An excellent quality dental impression is critical, because without it the dental lab cannot fabricate accurate, well-fitting restorations. Ideally, potential dental impression errors should be identified before the impression is sent to the lab. Otherwise, work may be delayed if the impression is rejected.
If the work goes ahead, there is an increased risk of spending precious chair-side time adjusting it, or it might need to be remade entirely.
The most common dental impression errors include:
Poor Tray Selection
Inadequate Impression Material Mixing
Poor Margin Detail
Choosing the Wrong Impression Material
Rushing the Impression
Failing to Keep the Patient Still
1. Poor Tray Selection
The tray selected must capture the necessary information without distortion. It should be large enough to cover all the teeth without contacting the soft tissues. Once the impression is taken, the tray should not show through.
Full arch trays are available in several sizes. The arch shape of trays varies according to the manufacturer as some are more square while others are more rounded. When selecting your tray, make sure it is long enough to capture the entire arch and wide enough to allow adequate seating of the tray. Metal trays are modifiable and may be widened in the posterior regions, but modifications to the anterior portion of metal trays are trickier. Plastic trays can be modified using an alcohol torch to heat the plastic, adapting the flanges to fit the patient.
Dual-arch trays (triple trays) come in a variety of widths, and it’s best to keep a selection in stock. When sizing a dual-arch tray, check the opposite side of the arch as the patient bites down on the empty tray. This will provide you with a good idea as to how the occlusion should look while taking the impression. Also, it allows the patient to feel what they need to do and will help ensure they bite down correctly during the actual impression taking. It is useful to provide a separate bite using rigid vinyl polysiloxane (VPS) material because wax bites tend to be less stable during transportation.
2. Inadequate Impression Material Mixing
When impression material is combined correctly, the color is uniform and without streaks. It’s more common to get streaking with hand-mixed putty materials, and hand-mixed materials should be kneaded quickly to ensure consistent color. When working with cartridge materials, bleed the cartridge before attaching the automix tip. This will ensure the catalyst and the base flow equally, avoiding issues with mixing. It is critical to use tray material and wash from the same manufacturer as these materials are chemically designed to work together. Using different materials could increase the risk of separation of the wash material from the tray VPS when the impression is poured in the dental lab.
3. Surface Contamination
Surface contamination can cause a tacky unset layer of impression material. It may be due to core build-up material, or composites or adhesives leaving a greasy coating on the tooth preparation. Any of these can prevent the material from setting correctly. Retraction cords and solutions containing aluminum chloride or ferric sulfate can transfer sulfur to critical areas, inhibiting the setting reaction of the marginal VPS material. Glove contact or rolling the retraction cord in gloved fingers or using a rubber dam can have the same effect. Putty can become contaminated with latex when mixing by hand.
To prevent surface contamination, rinse the area with water or mouthwash once the rubber dam is removed and make sure it is thoroughly dried. After using hemostatic agents, ensure the preparation is vigorously washed to remove any residual debris and hemostatic. If gingival oozing restarts, additional hemostatic can be burnished into the gingival to prevent further bleeding. Once bleeding has stopped, the prep is air-dried and ready for an impression. If the putty is mixed by hand, ensure gloved hands are washed, which will remove any residual surface sulfides and powder. Otherwise, choose powder-free or vinyl gloves.
4. Poor Margin Detail
The margin is one of the most critical aspects of a dental impression. Without an accurate marginal impression, problems such as overhanging or open margins, or an inadequately fitting restoration are more likely. Usually, voids at the margin are because of insufficient retraction or where fluid has accumulated, preventing the impression material from flowing around the margin.
Using retraction cords with syringeable hemostatics is the best way to solve this problem. One technique is to use a double retraction cord. Another approach is to use retraction pastes that are placed into the gingival sulcus after preparation. A diode laser can be helpful for widening the sulcus, improving visualization of the prepared margin while achieving hemostasis.
5. Internal Bubbles
If moisture, including blood, water, or saliva, is trapped in the impression material, it can cause bubbles in the impression. These bubbles may be large enough to affect the luting agent, increasing the space to be filled. When the luting material is thicker, the interface between the restoration and tooth is weaker. Also, the prosthetic material could be thinner, increasing the risk of failure; this is especially critical when using all-ceramic materials in which case a minimum thickness is critical. This problem can be solved by achieving hemostasis.
6. Marginal Tears
Marginal tears can occur when the wash material has insufficient tear strength. The strength of materials varies between manufacturers, and lower viscosity material is more likely to tear in the sulcus. When the sulcus is particularly deep, the wash material is thinner, and there is an increased risk that it will tear when removed. Also, removing the impression before the wash material is wholly set could cause marginal tearing. If you need to retake an impression because of marginal tearing, make sure any remnants are removed from the sulcus and consider additional tissue retraction to widen the sulcus. Also, choosing a more viscous wash material could help to improve the quality of the impression.
7. Choosing the Right Impression Material
Selecting the right impression material is crucial for achieving an accurate and predictable impression. Usually, a sort setting time is preferable for your patient’s comfort; in any case it is vital to know the working time for the material you have chosen. The impression must be inserted within the working time. If it is already setting, it may not seat fully and will not capture the required details. Choosing a material that is more hydrophilic will help it adapt to the prepared tooth more easily, especially sub-gingivally and where fluid may be present.
8. Trying to Rush the Impression
It is crucial to take your time, making sure the details are correct (for example packing the retraction cord). Ensure the cord is packed correctly and has retracted the tissue sufficiently. If you rush, the cord may not have enough time to sit, and it will do little other than traumatize the area around the sulcus.
9. Failing to Keep the Patient Still
Dental impressions can distort if the patient moves or begins to gag. One way to help keep the patient still is to chat with them; this will distract them so they are less likely to notice any discomfort. Keep reminding the patient to breathe through their nose. If they feel uncomfortable, wiggling their toes could help.
DDS Lab uses an impression rating system, and each impression is graded according to its quality. If you have questions, our experienced technical team can offer advice on how to achieve more predictable and higher-quality impressions.