Chapter 28: Fabrication and Trimming of Study Models

DHY 204

Chapter 28: Fabrication and Trimming of Study Models

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Step-By-Step Checklist for Trimming Models

1.) The Maxillary Base

• To make the maxillary arch parallel to the bench top, do one of the following after you have rocked the arch forward onto the anterior teeth

• Use a pencil and ruler to mark the model in four places at an equal height around the periphery

• Use a pencil between the pages of a book to the required height. Turn the model to mark it in at least four places

• The base should be approximately one-third of the total model height in the anterior region

• Trim the base of the maxillary model

• Place on grinding wheel and trim the base parallel to the pencil marks. The trimming goal is one flat surface. Rinse clean. Check periodically by laying trimmed surface on counter and attempt to "rock" the base from side to side. It should not have any bevels or exhibit a rocking motion

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2.) Maxillary Posterior Border

• Put both arches in occlusion and mark with a pencil the location on the maxillary cast of the most posterior anatomy that is present on either cast (so you know "how far in" to grind)
• Also mark the median raphe (midline of palate) with a pencil
• Now trim the maxillary model at a right angle to the penciled midline. Grind to the marked area identifying the most posterior anatomy

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3.) Mandibular Posterior Border

• Place the base of the maxillary model on the stage of the model trimmer. Occlude the mandibular model with the maxillary (mandibular model will be on top)
• Trim the back of the mandibular model until it is even with the back of the maxillary model, but do not trim away any anatomy
• Both posterior sides (backs) should be flat, and the occluded models should not open when set "on end" (on posterior sides)

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4.) Mandibular Base

• Keep both models in occlusion with the backs on the trimmer stage. Trim the base of the mandibular model so that it is parallel with the maxillary model

• The combined height should be 2 to 2½ inches

• The bases of the models and the occlusal plane should be parallel to the bench top

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5.) Lateral Borders

• Trim the sides of the widest cast first from canine to the last molar
• Trim parallel to the central grooves
• Next, put teeth in occlusion and trim the other arch to match

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6.) Anterior Borders

• Maxillary Cast

• Mark the canine eminence and the midline on the base portion of the model with a pencil

• Trim the anterior portion from the canine eminence to anterior midline on both sides

• These cuts will form a point with each other and with each lateral border

• Mandibular Cast

• Trim the anterior portion from canine to canine following the form of the arch

• It will be rounded in appearance

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7.) Posterior Angles

• Place the teeth in occlusion and trim the posterior angles (heels) of both casts to be even and symmetrical with the lateral and posterior sides of the casts

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8.) Finishing the Casts

• Trim and smooth the tongue area of the mandibular cast with a lab knife or large acrylic bur and lab handpiece

• Trim the excess “lip” (land area) of lateral and posterior borders of both casts

• Do not remove the mucobuccal fold unless instructed to do so

• Fill in voids with a putty of matching gypsum product in the base portion of casts

•  Use waterproof sand paper to smooth the base; do not round or sand the edges

• Remove any remaining nodules or blebs of gypsum from the cast with a lab knife

• Check casts for symmetry

• Label both casts with your name (for laboratory exercises) or the patient's name (for the clinical setting) and the date

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Tips for the Clinician- Trimming Models

• Make sure that adequate water is flowing through th model trimmer to prevent clogging
• Rinse the trimming debris from the model immediately after using the model trimmer
• Take care not to trim away teeth and other oral anatomy

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Precautions- Trimming Models

• Rest the pal of the hand on the trimmer’s table
• Keep fingers “tucked in” and away from the rotating wheel
• Do not use excessive force

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Summary- Pouring an Impression

• 1.) Mix the gypsum material
• 2.) Fill the impressions of the teeth
• 3.) Fill the rest of the impression
• 4.) Construct a base
• 5.) Wait 30-45 minutes for the gypsum material to set
• 6.) Separate the model from the impression by lifting the impression upward (do not use a tipping motion

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Summary- Trimming Models

• 1.) Trim the maxillary base parallel to the plane of occlusion
• 2.) Trim the posterior of the maxillary model perpendicular to its base
• 3.) Trim the posterior of the mandibular model even with that of the maxillary model
• 4.) Trim the base of the mandibular model perpendicular to the backs of both models
• 5.) Trim the sides parallel to the central grooves of the posterior teeth with the models in occlusion
• 6.) Trim the anterior of the models
• 7.) Trim the heels of the models while they are in occlusion
• 8.) Remove bless of excess stone or plaster
• 9.) Fill in pores, and sand smooth

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