Impression Techniques for Complete Dentures

What Is a Complete Denture Impression?

• Retention - resistance to removal along the path of insertion
• Stability - resistance to horizontal/rotational displacement during function
• Support - resistance to vertical occlusal forces
• Esthetics - reproducing natural contours of the face
• Preservation of residual structures - avoiding overloading and resorption

Classification of Impression Techniques

By Number of Steps

By Pressure Philosophy (Most Important Classification)

1. Mucostatic (Non-Pressure) Technique

• A custom tray is fabricated with adequate spacing
• Low-viscosity, fluid impression material is loaded into the tray
• Tray is seated with minimal force
• No border molding or active tissue manipulation performed

• High regard for tissue health and preservation
• Reduces pressure on vulnerable areas (thin mucosa, bony prominences)
• Less bone resorption over time (theoretically)

• Peripheral seal may be inadequate
• Poor retention if borders are not accurately extended
• Does not record tissues in functional positions

2. Mucocompressive (Pressure) Technique

• Tray seated with firm finger pressure, or patient bites down to generate pressure
• Tissues recorded in a compressed/displaced state

• Strong peripheral seal
• Better retention in the short term

• Can accelerate alveolar bone resorption
• Overloads areas with thin mucosa
• Retention is lost faster as bone resorbs

3. Selective Pressure Technique (Most Widely Used Today)

• Maxilla: hard palate, posterior palatal seal area
• Mandible: buccal shelf (retromolar pad region), residual ridge crest

• Incisive papilla, midline palatine suture, sharp bony ridges, tori, mental foramina

1. Custom tray fabricated with spacer uniformly except over the primary stress-bearing areas (tray contacts those areas directly, or spacer is thin there)
2. Relief holes or added thickness wax over relief areas
3. Medium-viscosity impression material (zinc oxide eugenol, polyether, or VPS medium body) loaded
4. Tray seated; more pressure is transmitted to stress-bearing areas naturally

• Distributes occlusal forces physiologically
• Protects vulnerable tissues
• Best long-term tissue health and denture stability

4. Functional (Mucodynamic) Impression Technique

a) Open-Mouth Functional Impression

• Patient actively performs functional movements while material sets
• Operator performs passive border molding (pushing cheeks and lips)
• The custom tray is held by the operator who controls pressure
• Most common technique in modern practice

b) Closed-Mouth Functional Impression

• Denture base with occlusal rims fabricated on primary cast
• Jaw relations registered first to capture correct vertical dimension
• Patient bites down into correct occlusion while functional impression material (fluid zinc oxide eugenol or soft liner) records the tissues
• Pressure is generated by the patient's own occlusal force

1. Primary (preliminary) impression taken with alginate in stock tray
2. Custom tray fabricated on primary cast with 1-2 mm spacer
3. Border molding: Thermoplastic material (green stick compound) added incrementally to the tray flanges; patient performs active movements (move jaw side to side, pucker lips, say "ah") while operator manipulates cheeks and lips - this records the functional extent of the sulcus
4. Spacers placed (0.5-1 mm) in mid-alveolar ridge areas
5. Tray adhesive applied
6. Low-viscosity elastomer (condensation silicone, polyether, or VPS light-body) washed over the entire tray
7. Tray seated; patient performs functional movements again
8. Impression removed, inspected, and poured in dental stone

5. Neutral Zone Impression Technique (for Atrophic Ridges)

1. Temporary denture base with occlusal rims fabricated
2. A soft/fluid material (tissue conditioner, zinc oxide eugenol, or impression plaster) is placed in the neutral zone area
3. Patient performs functional movements: chewing, swallowing, speech, smiling
4. The muscles carve out the neutral zone in the material
5. This carved form guides tooth positioning and polished surface contour of the final denture

Step-by-Step: The Two-Stage Standard Protocol

Stage 1 - Preliminary (Primary) Impression

1. Select appropriate stock tray size
2. Modify tray borders with rope wax if necessary (to extend short flanges)
3. Mix alginate to appropriate consistency
4. Load tray, seat in the mouth with even pressure
5. Hold tray stable while performing passive border molding (retract cheeks and lips)
6. Allow material to set, remove with single snap
7. Rinse, disinfect, pour immediately in dental plaster or stone

Stage 2 - Final (Master/Secondary) Impression

2a. Custom Tray Fabrication

• Custom tray made from autopolymerizing acrylic resin or light-cured resin on the primary cast
• Tray borders trimmed to be 2 mm short of the vestibular depth (to allow space for border molding material)
• Spacer (1-2 mm of wax) placed over the ridge before tray fabrication (to create space for impression material)
• Handle placed anteriorly

2b. Custom Tray Try-In

• Check tray extensions: should not impinge on frena, must not over-extend or under-extend
• Mark frenum and muscle attachments (Thompson stick/marker)
• Adjust with acrylic bur as needed

2c. Border Molding

• Green stick (thermoplastic) compound heated in water bath (65-70°C) or with torch flame
• Applied incrementally to one section of the tray border at a time
• Softened material seated and patient performs the relevant functional movement:
  • Anterior maxilla: pucker/relax lips
  • Posterior maxilla: open/close, cheek movements
  • Anterior mandible: lip, chin movements, protrusion
  • Posterior mandible: cheek blowing, "smile"
  • Retromolar pad: open wide
• Each section is chilled with cold water before moving to the next
• Objective: Accurately capture the depth and width of the functional sulcus - this creates the peripheral seal

