Why am I having trouble getting my crown impression to come out clearly?
There are a number of possibilities. This is a question that new dentists and dental students often wonder about. While it is possible to take a "bad impression" occasionally, if a dentist or a student is often not getting the results they want, they are probably doing something wrong.
The most common reasons a rejected impression are poor retraction, blood or saliva contamination, inadequate preparation, or an air bubble at the finish line. In this blog post I will detail some of the reasons these occur
First and foremost, each crown prep needs to have a clear and definitive finish line . This is created with a diamond bur and must be readable and continuous 360 degrees around the tooth. If this has not been accomplished, no amount of impressions will yield a usable good result. If this is not visualized before taking the impression how is a a dentist or a dental student sure that the preparation is adequate prior to taking an impression? Since a considerable amount of time is spent taking a silicone impression it always best to visually confirm that the preparations finish line is continuous and smooth prior to attempting an impression.
Assuming that a tooth has a continuous readable finish line then it must be visible to the naked eye before taking an impression. This is accomplished usually by placement of retraction cord either in a single cord or double cord technique. If after packing a single cord, the finish line is clearly exposed and visible, and hemostasis has been achieved, then the cord can be left in place and an impression taken.
If after a single cord is placed properly and the finish line is not visible then usually it is necessary to place a second , usually one size large piece of cord. The tooth should be isolated with cotton rolls if necessary and kept free of saliva. The impression material should be kept close by a be ready to be syringed immediately upon removal the cord. Usually before I remove the first layer of cord I moisten it with a couple of drops of water from the three way syringe, since removing slighltly moist cord is less likely to cause renewed bleeding when it is removed ( removal dry cord can cause the surrounding gingiva to stick to it and develop microscopic tears).
After its removal, I dry the area around the tooth in order to better visualize my preps margin . At this point, hopefully there is no tissue or blood resting on it ( tissue, clotted blood, or the first layer of smaller cord. If I can see my entire impression clearly and the prep is properly isolated I syringe my impression material slowly around the entire prep.
If after I remove my top cord I can not make out my entire finish line, then I gently and slowly remove the smaller first cord that I placed, making sure that it is moistened when it is removed. Once it is removed, I dry the prep and check to see that there is no bleeding and I can make out a clear finish line without any tissue, blood or clot adhering to the preparation. If the preparation needs cleaning I will wash it gently with with water from the three way syringe or carefully use an explorer to remove any clot that is sticking to the prep. Sometimes there are tissue tags that rebound over my finishcline and if this has occurred I will repack my smaller cord very gently and leave it in place for a minute before removing it again. This usually packs down any tissue tags. If there is any bleeding when the cord is removed it is usually helpful to take a small pledget of cotton dipped in a hemostatic agent and gently apply it to the sulcus to get the bleeding to stop.
When I am satisfied that I have a clear view of the finish line and no bleeding I will start syringing my impression material. It should be kept in mind that saliva is the "enemy" of making a good impression since it interferes with capturing an accurate copy of the prepped tooth. Proper isolation is key and only once it has been achieved should an impression be taken. I use dry cotton rolls that I place just before taking my impression to achieve my isolation, one on either side of the preparation. I do not turn my back to the tooth that I am impressioning and employ an "assistant to pass me my impression syringe(If you are a dental student try asking another student to assist.). I find that if I turn my back to the patient in order to grab my syringe, it is quite possible for my patient to move their tongue or close their mouth in order to swallow an rewet the entire area with saliva and move my cotton rolls so that they no longer have create the barrier I have created. It is important for the patient to remain open for a few minutes while you are isolating the tooth and stay that way so that the cotton rolls stay relatively dry and not get too wet. Dry pads can be placed over the opening of Stensen's ducts in order to limit salivation and the the low speed suction can be placed over the lingual salivary duct as well.
