Introduction

Changing attitudes to oral healthcare and dental treatments in developed countries have led to an increase in the prevalence of partially dentate adults.1,2,3 From April 2001 to March 2002, in excess of £300 million was spent on various items of prosthodontic treatment for partially dentate patients on the National Health Service in England and Wales.4 Data on the cost of similar treatments provided under private schemes is not available.

Design and prescription of prostheses are fundamental components of prosthodontic therapy, and should be completed mindful of mechanical and biological factors.5,6,7,8,9 This information should then be communicated in a clear and unambiguous manner to the dental technician.10 Inadequate communication of design information results in a prosthesis that has been fabricated with little reference to important clinical or biological information. The potential of poorly designed prostheses to cause tissue damage is well recognised.9,11

Investigations published in the literature have consistently demonstrated that the communication of design information for cobalt-chromium based removable partial dentures from clinician to dental laboratory is poor.6,12,13,14,15,16,17,18,19,20,21,22,23,24 This finding has been reported over the last 30 years in many countries including the United Kingdom,6,13,18,21 Sweden,14,23,24 Canada,12 South Africa,22 USA,15,19,20 and more recently in Ireland.16,17 A previous study by the authors17 also examined the selection of impression trays and materials for master impressions for cobalt-chromium based removable partial dentures and have found extensive use of inappropriate impression trays and materials.

There are few studies available in the area of fixed prosthodontics, but the more notable of these found that there was extensive use of inappropriate impression trays, and that impressions received by dental laboratories were not adequately disinfected.25,26,27 These studies were performed prior to the introduction of professional guidelines in this area.

Over the last 10 years, specific ethical and legal guidelines have been introduced that require the clinician to adequately design prostheses and communicate these design features to the technician. The British Society for the Study of Prosthetic Dentistry, a specialist society which promotes excellence in removable prosthodontics in the United Kingdom, states that '...the design of a partial denture is the duty and responsibility of the clinician...' and recommends that master impressions should be made using either a modified metal stock tray or preferably a rigid special tray, and recorded using a dimensionally stable elastomeric material. It also recommends that when alginate is used, impressions should be poured immediately to minimise dimensional changes.28 The British Society for Restorative Dentistry states that the purpose of written instructions for fixed prostheses is '...to record and communicate precise details of all aspects of the crown and bridgework required...'29

The European Union's Medical Devices Directive (Directive 93/42/EEC) places specific requirements on dental practitioners to provide adequate written instructions when a prosthesis is being manufactured, and that dental laboratories manufacture the prosthesis to this specification.30

The General Dental Council's guidelines for dental education state that to successfully complete dental undergraduate training, one should be competent at 'designing effective indirect restorations and ... partial dentures'.31

Most relevant studies in both fixed and removable prosthodontics reported in the dental literature were carried out prior to the introduction of the above-mentioned guidelines. No study has thus far examined design and prescription of both fixed and removable prostheses. Consequently, the purpose of this investigation was to examine the quality of written instructions and choice of impression trays and materials for fixed and removable prosthodontics.

Materials and methods

A pre-piloted questionnaire was distributed to eight large commercial laboratories with a broad geographical distribution and large 'catchment' area throughout the United Kingdom and Ireland. Information was sought regarding the quality of written instruction and selection of impression trays and impression materials for fabrication of three prostheses:

  • Cobalt-chromium based removable partial dentures (RPD);

  • Full veneer porcelain-fused-to-metal crowns (PFM);

  • Full coverage, porcelain-fused-to-metal fixed bridges (FB).

Written instructions were classified as:

  • 'Clear'— the design instructions are clear and unambiguous;

  • 'A guide'— most of the design instructions have been communicated, however minor decision-making on the design has been left to the technician;

  • 'Poor' — some of the design instructions have been communicated, however the major decision-making on the design has been left to the technician;

  • 'none'— no design instructions have been communicated.

Specific information was also sought in relation to each prosthesis type, such as:

  • The use of diagrams and provision of surveyed casts with RPD prescriptions,

  • Identification of 'metal-only' surfaces on PFMs,

  • Number of pontics on FBs.

Information was also sought relating to the 'disinfection status' of the supplied impressions. Data was recorded using a Microsoft Excel data sheet. Descriptive statistics are reported.

Results

Six hundred questionnaires were distributed, of which 447 were completed and returned (response rate= 75%). The distribution of completed questionnaires relating to each prosthodontic item examined is shown in Figure 1. Ninety-five per cent of items were privately funded, ie not covered under GDS regulations or the equivalent Irish Dental Treatment Services Scheme.

