Mandibular sagittal split osteotomy (SSO) is an operation performed for the correction of mandibular deformities. In this operation, sharp rotary tools are used during osteotomies and this can induce some complications. For example, if the inferior alveolar nerve is damaged, paralysis of the teeth, the lateral side of the tongue, and the corner of the lip can occur. To decrease the occurrence of such possible complications, we designed and manufactured a novel computer-assisted, patient-specific SSO guide and soft tissue retractor in our previous study. And, we first tested this apparatus on a cadaveric bone in vitro. Now, in this study, a surgical application of the instrument, which was designed and manufactured according to the requirements of the mandibular sagittal split osteotomies, was performed. This paper gives and discusses the results obtained from in vivo application of the apparatus.