J. Dent. 1991; 19: 160-163
A preliminary investigation into the longevity and causes of failure of single unit extracoronal restorations* G. S. P. Cheung Department
Faculty of Dentistry,
of Hong Kong, Hong Kong
ABSTRACT Four types of single unit restoration, including porcelain jacket, full and partial veneer, and metal ceramic crowns provided at the Prince Philip Dental Hospital in Hong Kong were investigated for evidence of failure. Of the 46.58units of restoration provided between 1981 and 1989,100 restorations of each type were selected at random by computer. The relevant patients were identified and invited to return for a review appointment. A total of 132 patients attended and 152 crowns were examined. The average length of service was 34 months at the date of examination. Using strict criteria, 21(14 per cent) restorations were deemed to have failed. Technical failure was the most prevalent cause of failure, followed by aesthetic complaints and endodontic problems. The failure rates ranged from 2.4 to 7.8 per cent per year for the different crowns in order of: partial veneer < full veneer < metal ceramic < porcelain jacket crowns. Fracture of restoration, which affected metal ceramic and porcelain jacket crowns, was the single most frequent cause of failure observed in this study. KEY WORDS: Crowns, Longevity, Failures J. Dent. 1991; 1990)
63 (Received 15 August 1990;
reviewed 5 October 1990;
accepted 4 December
Correspondence should be addressed to: Or G. S. P. Cheung, Department of Conservative Dentistry, Faculty of Dentistry, University of Hong Kong, 34 Hospital Road, Saiyingpun, Hong Kong.
INTRODUCTION The replacement of dental restorations is commonplace (Maryniuk, 1984). Restorations fail, necessitating their replacement, for many reasons and at different times since placement. A flaw may be present in a newly placed restoration (Lavelle, 1976) or in an existing restoration due to deterioration of the restorative material in service. The longevity of a restoration is, therefore, a function of: the clinical skill of the dentist who placed the restoration; the technical skill of the technician, in the case of indirect restorations; the properties of the restorative materials; the ability of clinicians to detect defects in restorations; the philosophy of dentists towards replacement of
This paper was presented at the 78th FDI Annual World Dental Congress, 1990 in Singapore. Supported in part by Hsin Chong-K. N. Godfrey Yeh Education Fund, University of Hong Kong. @ 1991 Butterworth-Heinemann 0300-5712/91/030160-04
restorations; and patient factors, such as oral hygiene, occlusal function and dietary habits. The longevity and causes of failure of amalgam and composite resin restorations have been reported extensively (Elderton, 1976; Lavelle, 1976; Crabb, 1981; Maryniuk, 1984; Klausner et al., 1987; Mjor, 1989). However, equivalent information on extracoronal restorations is limited. To the knowledge of the author, there is no report in the literature concerning the longevity and causes of failure of single unit extracoronal restorations, apart from several studies on crowns and bridges collectively (Schwartz et al., 1970; Walton et al., 1986; Glantz, 1989) or on all types of restorations (Bentley and Drake, 1986). The purpose of this study was to examine the longevity and causes of failure in a random sample of single unit crowns provided in the Prince Philip Dental Hospital, Hong Kong.
MATERIALS AND METHODS The Prince Philip Dental Hospital is the teaching hospital for the Faculty of Dentistry of the University of Hong Kong. Most dental treatment is provided by dental students under supervision, with a small proportion being carried out by registered dentists including lecturers. The personal and treatment records of every patient attending the hospital are managed by a computer system. From the computerized database, a list of all single unit extracoronal restorations which were provided between 1981 and 1989 was generated, including 565 porcelain jacket, 1308 full veneer, 502 partial veneer and 2283 metal ceramic crowns. A random sample of 100 restorations was selected for each type of crown. The relevant patients were identified and were invited to return for a review appointment. Since more than one restoration was chosen in certain individuals, less than 400 patients were invited to participate in the study. Each patient received a routine dental check-up during the review. The selected restoration(s) was then examined in detail, both clinically and radiographically. Data on patient’s particulars and the status of restoration(s) were recorded on a special form to facilitate analysis by computer. For the purpose of this study, an extracoronal restoration was deemed to have failed if any of the following complications were observed (Randow et al., 1986): 1. Technical-including fracture of restoration, tooth or root fractures, and loss of retention of the restoration. 2. Aesthetic-patient was not satisfied with the appearance of the restoration or restored tooth. 3. Cariologic-caries at the margin of the restoration. 4. Endodontic-the tooth became periapically involved and/or was root tilled after the crown had been cemented. 5. Others-such as tooth extraction for any other reasons. The collected data was analysed using a Sperry Univac computer. Patients found to require treatment at the time of review were offered further appointments.
fig. 7. Distribution of different restorations in the dental arches. E5,PJC; ?,? FVC; 0, PVC; !Zl,MCC. (Refer to Table I for definition of abbreviations.)
