L3 Secondary impressions

Complete Denture Overview¹²

NOTE

This lecture covers the oral anatomy relevant to secondary impressions for complete dentures, including the types of impressions, materials, and techniques used to accurately record the anatomy.

Treatment Schedule

NOTE

The secondary impression is the second clinical stage in the fabrication of complete dentures. It is made using a custom tray that was fabricated on a primary cast from the initial impression. This secondary impression serves as a highly accurate record of both the tissue surface and the full peripheral extent of the denture-bearing area.

Visit	Operator	Procedure
Visit 1	Clinician Laboratory	Patient evaluation & primary impressions Diagnostic casts & special trays
Visit 2	Clinician Laboratory	Secondary impressions Bead & box, master casts, base & rims
Visit 3	Clinician Laboratory	Contour rims, OVD, MMR, shade & mould selection Articulation & tooth setting
Visit 4	Clinician Laboratory	Wax try-in Finalisation & processing
Visit 5	Clinician	Assessment & issue Instructions for care
Visit 6	Clinician	Post operative review, post insertion instructions

Secondary Impressions

Intraoral Anatomy

NOTE

A thorough understanding of the intraoral anatomy is essential to comprehend the rationale behind specific secondary impression techniques.

Maxilla³⁴

• Buccinator

NOTE

The buccinator muscle arises from the outer surfaces of the maxillary and mandibular alveolar processes, attaching posteriorly to the buccinator crest of the mandible and the pterygomandibular raphe, and inserting anteriorly into the orbicularis oris. Its functions include:

• Flattening the cheek.
• Compressing the cheek against the molars during chewing to prevent food accumulation.
• Contributing to blowing, whistling, and sucking.
• Shaping the cheek and maintaining mouth shape during speech and facial expressions.

• Tuberosities
• Hamular notch

Posterior Palatal Seal⁵

• Posterior limit: The distal demarcation of the movable and non-movable tissues of the soft palate (vibrating line) – usually ~ 2mm anterior to Fovea Palatini
• Anterior limit: The junction of the hard and soft palates on which pressure, within physiologic limits, can be placed
• Lateral extension: Hamular notch

NOTE

The compressibility of these tissues can be assessed by gentle probing with a ball burnisher.

Maxillary Tuberosity⁶⁷⁸

NOTE

The maxillary tuberosity is the bulbous distal extension of the maxillary alveolar ridge, terminating in the hamular notch. A well-shaped tuberosity enhances denture retention and stability.

Enlarged or Flabby Tuberosity

An enlarged or flabby tuberosity can complicate denture fabrication by:

• Restricting the inter-ridge space.
• Forcing the occlusal plane to be tilted posteriorly to avoid the excess tissue, making occlusal balance difficult.
• Creating severe undercuts, especially if bilateral, which can prevent a rigid denture from seating properly.

Solution 1.

• Cut back the denture from one undercut area
• Seat this side first
• Then rotate into position

NOTE

The denture flange is not extended into one or both undercut areas. Typically, the flange on the side with the least undercut is shortened to minimize the reduction of the denture-bearing area. This avoids surgery but results in reduced stability and retention.

Solution 2.

• Block out one of the undercuts on the processing cast and fabricate a fully extended denture.
• Seat this side first
• Then rotate into position

NOTE

One undercut (usually the smaller one) is blocked out on the processing cast with plaster. This solution also diminishes stability and retention (though less than solution 1) and may lead to food packing in the blocked-out area.

Solution 3.

• Surgically remove one undercut prior to secondary impression-making and fabricate a fully extended denture.

NOTE

One or both undercuts are surgically reduced. Often, only the smaller undercut needs reduction if the denture can still be rotated into place. This procedure provides the best outcome for maximizing stability and retention.

Mandible^9101112

Impression Objectives¹³

• Retention
  • Is dependent upon atmospheric pressure, adhesion, cohesion (which depends on peripheral seal), mechanical locks and muscle control.
  • The tendency of different molecules to stick together
  • The tendency of like entities to stick together
• Stability
  • Close tissue adaptation reduces horizontal movement of the denture.
• Support
  • Maximum coverage ensures minimal pressure applied at any individual point.
• Aesthetics
  • Peripheral thickness should be varied according to the facial contours and lip support required.
• Ridge preservation
  • With the loss of stimulation of the natural dentition the alveolar ridge will atrophy or resorb, the process can be hastened or retarded by local factors.

