effect of impression tray design and impression technique upon the accuracy of stone casts produced...

J. Dent. 1991; 19: 283-289 283

Effect of impression tray design and impression technique upon the accuracy of stone casts produced from a putty-wash polyvinyl siloxane impression material

W. P. Saunders, S. W. Sharkey, G. McR. Smith and W. G. Taylor Department of Conservative Dentistry, Glasgow Dental Hospital and School, Glasgow, Scotland

ABSTRACT This study examined the accuracy of stone casts produced from impressions taken in stock polycarbonate trays, some of which had been strengthened with autopolymerizing polymethyl methacrylate resin. Three techniques were used to make the impression of an acrylic master model of the mandibular arch on which two extracoronal preparations for bridgework and one intracoronal inlay preparation had been carried out. Each preparation had been indented with a reference point for later measurement The impression material was a putty-wash polyvinyl siloxane material. Five impressions were taken for each type of tray for each impression technique and these were cast in die-stone after 24 h. The distances between the points were measured with a reflex microscope and the means determined for each design of tray. The mean difference between casts produced from the various tray designs and the acrylic master model were determined for each of the distances between the three measuring points for the various impression techniques. Statistical analysis showed that, with the polycarbonate stock trays, there were significant differences between some of the modifications and between them and the acrylic model, for the three distances (P < 0.05). These differences were limited to one measurement for one design of tray for each of the two-stage impression methods. With the one-stage technique the unreinforced tray and those reinforced with acrylic, over the heels and anteriorly, and the barred design were statistically significantly different from the acrylic model for measurement A-B. It would appear from this study that the design of tray, or the impression technique employed, has little effect on the accuracy of impressions made with the polyvinyl siloxane material tested when used as a putty-wash and a two-stage technique, but that the accuracy of the impression material within the bulk of the material (A-B) was affected adversely using a one-stage technique.

KEY WORDS: Impressions, A-silicones, Impression trays, Technique

J. Dent. 1991; 19: 283-289 (Received 18 February 1991; reviewed 28 March 1991; accepted 23 May 1991)

Correspondence shouldbe addressed to: Dr W. P. Saunders, Department of Conservative Dentistry, Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3J2, Scotland.

INTRODUCTION Dimensional accuracy of impression materials is essential for the accurate construction of crown and bridgework. Rubber-based elastomers are widely used in fixed prosthodontics and are highly accurate (Bergman et al., 1972; Eames et al., 1979a; Lacy et al., 1981). There are, however, a great number of variables which may affect the accuracy of impression materials. These have been outlined by Tjan et al. (1986) and include how the impression materials are used and the design of the

impression tray. Rubber base impression materials are reported to be most stable when an even thickness of 2-4 mm is present within the tray (Reisbick and Matyas, 1975; Eames et al., 1979b; McCabe and Storer, 1980), and this is said to be best achieved with a custom-made tray. Custom trays are, however, more time consuming and hence more expensive to construct than disposable stock trays. In addition, Bomberg et al. (1985) found that the differences in thickness of impression material in a stock tray compared with a custom-made tray was less than 1 mm.

@ 1991 Butterworth-Heinemann Ltd. 0300-5712/91/050283-07

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Several types of polyvinyl siloxanes are available for use in crown and bridgework. A heavily tilled putty may be used with either a low or medium viscosity wash material, either as a one-stage or two-stage technique. The putty- wash system was introduced to compensate for the problems of polymerization shrinkage that occurred with the condensation-cured silicone impression materials and this was applied later to the addition-cured silicones. In the one-stage technique both phases of the impression material are placed in the tray at the same time and the low viscosity material is injected around the preparations. There are two techniques by which the two-stage impression can be made. The first involves the use of a spacer. A thin sheet of polyethylene is placed over the teeth and this acts as a separator when the putty phase of the impression is being taken. The addition of the low viscosity material to the impression allows refinement. The second method is to make the impression with the putty phase and, when it is set, excess material is trimmed from the impression together with interdental tags. Sluiceways are made around the prepared teeth to allow the low viscosity material space to flow and the refining impression is made with the low viscosity material. The latter technique requires less volume of the low viscosity material. The net result of these techniques is that the putty acts as a form of custom-made tray.

It has been shown that a stock tray is acceptable when a heavy body-wash technique is used (Myers and Stockman, 1960) and Tjan et al. (1981) considered that although a custom tray was not necessary for use with the putty-wash system proper spacing of the tray and suitable rigidity was required for adequate accuracy. Burton et al. (1989) confirmed that a rigid tray was required for accurate impressions.

