James S. Brudvik, DDSga and David University of Washington, Seattle, Wash.
During the metal finishing phase of removable partial denture construction, metal is removed from all aspects of the casting. The amount removed from areas designed to contact prepared surfaces of abutments is critical to the resulting fit of the partial. Castings were evaluated before and during finishing procedures to measure metal loss at the tooth-removable partial denture interface. Finishing and fitting techniques for controlling the loss of metal are presented. (J PROSTHET DENT 1992;68:924-7.)
onstruction of removable partial denture (RPD) frameworks has been routinely delegated to the commercial dental laboratory without clear standards as to the quality of fit required from the casting.l The result has been RPDs with little, if any, frictional resistance to dislodgment at the tooth-frame interface. The resulting removable partial denture frameworks rely excessively on clasp arms to supply the required retention. With the need for a more sophisticated RPD, as may be required for implant prostheses, rotational path partial dentures and precision attachments, it is essential that the laboratory phases of construction be reconsidered to stress the development of controlled surface contacts at the tooth-frame interface. The fit of the chrome-based alloys used for RPDs may be compromised by poor mouth preparation, inaccurate master casts, errors in blockout and duplication, difficulty in obtaining sufficient expansion of the refractory, and the techniques used in fitting and finishing the metal framework.2 Almost every step in the construction process is additive, resulting in an ever decreasing quality of metaltooth contact at the interface. An evaluation of the body of published research indicates that the fit of the framework is poorly defined and that a consistent tooth-frame relationship is difficult to obtain. Pulskamp3 measured the differences between castings and the master casts from which they were made for the two types of nonprecious casting alloys and for type IV gold, finding a significant difference between the alloys. The Co-Cr alloy had the greatest shrinkage and type IV gold the least, with the Ni-Cr alloy between. Earnshaw4 has written extensively on cobalt-chrome alloys in dentistry with special interest in the inability of the refractory material to
Presented at the Pacific Coast Society of Prosthodontists meeting, Victoria, British Columbia. aProfessor, Department of Prosthodontics. bOwner, Edgewood Dental Laboratory, Everett, Wash. 16/1/41169
fully compensate for the casting shrinkage. Lanier et al.,” commented on the multiple areas where inaccuracy of the refractory mold may occur. Firtell et a1.5 described the difficulty of obtaining a repeatable fit of RPD castings. In a clinical evaluation, Stern et a1.6 measured the contact between the rests of clinically acceptable RPDs and the abutment teeth, and found that 20 % of the rests did not contact the rest preparation. The area of partial denture construction that has received the least attention in the literature is loss of metal in the finishing of the framework. Because clinicians seldom, if ever, finish and polish a metal framework, they are generally unaware of the potential for uncontrolled loss of metal during these laboratory procedures. This study evaluates the procedures and the resultant metal loss.
Standard laboratory practice at this stage includes the recovery of the casting and removal of the sprues, followed by gross finishing of the sprue attachment area. The framework is then electropolished to a bright finish in a heated acid bath while attached to an electrode at a given voltage, resulting in a loss of surface metal. The casting is next fitted to either the master cast or a duplicate cast by adjustment of the metal with various stones and burs, followed by rubber wheeling and final polishing. These steps all result in a loss of surface metal, so that when the finished casting is reseated on the master cast and returned to the clinician, all evidence of frictional contact with the teeth may have been lost.
A master model was constructed, consisting of four teeth placed in a block of resin leaving only their coronal portions exposed. The teeth were prepared with a smooth, straight (nontapered) diamond stone in a handpiece attached with a paralleling device to a dental surveyor to create parallel guiding planes on the proximal surfaces. The model was
Fig. 1. Extracted teeth mounted in resin block with conservative blockout limited to areas gingival to the guiding planes.
Fig. 3. Mitutoyo calipers measuring specimen positioned in individual resin block for accurate repositioning.
Fig. 2. Waxed specimen ready for spruing and investing.
Fig. 4. Specimen in as-cast condition showing naturally occurring nodules that require careful removal without alteration of the tooth/frame interface.
blocked out to keep wax below the height of contour during the blockout (Fig. l), and duplicated in hydrocolloid (Nobiloid, Nobillium Company, Albany, N.Y.) resulting in five refractory casts (Multi-vest, Ransom and Randolph, Los Angeles, Calif.) to which an analogue RPD form was waxed. The five waxed forms were invested and cast in J. D. partial denture alloy (J. F. Jelenko & Co. Armonk, N.Y.). The analogue form provided both a long and short span edentulous area for evaluation with a total of 10 posterior spaces (Fig. 2). A vernier caliper (Mitutoyo SR-44x2, Mitutoyo Corp., Japan), capable of measuring either 0.01 mm, or 0.0001 inch, was mounted on a sliding base and the castings were related with individual resin forms to allow their exact replacement for repeated measurements and were calibrated (Fig. 3). The castings were measured three times for each posterior edentulous space in their as-cast condition and the mean was taken (Fig. 4). The castings were then subjected to standard finishing techniques by one operator. They were returned to the vernier caliper for measurement
after each of three stages of finishing: (1) electropolishing. (2) stoning; and (3) rubber wheeling and final polishing. The casts were stoned with a fine white stone to simulate the removal of nodules and fitting the casting. The stoning was done under ~10 magnification and controlled so that when an obvious change in the metal surface was obtained, the operation was terminated. Rubberwheeling and final polishing was done with both low and high shine materials, again only to the point of an obvious change in the metal surface for each component. The surfaces created in the as-cast and the electropolished phase were photographed through an electron microscope at X500 power (Fig. 5). RESULTS Table I shows the mean changes in dimension between the 10 sets of minor connectors (guiding plane surfaces) for each phase of the finishing operation and as the mean total 925
Fig. 5. Right side of illustration shows metal in as-cast condition. Left side shows surface of metal after electropolishing. (Original magnification X500.)
