CHAPTER 11 Surveying
When a fixed partial denture (FPD) is prepared, the orientation of the diamond bur is controlled to remove an amount of tooth structure necessary to satisfy the requirements of the path of insertion for the prosthesis. Accomplishment of parallel preparations is ultimately verified by complete seating of the prosthesis, but could be verified on the master cast or dies by the use of the surveyor. Once the FPD is fabricated and completely seated, it is ensured full engagement of the entire circumference of and occlusal support from the abutment retainers. If adequate resistance form and fit of the prosthesis are provided, the chance for functional stability equivalent to natural teeth is good. This could not be ensured unless the relationship of the fixed prosthesis and the prepared teeth were carefully controlled.
For a removable prosthesis, the necessity for appropriately planned and executed tooth preparation, followed by verification of a well-fitting prosthesis that engages the teeth as planned, is equally important. As was briefly mentioned in Chapter 7, a dental surveyor is vitally important to the planning, execution, and verification of appropriate mouth modifications for a removable partial denture. Although it does not necessarily affect the occlusal rest preparations on abutment teeth, use of the surveyor is critical for planning the modifications of all tooth surfaces that will be involved in support, stabilization, and retention of the prosthesis. In this role, the use of a surveyor to determine the needed mouth preparation is vitally important in helping to provide stable and comfortable removable prostheses.
A dental surveyor has been defined as an instrument used to determine the relative parallelism of two or more surfaces of the teeth or other parts of the cast of a dental arch. Therefore the primary purpose of surveying is to identify the modifications of oral structures that are necessary to fabricate a removable partial denture that will have a successful prognosis. It is the modification of tooth surfaces to accommodate placement of the component parts of the partial denture in their designated ideal positions on abutment teeth that facilitates this prognosis.
Any one of several moderately priced surveyors on the market will adequately accomplish the procedures necessary to design and construct a partial denture. In addition, these surveyors may be used to parallel internal rests and intracoronal retainers. With a handpiece holder added, they may be used to machine internal rests and to make the guiding-plane surfaces of abutment restorations parallel.
The most widely used surveyors are the Ney (Figure 11-1) and the Jelenko (Figure 11-2). Both of these are precision-made instruments. They differ principally in that the Jelenko arm swivels, whereas the Ney arm is fixed. The technique for surveying and trimming blockout is therefore somewhat different. Other surveyors also differ in this respect, and the dentist may prefer one over another for this reason.
Figure 11-1 The Ney surveyor is widely used because of its simplicity and durability. Dental students should be required to own such a surveyor. By becoming familiar with and dependent on its use, they are more likely to continue using the surveyor in practice as a necessary piece of equipment toward more adequate diagnosis, effective treatment planning, and performance of many other aspects of prosthodontic treatment.
Figure 11-2 The Jelenko surveyor. Note the spring-mounted paralleling tool and swivel at the top of the vertical arm. The horizontal arm may be fixed in any position by tightening the nut at the top of the vertical arm.
Figure 11-3 Various tools that may be used with a dental surveyor. A, Ney undercut gauges. B, Jelenko undercut gauge. C, Ney carbon marker with metal reinforcement sheath. D, Jelenko carbon marker. E, Tapered tools, 2- and 6-degree, for trimming blockout when some nonparallelism is desired. F, Ney wax trimmer for paralleling blockout. G, Surveying blade used for trimming blockout.
The principal parts of the Jelenko surveyor are essentially the same as those of the Ney surveyor except that when the nut at the top of the vertical arm is loosened, the horizontal arm may be made to swivel. The objective of this feature, originally designed by Dr. Noble Wills, is to permit freedom of movement of the arm in a horizontal plane rather than to depend entirely on the horizontal movement of the cast. To some this is confusing because two horizontal movements must thus be coordinated. For those who prefer to move the cast only in horizontal relationship to a fixed vertical arm, the nut may be tightened and the horizontal arm used in a fixed position.
