Insufficiency of the posterior tibial tendon is challenging to treat. When the deformity is flexible, treatment options have included tendon transfer, often combined with a medial slide calcaneal osteotomy and/or a lengthening of the lateral column. Posterior calcaneal osteotomy has been shown to give correction, although not full correction. Lengthening of the lateral column also has been shown to give correction and has been used in the more severe flexible deformities, but it involves either fusion of the calcaneocuboid joint or risk of arthritis at this joint. An osteotomy combining the calcaneal medial slide with a lengthening of the lateral column at the same osteotomy site has been tested in the laboratory. This combined osteotomy provides a lengthening of the lateral column, but it is positioned away from the calcaneocuboid joint. In this study, the osteotomy was compared with a medial slide calcaneal osteotomy and an Evans lengthening of the lateral column, using a cadaver flatfoot model. Radiographic measurements were made to evaluate correction of the planovalgus deformity after each of these procedures. There was statistically significant improved correction with the new osteotomy compared with that in a standard medial slide, and correction was comparable to that in the lengthening of the lateral column. This combined osteotomy may be a reasonable alternative when more correction is desired than can be obtained from a medial slide alone and when the surgeon wishes to avoid an osteotomy near the calcaneocuboid joint.