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This course will give you basic techniques for taking a good impression for crown preparations, and dentures. Why are we constantly seeing bubbles and short margins in our impressions? Why are our finished crowns not seating when we try to insert them? The impressions are perfect. The laboratory completes the crown to fit the stone dies, but the crown does not fit the patient’s tooth. What happened between the laboratory and our perfect impression?
A good crown preparation will go a long way toward creating a good impression. A clear margin at the free gingiva or one millimeter below the free gingiva will give the best impression, but dentistry is not made up of ideal situations.
A deep mesial of distal amalgam or composite restoration may require a preparation with a feather margin to extend below the restoration. This will give you the seal you desire, but it will place the margin very close to the alveolar bone. The laboratory will have difficulty distinguishing between the preparation margin and the alveolar bone.
A good bevel margin should have the broken enamel rods removed with the light touch of a long diamond bur or a hatchet instrument. A ragged edge will give a poor seal and the laboratory will not have a good edge to bring the porcelain to. Sometimes we expect miracles from our laboratory.
We like our porcelain margins just slightly below the free gingiva, but it is very easy to leave a curve edge when we use a round diamond bur to place it. Even if you use a flat edge diamond bur, it is very easy to leave a lip on the margin. This should be removed with a narrow long diamond bur, or it could be done with a sharp hatchet instrument, but this will take a while.
You want a edge that is flat and free of overhanging enamel rods. Porcelain does not fit all that well to the flat porcelain margin on the tooth. It is the weakest point of the crown. Bacteria love to build their homes in such places.
Lower molars are notorious for being short. It is very difficult to place margins at the free gingiva and still have enough room for the occlusal reduction. The walls will be too short to hold the restoration. You will need to extend below the free gingiva enough to give you decent walls for retention. The margin will need to be a feather edge or slightly beveled. The slightly beveled margin will make it easier to distinguish the margin. It does not make a lot of sense to place lava crowns on lower molars when you are short on vertical length. Lava crowns need two millimeters of occlusal reduction.
The gingiva does respond well to abuse when you take the preparation below the free gingiva. The major draw back is the bleeding of the tissues making the impression difficult to take.
When you are doing the crown preparation, the gingival margins should be far enough away from the adjacent teeth to allow room for packing the cord. This can be a problem for the mesial of maxillary bicuspid teeth. You may need to take the preparation deeper then the recommended one millimeter below the free margin to accomplish this.
Sometimes it is necessary to cut the margins deeper then the ideal one millimeter below the free gingiva if the walls of the preparation are not long enough for retention. There is nothing wrong with a good metal feathered margin.
The object of placing the retraction cord is to expose the margin. This means the retraction cord needs to be taken below the preparation margin. This is not always that easy to do. When the tissue is very tight, you will need to push the tight tissue back to reveal the margin. When the margin is deep in the contacts, you will need to be below the margin with the packing cord.
A periodontal probe is good to use to check the depth of the pocket around the preparation margin. A depth of one millimeter will not take a number two cord well.
The recommended method of packing cord around a preparation margin is to use two cords. The first one is a number double (00) or triple 000 cord. This cord has to be packed below the preparation margin. If the pocket is shallow, the tissue will need to be compressed to place it. This does not translate into tearing the tissue. It does require a thin cord placement to expose the margin and give you some width for the impression material. The cord ends should not over lap. This will avoid any loose ends to catch the impression material.
The second cord can be a number two or three cord. This one does not need to be completely below the margin. It does more to hold the first cord down and soak up fluids coming from the wound. This cord does not overlap itself. The excess should be cut to allow it to fit the pocket around the tooth.
Starting with the double 00 cord, it is cut to overlap the circumference of the preparation. This will give you working room. The two ends of the cord are placed in a hemostat leaving a loop to be placed over the preparation, or you can leave enough length to hold the two ends with your left hand forefinger and thumb if you are right handed. You may need to place more cord in areas that are especially deep below the margin before you take the cord around the preparation. Ideally you want the cord to be one millimeter below the margin.
The hemostat with the cord loop in place on the preparation is moved to the mesial buccal (labial) gingival, and pressure is applied to the loop by pulling the cord tight. The hemostat (finger-thumb) holds the cord tight to the preparation margin as the cord is pushed into the sulcus beside the margin with a flat plastic instrument. I use a Hallenback instrument when the tissue is tight to the tooth.
When the cord is below the margin on the distal, mesial, and lingual side, the cord is removed from the hemostat and cut to fit the buccal (labial) margin. The cord is tucked into the buccal (labial) margin to finish the procedure. You need to feel or see the cord below the margin all the way around the tooth.. Your impression will not give you the margin if it is not exposed with the packing.
The larger #2 cord or #3 cord is looped and placed in the hemostat (finger-thumb). The loop is placed over the preparation and pressure is applied with the hemostat to hold the cord tight to the preparation. The cord is packed into the sulcus, and the excess is cut.