The purpose of this study was to determine the accuracy of stone casts poured from impressions made from a two- phase putty-wash polyvinyl siloxane material using a number of different designs of reinforcement of a stock tray. These were used in combination with three techniques for making the impression.

MATERIALS AND METHODS

The impression material used in this study was Express (3M Dental Products Division, St Paul, MN, USA). This material was supplied as a putty-wash system, the latter being mixed in an automixing syringe. A custom-made mandibular acrylic model was used as the master model (Fig. I). Preparation for a full veneer gold crown was carried out on 37, preparation for a metal-ceramic crown on 35 and preparation of a mesio-occlusal gold inlay on 46. Tooth 36 was absent from the arch. This provided a simple bridge preparation involving two extracoronal abutments and an intracoronal preparation in the opposite quadrant. The occlusal surfaces of the second molar and the second premolar on the left were indented with crosses, using a scalpel blade, to provide reference points for subsequent orientation and measurement. A similar mark was placed on the floor of the mesial box of the first molar on the right.

Stock mandibular polycarbonate trays with perforations (Eezitray, Wright Health Group, Dundee, UK) were used in this study. These were separated into four groups of five trays each (Fig. 2). In the first group the trays were used unaltered but in the other groups strengthening was added using autopolymerizing polymethyl methacrylate (Formatray, Kerr, Romulus, MI, USA). Auniform mixture of acrylic was prepared with the measured scoop provided and placed on the tray in one of three ways. In one group the heels of the tray were reinforced with the material and in the second the acrylic was added to both the heels and the labial and lingual flanges, anteriorly. In the third group struts of acrylic were placed across the trays at regular intervals (Tjan et al., 1981).

Each tray was coated on its inner surface with the adhesive provided with the impression material 5 min before the impression was taken. This was extended some 2 mm onto the outer surface of the tray, along the periphery.

Three methods were used to make the impression. In the first the putty-wash system was used in a single stage. The catalyst and base putties were mixed according to the

Fig. 7. Master resin model with reference points A (bottom left), B (top left) and C (right).

Fig. 2. Stock perforated polycarbonate tray to show various designs for reinforcement.

Saunders et al.: Accuracy of cases from polyvinyl siloxane impressions 285

Fig. 3. Measuring apparatus to show reflex microscope and microcomputer. A stone cast is on the microscope table.

manufacturers’instructions and placed into the impression tray. The wash component of the impression material was injected through an automixing syringe over the teeth ensuring that air voids were eliminated and a thin coating placed on the putty within the tray. The tray was seated onto the model and the material allowed to polymerize without pressure in an incubator at 37°C and 100 per cent relative humidity. In the other methods for making the impression the material was used in two stages. The base and catalyst putties were mixed as previously and placed in the tray. The tray was seated onto the model and allowed to set in an incubator at 37°C. The impression was removed from the tray and material trimmed away from the periphery, and interdentally, using a sharp scalpel. Sluiceways were also cut in the material adjacent to the prepared teeth. The low viscosity wash impression material was then injected around the prepared teeth and placed into the putty impression within the tray. The latter was reseated on the model and seated with firm pressure for 10 s. The impression was allowed to polymerize in an incubator at 37°C. The final technique for making the impression was similar to the second, except that the putty in the tray was separated from the model with a thin polyethylene spacer as supplied by the manufacturers. When the putty had set the spacer was removed and the excess material trimmed from the periphery with a scalpel. The second part of the impression was made with the wash material in the same way as before.

After removal from the acrylic model each impression was left for 24 h and then poured in a die stone, Velmix (Kerr Europe, Scafati, Italy), using water at a temperature of 18 “C and a water-powder ratio as recommended by the manufacturers. The stone was mixed mechanically with the water for 30 s under vacuum (Multivac 4, Degussa,

Geschaftbereich, Frankfurt, Germany) according to manufacturers’ instructions and five impressions were cast for each tray design. The stone was allowed to set for 30 min and then the casts were based using Kaffir D (British Gypsum Ltd, Newark, Notts, UK). The impressions were separated after 1 h and the excess material trimmed mechanically. The test samples were stored in air at room temperature for 24 h before being measured.

Measurements

The distances between the three reference points were measured using a reflex microscope (Reflex Measurement Ltd, Butleigh, UK) (Scott, 1981) as shown in Fig. 3. The potential dental applications and accuracy of the equip- ment has been described by Setchell (1984).

