There is nothing quite like the satisfaction of seating a case successfully without the need for time-consuming adjustments. Conversely, it can be very frustrating for the clinician and the patient when things don’t go smoothly. Perhaps a restoration fails to seat correctly or needs such significant adjustments that it must be returned to the lab for refinishing or entirely remaking. Poor impressions and incorrect shades are two of the most common failures resulting in remakes and adjustments, and there are many others.
Taking a perfect impression is a critical and technique-sensitive step when fabricating restorations. The clinician must identify possible complications that could affect the accuracy of the restoration or lead to the impression being rejected by the dental lab.
One of the most common restoration issues is with margin details, which, if not perfect, result in an inadequately fitting appliance that must be remade.
Here are a few of the most common margin errors and how to fix them:
Margin voids may be due to insufficient retraction or fluid accumulation which prevent the impression material from flowing fully around the margin. Margin voids can often be addressed by improving retraction techniques.
Fluid accumulation can also cause internal bubbles, which, when present on the margins of the preparation, can affect the fit of the prosthesis. These bubbles make the restorative material thinner than recommended, which might be critical when using all-ceramics. Rinsing and drying the preparation before impression-taking can solve the problem.
Air bubbles can be an issue for narrow, deep preparations. If an air bubble is present on the model, it can prevent the restoration from seating fully. Thoroughly rinsing and drying the preparation before taking an impression reduces the risk of air bubbles.
The most common reason for air bubbles/voids, is air trapped in the syringe, either during filling of the syringe or during application of the impression material around the sulcus.
If a syringeable material with poor tear strength is chosen, marginal tears are a risk, especially where there is a deep, thin sulcus. Removing the impression before it is completely set can also cause tears in the margin. Choosing a material with higher viscosity can prevent margin tears. Additional tissue retraction might also be needed.
Drags, pulls, or folds in the impression can develop when the impression material is no longer in its most fluid state when inserted into the mouth or when the impression tray is inserted in a single motion. Sometimes the impression material fails to adapt to the teeth. Drags aren’t easy to correct. You can sometimes remedy a pull by removing interproximal material that may have been hindering insertion and by covering the impression with a syringeable material, filling the depressions so that the impression can be reinserted into the mouth. This is known as relining the impression and causes more problems than it solves, such as poor fit.
Selecting the Wrong Tray
The impression tray should be large enough to hold enough impression material so that all the teeth are covered without the tray contacting the soft tissues. Once taken, the impression tray shouldn’t be visible through the impression material. Selecting the correct width of the impression tray is also important, because if it is too narrow, the tray might not seat fully. Plastic stock trays are much easier to modify than metal trays, which can normally only be widened in the posterior region of the mouth.
Impression trays can also distort, especially when the more flexible dual-arch trays are chosen. Selecting a rigid enough tray to resist distortion is crucial. When selecting dual-arch trays, the impression must capture at least one entire tooth mesially and distally of the tooth to be restored.
Impression Separating from the Tray
Sometimes the impression material may become separated from the tray, and the problem might not be detected until the restoration is tried in. Using tray adhesive each time can reduce this error. Use the adhesive made by the same manufacturer as the impression material, because the chemical composition will be compatible. Before applying the impression material, make sure the adhesive is dry. Some clinicians prefer to apply the adhesive at the beginning of the patient’s appointment, so it doesn’t delay impression taking.
Contamination of the Impression Surface
Less commonly, the surface of the impression material can become contaminated, affecting the material’s ability to set properly. Dental materials, including core build-up materials and composites and adhesives, can leave a greasy film on the preparation. Thoroughly cleaning the tooth with alcohol removes any film. Retraction cords and using a rubber dam may also contaminate the surface, so the areas should be rinsed with water or mouthwash and thoroughly dried. Anesthetic leaking from injection site is the most common reason for impression contamination.
Inaccurate Shade Matching
Shade taking is often subjective and involves many variables, not all of which can be easily controlled. Often the clinician must balance the patient’s expectations with realistic goals. Taking an exact shade can be elusive because of the layered quality of teeth.
Three different elements must be considered: hue, chroma, and value.
The hue is the color of the tooth.
Chroma is the concentration or saturation of the hue.
The value of a tooth is the lightness or darkness of the hue and is frequently the most critical factor when determining a shade.
Inaccuracies in shade taking occur if the lighting is incorrect and are affected by other factors such as color blindness and fatigue. Luckily, the tools and techniques for accurate shade taking are improving all the time.
Traditionally, shade tabs were held up to the patient’s mouth, and a single shade was selected. This technique still works reasonably well today, but it oversimplifies the process of shade-taking. Often, the patient’s teeth will be a combination of several different shades, and a standard shade guide only provides a starting point for determining the hue, chroma, and value of the tooth.
These days, digital imaging and hand-held image capturing devices are available, reducing human error in color perception. The images produced by these handheld devices are viewable on-screen by the technician, so the restoration can be checked for accuracy before sending the finished product to the clinician. However, their usefulness is limited unless the lab uses the same mapping Rx that goes with the clinician’s device. Otherwise, there is the risk of miscommunication of the shade.
Remakes and adjustments are frustrating for everyone, and being aware of the most common technique failures can help you avoid them. Your dental lab can also provide useful help and advice to improve accuracy in the future.
Please be reminded that should you wish to discuss a case in more detail, our experienced technical team is here to assist you.