The use of an mesial-occlusal-distal (MOD) restoration in repairing a large carious lesion depends on many factors. Biomechanical performance is one of the most important. It has been recognized that resistance to restoration failure is not solely a biological concern (e.g. toxicity), but that the cavity shape, dimensions, and the state of stress must all be taken into account. In the present study, a newly developed auto-mesh program was used to generate 30 three-dimensional (3D) finite element (FE) models simulating the biomechanics for multiple factorial design of the MOD gold restoration in a maxillary second premolar. Stress levels were related to individual design factors (e.g. pulpal wall depth [P], isthmus width [W] and interaxial thickness [T]) and to their interactions under the worst physiological scenario: a concentrated bite force acting on lingual cusp with debonded interfaces between cavity walls and restorations. The results showed that enlarging the volume of the MOD cavity significantly increased stresses in enamel but did not intentionally affect stresses in dentin. The alternation of individual design parameters significantly changed the peak stresses (P < 0·05). For all three parameters, except for the width, the peak stress increased as the cavity dimension increased. Stress elevation rate (termed as ‘volumetric stress rate’– stress elevation by increasing one unit volume of the restored materials) was different among three design factors. Depth was the most critical factor governing the stress elevation in enamel (1·76 MPa mm−3) while length (interaxial thickness) was the most important parameter in dentin (0·49 MPa mm−3). Width was the least compromising factor to the remaining tooth, 0·32 MPa mm−3 for enamel and −0·23 MPa mm−3 for dentin. The findings, at its core, did not fully agree with the traditional concept that the preservation of tooth substances will reduce risk of tooth fracture. This study leaves open possibility for the structural optimization of the MOD restoration.
