Tips on final impressions

Some points on final impressions from Lincoln Harris' course:

• Every step of the appointment is important in the lead up to the impression; Local anaesthesia,  final preparations, tissue and moisture control and the impression itself. The issue is that we complete the most important step at the end when we are the most tired. If any of these steps are lacking then the final impression will be affected.
• -Final preparations should be as smooth as possible as it allows wettability of the impression material and final cement. There is an argument as to smooth preps vs rough preps and wettability vs retention for cement but if there is a rubbish final impression you won't have a well fitting crown to cement in the end.
• If the gum is inflamed the cord won't stay in. The gum needs tightness and elasticity to hold the cord in place or the cord will fall out. This is why in the initial appointment it is important to produce well fitting temporary crowns that will allow good tissue health as well as providing periodontal treatment and oral hygiene instruction.Inflamed gingiva will not be conducive to retraction or good moisture control. In the temporary crown stage if there are signs of inflamed gingiva, remove the cause of the inflammation by recontouring the temporary crown, removing excess cement or providing oral hygiene instruction.
• You ideally want to wait 6 weeks for healing after crown lengthening to pack cord as the gingiva wont have the tensile strength to cold the cord in place as some elasticity is needed to hold onto the cord. Also the cord will slip in further into the attachment if packed in too early and won't provide the horizontal retraction needed.
• If there is bleeding after application of the first cord you are less likely to get good hemostasis after the second cord. A blood clot on the surface of the gums will stick to the impression, tear on removal and start bleeding which could affect your second go at the impression.
• When ready to take your impression, pull the cord out vertically and slowly. If there is any sign of moisture leakage or bleeding then immediately repack the cord and wait longer.
• Once you have packed cord, wait 10 minutes with cord in place for hemostasis. If anything is not right with your pre-impression stage, stop and do it again properly. spending some extra time waiting for the tissue to settle with the cords in is much more time saving than having to repack all the cords and waiting anyway after that.
• Putty and light body wash is easier to use than heavy body but not as accurate. Seat the tray slowly and evenly vertically down. If you seat posterior than anterior you will get drag marks. You must use a metal tray with putty as it is too rigid and will distort a plastic tray. Heavy body PVS can be used in a plastic tray but ensure you use a non perforated tray to provide the maximal pressure to displace the material when seating the tray.
• Silicone can only be seated once and in one direction. Can only flow upwards and outwards as it is a deadsoft material. Can't be jiggled down or moved in multiple directions like alginate can.
• Impressions on proclined incisors are a difficult to get right as there is a significant undercut on the labial surface compared to the angulation of the posteriors. If there are proclined incisors then insert the tray evenly along the axis of the anterior or place a huge amount of light body on the labial surfaces to ensure there is enough material. This is because light body can only flow up and outwards and will not flow around corners. If there is insufficient material labial to the incisors and you seat the tray vertically there will be a large void or drag in this area. This is the same situation at the lingual of the lower 7s.
• Squeeze some PVS on the bench after you have filled the tray.Wait till 30 seconds-1 minute after PVS sets on the bench before removing the tray. The more critical the impression is the longer you should wait. If a patient can tolerate 5 minutes with the tray in their mouth they can certainly tolerate 6 minutes. An extra minute waited can save a lot of time repacking cords and retaking the impression. You can't rely on feeling the impression in the mouth on the labial aspect because e.g if the suction is in the mouth it can lower the temperature and there is too much variability in the set. impression material on your gloved hand will be warmed by your skin. impression material on the bench is most reliable. The time given on the material for set time refers to the setting time outside the mouth.
• Soak cords in hemoastatic agent and dry them on gauze. Keep cords dry from the time that you pack  them in. The DA should be constantly holding the suction and keeping the tongue away from the preps. Cotton rolls can hold the cheeks away from the upper preps but the tongue must be controlled. If fluid enters the sulcus it will soak in and stay there even after you remove the first cord. This may affect the flow of your impression material
• The tongue needs to be pulled up to place cotton rolls underneath or it will keep slipping around. if the cotton rolls are too large you can cut them in half lengthways.
• Squeezing light body around the margins initially then going back around and putting it over the rest of the prep tends to make bubbles between these two applications on the vertical walls of the prep. instead, putting light body on the occlusal surfaces of the preps only and relying on the compression form the putty or heavy body to force the light body in around the margins is more successful. for this to be successful you need good retraction from the 2 cords to allow plenty of space the light body to flow in and capture the margins.
• If there is a problem with your impression, decide as soon as possible after taking it out. Staring at the impression won't remove the void on the margin. If there is anything obviously wrong take a new one straight away as the gums will start to collapse as soon as you remove the impression. 
• When the light body has started to set when you insert the tray there will be a failure of the material to mix. This will result in a defined line between the two. This is most likely to happen on the side that you started squeezing in the light body. When the tray is removed early before the light body sets you will get a gap between the two materials. This is most likely to happen on the side you finished applying the light body
• If you take the impression out early, sulcus will tend to tear off as it is the thinnest area of impression material. The junction between the light body and heavy body will tear off and fold over.
• PVS increases in strength greatly in the minute after the official set time so if there is any cord in the impression wait a minute before removing it. Early removal may result in tearing or deformation of the sulcus.
• If the impression is pretty good except for a few teeth, retake an impression and focus on getting a good capture of those few teeth. The copings can be made on separate models and combined on a final cast.
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