Effect of different onlay materials and resin core thickness on stress distribution in partial coverage restorations:...

To investigate the effects of onlay materials and resin core thickness on stress distribution in partial coverage restorations under static and thermal loading using finite element analysis (FEA).

A 3D mandibular first molar model was created from CBCT data. Four materials (zirconia, lithium disilicate [LD], polymer-infiltrated ceramic network [PICN], gold alloy [GA]) and five resin core thicknesses (1 mm to 4 mm, plus full crown control) were tested. A total of 20 models were loaded with 100 N occlusal force and thermal stresses (5-37 °C and 37-55 °C). The distribution characteristics of the maximum principal stress (MPS) were analyzed for different combinations of variables.

Under mechanical loading, thin resin cores (1-2 mm) performed best. Increasing thickness to 3 mm sharply raised enamel stress (zirconia: 77.35-80.72 MPa); at 4 mm, restoration stress peaked (zirconia: 150.12 MPa). Zirconia showed highest restoration stress (54.01-150.12 MPa) but lowest dentin stress (5.02-7.70 MPa); PICN had low and most stable restoration stress (55-91 MPa) but highest dentin stress (8-13 MPa). Under high-temperature thermal stress, enamel stress jumped sharply at ≥ 3 mm resin thickness, markedly higher than under low-temperature stress. GA with an extremely thin resin core (1R4O) caused abnormally high dentin stress (50.83 MPa) at low temperature.

A thin resin core (1-2 mm) offered better overall mechanical performance. Material type markedly influenced stress distribution. High-temperature thermal stress posed a greater threat to tooth structure than low-temperature stress. Thermal stress barely affected periapical/periodontal tissues.

For extensive MOD restorations, a 2 mm resin core is recommended as the safe threshold. Material selection should balance the protection of the restoration itself and the stress transfer to the tooth structure.