If all of these steps are taken properly, more often than not a clear impression can be taken and the final impression will be clean and the margin clearly readable. That being said, even though I am very experienced and take all of the above steps, I find that that on occasion, it is still necessary to take second and even third impression, for me to be satisfied with my final result.
from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2018/07/why-am-i-having-trouble-getting-my.html - http://lspindelnycdds.blogspot.com/
The most common reasons a rejected impression are poor retraction, blood or saliva contamination, inadequate preparation, or an air bubble at the finish line. In this blog post I will detail some of the reasons these occur
First and foremost, each crown prep needs to have a clear and definitive finish line . This is created with a diamond bur and must be readable and continuous 360 degrees around the tooth. If this has not been accomplished, no amount of impressions will yield a usable good result. If this is not visualized before taking the impression how is a a dentist or a dental student sure that the preparation is adequate prior to taking an impression? Since a considerable amount of time is spent taking a silicone impression it always best to visually confirm that the preparations finish line is continuous and smooth prior to attempting an impression.
Assuming that a tooth has a continuous readable finish line then it must be visible to the naked eye before taking an impression. This is accomplished usually by placement of retraction cord either in a single cord or double cord technique. If after packing a single cord, the finish line is clearly exposed and visible, and hemostasis has been achieved, then the cord can be left in place and an impression taken.
If after a single cord is placed properly and the finish line is not visible then usually it is necessary to place a second , usually one size large piece of cord. The tooth should be isolated with cotton rolls if necessary and kept free of saliva. The impression material should be kept close by a be ready to be syringed immediately upon removal the cord. Usually before I remove the first layer of cord I moisten it with a couple of drops of water from the three way syringe, since removing slighltly moist cord is less likely to cause renewed bleeding when it is removed ( removal dry cord can cause the surrounding gingiva to stick to it and develop microscopic tears).
After its removal, I dry the area around the tooth in order to better visualize my preps margin . At this point, hopefully there is no tissue or blood resting on it ( tissue, clotted blood, or the first layer of smaller cord. If I can see my entire impression clearly and the prep is properly isolated I syringe my impression material slowly around the entire prep.
If after I remove my top cord I can not make out my entire finish line, then I gently and slowly remove the smaller first cord that I placed, making sure that it is moistened when it is removed. Once it is removed, I dry the prep and check to see that there is no bleeding and I can make out a clear finish line without any tissue, blood or clot adhering to the preparation. If the preparation needs cleaning I will wash it gently with with water from the three way syringe or carefully use an explorer to remove any clot that is sticking to the prep. Sometimes there are tissue tags that rebound over my finishcline and if this has occurred I will repack my smaller cord very gently and leave it in place for a minute before removing it again. This usually packs down any tissue tags. If there is any bleeding when the cord is removed it is usually helpful to take a small pledget of cotton dipped in a hemostatic agent and gently apply it to the sulcus to get the bleeding to stop.
When I am satisfied that I have a clear view of the finish line and no bleeding I will start syringing my impression material. It should be kept in mind that saliva is the "enemy" of making a good impression since it interferes with capturing an accurate copy of the prepped tooth. Proper isolation is key and only once it has been achieved should an impression be taken. I use dry cotton rolls that I place just before taking my impression to achieve my isolation, one on either side of the preparation. I do not turn my back to the tooth that I am impressioning and employ an "assistant to pass me my impression syringe(If you are a dental student try asking another student to assist.). I find that if I turn my back to the patient in order to grab my syringe, it is quite possible for my patient to move their tongue or close their mouth in order to swallow an rewet the entire area with saliva and move my cotton rolls so that they no longer have create the barrier I have created. It is important for the patient to remain open for a few minutes while you are isolating the tooth and stay that way so that the cotton rolls stay relatively dry and not get too wet. Dry pads can be placed over the opening of Stensen's ducts in order to limit salivation and the the low speed suction can be placed over the lingual salivary duct as well.
If all of these steps are taken properly, more often than not a clear impression can be taken and the final impression will be clean and the margin clearly readable. That being said, even though I am very experienced and take all of the above steps, I find that that on occasion, it is still necessary to take second and even third impression, for me to be satisfied with my final result.
from Ask Dr. Spindel - http://lspindelnycdds.blogspot.com/2018/07/why-am-i-having-trouble-getting-my.html - http://lspindelnycdds.blogspot.com/
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