12.3 Fixed and Removable Prosthodontics & Orthodontic Assisting
Key Takeaways
Tooth shade selection for fixed restorations must be conducted at the start of the appointment in natural or 5500K color-corrected lighting prior to preparation, because tooth desiccation rapidly turns enamel chalky and artificially opaque.
Gingival retraction cords impregnated with epinephrine are strictly contraindicated in patients with cardiovascular disease or severe hypertension, requiring the use of aluminum chloride or ferric sulfate astringent cords.
Provisional crowns must satisfy five clinical benchmarks: correct occlusal contact to stop supraeruption, snug proximal contacts to prevent drifting, polished contours, and margins seated within 0.5 mm of the finish line without impinging on the periodontium.
Removable partial dentures (RPD) distribute functional masticatory forces through major and minor connectors, clasps for retention, and rests seated on prepared rest seats to transfer loads along the long axis of abutment teeth.
Orthodontic assisting progresses from elastomeric separator placement to molar band cementation with fluoride-releasing glass ionomer, direct bracket bonding with acid etch, archwire ligation with Mathieu pliers, and retention appliance delivery.
12.3 Fixed and Removable Prosthodontics & Orthodontic Assisting
Prosthodontics and orthodontics focus on replacing missing teeth, reconstructing damaged dentition, and correcting malocclusion to restore oral function, masticatory efficiency, and facial aesthetics. The chairside dental assistant plays a vital role across these disciplines, coordinating delicate impression procedures, fabricating custom provisional restorations, handling laboratory transfer records, and manipulating specialized orthodontic wires and brackets.
Fixed Prosthodontics: Indirect Restorations & Chairside Protocols
Fixed prosthodontics involves permanently cemented indirect restorations that cannot be removed by the patient. These include:
- Full Veneer Crowns: Completely encircle the clinical crown of a severely broken-down tooth.
- Inlays: Intracoronal cast or ceramic restorations that restore occlusal and proximal surfaces without covering cusps.
- Onlays: Restorations that cover one or more cusps, preserving sound tooth structure while protecting weakened cuspal walls.
- Fixed Partial Dentures (Bridges): Replace one or more missing teeth. Terminology: the abutment is the natural tooth or implant anchor supporting the bridge; the pontic is the artificial suspended tooth replacing the missing tooth.
- Laminate Veneers: Thin ceramic or composite facings bonded to the facial surfaces of anterior teeth for aesthetic enhancement.
Tooth Shade Selection Protocol
Accurate shade matching requires adherence to physiological and environmental guidelines:
- Timing: Shade selection must always be performed at the very beginning of the appointment, prior to tooth preparation, local anesthesia, or rubber dam isolation. During cavity preparation, teeth undergo desiccation (dehydration) from continuous air streams and high-speed evacuation. Dehydrated enamel loses translucency and turns chalky white within minutes, leading to an artificially light shade selection.
- Lighting: Evaluate the shade under natural daylight or color-corrected operatory lighting (5500 Kelvin). Standard dental operatory overhead lights distort color perception.
- Visual Environment: Remove brightly colored lipstick, and cover vivid clothing with a neutral gray patient napkin.
- Cone Fatigue Prevention: Make shade comparisons rapidly in 5-second glances. Gazing continuously at teeth exhausts the retinal cone receptors; rest the eyes by glancing at a neutral gray or blue background between comparisons.
Tooth Preparation & Gingival Retraction Cord Placement
To prepare a tooth for a full crown, high-speed diamond burs reduce occlusal height (typically 1.5 to 2.0 mm) and axial walls, terminating in a precise cervical finish line (chamfer for metal/porcelain-fused-to-metal crowns, or shoulder for all-ceramic crowns).
