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A 16 year old football player is brought in with severe right knee pain after being hit from the front. He states that he thought his knee was displaced initially, but now it looks back in place. He has no obvious deformity or effusion on observation, and no ecchymosis. Instability testing is difficult due to the patient’s pain. Radiographs are negative for fracture or dislocation. Dorsalis pedis pulses are palpable and normal bilaterally.
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September 30, 2018 at 8:44 pm
A) Lack of “hard” signs or evidence of poor perfusion on exam. The patient’s history is concerning for a posterior knee dislocation (since he was impacted anteriorly with a direct blow). One could show him a picture of posterior knee dislocation and ask if that is how his knee appeared. Up to half of knee dislocations are spontaneously reduced by ED presentation. Anterior and posterior dislocations are the most common, and carry a high risk of popliteal artery damage. Patients with no “hard” signs or obvious poor perfusion, and with palpable pulses, may still have vascular injury. Some centers perform CT angiogram on all patients with suspected or diagnosed knee (not patella) dislocation, but evidence is increasing that methods that are less costly and involve less radiation may be adequate, such as an ankle brachial index > 0.9 + observation / serial examination as inpatient and/or normal duplex ultrasound. MRI, which the patient will eventually need to assess the ligamentous damage, coupled with MRA is another option. Another common complication is peroneal nerve injury (25-35%), typically presenting with decreased sensation in the first web space and abnormal dorsiflexion of the foot.