The principle of operation has been described previously (Adams et al., 1989) and is that of a three-dimensional image of an object created in a semireflecting mirror. The object can be observed through the mirror to a measuring mark on an XYZ-encoded slide system. The mark is a light-emitting diode which can be moved in and around the image and can be set to coincide with any point on the image by means of stereoscopic depth perception. Each axis has a resolution of 0.001 mm. The microscope is connected to a computer which stores the recorded information. The software allows automatic alignment of the points being measured so that each model does not have to be placed in exactly the same position on the microscope stage.

The acrylic master model was measured six times and the mean values recorded. The mean distances between the reference points for each design of tray, and for each impression technique, were determined from measure-

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Tab/e 1. Mean distances (+ s.d.) on the master model from reference points A, B and C for the stock tray measurements

A-6 Distances (mm)

B-C C-A

Distances (mm) Mean 22.05 43.743 5 1.203 s.d. 0.027 0.039 0.059

Table 111. Mean distances (+ s.d.) on the stone casts when Express was used in stock tray with spacer and two-stage techniaue

Tab/e II. Mean distances (+ s.d.) on the stone casts when Express was used in stock tray with no spacer and a two- stage technique

Reinforcement of tray A-B

Distances (mm)

B-C C-A

Nil Mean s.d.

Bars of acrylic Mean s.d.

22.050 43.8 12 51.242 0.010 0.052 0.045

22.043 43.805 51.275 0.020 0.062 0.065

Reinforcement of tray

Nil Mean s.d.

Distances (mm) Heels only Mean 22.066 43.770 51.222

A-B B-C C-A s.d. 0.039 0.033 0.028

Heels, lingual and 22.042 43.770 51.235 labial, anteriorly

0.013 0.029 0.065 Mean 22.024 43.831 51.258 s.d. 0.018 0.085 0.08 1

Bars of acrylic Mean 22.040 43.778 51.214 s.d. 0.014 0.062 0.048 Table IV. Mean distances (+ s.d.) on the stone casts when

Heels only Express was used in stock tray and one-stage technique Mean 22.016 43.792 51.236 s.d. 0.049 0.068 0.045 Distances (mm)

Heels, lingual and Reinforcement

l$i%aI; anteriorly of tray A-8 B-C C-A

22.026 43.8 11 51.236 Nil s.d. 0.025 0.049 0.049 Mean 22.066 43.758 51.201

s.d. 0.018 0.01 1 0.020

ments of the five casts in each group. Each stone cast was measured three times and the mean values recorded. Differences between the mean distances on each of the groups of casts and the master acrylic model were calculated. Statistical analysis was carried out using a one- way analysis of variance and Student’s t-test to determine the effect of tray design on the accuracy of the casts.

Bars of acrylic Mean 22.014 43.758 51.202 s.d. 0.020 0.013 0.023

Heels only Mean 22.024 43.746 5 1.200 s.d. 0.018 0.01 1 0.013

Heels, lingual and labial, anteriorly Mean 21.992 43.743 5 1.200 s.d. 0.014 0.016 0.009

RESULTS

The mean distances between the reference points of the acrylic master model for the stock tray and its modification are shown in Table 1. The mean distances for the reference points for each group of casts are shown in Tables II-IV for each design of tray and the two-stage without spacer, two-stage with spacer and one-stage impression techniques respectively. The mean differences between the acrylic master model and the stone casts are shown in Tables V- W for the stock trays and the two-stage without spacer, two-stage with spacer and one-stage impression techniques respectively.

Statistical analysis showed there was only one measurement for either of the two-stage impression techniques that was significantly different from the acrylic master model. This was the case of the impression taken with a spacer using the tray reinforced with acrylic anteriorly and over the heels, for the measurement B-C (P = 0.037). There were significant differences between the acrylic

master model and the stone casts poured from single-stage impressions made in unreinforced trays, reinforced trays of the barred design and strengthened anteriorly and over the heels (P < 0.05) for the measurement A-B.

