Secondary Impressions in Forensic Science

Secondary Impression

• An ideal impression provides maximum extension without muscle impingement.
• It must have intimate contact with the tissue area covered.
• It must have proper form of the borders, including the posterior border of the maxillary denture.
• It should provide proper relief for hard and sensitive areas.
• It must equalize forces on the denture foundation area.

Custom Tray Ideal Requirements

• Individually made for each mouth.
• Rigid and stable, well adapted to the primary cast.
• Borders are slightly under-extended from the desired peripheral extension. It should have a 2 mm relief near the sulcus.
• Green stick compound can be used to do border molding.
• Frena should have adequate relief.
• The tray and handle must not interfere with functional movements of the mouth.
• It should be dimensionally stable on the cast and in the mouth.
• At least 2 mm thick in the palatal area and lingual flange for adequate rigidity.
• It should not bind to the cast.
• The tissue surface should be free of voids or projections.
• It should not react with the impression material and be easy to remove.

Special or Customized Tray Materials

• Types:
  • Shellac base plates (thermoplastic)
  • Light-cure acrylic resin
  • Auto-polymerizing (self-curing) acrylic resin
  • Cast or swaged metal
  • Vacuum-form poly vinyl
• Advantages:
  • More accurate fit to the patient's individual arch.
  • Reduces the bulk of impression material.
  • More accurate border contour.
  • Uses less impression material.
  • More comfortable for the patient.

Shim or Spacer

• A thickness of modeling wax or shellac base plate adapted on the study cast under the special tray.
• Advantages of Acrylic Resin Special Tray:
  • Easily constructed.
  • More rigid than shellac trays.
  • Accepts border tracing material.
  • Easily trimmed.

Custom Tray Types

• Spaced trays
• Fitted trays
• Windowed trays

Constructing the Custom Tray

• Outline the wax spacer on the cast.
• Posterior palatal seal area on the cast is not covered with wax spacer (maxilla).
• Buccal shelf is not covered with wax (mandible).
• Baseplate wax is placed on the cast (approximately 1 mm thick).
• Self-curing acrylic resin tray material is adapted uniformly over the cast.
• Tray thickness should be 2 to 3 mm.
• Resin handle is attached to the tray's anterior region.

Outline for Wax Spacer

• Outline is drawn on the cast.
• Depth of vestibule is outlined.
• Room is provided for frenum attachments.

Maxillary Cast

• A blue line is scribed 2 mm above the red line.
• The blue line determines the tray extension.

Relief Areas and Block Out Undercuts (Maxillary Cast)

• Frenum
• Buccal surface of the tuberosity
• Labial undercuts
• Rugae
• Flabby portions (are relieved)

Mandibular Cast

• The vestibular region is marked with red and blue pencil, as previously described.
• Undercuts are blocked out.
• The lingual side of the mandible, opposite the retromylohyoid space, is a relief area.
• Mylohyoid ridge
• Frenum
• Buccal shelf is not covered with wax.

Wax Spacer

• Provides room for impression material.
• Do not place relief over blockout.
• Already spaced from tissue.

Spacer Design

• Roy Mac Gregor recommends placement of a sheet of metal foil in the region of incisive papilla and mid palatine raphae.

Neill and Boucher Recommendations

• Neill recommends 0.9 mm adaptation of casing wax over all areas, except PPS area.
• Boucher recommends 1 mm base plate wax on all areas except PPS area.
• Posterior palatal seal area on the cast is not covered with wax spacer.

Morrow, Rudd, and Rhoads Recommendation

• Block out undercut areas with wax.
• Adapt full wax spacer (2 mm short of resin special tray border) across the impression.
• Place 3 tissue stops equidistant from each other.
• Sharry recommended Baseplate wax adapted over the entire area.
• Four stops of 2 mm width cut from wax (cuspid and molar region) extend from the palatal aspect of ridge to mucobuccal fold.

Space Required for Impression Materials

• Zinc oxide and Eugenol: No spacer (0.5–1 mm)
• Silicone (medium bodied): 1.5–3 mm (one layer of wax)
• Alginate: 3 mm (two layers of wax)
• Silicone (heavy bodied): 3–4.5 mm (three layers of wax)
• Impression plaster: 4.5 mm (three layers of wax)

Fabrication of Cold Cure Acrylic - Dough Technique

• Soak the cast in water for 5 minutes after completing wax blockout.
• Coat the cast surface with a separating medium (tin foil substitute, petroleum jelly, or Alcote).
• Stages:
  • Wet sandy stage
  • Early stringy stage
  • Late stringy stage
  • Dough stage
  • Rubbery stage
  • Stiff stage
• Material manipulated in late stringy & dough stages.
• Shaped into a 2 mm thick sheet.
• Excess material is cut out with a wax knife before the material sets.
• Trimmed to create a smooth surface with smooth margins.

