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Good design is the secret of success in complete denture prostheses but some of the techniques, that are commonly used prevent the construction of welldesigned dentures. It is generally accepted that complete denture prostheses should represent the lost tissues in the same amounts and in the same positions from which they were lost. This presents few difficulties when immediate dentures are made, but, with each successive denture it becomes more and more difficult to determine the size and shape of the lost tissues and thus the design of successional dentures deteriorates with time until it is difficult to make satisfactory dentures when extreme resorption has taken place. A 17-year longitudinal study of oral changes following the loss of teeth has provided a clear description of "The space in the mouth that was formerly occupied by the teeth and their supporting structures that have since been lost". I feel that this is a better definition of the "denture space" than those found in current glossaries of dental terms, as knowledge of it has led to a radical revision of our techniques for designing and making dentures to fill that space (Fig. 1). The measurements that were made to construct the average diagrams in Figure 1 showed a remarkable constancy of the bucco-lingual breadth of the alveolar ridge measured horizontally from the lingual gingival margins to the buccal mucosa midway between the buccal sulcus and the level of the lingual gingival margin, before the teeth were extracted (Fig. 1). The constancy of this measurement at each tooth position was so striking in the original sample that we measured altogether 100 patients to check that the variation was really as small as we observed. The results of those measurements are shown in Table 1. Note that the standard deviation is only about 1mm at each tooth position so that the average measurement plus or minus about 1mm will be correct for about 68% of patients and plus or minus about 2mm for about 95% of the patients. You can check this by measuring casts in your laboratory. Thus if we can locate the position of the lingual gingival margins in the edentulous mouth, we have a reasonable guide for repositioning the lips and cheeks correctly, because they lie against the buccal surfaces of the dentulous ridges. The remnant of the lingual gingival margins is represented in the edentulous mouth by a fibrous cord that crosses the ends of the rugae as it follows the line of the residual ridge (Fig. 2). It lies near the crest of the ridge when there has been little resorption but where resorption is extreme it lies near the vestibular sulcus. Difficulties in treating patients with marked resorption arise because a small impression tray is selected to fit the shrunken ridge, and, as it does not restore the full breadth of the sulcus, the impression material dribbles over its edge and consequently the sulcus on the cast is too narrow and it is then impossible to design the denture correctly. A denture made on a small cast will not be retentive as it is rather like a loose piston in the soft cylinder of the mouth-it can easily be 11 Lockharton Garden Edinburgh EH14 1AU. Scotland, UK.

Good design, the secret of successful complete denture prostheses.