Metal component of dental restorations and fixed prostheses in the oral cavity of 43 cases of adults aged 55 years or older in forensic autopsy

Abstract

Characterization of the metal component of dental restorations and fixed prostheses is useful for the treatment of dental metal allergies and personal identification. This study aimed to describe the composition of metal elements in dental restorations and fixed prostheses in the oral cavity of 43 cadavers of Japanese adults aged 55 years or older in forensic autopsies conducted at a university. In this study, Ag-Pd-Au alloys were most frequently detected, and the percentage of Ni alloys was smaller than that reported in patients with dental metal allergies. Furthermore, alloys containing other elements, such as Fe or Hg, were also detected in some cases.

Introduction

Dental metal materials are alloys that often remain even after fire accidents, unlike plastic materials such as composite resins [1,2]. Metal alloys have long been used in dental treatments in Japan. There are several reports on the composition of dental metal materials in the oral cavity of patients with suspected dental allergies [3,4,5,6]. However, there are few reports on metal materials used in common dental procedures since the composition of metal elements in dental restorations and fixed prostheses is not examined unless patients have dental allergies or some symptoms. The number of people aged 65 and over in Japan was 36.21 million in 2021, accounting for 28.9% of the total population [Cabinet Office (2022)]. The number of remaining teeth in older adults in Japan has been increasing, with 20.8 teeth in those aged 65-74 and 15.7 in those aged 75 and older based on the data from the latest Survey of Dental Diseases [Ministry of Health, Labor and Welfare (2016)]. Various dental metal materials are considered to reflect previous treatments remaining in the oral cavity, especially in older adults living in Japan.

This study aimed to clarify the elements of metal materials used for dental restorations and fixed prostheses in adults aged 55 years or older regardless of whether they have dental metal allergies.

Materials and Methods

Experimental protocols were approved by the epidemiological research ethics review committee of Hiroshima University (no. E2022-0087). This study was conducted following the “Ethical Guidelines for Medical and Biological Research Involving Human Subjects (Ministry of Education, Culture, Sports, Science and Technology; Ministry of Health, Labor and Welfare; Ministry of Economy, Trade and Industry, Japan).”

Subjects

Cadavers of deceased Japanese with dental metal restorations or fixed prostheses over the age of 55, on which both forensic autopsies and dental examinations had been carried out by forensic pathologists and forensic odontologists or licensed dentists, between September 2021 and December 2022 at the Center for the Cause of Death Investigation Research and Education of Hiroshima University were examined and 43 cadavers were selected under these criteria. Of the 43 cadavers, 35 had complete upper and lower jaws, while the other 8 had some post-mortem missing parts in the jaws. The distributions of age and sex of the cadavers are shown in Table 1. The mean age of the 43 cadavers was 76.4 ± 9.2 years (median 75.0). The 43 cadavers did not have a history of metal allergies according to information available prior to the autopsy. A total of 219 dental metal restorations and fixed prostheses were found (1-13 pieces/ cadaver), including 75 complete metal crowns, 26 resin-veneered crowns, six porcelains fused to metal crowns, 37 fixed partial dentures (fixed bridges), eight metal onlays (including a partial-coverage crown), 43 metal inlays, 19 amalgam fillings, four cast metal cores (including a cast metal core under a resin crown), and one porcelain fused to a fixed partial denture. The six porcelain samples fused to metal crowns were from the same cadaver (No. 12, Table 2).

Table 1 Age, sex, and condition of the jaws of the 43 cadavers

Age (years)	Cadavers with complete jaws (No. 1 to No. 35)		Cadavers with jaws missing parts post-mortem (No. 36 to No. 43)
	male	female	male	female
55-64	3	0	2	0
65-74	7	3	3	1
75-84	8	5	1	0
≥85	3	6	1	0
Subtotal	21	14	7	1
Total	35		8

Table 2 Metal elements in all dental restorations and fixed prostheses in the oral cavity of each case (No. 1 to No. 43)

