Introduction

The ideal denture base material should possess several key physical attributes.  Some of these properties include biocompatibility, good esthetics, high bond strength with available denture teeth, radiopacity, ease of repair, and should possess adequate physical and mechanical properties.¹  The denture base must be strong enough to allow the prosthesis to withstand functional and parafunctional masticatory forces.  In addition, because these prostheses are removable, shock induced fracture resistance, possibly due to patient abuse, is desirable.

Many different materials have been used for denture bases.  Historically materials such as bone, wood, ivory, and vulcanized rubber were utilized; now poly methyl methacrylate (PMMA) is used.^2-6  New materials such as polystyrene and light-activated urethane dimethacrylate have been developed, but PMMA remains the preferred material for removable complete and partial prostheses.⁷  The popularity of PMMA materials is based on its low cost, relative ease of use, and reliance on simple processing equipment.  There are, however, significant differences in the chemistry among denture materials based on PMMA chemistry.  Some materials rely on high levels of crosslink resin and heat activated initiators to maximize the physical properties of the processed materials, i.e., Sledgehammer Maxipak and ProBase Hot.  Other formulations like Lucitone 199 and Fricke Hi-Impact employ a PMMA polymer modified by adding a rubber compound to improve shock resistance and improve strength properties.

Causes of denture fractures are more often related to design errors rather than problems with the resin itself.  Denture failures can occur in excessively thin areas or weakened flanges around frenal notches.⁸  Midline fractures of denture base resins are especially troublesome, leading some to recommend selectively increasing the bulk of material in regions subject to deformation and fractures.  These locations include the palatal incisal junction, the posterior palatal midline, and the mandibular incisal area adjacent to the lingual and labial frenal attachments.⁹  Increasing bulk, however, can lead to other problems.  A denture base that is too thick can cause gagging or dislodgement of the denture when the patient opens wide or yawns.  Excessive thickness in a maxillary denture can interfere with the coronoid process during movement of the mandible.  Bulk in the area lingual of the maxillary anterior teeth can cause speech problems such as a slushy “S” sound by flattening the median grove of the tongue.  While minimizing the thickness of the denture base can lead to better patient acceptance, it also increases the potential for fracture making the use of a stronger acrylic resin very important.

These factors have led manufacturers to develop higher strength denture base materials.^{10, 11}  Considering the recent recommendations of the McGill Consensus Statement¹² suggesting implant supported overdentures be the standard of care for mandibular edentulous patients and the concomitant increase in the use of overdentures, the use of strong resins is imperative.¹³  A variety of physical properties can be used to assess the strength of denture materials.  The most common tests are impact strength; the ability of a material to resist a sudden high level force or ‘shock;’ flexural strength, the force needed to deform the material to fracture or irreversible yield; and flexural modulus, a measure of the stiffness of a material.  In addition the distance a material specimen can be deformed (yield distance) before failure also is an indication of the toughness of a material.

Because of the risk of fracture should a patient drop their denture, high impact strength is a desirable property.  Given the function of a denture base in a removable prosthesis, high flexural strength, flexural modulus, and a large yield point distance would help resist torsional forces in function leading to a longer clinical service life for the prosthesis.

The purpose of this study was to determine the impact fracture strength, flexural strength, flexural modulus, and the yield distance of high impact and conventional denture base resins.

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Citation Number: Vol. 6, No. 4, Page 094