Journal of Oral Rehahililation,

1992, Volume 19, pages 137-143

Use of shell crowns in Hong Kong dental hospital attenders E.F.

CORBET,

C..I.

H O L M G R E N and S . K .

P A N G Depattment of

Feriodontology and Public Health, Facully of Denlislry, University of I long Kong, Hong Kong

Summary ' ; ; ,r : ' > In 1984 an oral health survey of Hong Kong adults aged 35—44 years revealed that 15% of subjects presented with metal shell crown restorations, often as components of fixed-bridge reconstructions. The aim of this study was to describe the patterns and means of use of shell crowns, and to describe the patterns of tooth loss in patients who presented with stich restorations, attendit^g for treatment at the dental teaching hospital in Hong Kong over a 2-year period. A total of 1563 such patients attended. The records, which included radiographs, of 165 of these patients aged 35 years and above, obtained by means of a systematic sampling frame, wer"e studied by one trained examiner. On initial presentation each patient was found to have a mean of 4-0 shell erowns, 77% of these being eornponents of bridge restorations. The bridges involved a mean of only 3-9 units (retainers and pontics). The tooth type most commonly replaced by a bridge of this type was the first molar. It was concluded that most shell crowns used in this patient group are abuttnent retainers for predominantly short-span bridge restorations. Introduction An oral health survey of Hong Kong adults aged 35—44 years, eonducted in 1984 (Lind et al., 1987a), r-evealed that 15% of the subjeets wer'e in possession of full coronal 'shell' restorations constructed from various metals. These restorations did not appear to have been cast to fit an impression of a prepared tooth, but rather they had been fitted over presumably minimally prepared teeth. Consequently, they were not adapted to the tooth surfaee at the cervical region of the restoration, thereby producing marked overhangs. Details of the eonstructioti techniques for these types of restoration, given in the English language, can only be readily found in older textbooks, such as Goslee (1907), There is no aecepted name for this type of restoration. When wholly preformed and then only subsequently adjusted and adapted to fit minimally prepared teeth, these have been termed 'preformed' erowns or 'shell' crowns, and sometimes 'telescope' crowns. When other than the occlusal surface is constructed from a metal band soldered to fit the circumference of the minimally prepared tooth, the restorations have been termed 'band' (or 'banded') crowns. When the cervical one-third to one-half is constructed from a shaped seamless ring, these have been termed 'seamless shell' crowns. Disparaging sobriquets such as 'tin-can' and 'jam-pot' have also become attached to these types of crown, but the term shell Correspondetice: Mr Estnotidc Corbet, Departtnetit of Pcriodotitology atid Public Healtii, Prinee Philip Detital Hospital, Hospital Road, llotig Kong. 137

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crown appears to be an adequate, non-committal description for the restorations encountered in Hong Kong and many other parts of Asia. Each of the abovementioned forms of construction could be identified in the Flong Kong population group that was studied. Goslee (1907) noted that the seamless shell crown made possible the ultimate sueeess of permanent stationary bridgework. However, this favourable view was overtaken by advances in dental restorative technology. For example, Doxtater's standard textbook on crown and bridge work, published in 1931, stated that the shell crown was possibly the most conspicuous blot on the dental history of the previous generation, but that this type of restoration had been given a reputation that it did not deserve. In many parts of the world, including Asia, this type of restoration is still widely employed. Crowns, many of which it is assumed were of this design, wer-e found to be present in 15% of urban and rural northern Chinese subjects aged 20—80 years and bridges, presumably incorporating streh crowns, were particularly prevalent iti the rural group aged 3=30 years (Luan et al., 1989). These authors noted that the prevalence of such bridges in the rural population was strrprisingly high, and they suggested that this was pr'obably due to the access in rural areas of China to non-trained dental-care providers who have specialized in prosthetic treatments using a variety of materials and often rather specialized constrttction methods. Not all of those practitioners who provide such restorations in Hong Kong have been fortnally trained, nor are they necessarily registered with the Dental Council of Hong Kong. The aim of this study was to describe the patterns and means of use of shell crowns in patients who attended for dental treatment at the dental teaching hospital in Hong Kong over a 2-year period, and to describe the patterns of tooth loss in the patients with shell crowns. Materials and methods Over a 2-year period from 1985 to 1987, the presence of a sliell erown, either as a crown unit or as a bridge-abutment retainer, was entered in the patient screening record for eaeh hospital patient on initial attendance by the clinical dental surgeon. These recor'ds also contain demographic data such as age, sex and race, together with clinical data, such as the presenee of teeth, the presence of such crowns and the presenee of restorations. This information was subsequently stored in a computerized patient-record system. In addition, each patient was subjected to a radiographic examination procedure that ineluded an orthopantomogram. These were exposed by means of General Electric PANELIPSE II machines, which have a variable kilovoltage, employing a fixed time-cycle (26s) and fixed milliamperage (8mA) using mediumspeed film (Kodak DF 75) with regular intensifying screens. The radiographs were processed in an automatic processor (Kodak M7B). The aim of the sampling procedure was to identify sequentially from the computerized patient-record system, 50 patients in each of the following age categories 35—44, 45—54, 55—64 and 3=65 years. The sequence interval was determined by the total number of such patients in each age category. If a set of patient records was found to be missing or incotnplete, it was not replaced by those of another patient in the category. The Panelipse radiographs were all examined by one trained recorder (PSK), under identical physical conditions using constant illumination and without magnification. The presence or absence of each tooth was recorded, as was the presence of shell cr'owns and the presence and length of span of bridges employing shell crowns plus the teeth replaeed by pointies (Fig. 1). In the infrequent event of

