This is a single-center, ten-year, randomized controlled, split-mouth clinical trial. A total of 164 adult patients requiring full-cuspal coverage onlays will be enrolled. Each participant will receive one milled and one 3-D printed restoration, randomly assigned.

Restorations will be evaluated at 1 week, 6 months, and 1-10 years using the revised FDI criteria for indirect restorations. The primary outcome is the survival rate of the onlays.

Inclusion Criteria:

• The presence of two teeth requiring restorative treatment (onlays) because of primary or secondary caries or final restoration is needed after root canal treatment, with adjacent and antagonist teeth
• At least two walls (buccal and oral) with a minimum length of 2.5 mm and a width of 2 mm
• The deepest point of the tooth structure should be above a minimum of 3 mm of supracrestal
• The crown-to-root ratio is minimum 1:1

Exclusion Criteria:

• Inadequate oral hygiene
• Pregnancy
• Allergies to restorative materials
• Parafunctional habits (e.g., teeth grinding or clenching)
• Presence of orthodontic appliances
• Premolars that have undergone root canal treatment