WEEK 5: Impression taking and Mouthguards: Comprehensive Notes

Impression Fundamentals

• Definition of an impression: an anatomical model; a negative mold of the jaw used to create a replica of the patient’s dentition.
• Primary material: alginate. Reasons for use: cost-effective, easy to use, generally accurate for study models.
• Types of models after taking an impression:
  • Pour the impression with a stone cast to produce a study model (replica of teeth and supporting tissues).
  • A bite registration is taken after the study model to ensure appropriate occlusion (occlusion analysis, malocclusion assessment, and forces in occlusion).
  • Study models are used for planning and explaining treatment (especially by aesthetic dentists or prosthodontists).
  • Three-dimensional scanning is increasingly used to create digital study models that replace or augment physical models.
• Practical use of study models:
  • Used for treatment planning, rationale development, and showing before/after results.
  • External labs or in-house labs may fabricate restorations (onlays, crowns, dentures) based on study models.
  • Trays and study models are also used to fabricate bleaching trays, mouth guards, splints, etc.
• Materials and alternatives:
  • Alginate is the focus for study casts; cheaper and easier, but other materials (resin-based alginates or silicone-based materials) may be used when high precision is required (e.g., crowns, bridges) to minimize microporosities and achieve high accuracy.
  • Polyether is mentioned as used for fixed partial dentures in general knowledge.
  • Provisional use of denture materials by prosthodontists.
• Material origin and properties:
  • Alginate is generally extracted from seaweed; supplied as a powder.
  • Contains sodium and potassium salts of alginic acid; soluble in water; requires a water-to-powder ratio specified by the manufacturer.
  • Alginate impressions are elastic but tear easily; they do not bond to standard stock trays; may require tray adhesive spray.
• Trays and tray considerations:
  • Trays come in plastic (disposable) or metal (reusable) options.
  • A tray needs a gap of at least 3 mm between its edge and the oral tissues to avoid distortion and ensure proper capture of tissues; soft tissue markers should be present and not overly snug.
  • Upper trays typically have a palate area; lower trays are often a horseshoe shape.
  • If an upper tray cannot fit due to anatomy, a lower tray might be used on top in some cases; never place an upper tray on a lower arch.
  • Perforated trays help retain impression material; do not pull the impression material out of perforations during removal.
• Tray preparation and customization:
  • Some trays require trimming for patient comfort or to fit anatomy; carbide burs can be used on plastic trays to adjust the tray.
  • Wax build-ups can be used to increase rim height for comfort or better fit.
  • Resin sprays may be used on plastic trays to improve retention; metal trays rely on perforations for retention.
• Use of equipment and setup:
  • Armamentarium includes mixing bowl, spatula (plastic, resin, silicone, or metal), measuring cup, bite registration material (wax or gun dispensed material), and adhesive spray if required.
  • A leveled water-to-powder ratio is critical; manufacturer instructions should be followed precisely; if too thick or too runny, adjust by adding powder or water accordingly.
  • Mixing technique uses spatulation against the bowl; ensure an even, bubble-free texture.
• Setting reaction chemistry (alginate):
  • An elastic gel forms via a long-chain alginic acid reaction.
  • Sodium and sulfate interact to enable setting; sodium sulfate acts as a retarder (slows setting) to provide working time.
  • Different brands may have different setting profiles (fast-set vs regular-set) depending on the amount of retarder and other formulation differences.
  • Working time (WT) and intraoral setting time (IST) define the overall window; example profiles vary by brand and type; a common illustration includes:
  • WT \, \approx \; 1\,\text{min} \, 15\,\text{s} (mixing time)
  • IST \approx \; 1\,\text{min} \, 30\,\text{s} (time before impression becomes set in mouth)
  • Tot\; Time \approx \; 3\,\text{min} (total from start of mixing to complete set in mouth)
• Viscosity and fluidity:
  • Viscosity is influenced by water amount; follow manufacturer instructions for exact water amount.
  • The impression’s accuracy is affected by filler particles and polymeric macromolecules used in the alginate formulation.
• Dimensional characteristics and elasticity:
  • The set alginate has loosely bonded water between macromolecules; elasticity comes from cross-linked structure; setting is retarded by sodium sulfate.
• Disinfection and handling:
  • After impression is removed, disinfection is required due to saliva exposure (and occasional blood).