To capture an accurate elastomeric master impression (polyvinyl siloxane [PVS] or polyether), the marginal gingiva must be physically deflected away from the cervical finish line using gingival retraction cord:
- Mechanical & Chemical Action: The cord mechanically pushes the free gingival tissue outward and apical to the preparation finish line. Cords are typically impregnated with chemical astringents (such as aluminum chloride or ferric sulfate) that constrict local capillaries, achieving hemostasis and arresting crevicular fluid seepage.
Caution
Cardiovascular Hazard of Epinephrine Retraction Cords: Retraction cords impregnated with racemic epinephrine are strictly contraindicated in patients with cardiovascular disease, hypertension, hyperthyroidism, or severe anxiety. Epinephrine is rapidly absorbed through lacerated sulcular capillaries into the systemic bloodstream, precipitating acute tachycardia, cardiac palpitations, arrhythmia, and dangerous hypertensive emergencies.
- Clinical Technique: The retraction cord is looped around the prepared tooth and gently tucked into the gingival sulcus using a blunt, smooth or serrated cord-packing instrument (cord packer) with light, continuous rolling pressure. The cord remains in the sulcus for 5 to 7 minutes. Immediately prior to injecting the light-body wash impression material, the cord is moistened with sterile water (preventing tearing of the delicate junctional epithelium) and gently removed.
Provisional (Temporary) Restorations
A prepared tooth must never be dismissed without a cemented provisional (temporary) restoration while the dental laboratory fabricates the permanent crown.
- Types of Provisionals:
- Custom Provisionals: Fabricated using a pre-operative alginate impression, silicone putty matrix, or vacuum-formed clear plastic stent loaded with auto-cure acrylic resin (polymethyl methacrylate - PMMA) or bis-acryl composite resin (e.g., Protemp, Integrity).
- Preformed Crowns: Polycarbonate or cellulose acetate crowns (aesthetic anterior teeth) or aluminum / stainless steel crowns (posterior teeth), trimmed and crimped at the margins.
- Five Mandatory Criteria for a Provisional Crown:
- Occlusal Contacts: Must maintain precise occlusal contact with opposing teeth to prevent the prepared or opposing tooth from supraerupting.
- Proximal Contacts: Must establish snug contacts with mesial and distal adjacent teeth to prevent drifting or tipping and stop food impaction.
- Marginal Fit: Must seat cleanly within 0.5 mm of the finish line without overhangs or subgingival impingement. Overhanging margins cause localized gingival inflammation, bleeding, and permanent gingival recession; underextended margins leave sensitive dentinal tubules exposed.
- Contour & Polish: Must replicate natural anatomical tooth contours and be polished to a high luster to prevent bacterial plaque accumulation.
- Pulpal Protection: Must seal the cut dentin against thermal, chemical, and oral bacterial insults.
- Provisional Cementation: Cemented using temporary cement with low compressive strength (such as Type I zinc oxide-eugenol [ZOE] temporary cement). If the final permanent restoration will be bonded using resin cement, a non-eugenol temporary cement must be used, as eugenol inhibits resin polymerization.
Removable Prosthodontics: Complete & Partial Dentures
Removable prosthodontics encompasses appliances designed to replace missing teeth that can be readily removed and reinserted by the patient.
Removable Partial Dentures (RPD)
An RPD replaces multiple teeth in a partially edentulous arch:
- Metal Framework: Cast cobalt-chromium or titanium alloy skeleton.
- Major Connector: The rigid metal bar that joins the right and left sides of the prosthesis (e.g., palatal strap/bar on the maxilla, lingual bar on the mandible).
- Minor Connectors: Struts connecting clasps and rests to the major connector.
- Rests & Rest Seats: Small metallic extensions of the framework that seat into specially prepared recesses (rest seats) carved into the occlusal, cingulum, or incisal surfaces of abutment teeth. Rests are critical because they transfer masticatory biting forces along the long axis of the abutment teeth, preventing the partial denture from sinking vertically into the soft gingival tissues.
- Clasps (Direct Retainers): Flexible metal arms that grasp the cervical undercuts of abutment teeth to provide retention and prevent dislodgement.
