Increase in vertical dimension of occlusion dur­ ing the processing of complete dentures demands serious consideration. Inherent properties of the denture base resin and the dental stone used as an investing material are responsible for this frustrating outcome. Cured acrylic resin exhibits 2% linear or 6 to 7% volumetric shrinkage.1 Mohammed and Tsutsurm2 stated that the observed linear shrin­ kage, due to curing, is actually less than 2% proba­ bly because at the curing temperature, at which polymerization shrinkage would occur, the resin is quite soft and its shrinkage is resisted by the mold. Grunewald3 reported that the excess acrylic resin in the mold during final closure is responsible for the increased vertical dimension. Lerner and Pfeif-fer4 suggested a diaphragm, or layer of resin, on the land around the entire border of the denture which acts as a cushion, and possibly as a reservoir which helps to control the shrinkage. Pera5 advocated freezing the mold before packing and provision of central exhaust in order to avoid increase in the ver­ tical dimension. Vig6 proposed simple extension of the wax-up maxillary denture onto the posterior aspect of the cast to correct the shift of posterior teeth in processing. Such modifications in the pro­ cessing technique are only partially successful in compensating for the unfavourable properties of the acrylic resin. Taylor1 reported 4% elastic defor- mation of the dental stone from packing pressure. Grant7 reported tooth movement of 0.02 to 0.05 mm resulting from the setting expansion of the sec­ ond pour of the investing stone. A study by Becker et al8 showed 0.162 mm vertical change in the occlusal plane of the upper complete denture pro­ cessed using all stone mold. Wesley et al9 stated that the exothermic reaction of investing stone causes expansion of the wax leading to the change in tooth to master cast relationship.
This investigation was undertaken to compare the increase in vertical dimension of the complete dentures processed by: i) conventional method, and ii) by reduced amount of dental stone used for the second layer of the mold. The increase in the vertical dimension during processing of the upper complete dentures, lower complete dentures, and sets of the dentures is reported.

Fourteen sets of identical upper and lower stone casts were prepared using a flexible rubber mold (Whaledent International, New York, USA). Record bases were prepared of autopolymerizing acrylic resin and occlusion rims of the similar dimensions were made using pink baseplate wax. The casts were indexed to facilitate remounting. Using a face bow and centric relation records the upper and lower casts were mounted in alike posi­ tions in the Hanau articulator (Teledyne Hanau, Buffalo, NY, USA). Tooth arrangement of all the 14 sets of dentures was completed using Trubyte Bioform (Dentsply International, USA) acrylic resin anterior teeth and Biotone (Dentsply International, USA) acrylic resin 30 degree cusp angle posterior teeth. All sets of teeth were of the same mold. In centric occlusion, the contact of the supporting cusps namely; palatal cusps of the upper teeth and buccal cusps ofthe lower teeth with the respective fossae of the opposing teeth, was set-up and recon­ firmed using a thin articulating paper (Produits Dentaires SA Vevey Suisse). Positive contact of the incisal pin with the incisal table was rechecked and the thumb screw was tightened. The wax-up was accomplished to have similar contour and equal bulk to all of the upper as well as to the lower dentures. Varsity eject type Hanau flasks (Hanau Engineering Co. Buffalo, USA) were used for investing the dentures.
These sets of the dentures were equally divided into two groups. To begin with, 4 upper dentures of 4 sets and 3 lower dentures of the other 3 sets from group I were taken for processing. These trial dentures were invested in dental stone (Labstone, Columbus Dental Mfg. Co, St. Louis, USA). The three layer investing method was applied as described by Morrow et al.10 The second layer of the stone extended up to the occlusal level of the teeth having cusp tips and incisal edges exposed [Fig. 1], Wax elimination was carried out. The mold was packed using Permatone (Kerr Mfg. Co, Ml, USA) cross-linked acrylic resin at 3000 psi and was polymerized at 165°F for 9 hours. After cool­ ing, the mold was ejected from the flask, the invest­ ment was sectioned and pried out. Attached parti­ cles of investment were carefully removed from the exposed surfaces of the denture and cast. The pro­ cessed dentures attached to the casts were remounted on their original mountings and sec­ ured using plaster of paris. The rise of the incisal pin from the incisal table, in all of the seven cases was measured using a leaf gauge [Fig.2]. The leaf gauge was prepared from 4 ml (0.09978 mm) sheet of polyester transparency film Type 174 as suggested by Resenblum and Huffman.11 The leaf gauge con­ tained 50 leaves. Tests were carried out to deter­ mine the measuring accuracy of the leaf gauge. The leaves were divided into 5 groups, each containing 10 leaves and its thickness in each group was mea­ sured using verner caliper. Average of the reading indicated 1.0012 mm thickness to 10 leaves, and 0.10012 mm thickness to a single leaf. To start with, one leaf was placed on the incisal table and the upper jaw was swung forward. The leaf was gently pulled out and ease or resistance to pull is felt. If necessary, the procedure was repeated with an added leaf each time, till resistance, while pul­ ling, was experienced. Total thickness of the leaves required in the process denoted the distance between the incisal table and the incisal pin up to an accuracy of 0.1 mm. The increase in vertical dimension during the processing of the 4 upper complete dentures and 3 lower complete dentures was obtained. The procedure was repeated and the remaining 4 lower and 3 upper complete dentures of Group I were processed, remounted, and incisal pin rise from the incisal table was measured. The measurements indicated increase in vertical  dimension  of the sets of the dentures resulted by processing with conventional investing method.
The trial dentures from Group II were processed similarly but with modification in investing. The second pour of the investing stone extended only up to the maximum convexity on the labial, buc­ cal, and lingual surfaces of the teeth fFig.3]. The collected data included the increase in vertical dimension of 4 upper complete dentures, 3 lower complete dentures and 7 sets of the complete dentures, as a result of processing by the modified investing method.

