What are the common differential diagnoses for pain on the patella (kneecap)?

Common Differential Diagnoses for Patellar Pain

The most common differential diagnoses for patellar pain include patellofemoral pain syndrome (PFPS), patellar tendinopathy, patellofemoral osteoarthritis, patellar subluxation/dislocation, fat pad impingement syndromes (including Hoffa's disease), deep infrapatellar bursitis, osteochondritis dissecans, subchondral insufficiency fractures, and less commonly tumors, ganglion cysts, or pigmented villonodular synovitis. 1

Primary Patellofemoral Disorders

Patellofemoral Pain Syndrome (PFPS)

• Most common cause of anterior knee pain in adolescents and adults under 60 years, with incidence of 3-6% in the United States 2
• Cardinal feature is pain in or around the anterior knee that intensifies with knee flexion during weight-bearing activities 2
• Pain worsens with prolonged sitting ("theater sign") and descending stairs 2, 3
• Most sensitive physical examination finding is pain with squatting 2
• Results from imbalance of forces acting on the patellofemoral joint, leading to increased strain on peripatellar soft tissues 4

Patellofemoral Osteoarthritis

• Associated with patellofemoral cartilage loss and bone marrow lesions (BMLs) 1
• Patients aged 45-55 years with knee pain but normal radiographs may show elevated T2 mapping values on 3T MRI indicating early cartilage abnormalities 1
• Active knee pain correlates with presence of BMLs 1

Patellar Instability

Patellar Subluxation/Dislocation

• Bone marrow edema in classic locations for patellofemoral dislocation/relocation injuries identified by MRI 1
• Associated with anatomic measurements including patellofemoral subluxation and lateral patellofemoral friction syndrome 1
• Can occur from component malrotation in post-arthroplasty patients, with incidence of 1-12% 1

Tendon and Soft Tissue Pathology

Patellar Tendinopathy

• Causes activity-related anterior knee pain 1
• Occurs in both athletes and non-athletes, with repetitive loading placing jumping athletes at greatest risk 1
• Pain exacerbated by navigating stairs and prolonged sitting 1
• Physical examination elicits pain at inferior pole of patella with leg fully extended using resisted leg extension 1
• Decline squat test places increased load on patellar tendon and reproduces pain 1

Fat Pad Impingement Syndromes and Hoffa's Disease

• Various fat pad impingement syndromes are common etiologies of chronic anterolateral knee pain 1
• Enhancing synovitis thicker than 2mm in Hoffa's fat correlates with peripatellar pain 1
• Contrast-enhanced MRI more accurate for diagnosing Hoffa's disease 1

Deep Infrapatellar Bursitis

• Diagnosed on MRI as cause of chronic anterolateral knee pain 1
• Contrast-enhanced images more accurate for diagnosis 1

Bone and Cartilage Pathology

Osteochondritis Dissecans (OCD)

• Involves articular cartilage and subchondral bone 1
• Hyperintense rim or cysts at osteochondral fragment periphery on MRI indicate fragment instability 1
• MRI useful to determine best treatment method 1

Subchondral Insufficiency Fractures

• Previously termed spontaneous osteonecrosis of the knee 1
• Most commonly involve medial femoral condyle in middle-aged to elderly females 1
• MRI identifies these fractures earlier than radiographs, which are often initially normal 1
• Can progress to articular surface fragmentation, subchondral collapse, and osteoarthritis requiring total knee arthroplasty 1

Patellar Fracture

• Can occur in post-arthroplasty patients (up to 5.2% incidence) 1
• Many are asymptomatic, highlighting importance of radiography 1
• Transverse fractures associated with patellar maltracking; vertical fractures often occur through fixation holes 1

Less Common Etiologies

Synovial and Inflammatory Conditions

• Medial plicae - diagnosed on MRI and ultrasound 1
• Pigmented villonodular synovitis - requires contrast-enhanced MRI for diagnosis and quantifying synovitis 1
• Adhesive capsulitis - better diagnosed with contrast-enhanced MRI 1

Neoplastic Conditions

• Tumors - rare cause of chronic anterolateral knee pain 1
• Ganglion cysts - diagnosed on MRI; ultrasound can determine vascularity 1

Other Structural Abnormalities

• Osteophytes - identified on MRI 1
• Osteonecrosis - diagnosed on MRI 1
• Discoid meniscus - can cause anterolateral knee pain 1

Classification Framework

A comprehensive classification system divides patellofemoral disorders into eight groups 5:

1. Patellar compression syndromes
2. Patellar instability
3. Biomechanical dysfunction
4. Direct patellar trauma
5. Soft tissue lesions
6. Overuse syndromes
7. Osteochondritis diseases
8. Neurologic disorders

Clinical Pitfalls

• Meniscal tears are often incidental findings in older patients, with majority of people over 70 years having asymptomatic meniscal tears 1
• The likelihood of meniscal tear being present in painful versus asymptomatic knee is not significantly different in patients 45-55 years of age 1
• Plain radiographs are often initially normal in subchondral insufficiency fractures, requiring MRI for early detection 1
• Multiple pathologies can coexist, particularly in patients with osteoarthritis where both BMLs and synovitis/effusion may contribute to pain 1