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Patellofemoral Arthritis
Introduction
Anterior knee pain affects approximately 3% to 6% of the population, with a higher prevalence in young individuals.[1][2] Patellofemoral joint osteoarthritis is a common cause of anterior knee pain. It can occur in isolation or in combination with tibiofemoral joint osteoarthritis.
Patellofemoral Joint Anatomy
The patellofemoral joint is composed of the bony patella, 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 intercondylar or trochlear groove, where the lateral condyle is of a slightly greater diameter than the medial condyle. The articular facets consist of the inferior, superior, middle, and medial vertical.[3][4][5]
Patellofemoral Joint Static Stability
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. Secondary ligamentous stabilizers include the lateral patellofemoral ligament (LPFL), which 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.
Patellofemoral Joint Dynamic Stability
Dynamic stability of the patellofemoral arthritis is provided by the 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.[6][7][8]
Etiology
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Etiology
Patellofemoral arthritis refers to cartilage degeneration and loss specific to the structures of the patellofemoral joint; the underlying surface of the patella and the trochlear groove. 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 tibiofemoral joint arthritis. It may be unicompartmental or associated with femorotibial arthritis in either the medial or lateral compartments or both.
Patellofemoral arthritis can be primary or posttraumatic. Primary patellofemoral arthritis is typically found in older patients, often resulting from chronic, nontraumatic degeneration caused by malalignment or instability. Posttraumatic patellofemoral arthritis often presents in middle-aged patients who frequently have a history of instability, including dislocation or subluxation, laxity, or malalignment, which typically manifests as lateralization, muscle imbalance, or a high-riding patella, known as "patella alta."
Patellofemoral Arthritis Risk Factors
Key risk factors for developing patellofemoral arthritis include:
- Age
- Obesity
- A history of interarticular or patellar fracture
- Prior dislocation or subluxation
- Overuse from sports (eg, high-intensity running or weight training)
- A history of arthritis in other joints
Additionally, malalignment or lateral displacement of the patella depends on either static (bony and soft tissue) or dynamic (surrounding musculature) structures. Laterally displaced patella and increased lateral patellar tilt have been shown to be more strongly associated with the development of patellofemoral arthritis, especially isolated patellofemoral arthritis.[9][10] MPFL injury or laxity results in lateral patella instability and contributes to the later development of patellofemoral arthritis.
On the bony side, patella alta and trochlear dysplasia both negatively affect patellofemoral articulation, with a higher incidence of patellofemoral arthritis. Patella alta increases shear forces and stresses on the patellofemoral joint, and trochlear dysplasia is associated with low cartilage volume.[11]
Another risk factor for patellofemoral arthritis is anterior cruciate ligament (ACL) reconstruction, which can be performed using either a hamstring or a patellar tendon graft.[12] Furthermore, systemic inflammatory disorders, eg, rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, juvenile idiopathic arthritis, and systemic lupus erythematosus, are among other risk factors.[13][14]
Epidemiology
Isolated patellofemoral joint osteoarthritis is the second most common radiographic pattern of knee osteoarthritis (24%), following combined patellofemoral arthritis and tibiofemoral arthritis (40%).[15] Patellofemoral arthritis is more prevalent in females compared to males.[16] This condition is more commonly associated with the development of osteophytes compared to tibiofemoral joint arthritis, which acts as a significant source of pain.[17] Isolated patellofemoral joint osteoarthritis affects 9% of symptomatic patients older than 40, 13.6% of women, and 15.4% of men older than 60.[18]
Pathophysiology
The articular cartilage of the patella can reach up to 8 mm thick, which is considered the thickest articular cartilage in the body.[19] The reaction force of the patellofemoral joint increases with knee flexion and can reach 3-fold the body weight with stairs and 7- to 8-fold the body weight with squats.[20][21]
Patellofemoral arthritis is a broad term that reflects the osteochondral degeneration of the joint space. Arthritis is typically a combination of degeneration of the articular cartilage, known as chondromalacia, as well as abnormal biomechanical tracking of the patella within the trochlear groove, most often in the lateral patellar facet.
Chondropathy of the patellar cartilage is very frequently encountered in knee arthroscopy.[22] Patellofemoral arthritis is more prevalent compared to tibio-femoral arthritis in the middle-aged population.[23] Studies have proposed that patellofemoral pain syndrome eventually progresses to patellofemoral arthritis.[24][25]
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 are a risk factor for developing arthritis. Other contributors include patella instability, osteoarthritis or inflammatory arthritis, being overweight or obese, and the genetic quality of the cartilage. Trochlear dysplasia is identified radiographically in 3 out of 4 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 developing 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, an increased Q angle, also known as valgus malalignment, shifts pressure to the lateral facet. This is thought to increase the risk of patellar subluxation or dislocation, as well as the development of 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.