DISCUSSION

The results of this study show that the use of a stock polycarbonate tray, with or without modifications to reinforce the structure, did not affect the accuracy of the casts poured from impressions taken with a putty-wash polyvinyl siloxane impression material, Express, when used in a two-stage technique. However, the use of the impression material in a one-stage technique did affect the accuracy, but only for the measurement A-B, within the bulk of the material. As it would be expected that tray


Table V. Mean difference between acrylic model and stone casts for stock trays and two-stage impression with no spacer (- sign indicates acrylic < stone)

Reinforcement Distances (mm)

of tray A-B B-C C-A

Nil 0.000 - 0.068 (0.155) - 0.039 (0.076)

Bars of acrylic 0.007 (0.032) - 0.061 (0.139) - 0.072 (0.140)

Heels only - 0.016 (0.073) - 0.027 (0.062) - 0.020 (0.039)

Heels, lingual and 0.026 (0.1 18) labial, anteriorly

- 0.088 (0.201) - 0.056 (0.109)

Figures in parentheses indicate difference expressed as percentage change from acrylic model.

Table VI. Mean difference between acrylic model and stone casts for stock trays and two-stage impression with spacer (- sign indicates acrylic < stone)

Distances (mm) Reinforcement of tray A-B B-C C-A

Nil - 0.008 (0.036) - 0.027 (0.062) - 0.032 (0.063)

Bars of acrylic 0.0 10 (0.047) - 0.035 (0.08 1) - 0.01 1 (0.021)

Heels only 0.034 (0.156) -0.048 (0.111) - 0.033 (0.065)

Heels, lingual and 0.024 (0.1 1 1) - 0.068 (0.155) labial, anteriorly

- 0.033 (0.064)


Table VII. Mean difference between acrylic model and stone casts for stock trays and one-stage impression (- sign indicates acrylic < stone)

Reinforcement of tray A-B

Distances (mm)

B-C C-A

Nil - 0.016 (0.07 1) - 0.016 (0.035) 0.013 (0.003)

Bars of acrylic 0.036 (0.165) - 0.015 (0.034) 0.001 (0.002)

Heels only 0.026 (0.1 17) - 0.004 (0.008) 0.002 (0.004)

Heels, lingual and 0.058 (0.262) - 0.003 (0.006) 0.003 (0.005) labial, anteriorly


flexibility would affect the accuracy of the cross arch measurements, it would appear that the stock trays used in this study did provide sufficient rigidity to support this impression material, even without the addition of strengthening acrylic. It would seem that the way the material was manipulated was the more likely cause of the inaccuracies seen.

The differences in measurements between the acrylic master model and the casts are the most important to consider as any discrepancy may affect the fit of crown and bridgework clinically. There was only one difference that was statistically significant in the two-stage series, which was noted with the cross arch measurement B-C, in

the case of the tray reinforced anteriorly and at the heels used with a two-stage technique, with a spacer. It would be expected that cross arch measurements would show the greatest distortion due to tray flexibility. The differences between the acrylic model and the stone casts in the one- stage impression groups applied only to the measurement A-B. This distance involves the bulk of the stone cast where more die setting expansion could be expected to take place. In addition, there may be more contraction in this region as the impression material polymerizes.

When comparing the two-stage techniques the use of a spacer means that more low viscosity material is required for the refining impression and this may be subject to

288 J. Dent. 1991; 19: No. 5

distortion due to polymerization shrinkage (McCabe and Storer, 1980). The disadvantage of the two-stage technique, without spacer, is that considerably more seating pressure is required for the refining second impression stage. This may cause distortion of the tray and subsequent inaccuracies. This was not seen in this study, however, and this may have been due to the way the impression tray was seated for the second stage. Firm pressure was applied for 10 s and then all pressure was released so that the material was allowed to set without extraneous force acting upon it.

dimensional changes with variations in ambient temperature than a model constructed of low fusing metal.

Acknowledgements

The authors would like to thank Mrs Denice Strang for carrying out the statistical analysis of the data.

Valderhaug and Floystrand (1984) tested the accuracy of two impression materials and found that the stock tray produced as accurate an impression as a custom-made tray. However, they measured the impressions themselves and only in the horizontal plane. The measurements in the present study were made on stone casts. The accuracy of impression materials have been investigated using stone casts rather than measuring the impressions themselves (Lacy et al., 1981; Lin et al., 1988; Odman and Jemt, 1988). Other factors may affect accuracy including the way in which the impression materials are handled and compatibility of the impression material with the stone must also be considered. Previous research (Saunders et al., 1990) has shown that the impression material used in this study was compatible with the stone, and that casting the impressions using water at 18°C had no adverse effect on accuracy.

