What causes pain in the posterior knee when bending or walking?

Posterior Knee Pain: Causes and Diagnostic Approach

Pain in the back of the knee when bending or walking is most commonly caused by a Baker (popliteal) cyst, meniscal tear (particularly posterior horn tears), hamstring tendinopathy, or gastrocnemius strain, though referred pain from lumbar spine pathology must also be excluded if knee imaging is unremarkable. 1, 2

Primary Posterior Knee Pathologies

Baker Cyst (Popliteal Cyst)

• MRI accurately depicts the presence or rupture of a popliteal cyst and is the preferred imaging modality when radiographs are normal but symptoms persist 1
• Ultrasound can detect popliteal cysts, particularly when loose bodies might be within the cyst, and is useful for detecting synovial pathology 1, 3
• These cysts often communicate with the knee joint and may be associated with underlying intra-articular pathology 1

Meniscal Tears

• Posterior horn meniscal tears are common and can cause posterior knee pain, particularly with knee flexion 4
• Joint line tenderness has 83% sensitivity and 83% specificity for meniscal tears, while the McMurray test (knee rotation with extension) has 61% sensitivity and 84% specificity 4
• Important caveat: Meniscal tears are often incidental findings in older patients, with the majority of people over 70 years having asymptomatic tears 1
• The likelihood of a meniscal tear being present is not significantly different between painful and asymptomatic knees in patients 45-55 years of age 1

Hamstring and Gastrocnemius Pathology

• Hamstring tendinopathy or partial tears at the insertion can cause posterior knee pain with knee flexion 5
• Gastrocnemius muscle strain or tendinopathy affects the posterior knee and worsens with walking 5

Critical Differential: Referred Pain

Lumbar Spine Pathology

• In patients with chronic knee pain, referred pain from the lower back must be considered, especially if knee radiographs are unremarkable and there is clinical evidence or concern for lumbar spine pathology 1, 2
• This is a commonly overlooked diagnosis that can lead to unnecessary knee interventions 6
• When knee pain presents with normal radiographs, evaluation of the lumbar spine should be considered to identify possible origin of pain 2

Hip Pathology

• Referred pain from the hip must be considered, especially if knee radiographs are unremarkable and there is clinical evidence or concern for hip pathology 1

Diagnostic Algorithm

Initial Evaluation

• Radiography is the initial imaging study for chronic knee pain (pain >6 weeks) and should include frontal projection, tangential patellar view, and lateral view 1, 3
• For acute traumatic pain, radiographs should be reserved for patients meeting specific evidence-based criteria 3

When Initial Radiographs Are Normal or Show Only Effusion

• MRI without IV contrast is the next indicated examination when initial radiographs are normal or reveal a joint effusion but pain persists 1
• MRI accurately depicts the extent of effusion, presence of synovitis, and presence or rupture of popliteal cysts 1
• Subchondral cysts, articular cartilage, and meniscal abnormalities are easily detected on MRI 1

Alternative Imaging Considerations

• Ultrasound is useful for detecting synovial pathology, effusions, and can demonstrate meniscal extrusion (suggesting underlying meniscal tear) 1
• Ultrasound can localize suspected loose bodies, especially if they might be within a popliteal cyst 1
• CT arthrography may be used instead of MRI to evaluate menisci and articular cartilage when intra-articular abnormality is suspected 1

Common Pitfalls to Avoid

• Do not overlook referred pain from hip or lumbar spine pathology before attributing all symptoms to knee pathology 6
• Do not rush to imaging before completing a thorough clinical examination including assessment of lumbar spine and hip 6
• Avoid premature MRI use: approximately 20% of patients with chronic knee pain have had MRI performed without recent (within prior year) radiographs 1
• Do not assume all meniscal tears seen on imaging are symptomatic, particularly in patients over 45 years 1, 4

Age-Specific Considerations

• In adolescents, calcaneal apophysitis is the most common cause of heel pain (not posterior knee pain specifically), but knee extensor mechanism problems like Osgood-Schlatter disease and patellar tendonitis are common in teenage boys and young men 1, 7
• In patients >70 years of age, bilateral structural abnormalities can be present with primarily unilateral symptoms, limiting the ability to discriminate painful from nonpainful knees 1
• Active patients are more likely to have overuse injuries such as pes anserine bursitis 7