- Denture Base & Artificial Teeth: Pink acrylic resin mimicking oral mucosa, supporting acrylic or porcelain denture teeth.
Complete Dentures & Clinical Appointment Sequence
Complete dentures replace the entire dentition of an edentulous maxillary or mandibular arch. Fabrication requires five structured clinical appointments:
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| Complete Denture Clinical Workflow: |
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| Appointment 1: Preliminary Alginate Impressions (Diagnostic Casts & Trays) |
| Appointment 2: Final Impressions (Custom Trays + Border Molding + PVS Wash) |
| Appointment 3: Jaw Relations & VDO (Wax Rims, Centric Relation & Facebow) |
| Appointment 4: Wax Try-In (Aesthetic, Phonetic, & Occlusal Evaluation) |
| Appointment 5: Final Delivery (PIP Adjustments, Articulation & Instructions) |
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- Appointment 1 (Preliminary Impressions): Alginate impressions taken in stock edentulous trays; diagnostic casts are poured to construct custom impression trays.
- Appointment 2 (Final Impressions & Border Molding): Custom trays are border-molded using modeling compound or heavy-body elastomer to record the functional depth and width of the vestibules and muscle attachments, followed by a light-body PVS or polyether wash to create a master cast.
- Appointment 3 (Jaw Relations & Bite Registration): Baseplates with wax occlusal rims are adjusted in the patient's mouth to determine the Vertical Dimension of Occlusion (VDO), smile line, canine lines, and centric relation; a facebow transfer records the spatial relationship of the maxilla to the temporomandibular joints.
- Appointment 4 (Wax Try-In): The dental laboratory mounts artificial teeth in wax on the baseplates. The dentist, assistant, and patient verify facial aesthetics, lip support, phonetics (assessing "s" and "f" sounds), and centric occlusion.
- Appointment 5 (Delivery & Adjustments): The processed acrylic denture is seated. Pressure spots on the tissue surface are identified using Pressure-Indicating Paste (PIP) and relieved with an acrylic laboratory bur; occlusion is verified with articulating paper.
Denture Home Care Instructions
- Nightly Removal: Remove dentures at night to allow oral mucosa and alveolar ridges to recover from functional compression, preventing denture stomatitis and candida fungal infections.
- Cleaning Technique: Clean dentures daily over a basin filled with water or over a folded towel so that if dropped, the acrylic will not fracture. Use a specialized denture brush with a mild, non-abrasive denture paste or mild soap. Never use standard abrasive toothpastes or boiling water (which warps acrylic).
- Soaking: Store dentures in clean water or an effervescent denture cleanser in a closed container when not in the mouth to prevent the acrylic from drying out and warping. Never soak appliances with metal frameworks in bleach solutions, as chlorine corrodes base metals.
Orthodontic Specialty Assisting Protocols
Orthodontics diagnoses, prevents, and intercepts malocclusion and facial disharmonies through fixed and removable corrective appliances.
Diagnostic Records Armamentarium
- Cephalometric Radiograph: Standardized lateral skull radiograph used to trace and analyze skeletal, dental, and soft tissue angular relationships to the cranial base.
- Panoramic Radiograph: Assesses tooth eruption patterns, congenital absence, impactions, and root parallelism.
- Photographs: Standard series including three extraoral views (frontal smiling, frontal resting, profile) and five intraoral views (maxillary occlusal, mandibular occlusal, frontal occlusion, right lateral, left lateral).
- Diagnostic Casts: Accurately poured and trimmed orthodontic study casts or intraoral 3D digital scans.
Orthodontic Separators (Spacers)
Posterior molar teeth have broad, tight proximal contacts that prevent the seating of orthodontic bands. Orthodontic separators are placed 1 to 2 weeks prior to the band fitting appointment:
- Elastomeric Rings ("Donuts"): Stretched using separating pliers or two strands of dental floss and snapped around the mesial and distal contact points of molars. They wedge teeth slightly apart to create clearance.