Use of shell crowns in dental hospital attenders

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Fig. L Panelipse radiograph showitig a variety of bridges incorporating shell crowns.

any of these parameters not being amenable to clear determination on the basis of interpretation of the radiograph alone (e.g. due to the superitnposition of adjacent metal coronal restorations), then the screening data in the patient's hospital records were consulted. The data were analysed using the st'sst'c statistical package. Results A total of 1563 patients who attended the dental teaching hospital presented with shell crowns on their initial visit over the 2-year period. The sampling str'ategy produced 165 (rather than the target 200) complete patient records, and these patients wer'e evenly distributed between the four age categories with a female-to-male ratio of 2:1. The mean number of shell crowns in these patients was 4-0, 77% of which wer'e bridge abutment retainers. Table 1 shows the meati number of missing teeth, excluding unerupted teeth, lor those patients with shell crowns in each of the age eategories. The mean number of missing teeth was found to increase with age. Overall 36% of all missing teeth, exeluding third molars, in these patients had been replaced by a pontic of a bridge that incorporated a shell crown. Figure 2 shows the pr'oportions of each tooth type that were missing, and the proportions of these that were replaced by a pontic of such a bridge. Third molars, 60-80% of which were deemed to be missing or' unerupted, were excluded. Apart from first molars being the tooth type most eommonly found to be missing and to have been replaced by pontics of this type of bridge restoration, there was no other obvious pattern regarding the

Table 1. Mean number of missitig teeth Age group (years) 35-44 45-,54 55-64 3=65

Mean value ± SD 7-1 ±5-2 8-6 ±5-2 11-2 ±6-8 14-3 ±6-8

140

E.E. Corbet et al. Maxillary

60 50 40 30 20 10 8 7 6 5 4 3 2

I I

2 3 4 5 6 7 8

10 20 30 40

u

50 60 70

Mandibular

Fig. 2. f^roportiotis of unreplaeed missitig teetli atid teeth replaeed by a potitic according to tooth type: (•) = missing teeth, (0) = bounded pontic, (S) = cantilever pontic.

placement of these bridges. Bounded pontics, i.e. pontics of bridges that conformed to a fixed-fixed design, wer'e found to be more prevalent than eantilevered ponties, by an overall ratio of 4:1, although it can be seen that a cantilever design was more frequently used in posterior bridges. In general, with the exeeption of the molar regions, it was more common for bridges to be found in the upper arch. The mean number of units (retainer's and pontics) per bridge was found to be 3-9 (±1-7), of which a mean of 2-1 (±0-7) were retainer's and the remainder were pontics. Figure 3 shows that over 75% of all bridges incorporating shell crowns wer'e 2—4 unit restorations, with 63% being 3- or 4-unit bridges. Of the remaining 25% with wer'e longer span bridges, some were quite extetisive reconstructions. Figui'e 4 shows the distribution of the 23% of shell cr'owns that were not components of fixed-bridge reeonstructions. These were most frequently placed on maxillary anterior teeth. Not infrequently on posterior teeth these crowns were two shell crowns on adjacent teeth joined together. This appears to be a special design feature in the use of shell erown restorations on posterior teeth.

Use of shell crowns in dental hospital attenders

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40 r

30 D

S 20

10

J_l 2

3

4

U 5 6 7 Nunnberof units

8

9

>IO

Fig. 3. Proportiotis of bridges iticorporatitig shell crowns aeeording to the nutnber of units involved.

Di.scussion The aim of this study was to describe the use of shell crowns in a Southern Chinese dental hospital patient group in Hong Kong. It was not intended to describe the various metals used in the construction of shell erowns and bridges incorporating them, or to describe the special features of the construction of such crowns and bridges that are eneountered in a population served by dental-car'e providers who deliver restorations of this type. The latter ar-e quite common in many parts of the world, but there are no reports on their use in the literature. This study of the clinical use of shell crowns in Hong Kong forms the first part of an investigation into the periodontal consequences of restorations incorporating shell crowns, as part of which marginal alveolar bone levels will be measured. Panoramic oral radiographs have been shown to provide a high level of agreement with regard to diagnostic yield in the assessment of periodontal bone loss, when eompared to posterior bite-wing radiographs (Akesson et al., 1989). Panoramic radiographs were used in this study to assess marginal bone levels, but a pilot study showed that interpretation of the Panelipse radiographs provided a higher level of accuracy than a clinical examination in the assessment of bridges incorporating shell crowns. Due to the design features of such bridges, in a elinieal examination it can sometimes be very difficult to distinguish between abutment retainers and pontics, and thus to determine which teeth are missing. All radio-opaque intra-oral appliances are evident on panoramie radiographs, and shell crowns, due to their lack of adaptation to the tooth surface, can be readily identified. Almost 100% agreement between the findings of panoramic and full-mouth intra-oral r'adiographs with regard to the pr'esence of erowns and bridges has been r'eported (Ahlqwist et al., 1986). Difficulties in interpretation arose only with the possible overlap of adjacent radioopaque shell crowns in situations where they may have been either multiple abutment retainers or separate shell crowns mesial or distal to and in contact with an adjacent shell crown. It was possible to determine the extent of such restorations by referring to the patient's hospital reeords.

