November 1, 2004
by Bruce Glazer
With all the emphasis on cosmetics today, we seem to have lost touch with the need for articulation. This lack of interest is due primarily to the complexity of most articulation systems in use today.
The temporomandibular joint is the most complicated joint in the human body due to its three-dimensional spatial movements of rotation and translation. When prosthodontic treatment is needed due to breakdown of the stomatognathic system, for whatever reason, simulation of the patient’s mandibular movements is mandatory.
In the past, in order to transfer the relationship of the upper jaw to the temporomandibular joints, a facebow or earbow was needed. This facebow transfer was perilous and not easily transported to the laboratory, as the jig assembly was fragile and easily loosened.
With the ARCUSdigma, a digital movement analyzer, this relationship is captured and redefined with the use of a single stand-alone bitefork, which snaps into one of two positions on a special support jig. This support jig (Fig. 1) magnetically fastens to the lower member of the Protar articulator.
There is no need to transport the articulator to the lab, only the casts, which attach magnetically to the articulator, travel back and forth. Thus only one articulator is needed and the package transfer of models only is mail manageable. In the past, the major disadvantage of all articulator systems was the need to buy multiple units, since the case was not transferable between articulators.
Systems that claimed the ability to transfer did not stand the test of time and thousands of dollars were spent to acquire more and more articulators.
An equally frustrating requirement was the need for programming of the posterior determinants of occlusion; a daunting task, which required complicated intraoral appliances and hinge bows attached to recording styli and fixed tables.
Enter the ARCUSdigma, a handheld virtual articulator. A simple to place headbow is positioned behind the ears. The nasal support is adjusted to the patient’s nose and the registration can begin. The ARCUSdigma functions as a three-dimensional ultrasonic navigation system.
Using the new bitefork and headbow, an articulator-related registration is created which transfers the condylar centres and the horizontal plane of the articulator from the transmitter frame to the head of the patient.
The condylar spheres are exactly defined coordinates and by entering these coordinates into the software of the system,the condylar centres and the horizontal plane of the articulator become virtually connected to the transmitter-bitefork assembly.
The central aim of an articulator-related registration is to record sagittal and horizontal angles. These angles are essential in programming an adjustable protar so that a mounted cast will imitate the patient’s mandibular movements.
The Protar system of articulators was developed to orient the casts parallel to Campers Plane (Fig. 2) and thus parallel to the desktop.
This orientation is a decided advantage when using a smile back approach which uses the cant of the upper teeth to evaluate the occlusal plane. While the adjustable Protar is mandatory for extensive reconstructions its use in a single restoration is worthwhile if only to correctly develop tooth anatomy to harmonize opening and closing patterns and close cusp tip relations.
I encourage the constant use of the entire system in order for both the dentist and the technician to become comfortable and efficient in its use.
There are three steps needed to record the settings to program the Protar. The first step is to place a bitefork loaded with a fast setting registration paste into the mouth to capture an imprint of the upper teeth (Figs. 3 & 4). Next the head frame is connected to the four receivers magnetically; the three transmitters are attached to the upper bitefork.
The receiver frame, which is now attached gently to the patient’s head via the headbow, as opposed to the external auditory meatus, measures the spatial relation of the transmitter frame which is attached to the maxillary bitefork (Fig. 5).
This action transfers the condylar centres and the horizontal plane of the Protar which are virtually carried by the transmitter frame to the head of the patient (Fig. 6). The horizontal plane of the Protar serves as a maxillary reference plane to which jaw movements are related.1
The upper occlusal record is removed and the transmitter frame is transferred to the mandible by means of paraocclusal fixation (Fig. 7). With the jaws in maximum intercuspation the position of this transmitter frame is determined in relation to the condylar centres and the reference plane of the protar.
This measurement also virtually attaches the condylar centres of the protar as the posterior reference points to the patient’s mandible.
From a position of maximum intercuspation the patient is guided through a series of protrusive and lateroprotrusive movements. Using these traces the software determines the condylar and Bennett angles.
The upper bite plane is now snapped into one of two software determined positions on the support jig and the maxillary cast is mounted. The chair time needed to record the settings is minimal. The entire operation can be completed in less than ten minutes.
In combination, the ARCUSdigma and the Protar articulation system offers the dentist and the laboratory technician a simplified approach to creating a more aesthetic and functional prosthesis.
Dr. Glazer is the Prosthodontic Editorial Board Member for Oral Health.
1.Proschel Peter et al, Articulator-Related Registration-A simple concept for minimizing eccentric occlusal errors in the articulator. Inter Journal of Prosth: 15:#3, 289-294,2002.