NOTE

A well-fitting prosthesis can retard this resorption.

Impression Materials¹⁴

NOTE

Materials for secondary impressions are typically low-viscosity to achieve good detail reproduction without significant tissue displacement.

• Thermoplastic
  • Agar hydrocolloid (Stock Water-cooled Tray)
  • Impression waxes
• Chemical Reaction
  • Alginate (Custom Tray)
  • Zinc-oxide eugenol paste (Custom Tray)
  • Plaster of Paris (Custom Tray)
  • Polyvinyl siloxane (Custom Tray)
  • Polyether (Custom Tray)
• Digital
  • Intra-oral scan
  • Extraoral scan of secondary impression/cast

Impression Types¹⁵

Mucostatic

The state of the oral mucosa as recorded when not displaced by external forces.

• Advantages:

  • Dentures conform closely to the shape of the mucosal surface
  • Physical retention is optimal (when denture not under load)

• Disadvantages:

  • The occlusal loads will be uneven
  • Difficult to obtain true static impression

• An arbitrary relief area is often needed in maxillary impressions to prevent the denture from rocking, due to variations in mucosal thickness.

Functional

Tissue state recorded using a custom tray under “functional” load. All tissues loaded equally regardless of their load-bearing ability

NOTE

This technique records the tissue under what is assumed to be masticatory load, using an accurately fitting custom tray and ZOE paste. The disadvantage is that flabby tissues are not relieved, which can lead to an unstable denture base.

Selective Pressure

Tissue state recorded under load in custom tray with maximum coverage but relief in specific areas

• Advantages:
  • More even displacement under occlusal load
• Disadvantages:
  • Less retention to be obtained when the teeth are apart

---

No controlled studies have been identified to support an advantage due to any of the following factors: border moulding, post dam, mucostatic, functional or selective pressure impressions. Carlsson GE, 2013.

Mucostatic Impression Examples

Reversible Hydrocolloid

NOTE

This material (also known as agar hydrocolloid) can be used for mucostatic impressions, though it is more common for partially dentate arches.

Equipment & Procedure

• ==Equipment: Requires a water-cooled tray (double-skinned or with external cooling tubes), a water bath, and a metal syringe.==
• ==Procedure:==
  • The material (stick or tube) is boiled to liquefy it.
  • It is stored in a 65°C water bath to maintain its sol form.
  • Before use, it is tempered at 45°C for a few minutes.
  • The material is loaded into the tray, seated in the mouth, and allowed to cool to mouth temperature.
  • It is removed with a quick “snap” motion.
• ==Disadvantages: Requires costly equipment and careful handling to prevent syneresis (water loss) or imbibition (water uptake).==

Intraoral Scan¹⁶

NOTE

Digital scanning is becoming popular for its time-saving potential. The scan provides a mucostatic image of non-displaced tissue.

Challenges

• Retracting the lips and cheeks for camera access can displace peripheral tissues.
• Because of this, many operators prefer to scan a conventional impression or a cast made from one.
• Relief areas will almost certainly be needed, particularly in the maxilla, to compensate for varying tissue thickness.

Scan Strategy

Selective Pressure Impression¹⁷¹⁸

NOTE

This technique requires identifying areas that can tolerate masticatory loads and areas that need relief from pressure.

Denture Bearing Areas

Maxilla

NOTE

Several anatomical structures define the buccal and labial limits of the maxillary denture-bearing area. A transverse cross-section of the maxilla reveals varying thicknesses of the supporting mucoperiosteum and the underlying bone topography. This relationship dictates the impression technique, as areas with thin soft tissue perpendicular to masticatory forces may require relief in the impression.

