Radiographic evaluation of porosities in removable partial denture castings Arthur J. Lewis, D.D.S., M.D.Sc.* University of Western Australia, Dental School, Perth, Western Australia
The presence of porosities in a casting will predispose the prosthesis to failure in one of two ways. A large discrete void will so reduce the cross-sectional thickness of metal at some point that simple tensile failure may occur. Such voids are readily visible during an exammation of fracture sites. However, smaller intrametallic defects, frequently consisting of aggregations of micropores, may cause failure by acting as initiation sites for fatigue cracks. Such microporosity is not detectable at fracture surfaces since it is rendered inconspicuous by the invariably coarse and highly dendritic surface texture of castings. This study was undertaken primarily to determine by the use of radiography the total extent of porosity in removable partial denture castings. The distribution of porosities within partial denture frameworks was also investigated. MATERIALS A N D M E T H O D S The study was undertaken in two parts. Initially radiographs were made of 50 castings constructed of four different alloys. The distribution was as follows: Ticonium Premium 100,'~ 22; Vitallium,~ 9; Nobilium,w 6; and Crutanium,[ 13. All castings were technically sound, but each had been rejected for clinical reasons. During examination of the radiographs the following data on the castings were recorded: (1) whether the casting was for an upper or a lower partial denture, (2) number of radiolucencies considered definitely indicative of internal porosity, (3) morphology of the porosity, (4) occurrence of radiolucencies probably indicative of internal porosity,
*Chairman, Department of Prosthodontics. "~Ticonium Division, CMP Industries, Albany, N. Y. :~Austenal Products, New York, N. Y. w Products Inc., Chicago, Ill. HFried Krupp, GMBH Widia-Fabrik, Essen, West Germany. 278
and (5) location of radiolucencies Within the removable partial denture frameworks. For the radiography a Mueller Macrotank L* xray generator was used in conjunction with a standard 15 by 4 inch (38 by 10 cm) AGFA Gevaert Structurix D 7 t fine-grain film. Each sheet was kept in an individual black plastic light-opaque folder and was further protected by lead shields, which were removed only shortly before the film was exposed. The removable partial denture castings were positioned on the surface of the folder in such a way that a large amount of metal was close to the film plane. This usually resulted in occlusal rests and retainers being in contact with the folder, while palatal bars and other parts were kept away from it. Each denture was given a code number, and similar lead numerals were included in the exposure so that cross-identification was possible. Trials indicated that, when using a 24 inch (61 cm) target-film distance and when restricting the lateral spread of castings on the folder to a maximum width of 7 inches (18 cm), all sections of the castings could be recorded in focus on the film. The exposures were made at 4.0 mamp and 90 kV for 70 seconds. This produced maximum contrast between sound and porous metal. Following the radiography the 50 castings were fractured by holding each in the jaws of two longnosed pliers and then exerting a torsional load. Initial experimentation indicated that the castings should be held loosely to minimize any tendency to directly establish the fracture site. The castings broke into from four to seven fragments, with breaks usually occurring through the midline fracture of bars, at the separation of retainer arms, and at the retentive grids. All fracture surfaces were subse-
*C. H. F. Mueller, GMBH, Hamburg, West Germany. "~AGFAGevaert, N. V., Mortsel, Belgium. 0022-3913/78/0339-0278500.40/09 1978The C. V. MosbyCo.