Excessive bleeding may occur if the tissues are inflamed, or the preparations are taken deep. This can be controlled by placing a cotton two-by-two over the site and squeezing the tissue a few minutes. The bleeding will deplete the anesthesia in the tissues. You may need to re-inject with more anesthesia with epinephrine.
You can also soak the cords in hemodent before placing them. This will help to control the bleeding. Usually a tightly pack cord will eliminate a bleeding problem. If you are not in a hurry to take your impression, the bleeding will stop on its own in a few minutes. Excessive bleeding will interfere with the impression.
I usually start my temporary at this point because it allows the tissues to calm, and I can check for high spots on the preparation before I take the impression. It will also save time. The temporary can be worked while you are waiting for the impression to set.
When the gingiva is very thin or nonexistent (buccal of the mandibular third molar), care should be used when placing the Gingi-pak to avoid tearing or detaching the gingiva or mucosa. This is especially important where the gingiva is thin or nonexistent. The buccal gingiva attachment over the mandibular third molar is very thin. A preparation taken below the gingiva in this location will usually expose the mucosa fascia plane. It is very easy to open this fascia and deposit a load of impression material.
The gingiva should be re-injected with anesthetic before placing the Gingi-pak when the gingiva has been hemorrhaging, or the preparation has taken longer then normal.
Using the impression technique, the impression for the temporary crown should be taken before the preparation is started and before the old crown is removed. This allows you to make a temporary that will duplicate the tooth before you cut the preparation. Using DMG Luxatemp, Smar Temp or a material similar to them, the acrylic temporary is very easy to make.
An impression is taken for the temporary with a dual arch disposable tray. These are rigid plastic frames with nontearable mesh between the ledges. The trays come in full arches, quadrant, or anterior sizes. These will be used for the final impression, but they are very good for the construction of the temporary crown.
A heavy or medium fast set impression material is placed on both sides of the mesh between the ledges of the dual arch disposable tray. To obtain a sharper impression, some of the heavy body impression material can be placed directly on the tooth to be prepared before the impression-filled tray is placed. I push the tip into the embrasures and insert the material. The impression is placed over the tooth. The patient bites through the impression material until he touches the opposing teeth. It is important the maxillary and mandibular teeth are occluding correctly. Otherwise the temporary will need excessive grinding to bring the occlusion in properly. The impression material can be any one of many brands. The material should remain rigid enough to hold its shape during the process of making the temporary.
Sometimes it is necessary to fill in the holes or do a buildup before you take an impression for the temporary. It will give you a better wall for your crown preparation. Some doctors prefer to wait until the tooth is prepped before they do the buildup. It allows you to place your undercuts after the preparation to hold the buildup. This will avoid buildups popping out when you take the impression or when you remove your temporary crown and find the buildup came out with the temporary crown.
I had an older gentleman came into my office in a wheelchair. He had a fractured tooth on # 28. The tooth was root canaled and a composite buildup was placed in the tooth. The preparation was done and the impression was taken. The acrylic temporary was placed. Two weeks later the patient came to the office for his insertion. When I took the temporary off, the composite buildup came off with it leaving a large gapping hole.
I cut a dowel preparation in the canal, extended the preparation walls and retook the impression. I placed a temporary with an extension into the hole. To avoid contaminating the dowel I placed a piece of cotton in the deeper region. This eased the removal of the cement when the dowel and crown came back from the laboratory.
I took an X-ray to ensure the crown seated. The dowel and crown were cemented at one time to ensure the dowel seated correctly.
Two things are important to consider here. The composite buildup was not a good method. It insured the crown to failure. The composite did not adhere well to the walls of the root canaled tooth. An amalgam buildup may have been stronger, but it is difficult to place an undercut in canal that tapers. The dowel made the most sense because of its strength and the advantage of using cement to hold it in place.
You can fill in a hole with Cavit G or wax to give a better shape to your temporary before you take your temporary impression.
If you are doing a bridge, you can cut out the pontic portion in the impression itself. This will give you a bulk of material in the pontic portion to shape a tooth.
You can place a temporary crown in the pontic space using wax to hold it before you take the impression, or you can wedge and shape a piece of wax or Cavit G to fit the space before you take the impression for the temporary.
Using the previous impression before the preparation started, the Luxatemp material is placed in the tooth to be prepared and inserted in the mouth. The patient should bite down using a medium pressure. To obtain greater accuracy a small amount of the Luxatemp material can be place around the preparation before inserting the material filled impression.
The trick here is to remove the Luxatemp impression before it completely sets. I usually place a small amount of the material on the tray. When the material starts to set, I remove the impression. The temporary usually comes out with the impression. I do not remove it from the impression for a good five minutes to allow it to become hard.