  • Common disinfection approach: immerse in an aqueous disinfectant or spray with an aqueous disinfectant for a short time; wrap in mildly soaked paper towel and seal in a labeled plastic bag; refrigerate promptly.
  • Depending on the clinic, impressions may be poured in-house or sent to external labs; follow manufacturer instructions for disinfection and storage.
  • Potential consequences of improper disinfection/storage: shrinkage or inaccurate models due to improper handling.
• Temperature and speed considerations:
  • Temperature of water affects working time: cold water used to extend working time; warm/hot water is typically not used for alginates.
• Powder-to-water ratio considerations:
  • Too much powder yields thick mix; too much water yields runny mix and longer setting time; adjust gradually by following manufacturer instructions.
  • As experience increases, you’ll develop intuition for adjusting consistency by adding powder or water.
• Flavor and brand variations:
  • Some brands offer flavor options (e.g., peppermint); consider patient comfort and safety (kids and peppermint flavors may be problematic for certain ages).
• Impression evaluation after removal:
  • A good impression should show clear anatomic detail, no air bubbles, proper extension, and suction in the peripheral tissues.
  • Soft tissue markers (labial sulcus, frenum regions) and hard tissue markers should be visible; centering and proper orientation are essential for accurate pours.
• Impression orientation and anatomy references:
  • Upper anatomy: palate, palatine raphe, sulcus, labial sulcus, labial frenulum; for mouth guards, capturing the frenums and sulcus accurately is crucial.
  • Lower anatomy: labial frenum, buccal frenum, sulcus, mylohyoid ridge, retromolar pad; important for dentures and mouth guards to avoid tissue impingement.
• Intraoral procedure considerations:
  • Patient positioning: typically supine, stand behind the patient; avoid pulling the tray across the arch from front to back; ensure proper support of the tray during seating and removal.
  • Retraction: retract lips, rotate the tray into the mouth, seat posterior teeth first, then move to anterior regions.
  • Comfort and communication: maintain patient comfort, explain the procedure, and use reassurance techniques to reduce gag reflex; consider techniques: humming, breathing cues, salt on the tongue (clinical tricks for gag reflex management).
  • If gagging occurs or if the impression has not set, do not remove prematurely; the putty should be allowed to set to avoid a messy outcome.
• Retention and tray selection:
  • Test-fit and select a suitably sized tray before loading alginate; trays can be disposable or autoclavable.
  • Upper vs lower tray selection depends on anatomy and clinical need; ensure the tray is not too snug; provide space for the impression material.
• Practical tips for practice sessions:
  • Practice on peers to become proficient at tray handling, patient communication, and managing gag reflex.
  • Keep patient head stable; ensure a flat patient headrest when needed to prevent movement.
• Aftercare and workflow:
  • Once set, evaluate the impression for accuracy (availability of soft tissue markers, tooth detail, and absence of voids).
  • Decide whether to pour up in-house or send to a lab; pour-up involves using calcium sulfate materials (plaster or dental stone). The choice affects strength and trim-ability:
  • Plaster is easier to trim but weaker.
  • Stone is stronger and less prone to fracture; preferred for prosthodontic workflows or where durability is important.
  • Pouring up process overview:
  • Mix the stone/plaster with water in the recommended ratio; use a vibrating unit to reduce air bubbles; fill impression first with teeth, then bulk up to create a sturdy study model.
  • Create a mound around the impression to form the base; invert the impression onto the mound; allow to set; trim and finish the model; insert bite registration if needed.
• Model finishing and storage:
  • After setting, gently remove the impression tray; trim bases with appropriate tools; use bite registration to align upper and lower models.
  • Label both models with patient ID and name; store properly to avoid mix-ups.
  • Model trimming uses a rotating wheel with water lubrication; trim to create flat bases and appropriate angles (typically 45 degrees for maxillary base; mandibular base more curved).
  • Practical trimming differences:
  • Maxillary bases: sharper angles; anterior region; 45-degree posterior overlays.
  • Mandibular bases: more curved; 45-degree angle at the base.
• Practical summary points:
  • Follow manufacturer instructions for all products; measure and mix precisely; ensure proper seating, setting, and removal technique; always verify accuracy before using the model for treatment planning.
  • Understand the workflow from impression to study model to bite registration and occlusal analysis; plan for disinfection, labeling, and storage.

