Final answer:
Applying cavity varnish before cementing an onlay with resin cement reduces bond strength. Alternatives like calcium hydroxide or glass ionomer liners should be used to protect the pulp and reduce post-operative sensitivity. Resin cement with fluoride can reduce microleakage and strengthen the tooth.
Step-by-step explanation:
Prior to cementing an onlay in a vital tooth using resin cement, the application of cavity varnish is not recommended because it can reduce bond strength between the cement and the tooth structure. This is due to the layer it forms which can interfere with the adhesive bonding process that the resin cement requires. Instead, other measures should be taken to prevent post-operative sensitivity and protect the pulp, such as using liners like calcium hydroxide or glass ionomer. These liners help to provide a thermal barrier and release fluoride which can help in remineralization. Additionally, using a resin cement that contains fluoride can also help reduce microleavage by strengthening the tooth structure and preventing decay. It's important to ensure that the onlay is properly sealed to the tooth to prevent bacteria from entering and causing further decay or infection.