Fig. 6. Thick layer of wax is placed on interface surfaces of casting in preparation for electropolishing. Only sprues and rough flash have been removed at this point.
I. Mean values of metal lost in finishing for 10 edentulous spaces
Electropolish Stoning Rubber/polish Total Range of total
0.024 0.062* 0.127
loss 0.042 mm to 0.41 mm.
of metal removed for the entire process. It also shows the range of values obtained.
DIScUSSION Whether metal is removed from the RPD casting during the fitting and finishing phase is not in question. It is the amount of metal that is lost that raises the issue of clinical significance. The range of total loss values obtained (0.042 mm to 0.410 mm) was considerable, as might be expected from the amount of hand work involved. These values may be considered to represent the minimum loss of metal that would occur in a clinical situation because they were obtained with care in a controlled environment. Although the class I partial denture may require stress relief for satisfactory function,7 clinical control over the relief appears to be desirable. It is generally recommended that this relief be obtained clinically through the use of disclosing materials that mark the metal at the point of contact when the framework is loaded around its axis. The uncontrolled metal loss at the interface that occurs in the dental laboratory may give a degree of stress relief that exceeds the clinician’s desires. The resulting loose fit may require an even greater reliance on clasp retention to satisfy the patient. Class III and class IV partial dentures stand to benefit from as much frictional retention at the interface as can be preserved by controlling the loss of metal. A positive contact at the guide plane may eliminate the need for the of-
7. Polished framework shows high level of finishing up to the inferior border of the guide plane. The toothframe interface has not been touched beyond removal of surface nodules. Portion of frame that covers gingival tissue has received highest possible finish and polish. Fig.
ten unesthetic anterior clasp arm. In the rotational path of insertion partial denture, for example, even the loss of 50 pm of contact can result in unacceptable movement at the anterior component. Castings made for implant prostheses and for precision attachment procedures should also benefit from clinical control of the interface.
To protect the interface from the metal loss that occurs in electropolishing, the surface must be protected. A thick layer of hard baseplate wax or sticky wax is placed on all interface surfaces: guide planes, the underside of rests, and the tooth-contacting surface of clasp arms (Fig. 6). The casting can then be attached to the electrode and placed in the acid bath. It is also possible to protect the surface with adhesive tape placed on the critical areas of the casting.
The technician should. not adjust the interface contact areas other than to carefully remove all nodules with the smallest diameter stone possible. Finishing of all other surfaces can be standard. The casting is then either returned to the clinician without having been seated on the master cast or seated without regard to the master cast, because control of the interface demands that any fitting that alters the metal-tooth contact areas be done clinically. When the interface is left in the as-cast state, additional chair time is required to fit the frame to the teeth. The use of modern indicating materials such as Fit Checker (GC International Corp., Scottsdale, Ariz.) and Accu-Film tape (Parkell, Farmingdale, N.Y.) reduces the time needed to achieve a quality fit. Fit Checker disclosing material is first used to identify premature contacts that prevent complete seating of the casting (all that is needed in classIII and class IV situations). Small squares of the Acu-Film tape, placed inside the castings for class I and II partial denture situations will deposit die on the interface surface indicating functionally generated areas of excess contact. Both the tooth and the casting must be dry for this technique to be effective. The obvious roughness of the as-cast metal has an increased potential for plaque retention. The ideal finish at the interface of the minor connector is created by extending the rubberwheeling and polishing of the tissue surface of the major connector up to the gingival extension of the actual prepared guide plane, leaving the guide plane area in the as-cast state (Fig. 7).
CONCLUSION An actual contact between the guiding plane surface of a prepared abutment tooth and the minor connector of the RPD casting can be expected to aid in the retention of the partial denture. Control of a number of laboratory procedures is essential to obtain this level of contact. Precision at the tooth-RPD interface offers multiple advantages for all removable partial dentures. REFERENCES I. Taylor DT, Matthews AC, Aquilino SA, Logan NS. Prosthodontic survey. Part I. Removable prosthodontic laboratory survey. J PROSTHET DENT 1984;52:598-601. 2. Lanier B, Rudd K, Strunk R. Making chromium-cobalt removable partial dentures. J PROSTHET DENT 1971;25:197-205. 3. Pulskamp F. A comparison of the casting accuracy of base metal and gold alloys. J PROSTHET DENT 1979;41:272-5. 4. Earnshaw R. Cobalt-chromium alloys in dentistry. Br Dent J 1956;101:67-75. 5. Firtell DN, Muncheryan AM, Green AJ. Laboratory accuracy in casting removable partial dent,ure frameworks. J PROSTHET DENT 1985; 54~856-61. 6. Stern MA, Brudvik JS, Frank RP. Clinical evaluation of removable partial denture rest seat adaptation. J PROSTHET DENT 1985;53:658-61. 7. Hindels GW. Stress analysis in distal extension partial dentures. J PROSTHET DENT 1957;7:197-205. Reprint requests to: DR. JAMES S. BRUOVIK UNIVERSITY OF WASHINGTON DEPARTMENT OF PROSTHODONTICS, SCHOOL OF DENTISTRY SEATTLE, WA 98195