Materials Spotlight: Alginate in Depth

• Alginate basics:
  • Alginate is typically supplied as a powder; water-to-powder ratio must be per manufacturer.
  • Uses include study casts and duplicating models; for high-precision restorations (inlays, crowns, bridges), alginate alone is often insufficient due to potential porosities.
• Setting reaction and working time:
  • Reaction forms an elastic gel from a cross-linked alginate network; retarder (e.g., Na2SO4) slows setting; faster-set formulas exist with different retarder behavior.
  • Working time depends on water temperature and brand; cold water typically extends working time; hot water is not standard practice for alginate.
• Mechanical and dimensional properties:
  • Elastic, tear-prone; not ideal for very precise work due to potential distortion or tearing during removal.
  • Dimensional stability depends on proper mixing, pouring, disinfection, and storage.
• Practical handling tips:
  • Fluff or shake the alginate powder before dispensing; use leveled scoops to ensure consistent measurements.
  • Use a measuring cup for accurate water volume; keep mixing bowl dry.
  • Maintain proper isolation and tray selection for comfort and accuracy.
• Storage and disinfection specifics:
  • Follow brand-specific disinfection and storage instructions; refrigerate after disinfection when appropriate; pour or ship to lab within required timeframes.

Bite Registration and Occlusion

• Bite registration purpose:
  • To capture occlusal relationships and ensure proper alignment of the upper and lower arches in the final study models.
  • Typically done with wax (a wax bite block) pressed between the teeth; patient bites down to produce a record of their occlusion.
• Practical tips:
  • Remind patients, especially children, to bite down properly; physically assist by placing fingers on the inner cheeks to guide occlusion; have them practice a few bites before recording.
• Occlusal considerations for device fabrication:
  • For mouth guards, ensuring correct occlusion helps seating and comfort; for dentures or splints, occlusion must be recorded accurately for proper fitting.
  • When labs fabricate units (e.g., mouth guards or dentures) from the study models, precise occlusion data ensures proper fit and function.

Disinfection, Labeling, and Storage Practices

• Disinfection workflow:
  • After impressions, disinfect according to product instructions (immersion or spray); rinse as required; air dry or use absorbent materials.
  • Label each bag with patient information; avoid cross-contamination.
  • Store in a fridge if needed; determine whether to pour in-house or send to an external lab.
• Transportation and handling:
  • Refrigeration and timely pouring or shipping are important to maintain dimensional stability.

From Impression to Model: Pouring Up and Model Finishing

• Pouring materials:
  • Calcium sulfate-based materials (stone or plaster): plaster is easier to trim; stone is stronger and less prone to fracture, making it suitable for prosthodontic planning or long-term storage.
• Pouring technique:
  • Place material into the impression in stages; initially fill the teeth impressions, then gradually fill around them.
  • Use a vibrating table to minimize air bubbles; ensure the material flows into all interproximal spaces and anatomy.
  • Build a mound around the impression to form the base; invert the impression onto the mound and allow to set.
  • After set, remove the impression tray gently; trim the base and rough edges; insert bite registration if needed.
• Model trimming:
  • Use a model trimmer with water lubrication; avoid bare fingers in contact with the wheel.
  • Trim bases to create stable, flat surfaces for storage and comparison.
  • 45-degree angular cuts are typical for upper and lower bases; uppers tend to have sharper posterior angles; lowers are more rounded.
• Labeling and storage:
  • Always label upper and lower models with patient ID and name; store properly to prevent mix-ups.
  • Digital storage and 3D scans are increasingly used to maintain patient records without physical storage burdens.

