Which furcation of the maxillary molar is closest to the CEJ?

In the maxillary first molar, the furcation entrance distances from the cementoenamel junction (CEJ) are: mesial = approximately 3 mm, buccal = approximately 4 mm, and distal = approximately 5 mm. Therefore, the mesial furcation is the closest to the CEJ. This is clinically significant because the mesial furcation is the first to be involved in periodontal disease as attachment loss progresses apically. Knowledge of these distances guides probing technique and surgical planning for furcation involvement.