Self-curing acrylic resin tray material

• Uniformly adapted over the cast
• Tray thickness: 2 to 3 mm
• Resin handle is attached in the anterior region of the tray

Border Molding of the Special Tray

• Performed with thermoplastic modeling compound, waxes, and impression materials.
• Borders of the impression are in harmony with the physiologic action of the limiting anatomic structures.
• Sectional border molding: Labially, posteriorly, and lingually
• One-step border molding: Records all borders simultaneously

Ideal Impression Material Features

• Dimensional accuracy
• Dimensional stability
• Hydrophilic properties
• Wettability
• Elastic recovery
• Flexibility
• Tear strength
• Biocompatible and hypoallergenic
• Non-toxic; pleasant odor and taste
• Inexpensive
• Long working time, short setting time, and long shelf life
• Disinfection should not reduce surface detail or accuracy.
• Should be pourable multiple times without losing accuracy.

Impression Techniques Classifications

• Preliminary impression Materials: Impression compound, Alginate, and Putty
• Final impression Materials: Plaster of Paris, Zinc Oxide-eugenol Paste, Irreversible Hydrocolloid, Silicone, Polysulfide Rubber, Polyether, and Tissue-conditioning material

Diagnostic Impression

• Negative replica of oral tissues used to prepare a diagnostic cast.
• Used for study purposes.
• Made as a part of treatment plan and to estimate the amount of pre-prosthetic surgery.
• Articulates casts on tentative jaw relations to evaluate inter-arch space.

Impression Techniques Based on Pressure

• Pressure technique
• Minimal pressure technique
• Selective pressure technique

Impression Techniques Based on Mouth Position

• Open mouth
• Closed mouth

Impression Techniques Based on Border Molding Manipulation

• Hand manipulation
• Functional movements

Classification of Final Impression Theories and Techniques

• Minimal pressure impression technique (Mucostatic impressions or open mouth impression)
• Mucofunctional (Mucocompression or Pressure Impression Technique)
• Selective pressure impression technique
• Functional Mandibular Impression
• Other alternative techniques

Techniques for Final Impressions

• Construction of the trays
• Impression material used
• The way of impression making

Pressure Control

• Spacer
• Perforations
• Material viscosity

Relationship of Wax Relief and Impression Techniques

• The thickness and the position of wax relief in a custom tray allow the dentist to control how much and where pressure is placed during impression.

I. Minimal Pressure Impression Technique (Mucostatic Impressions/Open Mouth Impression)

• Soft tissues are not compressed or distorted.
• The impression material must flow readily.
• Trays require a spacer with stopers and 1-2 holes for material escape.
• Plaster of Paris was the true mucostatic impression material, though hydrocolloids often yield equally good clinical outcomes.

I. Minimal Pressure Impression Technique (Mucostatic Impressions/Open Mouth Impression)

• Soft tissues that are displaced during function will return to their undisplaced position when forces are released.
• Dentures will be unseated by tissue action (tissue rebound).

The Mucostatic Technique Results

• Results in a denture closely fitting the mucosa of the denture-bearing area, but with poor peripheral seal.
• Retention is mainly due to interfacial surface tension.

Minimal Pressure Impression Technique (Mucostatic Impressions/Open Mouth Impression): Tissue Stops

• 4mm from posterior extension
• 4mm from depth of vestibule

Plaster of Paris Impression

• Images of trays (upper, lower, and sectional views).

I. Minimal Pressure Impression Technique (Mucostatic Impressions/Open Mouth Impression): Advantages

• Operator can see and ensure proper border molding and muscle movements are easily accomplished.
• Less distortion of the mucosa and high regard for tissue health.
• Technique of choice for flabby and thin wiry ridges.

I. Minimal Pressure Impression Technique (Mucostatic Impressions/Open Mouth Impression): Disadvantages

• Mucosal topography is not static over a 24-hour period.
• Insufficient border molding reduces peripheral seal and retention, with potential for food slip.
• Short denture borders may reduce support for the face and affect esthetics.
• Shorter flange means less lateral stability.