Practical Considerations for Clinical Practice

• Standing and positioning:
  • Clinicians typically stand behind the patient; the patient is positioned with a headrest to minimize movement.
  • For upper impressions, ensure a consistent insertion path; for lower impressions, ensure stable seating and avoid forcing the tray.
• Communication and patient comfort:
  • Reassure patients during the procedure; use distraction or breathing techniques to minimize gag reflex.
  • Explain the sensation and set time; discuss aftercare and what to expect during and after the impression.
• Common pitfalls and troubleshooting:
  • Air bubbles, voids, or tears indicate an inadequate impression; retake if necessary.
  • If the tray is too small or too large, a poor impression results; consider alternative trays (including using a lower tray for an upper impression when necessary).
• Summary of workflow:
  • Prepare patient and explain procedure.
  • Select tray size; apply adhesive if needed.
  • Mix alginate per manufacturer instructions; record working time and intraoral setting time.
  • Load tray with alginate; seat and wait for set.
  • Remove carefully; evaluate impression; disinfect and label.
  • Pour up or send to lab; create study models; trim and finish.
  • Capture bite registration and ensure proper occlusion; store models or scan digitally.

Mouth Guards: Why and When

• Purpose:
  • Prevent sports-related dental injuries; protect hard and soft tissues; educate patients about prevention.
  • Mouth guards distribute and absorb forces; reduce risk of fractures, avulsions, soft tissue injuries, and TMJ damage.
• Real-world relevance and ethics:
  • Evidence shows mouth guards reduce injury risk but do not guarantee prevention; emphasize proper fit, care, and compliance.
  • Clinicians must balance best practice with patient-specific factors (age, dentition, growth, finances).

Mouth Guards: Types and Indications

• Stock mouth guards:
  • Over-the-counter, one-size-fits-all; advantages: low cost, immediate availability.
  • Disadvantages: poor fit, reduced retention, potential breathing/speech difficulties, less protection, not ideal for most patients.
• Boil-and-bite mouth guards:
  • Thermoplastic; heated to soften and molded at home (or in clinic with guidance).
  • Advantages: more adaptable fit than stock; relatively affordable.
  • Disadvantages: variable fit, may still slip; depends on patient technique and supervision.
• Custom-made mouth guards:
  • Gold standard in protection, retention, comfort, and durability.
  • Fabricated from an accurate impression and dedicated lab fabrication; often single-arch (maxillary) but can be designed for mandibular use in special cases.
  • Advantages: superior retention, comfort, speech and breathing, protection, longevity, better shock absorption.
  • Disadvantages: higher cost and longer lead time; requires impressions and lab involvement.
• Thickness considerations:
  • Standard recommended thickness around t \approx 4\text{ mm}; thickness may vary based on sport, risk, and age.
  • Multilayer designs or thicker sections may be used for higher-risk activities or professional athletes.
• Materials:
  • EVA (ethylene-vinyl acetate) is common for custom guards; multiple layers may be bonded with heat and pressure to increase strength.
  • Vacuum-formed or heat-pressed laminate processes are typical for custom guards; designs can include multiple layers and colors.
• Design considerations:
  • Upper arch is traditionally favored for better fit and protection; lower-arch guards are less common.
  • Custom fits should be evaluated for retention, comfort, speech, breathing, odor, and taste; ensure not to impede respiration or confidence in wearing.
  • Mouth guards must be durable, easy to clean, and non-irritating to tissues.
• Sports-specific and patient-specific considerations:
  • Ortho patients may require mouth guards that accommodate orthodontic appliances or emerging dentition.
  • Mixed dentition or erupting teeth may call for bite-and-bite or staged approaches.
  • Growth and eruption patterns must be considered when planning long-term guard use.

