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Alginate Impression and Diagnostic Study Model Techniques (AGD 250)

Presenter: Ellen Gambardella, CDA, RDA, M.Ed., FADAA and Rita J. Johnson, COA, RDH, MA Bus


Accumulation of dental data provides the foundation for comprehensive dental care. Alginate impressions and study models have been used in dentistry for years, primarily to aid in diagnosis and treatment planning. Alginates are elastometric by nature and offer duplicating properties necessary for obtaining an accurate impression. These impressions are poured up in gypsum (plaster and stone) to obtain models or casts. Study models are frequently used for presentation purposes and as a visual aid for patient education. These models provide excellent legal documentation and serve as one component of the permanent record of a patient’s oral condition. Study models also provide occlusal representation to identify Angle’s classification of (mal)occlusion, which defines the relationship of the maxillary and mandibular teeth in the sagittal plane. Additionally, they provide recognition of wear patterns, missing teeth, tooth malignment, arch width, arch length, drifting teeth, actual size reproductions of anatomical structures including tooth size, shape, positions, gingival margins, interdental papillae, and the freni. Alginate impressions and study models are a valuable adjunct in providing optimal patient care.


Ellen Gambardella, CDA, RDA, M.Ed., FADAA  In a nationwide search, Ellen was selected as the “Most Effective Dental Assistant Educator in the United States”. The Massachusetts Dental Society presented her with the “Special Award for Teacher of the Year” and the “Volunteer Auxiliary of the Year” award. She is the recipient of the Goldin Foundation Award for Excellence in Education and the Dr. LeClaire Dental Health Professional Award for innovative and impassioned teaching. The United States Air Force recognized Ellen for her educational support of the 66th Dental Flight and Medical Group. She has been appointed by the Massachusetts Department of Education to develop Dental Assisting teacher licensing examinations. A former faculty member of Tufts and Northeastern Universities, Ellen has lectured nationally and internationally, is an author, and has vast experience in the area of accelerated learning. She has been featured in The Dental Assistant Journal on “Maximizing Efficiency in the Dental Office” and was awarded the title of “Fellow” from the American Dental Assistants Association. DANB has enrolled Ellen in the Dr. Samuel Harris National Museum of Dentistry Commemoration Program where an engraved brick with her name is displayed for her outstanding commitment to the profession.

Rita Johnson, COA, RDH, MA Bus, is a former treatment coordinator in a private orthodontic practice and a former professor at Middlesex Community College, where she received the Outstanding Faculty Recognition Award. Rita is also the recipient of the “Volunteer Contributions for Auxiliaries” award, given by the Massachusetts Dental Society. Rita worked at Brontes Technologies, where she was involved in the research and development of digital orthodontics. She is currently employed by 3M Oral Care Solutions Division, where she serves as Digital Practice Specialist. Rita has written RDH curriculum, authored articles, patented orthodontic products, and has vast experience in the area of practice management.

The editor wishes to thank Michael Durda of Dux Dental for his countless hours editing portions of this manuscript and supplying photographs.


Upon completion of this course, the dental professional should be able to:

• State at least four reasons for obtaining diagnostic study models.
• Compare and contrast what is meant by a positive reproduction versus negative reproduction of the oral cavity.
• Describe the properties of an irreversible hydrocolloid.
• List six characteristics desired for an ideal alginate impression material.
• Define working time and setting time as these terms relate to working with alginate and gypsum products.
• Describe the effects of humidity on the spatulation and setting time of alginates and gypsum materials.
• Identify at least three reasons for the operator to wear Personal Protective Equipment (PPE) while working with alginate and gypsum products. 
• Explain the mechanisms for disinfecting alginate impressions, including the recommended types of disinfectants and time required.
• Explain the purpose of chromatic agents in the powder of some alginates.
• List all supplies needed for obtaining an alginate impression.
• Explain the rationale for instructing the patient to rinse with a mouthwash prior to taking the alginate impression.
• Identify the different varieties of impression trays available for use in dentistry.
• Discuss the guidelines used in impression tray selection.
• Describe the purpose of adhesives prior to loading the impression tray with alginate.
• Identify three reasons for using utility/beading wax on the periphery of impression trays.
• Describe the rationale for obtaining the mandibular impression prior to the maxillary impression.
• Identify three mechanisms available for mixing alginate material.
• Comment on the purpose for “fluffing” the alginate powder prior to use.
• Differentiate between a smooth vs. a grainy mix of alginate material and what precautions should be taken for insuring a homogeneous consistency.
• Describe how to load a mandibular impression tray with alginate.
• Describe how to load a maxillary impression tray with alginate.
• Explain how to seat a mandibular impression including the patient and operator positions.
• Explain how to seat a maxillary impression including the patient and operator positions.
• Describe the appropriate mandibular impression removal technique.
• Describe the appropriate maxillary impression removal technique.
• Comment on what is meant by the impression inspection.
• Elaborate on the protocol for impression storage.
• Explain the purpose of a bite registration.
• List three means of obtaining a bite registration.
• State six desirable characteristics required of gypsum products.
• Define accelerators and retarders as applicable to gypsum products.
• Name four examples of accelerators.
• Name four examples of retarders.
• List all supplies needed in order to pour diagnostic study models.
• Describe what is meant by a W/P ratio of 50:100.
• Explain the rationale for gradually sifting the plaster or stone powder into the water in the mixing bowl.
• Elaborate on the rationale for using the vibrator on the mixed gypsum product.
• Describe the technique for filling the teeth in the impression with plaster.
• Differentiate between the anatomical portion and the base portion of the study model.
• Identify and describe four methods of forming a study model base.
• Cite the precise W/P ratio for the anatomical portion verses the base portion of the study model and explain the rationale for the difference.
• Describe the separation procedure that should be followed to remove a cast from the impression and detail the necessary precautions that should be taken.
• State the purposes for trimming the study models.
• Explain the rationale for wearing PPE while using the model trimmer.
• Describe the trimming procedure for the maxillary model including the appropriate angles engineered for the Tweed vs. the Ricketts technique.
• Elaborate on the trimming procedure for the mandibular cast including appropriate angulation.
• Explain why we simultaneously cut the “heels” of the mandibular and maxillary casts.
• Describe the sanding technique used for finishing diagnostic study models.
• Differentiate between two acceptable polishing techniques for finished casts.
• Identify two means of labeling diagnostic casts.•Discuss the proper storage of study models.


The ADAA has an obligation to disseminate knowledge in the field of dentistry.  Sponsorship of a continuing education program by the ADAA does not necessarily imply endorsement of a particular philosophy, product or technique.

The ADAA cautions participants taking this course on the hazards of using limited knowledge when integrating new techniques into their practices.

Credits earned upon completion of the course may be used to meet DANB’s Recertification Requirements.


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