UNLABELLED: Malunion, nonunion, congenital abnormalities, and osteometabolic diseases are the main causes of long bone deformities. Although the exact incidence is unclear, it is estimated that about 10% of all fractures have some complication in terms of fracture-healing. In addition to the aesthetic impact, malunions generally substantially impair function and quality of life. Every malunion is unique, and treatment is usually planned according to the degree of deformity and the postoperative expectations of the patient. However, it is noteworthy that deformity correction usually requires a high degree of surgical expertise. Several techniques have been proposed over the years, and new techniques that utilize current technologies are available, such as computer-assisted single-cut osteotomy. In 2009, Russell et al. proposed the clamshell technique for diaphyseal malunions. This technique is our preferred treatment for diaphyseal malunions and acute fractures in the setting of a previous malunion or deformity. The following videos will thoroughly describe the steps to perform this useful and effective surgical technique for malunion correction. DESCRIPTION: The key principle of the "clamshell osteotomy" is to create a comminuted fracture at the malunion site and utilize an intramedullary rod as a template for deformity correction. ALTERNATIVES: Multiple osteotomy types and fixation methods are currently available for diaphyseal malunion correction. Among the osteotomies, opening or closing wedge, uniplanar, multiplanar, oblique, and dome methods may be utilized. In addition, several fixation methods can be utilized, including plates and screws, intramedullary rods, and external fixators. RATIONALE: The clamshell technique is a useful and effective treatment option for diaphyseal malunions of the lower extremity. The ability to utilize an intramedullary nail as a template for deformity correction makes the procedure simpler than previously described techniques, which require perfect preoperative planning to avoid over- and undercorrection. The versatility of this procedure justifies its incorporation into the therapeutic arsenal for treatment of complex diaphyseal malunions. EXPECTED OUTCOMES: To our knowledge, all previously reported cases utilizing the clamshell osteotomy have resulted in positive outcomes. Russell et al. presented a case series of 10 patients with posttraumatic diaphyseal malunions (4 femoral and 6 tibial), in which all patients showed coronal and sagittal-plane correction to within 4°, limb-length inequality correction to within 2 cm, and complete correction of translation, rotation, and joint-line orientation angles. In addition, all osteotomies healed uneventfully. The reported complications included broken interlocking screws in 1 case, need for dynamization in 1 case, and superficial wound dehiscence in 2 cases (1 of which required surgical debridement). Pires et al. presented 4 cases of clamshell osteotomies performed for the treatment of acute fractures in the setting of a previous malunion. All osteotomies healed by 15 months (mean time to healing [and standard deviation], 6.8 ± 4.4). One of these 4 cases was a Gustilo-Anderson grade-IIIB open fracture that required muscle flap coverage and a subsequent Hernigou procedure. When discussing treatment options with patients, it is important to clarify that there is currently no clear best technique to treat complex malunions; however, the clamshell osteotomy is a simpler procedure compared with others that have previously been described and has the benefits of quick rehabilitation and good deformity correction without the drawbacks of an external fixator. IMPORTANT TIPS: Preserve the blood supply in the opposite cortex.Close the fascia before reaming the medullary canal.Do not ream the osteotomy site.Be sure to perform a bicortical osteotomy.Create a stable construct.