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References

McCord JF, Grant AA. Prosthetics: impression making. Br Dent J. 2000; 188
McCord JF, Tyson KW. A conservative prosthodontic option for the treatment of edentulous patients with atrophic (flat) mandibular ridges. Br Dent J. 1997; 182:469-472
Cawood JI, Howell RA. A classification of the edentulous jaws. Int J Oral Maxillofac Surg. 1988; 17:232-236
Scott BJ, Hunter RV. Creating complete dentures that are stable in function. Dent Update. 2008; 35:259-267
Feine JS, Carlsson GE, Awad MA, Chehade A, Duncan WJ, Gizani S, Head T, Lund JP, MacEntee M, Mericske-Stern R, Mojon P. The McGill consensus statement on overdentures. Mandibular two-implant overdentures as first choice standard of care for edentulous patients. Montreal, Quebec. 2002; 17
Thomason JM, Feine J, Exley C, Moynihan P, Müller F, Naert I Mandibular two implant-supported overdentures as the first choice standard of care for edentulous patients – the York Consensus Statement. Br Dent J. 2009; 207:185-186
McCord JF, McNally LM, Smith PW, Grey NJ. Does the nature of the definitive impression material influence the outcome of (mandibular) complete dentures?. Eur J Prosthodont Rest Dent. 2005; 13:105-108
Guidelines in Prosthetic and Implant Dentistry. In: Ogden A (ed). London: Quintessence; 1996
Field J. First impressions count: how to take a primary impression. Dent Nursing. 2016; 12:72-79
Turner JW, Moazzez R, Banerjee A. First impressions count. Dent Update. 2012; 39:455-471
Basker R. Prosthetic Treatment of the Edentulous Patient, 5th edn. Oxford: Wiley Blackwell; 2011

Technique Tips Prosthodontics: The Admix Impression

From Volume 45, Issue 10, November 2018 | Pages 991-993

Authors

Kasim Butt

BDS, MJDF RCS Eng, PgCert Dent Ed

Specialty Registrar in Restorative Dentistry, Sheffield Teaching Hospitals NHS Foundation Trust

Articles by Kasim Butt

Email Kasim Butt

Karun Dewan

BDS, MFDS RCSEng, LDS RCSEng, MSc(Prosth Dent), FDS RCS(Rest Dent)

Staff Grade, Department of Restorative Dentistry, Morriston Hospital, Swansea SA6 6NL, UK

Articles by Karun Dewan

Article

The ‘admix impression’ is a definitive secondary impression technique used in the management of severely resorbed mandibular ridges covered with atrophic mucosa.1 First described by McCord and Tyson, it involves the use of a viscous admix of impression cake compound and greenstick tracing compound.2 Mandibular ridges equating to Cawood and Howell ridge classification V and VI often pose clinical challenges when constructing a satisfactory conventional complete denture.3 As the mandibular alveolar ridge resorbs there is a reduced area of support available for a mandibular complete denture.4 The ridge may be complicated further with folds of thin atrophic (non-keratinized) mucosa, which may cause pain and discomfort upon contact with a denture base.1

The McGill Consensus in 20025 and the York Consensus in 20096 concluded that a two implant-supported mandibular overdenture is the first choice standard of care, as opposed to a conventional mandibular denture for edentulous patients. However, this option may not always be feasible in some cases owing to financial constraints, patient choice or systemic medical conditions. A randomized control trial found that patients with a mandibular ridge equating to Cawood and Howell ridge classification V (Figure 1) and VI preferred mandibular dentures constructed with definitive impressions made using the admix impression technique.7 The aim of this paper is to re-visit the ‘admix impression’ technique to aid the practitioner in the construction of a conventional mandibular complete denture in patients with a severely resorbed atrophic mandibular ridge.

Figure 1. A severely resorbed mandibular ridge equating to Cawood and Howell ridge classification V. Note the presence of folds of thin atrophic mucosa.

