Crowns and other extra-coronal restorations: Impression materials and technique

PRACTICE IN BRIEF ● An overview of impression materials ● A rationale is put forward for the choice of elastomeric materials ● Solutions are provided to common problems encountered by the clinician in producing good, accurate impressions ● An emphasis on the need for feedback between the laboratory and the dentist 10 Crowns and other extra-coronal restorations: Impression materials and technique R. W. Wassell1 D. Barker2 and A. W. G. Walls3 Well-fitting indirect restorations can only be made if there are accurate models of the oral tissues available, made from high quality impressions. Waiting for an impression to set may be more stressful for the dentist than the patient. Should the impression need to be repeated there is the embarrassment of having to explain this to the patient, the cost implications of material and time wasted and the aggravation of running late for the next appointment. Yet, if a ‘Nelsonian’ eye is turned to a defective impression we can only expect a substandard restoration in return. CROWNS AND EXTRA-CORONAL RESTORATIONS: 1. Changing patterns and the need for quality 2. Materials considerations 3. Pre-operative assessment 4. Endodontic considerations 5. Jaw registration and articulator selection 6. Aesthetic control 7. Cores for teeth with vital pulps 8. Preparations for full veneer crowns 9. Provisional restorations 10. Impression materials and technique 11. Try-in and cementation of crowns 12. Porcelain veneers 13. Resin bonded metal restorations 1Senior Lecturer in Restorative Dentistry, 2Higher Specialist Trainee, 3Professor, Department of Restorative Dentistry, The Dental School, Newcastle upon Tyne NE2 4BW; *Correspondence to: Dr R. W. Wassell, Department of Restorative Dentistry, The Dental School, Newcastle upon Tyne NE2 4BW E-mail: Refereed Paper © British Dental Journal 2002; 192: 679–690 Sometimes impression defects only come to light after the cast has been poured. Thus laboratory inspection is an important part of quality control, but many technicians find it difficult to feed back to their dentists for fear of the messenger being shot. Feedback between laboratory and dentist is critical to the establishment of an open and honest relationship. Hopefully, an improved understanding of impression materials coupled with techniques to overcome problems will encourage higher standards and the confidence to accept appropriate advice. The first part of this article considers the factors influencing choice of impression material. To have a practical understanding of impression materials it is not necessary to have a PhD in dental materials science. Nevertheless, to select an appropriate material it does help to have a feel for the classification of impression materials as well as concepts such as working time, setting time, permanent deformation and dimensional stability. There are certainly other important factors that will influence your decision such as ease of manipulation, taste and tackiness but these have so far eluded quantitative measurement. The putty-wash technique has proven popular not least for its ease of handling, but there are drawbacks that can have significant effects on its accuracy and these will be discussed. The final part of the article will consider how to overcome the problems routinely encountered in recording impressions. CLASSIFICATION OF IMPRESSION MATERIALS Impression materials are commonly classified by considering their elastic properties once set. Therefore, they can be broadly divided into BRITISH DENTAL JOURNAL VOLUME 192 NO. 12 JUNE 29 2002 non-elastic and elastic materials as shown in Figure 1. Non-elastic impression materials are generally not used for obtaining impressions of crown preparations because of their inability to accurately record undercuts. The elastic impression materials can be divided into two groups: the hydrocolloids and the synthetic elastomers. Table 1. details the physical properties of the major groups of elastic impression materials available. Hydrocolloid impression materials The two types of hydrocolloids used in dental impressions are agar and alginate. Agar is a reversible hydrocolloid because it can pass repeatedly between highly viscous gel and low viscosity sol simply through heating and cooling. However, alginate once converted to the gel form cannot be converted back into the sol, and is therefore said to be irreversible hydrocolloid material. Agar and alginate may be used independently or in combination to record crown impressions. Agar was first introduced into dentistry for recording crown impressions in 1937 by Sears1 and was the first elastic impression material available. It is not commonly used in dental practice today however, because of the need for expensive conditioning baths and water cooled trays. Alginate, unlike agar, does not require any special equipment. Being easy to use and inexpensive it is popular for less critical applications eg opposing casts and study models. Alginate and agar produce impressions with reasonable surface detail. They are both relatively hydrophilic and are not displaced from wet surfaces as easily as the elastomers.2 However, in respect of recording crown prepara679 PRACTICE lems associated with the hydrocolloids, namely poor dimensional stability and inadequate tear resistance. IMPRESSION MATERIALS Non-elastic materials Impression plaster Impression compound Impression waxes Synthetic elastomers Hydrocolloids Reversible Irreversible Polysulphides Polyether Silicones Condensation Addition Fig. 1 Classification of impression materials tions these materials have two major disadvantages. Firstly, very poor dimensional stability because of the ready loss or imbibition of water on standing in dry or wet environments respectively. Secondly, low tear resistance which can be a real problem when attempting to record the gingival sulcus. Some work supports the use of combined reversible and irreversible hydrocolloid impression systems.3,4 These systems are used in a way similar to the putty-wash technique for silicone rubbers described later in this article, with the agar injected around the preparation to capture surface detail and the more viscous alginate in the impression tray. The advantages of this combination system compared with agar or alginate used individually is the minimisation of equipment required to record an agar impression (no water cooled tray is needed) and the fact that agar is more compatible with gypsum model materials than alginate. It is also relatively cheap in comparison to many synthetic elastomers. Lin et al.5 demonstrated that the accuracy of this combination system is better than either the reversible or irreversible materials used separately and is comparable to that of polysulphide impression materials. However, the problems of low tear resistance and poor dimensional stability still apply resulting in the need for impressions to be cast up immediately. For these reasons, most practiti (...truncated)