The proximal lesion has now advanced to the point where there is surface cavitation and, until that surface is smooth again, plaque will continue to accumulate and the caries will progress. Once the cavitation reaches the dentine bacteria will become involved in destruction of the collagen framework. In this case the marginal ridge is grossly undermined and will need to be removed but is it necessary to remove all the demineralised enamel at the gingival margin? The decision will be made during cavity preparation but it is likely that some or all can be remineralised thus limiting the extent of the cavity.