Periodontal bone loss assessed on radiograph and at surgery

Correct answer: A. Vertical (angular) bone loss

• A. CORRECT. VERTICAL or ANGULAR bone loss is present when the crest slopes APICALLY along the root surface, so that the base of the defect lies apical to the surrounding crest and an oblique angle forms between the root and the bone. Vertical defects are the ones that may be amenable to regeneration, which is why the distinction is worth making.
• B. Horizontal bone loss keeps the crest PARALLEL to a line joining the CEJs of adjacent teeth. The whole crest drops together and no angular defect forms. That is not what is described.
• C. Furcation involvement is bone loss into the interradicular space of a MULTIROOTED tooth, seen as a triangular radiolucency between the roots. The stem describes loss on the mesial surface, not between the roots.
• D. Physiologic crest sits 1–2 mm apical to the CEJ, runs parallel to it, and is corticated. Anything beyond that is disease.

Note: Radiographs systematically UNDERSTATE bone loss, because the buccal and lingual plates superimpose on the defect. The interproximal picture is the only part a two-dimensional film reads reliably.

Correct answer: C. An interdental crater

• A. A one-walled or hemiseptal defect has lost the interproximal septum together with one of the facial or lingual plates, leaving a single bony wall. It is the least favourable defect for regeneration.
• B. A three-walled intrabony defect is bounded by the root plus three bony walls, most often on the mesial or distal of a single tooth. It has the best regenerative prognosis, but it is not the pattern described.
• C. CORRECT. An INTERDENTAL CRATER is a two-walled defect in which the interproximal bone is resorbed while the FACIAL and LINGUAL walls remain standing, producing a bowl or trough between them. Craters are the commonest osseous defect, accounting for roughly a third of all defects and about two-thirds of those in the mandible, because the interdental septum has the thinnest cortex and the richest vascular supply.
• D. A circumferential defect surrounds the root on all sides in a moat pattern; it involves a single tooth rather than the septum between two.

Note: Count the remaining WALLS, not the missing bone. Regenerative predictability rises with wall count and with narrowness: three-walled and narrow does best, one-walled and wide does worst, and craters are usually managed by osseous recontouring rather than regeneration.

Correct answer: B. 3 mm

• A. 5 mm results from ADDING the 1 mm instead of subtracting it. Addition is correct only when there is true RECESSION, with the margin apical to the CEJ.
• B. CORRECT. CAL is measured from the CEJ to the base of the pocket. When the gingival margin is CORONAL to the CEJ — overgrowth or oedema rather than recession — that overlying distance is SUBTRACTED: CAL = probing depth − distance from CEJ to gingival margin = 4 − 1 = 3 mm. Recorded conventionally, a coronal margin is a NEGATIVE recession value, and CAL = PD + (−1) = 3 mm.
• C. 6 mm does not follow from any correct handling of these two numbers.
• D. 4 mm is the probing depth itself. Probing depth equals CAL only when the gingival margin sits exactly AT the CEJ, which is not the case here.

Note: Fix the sign convention and the arithmetic follows every time. CAL = probing depth + recession. Margin APICAL to the CEJ (recession) is a POSITIVE value and is added. Margin CORONAL to the CEJ (enlargement or oedema) is a NEGATIVE value and is subtracted. CAL, not probing depth, is what measures true destruction.