The admix impression technique

  • A primary impression should be taken of the edentulous mandibular ridge for the construction of an accurate close-fitting customtray. The British Society for the Study of Prosthetic Dentistry (BSSPD) states that primary impressions should accurately record clinically relevant anatomical landmarks of the edentulous mouth without excessive tissue distortion.8 Rigid stock trays may need to be modified to achieve this.9 Clinical landmarks that should be recorded when making a primary impression of an edentulous mandible are highlighted in Table 1 and Figure 2.
  • Following a primary impression, it is critical that a detailed design for a custom-tray is outlined to the technician. A close-fitting, non-perforated, light-cured, acrylic custom-tray, which covers all the clinically relevant landmarks and is 2 mm short of the functional depth of the sulcus, is required. To avoid distortion of the lower lip during impression taking, and to allow accurate functional movements to take place, a stub handle design should be incorporated.11 In addition, finger rests in the premolar region allow the tray to be stabilized during the impression without restricting border-moulding movements.11 An example of a mandibular custom-tray with a stub handle and finger rest design is shown in Figure 3.
  • The British Society of Prosthetic Dentistry (BSSPD) guidelines state that definitive secondary impressions ‘should record the entire functional denture-bearing area to ensure maximum support, retention, and stability for the denture during use’.8 The custom-tray is tried in the mouth and extensions are checked. The tray should be covering the clinical landmarks highlighted above in Figure 2 and also be 2 mm short of the depth of the sulcus to allow border moulding to take place in order to record the functional depth and width of the sulcus.
  • Three parts by weight of (red) impression cake compound to seven parts by weight of greenstick are required.2 These impression materials are coated in Vaseline, and placed in a water bath set to a temperature of 68–70 degrees Celsius.2Figure 4 shows the materials needed for an admix impression.
  • Once the separate constituents are warmed evenly, the admix is created by kneading the materials together with gloved fingers coated in Vaseline. The final colour is a browner green than the original greenstick (Figure 5).
  • The impression material is quickly loaded into the tray (with a well-fitting special tray a layer of 3–4 mm is usually sufficient). Once the tray is loaded, it should be soaked back in the water bath for 30 seconds. Once removed, it is promptly transferred to the patient's mouth.
  • Using a standard impression technique, the tray is stabilized with pressure on the finger rests and border moulding is undertaken. Patients are instructed to carry out functional movements such as licking their upper lip, swallowing and raising their tongue to the roof of the mouth. The working time for this admix material is 1–2 minutes and therefore it is important that these movements are rehearsed prior to definitive impression-taking and conducted in a prompt manner.
  • On removal, the impression is chilled in water, and then re-inserted. Clinicians should press on the finger rests in the premolar region and reciprocate this pressure with their thumbs on the inferior body of the mandible. Any discomfort in the denture-bearing area which patients feel at this point can either be highlighted to technicians for the application of relief on the master cast, or the impression surface can be adjusted with a hot wax knife.2 This provides an objective test as the pressure placed using this technique on the chilled impression simulates the pressure on the completed denture in function. This affords credibility to patients that they should be able to bite without discomfort.

    • Anatomical Landmark Clinical Relevance to Prosthesis
    Retromolar Pads Retention/Peripheral seal – The posterior border of the mandibular complete denture should extend up to between half to two-thirds of the retromolar pads, displacing them and providing a peripheral seal.
    Buccal Shelves Located between the alveolar ridge crest and the external oblique ridge. Provides primary support for a mandibular complete denture.
    Residual Alveolar Ridge Provides stability and secondary support if the ridge is well formed with good height and width.
    Retromylohyoid Fossa Located distal to the attachment of the mylohyoid muscle. Provides stability when engaged bilaterally by the disto-lingual aspect of the denture flange.
    Mylohyoid Ridge The mylohyoid muscle attaches the mylohyoid ridge. When the inferior border of the denture flange contacts the contracted mylohyoid muscle a border seal may be achieved, contributing to retention.
    Buccal and Lingual Sulci The denture flange needs to fill the width and depth of the sulci to achieve a peripheral border seal, contributing to retention.
    Fraenum Attachments The denture needs to be notched around these to prevent trauma and displacement during function.
    Figure 2. An edentulous mandible illustrating the following anatomical landmarks relevant to mandibular complete denture construction: 1 – Retromolar pad; 2 – Residual alveolar ridge; 3 – Mylohyoid ridge; 4 – Buccal shelf; 5 – Retromylohyoid fossa.
    Figure 3. Mandibular custom-tray with a stub handle and finger rest design.
    Figure 4. The materials needed for an admix impression.
    Figure 5. A completed admix impression of a severely resorbed edentulous mandibular ridge.

    Conclusion

    The admix impression is a useful alternative prosthetic technique in the management of severely resorbed mandibular ridges when finances, patient choice or systemic medical conditions prevent dental implant treatment. It provides an accurate impression of the severely resorbed atrophic mandibular ridge. It also allows thin folds of non-keratinized mucosa, which may be painful upon contact with the denture base, to be identified and relieved prior to denture base construction.