Coronal Stiffness and Failure Load in Prepared and Restored Teeth

Objectives: Caries-infected teeth require removal of tooth structure before restoration, and thus weaken the tooth and may reduce longevity. This study evaluated how coronal stiffness of teeth changed after preparation and restoration.
Methods: Thirty extracted human molars (IRB18-05872-NHSR) were embedded in acrylic to fit a jig that ensured repeatable measures. Each molar was loaded and unloaded (repeated 3 times) in a universal testing machine (Instron) at a 45° on the buccal cusp ridge at 100 N/min with maximum load of 200N at 3 stages: baseline, MOD preparation, and 24-hr after restoration. Preparation dimensions were scaled according to bucco-lingual width. The molars were randomly divided into three restoration groups (N=10): TPH Spectra composite (Dentsply Sirona); ACTIVA BioACTIVE restorative composite (Pulpdent); and Valiant PH.D. amalgam (Ivoclar Vivadent). Scotchbond Universal Adhesive (3M ESPE) was used for the composites. Coronal stiffness was obtained from the slope of the third load-displacement curve and normalized to the baseline value of each tooth. Finally, the teeth were loading until failure. Fracture type (interface I, cuspal fracture CF, tooth split S) was recorded.
Results: Coronal stiffness decreased 7-9% after preparation; restorations recovered tooth stiffness ≥99% (see Table), which was a statistically significant increase (two-way ANOVA; P=0.0001). ACTIVA BioACTIVE composite restored closest to original stiffness, followed by the Valiant PH.D. amalgam and TPH Spectra ST composite; but differences among restorative materials were not significant (two-way ANOVA; P=0.7472). Fracture loads were not significantly different (one-way ANOVA; P=0.5946). Most fractures were at the interface.
Conclusions: Loss of tooth structure after cavity preparation reduced coronal stiffness. After restoration, all three restorative materials recovered tooth stiffness close to the original values of the sound teeth. Both composite groups fractured at the interface similar to amalgam. This indicates that the interface, even in materials that are bonded, remains a weak-point in all restored teeth.