The juvenile Tillaux fracture and the triplane fracture of the distal end of the tibia in adolescents are considered uncommon injuries and the mechanism of both injuries has been uncertain. It has been postulated that both fractures probably are caused by external rotation of the foot. We atempted to evaluate the mechanism of injury as well as to conform the role of the external rotation of the foot for the juvenile Tillaux fracture and triplane fracture. From 1983 to 1988 at the department of orthopedic surgery, college of medicine, Hallym University, we evaluated the cases of nine adolescent patients with the juvenile Tillaux fracture and nine patients with the triplane fracture of the distal tibial epiphysis. We confirmed that external rotation force produced the triplane fracture as well as juvenile Tillaux fracture. And we could postulate that the triplane fracture needed further external rotation force, less closure of the distal tibial epiphysis, and more complicated mechanism of injury than the juvenile Tillaux fracture. The knowledge of these mechanism of injury enables us to use a rational manipulative approach to reduce these fractures.