Another difference between Ney and Jelenko surveyors is that the vertical arm on the Ney surveyor is retained by friction within a fixed bearing. The shaft may be moved up or down within this bearing but remains in a vertical position until again moved. The shaft may be fixed in any vertical position desired by tightening a setscrew. In contrast, the vertical arm of the Jelenko surveyor is spring mounted and returns to the top position when it is released. It must be held down against spring tension while it is in use, which to some is a disadvantage. The spring may be removed, but the friction of the two bearings supporting the arm does not hold it in position as securely as does a bearing designed for that purpose. These minor differences in the two surveyors lead to personal preference but do not detract from the effectiveness of either surveyor when each is properly used.
Because the shaft on the Ney surveyor is stable in any vertical position—yet may be moved vertically with ease—it lends itself well for use as a drill press when a handpiece holder is added (Figure 11-4). The handpiece may thus be used to cut recesses in cast restorations with precision with burs or carborundum points of various sizes in a dental handpiece.
Figure 11-4 Lab handpiece clamp. Handpiece holders attach to the vertical spindle of surveyors and may be used to create and refine any parallel surface on a surveyed crown, as a drill press to prepare internal rests and recesses in patterns and/or castings, and to establish lingual surfaces above the ledge that are parallel to the path of placement in abutment restorations.
The surveyor may be used for surveying the diagnostic cast, recontouring abutment teeth on the diagnostic cast, contouring wax patterns, measuring a specific depth of undercut, surveying ceramic veneer crowns, placing intracoronal retainers, placing internal rests, machining cast restorations, and surveying and blocking out the master cast.
Figure 11-5 Tilt of the cast on the adjustable table of a surveyor in relation to the vertical arm establishes the path of placement and removal that the removable partial denture will take. All mouth preparations must be made to conform to this determined path of placement, which has been recorded by scoring the base of the cast or by tripoding.
Figure 11-6 A, Solid line represents the height of contour on the abutment at selected orientation of the diagnostic cast to the vertical spindle of the surveyor. Dotted line represents the desirable height of the contour for optimally locating components of the direct retainer assembly. A 0.01-inch (0.25-mm) undercut gauge is used to mark the location of the tip of the retentive arm of the direct retainer. B, By reducing the axial contour of the tooth by only 0.01 inch, the optimum height of contour can be achieved without exposing the dentin. C, Stone tooth is trimmed with a surveyor blade to the desired height of contour. The trimmed area is marked in red pencil and serves as a blueprint for similar recontouring in the mouth. If one can safely assume that the enamel is 1 to 1.5 mm thick in the area of contemplated reduction, only 0.25 mm of enamel needs to be removed to achieve the optimum height of contour.
The surveyor blade is used as a wax carver during this phase of mouth preparation, so that the proposed path of placement may be maintained throughout the preparation of cast restorations for abutment teeth (Figure 11-7).
Figure 11-7 After the cast has been oriented to the surveyor at the predetermined path of placement, designated axial surfaces of the wax pattern are altered with the surveyor blade to meet specific requirements for placement of framework components. A, Wax pattern is carved with the surveyor blade to produce a distal guide-plane surface parallel to the selected path of insertion. B, Same pattern is modified from the distal guide plane along the buccal surface to align the surface with the height of contour most favorable to the direct retainer specifications.
Guiding planes on all proximal surfaces of wax patterns adjacent to edentulous areas should be made parallel to the previously determined path of placement. Similarly, all other tooth contours that will be contacted by rigid components should be made parallel. The surfaces of restorations on which reciprocal and stabilizing components will be placed should be contoured to permit their location well below occlusal surfaces and on nonretentive areas. Those surfaces of restorations that are to provide retention for clasp arms should be contoured so that retentive clasps may be placed in the cervical third of the crown and to the best esthetic advantage. Generally, a small amount of undercut from 0.01 to 0.02 inch (0.250 to 0.50 mm) or less is sufficient for retentive purposes.
