The minimum thickness of dentin required below a cavity preparation is 2 mm to ensure adequate strength and support for the remaining tooth structure. This thickness prevents fracture and maintains the integrity of the tooth under masticatory forces. Less than 2 mm of dentin can compromise the tooth’s resistance and increase the risk of pulp exposure or postoperative sensitivity. Maintaining this minimum thickness is crucial for the long-term success of restorative procedures and preservation of tooth vitality.

Reference: Principles and Practice of Operative Dentistry, 5th Edition, James B. Summit.