A 12-year-old boy diagnosed with osteosarcoma of the left distal femur, who was treated by osteoarticular resection followed by modular prosthesis reconstruction. (A) Preoperative anteroposterior radiographic view of the knee showing a permeating sclerotic lesion of the distal femur. (B) An immediate postoperative radiograph showing reconstruction of the distal femur using a modular prosthesis with a smooth tibial stem. The modular distal femur prosthesis with smooth tibial stem was able to preserve the growth of the growth plate of the proximal tibia in children with sarcoma of the distal femur.