What causes pain directly over the patella (kneecap)?

Pain Directly Over the Kneecap: Differential Diagnosis

Pain directly over the patella most commonly indicates patellofemoral pain syndrome (PFPS), patellar tendinopathy, or patellofemoral osteoarthritis, with PFPS being the leading diagnosis in patients under 40 years and patellofemoral OA more likely in those over 45 years. 1, 2, 3

Primary Diagnostic Considerations by Age

Younger Patients (Under 40 Years)

• Patellofemoral pain syndrome is the most common cause, characterized by retropatellar or peripatellar pain that worsens with stair climbing, squatting, or prolonged sitting with flexed knees, and has a lifetime prevalence of approximately 25%. 2, 3
• Patellar tendinopathy frequently causes anterior knee pain directly over the patellar tendon insertion. 1
• Osgood-Schlatter or Sinding-Larsen-Johansson syndrome should be considered in adolescents and can be evaluated with MRI if radiographs are inconclusive. 4

Middle-Aged to Older Patients (Over 45 Years)

• Patellofemoral osteoarthritis becomes increasingly likely, though it may present identically to PFPS even in older patients with normal radiographs but elevated T2 mapping values on MRI indicating early cartilage changes. 2
• Subchondral insufficiency fractures (previously termed spontaneous osteonecrosis) most commonly involve the medial femoral condyle in middle-aged to elderly females, with radiographs often initially normal. 1

Additional Patellofemoral Pathologies

• Patellar subluxation or dislocation may present with anterior knee pain, particularly if there is a small osseous fragment along the medial patellar margin on radiographs, which warrants MRI to assess the medial patellofemoral ligament and cartilage injury. 4, 1
• Fat pad impingement syndromes and Hoffa's disease (characterized by enhancing synovitis >2mm in Hoffa's fat) correlate with peripatellar pain. 1
• Deep infrapatellar bursitis is a potential cause of anterior knee pain. 1
• Medial plicae can cause anterior knee pain and may be identified on MRI. 1
• Osteochondritis dissecans (OCD) can cause anterior knee pain and may produce loose bodies. 1

Critical Pitfall: Referred Pain

Before attributing pain solely to the patella, you must clinically evaluate the hip and lumbar spine, as both commonly refer pain to the anterior knee. 1, 2

• Hip pathology refers pain to the knee and should be evaluated if knee imaging is normal. 1, 2
• Lumbar spine pathology must be considered when knee radiographs are unremarkable and clinical evidence suggests spinal origin. 1, 2
• A thorough clinical examination of the lumbar spine and hip should precede knee-focused imaging. 1

Diagnostic Algorithm

Step 1: Initial Imaging

• Obtain anteroposterior and lateral knee radiographs first to exclude fractures, osteoarthritis, osteophytes, loose bodies, and osseous fragments. 4, 1
• Approximately 20% of patients with chronic knee pain undergo MRI without recent radiographs, which is premature. 1

Step 2: Physical Examination Findings

• Anterior knee pain during squatting is approximately 91% sensitive and 50% specific for patellofemoral pain. 3
• Assess for knee effusion, as its presence guides treatment approach and may indicate synovitis in osteoarthritis. 2, 5
• Evaluate hip and lumbar spine clinically before attributing symptoms solely to knee pathology. 1, 2

Step 3: Advanced Imaging When Indicated

• MRI without IV contrast is indicated when radiographs are normal or show only joint effusion but pain persists, or when radiographs demonstrate a small osseous fragment along the medial patellar margin suggesting prior patellar dislocation. 4, 1
• CT without IV contrast may be indicated to evaluate patellofemoral anatomy in the setting of chronic knee pain related to repetitive patellofemoral subluxation. 4
• MRI can identify bone marrow lesions and synovitis/effusion, which are potential pain generators in osteoarthritis patients. 5

Age-Specific Clinical Patterns

• Teenage girls and young women are more likely to have patellar tracking problems such as patellar subluxation and patellofemoral pain syndrome. 6
• Teenage boys and young men are more likely to have knee extensor mechanism problems such as tibial apophysitis (Osgood-Schlatter lesion) and patellar tendonitis. 6
• Patients aged 45-55 years with knee pain but normal radiographs can have early cartilage changes that still present clinically as PFPS. 2
• Patients over 70 years may have bilateral structural abnormalities with primarily unilateral symptoms, limiting MRI's ability to discriminate painful from nonpainful knees. 5

Common Diagnostic Pitfalls

• Not all meniscal tears are symptomatic, particularly in patients over 45 years, so incidental findings should not be over-interpreted. 1
• Radiographs may be initially normal in subchondral insufficiency fractures, which later show articular surface fragmentation and subchondral collapse. 1
• MRI findings do not always correlate with symptoms, even when synovitis/effusion is present. 5