A Novel Technique for Complete Denture Construction
Using a central bearing point tracing device with stock impression trays
LeRoy Selvidge, PhD, DDS | Russell Wicks, DDS, MS
Obtaining accurate final impressions and precise interocclusal records is a critical step in the construction of complete dentures. This article presents a novel clinical technique that adapts a central bearing point tracing device to stock edentulous impression trays. This method saves time in combining two traditional appointments in complete denture therapy for final impressions and centric relation (CR) records, and provides the advantage of verifying a repeatable CR position.
Determining Centric Relation
Centric relation has been defined as the position of the mandible in relation to the maxilla that results in the condyles of the mandible being located in the most anterior and superior portion of the maxillary fossae.1 This position is considered important in constructing complete dentures because it defines a repeatable point of origin from which mandibular movements may be initiated. It is imperative to capture this position with complete accuracy so that opposing dentures can be created in proper articulation.2
There are several methods for determining this centric position, including the use of a central bearing tracing device (CBTD). The first patent was assigned to Alfred Gysi in 1916. The use of an intraoral CBTD in a complete denture construction was reported by Phillips in 1927.3 Other noteworthy authors have further described the development and use of CBTDs for complete dentures.4-7
A common method to incorporate a CBTD is to make record bases on the casts of the final or master impressions and attach the pin and plate to wax rims.8 Swanson9 modified the use of a CBTD by attaching it to custom impression trays and using the pin to maintain consistent vertical dimension of occlusion (VDO) in conjunction with a functional stereographic recording for eccentric jaw relationships. He created master impressions and jaw relationship records at the same appointment. During the procedure he described that he often had to reduce the thickness of the trays in the maxillary tuberosity and mandibular retromolar pad areas to allow unrestricted movement.
CBTD components (Massad® Jaw Recorder, Nobilium, www.nobilium.com) were adapted to stock impression trays (Massad® Edentulous, Nobilium) as follows. Three short, thin strips of light-activated tray material (Triad®, Dentsply Sirona Restorative, www.dentsply.com) were arranged carefully along the inner arch of both upper and lower trays to complement the size and placement of the CBTD components. A separating material was applied on the underside of each component and they were pressed in place. Excess material was trimmed away with a sharp knife and the remainder was light-cured, forming a three-point baring shelf supporting the components. A separator was then applied to the top of the components. Additional tray material was cut into approximately 3-mm x 5-mm strips and layered over the components in the areas supporting their underside. Portions of these strips were engaged into vents in the trays to obtain retention. Upon curing, slots were effectively formed so that the CBTD components could be removed from the trays and replaced in a reproducible fashion (Figure 1).
The trays were shortened in their posterior regions and the tray handles removed using an acrylic bur. The CBTD components were attached and both trays placed into the mouth. Modifications were made intraorally by adjusting tray peripheries and adding modeling compound to a controlled thickness (Figure 2). Separate final impressions were made using variable viscosity vinyl polysiloxane materials10 (Aquasil®, Dentsply Sirona Restorative, www.dentsply.com) (Figure 3). Both impressions were replaced in the mouth with the centric pin in an open rest position. The pin height was reduced to the determined VDO. The posterior areas of the seated trays were observed for clearance, as the patient was asked to move his mandible forward, backward, and side-to-side (Figure 4).
The striking plate was coated using an indelible marker and a tracing was obtained of the eccentric jaw movements. At the point of the tracing, an indented disk was placed to help the patient relocate CR, and a jaw relationship record was made at this position with a rigid vinyl media (Megabite®, DenMat, www.denmat.com). The record was removed with the trays united (Figure 5).
The land areas of the trays were identified and stone containment box areas constructed in wax (Figure 6 and Figure 7) to provide specific dimensions for cast bases. The impressions were poured individually without separating the record (Figure 8). To promote accuracy, the casts were not removed from the impressions until after mounting. Casts were mounted on a semi-adjustable articulator in hinge lock position (Figure 9). A monoplane, non-balanced occlusion was planned for this case, so the casts were arbitrarily mounted in a central position within the articulator. If a balanced occlusal scheme is preferred, a facebow transfer and protrusive record can be added. Traditional techniques were employed to finish complete denture construction, including occlusion rims to verify the mounting and map anterior tooth position, and a wax try-in of the arranged prosthetic teeth for the proposed denture.
Heartwell and Rahn2 list three primary requirements for making CR records. The technique described lends favor to these tenants.
1. Record the correct horizontal relationship of the jaws. The central bearing point technique graphically illustrates a specific origin of movement position of the mandible relative to the maxillae and confirms this spatial position as the centric record is captured.
2. Exert equalized vertical pressure. Using the impressions as the platforms for the centric record has the benefit of maximum residual ridge11 adaptation for stability. Combining an attached CBTD to the impressions creates mechanical advantage for equal pressure distribution on jaw closure. Traditional record bases with wax rims constructed from master casts may be difficult to establish equal pressure distribution when deriving a record for a variety of reasons.
3. Retain the record in an undistorted position until the casts are mounted. The impression material has an intimate tissue contact, and is able to compensate for variations in the contour of the ridges because of elasticity. Construction of record bases involves laboratory procedures subsequent to the master impressions. Each of these procedures has inherent inaccuracies. The casts are poured and never separated from the impression or the record until after mounting to the articulator in the technique described.
When CBTDs are fitted to traditional record bases, additional appointments are required. One of the primary advantages of the technique described here is convenience in the form of reduced chairtime. When applicable, the use of stock trays for master impressions forgoes the need for primary impressions and casts. Due to wide variations in edentulous anatomic form,6 mounted casts acquired in this manner are also useful in reducing the time required to define the dimensions for subsequent occlusion rims and prosthetic tooth location.
Despite its expediency, this technique may be contraindicated in patients exhibiting spatial constraints in the posterior region of the edentulous jaws, or other abnormal anatomic or skeletal presentations that complicate the determination of centric and vertical jaw relations. It may also have limited use in patients who are unable to execute repetitive jaw movements.