A 10-year-old boy, diagnosed with osteosarcoma of the left distal femur, who was treated by osteoarticular resection followed by reconstruction using an expandable modular prosthesis. (A) An anteroposterior radiographic view showed osteosarcoma of the distal femur infiltrating the growth plate. (B) Coronal-fat saturation magnetic resonance image showing the location of the tumor at the distal femur. (C) Postoperative radiographs showing reconstruction of the distal femur with an expandable modular prosthesis at the 5-month follow-up. (D) Radiograph of the lower limbs showing elongation of the modular prosthesis at the 2-year follow-up. The extendable prosthesis was able to avoid discrepancies in children undergoing resection of the distal femur with sacrifice of the growth plate.