• ==Lateral and Medial Pterygoid Muscles: These have a lesser influence on the denture borders.==
• ==Tensor Veli Palatini: This muscle tenses the soft palate. The vibrating line, which is the line where this movement is first discernible, marks the posterior limit of the maxillary denture-bearing area.==
• ==Pterygomandibular Raphe: This is the common attachment point for the buccinator and superior pharyngeal constrictor. It is located adjacent to the hamular notch.==

• ==Primary Stress-Bearing Areas: Crest and sides of the alveolar ridge.==
• ==Secondary Stress-Bearing Areas: Buccal and lingual flanges, and the medial aspects of the palate.==
• ==Relief Areas: Incisive papilla, midline of the palate, any severely undercut areas, and areas where the alveolar ridge is flabby.==

Mandible

NOTE

The mandible is a horseshoe-shaped bone and the only mobile bone in the skull. It consists of a body and two rami that articulate with the skull base. The body includes the alveolar ridge (which resorbs after tooth loss) and the denser basal bone (less subject to resorption).

Key Bony Landmarks

External Surface:

• ==Mental Foramen: Located below the second premolar root. With severe resorption, it can move to the crest of the ridge, causing significant pain under denture pressure.==
• ==Oblique Ridge==
• ==Retromolar Area== Internal Surface:
• ==Mylohyoid Ridge==
• ==Mandibular Foramen==
• ==Genial Tubercles==

Key Muscle Attachments

• ==External (Buccal/Labial Limits): Mentalis, Labii Inferioris, Triangularis, Buccinator, and Masseter. The retromolar pads mark the posterior limit of the denture-bearing area.==
• ==Internal (Lingual Limits): Genioglossus, Geniohyoid, and Mylohyoid.==

• ==Primary Stress-Bearing Areas: Sides of the alveolar ridge, buccal flanges, and lingual flanges down to the mylohyoid line.==
• ==Relief Areas: The crest of the alveolar ridge (as it is often narrow and sharp), retromolar pads, and significant undercuts (especially on the lingual aspect below the mylohyoid line).==

Maxillary Relief Area¹⁹²⁰²¹

• Simple geometry and the histology of the area dictates that the width of the maxillary relief area is related to the cross-sectional shape of the palate and thickness of the underlying tissue
• Denture stability is enhanced by appropriate relief

Rationale & Extent

• ==Rationale: Seating forces are generally perpendicular to the denture-bearing areas. Surfaces at a right angle to these forces receive the greatest load. To prevent the denture from rocking around the thin tissue of the midline, this area must be relieved.==
• ==Anteroposterior Extent: From the incisive papilla to just short of the posterior palatal seal’s anterior border.==
• ==Lateral Extent: Extends across the palate between the points where the surface changes from nearly flat to angled. The width of the relief area increases as the palate shape goes from narrow to flat.==

= Wax Relief

• NARROW (V-Shape)
• NORMAL
• FLAT

Alveolar bone Mucosa Impression tray

Tray Fabrication

Design Objectives²²

• Custom-made for final impressions to address individual anatomical variations
• Well adapted to tissues
• 2 – 3 mm thick
• Border extensions 1 mm short of the vestibule when the intraoral tissues are at rest and 1mm short of the frenula when they are active
• Handle design should not impinge on the vestibule nor distort the lips
• Fabricated of strong, stable, biocompatible material
• Relief provided where indicated
• Maximum coverage to minimise the pressure applied to the underlying tissues

Design Outline

NOTE

1. Trace the full extension of the proposed denture on the primary cast.
2. Draw a second line 1 mm short of the first line; this will be the final extension of the impression tray.
3. Indicate any areas requiring wax relief on the cast.
4. Write a detailed laboratory prescription, including the date, patient details, item required, material, procedure, and any special design instructions. The technician relies entirely on these instructions.

• Outline to full extension
• Identify the cut-back requirements
• Identify areas on the cast that require wax relief
• Produce a laboratory prescription that outlines a request for a special tray

Wax Relief²³²⁴

• Undercut on labial of alveolar ridge has been blocked out with wax.
• Outline for wax relief of incisive papilla and central palate shaded in pink.
• Minimal wax block-out of lingual undercut on left alveolar ridge and lingual aspect of the ridge in the midline.
• Outline for wax relief of crest of alveolar ridge shaded in pink. Note the minimal width.

