MMP Hybrid Layer Degradation — ORE Part 1 MCQ

Correct answer: E — MMP-2 and MMP-9 (gelatinases)

MMP-2 and MMP-9 (gelatinases A and B) are the predominant endogenous MMPs in dentin and the primary drivers of hybrid layer collagen degradation in composite restorations. Immunohistochemical and zymographic studies confirm these enzymes are intrinsic to the dentin organic matrix. Upon acid demineralisation (which accompanies etch-and-rinse adhesive placement), low pH activates latent MMP-2 and MMP-9, which then degrade denatured (gelatin-like) collagen in the hybrid layer over time. Selective gelatinase inhibition reduces collagen breakdown by up to 90%, demonstrating their critical role. Distractor B (MMP-1, interstitial collagenase) and D (MMP-13, collagenase-3) have minimal activity in dentin and are less implicated in hybrid layer degradation. Distractor C (MMP-7/matrilysin) has poor collagenolytic activity. Distractor E (TIMP-1) is a *tissue inhibitor* of MMPs, not a metalloproteinase, and acts to protect the hybrid layer—the opposite of the question premise.

Reference: Frassetto A, Breschi L, Turco G, et al. Mechanisms of degradation of the hybrid layer in adhesive dentistry and therapeutic agents to improve bond durability. J Dent Res. 2016;95(5):497–506. PubMed: 23953737 (Tjäderhane L, et al. Strategies to prevent hydrolytic degradation of the hybrid layer—a review. J Dent Res. 2013;92(2):129–135.) https://pubmed.ncbi.nlm.nih.gov/18335530/ (MMP-2/-9 immunolocalisation); https://pubmed.ncbi.nlm.nih.gov/19410083/ (zymographic confirmation in radicular dentin).