The effect of endodontic access cavity preparation and subsequent restorative procedures on molar crown retention

Preparation of an endodontic access cavity through a full crown may affect its retention. This study was undertaken to investigate the effects on molar crown retention of endodontic access cavities and their subsequent restoration. Thirty human molars were mounted in resin, crown preparations were cut and their surface areas were determined. Vented metal copings were cemented with zinc phosphate and the forces required to displace each coping were measured using a tensile-testing machine. The copings were recemented, access cavities were cut and their surface areas determined prior to the displacement forces being re-measured. The copings were recemented, assigned to two groups, and the access cavities were restored - Group 1 with amalgam; Group 2 with glass ionomer (GIC). Displacement forces were re-measured and the copings were recemented. The occlusal margins of the access cavities were bevelled and restored again prior to displacement forces being remeasured. Mean displacement forces were - Group 1: Original (kg force), 37.86 +/- 3.97; After access cavity, 29.28 +/- 3.22; Amalgam, 50.21 +/- 4.71; Amalgam + bevel, 46.45 +/- 6.21. Group 2: Original, 42.77 +/- 4.49; After access cavity, 39.25 +/- 5.91; GIC, 48.11 +/- 3.55; GIC + bevel, 39.63 +/- 5.31. Statistical analyses with paired t tests showed that retentive values with access cavities were significantly lower than with intact crowns. Amalgam or GIC restorations increased retention beyond original values, significantly with amalgam. Bevelled occlusal margins decreased retention of crowns with restored access cavities but this was not significantly different from the original values. A significant relationship existed between total surface areas of the crown preparations, areas of the occlusal tables, and retentive values for crowns without access cavities. The access cavity area, as a proportion of the total area of the preparation, was related to the decrease in retention.