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What is A condition where there is loss of interdental bone on facial and lingual plate, without involvement of radicular bone is: and explain in detail?
Reversed Architecture
Reverse (or negative) alveolar bone architecture is the result of a loss  of  interdental  bone,  without  a  concomitant  loss  of  radicular (buccal or lingual/palatal) bone, thereby reversing the normal (or positive)  architecture .  Negative  architecture  is  more common in the maxilla of patients with periodontitis.
Ledges
Ledges are plateau-like bone margins that are caused by the resorption of thickened bony plates .
Osseous Craters
Osseous craters are a specific type of two-wall defect; they present as concavities in the crest of the interdental bone that is confined within the facial and lingual walls. Craters have been found to make up about one-third (35.2%) of all defects and about two-thirds (62%) of all mandibular defects; they occur twice as often in posterior segments as in anterior segments.
Ref: Newman and Carranza’s Clinical Periodontology, thirteenth edition; page no 325