2d. Final Wash Impression

• Apply tray adhesive to the entire internal surface
• Mix final impression material:
  • Zinc oxide eugenol (ZOE) paste - classic, good detail, requires no spacer, not used in ZOE-sensitive patients
  • Polyvinyl siloxane (PVS/VPS) light-body - excellent detail, dimensional stability
  • Polyether - good dimensional accuracy, slightly stiff
  • Zinc oxide eugenol paste (Impression Paste) - fluid, mucostatic
• Load tray, seat, apply gentle pressure, patient performs border-molding movements
• Allow complete setting, remove in one smooth motion

Impression Materials Summary

Special Situations

Flabby (Hyperplastic) Ridge

• Arises from long-term wearing of ill-fitting dentures; the anterior maxillary ridge becomes soft and displaceable
• Two-step final impression: first record the non-displaceable hard palate under normal pressure (polyether), then window the impression tray over the flabby area and record it with a low-viscosity mucostatic material
• This avoids displacing the flabby tissue and causing a rocking, unstable denture

Severely Resorbed Mandibular Ridge

• Neutral zone technique (see above)
• Selective pressure over buccal shelf only
• Consider functional impression with tissue conditioner

Bony Prominences / Tori

• Relieve the impression tray and impression material over tori
• Use mucostatic material in these areas

Key Points for Retention and Peripheral Seal

• Accurate border molding extending the flanges to the functional sulcus depth
• Posterior palatal seal (post-dam) for the maxillary denture, placed at the junction of hard and soft palate (vibrating line located by asking the patient to say "ah")
• The final wash impression recording the residual ridge and palate at the correct pressure level

Summary Table: Techniques Compared

Acrodynia (Pink Disease / Feer Syndrome)

Definition and Synonyms

• Pink disease (for the characteristic pink discoloration of the extremities)
• Feer syndrome (after Swiss pediatrician Emil Feer)
• Feer-Swift disease
• Swift disease

Etiology and Sources of Mercury Exposure

• Mercurial teething powders (calomel - mercurous chloride) - the classic historical cause; widespread in the early-to-mid 20th century
• Diaper (nappy) ointments containing mercury compounds
• Mercurochrome and other topical antiseptics
• Broken thermometers and barometers (elemental mercury vapor inhalation)
• Latex paint (phenylmercury fungicide, now banned)
• Ambient mercury vapor from industrial or domestic sources

Why Does It Occur? Pathophysiology

1. Widespread enzyme dysfunction, inactivation, and denaturation - because sulfhydryl groups are ubiquitous in metabolic enzymes
2. Inhibition of catecholamine-O-methyltransferase (COMT) - the key enzyme degrading catecholamines (epinephrine, norepinephrine). This results in elevated circulating catecholamines, causing:
   • Hypertension
   • Tachycardia
   • Sweating (diaphoresis)
3. Idiosyncratic hypersensitivity component - not all children exposed to the same mercury levels develop acrodynia, indicating an individual immune/hypersensitivity response to mercury ions. This explains why acrodynia is not a simple dose-response toxicity but rather an idiosyncratic reaction.

"Acrodynia is also associated with an idiosyncratic hypersensitivity to mercury ions and mercury poisoning, which itself can increase circulating catecholamines and mimic pheochromocytoma." - Goldman-Cecil Medicine

Clinical Features

Cardinal Features (SWEATING + PINK EXTREMITIES + NEUROPSYCHIATRIC)

The Classic Triad of Chronic Elemental Mercury Toxicity (not just acrodynia):

1. Tremor (mercurial tremor - both static and intentional; "tetanus mercurialis" for episodic bursts)
2. Gingivostomatitis
3. Neuropsychiatric disturbances (erethism)

Acrodynia vs. Erethism - Distinguishing the Two Syndromes

Mimics Pheochromocytoma

• Hypertension
• Tachycardia
• Sweating
• Elevated urinary catecholamines

Diagnosis

Note: Correlation between mercury levels and symptoms can be inconsistent, especially in the idiosyncratic acrodynia phenotype.

Treatment

1. Remove the source of exposure - immediately and completely
2. Environmental decontamination - if ambient mercury contamination is present
3. Supportive care - fluid resuscitation, hemodynamic support
4. Chelation therapy:
   • Oral DMSA (dimercaptosuccinic acid / succimer) - chelator of choice; generally well tolerated orally
   • Dimercaprol (BAL - British Anti-Lewisite) - used if oral administration is not possible
   • CONTRAINDICATION: BAL/dimercaprol is contraindicated in methylmercury poisoning because it may redistribute/shift mercury into the brain, worsening CNS toxicity
   • Chelation increases urinary elimination of mercury

Prognosis

• Acrodynia: Patients typically recover completely after chelation therapy and removal of mercury exposure - Goldman-Cecil Medicine
• Erethism/neurotoxicity: May be irreversible, particularly with organic mercury poisoning where diagnosis is often delayed
• Acute renal failure from inorganic mercury can sometimes resolve

Key Exam Points (Forensic / Clinical)

• Acrodynia = Mercury poisoning (Answer B in the forensic toxicology MCQ above)
• Classically caused by calomel (mercurous chloride) in teething powders
• Synonyms: Pink disease, Feer syndrome, Feer-Swift disease
• Mechanism: COMT inhibition → elevated catecholamines → mimics pheo
• Chelator of choice: DMSA (oral); BAL contraindicated in methylmercury
• Complete recovery possible with chelation + exposure removal
• Plumbism = lead poisoning (not mercury)