The primary mechanism behind the development of pit and fissure caries is the physical impaction of food debris in the narrow grooves of the occlusal surface. These fissures are anatomically deep and difficult to clean, allowing food particles to accumulate and creating a conducive environment for bacterial proliferation. This leads to localized acid production and enamel demineralization. Unlike smooth surface caries, the caries initiation here is primarily due to food retention rather than direct bacterial invasion through enamel cracks. Therefore, pit and fissure areas require special preventive measures such as sealants and meticulous oral hygiene to minimize debris retention.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.