Gothic arch tracing
Creation of prostheses for fully edentulous cases requires an authentic replication of centric relation and vertical dimension.
Wax bite rims have long been the main means of recording vertical dimension and centric relation for fully edentulous patients. Unfortunately, this method presents many challenges to establishing an accurate recording.
The success of any prosthesis, particularly with respect to removables, depends on an authentic replication of centric relation and vertical dimension. One reason the wax rim method is not preferred is the patient’s proprioceptive nervous system still has great influence on mandibular movements and condylar position, and a number of processes are responsible for an inaccurate, acquired bite.
For instance, as the patient wears his or her old dentures, the teeth wear. As a result, vertical dimension collapses, the alveolar ridges continue to resorb, the condyles move down onto the eminence causing the mandible to move forward, and the patient acquires a new centric relation that is neither healthy nor esthetically pleasing.
The need to address these problems was voiced as far back as 1910. Prof. Alfred Gysi¹ contended that if accurate alignment of the maxilla and mandible could be achieved, it would result in great improvements in function, phonetics and esthetics in prostheses design and fabrication.¹
What is the solution? Imagine if we could remove the traditional wax bite rim and restrict masticatory forces to a central bearing point in the mouth, creating a fulcrum of support for the mandible. By doing this, the muscles of mastication encompassing the TMJ would be free to return to their correct physiological position, and a patient-generated vertical would be easily and precisely recorded. If there was some way of recording the path of this central bearing point through protrusive and excursive movements, undoubtedly the point at which these movements intersect would precisely record centric relation.
This is exactly what the gothic arch tracer does. A gothic arch is derived from the surface of revolution of ogives, the intersecting transverse ribs of arches that establish the surface of a gothic vault. An ogive or ogival arch is a pointed arch, drawn with compasses, or with the arcs of an ellipse (Fig. A), and is one of the most defining characteristics of gothic architecture as seen in the historical city hall, Münster, Germany (Fig. B), hence the term, gothic arch tracer. There are a number of gothic arch tracers on the market. In this case, we used the Geneva 2000 Vertical and Centric Recorder available through Candulor USA (Fig. C).
The patient presents with a fully edentulous maxilla and mandible with two implants in the positions of tooth Nos. 22 and 27 (Figs. D and E). A good option to aid you in positioning the arch tracer is to obtain a preliminary bite record from the clinician using a Centric Tray (Ivoclar Vivadent), and mounting the casts on an articulator accordingly. When creating the bases for the maxillary and mandibular components of the arch tracer, it is preferred to follow the Myostatic or immobile outline, avoiding coverage of any mobile tissue surfaces. For these bases we used Triad VLC material (DENTSPLY Trubyte). On the mandibular arch, Zest locator housings and black males for attachment to the locator abutments in the positions of tooth Nos. 22 and 27 were incorporated into the VLC base using Triad gel.
01 Position the bar so the pin will be at the position of the second bicuspids and centered on the cast directly between the residual ridges. Use Candulor setup wax to position the bar (Fig F). The top of the bar should be level with the highest point of the retro-molar pad. There also should be a downward tilt to the bar toward the anterior region so the bar parallels the ala-tragus or Camper’s plane.
02 On the maxillary cast, create bases using the Myostatic outline and position the striking plate in the palatal area of the maxillae. Position the plate firmly into a ball of warm setup wax. The strike plate should again be parallel to the Camper’s plane and can be visually verified using a rim former set into the hamular notches (Fig. G).
03 Using the preliminary articulation, set the intervestibular distance to 37 mm by screwing or unscrewing the ball bearing pin on the mandibular assembly thereby opening or closing the vertical dimension. (This is an approximation and the final calibration will be performed by the practitioner in the mouth.)
04 The last step is to fabricate an Esthetic Control Base or ECB. This will give the practitioner the ability to determine esthetic proportions and considerations independent from the arch tracing. In addition, the practitioner will be able to more accurately assess proper anterior position for optimum phonetics. Using thermoformed pink base material, fabricate a base and trim back 1 mm short of mobile tissue.
05 Use wax to form the peripheral borders of the base, and bite rim using a rim former. Note: Remember this bite rim is not used for bite registration, but is used only as an esthetic guide (Fig. H). Make sure the incisal area is thin. The dimensions of incisal length also can be determined by the papillameter reading sent by the practitioner. At this point the case is ready to be sent out.
06 The practitioner will check proper parallelism of the strike plate and lower bar intraorally. At this point vertical dimension will be determined through a process of visual indicators, as well as patient feedback.
07 The strike plate is marked, the apparatus placed intraorally (Fig. I), and the patient is instructed and shown by the practitioner how to reproduce protrusive and excursive movements with the ball bearing coming into contact with the strike plate.
08 The maxillary base with the strike plate is removed and a defined arrow point will be visible on the marked strike plate (Fig. J). At this point, the plastic centric stop is luted to the strike plate with the hole directly over the apex of the arrow point using wax (Fig. K). The arch tracer is again placed in the mouth, and the centric relation is verified by having the patient close, at which point the ball bearing pin should come into contact with the strike plate directly through the hole on the centric stop.
09 The entire assembly is luted together intraorally with plaster or Luxabite (DMG) and removed upon setting.
10 The ECB is inserted and appropriate changes and/or notations are made with regard to phonetics, and esthetic considerations (i.e., incisal length, smile line, midline, canine position, and buccal corridor).
11 Once the case comes back into the laboratory, the maxillary cast is definitively mounted using a facebow record provided by the clinician, and the mandibular cast is mounted to the maxillary cast using the arch tracing (Figs. L and M).
Fig. N shows the finalized setup using Vita Physiodens anterior teeth and the requested lingualized occlusal scheme using Vita Lingoform posterior teeth.
It may be a challenge to help your clients see how this method can benefit them, but the security of having a system that results in greater predictability and improved accuracy of vertical and centric recordings is worth the effort.
Arian Deutsch, CDT, is a member of the Dental Technician’s Alliance of the American College of Prosthodontists and of the Arizona Dental Association. He has been involved with removable prosthetics for more than 18 years and owns Deutsch Dental Arts in Youngtown, Ariz.