The restoration of endodontically treated teeth (ETT) has been a controversial topic for many years. It is known that vitality loss causes physical and structural changes affecting the dentin properties such as micro-hardness, modulus of elasticity and fracture toughness. Root filled upper premolars present specific challenges for the restorative dentist because in addition to esthetic considerations, cusp fracture was found to be more concentrated in these teeth. Furthermore, longitudinal root fractures are more common in upper premolars with narrow roots in the mesiodistal dimension.

Purpose: The aim of this in-vitro study was to measure the fracture resistance and relative stiffness of root canal treated premolars restored with two different preparation designs of ceramic restorations.

Methods: The present study was conducted using 16 extracted human maxillary premolars. Teeth were divided into two equal groups (N=8): Group (A) received an onlay with palatal cusp coverage and Group (B) received an overlay with total cuspal coverage. Lithium di-silicate CAD blocks were used to fabricate all the restorations and cemented using self-adhesive dual cure resin cement. All Specimens were then subjected to fracture with cross-head speed 1.00 mm/min.

Results: There was no statistically significance between the two preparation designs concerning the fracture resistance. While the relative stiffness results showed that the restored overlay (Group B) is significantly higher than the restored onlay (Group A).

Conclusion: Both partial coverage restorations designs (overlay and onlay) restored the relative stiffness of the tooth structure and could work as an alternative line of treatment for restoring endodontically treated maxillary bicuspids.