The temporary may remain on the preparation. If this occurs, there is probably an undercut holding it. This usually takes place in the contact areas. The material is still soft and can be removed easily. This allows you to check and remove the undercut. If you wait too long, the undercut material will remain. You will need to cut out the temporary material with a long diamond bur. The possibility of nicking the preparation is high here, and you may want to remove the undercut. If you have not taken your impression, this is not a problem.
You may find holes or some thin areas in the temporary. You can add to the material after you dry it off good. This can be done with the Luxatemp material or with the light cure flowable composite.
If you find a hole or thin area on the occlusal, you need to reduce the preparation before you take the impression. It will usually be the lingual cusp.
The temporary may be difficult to seat after it is removed and the material has hardened.
The embrasures need to be checked. The undercuts can be removed with a flame-shape diamond bur.
The preparation may carry an undercut where the decay existed. This will show up on the inter-surface of the temporary as a bulge of acrylic. This area will need to be removed with the flame-shape diamond before the temporary will seat.
There is nothing wrong with hollowing out the inside of the temporary some to make it seat better. It will be filled with cement.
If an undercut should occur and the acrylic temporary cannot be removed, the temporary will have to be cut free with a long narrow round-end diamond using plenty of water. Sometimes removing the excess in the embrasures will free it. A large spoon can be used to apply pressure on the distal buccal margin and on the mesial buccal margin. You need to be sure the spoon is well beneath the lip of the acrylic margin. The patient should be protected in case the spoon slips or the acrylic temporary pops off. The spoon can cause a nasty wound to the mucosa. Cutting the temporary off with a 557 bur and remaking another temporary is the treatment of choice. Use plenty of water and care to avoid nicking the preparation.
It is also a good idea to keep the temporary impression. If the crown delivery is delayed more then two weeks you may find the temporary coming apart from the constant attack of the wet salvia (water absorption), and the patient’s eating habits. You may need to make another temporary.
The bite is the most important phase of making a temporary. If it is left too high, it can cause the patient pain, or at the very least a fractured temporary.
A middle age Latino woman came into the office requiring three crowns numbers 15, 18, and 19. I cut the preparations, and took the impressions. I left my assistant to construct the temporaries while I moved to another patient. Ten minutes later the woman came out of the operatory and moved quickly passed saying she was sorry, but she had to take her daughter to the doctor’s office to have her sutures removed.
She came back a few days later with her teeth hurting. My assistant had not reduced the temporaries enough leaving them high. The patient was a clincher and drove her bite into the temporaries. Her teeth became super sensitive to touch, cold water, and air.
I reduced the temporaries, but her teeth remained sensitive. She finally went to another dentist and had all three teeth root canaled. I lost the patient here because the temporary was left high and the patient was a clincher. The combination caused the nerves to swell and give the patient a great deal of pain. If I could have convinced the patient to allow me to make her a bite appliance, I think the problem would have solved itself, but she didn’t think it was her bite. She would not allow me to make her a bite appliance for her. We also had a communication problem. The lady spoke only Spanish. I had to talk through my assistant who spoke Spanish.
The preparation is finished and ready for the impression. The temporary is started, and any necessary occlusal reduction is made. A special note: You need to look at the lingual cusp. This is where most of the high spots will occur. You may want to pack the preparation a second time, but this is usually not necessary unless the tissues are tight.
There are many techniques and materials for taking an impression. This article will deal with only one technique and two impression materials. The procedure for taking the impression is similar regardless of what material or techniques are used. The article will discuss what is important to obtain a good impression of the preparation.
The combination of a light syringe material and heavy tray material is ideal for a good impression even though a heavy material by itself may work. The light syringe material should be from the same company as the heavy tray material, or material that work well together can be used.
Materials
Light fast set impression material that mixes upon delivery, gun, tip to reach the margins
Medium fast set impression material that mixes upon delivery, gun
Quadrant dual arch disposable trays
This is a very easy and quick method of taking an impression. The bite registration and the impression in obtained in the one procedure. The procedure is patient friendly. The patient will not have a gagging problem with the quadrant tray. The amount of material needed is small, and impression time is relatively fast. The procedure is similar to the impression for the temporary except the light fast set impression material is added to the process.
The tray selection can be difficult when you have a bicuspid preparation and neither the posterior nor anterior quadrant trays reach it. Tray selection is important. I like the Triple Tray by Premier Dental because of the wide selection of trays to prevent the above problem. The mesh needs to be strong enough to hold the impression material and not impede the bite registration.
You are going to take the bite registration at the same time as you take the impression. This is a relatively new concept. It works very well, but takes away the laboratory technician ability to adjust the bite. Everything lies with technique done in the office.
The trays should be tried in before the impression material is placed. The patient may not be able to bite down into occlusion. When the tray is full of impression material, it is not the time to find this out. You should check the occlusion on the opposite side. You should note how the teeth come together. The patient may have a weird bite. The technique depends on the patient biting correctly with the impression material in the tray.
The tray should fit over the preparation and include as many teeth as possible mesial and distally to the preparation to give the laboratory technician something to work with. Sometimes the distal end of the tray pinches the tissue around the third molars.
