The authors describe two cases of severely angulated tibial non-unions after proximal tibia stress fractures associated with ipsilateral osteoarthritis treated with modular knee endoprosthesis with a long tibial stem to stabilize non-union fragments. During the procedure, no additional osteosynthesis or bone grafting was added. Both non-unions healed after 6 months with no post-operative complications. The authors suggest implantation of modular knee endoprosthesis, as a single procedure, in treatment of proximal tibia non-union after a stress fracture as a result of severe varus/valgus deformity. It provides a solution for osteoarthritis treatment, axis correction and non-union osteosynthesis.