RESULTS One hundred and thirty-two patients (85 women and 47 men) attended review appointments. The average age of the respondents was 37.3 years, ranging from 17 to 73 years. A total of 153 restorations were examined (38 per cent of the sample, Table I). One restoration which was an inlay with occlusal coverage was selected in error and excluded from the database. Of the 152 restorations included for further analysis, 73 crowns (48 per cent) restored vital teeth. The average length of service of all restorations was 34 months (Table I>. The distribution of the teeth restored is illustrated in Fig. 1. In general, porcelain jacket crowns were placed on upper incisors; full and partial veneer crowns on posterior teeth and metal ceramic crowns tended to be present on both upper and lower anterior and posterior teeth. A total of 21 restorations were considered to have failed according to the criteria adopted. Most of these failures being technical, aesthetic and endodontic (Table ZZ).The computed failure rates (Roberts, 1970) ranged from 2.4 to 7.8 per cent per year for the four types of extracoronal restorations investigated (Table ZZZ).Fracture was the Tab/e II. Summary of all causes of failure
Tab/e 1. Length of service at the date of examination
Type of crown *
Sample size (no.1
PJC FVC PVC CMC Other
100 100 100 100 -
Total number examined 34 32 38 48 1 153
Length of service (months) 40.1 31.4 32.8 33.4 24.0 34.1
‘PJC, porcelain jacket crown; FVC, full veneer gold crown; PVC, partial or three-quarter veneer crown; MCC, metal ceramic crown.
Types of failure Technical Fracture of restoration Fracture of tooth Loss of retention Aesthetic Caries Endodontic Others (tooth extracted) All Total no. of restorations examined
8 1 3
5.3 0.7 2.0
: 3 2
z 2:o 1.3
J.Dent. 1991; 19: No.3
Table 111.Amount of failures and failure rate of each type of restoration Types of
Porcelain jacket Full gold veneer Partial veneer Metal ceramic Total
No. of restorations Examined failed 34 32 38 48 152
9 2 3 7
(% per year)
(26.5%) ( 6.3%) ( 7.9%) (14.6%)
7.8 2.4 2.9 5.2
single most frequent cause of failure (38 per cent). Typically, fracture involved a porcelain jacket crown (Table Iv). The other two forms of failure were relatively less frequent. Four restorations were considered by patients to have unacceptable aesthetics, one of which was a full gold veneer crown on a lower first molar (Table IV). Tooth fracture resulted in the failure of one (Table II). Three vital teeth became non-vital following crowning, necessitating root canal therapy. Two crowned teeth were extracted for other reasons (Table II).
DISCUSSION This cross-sectional study was designed to assess the incidence and causes of failure of single unit extracoronal restorations. The response rate was disappointing. Although a random sample of restorations was investigated, it remains unknown as to whether review of the non-respondents would have influenced the results. Hence, this report is regarded as preliminary. The four types of crown included in the current study constitute almost all single unit extracoronal restorations provided in the Prince Philip Dental Hospital. Other extracoronal restorations were either too few in number or too recently introduced to be studied. Comparing the different crown designs, full gold veneer and partial veneer crowns were found to have a failure rate of less than 3 per cent per annum. A relatively high failure rate was observed for porcelain jacket and metal ceramic crowns (Table III). This trend has been observed in other studies (Schwartz et al., 1970; Walton et al., 1986). The major causes of failure observed do not agree with most other studies which generally concluded that secondary caries is the single most frequent cause of failure in both single unit crowns and fixed bridges (Kantorowicz, 1968; Schwartz et al., 1970; Walton et al., 1986; Randow et al., 1986; Glantz, 1989). It is suggested that this may, in part, be due to Hong Kong having had fluoridated water since 1961. The incidence of technical failures was greater than that for all other causes, with fracture of restoration being the commonest cause of failure. This may be related to the dietary habit and the types of food common in Hong Kong. Both porcelain jacket and metal ceramic crowns suffer failures due to material fractures (Table IV); however, as shown in this study, metal ceramic crowns
Tab/e IV. Details of the causes of failure Cause of failure
Type of crown* WC WC CMC Total
Fracture of restoration Aesthetics Endodontic Loss of retention Tooth extraction Fracture of tooth
5 2 1 1
0 1 1 0
: 1 1
3 1 0 1
8 4 3 3
*For definition of abbreviations,see notes to Table 1.
suffer fewer fractures than porcelain jacket crowns (cf. Table II). Crowns with poor aesthetics were regarded as failures in the present investigation (Table IV). In contrast, aesthetics was not considered to be a cause of failure in the studies by Roberts (1970), Reuter and Brose (1984), Bentley and Drake (1986) and Cheung et al. (1990), which could have resulted in an underestimation of failure. Failures of endodontic origin comprised a small proportion of the failures observed (4.1 per cent of vital teeth). This figure compares favourably to those reported for the abutments of fixed bridges (Karlsson, 1986; Cheung et al., 1990; Foster, 1990). It is suggested that during the fabrication of a bridge, aligning the preparations of several abutments often requires additional removal of sound tooth structure. The deeper and the more extensive the tooth preparation, the greater the degree of inflammatory pulpal 1:sponses (Kim and Trowbridge, 1987). This may have contributed to the higher incidence of pulpal death after fixed prosthodontic treatment in comparison to single unit restorations. The remaining two cases of failure were attributed to tooth extraction. The affected teeth, which had been root filled and later apicectomized, were extracted following further periapical pathology. The crowns were intact in these cases and as such would not have been regarded as a primary failure by other investigators (Reuter and Brose, 1984; Walton et al., 1986; Foster, 1990).
Acknowledgements This study was carried out in conjunction with four elective dental students, MS G. Francis, F. M. Lovat, E. Lane and Mr J. R. P. Jones from the University of Newcastle upon Tyne, and a number of local dental undergraduates who helped with the organization, booking and reception of the patients. The author is indebted to Dr A. Dimmer, Senior Lecturer, Department of Conservative Dentistry, University of Hong Kong, who participated in the clinical examination of patients.
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