Fracture resistance of molar teeth with mesial-occlusal-distal (MOD) restorations

Filled MOD restorations show near-complete recovery of tooth strength relative to the newly prepared, unfilled state. The present study examines the underlying mechanics of this recovery by more closely quantifying the mode of splitting fracture from the cavity base. By understanding the role of specific cavity dimensions on fracture resistance, useful clinical guidelines concerning MOD morphologies are formulated.

A systematic in vitro study is made of the load-bearing capacity of filled and unfilled MOD cavities by axially loading extracted molar teeth with a hard metal ball. Filled and unfilled cavities are considered as bounding cases. Focus is placed on drillings with rectangular or rounded tips, covering a range of cavity widths and depths. The failure process is monitored during loading by a video camera, enabling the entire damage evolution from first contact to ultimate failure to be recorded.

While respecting the widely accepted clinical practice of drilling cavities with internal widths less than one third that of the entire tooth, a stronger correlation is obtained between critical splitting load P_C and ratio of cavity wall thickness h (distance between cavity wall and outer tooth surface) to cavity depth D. Imposing a conservative upper limit on P_C for tooth survival, the study recommends that MOD cavities be prepared such that the ratio remains in the region h>D, regardless of the tooth size.