The tip is placed in the gun with the light fast set impression material. The tip has a tendency to break free of the mixing tip. A little care here will prevent the tip from coming loose and placing the impression everywhere except in the preparation.
The packing cords are removed. If the pocket depth is below two millimeters, you may want to leave the first packing cord in place when you take the impression. The cord should not have any loose ends. This will give you the advantage of keeping the sulcus open and the margins exposed.
If the sulcus is less then two millimeters, you may encounter difficulty in exposing the margins. In this case it would be wise to remove all of the packing material to expose the margin. If you leave the packing material in the sulcus for ten minutes, the sulcus will remain open long enough for the impression.
There are all kinds of impression material. Everyone has his favorite. I would recommend one with a gun and a tip for the light impression material. This allows you to take the light body into the sulcus of the free gingiva around the preparation.
The heavy body is first placed in the tray. Both sides of the tray are filled. One side is for the bite and other side will have the impression of the preparation. Unless it is a full arch impression, you will have plenty of time to take the light body around the preparation. The heavy body impression material sets slower with this in mind.
The preparation is air dried. Do not rinse with water first to remove the blood that may have accumulated. A small amount of blood will not interfere with the impression, but water will allow the sulcus to close.
The tip of the light body impression material starts in the (mesial or distal) embrasure. The light body impression material is forced into embrasure with enough pressure for the material to reach the gingival sulcus and extrude to the lingual side. Keeping the tip in the sulcus, the material is carried around the preparation.
The tip should be perpendicular in the sulcus as much as possible to force the impression material into the sulcus. This is not a heavy pressure. You do not want to overflow the sulcus and lose sight of it in the process. If this should happen, you should wipe out the overflow with your finger until you see the sulcus. You do not get a good impression when you are fishing for the sulcus.
You should continue a steady pressure to force the material beyond the preparation’s margins. When you reach the mesial embrasure, the light body material is forced into it until you see it extruding out on the lingual side. Keeping the tip in the material, you move the tip to the lingual side. The tip is placed in the lingual sulcus, and the material is applied as you move toward the distal. You want to avoid bubbles in the impression material.
Again, the trick here is to not overload the sulcus with impression material to the extent you can no longer see the sulcus. You need to move along with the tip, and keep it in the sulcus. You may need to wipe off an overloaded portion to regain access to the sulcus.
The heavy body and tray are placed over the light body material, the patient bites down, and the occlusion is checked on the opposite side to insure it is seated. You may need to add some more heavy body material to the impression to cover the exposed teeth. Often the heavy body will pull away from the teeth when the patient bites down leaving a gap at the gingiva.
Some material will give a two to three minute setting time, but I have found errors in taking the impression out too soon. It is better to wait five minutes to insure the material has set. This will prevent distortions. Besides, you need to finish the temporary you started earlier.
I like to take a second impression to give the lab another die to check for any distortions. It is better then having the patient back for another impression. I also like to give the lab one impression using the compression technique. This may incur a distortion error, but it also may give you a very accurate impression of the preparation’s margins. I usually mark this impression so the lab will know which one is the compressed impression. If a decision needs to be made as to which impression is accurate, he will know the compression impression probably has the error.
The quadrant impression is rinsed, dried, and examined. If there is a discrepancy, the impression can be taken again using the defective impression. The impression is tried in the mouth to ensure it will seat and to locate a few guides to ensure an easy insertion. Undercuts preventing the impression from seating correctly is removed with a pair of scissors.
A small amount of the light fast set impression material is placed on the margins of the preparation impression. You should avoid an excessive amount and placing the material on the pulpal walls.
Too much material will distort the impression. This is only a drop of material to fill in the discrepancy and cover the margins of the preparation impression. The quadrant impression is placed in the mouth over the preparation and seated. The patient bites to finish setting the impression. The bite is checked. The quadrant needs to be seated until the opposing teeth touch. This does not equate into clinching.
It is very easy to overfill the preparation and distort the impression. It is only a small amount covering the gingival margins. The compression will take the material down the sides of the preparation.
It is very critical the patient closes all the way after the impression is placed. If the impression will not seat, or the patient cannot close his mouth, the impression will need to be retaken.
Sometimes it is difficult to place the quadrant a second time. When the dry try-in is done, the end of the quadrant should be noted. It may be necessary to mark it.
The most important step to remember: Do not to have the patient clinch his teeth when he closes on the impression material. This will distort the impression.
I had a man in his mid thirties come into the office. He needed a bridge from number twelve to fourteen. It was a routine bridge. I had the patient bite down when I placed the tray full of impression material in his mouth. I told him to hold it firm and not change his bite. The impression was removed and I sent it to the laboratory. When I attempted to deliver the bridge, it would not fit. I looked like the bridge was made for another patient.