Technique

• Primary impression is made with alginate.
• A baseplate wax space is adapted.
• A special tray is adapted over the wax spacer.
• Escape holes are made for relief.
• The spacer is removed, and an impression is made with a free-flowing material.

II. Mucofunctional or Mucocompression Impressions: A-Closed Mouth Impression Technique

• Proposed by Greene in 1896.
• Tissues are recorded under functional pressure to provide better support and retention.
• Records tissues in their functional/supporting form.
• Impresses all tissues under equal pressure irrespective of anatomy.
• Is mechanical rather than biological.

II. Mucofunctional or Mucocompression Impressions: A-Closed Mouth Impression Technique

• Impression material most commonly used is zinc oxide and eugenol paste.
• Trays require occlusion blocks set at the required vertical dimension.

Technique

• Primary impression made with impression compound.
• Special tray used with bite rims and uniform occlusal surfaces with appropriate vertical dimension.
• Secondary impression made using Zinc Oxide and Eugenol impression material.
• Impression is inserted in the mouth and held under biting pressure for 1-2 minutes.
• Mold borders by asking the patient to perform functional movements while biting (close) on the wax rims.

Border Mold 

• Accurate registration of peripheries for retention.
• Study cast image. 

Advantages

• Better retention and support
• Patient can exert masticatory force on material during function making it stable.

Disadvantages of the theory

• Excess pressure may cause alveolar bone resorption.
• Excess pressure on peripheral tissues interferes with blood supply, causing transient ischemia which accelerates ridge resorption.
• Pressure applied during impression-making is not identical to functional load.
• Dentures constructed may not fit well at rest.
• Pressure on sharp bony ridges may result in pain.
• Improper border molding may lead to overextended denture.

II. Mucofunctional or Mucocompression Impressions: b- Opened Mouth Impression Technique

• Images of trays.

Metallic Oxide Paste (Zinc oxide/Eugenol Impression Paste), Green Stick Impression Compound, (Close Fitting Tray)

• Images showing the use of the material

When Tissues are Held

• When tissues are held in a displaced position, pressure limits normal blood flow.
• Depriving tissues of blood supply leads to resorption.

III-Selective Pressure Impression: Applied Aspect

• First advocated by Boucher in 1950.
• This technique combines the principles of pressure over areas with minimal pressure on others.
• The tray is constructed with relief over sensitive areas and closely adapted over stress-bearing areas.

Boucher's Basal Seat Division

• Basal seat areas are divided into zones based on their capacity to withstand masticatory loads without resorption.
• Regions consist of primary stress bearing area, secondary stress bearing area and relief areas.

III-Selective Pressure Impression: 1. The Plaster Wash Impression (Oldest)

• Oldest technique of selective pressure impression technique.
• Compound impression
• Scraping the compound to create space for the impression material.
• Plaster wash impression

III-Selective Pressure Impression: 2. Modified Impression Procedures for Perceived Ridge Support Problems

• Displaceable (flabby) anterior maxillary ridge.
• Window tray impression technique using zinc oxide or polysulfide and zinc oxide imp. material
• Window tray impression technique using a medium-bonded\light-bonded PVS imp. material.
• Fibrous (unemployed) posterior mandibular ridge.
• Flat (atrophic) mandibular ridge covered with atrophic mucosa.

III-Selective Pressure Impression: 2- Modified Impression Procedures for Perceived Ridge Support Problems

• Techniques for displaceable (flabby) anterior maxillary ridge.
• Techniques for fibrous (unemployed) posterior mandibular ridge.
• Techniques for flat (atrophic) mandibular ridge covered with atrophic mucosa.

Support Problems May Be Overcome By

• Appropriate relief of the master cast.
• Modified impression techniques.

i. Displaceable (Flabby) Anterior Maxillary Ridge: 1. Window Tray Impression Technique Using Zinc Oxide Imp. Material

• This technique is used for flabby tissues in the anterior portion of the mouth.
• A close-fitting tray is constructed using cold-curing acrylic resin, exposing the flabby ridge area.

Window Tray Impression Technique

• Steps:
  • Outline the mobile tissue on the preliminary cast.
  • Construct the custom tray with a window over the mobile tissue.
  • Place the handle in the middle of the palate.
  • Border mold and make the zinc oxide impression in the usual manner.
  • Cut out the zinc oxide impression material in the window with a sharp scalpel.
  • Record the mobile tissue area with plaster of Paris impression material seated in the patient's mouth using a brush or syringe.
  • End with the completed impression.

