Soft Denture Lining Materials

Introduction

Soft denture lining material is defined as a polymeric material that is placed on the tissue-contacting surface of a denture base to absorb some of the mastication impact energy by acting as a type of “shock absorber” between the occlusal surfaces of a denture and the underlying oral tissues. A denture soft liner also may be used to engage natural or prosthetic undercuts so as to provide retention, stability and support. As masticatory forces are directly transmitted to the underlying tissues through the denture, the residual ridge becomes burdened with increasing years of edentulism, diminishing the shock absorbing capacity of the mucosa. A soft liner is suggested to compensate for the lost thickness and function of the mucosa.1,2

They can cause debonding of softliner from denture base. Factors which can affect the bond strength between lining material and acrylic resin include geometry of bond surface whether it is surface pretreatment of denture base resin, thickness of the lining material, water absorption, use of bonding agents and denture base composition.5

Lammie and Storer classified the processed resilient materials as follows.

1. natural rubber, 2. polyvinylchloride, 3. polyvinylacetate, 4. methyl methacrylate copolymer, and 5. silicone.6

Silicone-based soft denture liners have little or no chemical adhesion to denture base resin so, an adhesive primer is used to aid bonding to the denture base resin. Bond strength can be altered by various methods, namely mechanical roughening, chemical treatments and mechano-chemical treatment. Adequate literature is available regarding mechanical surface roughening of denture bases. But, there is paucity of information about the chemical treatment particularly acid etching.16, 17 The purpose of present study was to evaluate the effect of mechanical and chemical surface pre treatments on the bond strength of heat polymerized and auto polymerized silicone based soft liners to PMMA and to assess the effect of saliva on this

These results agree with those of other investigators,43,56,63 who suggested that water storage reduced resilient liner bond strength. The lower bond strength may result from the swelling and stress formation at the bond interface, or from a change in the viscoelastic properties of the liner, rendering the material stiffer and better able to transmit external loads to the bond site. Water sorption by lining material leads to change in dimension and stress concentration at liner- denture base interface reducing the bond strength.69Filler type and its bonding to the polymer is responsible for water absorption seen in auto polymerized silicone. Heat cured silicone has better bond to filler and greater cross linking leading to the production of a denser material which are devoid of micro pockets of water within the material.70 The residual cross linking agent within the material also contributes to absorption of