I took another impression and told the patient to hold it firm to avoid distortion. I took another impression and finished it with the compression technique. Two weeks later the patient was back. The same thing happened. He bridge fitted the stone dies perfectly, but not the patient’s mouth. The bridge appeared distorted. It would not even start to seat. What was happening? The laboratory was charging me for the redo’s. Everything appeared correct.
When I was taking the third impression on the patient, I remained in the operatory and watched him. I had done everything correct. The heavy body impression material was loaded in the quadrant tray. The light body impression material was placed around the teeth. The tray with the heavy body material was placed in the patient’s mouth. He was told to bite down. Then it happened.
He was not only biting down, but he was clinching his teeth tight together thinking he was helping to make a better impression. There was my distortion. He was distorting the impression by clinching during the impression. Then upon releasing the impression, it distorted. The patient was the problem.
This problem can be avoided by taking one impression without having the patient close his teeth. The second impression can be taken with the mouth closed to obtain the bite.
Materials
In the above technique the bite is established when the impression is taken. If the impression is taken with a solid tray instead of a mesh tray, then a bite registration is necessary. The bite can be recorded on a piece of base plate wax.
Most laboratories ignore the wax bite because they are usually not good. If a full mouth impression is taken, the laboratory can usually figure the bite out by using the opposing full arch stone model. The reason for this is the difficulty of securing a good registration with wax. Wax usually leaves a slight opening in the bite registration. The patient simply does not bite all the way through the soft wax.
When the quadrant technique described above is used, the impression and the bite registration are taken at the same time. This leaves the laboratory no other choice but to accept the bite registration handed to him.
When the full arch impression is taken with a single crown, the laboratory has a method of establishing the bite. Matching the maxillary and mandibular teeth works well with full arch impressions. This is especially true for a person who clinches his teeth. The bite is tight and is easy to articulate.
It is still recommended to give the laboratory a good wax bite registration. This can be accomplished by slightly heating a sheet of base plate wax over the Bunsen burner and splitting it in half with a laboratory knife. The wax base plate will fracture into many pieces if it is not heated first.
The remaining wax half is heated again over the Bunsen burner until the wax is soft. This will require moving the wax sheet constantly over the flame to avoid overheating it.
Ten millimeters of the preparation side of the wax is folded over to give a double thickness. This is placed in the patient’s mouth. Keeping the wax flat as possible, the patient bites into the wax several times until the sound of teeth touching is heard.
The wax is removed from the patient’s mouth and placed under cold water until the wax sets. This will distort the wax slightly, and require another registration to straighten the wax. The cold wax is returned to the patient’s mouth and reinserted in the same location. The patient bites into the wax as before until the sound of teeth touching is heard.
Another method is the old technique of rolling the soft base plate wax into a long ball of wax (mush-bite). This is placed over the patient’s maxillary teeth. The patient is instructed to bite through the wax. While the patient is biting and holding the wax in place, the soft wax is pushed into the teeth to register the buccal side of the teeth.
This will give a good registration of the teeth’s position, but it will not give an accurate occlusal bite registration. Again, the hardened wax will leave a slight occlusal opening, and it is difficult to see the teeth beneath wax. The laboratories usually throw these aside and eye in the opposing stone model.
There is a good method of taking the bite. It is a minnie version of the mush-bite. A small piece of base plate wax is heated and rolled into a small piece of wax. The wax is place on the preparation only. The patient bites through the wax until he occludes with his natural teeth. The wax is pushed into the preparation to register the buccal wall, and removed from the mouth while it is still warm.
The excess wax covering the occlusal of the mesial and distal teeth is removed with a lab knife or scissors, and reinserted over the preparation. The patient bites the wax several times to insure no wax is holding up the registration.
The wax registration is removed and the occlusal excess wax on the lingual is removed with a lab knife or scissors until only the portion of the wax touching the opposing teeth and the buccal surface of the preparation remains.
The wax bite is cooled with water and reinserted into the patient’s mouth. Any distortions created from the cold water are corrected. The bite wax is removed.
The laboratory can now set the bite registration with the opposing study model using the wax registration and the other teeth in the impression as a guide. He will be able to see where the registration and other teeth in the impression line up with the opposing teeth.
This is similar to the short wax bite technique except acrylic is used instead of wax. The acrylic is mixed by incorporating the acrylic powder into the liquid until it is saturated with the powder. The excess is removed by turning the mixture over and tapping the bench. This will leave a saturated powder to liquid mixture at the correct consistency.
After mixing the acrylic mixture, it is allowed to sit until it becomes doughy. The doughy mixture is kneaded with the fingers until the acrylic mixture snaps when it is pulled apart. The acrylic doughy mixture is placed over the preparation. The patient bites the acrylic mixture to place. Using your finger, the acrylic mixture is push against the buccal wall of the preparation to register it.
The imprinted acrylic mixture is pulled from the preparation and the curved scissors removes the buccal, lingual, mesial and distal overhangs. Any portion of the acrylic mixture that is not included in the bite registration is removed.
