A 22-year-old footballer pivots acutely and hears a 'pop'. He develops rapid knee haemarthrosis and cannot bear weight. Lachman test is positive (grade II, soft end-point). MRI confirms ACL tear with bone bruising of the lateral femoral condyle and posterior lateral tibial plateau. This bone bruise pattern on MRI suggests which injury mechanism?
- A Direct valgus force causing medial compartment impaction
- B Posterior tibial translation causing posterior horn of medial meniscus impaction
- C Anterior tibial subluxation on the femur with posterior lateral tibial plateau impacting the lateral femoral condyle (classic pivot-shift mechanism) ✓
- D Hyperextension injury with central femoral notch impaction on the ACL
Explanation
The classic 'kissing contusion' pattern in ACL tears — bone bruising of the posterior lateral tibial plateau and anterior lateral femoral condyle — results from the pivot-shift mechanism: the tibia subluxes anteriorly and laterally under the femur during internal rotation and valgus stress, causing impaction between the posterior-lateral tibial plateau and the anterior-lateral femoral condyle at the moment of giving way. This pattern confirms the non-contact pivot mechanism and correlates with concurrent lateral meniscal injury risk. Valgus force impaction affects the medial compartment; hyperextension causes central notch bruising.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.