What is the differential diagnosis for a patient presenting with posterior knee pain?

Differential Diagnosis for Posterior Knee Pain

The differential diagnosis for posterior knee pain includes popliteal (Baker's) cysts, hamstring tendinopathy, meniscal tears with posterior horn involvement, gastrocnemius or popliteus tendinopathy, deep vein thrombosis, referred pain from lumbar spine or hip pathology, and less commonly popliteal artery aneurysm, nerve entrapment, or masses such as ganglion cysts. 1, 2, 3

Primary Posterior Knee Pathologies

Most Common Causes

• Popliteal (Baker's) cysts are among the most frequent causes and often communicate with the knee joint, frequently associated with underlying intra-articular pathology 1
• Hamstring tendinitis affecting the semimembranosus, semitendinosus, or biceps femoris insertions 2, 3
• Meniscal pathology, particularly posterior horn tears or associated ganglion cysts 2, 3
• Knee joint effusions causing posterior fullness and discomfort 2

Musculotendinous Causes

• Gastrocnemius tendinopathy or strain, particularly at the medial or lateral head origins 2, 3
• Popliteus tendinopathy or ganglion cysts of the popliteus tendon sheath 2
• Semimembranosus bursitis (distinct from Baker's cyst) 2, 3

Less Common but Important Causes

• Posterior cruciate ligament sprains 2
• Arthrofibrosis following trauma or surgery 2
• Common peroneal nerve irritation 2, 3
• Deep venous thrombosis (critical not to miss) 2, 3
• Popliteal artery aneurysm (vascular emergency consideration) 3
• Ganglion cysts arising from joint capsule or tendon sheaths 2

Critical Differential: Referred Pain

Before attributing all symptoms to local knee pathology, referred pain from the lumbar spine or hip must be excluded, especially when knee radiographs are unremarkable. 1, 4

• Lumbar spine pathology can refer pain to the posterior knee through L5-S1 nerve root irritation 5, 1
• Hip joint pathology commonly refers pain to the knee, particularly in adolescents with slipped capital femoral epiphysis 5, 6
• A thorough clinical examination including assessment of lumbar spine and hip should be completed before rushing to imaging 1, 4

Diagnostic Algorithm

Initial Evaluation

• Start with plain radiographs including frontal projection, tangential patellar view, and lateral view of the affected knee 1, 7
• Consider hip radiographs if knee films are unremarkable and clinical suspicion exists for hip pathology 5, 1
• Consider lumbar spine imaging if there is clinical evidence of radiculopathy or spinal pathology 5, 1

Advanced Imaging When Indicated

• MRI without IV contrast is the next examination when radiographs are normal or show only effusion but pain persists 1, 7
• MRI accurately depicts popliteal cyst presence or rupture, meniscal tears, and other soft tissue pathology 1
• Ultrasound can effectively detect popliteal cysts, synovial pathology, and guide aspiration if needed 1, 7
• Ultrasound is particularly useful when loose bodies might be within a cyst 1

Age-Specific Considerations

Adolescents and Young Adults

• Tibial apophysitis (Osgood-Schlatter) more common in teenage boys 6
• Slipped capital femoral epiphysis presenting as referred knee pain 6
• Consider overuse injuries in active patients 6

Middle-Aged to Elderly

• Subchondral insufficiency fractures most commonly involve the medial femoral condyle in middle-aged to elderly females 5
• In patients over 70 years, bilateral structural abnormalities can be present with primarily unilateral symptoms 1
• Meniscal tears are often incidental findings in older patients, with the majority over 70 having asymptomatic tears 5

Common Pitfalls to Avoid

• Do not overlook referred pain from hip or lumbar spine before attributing symptoms solely to knee pathology 1, 4
• Avoid premature MRI without recent radiographs, as approximately 20% of patients with chronic knee pain undergo MRI without initial plain films 1
• Not all meniscal tears are symptomatic, particularly in patients over 45 years of age 1
• Do not miss vascular emergencies such as popliteal artery aneurysm or deep vein thrombosis 2, 3
• Complete a thorough neurovascular examination to avoid missing nerve entrapment or vascular pathology 2, 3