The acrylic bite registration is reinserted. The occlusion of the mesial and distal teeth should be seen when the bite is closed. The acrylic mixture is allowed to cure while it remains in place.
Once the acrylic registration is cured, it is checked with blue articulating paper. Any discrepancies should be corrected with a green stone using plenty of water. This will give you an accurate bite registration that is relatively permanent. This is a good technique to remember when you are doing a full mouth reconstruction.
It is also recognized there are many ways to take the bite registration including an applicator gun and cartridge. The difficulty with the technique is the assumption the patient is biting in centric, and that the patient will hold this position for a minute or more while the material sets.
There may not be enough tooth structure to obtain an adequate occlusal space to place the porcelain crown. This is especially evident in third molar crowns.
This problem can be corrected by using a metal occlusal that requires less thickness.
The problem can also be solved by reducing the opposing tooth especially if it has a prominent cusp.
The laboratory may miss the bite. This is usually the result of the laboratory eyeing in the bite registration incorrectly.
This can be corrected by giving the laboratory a good bite they can use.
This can be corrected by setting the study models before the case is sent to the laboratory, or vertical pencil lines can be placed on the study models once they are occluded.
This portion of the article will deal the basic technique for taking a good impression for an extended denture. The denture is only as good as the impression using the proper extensions.
This critical before you start the impressions.
Areas of concern are:
What is the condition of the bone and the tissue over the bone?
Has the bone receded leaving behind loose tissue? Is there enough ridge left to hold a denture? The loose tissue may be the only thing to get a hold of to support the denture. Ideally you would want to remove the tissue to place the denture against solid bone, but you do not want to remove your retention. This is a hard call. A little bead of tough tissue over the alveolar bony ridge should probably be retained for support of the denture.
When excessive amounts of bone are lost, you can expect a very thick denture to maintain the vertical. You will need every bit of bone to support this denture. It is also very important the denture is over the ridges. Maxillary posterior teeth placed off the ridge to accommodate a wider mandibular arch will unseat the denture when it goes into function.
How do the ridges line up? Is this a class II or a Class III?
The denture needs to keep the class II or the class III to keep the teeth on the ridge. The bite needs to seal the denture every time they occlude. This will not happen if the teeth are not over the ridge.
This is especially important for the anterior teeth. If the maxillary anterior teeth are placed way off the ridge to accommodate and class I position, the maxillary denture will break the post dam seal and come loose. If the mandibular teeth are forced off the ridge trying to change a class II relationship to a class I, the mandibular denture will unseat every time the patient closes his teeth. The teeth need to be on the ridge.
Does the patient have tori protrusions? These would include the mandibular lingual area, palatal, maxillary buccal and mandibular buccal protrusions.
These should be removed before the denture is started because they will be the source of sore spots though the process of making the denture and after the denture is completed.
Maxillary Buccal tori usually extend from the first bicuspid to the third molar area. They make it very difficult to place a denture that does not create a sore spot on the thin tissue over the tori. This is the main problem with a tori. The tissue is thin over the bone. There is very little connective tissue. Once the tissue is cut, it takes a long time to heal because of the lack of tissue and the poor blood supply to the area.
If the patient has submandibular tori, you will need to remove it with a surgical procedure. You cannot place a denture in the natural undercut the tori provides. The best you can do is go over the top of it and cut out the tori portion, or you can keep the denture above the tori and sacrifice the extension and retention.
d. If the palatal tori is not too large and has smooth edges, you may be able to keep the tori and place the denture. Though, it is best to remove it.
Does the patient exhibit PEH Papillary Epithelial Hyperplasia? This is usually caused from wearing a loose denture. The condition will improve when the patient stops wearing the loose denture for three weeks. If the patient refuses to leave his denture out, then you will need to surgically remove it before the new denture is placed.
Is the palate broad and flat, or is it steep and constricted? The more constricted the palate, the better will be the retention, because you will have more tissue surface.
The mandibular extended denture takes advantage of the diverging angle of the mandible. Distal to the mylohyoid line, the angle of the mandible diverges outward. By extending the denture into this area, the denture will incorporate an undercut.
The mandibular denture is brought forward into the undercut and slipped over the anterior alveolar bone. If the patient can tolerate this mandibular extension of the denture, the denture will be locked into place. For the impression to reach this area, the impression tray must be extend with compound and pushed into the undercut.
The muscles in the floor of the mouth will dictate the extent of the extension and the amount of undercut the patient can tolerate. The mandibular denture is held in place by the amount of undercut and the lingual seal created by the tongue at the plica sublingualis.
When the tongue lies on the lingual side of the anterior mandibular denture, it creates a seal similar to the post dam of the maxillary denture. The amount of extension of the anterior lingual side of the mandibular denture toward the plica sublingualis is determined by the muscles in the floor of the mouth. The more surfaces the tongue has to cover, the better the seal. The plica sublingualis should not be covered.
