A 25-year-old footballer twists his knee and feels a 'pop' followed by acute haemarthrosis. Lachman's test is positive (2+). MRI confirms complete ACL tear with a concomitant bone bruise (contusion) at the lateral femoral condyle and posterolateral tibial plateau. The typical mechanism of injury causing this MRI pattern is:
- A Hyperextension injury with contact on the anterior tibia
- B Non-contact deceleration and pivoting with valgus collapse and internal tibial rotation ✓
- C Dashboard injury (posterior force on flexed knee)
- D Direct varus force on the lateral knee
Explanation
The 'kissing contusions' pattern of bone bruising on MRI (lateral femoral condyle + posterolateral tibial plateau) is pathognomonic of non-contact ACL injury mechanism: sudden deceleration with knee near extension, valgus collapse and internal tibial rotation (combined forces cause anterior subluxation of lateral tibial plateau against lateral femoral condyle). This mechanism accounts for >70% of ACL tears in sports. Dashboard injury causes posterior tibial force and PCL injury. Hyperextension with contact causes ACL/posterior capsule injury pattern. Direct varus force causes lateral collateral ligament injury, not the ACL kissing contusion pattern.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.