It is essential that the impression material is securely attached to the tray, especially when the set material is removed from the mouth. This is achieved by using a paint-on adhesive with or without mechanical devices such as perforations. Bomberg et al. (1988) found that impressions were more accurate when taken in trays with perforations than those without, but Tjan and Whang (1987) found no difference between perforated and unperforated trays when adhesive had been applied. In the present study, although the trays were perforated they were coated with adhesive as provided by the manufacturers.

The time after the impression was taken until poured may affect the accuracy of the cast. Twenty-four hours was chosen in this study to simulate clinical conditions. Johnson and Craig (1986) showed that a delay of 24 h did not affect the accuracy of addition-cured silicone materials and Tjan et al. (1986) found that impressions made from polyvinyl siloxane materials remained accurate for a week.

The method of measuring the models in this study was straightforward and has been used previously (Saunders et al., 1990). The advantage of this system was that all measurements were made in three dimensions, and it was not necessary to orientate each cast in the same way prior to measurement. The associated computer software was able to compensate for these differences in orientation.

An acrylic typodont model has been used in other studies of impression accuracy (Tjan et al., 1987; Lin et al., 1988; Burton ef al., 1989; Gordon et al., 1990) but it is conceded that such a model may be subject to more

References Adams L. P., Jooste C. H. and Thomas C. J. (1989) An

indirect in vivo method for quantification of wear of denture teeth. Dent. Mater. 5, 31-34.

Bergman B., Olsson K A_ and Stenberg T. (1972) Dimensional stability of a rubber impression material. Sven. Tandlak. Tidskr. 65, 559-568.

Bomberg T. J., Hatch R. A and Hoffman W. (1985) Impression material thickness in stock and custom trays. J. Prosthet. Dent. 54, 170-172.

Bomberg T. J., Goldfogel M. H., Hoffman W. et al. (1988) Considerations for adhesion of impression materials to impression trays. .Z. Prosthet. Dent. 60, 681-684.

Burton J. F., Hood J. A A., Plunkett D. J. et al. (1989) The effects of disposable and custom-made impression trays on the accuracy of impressions. .Z. Dent. 17, 121-123.

Eames W. B., Wallace S. W., Suway B. S. et al. (1979a) Accuracy and dimensional stability of elastomeric impression materials. .Z. Prosthet. Dent. 42, 159-162.

Eames W. B., Sieweke J. C., Wallace S. W. et al. (1979b) Elastomeric impression materials; effect of bulk on accuracy. J. Prosthet. Dent. 41, 304-307.

Gordon G. E., Johnson G. H. and Drennon D. G. (1990) The effect of tray selection on the accuracy of elastomeric impression materials. J. Prosthet. Dent. 63, 12-15.

Johnson G. H. and Craig R. G. (1986) Accuracy of four types of rubber impression materials compared with time of pour and a repeat pour of models. J. Prosfhet Dent. 55, 197-203.

Lacy A. M., Fukui H., Bellman T. et al. (1981) Time dependent accuracy of elastomer impression materials. Part II: Polyether, polysulphides and polyvinylsiloxane. .Z. Prosthet. Dent. 45, 329-333.

Lin C. C., Ziebert G. J., Donegan S. J. et al. (1988) Accuracy of impression materials for complete-arch fixed partial dentures. J. Prosthet. Dent. 59,288-291.

McCabe J. F. and Storer R. (1980) Elastomeric impression materials. The measurement of some properties relevant to clinical practice. Br. Dent. J. 149, 73-79.

Myers G. E. and Stockman D. G. (1960) Factors that affect the accuracy and dimensional stability of the mercaptan rubber-base impression materials. J. Prosthet. Dent. 10, 525-535.

Odman P. A. and Jemt T. M. (1988) Accuracy of impression materials in a semi-clinical model. Dent. Mater. 4, 64-67.

Reisbick M. H. and Matyas J. (1975) The accuracy of highly tilled elastomeric impression materials. .Z Prosthet. Dent. 33,67-72.

Saunders W. P., Sharkey S. W. and Smith G. McR. (1990) The effect of water temperature on the accuracy of stone casts recovered from addition-reaction elastomeric impression materials. Znt. J. Prosthodont. 3, 577-581.

Scott P. J. (1981) The reflex plotter: measurement without photographs. Photogrammetric Rec. 10, 435-445.

Setchell D. J. (1984) The reflex microscope-an assessment of the accuracy of 3-dimensional measurements using a new metrological instrument. .Z. Dent. Res. 63, (Abstr. 32). 493.


