Clinical outcome of the altered cast impression procedure compared with use of a one-piece cast

STATEMENT OF THE PROBLEM: An altered cast impression procedure to improve the support of distal extension removable partial dentures is widely taught, but not often used in dental practice. PURPOSE: The purpose of this study was to determine the efficacy of an altered cast compared to a one-piece cast with regard to base support, abutment health, and patient comfort over time. MATERIAL AND METHODS: Seventy-two patients receiving a mandibular bilateral distal extension removable partial denture were assigned randomly for treatment using either a one-piece or an altered cast. All impressions and associated laboratory procedures were made by one investigator. A second investigator evaluated extension, support, and adaptation of the denture bases by observation of border length and lifting of the indirect retainer from its seat. The space between the soft tissues and the base when the framework was related to the teeth was measured cross-sectionally at half the length of the denture base. Mobility, gingival index, and sulcus depths at 6 locations around each abutment tooth were recorded at insertion and again 1 year later. Chi-square tests were used to evaluate differences between the treatment groups (alpha=.05). RESULTS: There was 0.15 mm less space between the ridge crest and base in the altered cast group (P<.01), and underextension of the base occurred only in the one-piece cast group (P=.01). At insertion, no tissue-ward movement was observed in 85% of the prostheses when anterior-posterior rotation was attempted. Fifteen subjects (21%) were lost to recall at 1 year, but they were distributed equally between the 2 groups. Of the remaining 57 prostheses, 42% exhibited decreased base support and 33% had increased gingival inflammation; the deepest probing depth decreased in 61%, mobility decreased or remained the same in 80% of the direct abutments, and 88% of the subjects were satisfied. None of these findings were related to the impression procedure. CONCLUSION: The altered cast impression procedure does not offer significant advantages over the one-piece cast, provided the standards used in this study are met. These include a completely extended impression, use of magnification to adjust and ensure complete seating of the framework, and coverage of the retromolar pad and buccal shelf by the base.