No.	Age (years)	Sex	*	Cu	Zn	Pd	Ag	Au	Sn	Cr	Co	Ni	In	Fe	V	Hg	Ga
Cases with complete jaws
1	86	F	7	+	+	+	+	+					+	+
2	81	F	3	+	+	+	+	+
3	71	M	10	+	+	+	+	+						+
4	87	F	7	+	+	+	+	+					+
5	89	F	3	+	+	+	+	+
6	73	M	3	+	+	+	+	+	+
7	77	F	6	+	+	+	+	+	+	+	+	+
8	78	F	5	+	+	+	+	+
9	78	M	2	+	+	+	+	+
10	68	M	4	+	+	+	+	+	+				+			+
11	71	F	6	+	+	+	+	+								+
12	73	F	11	+	+	+	+	+									+
13	79	F	5	+	+	+	+	+
14	75	M	4	+	+	+	+	+
15	75	M	1	+	+	+	+	+
16	70	M	4	+	+	+	+	+
17	85	M	6	+	+	+	+	+	+				+			+
18	75	M	7	+	+	+	+	+					+
19	62	M	1	+	+	+	+	+
20	72	M	4	+	+	+	+	+
21	73	M	7	+	+	+	+	+
22	78	M	1	+	+	+	+	+
23	66	M	6	+		+	+	+
24	99	F	3	+		+	+	+		+	+	+
25	89	F	6	+	+	+	+	+
26	89	F	5	+	+	+	+	+
27	78	M	5	+	+	+	+	+
28	80	F	8	+		+	+	+	+							+
29	93	M	8	+	+	+	+	+
30	64	M	7	+	+	+	+	+
31	68	F	6	+	+	+	+	+					+	+
32	84	M	2	+	+	+	+	+
33	58	M	5	+	+	+	+	+					+
34	86	M	4	+	+	+	+	+
35	77	M	7	+	+	+	+	+								+
Cases without complete jaws
36	82	M	3	+	+	+	+	+
37	71	M	5	+	+	+	+	+
38	89	M	3	+	+	+	+	+							+
39	63	M	13	+	+	+	+	+	+			+				+
40	71	M	7	+	+	+	+	+								+
41	62	M	1													+
42	69	F	6	+	+	+	+	+
43	73	M	2	+		+	+	+								+

*Number of dental metal restorations and fixed prostheses in the oral cavity

 

Analysis of metal elements

Metal elements were analyzed on metal materials in the 219 dental restorations or fixed prostheses of the 43 cadavers, in which a trace amount of metal was collected from each dental metal in the oral cavity of the cadavers by polishing with a silicone rotary instruments (M3#28; SHOFU Inc., Kyoto, Japan). Subsequently, the silicone rotary instrument with the polished metal samples were tested using a fluorescence X-ray analyzer (MESA-500W; HORIBA, Ltd., Kyoto, Japan). The silicone rotary instruments without metal samples were also analyzed to examine the metal components, whose results were used as backgrounds and subtracted from the value of the tested silicone rotary instruments. The process and evaluations were performed correctly by an experienced forensic odontologist and oral examiners, all of whom were licensed dentists.

Results

Ag-Pd-Au-Cu-Zn alloy or Ag-Pd-Au-Cu alloy was detected in 92.0% of complete metal crowns, 92.3% of resin-veneered crowns, 83.8% of fixed partial dentures, 87.5% of metal onlays, and 76.7% of metal inlays (Table 3). Ag, Pd, Au, Cu, and Zn were detected in the oral cavities of 42 of the 43 cadavers, excluding one without complete jaws (No 41, Table 3). Ni alloys were found in fixed dental prostheses of two of the 35 cadavers with complete upper and lower jaws, as well as one of the eight cadavers with some post-mortem missing parts in the jaws (Table 3). Additionally, Sn, Cr, Co, In, Fe, V, Hg, and Ga were detected (Table 3). Ga was detected only in the six metal crowns fused to porcelain in one cadaver (No. 12), and V was detected in only one cast metal core (No. 38).

Table 3 Breakdown of alloys in the 219 dental metal restorations and fixed prostheses

	(%)	Cu	Zn	Pd	Ag	Au	Sn	Cr	Co	Ni	In	Fe	V	Hg	Ga
Complete metal crown (n = 75)	61.3	+	+	+	+	+
	30.7	+		+	+	+
	1.3	+	+	+	+	+						+
	1.3	+		+	+	+						+
	1.3	+	+	+	+	+								+
	1.3	+						+	+	+
	1.3								+
	1.3		+		+
Resin-veneered crown (n = 26)	53.8	+	+	+	+	+
	38.5	+		+	+	+
	3.8			+	+		+
	3.8	+	+		+
Porcelain fused to metal crown (6) or fixed partial denture (1) (n = 7)	85.7	+		+											+
	14.3			+		+
Fixed partial denture (n = 37)	62.2	+	+	+	+	+
	21.6	+		+	+	+
	2.7	+		+	+	+						+
	2.7			+	+	+
	2.7			+		+					+
	2.7	+	+	+	+	+					+
	2.7	+						+	+	+
	2.7	+	+		+	+
Metal onlay (n = 8)	50.0	+	+	+	+	+
	37.5	+		+	+	+
	12.5		+		+						+
Metal inlay (n = 43)	30.2	+	+	+	+	+
	46.5	+		+	+	+
	2.3	+	+	+	+	+					+
	4.7		+		+						+
	4.7	+	+	+	+
	2.3	+			+	+
	2.3	+		+		+
	2.3	+		+	+
	2.3	+	+		+						+	+
	2.3		+		+						+
Amalgam filling (n = 19)	47.4													+
	21.1				+									+
	10.5	+			+		+							+
	10.5						+							+
	5.3				+									+
	5.3	Undetectable
Cast metal core (n = 4)	25.0		+		+						+	+
	25.0	+	+		+	+
	25.0		+		+								+
	25.0	Undetectable

Discussion

There is little information concerning the components of dental metal materials in the actual oral cavity without dental allergies since it is difficult to take materials from dental restorations and fixed prostheses in the oral cavity without appropriate reasons. In the present study, there was no history of metal allergies according to information available prior to the autopsy.