 
Table 1 shows the incisal pin rise resulted from processing the dentures in Group I by conventional investing methods. The mean increase in vertical dimension of upper dentures processed by the conventional method was 0.675 mm, and that of the lower dentures was 0.233 mm. The mean increase in vertical dimension resulted from pro­ cessing of upper dentures was 0.650 mm, and that of lower dentures was 0.266 mm. Changes in verti­ cal dimension of the upper dentures, as well as that of the lower dentures of the two groups, were com­ pared by applying t-test statistics. The difference between vertical dimension, as a result of proces­ sing the upper dentures by two different investing methods, was not significant (t = 0.195 and P < 0.5). The difference in the change, as a result of processing the lower dentures by two different investing methods, was also insignificant (t = 0.247 and P < 0.5). Average increase in vertical dimen­ sion of the sets of dentures processed by conven­ tional investing method was 0.871 mm, and that of those processed with the modified investing method was 0.671 mm. The difference between the two groups was insignificant (t = 0.428 and P < 0.5).
Comparison of the average increase in vertical dimension of the upper dentures with that of the lower dentures processed by conventional invest­ ing method showed statistically significant differ­ ence. The computed t value was 3.100 and the level of significance was P > 0.02. This difference between upper and lower dentures was also signif­ icant in the dentures processed by the modified investing method (t = 4.590 and P > 0.001). Since the two  investing  methods  used  in this study proved to be similar, the average increase in vertical dimension of all the dentures from both groups was calculated. It was 0.662 mm in upper denture, 0.250 mm in lower denture, and 0.771 mm in a set of complete dentures [Table 2).

 
This study revealed that the average increase in vertical dimension of occlusion, after processing the upper complete denture, was more than that resulted after processing the lower complete denture. This study does not offer plausible expla­ nation for the difference in vertical dimension change of upper and lower complete dentures due to processing. However, difference in the amount of acrylic resin required and the polymerization shrinkage exhibited in different areas of the upper and lower complete dentures may be responsible for the difference. No reports were available in the literature, comparing processing induced vertical dimension change in the upper vs that in the lower complete dentures. Dental stone of the second layer of investment makes direct contact with the wax of trial denture. Heat evolved during setting of the stone softens the wax and expansion of the den­ tal stone causes tooth movement. Part of this study was aimed towards reducing in the amount of den- tal stone used for pouring the second layer of the investment in order to reduce the total amount of exothermic heat and setting expansion. The results did not show statistically significant reduction in the vertical dimension change. The quantity of dental stone lessened was small, possibly 4 to 5 in. in height. Further, lowering of the second layer of the investment will terminate it into the undercut area and deflasking will become difficult. Curtailing the exothermic heat and setting expansion, possibly was not sufficient to minimize the vertical dimen­ sion change.
Application of different investing media, use of various denture base resins and employment of sev­ eral processing techniques had been tried to van­ quish vertical dimension increase during processing of the dentures. In the literature, no report was cited illustrating a system for abrogation of this conse­ quence. The studies reporting increased vertical dimension of complete dentures, processed using dental stone investing medium and compression molding technique, are summarized in Table 3.
In the present study the increase in vertical dimension of occlusion (measured at incisal pin) of all the 14 sets of complete dentures ranged between 0.3 to 1.2 mm, with an average of 0.77 mm. The result compared favorably with previous reports.

  The investigation yielded the following informa­ tion:

summary

An investigation was conducted to evaluate the change in vertical dimension of the upper complete dentures, lower complete dentures, and the sets of dentures processed by conventional investing method and by a modified investing method. No difference was found between the results obtained by processing of the dentures by the two investing methods. The increase in vertical dimension of upper complete dentures as a result of processing was more than that of the lower complete dentures. The vertical dimension change in the sets of the complete dentures is comparable with previous reports cited in the literature.

The authors would like to thank F. Balucas, R.D. Sokyawan, T. Corpus, and R.A. Fasan for their assistance in the laboratory procedures.