Tjan A. H. L. and Whang S. B. (1987) Comparing effects of tray treatment on the accuracy of dies. J. Prosthet. Dent. 58, 175-178.

Tjan A. H. L., Whang S. B. and Miller G. D. (1981) Why a rigid tray is important to the putty wash silicone impression method. J. Calif: Dent. Assoc. 9, 53-58.

Tjan A. H. L., Whang S. B., Tjan A. H. et al. (1986) Clinically oriented evaluation of the accuracy of commonly used impression materials. J. Prostbet. Dent 56, 4-8.

Valderhaug J. and Floystrand F. (1984) Dimensional stability of elastomeric impression materials in custom-made and stock trays. J. Prostbet. Dent. 52, 514-517.

Book Reviews

Orale Implantologie, Allgemeine Grundlagen und Iti- Hohlzylindersystem. A. Schroeder, F. Sutter and G. Krekeler. 1988. George Thieme, Stuttgart. Softback.

The rapid developments now taking place in dental implantology are usually reported as conference papers, scientific articles or manufacturers’ literature. More extensive discussions, like the book under review, are less frequent. Such books have the advantage that the vision behind the system can be described: the disadvantage is that such a vision can never be considered objective. None the less, this book deserves a place next to other, well-established system overviews. It falls into four sections. In the first, ‘Algemeine Grundlagen’, the authors describe the development and use of titanium as a biomaterial. Intimate bone contact is possible but questions of long-term stability and of pocketting in the perimucosal area leading to loss of the implant continue to be a source of clinical concern. The existence of other bioactive materials such as hydroxylapatite or bioglasses are almost completely ignored. Coating with titanium-plasma is favoured for its influence on stability of bone-contact and long-term retention. The richly illustrated discussion of anatomical principles indicates the technically impossible situations that are sometimes found. The next three sections deal specifically with the ITI system based on hollow cylinders. The surgical techniques and preferred superstructures are discussed with particular relevance to the multicylinder system although these are relatively seldom indicated. In the edentulous mandible, four single cylinders are placed in most cases and later loaded and connected with a Dolder-like bar. The rationale behind this choice for a loading system is not explained. In the partially edentulous cases, most attention is paid to the free-end situation, here treated with bridges on either implants alone or on implants combined with natural teeth. The final section covers dental-forensic aspects and statistics with guidelines for registration and documentation. To the authors’ credit, they do not claim high levels of success without exactly defining the criteria they use. Although this book does not cover the latest developments for all biomaterials systems and evaluation techniques, it is to be particularly recommended for all dentists interested in implantology for its clinical, anatomical and prosthetic sections. C. de Putter

Functional Anatomy of the Masticatory System. W. E. McDevitt. Pp. 122. 1989. Oxford, Butterworth- Heinemann. Softback, f 12.95.

As the head of a department of anatomy I approached this book with considerable anticipation. ‘Functional anatomy is the Cinderella branch of the subject’ are the first words of the Preface. How true. A quick scan through the book revealed plenty of meat (no pun intended) and numerous clear illustrations. It was only when I settled down to a fuller appreciation of the work that I began to lose faith. The splendid black and white diagrams are labelled in a system whose complexity defies description. Anatomical points are initially labelled with numbers which seem to be related neither to alphabetical order (Figure 1: (64) genial tubercle, (65) symphysis menti) nor to order of appearance (Figure 1 contains (6), (7), (60-66)). There is no overall key. Later we find upper case letters corresponding to standard planes (and other things): here and there (Figure 38) lower case letters corresponding to anatomical features are added. Personally, I would find a structure labelled ZY easier to equate with the zygoma than one labelled 9. The book designer has also done a disservice to the illustrators in aligning all figures with the left-hand text margin. A small figure (e.g. Figure 31) on a right-hand page is thus squashed into the binding, with a generous expanse of white space to its right. The biggest disappointment, however, was the failure of the author to deliver the functional anatomy. The descriptive sections on the muscles, bones and joints of the masticatory area are clear and detailed but the pith seems to be very sparse. The chapters on collective action of the mandibular muscles (Chapter 6) and functional movements (Chapter 1 1) are both very short, and I failed to find details of the feedback between nerves, muscles and sense organs which ensures, for instance, that we do not destroy our molar teeth each time we close our mouths. As a detailed account of the region therefore, the book is to be recommended (if you can live with the labelling): but I am afraid that the definitive book on function is still waiting to be written. D. R. Johnson

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