The smear layer is formed by the action of which on dentin?

• A. Ultrasonic vibrations
• B. Bur or hand instruments
• C. Laser
• D. Chemical etchants

Answer: (B)

Smear layer forms when dentin is cut or shaped by mechanical instrumentation, such as a dental bur or hand cutting instruments. The friction and scraping generate a thin film of debris—dentin shavings mixed with organic material like collagen, minerals, and sometimes salivary or bacterial remnants—that coats the surface and can plug the dentinal tubules. This layer influences how substances interact with dentin, affecting bonding and sealing in restorative or endodontic procedures. The other options don’t describe the normal formation process: chemical etchants remove or demineralize surfaces, lasers can ablate tissue without the same classic smear, and ultrasonic vibration can remove debris or alter surfaces but is not the standard cause of the smear layer.