Figure 1
figure 1

Distribution of completed questionnaires relating to each prosthodontic item examined

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The impression tray used for all items examined is shown in Figure 2, and choice of impression material for all items examined is shown in Figure 3. The combinations of impression trays and materials selected are shown in Table 1. The minimum time between making an impression at the chairside and subsequent pouring of the master cast in the laboratory was 72 hours. The quality of written instructions for all items examined is shown in Figure 4.

Figure 2
figure 2

Selection of impression tray for all prosthodontic items examined

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Figure 3
figure 3

Choice of impression material for all prosthodontic items examined

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Table 1 Combination of impression tray and materials for all prosthodontic items examined.
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Figure 4
figure 4

Quality of written instructions for all prosthodontic items examined

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The selection of impression tray classified according to each item examined is shown in Table 2. The choice of impression material classified according to each item examined is shown in Table 3. The quality of written instructions classified according to each item examined is shown in Table 4.

Table 2 Selection of impression tray classified according to each prosthodontic item examined
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Table 3 Choice of impression material classified according to each prosthodontic item examined
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Table 4 Quality of written instructions classified according to each prosthodontic item examined
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For chrome-cobalt RPDs, the most common combination of impression tray and material was a special tray and alginate (61%, n=83). A diagram of the planned prosthesis was included in 40% (n=54) of cases. Nine per cent (n=12) of prescriptions included surveyed study casts. Twenty-eight per cent of cases (n=38) included a request that the technician design the RPD.

For PFMs, the most common combination of impression tray and material was a plastic stock tray and silicone (48%, n=93). Seventy per cent (n=134) of prescriptions did not specify which surfaces were to be covered with metal only. In 14% of cases (n=26) the technician had to contact the dentist to clarify the written instructions supplied.

For FBs, the most common combination of impression tray and material was a plastic stock tray and silicone (51%, n=61). Sixty-three per cent (n=75) of prescriptions did not specify how many pontics were to be included in the planned FB. In 11% of cases (n=13) the technician had to contact the dentist to clarify the written instructions supplied.

The 'disinfection status' of master impressions examined (n=447) is shown in Figure 5. Fifteen per cent of impressions examined were clearly contaminated with evidence of blood or other debris attached. In 28% of cases, the technician was unsure that the impression had been sufficiently decontaminated. The technician was sure that the master impression had been disinfected in 57% of cases.

Figure 5
figure 5

The 'disinfection status' of master impressions examined

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Discussion

The findings of this investigation are quite revealing as to the attitudes and habits of dental practitioners in the field of fixed and removable prosthodontics. Cobalt-chromium based removable partial dentures, full-veneer porcelain-fused-to-metal crowns, and fixed bridges could each be considered a 'dental precision casting', ie abutment teeth are prepared in the mouth, a suitable impression is made, a cast is made from this impression, and the prosthesis is made on this cast. Clearly, the need for a dimensionally stable and accurate impression technique is an important factor when fabricating a 'good quality' prosthesis.28,32,33,34 Despite the advice given by specialist organisations,28,29 and various authoritative textbooks in the areas of fixed and removable prosthodontics, 32,33,34 there was extensive use of plastic stock trays (50% of sample investigated). Furthermore, one-fifth of all impressions were made using alginate. While the use of alginate is acceptable for master impressions when used with a rigid special tray and poured immediately,28 the casts in this study were not made for at least 72 hours. What is perhaps more revealing is the difference in choice of impression trays and materials between fixed and removable prostheses. The most commonly used impression material for making a master impression for an RPD was alginate (61% of RPDs), while the most commonly used material for PFMs and FBs was silicone (83% of PFMs, 88% of FBs). The most commonly used impression tray for RPDs was a special tray (74% of RPDs), while the most commonly used impression tray for PFMs and FBs was a plastic stock tray (63% of PFMs, and 60% of FBs). The reasons underlying such divergence in approach between making a master impression for a fixed prosthesis and a removable prosthesis remains unclear.

Poor or no written instructions were provided in 47% of cases (50% of RPDs, 50% of PFMs, 39% of FBs). Twenty-eight per cent of RPD prescriptions requested that the technician design the RPD. In 14% of PFM prescriptions and 11% of FB prescriptions the technician had to contact the dentist to clarify the written instructions supplied.

It is disappointing that even though the problems of inadequate prescription and communication between clinician and laboratory were first highlighted almost 30 years ago,23 there is still evidence to demonstrate that these problems still persist. Owing to the lack of adequate instruction, the practice of devolving prescribing discretion to the dental technician is still evident. The design of any prosthesis is based on mechanical and biological principles.5,6,7,8,9 It follows that if master impressions are being sent to dental laboratories with inadequate design information, the technician does not have access to crucial information relating to the nature and health of the periodontal tissues.18 The potential for unnecessary and unjustifiable tissue damage is clear.