142

E.E. Corbet et al. Maxillat7

7 6 5 4 3 -

nf i

2 t -

8

7

6

5

4

3

2

1

1

mi

2

3

4

5

6

7

8

I 2

L

3 4 5 6 7 8

• Mandibular

,

Fig. 4. Proportions of teeth with shell crowns not incorporated into bridges according to tooth type.

The female-to-male ratio of 2:1 reflects the pattern of patient attendanee at the dental teaching hospital, and for individuals aged 5=35 years it was 1-7:1, so should not be interpreted as indicating that shell erowns are necessarily tnore prevalent in Hong Kong females over 35 years of age. The investigation of tooth mortality and prosthetie treatment patterns in northern Chinese subjects by Luan et al. (1989) was a population-based study and, apart from stating that bridges often had rather specialized construction principles, the report did not speeifieally mention that shell crowns were the type of retainer used. Therefore any comparisons between northern Chinese and southern Chinese in the pr'esent study should be made with eaution. For subjects aged 30—59 years, the mean number of teeth replaeed by a bridge was 2-1 in northern China, confirming that, on average, bridges of this type in the Chinese cotntnonly have a short span. In the Flong Kong patients, bridges incorporating shell crowns were most often of a conventional fixed-fixed short-span design. Furthermore, the

Use of shell crowns in dental hospital attenders

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pattern of plaeement of these bridges refleeted the pattern of missing teeth, with the exception of the second molar. The high proportion of missing teeth replaced by bridges incorporating shell crowns in these patients perhaps reflects a preference among this patient group in addition to the advice received from their dental-care providers. It is unlikely that all of the shell crowns wottld have been provided by untrained or unregistered dental practitioners. The nutnber and practice styles of unregistered practitioners has not been adequately documented, but in 1984 10% of the last dental visits paid by individuals aged 35—44 years had been to unlieensed practitioners (Lind et al., 1987b). The use of shell crowns other than as components of bridge reconstructions was found to be surprisingly prevalent, with altnost 25% of all shell crowns not being associated with bridges. The use of shell crowns not incorporated into fixed bridges on anterior teeth was expeeted, par'ticularly when constructed of a gold alloy, as this display of gold can have a cultural significance over and above a need for the restoration of teeth due to dental disease or loss of tooth substance. The finding of their apparent use as restorations on posterior teeth was unexpected. Further research to assess the periodontal eonsequences of restorations incorporating shell crowns is in progress. It was concluded from this part of the study that shell crowns are principally, but not exclusively, used as bridge abutment retainers, and mostly for bridges of a eonventional fixed-fixed design and of a short span. References Attt.owtsr, M., HAt.t.tNG. A. & Hot.t.tiNoiit! L. (1986) Rotational patioratnic radiography iti epidetniological studies of dental health: comparison between panoramie radiographs and intra-oral full tnouth surveys. Swedish Denial Journal, It). 79. AktiSsoN, L, RottLiN, M., HAKANSSON, .1., HAKANSSON, H . & NAssttiAM, K. (1989) Coinparisoti between panoramie and posterior bitewitig radiography in the diagtiosis of periodotital bone loss. Journal of Denlislry, 17, 266. DoxTATtit;, L.W. (1931) Procedures in Modern Crown and Bridgework, p. 2(B. Detital Items of Interest Publishing Co., Brooklyn, New York. Cost.tit-, H..I. (I9t)7) Principles and Fractice of Crown and Bridgework, p. 5. Consolidated Detital Manufaeturitig Co., New York. LiNU, O.P.. EVANS, R.W., Cot;t)t!T, E.F., Hot.MiaftiN, C.l., LtM, f..P. & MAK, K. (1987a) Hong Kong Survey of Adult Oral Health. Part 2. Oral health related perccptiotis, ktiowledge and beliaviour. Communily Denial Health, 4, 367. LtNtx O.P., Hot.MGt«UN, C..I., EVANS, R.W., CotuitiT, E.F., LtM, L.P. & DAVtus, W.l.R. (1987b) Hong Kotig Survey of Adult Oral Health. Part 1. Clinical Findings. Community Dental Health. 4, 351. LUAN, W.M., BAtit.UM, V., CtitiN, X. &. Fetittstcov, O. (1989) Tooth mortality and prosthetie treatment pattcrtis in urbati atul rural Cliinese agetl 2t)-8t) years. Community Dentistry and Oral Epidemiology, 17, 221.