What is the relationship between probing depth and CAL when the gingival margin is at the CEJ?

When the gingival margin is at the cementoenamel junction (CEJ), the probing depth and clinical attachment level (CAL) are equal because there is no displacement of the gingival margin.

Probing depth is the measurement from the gingival margin to the bottom of the periodontal pocket. Clinical attachment level measures from a fixed reference point, typically the CEJ, to the bottom of the pocket. When the gingival margin is positioned precisely at the CEJ, the two measurements align perfectly: the distance from the CEJ (where CAL is measured) to the bottom of the pocket (where probing depth is measured) is the same as the distance from the gingival margin to the bottom of the pocket.

This relationship of equality holds true under these specific circumstances. When the gingival margin is above or below the CEJ, the relationship between CAL and probing depth would change, influencing calculations accordingly. Therefore, when the gingival margin is at the CEJ, no additional calculations are necessary, confirming that both measurements are indeed equal.