Posterolateral rotary knee dislocations are irreducible due to soft tissue incarceration within the joint. [3][4][5][6]8,11,13 Most commonly, the injury traps the free end of the torn medial collateral ligament (MCL) in the intercondylar notch. We present a previously unreported finding of vastus medialis interposition. The MRI, intraoperative findings, and clinical course unique to this case are described as well as the findings most common to posterolateral knee dislocations. CASE REPORTA 17-year-old male injured his right knee falling from the hood of a moving vehicle. The patient received initial care at an outside institution. A dislocation was recognized, but 2 attempts at closed reduction failed. The patient arrived at our institution 24 hours after the injury for definitive treatment.Physical examination revealed the right knee fixed at 30° of flexion with lateral subluxation of the patella and a prominent medial femoral condyle. A large skin pucker ( Figure 1) was noted over the medial joint line. The medial skin furrow was recognized as a sign of irreducibility, and no closed reduction was attempted. The limb was neurovascularly intact and the skin viable. Despite a palpable pulse in the foot, the concern for an occult vessel injury prompted further investigation. An arteriogram was normal.Plain radiographs showed widening of the medial compartment (Figure 2) of the knee joint. The patella was subluxated laterally. No evidence of bony ligamentous avulsion was found.The MRI demonstrated a soft tissue signal consistent with muscle within the notch and medial compartment.The torn posterior cruciate ligament (PCL) rested on the superior portion of this mass (Figure 3). The anterior cruciate ligament (ACL) was torn from its origin and was less clearly defined. The MCL and capsule did not appear on the cross sections and were obscured by the large muscle mass. The menisci, bone, and cartilage surfaces were unremarkable.A gentle examination under anesthesia accentuated the medial skin furrow, confirming the irreducibility of the injury. An open reduction was done 36 hours following the injury. The delay was secondary to failed attempts at closed reduction at an outside institution followed by a transfer to our institution. The medial femoral condyle rested deep to the subcutaneous tissue through a rent in the vastus musculature. The muscle belly was removed from the notch and medial compartment of the joint to expose the incarcerated MCL and capsule. A congruent reduction was achieved following removal of these structures. The PCL was repaired to its avulsed insertion site through a posteroanterior bony tunnel and tied over a post on the anterior cortex of the tibia (Figure 4). The MCL was Figure 1. The arrow indicates the pathognomonic sign of irreducibility, the medial skin furrow.