Many authorities in the area of removable partial prosthodontics have advised the use of diagrams when prescribing RPDs.10 In the sample examined, only 40% of RPD prescriptions included such a diagram. The use of surveyed preliminary casts is extremely beneficial prior to planning RPDs — allowing identification of the most suitable path of insertion prior to making tooth preparations and master impressions.10 Surveying avoids unnecessary removal of tooth substance, while identifying the optimal path of insertion.35 However in this sample only 9% of RPD prescriptions included surveyed preliminary casts.

As previously mentioned, the 'Guidelines for Crown and Bridge' published by the British Society for Restorative Dentistry explicitly states that the purpose of written instructions is to communicate 'precise details of all aspects of the crown and bridgework required'.29 In this investigation, 70% of prescriptions for full-veneer porcelain-fused-to-metal crowns did not identify the surfaces that were to be covered with metal only. While provision of porcelain 'tooth-coloured' surfaces are requested by patients in aesthetic regions, these can sometimes have deleterious effects.36 Responsibility rests with the clinician to clarify which surfaces are not suitable for porcelain coverage, and to instruct the technician accordingly. Similarly, 63% of prescriptions for fixed bridges did not specify how many pontics were to be included in the planned prosthesis. This finding has functional and financial implications. Most laboratories charge a fee per pontic included, which the clinician in turn passes to the patient.

Approximately 50% of the master impressions and written instructions examined in this investigation failed to meet the relevant selected ethical and legal guidelines. Various reasons have been advanced for this over the years, and relate mainly to educational6,10,18,19,20,21,22,23,37 and financial6,26,38 issues. Historically, in the field of removable partial prosthodontics, investigators have highlighted a lack of adequate educational exposure at dental undergraduate level;6,19,20 a lack of confidence in designing dentures related to a lack of clinical exposure;6,37 and a discrepancy between the principles of removable partial denture design taught in dental schools and those subsequently employed in clinical practice.18,21,22 The benefits of postgraduate and continuing educational courses in removable partial denture design in overcoming this difficulty have been highlighted.22,23,26 It is clearly evident that despite the development of national guidelines on dental undergraduate training, the introduction of vocational training over ten years ago, the recent introduction of continuing professional development, and the consistent documentation of poor quality prescription of fixed and removable prostheses over the last 30 years, there are still significant problems of poor quality prescriptions and use of inappropriate impression trays and materials.

Some authors have claimed that the lack of proper prescription and use of inappropriate materials relates to inadequate funding of the provision of prostheses.6,26,38 This criticism is usually directed at services provided on the National Health Service, inferring that if the fee were increased then the quality of the service provided would also increase. The authors feel that this argument may not be entirely valid. Firstly, almost all (95%) the cases examined in this study were funded on a private basis, where GDPs have full discretion regarding the fee charged to the patient. Secondly, previous work by the authors that demonstrated inadequate prescription of removable partial dentures in Ireland, where all cases were privately funded.16,17 Finally, by making a comparison with another discipline in restorative dentistry, investigations into the quality of endodontic treatment on the National Health Service found that increasing the fee alone might not lead to an increase in the quality of treatment provided.39

A disturbing finding from this investigation relates to the lack of, or inadequate, disinfection of the master impression. In only 57% of cases was the technician sure that the materials received had been disinfected. While clinicians may be aware of the need of appropriate disinfection protocols for master impressions, for unknown reasons they are not following the relevant protocols. It was reported in this study that both saliva and blood were visible on 15% of all impressions examined. In 28% of cases the technician reported that they were 'unsure' that materials had been disinfected – reportedly this finding related to lack to information on the written instructions confirming disinfection of the impression; presence of liquids attached to the impression which were not blood-stained, but may have been saliva; and a lack of certainty regarding certain practitioners given previous experience. Sending contaminated materials via the postal system is unethical (not to mention hazardous and illegal) in both Ireland and the United Kingdom.25,40 The potential for transmission of infective agents from the dental surgery to the dental laboratory via inadequately disinfected impressions has previously been demonstrated.41 This is a sobering thought in contemporary dental practice where infection with diseases such as HIV, Hepatitis B, and Hepatitis C, present real threats to all members of the oral healthcare team.

Conclusion

In a significant proportion of cases, the written instructions and master impressions examined in this study failed to meet the relevant ethical and legal guidelines. The quality of written prescription for both fixed and removable prostheses was poor. Even though cobalt-chromium based removable partial dentures, full-veneer porcelain-fused-to-metal crowns, and fixed bridges are regarded as dental precision castings, plastic stock trays were widely used for recording one-half of master impressions. Alginate was used for recording nearly two-thirds of master impressions for RPDs, which were not poured for at least 72 hours. In only 57% of cases the technician felt that non-infected material had been sent to the laboratory. Further investigation is warranted to establish why recently published guidelines are not being followed.