Some causes of failure of crowns, bridges and dentures

With increasing emphasis being placed on precision in the construction of prostheses the importance of the visco-elastic characteristics of tooth support should be understood. Since the tooth returns to its resting position quite slowly, especially when repeated or sustained forces have been applied, an impression prepared prior to fully recovery will convey erroneous relationships of the tooth with its neighbours to the working model and, eventually, to the prosthesis. Such an error has been identified in young human subjects when inter-dental wedges, gingival retraction cord, and rubber dam are placed. The preparation of conventional alginate impressions in special trays can displace abutment teeth to a clinically significant degree as was recorded for seven partially dentate patients. Simple precautions will circumvent these potential sources of error.