NOTE

After blockout, a thin layer of Vaseline is applied to the cast’s fitting surface to aid in separating the processed tray.

Light curing material

NOTE

• ==Material: Palatray material is supplied as a uniform 2 mm thick, anatomically shaped blank.==
• ==Fabrication:==
  • The blank is molded to fit the master cast.
  • Excess material is trimmed with a scalpel.
  • The excess can be used to form a handle, which is attached to the tray.
  • The tray and cast are placed in a light box to cure.
  • Final trimming of the peripheral margins is done after curing.

Palatray light-curing tray material	Polylux light-curing box	Completed Tray

Completed Tray (3D Printed)²⁵²⁶

NOTE

For this exercise, pre-made 3D printed trays will be provided. Before use, each tray must be checked on the phantom jaws for:

Check

• Stability
• Rigidity
• Even thickness
• Accurate adaptation to cast
• Maximum surface area coverage
• No over-extensions in sulcus or at frena
• Smooth to touch
• Adequate space to allow accurate muscle moulding

Materials and Techniques

Zinc Oxide Eugenol Paste

NOTE

Though not used in this exercise, ZOE paste has long been used for secondary impressions.

Properties & Technique

• ==Properties: Low viscosity, excellent record of tissue detail, cannot support itself.==
• ==Technique:==
  • Equal lengths of base and accelerator are mixed until homogeneous.
  • The tray borders are first adapted with green compound tracing stick (border moulding).
  • The posterior palatal seal is often traced with higher-viscosity brown compound for better tissue compression.
  • The ZOE paste is applied as a thin wash layer inside the border-moulded tray.
  • The tray is seated and muscle-moulded.

Palatal Vent Holes²⁷²⁸

palatal vent hole allows for:

• escape of air and excess material
• minimizes aspiration risk
• reduces pressure

NOTE

After verifying the fit of the maxillary tray, a vent hole must be cut in the palate with a bur. The purpose is to allow excess impression material to escape centrally, preventing it from flowing over the posterior border and posing an aspiration risk. It also provides pressure relief, resulting in a thinner, more even layer of impression material.

WARNING

If the patient has a flabby ridge and the tray has been relieved, an additional vent hole should be placed in the center of that relief area.

Polyether Material²⁹

Armamentarium

NOTE

The material for this exercise is Impregum Soft, a hydrophilic polyether.

1. Adhesive: The tray must be painted with the correct polyether adhesive. Allow it to dry completely; wet adhesive acts as a lubricant and will cause the material to separate from the tray.
2. Mixing: The material is machine-mixed using a Pentamix machine. - Ensure the correct base and catalyst packets are loaded. - A mixing nozzle is attached to the dual cartridge. - The machine mixes and extrudes the material quickly. Extrude only a small amount to avoid an overly thick impression or choking the patient.

Loading tray & seating in mouth³⁰

• Add Impregum Soft into the tray
  • Remember – only small amount required
• Spread evenly to avoid air bubbles
• Ensure that material is extended onto the margins
• Seat tray from the posterior forward to reduce the risk of aspiration
• Remove excess with a dental mirror head
• If imminent aspiration risk – use suction

Border Moulding

NOTE

Border moulding is performed while the tray is firmly seated and continues until the material has set.

• ==Maxilla: Seat the tray with one hand while gently stretching the lips and cheeks of the contralateral side downwards.==
• ==Mandible:==
  • Seat the tray with one hand.
  • Gently stretch the lips upwards from one buccal frenum to the other.
  • Distal to the buccal frena, gently massage the external facial tissues against the tray. This allows material to flow correctly relative to the buccinator muscle fibers.
  • The lingual periphery is moulded by having the patient perform lateral and protrusive tongue movements over the tray.

