From changing techniques to seating trays, Mohammad Khalfan reveals his top clinical tips and advice when it comes to impression taking
Know why you are taking the impression:
Although this one may seem repetitive, knowing what you want to do with the impression will dictate your anatomy. I cannot change that, unfortunately. Is it for a partially dentate patient? (What teeth do we want to replace?) Or a completely edentulous patient? Study models? Answering these questions will help you know what structures you need to capture with your impression.
Generally, we want to capture the functional depth and width of the sulci with the denture-bearing areas and the teeth. The denture-bearing area consists of the buccal shelf, residual ridge and retromolar pad (mandible) and maxillary tuberosity, alveolar ridge and rugae (maxilla).
You will not worry about the retromolar pad for a lower partial denture unless it involved a free-end saddle. Yet, you will need it for every case of complete lower denture because of its role in peripheral seal, retention and stability. However, sometimes compromises are needed. For example, we can occasionally ignore capturing a maxillary torus when taking an impression of a severely retching patient.
How to find areas of overextension
Overextended trays will mostly interfere with border moulding and achieving muscle balance. If the primary tray is overextended, you may not capture high frenal attachment for relief. Hence, you will need to reduce the special tray.
Sometimes tray extensions can be checked with direct vision and good lighting but it can also be tricky, especially with lingual flanges of lower special trays. An easy way to check areas of overextension is to apply a thin layer of light body silicone onto the tray and the overextended regions will rub off. This will guide you to areas you will need to reduce more with a straight handpiece and an acrylic bur.
Extending the secondary tray
The quickest way to rigidly mould a tray is with greenstick. However, using it can be a technically challenging. A straightforward way to do this is to extend the tray outside the mouth. Use a hot air heater to heat the greenstick, and while doing this, gently keep rotating the greenstick; it should not remain still in flame longer than a second. Soon enough, the shiny surface will be tacky to touch, and the greenstick will be ready to use once it starts to sag.
Wipe off the warmed greenstick onto the tray, and it will effectively adhere to it when the tray is dry. Use lubricated gloves with Vaseline, and, under warm water, mould the greenstick to the desired shape; if you plan for a complete upper denture and want to capture the tuberosity, your thumb will make an ideal contour.
Another effective technique involves placing the greenstick in warm water, giving it some time to sag, and then shaping it onto the tray under warm water with lubricated gloves. You need to be quick when it starts to sag; otherwise, you will struggle more than ever.
Seat your trays posteriorly first then anteriorly
It is common for students to seat the tray randomly once it is inserted into the oral cavity, and a big part of the reason is anxiety. Next time, try to seat the posterior aspects of the tray first, then anteriorly. This ensures any excess flows towards the front of the mouth and allows you to watch the tray being seated over anterior teeth/ridge.
If you notice your tray is close to the labial surfaces of anterior teeth, it is likely the tray is too far posteriorly.
The power of the tray handles
The handles of the tray are a bit under-appreciated. Use it to check the orientation of the tray relative to the facial midline and occlusal planes.
Further, when seating the tray, try to use the handle rather than allowing your fingers to sit onto the palatal vaults, as this will prevent the material from engaging with the retentive features of the tray and may detach upon removal.
For your ease, you can ask the technician for a stub handle and finger rests with lower special trays, which provide better control and grip of the tray without restricting border moulding movements particularly of the lower lip to avoid overextension.
Mobile teeth
A very tricky problem to deal with. While it is sometimes necessary to discuss the risk of potential inadvertent extraction and/or discomfort when taking the impression, a few things can lower this risk. First and foremost, if the teeth are sore, you may need to give local anaesthetic. Then consider using some or all three of:
- Block out significant undercuts with soft wax
- Apply a thin film of Vaseline over mobile teeth
- Remove the tray in an upward sharp motion rather than twisting.
Alternatively, in difficult cases you occasionally have to consider splinting teeth before taking the impression. Any of these methods can make your life easier when taking an impression.
Change your technique if it does not work, using the admix impression for complete dentures
The most common reason for secondary care referral of edentate patients is resorption of the lower ridge. Patients with flat ridge form as well as those with depressed ridge form (class V and VI of Cawood-Howell classification) are particularly challenging when constructing complete dentures. This is because of the reduced area of support available. For this technique, you will need red impression compound, greenstick, warm water and Vaseline.
You will use 30% impression compound for 70% greenstick. But first, separately heat them with warm water. Once they have heated up evenly, the admix is created by kneading the materials together with gloved fingers coated in Vaseline.
The final colour will be browner green when compared to the greenstick colour. The material is then loaded on the tray, placed in a warm bath for 30 seconds, and promptly transferred to the patient’s mouth. The working time is often one to two minutes.
On removal of impression, it is chilled in water and then reinserted with pressing in the premolar region to reciprocate the pressure on the mandible. Relieve areas of pain with a hot wax knife or highlight them for the technician to relieve on the master cast.
For references, email [email protected].
Acknowledgements:
Dr Andrew Paterson, Glasgow Dental School
Dr Danah Alkhateeb, Manchester Dental Hospital and School
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