Determination of the minimum number of marginal gap measurements required for practical in vitro testing

Statement of problem. Gap measurements along margins are frequently used to assess the quality of single crowns. However, the number of gap measurements required for clinically relevant results in laboratory studies is not known. Purpose. This study estimated the minimum number of gap measurements on margins of single crowns to produce relevant results for gap analysis. Methods and material. Ten all-ceramic crowns were fabricated on a master steel die. Gaps along crown margins were investigated in a scanning electron microscope on the master steel die without cementation and on replica dies after conventional cementation, Measurements were made in 100 mu m steps according to 3 gap definitions. The: initial number of measurements per crown (n = 230) n as reduced to smaller subsets using both systematic and random approaches to determine the impact on the quality of results. Results. On the data of gap definition 1, reduction from 230 to about 50 measurements caused less than +/-5 mu m variability for arithmetic means. Analysis of standard errors showed slowly increasing values smaller than 3 mu m, both indicating no relevant impact on the quality of results. Smaller data sizes yielded accelerated increase of standard errors and divergent variabilities of mean. The minimum of 50 measurements did not depend on gap definition or on cementation condition. Conclusion. Fifty measurements are required for clinically relevant information about gap size regardless of whether the measurement sites are selected in a systematic or random manner, which is far more than what current in vitro studies use.