventing lateralization of the patella and is termed McConnell taping. A simple knee sleeve with compression may provide relief. Patella stabilizing braces and hinged braces can also play a role in treating patellofemoral osteoarthritis although not all patients will gain relief.
Surgical management is indicated in select patients. Generally, surgical candidates have not received any benefit from nonoperative management. Arthroscopy with debridement of the patellar cartilage is controversial. Soft tissue realignment of the extensor mechanism is generally directed at decreasing lateralization of the patella in the trochlear groove. This procedure varies at the discretion of the surgeon and patient and includes the release of the lateral patellar retinaculum, reattachment or reconstruction of the MPFL, or advancement of the vastus medialis obliquus.
Tibial tubercle osteotomy was designed to relieve the load of patellofemoral arthritis on the lateral facet. This procedure involves realignment or transfer of the tibial tubercle more anteriorly or medially to reduce the load on the patellar cartilage. In the appropriate patient, this procedure can reduce the pain of patellofemoral arthritis, particularly in young patients.
Autologous chondrocyte implantation is aimed at increasing the amount of articular cartilage of the patella by transferring it from an area with less weight-bearing function. Patelloplasty or patellofemoral replacement with a prosthesis may also be indicated in patients with severe, isolated patellofemoral arthritis with preservation of femorotibial joint space. Total knee arthroplasty tends to have better outcomes in patients than patelloplasty; however, this intervention is usually reserved for older patients with tricompartmental osteoarthritis.
The differential diagnosis of patellofemoral osteoarthritis includes primary or secondary osteoarthritis, Iliotibial band syndrome, L3-L4 radiculopathy (disc herniation), and tendonitis (quadriceps, patella). Other causes of anterior knee pain include neuroma, crystal arthropathy (gout, pseudogout), infectious arthropathy (Lyme disease, septic, gonococcal) and inflammatory (rheumatoid, psoriatic, seronegative) arthropathy.
Radiographically, patellofemoral arthritis is classified into four stages based on the merchant view:
Mild - more than 3 mm of joint space
Moderate - less than 3 mm of joint space but no bony contact
Severe - bony surfaces in contact over less than one-quarter of the joint surface
Very severe - bony contact throughout the entire joint surface
The prognosis of patients who develop patellofemoral arthritis is variable. Generally speaking, it is a progressive disease that requires more aggressive intervention as the degenerative changes worsen. In younger or otherwise healthy patients, conservative management may reduce their symptoms to a point where it does not affect their activities of daily living. In other patients who have more progressed disease state, involvement of the femorotibial joint, obesity, or other chronic medical conditions, they will generally require more aggressive management.
In patients managed conservatively, complications are limited to persistent pain and instability and progression of the disease. Complications of surgical management include infection, quadriceps weakness, loss of normal function, and persistent pain despite the procedure. Patients may develop femorotibial osteoarthritis requiring total knee arthroplasty.
In general, rehabilitation of patellofemoral arthritis, whether operative or nonoperative, is directed at strengthening the quadriceps muscle, specifically the vastus medialis oblique, hip muscles, and core. The goals are to decrease the Q angle, lateralize the patella, and improve racking in the trochlear groove. If surgery is performed, the surgeon will have protocols specific to their procedure.
Most patients with patellofemoral arthritis can be managed conservatively without consulting an orthopedic physician. Patients with refractory symptoms requiring injections or who may be candidates for surgical intervention should be consulted accordingly.
The best treatment for patellofemoral arthritis is prevention. Prevention is aimed at maintaining the strength of the quadriceps, hip, and core. Activity modification in patients developing patellofemoral pain can slow progression of the disease. Maintaining a healthy weight can also reduce the stress load on the patellofemoral joint.
Patellofemoral arthritis is a common cause of anterior knee pain.
Development is often multifactorial and involves overuse and biomechanical components.
It may be unicompartmental, meaning limited to the patellofemoral joint space, or bi- or tri-compartmental, involving the femorotibial joint space.
Diagnosis is usually made clinically in conjunction with standard knee radiographs.
Conservative management includes physical therapy, activity modification, analgesics, bracing, and injections.
Surgical management is an option in select patients.