Histopathology
Histological evaluation of the joint reveals chondrocytes, collagen fibers, proteoglycans, and water.
History and Physical
Clinical History
Anterior knee pain is the most common clinical complaint of patients with patellofemoral arthritis. This pain is typically exacerbated by activities, eg, standing up or sitting down, walking up or down stairs or hills, and kneeling. Exercises that exacerbate the condition 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 symptoms of popping, cracking, and grinding. They may report a history of stiffness or pseudo-locking due to the friction of the patella in the trochlear groove, as well as a sensation of instability or a feeling of the knee "giving way." Clinicians should also clarify if the patient has had any subluxation or dislocation events.[26][27]
Physical Examination
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 clinician may measure the Q angle. The clinician may observe an effusion, atrophy of the quadriceps or hamstrings. The patella may be "squinting" or tilted inward, or "frog-eyed" and tilted outward. Patella tracking and mobility during passive flexion and extension should be observed.
Palpation during passive flexion-extension of the joint may reveal crepitus, the most suggestive and first indication of patellofemoral arthritis.[28] In patellofemoral arthritis, tenderness around the medial or lateral patellar facet or on the medial or lateral femoral condyles can be present. The range of motion is usually reduced, especially in flexion, due to pain. A J-sign can indicate patellar maltracking and often correlates with the severity of trochlear dysplasia or patella alta.[29]
The patellar grind test, also known as the Clarke test, is performed with the patient supine with the knee in full extension. The clinician places their hand on the proximal patella and asks the patient to slowly contract their quadriceps muscle while resisting the 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 applying a laterally directed force to the patella while the knee is in full extension and at 90 degrees of flexion. A positive test is pain or quadriceps recruitment to avoid pain. This test primarily looks for patellar laxity or mobility, but can also be suggestive of patellofemoral arthritis.
Evaluation
Radiographic Imaging
Standard radiographs are typically sufficient to evaluate the patellofemoral joint. Anterior-posterior, 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 determine the alignment of the patella (alta, normal, or baja), femoral condylar dysplasia, and arthritis. The axial view, also termed the "merchant" or "sunrise" view, can evaluate for patellar malalignment, trochlear groove depth, and arthritis.
Both lateral and axial views can be used to evaluate the patellofemoral joint space and assess the degree of arthritis. The Rosenberg or anterior-posterior 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 older than 55.
Additionally, 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 typically -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 the patella bone measured on the lateral view. The patellofemoral index ratio compares the distance between the medial and lateral articular spaces.
Additional Imaging Studies
If the diagnosis is in doubt, a computed tomography (CT) scan can provide a more accurate assessment of the patellofemoral joint. However, this is typically 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, magnetic resonance imaging (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 its diagnostic utility.
Laboratory and Arthroscopic Studies
Inflammatory causes of patellofemoral arthritis should be considered in the proper clinical context. Serology for conditions, eg, Lyme disease, rheumatoid arthritis, and gonococcal arthritis should be considered when appropriate.
In some patients, arthroscopy can also be used for simultaneous diagnosis and therapy.
Treatment / Management
Conservative Patellofemoral Arthritis Management Approaches
Treatment of patellofemoral arthritis has been challenging for clinicians. This is partly due to its variety of causes as well as the growing but inadequate understanding of cartilage regeneration. In most cases, management is nonoperative, and conservative treatment is indicated. No specific conservative management has been established for addressing isolated patellofemoral joint osteoarthritis; instead, the same management applies to knee osteoarthritis.
Physical therapy
Physical therapy is a mainstay of treatment and can alleviate patellofemoral pain by strengthening the quadriceps femoris complex, particularly the vastus medialis, as well as stretching the lateral patellar retinaculum. This can help with maltracking and the 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 following treatment. Weight loss can decrease force loads on the anterior knee and alleviate pain. Activity modification may be helpful, including reducing the frequency of squats, lunges, and jumps, as well as other activities that involve prolonged flexion, and increasing other activities that place less stress on the anterior knee.
Nonopioid therapies
Analgesics, preferably nonopioids, eg, NSAIDs and acetaminophen, can be used to help treat pain. Glucosamine and chondroitin sulfate may help reduce or alleviate 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. Platelet-rich plasma and other regenerative medicine modalities may play a role in treating patellofemoral osteoarthritis; however, the evidence supporting these options is mixed, and further research is generally required.
