Background: Crown lengthening procedures are a significant part of the periodontist's armamentarium. In order to recreate the clinical space lost to caries or trauma necessary for prosthetic restoration, osseous surgery is often required. If the procedure is not carefully planned, it may result in furcation involvement of multirooted teeth. Methods: Twenty-six subjects with 26 mandibular molar teeth requiring crown lengthening procedures prior to prosthetic crown placement were evaluated. Nineteen subjects with 24 prosthetic crowns on lower molars which had not undergone crown lengthening were included as control teeth. Bite-wing radiographs prior to surgery (for the test group) or placement of the crown (control group) and 5 years after completion of the prosthesis were compared and analyzed. Results: Of the 26 test teeth, 10 teeth (38.5%) were found to have radiographic evidence of furcation involvement, whereas none of the control teeth developed furcation invasions. Conclusion: A critical distance from the furcation (CDF) of 4 mm was established as a landmark under which, if surgery was performed on mandibular molars, chances of furcation involvement in the future were very high.
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