Completed Polyether Impression³¹³²

• Separate tray from model
• Check for “burn through” and other defects
• Trim excess with scalpel or scissors
• Reinsert to check the retention and stability (only for clinical cases)

Removal and Inspection

• ==Removal: Removing the impression from the model in the phantom head is difficult. It is easier to remove the model from the phantom head first, then separate the impression.==
• ==Inspection: Check the impression for defects like burn-through (areas where the tray shows through), inadequate peripheral extension, or bubbles.==
• ==Trimming: If the impression is acceptable, trim any peripheral excess with a scalpel or curved scissors to create a uniform peripheral thickness. The same procedure is followed for the mandibular impression.==

Polyether Impression

Summary³³

NOTE

The overall procedure involves using a custom-made tray to obtain a detailed impression of the denture-bearing area. This impression is then cast in Type III gypsum to create the final master cast for denture fabrication.

special tray | final impressions | master models

Flashback³⁴³⁵

Dental Student Clinic

Operative dentistry student clinic circa 1960’s at Perth Dental Hospital in Wellington St.

NOTE

Significant changes have occurred in dentistry over time. In the past, all procedures, including crown preparations, were performed with the dentist standing and the patient seated upright. Belt-driven slow-speed drills were used without high-speed evacuation, and lighting was poor. Instruments were sterilized by boiling. This serves as a reminder that dentistry is a profession that demands lifelong learning to keep up with advancements.

Conclusion³⁶

NOTE

This concludes the session on secondary impressions.

Thank You

Any questions? Contact me

Dr Ian Lander ian.lander@uwa.edu.au

Footnotes

1. Original PDF page 1: L3 SecondaryImpressions, p.1 ↩

2. Original PDF page 2: L3 SecondaryImpressions, p.2 ↩

3. Original PDF page 3: L3 SecondaryImpressions, p.3 ↩

4. Original PDF page 5: L3 SecondaryImpressions, p.5 ↩

5. Original PDF page 4: L3 SecondaryImpressions, p.4 ↩

6. Original PDF page 6: L3 SecondaryImpressions, p.6 ↩

7. Original PDF page 7: L3 SecondaryImpressions, p.7 ↩

8. Original PDF page 8: L3 SecondaryImpressions, p.8 ↩

9. Original PDF page 9: L3 SecondaryImpressions, p.9 ↩

10. Original PDF page 10: L3 SecondaryImpressions, p.10 ↩

11. Original PDF page 11: L3 SecondaryImpressions, p.11 ↩

12. Original PDF page 12: L3 SecondaryImpressions, p.12 ↩

13. Original PDF page 13: L3 SecondaryImpressions, p.13 ↩

14. Original PDF page 14: L3 SecondaryImpressions, p.14 ↩

15. Original PDF page 15: L3 SecondaryImpressions, p.15 ↩

16. Original PDF page 16: L3 SecondaryImpressions, p.16 ↩

17. Original PDF page 17: L3 SecondaryImpressions, p.17 ↩

18. Original PDF page 18: L3 SecondaryImpressions, p.18 ↩

19. Original PDF page 19: L3 SecondaryImpressions, p.19 ↩

20. Original PDF page 21: L3 SecondaryImpressions, p.21 ↩

21. Original PDF page 20: L3 SecondaryImpressions, p.20 ↩

22. Original PDF page 22: L3 SecondaryImpressions, p.22 ↩

23. Original PDF page 23: L3 SecondaryImpressions, p.23 ↩

24. Original PDF page 24: L3 SecondaryImpressions, p.24 ↩

25. Original PDF page 25: L3 SecondaryImpressions, p.25 ↩

26. Original PDF page 26: L3 SecondaryImpressions, p.26 ↩

27. Original PDF page 27: L3 SecondaryImpressions, p.27 ↩

28. Original PDF page 28: L3 SecondaryImpressions, p.28 ↩

29. Original PDF page 29: L3 SecondaryImpressions, p.29 ↩

30. Original PDF page 30: L3 SecondaryImpressions, p.30 ↩

31. Original PDF page 31: L3 SecondaryImpressions, p.31 ↩

32. Original PDF page 32: L3 SecondaryImpressions, p.32 ↩

33. Original PDF page 33: L3 SecondaryImpressions, p.33 ↩

34. Original PDF page 34: L3 SecondaryImpressions, p.34 ↩

35. Original PDF page 35: L3 SecondaryImpressions, p.35 ↩

36. Original PDF page 36: L3 SecondaryImpressions, p.36 ↩