Which ligament or meniscus is likely affected by pain behind the knee when bending?

Pain Behind the Knee When Bending: Likely Structures Involved

Posterior knee pain with knee flexion most commonly implicates the posterior horn of the medial meniscus, followed by the posterior cruciate ligament (PCL), Baker's cyst, or popliteus tendon pathology.

Primary Differential Diagnosis

Most Common: Posterior Meniscal Pathology

• The posterior horn of the medial meniscus is the most frequent source of posterior knee pain during flexion 1
• The medial meniscus lacks direct femoral attachment, making it more vulnerable to injury compared to the lateral meniscus 2
• Meniscal tears are highly prevalent in acute knee trauma with hemarthrosis, occurring in over one-third of cases 3
• Pain typically worsens with knee flexion as the posterior horn is compressed between the femur and tibia 1

Second Most Common: Posterior Cruciate Ligament (PCL)

• PCL injuries characteristically produce posterior knee pain, particularly during flexion 4, 3
• The PCL is the strongest knee ligament, preventing posterior tibial displacement and providing 95% of resistance to posterior drawer 4
• Classic injury mechanism involves direct anterior tibial blow with knee flexed (dashboard injury, soccer trauma) 4
• The "posterior sag sign" is pathognomonic: disappearance of tibial tubercle prominence when knee is flexed to 90 degrees due to gravity-assisted posterior tibial displacement 4

Other Important Causes

• Baker's cyst (semimembranosus cyst) causes posterior pain, especially with flexion that compresses the cyst 1
• Popliteus tendon pathology including tendinitis or ganglion cysts can produce deep posterolateral pain with flexion 1
• Hamstring tendinitis affects the posterior knee but typically involves the insertion sites rather than deep joint pain 1

Clinical Examination Approach

Key Physical Findings to Assess

For PCL injury:

• Perform posterior sag sign test with patient supine, knee flexed to 90 degrees—observe for posterior tibial displacement 4
• Posterior drawer test at 90 degrees flexion (sensitive but not specific in acute injuries) 4
• Knee hyperextension sign 4

For meniscal pathology:

• Joint line tenderness, particularly posterior medial or lateral 1
• McMurray test with knee flexion and rotation
• Pain with deep squatting or kneeling positions

For popliteus/Baker's cyst:

• Palpable mass in popliteal fossa 1
• Pain with resisted knee flexion and external tibial rotation (popliteus)

Diagnostic Imaging Recommendations

MRI is the definitive imaging modality for evaluating posterior knee pain with suspected internal derangement 5

• MRI demonstrates superior accuracy for meniscal tears (sensitivity 88-90%, specificity 90%) 5
• MRI is highly accurate for PCL injuries (sensitivity 90%, specificity 97%) 5
• MRI can identify Baker's cysts, popliteus pathology, and bone marrow edema 5

Critical Clinical Pitfalls

• False negative posterior drawer tests are frequent in acute PCL injuries due to pain, guarding, and hemarthrosis 4
• Isolated PCL tears may not cause significant functional instability despite posterior laxity, particularly in lower-demand patients 4
• Posterior horn meniscal tears can be missed on physical examination alone, requiring MRI confirmation 1
• Multistructure injuries (PCL + meniscus, meniscus + cartilage) significantly increase long-term osteoarthritis risk (OR 2.31-3.14) 5, 6, 7

Management Implications

For isolated PCL midsubstance tears:

• Conservative treatment with intensive quadriceps strengthening is recommended for most patients 4
• Immobilization should be brief (under 2 weeks) followed by early controlled weightbearing 4
• Functional outcome depends more on quadriceps strength than residual laxity 4

For meniscal tears:

• Arthroscopic evaluation and treatment as indicated 1
• Meniscectomy combined with other injuries dramatically increases osteoarthritis risk (partial meniscectomy OR 1.87, total medial meniscectomy OR 3.14) 5, 6

For PCL avulsion fractures:

• Surgical fixation is recommended when bony fragment is displaced 4