Fabrication and Lab Considerations for Custom Mouth Guards

• Materials and layering:
  • EVA primary material; thickness and layering may vary by lab and patient needs; multilayer guards may use softer inner layers with harder outer layers for impact resistance.
  • Colors and designs are available; for kids, color and team logos can be motivating.
• Fabrication methods:
  • Vacuum forming: forms a single-piece guard over a model; cost-effective and common.
  • Heat-press lamination: used for multi-layered guards with higher strength.
  • Advanced labs may employ specialized equipment to create segmented or multi-layer guards with optimized thickness in incisal edges and posterior regions.
• Clinical workflow for custom guards:
  • Obtain accurate impressions (as described above) to create a precise model.
  • Send to lab with patient details and sport requirements; discuss layer composition, thickness, color, and anticipated wear.
  • Lab returns the finished guard; verify fit, comfort, and occlusion; check for retention and any tissue irritation; adjust as needed.
• Practical notes:
  • Minor adjustments may be needed to seating or trimming for comfort; ensure guard does not impinge on tongue space or block breathing.
  • Parents/patients should be instructed on cleaning and maintenance; regular checkups are recommended, especially during growth spurts or orthodontic changes.

Sports-Specific Evidence and Best Practices

• Rugby study and coaches:
  • Amateur rugby study: higher dental trauma when mouth guards were not worn.
  • 50% wore mouth guards during training; 93% of coaches encouraged use; 75% recommended custom-made guards; 68.2% of players wore mouth guards during training or play (2016 data).
• Implications for practice:
  • Emphasize prevention with patients and families; tailor recommendations by sport, age, and dentition status.
  • Engage coaches and families to improve compliance with mouth guard use.
  • Reinforce that mouth guards reduce trauma but do not guarantee prevention; discuss proper use and care.
• Sport-specific considerations:
  • Football, hockey, rugby often require high-level protection; mouth guards should be robust and well-fitted.
  • Netball and other non-contact or lower-risk sports may still benefit from protection, depending on head contact risk.

Maintenance, Follow-Up, and Education

• Maintenance of mouth guards:
  • Regular cleaning and inspection for wear; check for fit at recall visits, particularly during orthodontic treatment or growth.
  • If discomfort, misfit, or wear occurs, arrange replacement or adjustment.
• Patient education and documentation:
  • Provide instructions for care, cleaning, and wear guidelines.
  • Document discussions about sport risk, mouth guard selection, and follow-up plans.
  • Include notes about growth, dentition changes, and orthodontic status in the plan.

Practical Ethical and Practical Considerations

• Communication:
  • Transparent discussion about benefits, limitations, and cost of mouth guards; avoid promising 100% prevention of injuries.
  • Involve patient and guardians in decision-making about material, design, and maintenance.
• Safety and hygiene:
  • Always follow safety and infection control protocols for impression taking and guard fabrication.
• Future trends:
  • Emphasis on digital impressions and 3D scanning for faster, accurate guard fabrication;
  • Digital storage of impressions and model data improves accessibility and reduces physical storage needs.

Quick Reference: Key Numbers and Facts

• Gap required for tray edge clearance: at least 3\text{ mm} between tray edge and oral structures.
• Mouth guard thickness: commonly t \approx 4\text{ mm}; thickness may be adjusted for risk and sport.
• Common working and setting times (brand-dependent):
  • Example profile: WT \approx 1\,\text{min} \, 15\,\text{s},\; IST \approx 1\,\text{min} \, 30\,\text{s},\; Tot \approx 3\,\text{min}
• Rugby-related efficacy data (2016):
  • % wearing mouth guards during training: 50\%
  • % coaches encouraging use: 93\%
  • % coaches recommending custom-made guards: 75\%
  • % players wearing guards during training or play: 68.2\%

End of Notes

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