A Simple and Accurate Method for Mounting Models in Orthognathic Surgery

Abstract

Traditionally, a facebow device is used to register the 3-dimensional relationship of the maxillary dental arch to the Frankfort horizontal plane (FHP) using either the patient’s external meati or the condylar heads (depending on the requirements of the facebow) as the posterior reference. Auxiliary components can be attached to the facebow to aid in the anterior alignment using nasion or infraorbitale. This facebow registration is transferred to the articulator to position and mount the maxillary dental model. The mandibular dental model is then mounted with an interocclusal registration. Unfortunately, this facebow registration method often creates inaccuracies in the mounted maxillary model position, thus presenting the models in a significantly different orientation compared with the actual relationship of the patient’s jaws to the cranial base structures. 2-5 The predetermined surgical movements derived from the patient evaluation and prediction tracing, when performed on malaligned dental models, may result in significant malpositioning of the jaw structures at the actual surgery, producing compromised functional and esthetic outcomes for patients. There are reasons for inaccuracies in the traditional facebow mounting of dental models: 1) the vertical, anteroposterior (A-P), and/or mediolateral position of the patient’s external meati or condyles (depending on the reference for the facebow) may be asymmetric from side to side compared with the fixed symmetric position of the facebow mounting rods on the articulator; 2) due to anatomical variances, the patient’s FHP as determined by the facebow may be significantly different than the fixed FHP of the articulator; 3) the facebow may be improperly positioned on the patient, or facebow components could shift when tightening the bolts, nuts, and/or screws during the registration procedure; 4) cranial base and jaw aberrations may be present that are not reproducible on the articulator; 5) anatomical structures may be absent (ie, hemifacial microsomia), rendering the facebow mounting totally arbitrary; and 6) shifting of the facebow components can occur with inadequate tightening of the bolts, nuts, and/or screws at registration acquisition or with rough handling during the mounting of the maxillary model. The purpose of this article is to describe a simple and accurate method for mounting dental models using the SAM Occlusal Plane Indicator (OPI) device (Figs 1, 2) for the SAM articulator (SAM-Prazisionstechnik, Munchen, Germany: US distributor: Great Lakes Orthodontics, LTD, Tonawanda, NY). The OPI