Viscoelastic supplementation
Viscosupplementation is an alternative option for some patients, but the evidence regarding its efficacy is mixed. Hyaluronic acid injection has been shown to have anti-inflammatory, analgesic, and chondroprotective effects due to its modulation of the intra- and extracellular inflammatory cascade. Therefore, the safety and efficacy of hyaluronic acid injection have been reported in the management of patellar chondropathy.[30](B3)
Kinesio-taping of the patella
Kinesio-taping of the patella is designed to prevent lateral displacement of the patella and is commonly referred to as McConnell taping.[31][32] A simple knee sleeve with compression may provide relief. Patella stabilizing braces and hinged braces can also play a role in treating patellofemoral osteoarthritis. Stabilizing patella braces reduce pain by increasing the patellofemoral joint loading area.[33] (B3)
Other types of braces that may help alleviate patellofemoral joint pain include alignment correction braces, such as proprioceptive sleeves, wedged orthotics, or valgus offloading braces, as patellofemoral joint degeneration is often associated with valgus malalignment of the lower limb.[34](A1)
Surgical Management Approaches
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 a controversial procedure. Soft tissue realignment of the extensor mechanism is generally directed at decreasing lateralization of the patella in the trochlear groove. These procedures vary at the discretion of the surgeon and patient and include the release of the lateral patellar retinaculum, reattachment or reconstruction of the MPFL, or advancement of the vastus medialis obliquus.
Lateral retinacular lengthening
Lateral retinacular lengthening is preferred over lateral retinaculum release due to the potential iatrogenic medial instability with lateral release. Nevertheless, medial instability can be effectively managed with lateral patellofemoral ligament reconstruction.[35][36]
Lateral patellar facetectomy
Lateral patellar facetectomy (arthroscopic or open) is indicated for lateral-sided patellofemoral arthritis and can provide short-term relief. When combined with a lateral release, lateral patellar facetectomy has been shown to result in significant improvements in knee clinical outcome scores and pain.[37]
Tibial tubercle anteromedialization osteotomy
Tibial tubercle anteromedialization osteotomy (TTO) was designed to relieve the load of patellofemoral arthritis on the lateral facet by altering patellofemoral tracking. This procedure involves the realignment or transfer of the tibial tubercle more anteriorly or medially to reduce the load on the patellar cartilage. TTO is indicated in skeletally mature patients younger than 65 years, osteoarthritis grade 3 Kellgren-Lawrence, and isolated patellofemoral pain for ≥6 months after exhausting the conservative measures.[38]
However, the following factors were reported to be associated with a poorer prognosis and should be noted by the treating surgeon: advanced age, excessive femoral anteversion, foot pronation, and postoperative patellofemoral crepitus.[38] TTO is often used in conjunction with soft tissue and bony realignment procedures, as well as cartilage resurfacing procedures.[39](A1)
Cartilage regeneration techniques
Currently, a variety of viable cartilage regeneration techniques are available. Therefore, the key to managing patellofemoral degeneration is to identify early cases of chondropathy that can be potentially cured versus those that require salvage procedures, particularly when advanced arthritis is present, characterized by a combination of inflammation and complete cartilage loss.[40][41](B3)
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.
Patellofemoral arthroplasty
Patellofemoral arthroplasty (PFA) is a partial knee replacement developed as a joint-preserving procedure with the same principles of maintaining natural knee, stability, kinematics, and proprioception as it preserves the tibiofemoral articulation as well as the cruciate ligaments. PFA is indicated in middle-aged, active patients with isolated patellofemoral arthritis with symptoms recalcitrant to nonoperative management.[42][43] (A1)
Contraindications to PFA include the presence of tibiofemoral arthritis, which is the most common relative contraindication. Other contraindications include inflammatory arthritis, mechanical issues, or psychosocial factors, in which case total knee arthroplasty (TKA) is usually better indicated.[44][45](B2)
Historically, first-generation PFA implants had a poor reputation due to a higher complication rate and early conversion to TKA. This is being countered by second-generation implants, which have led to significant improvements in patient-reported outcome measures (PROMs), characterized by low complication rates and higher implant survivorship. PFA can be performed in isolation or in combination with unicompartmental tibiofemoral replacement.[46] Additionally, it can be combined with patellar stabilizing procedures in cases of associated patella instability, eg, MPFL reconstruction or TTO.[47] Second-generation PFA implants feature a wider trochlear component, a longer proximal anterior flange, and a more conforming radius of curvature.[48] It includes both onlay and inlay designs.