RADIOGRAPHIC EVALUATION OF DENTURE CASTING POROSITIES
Table I. Evaluation of porosity in removable partial denture castings as determined by radiographs Alloy
No. of castings Upper Lower
22 8 14
9 7 2
6 4 2
13 5 8
3 5 2 -2 -4 3
1 3 1 2 --1 1
2 3 --1 -
2 3 2 2 -2 5 -
No. of flaws N o flaws detected No definite flaws detected 1 flaw 2 flaws 3 flaws 4 flaws 5 flaws In excess of 5 Multiple areas Probable m u l t i p l e areas Voids
quently examined under a stereo microscope. In the second part of the project radiographs were taken of approximately 200 castings, half of which were of a nickel-base alloy* and the remainder of a cobalt-base alloy.~ All castings were technically and clinically acceptable. The radiographs were analyzed for the desired data with a high-intensity viewer designed for industrial radiography. RESULTS The results of the radiographic examination are listed in Table I. Examination of radiographs showed three types of radiolucencies. Large radiolucencies with well defined borders. This type is indicative of the presence of a large void within the casting (Fig. 1). Excluding the Crutanium samples only three were found in the 37 radiographs examined. There was no consistency in their distribution throughout the removable partial denture framework. One occurred in the shoulder region of a buccal arm, the second in an occtusal rest, and the third in the region of connection between a metal backing and an anterior palatal bar. Small radiolucencies with diffuse outlines. These occurred more frequently and most of the castings * T i c o n i u m P r e m i u m 100, T i c o n i u m Division, C M P Industries, Albany, N. Y. ~'Nobilium, N o b i l i u m P r o d u c t s Inc., Chicago, Ill.
THE JOURNAL OF PROSTHETIC DENTISTRY
F i g . 1. R a d i o g r a p h s h o w i n g a l a r g e v o i d i n t h e s h o u l d e r r e g i o n o f a r e t a i n e r . T h e v o i d is c o m p l e t e l y c o n t a i n e d within the casting; the apparent lack of metal on one side resulted from the exposure conditions.
Fig, 2. Radiograph showing widespread diffuse porosity within a removable partial denture casting. showed at least one such radiolucency. Only four of the 50 castings were considered to be entirely free of such defects, although in five castings it was felt that the radiolucencies could not definitely be attributed to the presence of porosity. Widespread diffuse porosity with several voids is shown in Fig. 2. Aggregates of small radiolucencies with diffuse outlines. These aggregates tend to occur in characteristic locations. The classification of "multiple areas" used in Table I refers to this type of complex. An example is shown in Fig. 3. After the fragmentation of the frameworks the coarse dendritic texture at most of the fracture surfaces made detection of internal voids difficult. No problem was encountered when the voids were larger than any part of the dendritic geometry. However, as size diminished it was not possible to determine whether a particular small surface indentation was part of the wall of an internal cavity or 279
Fig. 5. Radiograph showing three multiple regions along the posterior border of a palatal bar. Each corresponds to the point of entry of a sprue.
Fig. 3. A "multiple area" within the retentive grid of a removable partial denture casting.
Fig. 4. Radiograph showing internal porosity at the sprue-lingual bar junction. The bulk of the porosity is contained within the sprue. caused by the tearing away of that section of the dendritic structure which then became part of the opposing fracture face. Definite voids were detected in the Crutanium castings and 11 possible voids were noted in castings of the other three alloys. Fracture of the castings tended to occur within the experimental partial denture frameworks at the same places as reported in clinical failure cases, 1 indicating the existence of similar points of stress concentration. Inclusions were identified at three fracture sites; they all appeared gray and devoid of surface detail. Some of the fracture faces examined showed a staining of the metal immediately beneath the external surface or deeper, but always with a definite connection to the surface. The stain tended to follow the orderly crystallographic orientation of the associated dendritic structure. The staining usually occurred at the midline of major connectors. However, in one instance staining was detected at the shoulder of a retainer. Stain distribution was as follows: Ticonium Premium 100, 2; Vitallium, 3; Nobilium, 0; and Crutanium, 4. The results obtained in the second part of the study, which involved the taking of radiographs of 280