Ceramic veneer crowns are often used to restore abutment teeth on which extracoronal direct retainers will be placed. The surveyor is used to contour all areas of the wax pattern for the veneer crown except the buccal or labial surface. It must be remembered that one of the principal goals in using a porcelain veneer restoration is to develop an esthetic replica of a natural tooth. It is unlikely that the ceramic veneer portion can be fabricated exactly to the form required for the planned placement of retentive clasp arms without some reshaping with stones. Before the final glaze is accomplished, the abutment crowns should be returned to the surveyor on a full arch cast to ensure the correct contour of the veneered portions or to locate those areas that need recontouring (Figure 11-8). The final glaze is accomplished only after the crowns have been recontoured.
Figure 11-8 Resultant metal-ceramic surveyed crown from Figure 11-7, which is being refined to maintain the distal guide plane and buccal height of the contour previously designed. Final glaze has not been placed on the veneer crown and required alterations of surfaces to conform to ideal placement of the retainer (solid line) can be performed by machining. Final glaze is produced only after necessary recontouring is accomplished.
The surveyor may be used as a drill press, with a dental handpiece attached to the vertical arm by a handpiece holder. Internal rest seats may be carved in the wax patterns and further refined with the handpiece after casting, or the entire rest seat may be cut in the cast restoration with the handpiece. It is best to carve the outline form of the rest seat in wax and merely refine the casting with the handpiece.
An internal rest differs from an internal attachment in that some portion of the prosthesis framework is waxed and cast to fit into the rest seat rather than a matched key and keyway attachment used (see Figures 6-13 and 6-14). The former is usually nonretentive but provides a definite seat for a removable partial denture or a cantilever rest for a broken-stress fixed partial denture. When they are used with fixed partial dentures, nonparallel abutment pieces may be placed separately.
The internal rest in partial denture construction provides a positive occlusal support that is more favorably located in relation to the rotational axis of the abutment tooth than the conventional spoon-shaped occlusal rest. It also provides horizontal stabilization through the parallelism of the vertical walls, thereby serving the same purpose as stabilizing and reciprocal arms placed extracoronally. Because of the movement of a distal extension base, more torque may be applied to the abutment tooth by an interlocking type of rest, and for this reason its use in conjunction with a distal extension partial denture is considered to be contraindicated. The ball-and-socket, spoon-shaped occlusal, or noninterlocking, rest should be used in distal extension partial denture designs. Use of the dovetailed or interlocking internal rest should be limited to tooth-supported removable restorations, except when it is used in conjunction with some kind of stress-breaker between the abutments and the movable base. The use of stress-breakers has been discussed in Chapter 9.
Internal rest seats may be made in the form of a nonretentive box, a retentive box fashioned after the internal attachment, or a semiretentive box. In the latter, the walls are usually parallel and nonretentive, but a recess in the floor of the box prevents proximal movement of the male portion. Internal rest seats are cut with dental burs of various sizes and shapes. Tapered or cylindrical fissure burs are used to form the vertical walls, and small round burs are used to cut recesses in the floor of the rest seat.
With a handpiece holder attached (see Figure 11-4), the axial surfaces of cast and ceramic restorations may be refined by machining with a suitable cylindrical carborundum point. Proximal surfaces of crowns and inlays, which will serve as guiding planes, and vertical surfaces above crown ledges may be improved by machining, but only if the relationship of one crown to another is correct (see Figure 14-9). Unless the seating of removable dies is accurate and they are held in place with additional stone or plaster, cast restorations should first be tried in the mouth and then transferred, by means of a plaster or acrylic-resin index impression, to a reinforced stone cast for machining purposes. The new cast is then positioned on the surveyor, conforming to the path of placement of the partial denture, and vertical surfaces are machined with a true-running cylindrical carborundum point.
Although machined parallelism may be considered ideal and beyond the realm of everyday application, its merits more than justify the additional steps required to accomplish it. When such parallelism is accomplished and reproduced in a master cast, it is essential that subsequent laboratory steps be directed toward the use of these parallel guiding plane surfaces.
Because surveying the master cast follows mouth preparations, the path of placement, the location of retentive areas, and the location of remaining interference must be known before the final design of the denture framework is completed. The objectives of surveying the master cast are as follows: