| Objective: To study the effect of different post and cores systems on resistance to fracture of the remaining dental tissue of endodontically treated teeth and analyze the relationship of the combined effect of crown lengthening and placement of a ferrule and teeth's failure resistance.Methods: The experiments were performed with a homogeneous sample of sixty intact single-rooted human mandibular premolars of similar dimensions, extracted recently from adolescents who were receiving orthodontic treatment. Teeth were cut off from CEJ and randomly divided into six groups with ten each. Teeth in group A were prepared with no ferrule and restored with cast Ni-Cr posts and cores and full crown casts. Teeth in group B were prepared with 1.5mm ferrule and received cast Ni-Cr posts and cores and full crown casts. Teeth in group C were prepared with no ferrule and received prefabricated metal posts and composite resin cores. Teeth in group D were prepared with 1.5mm ferrule and received prefabricated metal posts and composite resin cores. Teeth in group E were prepared with no ferrule and received fiber glass posts and composite resin cores. Teeth in group F were prepared with 1.5mm ferrule and received fiber glass posts and composite resin cores. Specimens then were embedded vertically in autopolymerizing acrylic resin, which had a flat surface 2mm below the facial cemento-enamel junction. Then specimens were secured in a universal load-testing machine with the use of a device that allowed loading of the tooth facial cusp at 30 degrees to the long axis of the tooth at a crosshead speed of 1mm/min until fracture occurred. The magnitude of the force causing fracture and the mode of fracture were recorded and classified as favorable (would allow repair) or catastrophic (non-restorable). Statistical analysis of the data was computed to reveal significant correlations. One-way ANOVA and t-test test were used to determine the significance of the failure resistance loads. A nonparametric chi-square test and Fisher's exact test were conducted for statistical evaluation of the mode of fracture. Results:1. Mean failure loads were calculated for all groups. Group A has the highest fracture resistance with the value of 3.0369±0.3388KN while Group F has the lowest fracture resistance with the value of 2.0188±0.3864KN. The values of other three, group C, group E and group B are 2.7703±0.3603KN, 2.5007±0.5429KN and 2.4780±0.5614KN, respectively.2. One-way ANOVA was used to determine significant differences of the fracture resistance loads among A, C, E groups. The results showed the differences were significant (P<0.05). Then a LSD test was performed to compare the fracture resistance loads between these groups. There was a significant difference between group A and group E (P<0.01). But the differences were not evident between group B and group E or group A and group C (P>0.05). One-way ANOVA was used to determine significant difference of the fracture resistance loads among B, D, F groups. The results showed the differences were not significant (P>0.05).3. T-test was used to determine significant difference of the fracture resistance loads among A and B groups, C and D groups, and E and F groups. The differences in the fracture resistance of teeth were significant (P<0.05).4. Mode of fracture: Teeth restored with cast Ni-Cr posts and cores or restored with prefabricated metal posts and composite resin cores displayed more catastrophic fractures than teeth restored with fiber glass posts and composite resin cores. It can be speculated that the fiber glass posts and composite resin cores can protect the residual root.Group A: Two teeth had oblique fractures in the middle part of one third of the root. Five teeth had longitudinal fractures extending the entire length of the root. Three teeth had fractures at dental cervix. All fracture patterns of group A were catastrophic.Group B: One tooth had fracture at the root tip. Two teeth had oblique fractures in the middle of one third of the root. Three teeth had longitudinal fractures extending the entire length of the root. Four teeth had fractures at dental cervix. All fracture patterns of group B were catastrophic.Group C: One tooth had oblique fracture in the middle of one third of the root. Four teeth had longitudinal fractures extending the entire length of the root. Three teeth had fractures at dental cervix. Two teeth had fractures at their posts or cores. Two fracture patterns of group C were favorable.Group D: One tooth had fracture at the root tip. One tooth had oblique fracture in the middle of one third of the root. Two teeth had longitudinal fractures extending the entire length of the root. Three teeth had fractures at dental cervix. Three teeth had fractures at their posts or cores. Three fracture patterns of group D were favorable.Group E: Four teeth had fractures at dental cervix. Six teeth had fractures at their posts or cores. Six fracture pattern of group E were favorable.Group F: One tooth had fracture at the root tip. Two teeth had fractures at dental cervix. Seven teeth had fractures at their posts or cores. Seven fracture pattern of group F were favorable. Conclusions:1. Teeth prepared with no ferrule and restored with cast Ni-Cr posts and cores show higher resistance to occlusal forces than those prepared with no ferrule and restored with fiber glass posts and composite resin cores. The latter has superiority in teeth protection. 2. When teeth are subjected to CEJ level, the combination of crown lengthening and placement of a ferrule can decrease the teeth's failure resistance.3. Teeth restored with fiber glass posts and composite resin cores have higher failure resistance than those restored with the other two restoration methods. The ferrule has no effect on the failure resistance.4. Prefabricated metal posts and composite resin cores are useful and economical posts and cores systems.
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