How To Take A Good Dental Impression With Difficult Access

As a dentist, I strive for excellence in all aspects of dentistry. In all indirect restorations, superb impressions are paramount for accurate recording of the prepared tooth/ teeth and surrounding soft mucosa, gingiva. It is all too easy to accept a sub optimal impression and ask the technician to 'do his best'. This is not acceptable. The working impression should replicate the patient as closely as possible. This will allow your technician to case an accurate impression and ultimately produce a well-fitting restoration with minimal or no adjustment. I will describe a technique that will allow an accurate impression in those cases with difficult access.

1. Ensure that the margins are clear and visible.

2. A dry field is required.

3. Use retraction cord with a haemostatic agent or a soft tissue diode laser if you have one. This will allow removal of excess tissue, provide haemostasis and most importantly allow the recording of at least 1mm of tooth tissue below the margin line.

4. Ensure there is adequate occlusal clearance. Most technicians say that the most common error is lack of occlusal space. Use clearance guides if you have them or use parallel sided hand instrument which is pre measured using calipers and slide along the medial and distal aspect of the preparation to verify clearance.

5. Use loupes to ensure your margins are clear and even. If you have a intra oral camera, take an image from an occlusal view and ensure you have 360 degree visualisation of the preparation.

6. Block out any undercuts and large inter dental spaces using block out resin. This is a step that is often overlooked but vital to ensure the impression can be removed with ease. Your patient will love you for this!

1. These areas can be very difficult due to the limited access and muscular actions of the tongue.

2. Initially choose your tray carefully and ensure it covers all aspects of the preparation and the entire arch.

3. Take a PVS impression of the entire arch in heavy and light body impression material. My material of choice is Honigum by DMG. This should be accurate as possible.

4. Trim the excess around the borders.

5. Re seat and ensure the impression re locates with ease.

6. Remove and bur away the preparation site using an acrylic bur. Drill a hole into the occlusal aspect of the impression tray of the prepared tooth and a channel buccally from the impression material.

7. Re seat the impression and syringe into the access hole light body material until the excess flows through the pre drilled channel. Allow to set.

8. Remove and you should have recorded an accurate impression of the preparation with NO airflows, NO drags, clear margins and at least 1mm below the impression line. This technique will allow you take impressions in the most difficult access cases- please try it the next time you have a crown or bridge prep.

If there are any questions on this technique, please do not hesitate to contact me: nissit@progressivedentistry.co.uk

The future is digital. Intra oral scanners will become a greater part of dentistry in the next few years which will eliminate the need for conventional dental impressions. This is incredibly exciting and there are huge benefits with this technology. Having tested this in practice, I can say the techniques are incredibly accurate. However, at present the systems are too costly for most dental practices and time consuming. This will change as we have seen with digital radiography. Once it does, we will convert to intra oral scanning for all of our impressions. I will keep you posted.

This article is for dentists, written by Dr Nissit Patel.