- Steel Separating Springs: Pliable wire springs placed with bird-beak pliers for heavy contact resistance.
Molar Band Fitting & Cementation
Preformed stainless steel molar bands are sized to fit snugly around the crown of first and second molars. The band is pushed into position using a band pusher along marginal ridges and driven fully subgingival by having the patient bite down on a band bite stick. Bands are permanently cemented with Glass Ionomer Cement (Type I), which adheres to enamel and continuously releases fluoride beneath the band to prevent enamel decalcification and white spot lesions.
Direct Bonding of Orthodontic Brackets
Brackets provide the handle through which archwires transmit biomechanical forces to move teeth:
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| Direct Orthodontic Bracket Bonding Sequence: |
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| 1. Polish Enamel with Fluoride-Free Pumice & Rinse |
| 2. Acid-Etch Enamel (37% Phosphoric Acid for 15-30 Seconds) |
| 3. Rinse Copiously & Desiccate Enamel Until Frosty/Chalky White |
| 4. Apply Liquid Primer/Bonding Resin & Air-Thin |
| 5. Apply Composite Bracket Adhesive to Mesh Base of Bracket |
| 6. Transfer Bracket with Placement Tweezers & Orient Along Crown Long Axis |
| 7. Press Bracket Firmly Against Enamel & Remove Excess Flash with Explorer |
| 8. Photocure Thoroughly with Curing Light from Occlusal & Gingival Angles |
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Archwires, Ligation & Retention
- Archwires:
- Nickel-Titanium (NiTi): Highly flexible with shape memory; used during early treatment phases for initial dental arch leveling and rotational correction with light, continuous forces.
- Stainless Steel: Rigid and stiff; used in intermediate and finishing phases for major space closure, torque control, and archform stabilization.
- Ligation: Archwires are secured into bracket slots using:
- Elastomeric Modules (O-Rings): Stretched over bracket tie wings using Mathieu needle holder pliers.
- Ligature Wire Ties (0.010-inch Stainless Steel): Twisted securely with a hemostat, trimmed to a 3 to 4 mm pigtail with pin-and-ligature wire cutters, and tucked underneath the archwire toward the gingiva using a ligature director to prevent soft tissue laceration.
- Retention: Following debonding and adhesive removal with multi-fluted carbide burs, retainers prevent relapse. Common retainers include the removable Hawley retainer (acrylic palate with a labial wire bow), clear Essix retainers (vacuum-formed thermoplastic shells), and fixed bonded lingual wires bonded from canine to canine.
Why is it clinically critical to perform tooth shade selection at the very beginning of a fixed crown preparation visit?
Local anesthetic vasoconstrictors cause immediate permanent discoloration of the dental pulp.
Color shade tabs must be sterilized in an autoclave between initial viewing and preparation.
Teeth dry out during preparation, turning lighter and chalky, so a later shade match is inaccurate.
Dental impression materials react exothermically and bleach adjacent enamel surfaces.
A patient with a documented history of severe hypertension and cardiac arrhythmia presents for a master crown impression. Why must the dental assistant ensure that the retraction cord does NOT contain racemic epinephrine?
Epinephrine reacts with polyvinyl siloxane impression materials, releasing hydrogen gas voids.
Epinephrine stains the cervical margin of ceramic restorations a dark brown color.
Epinephrine prevents the temporary crown cement from achieving full compressive strength.
Absorbed epinephrine can cause tachycardia and a dangerous rise in blood pressure.
What is the primary biomechanical function of an occlusal rest incorporated into a removable partial denture (RPD) cast framework?
To join the right and left quadrants of the prosthesis across the midline of the dental arch.
To direct chewing forces along the abutment's long axis and keep the denture from sinking into the tissue.
To mechanically bond the artificial acrylic teeth directly to the cobalt-chromium metal framework.
To provide spring-like friction retention beneath the cervical undercut of anterior abutments.
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