Purpose: Treatment of complex diaphyseal malunions is challenging. It requires extensive preoperative planning and precise operative technique. A single technique has not been generalizable secondary to the limitations of each type of osteotomy. A simpler method was developed to manage these deformities. Method: Ten patients with complex diaphyseal mal-unions (4 femoral, 6 tibial) underwent a clamshell osteotomy. Indications for surgery included pain at adjacent joints and deformity. Preoperative evaluation included deformity characterization. The malunited segment was identified on biplanar radiographs. After exposure the malunited segment was transected perpendicular to the normal diaphysis proximally and distally. The transected segment was again osteotomized about its long axis and wedged open with a lamina spreader, similar to opening a clamshell. The surgical approach was sealed to retain the subsequent reamings. The proximal and distal segments of the diaphysis were aligned using the intramedullary nail as an anatomic axis template and the opposite extremity as a length and rotation template. Partial weight-bearing mobilization with crutches began immediately and progressed based on clinical and radiographic evaluation. Followup ranged from 6–52 months. Results: Radiographic angular corrections were complete in each case and ranged from 2–20 degrees in the coronal plane, 0–32 degrees in the sagittal plane, and 0–25 degrees in the axial plane (rotation). Correction of length ranged from 0–5 centimeters, restoring leg length to within 2 centimeters in all cases. All osteotomies were healed both clinically and radiographically by 6 months. All patients were ambulatory without assistive devices by the time of the most recent followup. Conclusion: The proposed osteotomy provides a generalizable way to correct many forms of diaphyseal mal-unions by acting as a bypass in realigning the anatomic axis of the long bone using a reamed intramedullary nail as a template