An 18-year-old male suffered a valgus injury to the right knee due to a fall during a bigfoot race he took part in when he was 15 years old. He visited a different hospital at the age of 15. No obvious ligament injury or fracture was noted on MRI and physical examination. However, he gradually became aware of the valgus deformity of the right knee. Finally, he could not take part in a sports activity because of right knee pain. X-ray images at the age of 18 at an initial visit to our department showed severe valgus deformity with mechanical lateral distal femoral angle (mLDFA) of 71 degrees in contrast to left mLDFA which was 87 degrees. We performed a biplane-cut distal femoral varus osteotomy (DFO). Postoperative X-ray images showed an improvement of 86 degrees in mLDFA. Bony fusion was achieved six months after surgery, and he could play several sports activity.