Cuspal deflection of maxillary premolars restored with bonded amalgam.

The aims of this study were to measure cuspal deflection of premolars restored with bonded amalgam and to investigate bond resistance to thermo-cycling and cyclic loading. Strain gauges were used to measure cuspal deflection of maxillary premolars restored with MOD bonded amalgam restorations. A nondestructive method was used in which teeth were loaded repeatedly to record cuspal deflection following different restorative procedures. Ten extracted premolars with similar dimensions were selected and their roots mounted in resin bases 2 mm below the CEJ. Two single-element strain gauges were bonded to the buccal and lingual surfaces of the cusps of each tooth at a level that corresponded to the pulpal floor of MOD cavities. These were connected to a strain indicator with a built-in wheat-stone bridge. An Instron machine was used to apply a 100 N compressive load. Micro-strain readings were recorded with each loading at the following stages: (1) sound unprepared teeth (baseline reading), (2) following preparation of a medium-size MOD cavity, (3) 24 hours following restoration with amalgam, (4) following amalgam removal, (5) 24 hours following restoration with bonded amalgam. Durability of the bond was further tested by cyclic loading of 2000, 4000, 6000, and 8000 load cycles. Mean micro-strain values recorded at the buccal cusp were: 48.0 (21.6), 126.8(57.2), 121.4(53.3), 120.8(56.1), and 65.2(36.5) for test stages 1, 2, 3, 4, and 5 respectively. Cuspal deflections following cyclic loading recorded at the buccal cusp were: 60.0(41.0), 63.6(51.9), 59.6(36.3), and 61.6(36.8) at the above four cyclic loading stages respectively. A similar trend was also observed for measurements of the lingual cusp. It was concluded that bonding amalgam restorations decreases cuspal deflection and consequently may assist in restoring tooth strength under conditions of the oral environment.