The undercut and the anterior lingual extensions of the mandibular denture are adjusted to accommodate the patient. Both areas will need to be shortened until the patient can tolerate the extension. This will vary with each patient. Before the shortened process becomes too aggressive, the patient should be given the opportunity to adjust to the extended denture. Most patients can tolerate a degree of the undercut and anterior lingual seal.
The maxillary-extended denture extends as high up on the tuberosity of the maxilla as the tissue will allow going from one side of the maxilla to the other side. The hard palate is completely covered with the beaded post dam sealing the posterior portion of the denture in the tissue of the soft palate.
The seal of the denture depends on the beaded post dam and the amount of extension the patient can tolerate over the tuberosity of the maxilla. The goal in the construction of the maxillary denture is to create a suction that will hold the maxillary denture in place. The maxillary denture is extended into the vestibule as far as the tissue will allow.
It usually takes five appointments to make a denture. Two of these appointments deal with taking impression. This course will be limited to the impression aspect of the denture process. The impressions must incorporate the lingual extensions of the mandibular arch to construct a mandibular denture. It must cover the tuberosity of the maxillary bone to construct a maxillary denture. The first appointment is the initial impressions the trays will be constructed from.
Materials needed
An alginate impression is taken of the maxillary and mandibular arches. The mandibular impression is taken first to allow the patient to become used to the material you are using. The patient lifts his tongue to allow the impression tray to slip below it. This should be practiced without the alginate impression material first to correct any problems the patient may experience.
If the patient has existing teeth (immediate dentures), the wet alginate impression material is scooped up with a finger and spread over the labial side of the mandibular anterior teeth. This is important to ensure the lower lip will not remove the alginate. This becomes especially important for patients with tight lower lips.
The maxillary impression is taken with the patient sitting almost all the way forward. He is instructed to breathe through his nose. Most of the gagging problems can be attributed to the patient trying to breathe through his mouth creating an air hole when the impression material is placed. The impression material slips down the air hole causing the patient to gag.
The patient becomes anxious when he cannot breathe through his mouth, and the loose impression material has moved down his throat. The patient is instructed to hold his breath, think of his nose, and then breathe through his nose. You may need to talk him through the process. The patient can breathe anytime he chooses through his nose while the impression material is in his mouth.
On extremely rare occasions, when the patient is going to upchuck the impression material before it is completely cured, you might try to pull the offending piece of material from the patient’s throat with a finger sweep.
No matter how much material enters a patient’s mouth, he will tolerate it as long as tray or material is not loose. If the patient feels something loose, he will attempt to swallow it to breathe, bringing on the gagging reflex.
When the impressions are completed, they are wrapped in a wet paper towel, placed in a plastic zip-lock bag and sent to the laboratory. At this stage it is good to do the lab work in the office for a quick turn around time. This course will not go into this technique. Instead, we will assume the laboratory made trays with good extensions.
A large number of offices take only a primary impression for the denture and then wonder why the dentures do not stay up. In some mouths the trays may include the extensions and the next step may not be necessary, but these patients are few and far between. To obtain good impressions it is necessary to include extensions as described below.
The laboratory trays are tried in the mouth to ensure they will seat without pinching the tissue. This is especially important in the mandibular lingual undercut area.
Materials Needed
The goal of the second appointment is to take a very detailed impression of the maxillary and mandibular alveolar surfaces and their extensions.
The trays are tried in the mouth and trimmed to fit. The trays occasionally may need more acrylic removed where the teeth are located for immediate dentures. There should be enough room for the alginate to flow around the teeth. A slight hole in the tray can be tolerated, but an undercut will interfere with the impression.
The water is heated and poured into the pan with the cloth dipper. A stick of red or green compound is placed on the cloth dipper and lowered into the hot water for a few seconds. The cloth dipper carrying the compound stick is removed from the hot water and placed across the pan. The hot water remaining in the cloth will continue to heat the compound.
Or, you can place hot water in a green bowl, and microwave it two minutes. The compound is place in the hot water to soften it. It is really important to always use wet gloves to handle the compound. Otherwise the material will be sticking to your fingers.
When the red stick is pliable, it is lifted from the cloth dipper and applied to the acrylic tray. Starting at the anterior frenum, the compound is shaped over the edge of the acrylic tray and carried toward the post dam area with moist gloved fingers. It is better to do a quadrant at a time to keep control of the material.
The red compound on the tray is placed above the Bunsen burner or flame heater for a few seconds to glaze the material. Wetting your gloved fingers, the compound is blended into the acrylic tray and shaped with gloved fingers. The tray with the compound is returned to the hot water to cool the compound and to make it uniformly hot. The soft compound is part of the acrylic tray and extends above the edge. The compound should be touched with the back of your finger to insure the compound is not too hot to place in the patient’s mouth. Then it is placed in the patient’s mouth. The cheek is brought over the edge and massaged. This shapes the compound to the maxillary tuberosity.
