Class I amalgam restorations in posterior positions are often replaced with no heed to the presence of hairline fractures in the marginal ridges which are a result of the Poisson Effect, whereby a solid deforms under stress and the cyclical fatigue of which leads to formation of these microcracks. The hairline fractures often not only are weak points with risk of further apical propagation, they also act as superhighways for bacterial ingress, placing the tooth at greater risk of secondary caries. This tooth #18 featured both M and D marginal ridge hairline fractures both of which penetrated past the DEJ. The goal is to remove if possible all cracks within 2mm of the DEJ in dentin (PSZ) (peripheral seal zone) and as apically as possible within reason.
