Load Symmetry on the Failure Modes of MOD Composite Restorations

Objectives: Evaluate fracture modes of human molars with MOD composite restorations according to loading symmetry due to variations in occlusal anatomy: load positioned on the occlusal central groove (axial loading) or proximal groove (off-axis loading). Unrestored molars with MOD cavities and intact molars were used as reference.
Methods: Molars with symmetric and asymmetric occlusal planes were subdivided into Intact and Restoration groups. Restoration group received standard MOD preparations with 3mm pulpal depth and buccolingual width equivalent to 40% of the tooth. Direct resin composite restorations were carried out with Scotchbond Universal + Filtek Z350XT (3M ESPE). Additional 10 asymmetric molars received MOD preparations, either identical to above described (Cavity group) or with pulpal depth extended to expose pulp chamber (Cavity/pulp group). All 30 teeth were aged in water for 2.5-years, then loaded to fracture using a WC spherical indenter.
Results: Axial loading of intact molars results in not-reparable bulk fractures with extensive occlusal deformation. While off-axis loading causes reparable cusps chipping, but failure occurs at half the load to fracture in axial loading (3374 ±544N). For the restoration group, loading symmetry did not affect the load threshold to fracture (~1800N), but the fracture patterns observed were different: proximal chipping of the restoration was consistent within off-axis loading; while for axial loading, proximal chipping of the restoration, restoration/tooth splitting, or wall fractures were observed. Wall fracture was observed in all samples of the Cavity group, at loads ranging from ~500N to ~1100N. Cavity/pulp group showed the lowest load to fracture (495 ±72N), with wall fracture of all samples.
Conclusions: Direct composite restoration adds increased strength to molars with extensive MOD cavities, approaching that of intact tooth. The loading symmetry plays an important role in fracture modes of both intact and restored molars, governing the extent of damage and the reparability of the tooth.