Predictable Impression Technique for Indirect Restorations
Douglas A. Terry, DDS
accurate impression is the foundation for success with indirect restorations.1,2
Modern impression techniques used in restorative dentistry require displacement
of gingival tissue to record the exact dimensions of the preparation, the architecture
of the margins of the preparation, and the relationship of the prepared teeth
to the soft tissue and surrounding dentition.3 Restorative and periodontal
complications can occur from improperly positioned subgingival margins, thin
tissue biotype, bulky fibrous papilla, and traumatic manipulation of the soft
tissue.2 Achieving an accurate final impression is the result of
properly integrating multiple interrelated steps throughout the process.
Criteria for an Accurate Impression
of factors influences the quality and predictability of a final impression,
such as moisture control, gingival retraction method, displacement time, size
and type of impression tray, volume of the impression material, and tissue
management.1,4,5 According to the literature, the disparate adoption
of the latest materials may not lead to clinical success, but the accuracy of
the impression may be controlled by technique.6 An accurate
impression should provide the following:
- Adequate wash thickness to withstand
distortion and tearing when intraorally removed;
- No evidence of voids, bubbles, drags, or
- Displays a uniform homogenous mix of materials;
- Uniform bond between the impression material,
adhesive, and tray;
- Reproduces fine surface details, free of debris
such as saliva and blood; and
- Distortion free, and completely set upon
Managing the Periodontal Complex
periodontal tissue is a prerequisite for success. Inflammation of gingival
tissue prior to impression-taking can complicate the procedure. Bleeding and
moisture from crevicular fluid can displace impression material, resulting in
voids and rounded indistinct finish lines that can cause an inaccurate castand an improperly fitting final restoration.4 Furthermore, if
a subgingival margin is placed in the presence of inflammation, there is
potential for gingival recession and exposure of the restorative finish line.7,8
The soft tissue must, therefore, be properly managed.
preoperative consideration during initial therapy is control and elimination of
all sources of irritation and inflammation. Unfortunately, this may require delaying
the impression procedure after tooth preparation to allow for improvement in
the soft tissue condition. The provisional restoration is an essential
component of this initial therapy and can improve the quality of the
impression. It preserves the position, form, and color of the gingiva and
maintains the periodontal health prior to impression-taking and while the
definitive restoration is being fabricated.
tissue management during the preparation and impression-making stages requires
an understanding of the gingival tissue architecture. The most important
determining factor to predicting how the tissue will respond to preparation and
impression techniques begins with the relationship of the free gingival margin
to the osseous crest. Preoperative recordings of facial and interproximal bone
height, and determination and preservation of the biologic width, can provide
predictable postrestorative gingival margin levels and periodontal health.
Clinical Guidelines for Impression Techniques
Various impression techniques can be utilized
for different restorative and prosthetic reconstructive procedures. These
techniques include the putty-wash one- and two-step system, dual-arch method,
and simultaneous dual-viscosity--designed procedures such as the
one-step/double-mix impression method. Impression materials will only make
accurate impressions of tooth surfaces that are clean, visible, and dry. Tooth
preparations with subgingival finish lines, therefore, require displacement of
gingival tissue. Several techniques have been proposed in the literature, such
as electrosurgery, rotative gingival curettage, and diode laser-assisted
gingival troughing.9-11 To achieve a gingival displacement with
minimal trauma, the retraction cord technique is most reliable and has demonstrated
a high degree of predictability in regards to control of gingival recession
caused by the impression. One study indicates that a minimal crevicular width
of 0.20 mm is required for successful impressions.3
measurements on the facial and interproximal regions of the unprepared tooth
can provide predictable categorization. These osseous crest positions can be
divided into three categories: normal, low, and high. For a normal crest
position (ie, 85% of patients), these two measurements for anterior teeth
should be approximately 3 mm on the facial and 4 mm on the interproximal
aspects when adjacent teeth are present.2 When the depth of the
osseous crest to gingival margin is greater than these measurements, it is
considered a low crest position.2 If the depths are less than these
measurements, it is considered a high crest position.2 In a normal
osseous crest position, the gingival complex will return to a normal crest
relationship after tissue manipulation and impression-taking.1,12 In
a high crest relationship, however, traumatic manipulation and placement of
subgingival preparation margins will position the final restoration too close
to the osseous crest, creating a violation in biologic width.1,13
The low crest position with a thin biotype is considered the most unstable
clinical situation and can result in the most variation in final gingival
position.2,14 For optimal restorative results, the low and high
osseous positions should be corrected through osseous and/or orthodontic
treatment prior to restorative treatment. In clinical situations where
adjunctive therapy is not planned, careful and gentle manipulation is critical.
During gingival displacement, therefore, the number of retraction cords and the
size should be modified according to the osseous crest position and tissue
following clinical protocol demonstrates the one-step/double-mix impression
with a double-cord gingival displacement for a normal crest position. During the
restorative phase and after the onset of anesthesia, the tooth is prepared
relative to the osseous crest with the finish line following the scallop of the
gingiva. A primary compression cord of small diameter is soaked in a plain
buffered aluminum chloride solution and gently placed in the bottom of the
sulcus around the preparation with light pressure from a cord-packing
instrument (Figure 1A). The finish line of the preparation is extended to the coronal
aspect of the cord, which places the finish line of the final restoration
approximately 0.5 mm to 1 mm below the gingiva. This initial placement of the
retraction cord provides a seal to the sulcus to prevent contamination of the
margins by blood or crevicular fluid. The first cord layer is a sulcus liner,
to prevent tearing of the sulcular epithelium and bleeding, which can be a concern
with the single-cord technique. Additionally, it retracts the tissue so as to
prevent contact of the diamond bur with the gingival epithelium during final
margin placement(Figure 1B).9
retraction cord is then inserted to displace the tissue apically and laterally
(Figure 2A). The gingival retraction is allowed to remain for five minutes to
allow water absorption by the superficial cord. This generates expansion of the
superficial cord and increases the crevicular width.Prior to
taking the impression, any excess moisture is eliminated. The second retraction
cord is removed, and a low-viscosity impression material is immediately injected
into the sulcus (Figure 2B). The entire preparation is covered with the low-viscosity
material and directly followed by the placement of the tray, which has been
loaded with a more viscous material. The tray is removed along the path of
insertion after inspection of the set material and upon reaching the specified
setting time. The impression is then confirmed for accuracy with magnification.
The ideal registration is created with a laterally deflected sulcus greater
than 0.5 mm in width, and more than 0.5 mm of apical deflection sufficient to
record an adequate amount of unprepared tooth structure apical to the margin.
quality and accuracy of the final impression plays a significant role in
restorative success. An understanding of gingival tissue architecture and how
it influences restorative margin placement can provide insight into proper
management of soft tissue and improved impression techniques. One should not
discount, however, the influence of a clinician’s skill and experience with a
specific material and technique on the quality of the final impression and
Professor, Department of Restorative Dentistry and Biomaterials, University of
Texas Health Science Center Dental Branch, Houston, TX; private practice,
Institute of Esthetic and Restorative Dentistry, Houston, TX.
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