The most frequent cause of ulcers and abrasions beneath denture bases is improper occlusion. When dentures do not have correct occlusal relationships, uneven pressure is exerted on the mucosa during mastication. This leads to repeated trauma and irritation, resulting in ulcerations and abrasions. Proper occlusal adjustment ensures even distribution of masticatory forces, preventing localized soreness and tissue damage. Other causes like flange overextension or allergies are less common and typically present differently. Thus, meticulous occlusal balancing is critical for denture comfort and tissue health.

Reference: Prosthodontics, S.C. Rangarajan, 3rd Edition.