What is the likely diagnosis and initial management for a healthy 14‑year‑old female with acute posterior knee pain after a basketball injury, mild swelling, and negative anterior/posterior drawer and varus/valgus stress tests?

Likely Diagnosis: Popliteal Muscle or Tendon Strain

Based on the clinical presentation of isolated posterior knee pain after basketball trauma with negative ligamentous testing and minimal swelling, this 14-year-old most likely has a popliteal muscle or tendon strain, though meniscal injury (particularly posterior horn tear) remains an important differential diagnosis.

Initial Diagnostic Approach

Imaging Decision

• Obtain plain radiographs (AP and lateral views) as the initial imaging study to exclude fracture, even though Ottawa Knee Rules may not be strictly positive 1, 2
• The American College of Radiology recommends radiographs as first-line imaging for acute knee trauma, particularly when there is focal tenderness or effusion 1, 2
• If the patient cannot bear weight for 4 steps, has focal tenderness at the patella or fibular head, cannot flex to 90 degrees, or is ≥55 years old, radiographs are mandatory per Ottawa Knee Rules 2, 3

Physical Examination Specifics

• Re-examine in 3-5 days if initial assessment is limited by pain and swelling, as examination accuracy improves significantly after acute inflammation subsides 3, 4
• Palpate specifically for joint line tenderness (suggests meniscal injury), popliteal fossa tenderness (suggests popliteal strain), and assess for effusion 3
• Test knee flexion against resistance with the knee at 90 degrees—pain suggests popliteal muscle involvement 4

Initial Management Protocol

Immediate Treatment (First 48-72 Hours)

• Implement PRICE protocol: Protection, Rest, Ice, Compression, and Elevation 1
• Apply ice for 15-20 minutes every 2-3 hours during the first 48 hours to control swelling 1
• Prescribe NSAIDs to reduce pain and swelling and accelerate return to activity 1
• Limit weight-bearing as tolerated; crutches if needed for comfort but encourage early mobilization 4

Functional Rehabilitation (After 48-72 Hours)

• Begin motion restoration and strengthening exercises as soon as pain allows—functional rehabilitation is superior to immobilization 1
• Focus on quadriceps activation and maintaining range of motion 4
• Progress to proprioceptive training and sport-specific exercises as symptoms improve 1

Indications for Advanced Imaging

When to Order MRI Without Contrast

• If significant joint effusion persists, inability to fully bear weight after 5-7 days, mechanical symptoms (locking/catching), or joint instability develops 2
• MRI is the appropriate next study after negative radiographs when internal derangement (meniscal or ligamentous injury) is suspected 1, 2
• The American College of Radiology classifies non-contrast knee MRI as "usually appropriate" for suspected soft-tissue injury after negative X-rays 2

Red Flags Requiring Urgent Evaluation

• Palpable gap in tendon, gross deformity, complete inability to bear weight, or fever with joint effusion require immediate specialist referral 3
• Persistent instability with walking after initial swelling resolves suggests significant ligamentous injury and warrants MRI or orthopedic consultation 2, 3

Common Pitfalls to Avoid

• Do not skip radiographs and proceed directly to MRI—plain films must be obtained first to exclude fracture and comply with evidence-based guidelines 1, 2
• Do not immobilize the knee unless fracture is confirmed—functional rehabilitation with early motion is superior to prolonged immobilization 1
• Do not dismiss posterior knee pain as minor—while popliteal strains are common, meniscal tears (especially posterior horn) can present similarly and may require surgical management if mechanical symptoms develop 4
• Adolescents with persistent knee pain and inability to bear weight should have hip examination to exclude slipped capital femoral epiphysis, which commonly refers pain to the knee 3

Expected Recovery Timeline

• Most isolated soft-tissue strains improve within 1-2 weeks with conservative management 4
• If symptoms persist beyond 7-10 days or worsen, obtain MRI to evaluate for meniscal tear or other internal derangement 2, 4
• Basketball players can typically return to sport when they achieve full range of motion, no effusion, and can perform sport-specific movements without pain 1, 4