Award Date


Degree Type


Degree Name

Master of Science in Oral Biology



First Committee Member

Ronald Lemon, Chair

Second Committee Member

Katherine Howard

Third Committee Member

Bob Martin

Fourth Committee Member

Karl Kingsley

Graduate Faculty Representative

Patricia Cruz Perez

Number of Pages



Retention is required in the majority of orthodontic patients throughout the remainder of their life. The two primary removable appliances are known as the traditional Hawley retainer or the vacuum formed retainer. These appliances were developed to maintain the position of the dentition without sacrificing oral health. The orthodontic population is at a higher risk for caries due to plaque accumulation from poor diet, suboptimal oral hygiene and often lack of motivation. These two retainers occupy different niches and are comprised of different materials; therefore the retainers' effect on oral health could be very different. An understanding of which bacteria and to what extent the bacteria adhere to these two retention appliance materials could ultimately provide clinicians with another factor to consider when choosing a specific retainer.

Two common caries bacteria, Streptococcus mutans and Lactobacillus acidophilus , were chosen to study their adherence properties on two common retention materials; polymethyl methacrylate in the traditional Hawley retainer and a thermoplastic polymer made of polypropene in the vacuum formed retainer. Bacterial adhesion tests on both materials were run either with or without prior coating in saliva and the number of adhered bacteria was determined by both directly counting colony-forming units of bacteria swabbed from the materials and by inference from total metabolic activity of the adhered bacteria as determined by incubation with the tetrazolium dye,sodium2,3,-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)-carbonyl]-2H-tetrazolium inner salt)(XTT) reagent.

Culture analysis from adhesion testing determined through colony forming units, showed an increased adherence of both bacteria to polymethyl methacrylate compared to Polypropene. This was reflected in a 3-fold increase for Lactobacillus acidophilus and 7-fold increase for Streptococcus mutans on the polymethyl methacrylate. Bacterial adhesion testing performed using the metabolic XTT proliferation assay also demonstrated increased adhesion on polymethyl methacrylate. Bacterial adhesion to polypropene was decreased by 30% for Lactobacillus acidophilus and 27% for Streptococcus mutans compared to polymethyl methacrylate. XTT assay also indicated that prior coating of materials to saliva had little effect on the extent of bacterial adhesion.

In conclusion, bacterial adherence is increased for polymethyl methacrylate when compared to polypropene, regardless of the assay technique used to determine the number of adhered bacteria. Further research needs to be conducted to determine if increased adherence to polymethyl methacrylate is significant enough to influence choices for orthodontic retention.


Bacterial adherence; Biological sciences; Health and environmental sciences; Lactobacillus acidophilus; Mouth — Microbiology; Orthodontic appliances; Removable; Orthodontics; Corrective; Polymethylmethacrylate; Retention; Streptococcus mutans; Teeth — Care and hygiene


Dental Materials | Oral Biology and Oral Pathology | Orthodontics and Orthodontology

File Format


Degree Grantor

University of Nevada, Las Vegas




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