Fig. 6. Radiograph showing gross shrinkage porosity associated with extreme variations in bulk throughout a casting. approximately 200 castings, are presented as Table II. DISCUSSION A major difficulty was experienced in establishing the relationship between observable radiolucencies and the presence of internal porosity. Surface indentations and regional reductions in thickness also produce localized radioluceneies in certain situations. Therefore the castings were examined where any doubt existed. Nevertheless it was still found that some radiolucencies could not be related with certainty to the presence of porosity, and they are included in the tables as "probable" areas. Radiolucencies possessing well defined margins indicate the presence of discrete voids and they tend to have a random distribution. The author has demonstrated, however, that the diffuse variety is the result of aggregations of interdendritic porosity. 2 They occur with a characteristic distribution at abrupt changes in cross-sectional thickness in the clinical casting or at the casting-sprue junction. In these situations they result from localized metal shrinkage caused by an interruption in the process of unidirectional solidification. For example labial and buccal arms are generally free from porosity, MARCH 1978
STRESSES FROM LOADING DISTAL-EXTENSION REMOVABLE PARTIAL DENTURES
Table II. Results of a numerical analysis of porous regions in clinical partial denture castings Ticonium
Upper No. No. of castings No flaws detected 1 flaw 2 flaws 3 flaws 4 flaws 5 flaws In excess of 5 Flaws per casting Definite Probable Voids Multiple areas Probable multiple areas
27 4 3 4 1 1 1 0 1.0 0.9 2 9 9
13 17 4 4 4 0
81 13 24 15 8 1 2 1
9 39 39
1.5 0.7 8 15 3
35 22 12 1 3 1
59 16 13 5 1 1 0 0
12 22 4
0.7 0.7 2 19 3
30 12 2 2 0 0
38 2 17 6 3 1 0 0
47 17 8 3 0 0
5 44 7
1.2 0.9 2 3 0
6 8 0
*All percentages were calculated by using the number of castings actually showing flaws, not the total n u m b e r of castings.
probably due to their even taper. The shoulder regions, however, often show porosity due to the greater bulk of metal frequently associated with occlusal rest complexes. The greater the cross-sectional variation and the more metal involved in the region the more pronounced are the shrinkage effects. It is in such situations that "multiple areas" develop. These are aggregations of diffuse radiolucencies and they occur particularly at casting-sprue junctions. At the point of entry of a sprue into a lingual bar (Fig. 4) the porosity may be totally confined within the sprue; yet after polishing the bar it is not unusual to find a pit or a pinhole porosity in this region. Multiple areas at the surface show up as pinhole porosity. Frequently three radiolucencies are seen in lingual bars, each corresponding to the point of entry of a sprue. Multiple areas are most commonly found in palatal bars and the sites correspond to the points of entry of sprues (Figs. 5 and 6). In lower distal-extension frameworks the retentive grids for acrylic resin bases are the most common sites of porosity because of the many changes in cross-sectional thickness (Fig. 3). In upper castings multiple radiolucencies at the sites of sprue attachments are most noticeable. In both upper and lower castings isolated porosities are frequently found adjacent to the shoulder region of retainer arms. Porosity was rarely found at any point within the partial denture frameworks other than at abrupt changes in cross-sectional thickness. The only exceptions were two small regions detected within indirect
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retainers that contacted the lingual surfaces of lower anterior teeth and three equally small areas within embrasure rests. The subsurface staining that has been described was probably caused b y the entry of the staining element or oxygen through minute surface cracks. These could represent an early stage in the development of fatigue failure and their location tends to support this possibility. SUMMARY Most of the removable partial denture castings examined showed radiographic evidence of internal porosity. In general the porosity occurred in characteristic regions associated with abrupt changes in cross-sectional thickness. These were located either within the partial denture framework or at the casting-sprue junctions. The porosity is due to metal shrinkage caused by a premature solidification at some point, which in turn interferes with a controlled unidirectional solidification of the casfing. REFERENCES 1.
Lewis, A.J.: Failure of removable partial denture castings during service. J PEOSTnET DENT 39:2, 1978. 2. Lewis, A. J.: Microporosity in casting alloys. Aust Dent J 20:161, 1975.
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DR. ARTHURJ. LEwis UNIVERSITYOF WESTERN AUSTRALIA DENTAL SCHOOL 179 WELLINGTONST. PERTH, WESTERNAUSTRALIA6000