Impression Flaws During Dental Impressions
The term “impression flaws” is an umbrella term referring to all unwanted complications that may arise when dental impressions are taken.
The impression is the basis for your dental prosthesis; mistakes that occur during this step of the procedure will inevitably lead to an ill-fitting prosthesis. Here you see a brief summary of the work steps involved after the impression has been taken. The impression is removed from the patient’s mouth and sent to the dental technician, who pours plaster into the impression, thus obtaining a replica of the patient’s mouth; this is called a plaster model. The dental technician uses this model to produce the prosthesis.
One of the most common mistakes when taking impressions is the presence of air bubbles. By using a fine syringe most air bubbles can be avoided. Distributing the impression material with the aid of the air blower further decreases the presence of air bubbles. Another frequent mistake is the lack of preparation margins. When the dentist cuts the tooth for a crown, a preparation margin results. A preparation margin refers to the point where the uncut tooth transitions into the cut tooth.
It is imperative that this preparation margin is apparent in the impression; otherwise, the dental technician does not know up to what point he should prepare the crown margins. In many cases, blood, saliva, or the gums themselves obscure the preparation margins. Thus, if the impression is taken without the appropriate preliminary work, the impression material is not able to surround the preparation margins. This results in a dental impression with missing borders. In practice, the dental technician often can’t recognize preparation margins because of bad impressions, so he determines them himself. Normally, the margins are then placed a few millimeters below the gum line, where you won’t notice the leaking crown margins. The consequence is a dental prosthesis (in this case, a crown), that is not closed off tightly and, moreover, irritates the gums – you will notice bleeding and receding gums. This mistake can be avoided by laying threads prior to the execution of the impression and preparation. More about that in the video entitled "Dental Impression".
Of course, the impression material may become deformed; this can happen primarily if the material has exceeded its shelf-life, if the processing times are not observed, or if the impression material is handled incorrectly. Thus, e.g., because of the occlusion of the teeth, the impression in the upper jaw should be taken on the opposite side of the cut teeth first, because this way, the impression is pulled off in a lengthwise direction from the cut tooth and is less likely to become deformed.
The teeth in the lower jaw are leaning towards the inside – therefore, the impression in the lower jaw should be prepared and pulled off on the side of the cut teeth first. If the impression tray is removed on the wrong side, the impression material is then more likely to become deformed. Taking such details into consideration prevents the impression from becoming deformed. Furthermore, impressions should always be taken with full trays! As a rule, partial trays are not exact, because the position of the plaster models cannot be precisely aligned with one another.
With a partial tray, the dental technician obtains partial models; these can be swiveled and thus can’t be positioned accurately. The dental prosthesis can’t be shaped precisely. A full impression yields complete jaw models and thus, a so-called three-point contact. Therefore, the models cannot be swiveled. It is only then that the technician is able to produce an exact-fitting dental prosthesis. A pre-impression – the pink impression material in the image – prevents the teeth from touching the impression tray (which is called push through). If the impression shimmers through the metallic tray, then the impression must be repeated due to the push through.
In case of push through, the respective tooth is deflected a bit, resulting in an incorrect impression. The dental technician would then inevitably produce an ill-fitting dental prosthesis – a long cutting process and/or leaking crown margins would be the consequence. Finally, let’s not forget faults that may occur during the transportation of the impression (e.g., transport routes that are too long or less than perfect packaging of the impression) and/or at the dental technician’s office. But fortunately, all these faults can be avoided with good cooperation between the doctor, the technician, and the dental assistant!