Background: Paralysis of the crural nerve secondary to a compressive haematoma of the psoas in the pelvis is a well-known complication of anticoagulant therapy. This complication has also been described after hip or pelvic surgery. Its occurrence in a context of trauma is exceptional.

Case report: A 16-year-old female adolescent sought emergency care for total deficit of knee extension. The patient had an enlarged painful knee subsequent to a skateboard fall. She reported knee trauma involving the patella and a direct shock to the homolateral hip, on the trochanter. Physical examination confirmed the knee and hip pain. Rest was advised. One and a half month after the accident, the patient again consulted for total deficit of active knee extension. The initial diagnosis suggested was posttraumatic rupture of the patellar tendon. An emergency MRI was normal, ruling out this diagnosis. More attentive physical examination revealed the presence of a complete paralysis of the quadriceps muscle by crural nerve palsy. MRI of the pelvic region revealed the presence of a voluminous haematoma of the psoas compressing the crural nerve. Emergency evacuation of the haematoma was performed. The patient underwent rehabilitation for one year and achieved progressive and complete recovery of the quadriceps function. An electromyogram obtained at one year was normal.

Conclusion: This was an exceptional case of crural nerve palsy secondary to a posttraumatic haematoma of the psoas, with no notion of anticoagulation therapy. The initial knee injury was misinterpreted as involving a local patellar problem but in reality had caused a paralysis of the quadriceps muscle. MRI provided the diagnosis of psoas haematoma.