From the 219 dental metal restorations and fixed prostheses of the 43 cadavers of Japanese adults aged 55 years or older, Ag-Pd-Au-Cu-Zn alloy or Ag-Pd-Au-Cu alloy were the most frequently detected materials. The cost of dental materials is covered by the universal health insurance system in Japan. In universal health insurance, claims for dental Ag-Pd-Au alloys (main composition: Ag content of at least 40%, Pd content of at least 20%, Au content of at least 12%) for metal inlays and crowns in private dental clinics in Japan have been the largest in recent decades [Ministry of Health, Labour and Welfare. Statistics of Medical Care Activities in Public Health Insurance FY 2021]. Hence, Ag-Pd-Au alloy is considered the metal most often used in dental restorations and fixed prostheses covered by universal health insurance in Japan. Amalgam fillings were also present in the cadavers because amalgam fillings were often used in Japan for dental restoration covered by universal health insurance until 2016.

On the other hand, Ni alloys are well known to cause allergies. The percentage of Ni alloys in this study (two out of 35 cadavers with complete upper and lower jaws and one out of eight cadavers with some post-mortem missing parts in the jaws) was smaller than that in a report on people with suspected dental metal allergies (57 persons in total 187 subjects) [6]. The Ag-Au-Pd alloys appeared as a grossly similar “silver” color to Ni alloys, other Ag alloys, and polished amalgam; hence, it is difficult to distinguish dental metal materials with the naked eye. Thus, the results of this study are important in terms of discussing the oral condition more realistically.

The Ministry of Health, Labor, and Welfare in Japan controls the approval of the manufacturing and marketing of dental devices and materials, including dental treatment materials, based on the Japanese Industrial Standard (JIS) and International Organization for Standardization (ISO). Recently, with the development of computer-assisted design and manufacturing, as well as adhesive-based materials, non-metal materials have become more widely used in Japan. The reasons for the use of non-metal materials are not just the lower cost and shorter treatment periods, but also minimizing the risk of dental metal allergies associated with Pd, Ag, Ni, and Zn [3,5,6,7], as well as Hg toxicity [8,9,10]. Since 2020, some dental treatments using titanium (JIS H4650) for dental casting have also been covered by health insurance in Japan. Therefore, in the future, a more diverse combination of materials newly covered by health insurance will be found in the oral cavity of people with a history of treatment in Japan, including various non-metal materials and Ti alloys.

This study, however, has some limitations. One main limitation was the small number of cases included. However, there is little information concerning the components of dental metal materials in the actual oral cavity of patients with non-metal allergies as described. This study shows rare information on people without an obvious history of dental metal allergies. Moreover, the study evaluated the composition of metal materials for dental restorations and fixed prostheses in adults aged 55 years or older in Japan. Second, metals that tended to be lower in composition than other metals, such as Zn in Ag-Pd-Au alloys, might not be detected owing to the limitations of the measurement instrument. Third, to avoid destroying the teeth, a very small sized amalgam filling and an exposed cast metal core under a resin crown could not be sampled in quantities by polishing with the silicone rotary instrument. However, the major composition of metal elements in almost all of the dental metal treatments in this study were detected.

In conclusion, a high proportion of alloys composed of Ag, Pd, Au, and Cu was found in metals for dental restorations and fixed prostheses in the oral cavity of cadavers of Japanese adults aged 55 years or older in forensic autopsy sometime after 2022 when this study was conducted, as expected in universal health insurance system in Japan. The percentage of Ni alloys in this study was smaller than that reported in patients with dental metal allergies. However, alloys containing other elements, such as Fe or Hg, may also be detected, even if dental restorations and fixed prostheses appear to be similar to silver in color.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Funding

This study was partially supported by a research grant from Pfizer Health Research Foundation FY 2020.

Author contributions

H.O. and M.N. conceived the ideas; H.O., N.O., Nar K., Nao K. and M.N. collected the data; H.O., N.O. analyzed the data; H.O. and M.Y. led the writing; and N.O., Nar K., Nao K., and M.N. reviewed and edited the manuscript; all authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors are grateful to Dr. Masae Kitagawa (Hiroshima University) for years of collaboration, technical support, and carefully proofreading the manuscript. We also thank Dr. Naoki Ishiuchi for approving this study. Additionally, many thanks go to Ms. Rie Miyata (Hiroshima University Hospital) for technical assistance.

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