Why are liners and flowable composites often placed under composites?

Not all fillings are shallow and some are placed in teeth that have extensive and "deep" decay. Often when cavities are deep dentists must decide how much of a tooth needs to be cleaned out in order to achieve a successful restoration. Its easy with a relatively small cavity where the caries is not close to the nerve (more than one mm away). Most dentists will remove all softened tooth structure until sound unaffected dentin is reached. The situation is more complicated the closer the dentist is forced to go as he or she approaches the pulp. This is because the pulp doesn't do so well when microscopic or macroscopic "exposures" occur. Pulps, tissues inside teeth composed primarily of connective tissue, blood supply and nervous tissues, will sometimes die (become non vital because they loose their blood supply). When this happens a tooth will need a root canal.

Most dentists will try to avoid exposing the pulp if possible when cleaning out a cavity and when I believe I am close , I will more likely leave "affected dentin " present. This dentin visually looks different from normal dentin which is harder and orange colored. Instead it can present as a darker brown or black and is softer when scratched with a spoon excavator or an explorer.

Now dentists normally do not use a microscope to verify whether tooth structure that is left has bacteria  present in it or not, but instead must use "clinical judgement" in determining when to stop removing this affected dentin. It should not be soft, but it can be somewhat "tacky" when scraped by a spoon excavator. Most dentists will use Dycal or Vitremer liner ( or some other related products) to cover this affected dentin if it will remain under a filling. These "pulp capping and liner materials tend to seal and dry out this softer dentin and encourage the formation of secondary dentin to form. Also, any bacteria that remain embedded in the remaining tooth structure can become inactivated since they are not in communication with oral fluids and are effectively entombed beneath the liner or pulp capping media.

Flowable composites are also used as a type of liner since they can be dabbed onto the floor and some of the walls of the preparation after a primer has been used and set prior to the bulk fills of more sturdy composites. The flowables are applied so that a dentist can be certain that these areas have not voids and are really sealed well. Bulk composites that are added afterwards can shrink and sometimes pull away from the walls of the preparation and can create areas that are not properly sealed.  Also The flowable composite, if kept away from the margins of the preparation, can help ensure a better fitting filling and help ensure that the patient will not experience sensitivity afterwards. Also flowable composite can get into small irregularities in the internal preparation more easily than a packable hybrid composite.

Of course dentists as a rule all have their own favorite recipes for placing fillings and most feel they have developed techniques that work well in their hands. The use of Dycal, Vitremer and flowables can vary from dentist to dentist and as a rule most have good results with their restorative dentistry. When properly designed and executed, posterior and anterior composite restorations can be extremely durable and last for more than a decade( sometimes two or three!)




from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2018/02/why-are-liners-and-flowable-composites.html - http://lspindelnycdds.blogspot.com/