Introduction
When pit-and-fissure caries runs from the occlusal surface along a groove onto the buccal or lingual surface of a molar, the occlusal Class I cavity is continued on that surface as a narrow step: an occluso-buccal or occluso-lingual (occluso-palatal) cavity. When only the blind pit at the end of the groove is carious, it gets its own small foramen caecum cavity. Both lie where cusps are easily weakened and the pulp is close, so bur angle and depth are critical.
Where these cavities occur
Black's Class I includes all pits and fissures: on the occlusal surfaces of premolars and molars, on the occlusal two thirds of the buccal and lingual surfaces of molars, and on the lingual surfaces of maxillary incisors. The grooves that carry caries off the occlusal surface are:
- Mandibular molars – the buccal groove (between the mesiobuccal and distobuccal cusps), ending in the buccal pit, usually in the middle third → occluso-buccal cavity; less often the lingual groove → occluso-lingual cavity.
- Maxillary molars – the distopalatal groove, running from the distal pit between the mesiopalatal and distopalatal cusps onto the palatal surface, sometimes ending in a palatal pit → occluso-palatal cavity.
- Maxillary incisors, above all lateral incisors – the lingual pit, just incisal to the cingulum.
These blind pits are the foramen caecum (Latin: "blind opening") of the tooth. A carious pit or fissure on a buccal or lingual surface is Class I; caries starting on the smooth gingival third of the same surface is Class V. If the groove leading to a pit is sound, the pit is prepared on its own and the groove is sealed; if the groove is carious or deeply fissured, the cavity runs through it to the pit.
Types one and two
As in the occlusal Class I cavity (previous topic), the type describes the occlusal part: type 1 – limited to the pit next to the groove; type 2 – following the carious fissure system. The step is the same in both.