Onlay designs
The majority of the second-generation implants utilize an onlay technique, eg, the Avon prosthesis.[49] This involves performing an anterior bone cut to remove the proximal trochlea cartilage and bone. The design involves a broad trochlea with a less-constrained proximal flange. Additionally, a valgus angle was created to improve patella tracking, and further modification was made to place the trochlear component in 3 to 6 degrees of external rotation relative to the posterior condylar axis. The size of the used patellar button should be selected to account for the original thickness to avoid overstuffing the patellofemoral joint. Bony and soft tissue patellar releases should include lateral patella facetectomy, release of the lateral patellofemoral ligament, and lateral retinacular release.[49]Inlay designs
These rely on more patient-specific anatomic implants. Theoretically, this design offers better stability, maintains soft tissue tension, reduces the risk of overstuffing the patellofemoral joint, and minimizes mechanical complications in the patellofemoral joint.[50] It involves a larger selection of trochlear sizes with varying offsets and radii of curvature.[51][52] The troclear implant is selected to match the patient's native anatomy with minimal bone resection. So, inlay designs can be more practical in cases of focal cartilage defects. Inlay designs have demonstrated significant improvements in short-term outcomes, with no mechanical failures reported at 2 years.[50][53](B2)
Differential Diagnosis
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 (eg, gout or pseudogout), infectious arthropathy (eg, Lyme disease, septic arthritis, or gonococcal arthritis), and inflammatory arthropathy (eg, rheumatoid, psoriatic, or seronegative arthritis).
Staging
Radiographically, patellofemoral arthritis is classified into the following 4 stages based on the Merchant view:
-
Mild: >3 mm of joint space
-
Moderate: <3 mm of joint space but no bony contact
-
Severe: Bony surfaces in contact over <25% of the joint surface
-
Very severe: Bony contact throughout the entire joint surface
Prognosis
The prognosis of patients who develop patellofemoral arthritis is variable. Generally speaking, this condition is a progressive disease that requires more aggressive intervention as the degenerative changes worsen. In younger or otherwise healthy patients, conservative management may reduce symptoms to a point where they do not affect daily activities. In other patients who have a more progressed disease state, involvement of the femorotibial joint, obesity, or other chronic medical conditions, they will generally require more aggressive management.
Complications
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, which may require total knee arthroplasty.
Postoperative and Rehabilitation Care
In general, rehabilitation for patellofemoral arthritis, whether operative or nonoperative, focuses on strengthening the quadriceps muscle, specifically the vastus medialis oblique, as well as the hip muscles and core. The goals are to decrease the Q angle, lateralize the patella, and improve tracking within the trochlear groove. If surgery is performed, the surgeon will have protocols specific to their procedure.
Consultations
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.
Deterrence and Patient Education
The best treatment for patellofemoral arthritis is prevention. Prevention focuses on maintaining the strength of the quadriceps, hips, and core. Additionally, activity modification in patients at risk for developing patellofemoral pain can slow the progression of the disease. Maintaining a healthy weight can also reduce the stress load on the patellofemoral joint.
Pearls and Other Issues
The following factors should be kept in mind with patellofemoral arthritis:
- Patellofemoral arthritis is a common cause of anterior knee pain.
- Development is often multifactorial and involves overuse and biomechanical components.
- Patellofemoral arthritis 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.
Enhancing Healthcare Team Outcomes
Enhancing patient-centered care and outcomes in patellofemoral osteoarthritis requires a coordinated, interprofessional approach among physicians, advanced practitioners, nurses, pharmacists, and other healthcare professionals. Each team member brings a unique set of skills and responsibilities critical to managing this multifactorial condition. Physicians, including internists, rheumatologists, and physiatrists, are responsible for accurate diagnosis and comprehensive care planning, while orthopedic surgeons provide surgical expertise when conservative treatments fail. Advanced practitioners and nurses support these efforts by monitoring patient progress, reinforcing treatment plans, educating patients about self-care strategies, and promoting adherence to prescribed therapies. Pharmacists contribute by ensuring safe and effective medication use, counseling patients on pain management options, and identifying potential drug interactions.
Interprofessional communication is essential in aligning strategies across disciplines, ensuring seamless transitions in care, and maintaining consistent messaging to the patient. Physical therapists play a vital role by designing individualized rehabilitation programs that emphasize strengthening the quadriceps femoris, particularly the vastus medialis, and stretching tight structures such as the lateral patellar retinaculum. Nurses and advanced practitioners coordinate follow-up care and facilitate timely referrals when more invasive interventions are needed. Together, these professionals work to ensure optimal pain control using nonopioid analgesics, assess the appropriateness of intra-articular therapies such as corticosteroids or viscosupplementation, and explore emerging options like platelet-rich plasma. A shared commitment to communication and care coordination not only enhances patient safety and satisfaction but also improves long-term functional outcomes in individuals with patellofemoral osteoarthritis.
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