2. Window Tray Impression Technique Using Polysulfide Impression Material

• Used for highly mobile or hypertrophic tissues with minimal displacement.
• Mobile tissues are commonly seen anteriorly in patients with combination syndrome.
• Underlying bony ridges are typically knife edged, and these tissues act as a cushion, rarely impinging on the interocclusal space.
• Steps (similar to zinc oxide technique):
  • Outline mobile tissue on preliminary cast.
  • Construct tray with a window over mobile tissue.
  • Place handle in middle of palate.
  • Border mold and make the polysulfide impression.
  • Cut out polysulfide impression material in the window (using a scalpel).
  • Record the mobile tissue area with zinc oxide impression material.
  • Obtain the completed impression, master cast.

3. Window Tray Impression Technique Using Medium-Bonded PVS Impression Material

• Displaceable area is removed from the special tray.
• A light-bodied PVS impression material is used.
• The completed impression is obtained.

Demerits

• Some feel that it is impossible to record areas with varying pressure.
• Some areas still receive functional load, leading to potential danger of rebounding and loss of retention.

III-Selective Pressure Impression: 2. Modified Impression Techniques: ii. Fibrous (Unemployed) Posterior Mandibular Ridge

• The ridge is not useful for support.
• Recognized by a thin, mobile thread-like ridge (essentially fibrous).
• Modified techniques address this fibrous ridge.

III-Selective Pressure Impression: 2. Modified Impression Techniques: iii. Flat (Atrophic) Mandibular Ridge Covered with Atrophic Mucosa (Atwood's Ridge)

• Complicated by folds of atrophic and/or non-keratinized tissue lying on the ridge.
• A viscous admixture of impression compound and tracing compound removes soft tissue folds and smooths them over the mandibular bone, reducing the potential for discomfort from the atrophic sandwich (mucosa lying between the denture and mandibular bone).

An Admix (Impression Compound & Greenstick)

• An admixture (admix) of 3 parts by weight of impression compound to 7 parts by weight of greenstick (using hot water and kneading with vaselined, gloved fingers) is created.

III-Selective Pressure Impression: 1, 2, 3

• 1. The plaster wash impression i.,ii.,iii. Modified impression procedures for perceived ridge support problems
• 3. The light body wash selective pressure impression technique (conventional technique).

III-Selective Pressure Impression: 3. The Light Body Wash Selective Pressure Imp. Tech

• Beumer et al (2011) developed this method.
• One of the most common and conventional methods used.
• Aims to record tissues at rest with the only exception being the posterior palatal seal area.
• Impressions are made using a lightweight material.

The Selective Pressure Technique

• Combines extension(for max coverage within tissue tolerance) with light pressure or intimate contact(with movable, loosely attached tissues in the vestibules).
• Utilizes a light body impression.

Checklist for Maxillary Final Impression Tray

• Extends to the junction of hard and soft palate.
• Extends into the pterygomaxillary notches.
• Covers maxillary tuberosities.
• Extends into the buccal and labial vestibules.
• Provides relief for labial and buccal frena.
• Be neat, clean, and free from sharp or rough areas.

Areas Requiring Special Attention

• Posterior palatal seal area
• Incisive papilla
• Buccal and labial vestibule
• Mobile, hypertrophic tissue
• Palatal torus

Tray Extension Beyond Vibrating Line

• Typically extends 2–3 mm beyond the vibrating line. 
• Note the pterygomaxillary notch.

Final Impressions: Boxing & Pouring

• Inspect the impression for voids or bubbles.
• Box the impression and pour the master cast.

Box Impression Advantages

• Preserves the border of the impression.
• Allows control of cast base thickness.
• Enables stone material vibration into the impression.
• Saves time and conserves material.

Checklist for Mandibular Final Impression Tray

• Extend to the external oblique ridge.
• Extend into the retromylohyoid space.
• Extend into the alveololingual sulcus
• Be neat, clean and free from sharp areas.

Custom Tray Outline

• The custom tray outline is typically drawn 2–3 mm short of the denture outline.
• Baseplate wax is added for relief over tori and flabby ridges.
• Undercuts of the anterior labial and posterior lingual regions are blocked out with wax.
• Note the finger rests and the size and position of the handle. 

Undercuts to Block Out

• Lingual side of the mandible opposite the retromylohyoid space
• Mylohyoid ridge
• Frenum
• Buccal shelf not covered by wax

This quiz explores the fundamental concepts of secondary impressions in forensic science. Answer questions related to their characteristics, techniques used to create them, and the comparisons with primary impressions. Test your understanding of the details and drawbacks associated with secondary impressions.