The dual-arch impression.

Introduction also be used for multiple restorations! or fixed partial dentures up to four units. However, if the most posterior tooth in the arch is prepared for a fixed partial denture, the patient may close at a decreased vertical dimension of occlusion, which would result in a significant negative occlusal error. Care must be excercised when this type of impression tray is used. Before the impression is made the tray must be placed in the mouth to ensure that the impression will include a sufficient number of teeth to provide a definite occlusal relationship of the casts. Ideally, there should be at least two teeth in the same quadrant which have contact with the opposing arch. Sufficient room behind the most distal tooth in the quadrant is needed to accommodate the posterior bar of the tray when the mouth is completely closed. If not, the patient can occlude on the bar, resulting in an increased occlusal vertical dimension. Previous literature, however, does not mention the importance of h()rizontal clearance between the vertical rim of the tray and the alveolar ridges. If the bucco-lingu~l width of the alveolar ridge is wider than the width of the tray, the plastic outer rims will be wedged apart when the patient closes into the impression material (Fig. 1). The tray will be expanded during the set of the impression material, and upon tray removal, the outer rim will return to its original shape and distort the impression. Once poured, the resulting die will be smaller than (or of different configuration from) the tooth preparation, yielding a casting that is too small. If this situation occurs, a new tray may be modified by cutting the mesh and spreading the tray several millimeters. This prevents hard contact between the outer rims and the soft tissue during the impression procedure (Fig. 2). The remainder of this article will present a clinical situation in which this problem occurred.