The ulna is a long bone characterized by rotational motion with the radius, and the ulna is thin and not very rich in blood circulation. Internal fixation of the ulna fracture with a steel plate has been shown to be effective. However, if the initial surgery fails and a bone nonunion develops, the fracture cannot be fixed firmly with nails or plates. We used locking intramedullary nails to treat ulnar nonunion with good results. The implantation of intramedullary pins, which can distribute the load, allows the patient to perform early activities. The uniform distribution of elastic stress can avoid extensive soft tissue stripping during internal fixation of steel plate, good stability of the broken end, early functional exercise, high rate of bone healing, and fast recovery. Pre-operative photo of ulnar osteoarthritis Case description: This patient is a 24-year-old male, who underwent internal fixation of ulnar fracture in a foreign hospital 9 years ago, but failed and formed osteoarthritis. 8 years ago, I used the treatment of internal fixation of plate bone grafting + BMP injection, and the osteoarthritis healed. 1 year ago, due to the growth of a lot of scabs to cover the plate, and the new fracture was caused by the inadvertent removal of the plate in the local area. The broken end of the fracture had residual nail holes, and the fracture area was obviously osteoporotic, with resorption of the stump, which was a typical atrophic type of bone nonunion, and the broken nail was left behind. Obviously, the fracture could not be fixed with nails and plates, and of course could not be healed in the end. Post-operation photo Stem cell transplantation + locked intramedullary nailing for ulnar atrophic osteochondroma We used locked intramedullary nailing for the treatment. We used locking intramedullary nails to open the occluded medullary cavity and implanted intramedullary pins that can spread the load and protect the blood flow of the surrounding periosteum. It avoids extensive soft tissue stripping during internal fixation of steel plate and provides good stability of the broken end, early functional exercise and high bone healing rate. The postoperative treatment combined with autologous bone marrow stem cell injection for 2 times resulted in rapid healing. at 3 months there was a large number of bone scabs formed at the fracture end, continuous bone scabs, and clinical healing.