Another section is heated over the Bunsen burner. This is not an attempt to melt the compound. You are trying to make the compound more pliable. The compound is passed over the flame slightly to accomplish this. The wet fingers work the compound over the periphery of the acrylic tray.
When the tray with the warm compound is placed in the patient’s mouth, it is pushed up into the vestibules. The cheek is pulled down across the compound to contour the attached tissue into the soft compound.
The tray is removed and inspected. The areas of over flow are heated in the Bunsen burner flame slightly and smooth over. The tray is placed in the hot water a few seconds, and reinserted into the patient’s mouth to contour the compound. The tray is removed and inspected. The compound should conform to the attached tissue. Always check the compound with the back of your finger before you place it in the patient’s mouth.
In areas where the compound is short, more compound needs to be added. The end of the stick of compound is placed over the Bunsen burner flame. When the compound starts to melt, it is dripped over the compound on the tray where it is short.
When enough compound is added, it is shaped with a moist gloved finger. The tray is placed in the hot water bath a few seconds to cool the compound. Then the tray is placed in the mouth, and the cheek is stretch over the compound to contour it to the attached tissue.
The area of most concern is the posterior maxillary tuberosity as it flows into the post dam area. A good extension into this area will help to create a seal for the maxillary denture. If the compound is still short more can be added until the compound has a good impression of the area.
The other side is done in a similar matter. If the compound is difficult to handle as one piece it can be split in half. The first piece of compound starts at the frenum and works it way back toward the post dam. The second piece is added to the first piece and continues toward the post dam.
A piece of the red compound is heated and added to the post dam. The compound on the post dam is passed over the Bunsen burner flame until the compound has a glaze. The soft compound is shaped and pressed to the tray with moist gloved fingers.
The compound is passed over the flame a second time to soften it more. Before the tray is placed in the mouth, it goes into the hot bath of water to cool the compound and give it a uniform temperature.
The temperature of the compound is checked with the back of your finger. When it will not burn the patient, the compound tray is placed in the mouth, and the post dam is pressed to the palate with moist gloved fingers. This will give a good impression of the post dam and ensure a tight fit.
The tray is sprayed with Spray-on Tray Adhesive and dried with the air syringe. It is now ready for the impression.
This impression can be taken with alginate or Coe Flex type 3 light body impression material. If you plan on pouring the impression immediately the alginate is preferred. Most laboratories like the Coe Flex because it does not need to be poured immediately and holds its accuracy for long periods.
The adhesive is poured into the acrylic tray, spread with a cotton roll, and air dried. Enough base and catalyst are mixed to cover the tray. This is not an excessive amount. The tray fits fairly tight. It needs to cover all of the impression side of the tray going up over the edges slightly.
When it is seated in the patient’s mouth, it needs to go all the way to insure the material flows over the edges slightly. It takes eight minutes for the material to harden totally. This will depends on the conditions in your office. Finger test it. If the material feels hard, you may be able to remove it earlier.
The patient will tolerate the material in his mouth as long as he breathes through his nose and the tray is tight to his palate. The tray is removed and sent to the laboratory for bite blocks.
A small amount of alginate is mixed in the green bowl to a watery consistency. It needs to be thoroughly mixed. The thin mixture is poured into the acrylic tray. This impression does not require a large amount alginate. The tray already fits tight to the palate. A thin layer of alginate is flowed over the tray.
The patient is brought forward and reminded to breathe through his mouth when the tray and impression material are placed. The excess alginate is removed and the tray is inserted into the patient’s mouth. The tray is pressed tight to the palate starting from the post dam and going forward to move as much of the alginate to the anterior.
The alginate will flow through the bur holes in the acrylic tray to lock the impression in place. The excess alginate will flow into the patient’s mouth, but the patient is breathing through his nose and tolerates the alginate in his mouth.
The alginate hardens and is removed from the patient’s mouth by applying pressure on the handle. The impression should pop loose. The excess post dam alginate is cut free of the impression with a pair of scissors. The trimmed impression is wrapped in a wet paper towel and placed in a zip-lock bag. The impression needs to be poured immediately.
The alginate sets in about two minutes. It takes an accurate impression, and flows easily over the acrylic tray. The area behind post dam has a tendency to snap free. If you go ahead and remove this portion, this should not be a problem.
When the preliminary mandibular impression is poured, the soft stone should be removed in the lingual submandibular fossia on both sides to form a deep sulcus beside the mandible. This will allow the acrylic to flow into this area when the tray is made.
The tray with the two submandibular lingual wings is tried in the mouth to be sure there are no undercuts. You may need to remove some acrylic to seat it. You want the tray to fit loose to allow room for the compound.
Compound is added to the mandibular tray in a like matter as the maxillary tray except the compound is carried into the lingual submandibular fossia (undercut). The right side is completely done before the left side is started. This will allow you to keep a point of reference when the tray is tried in the mouth with the compound. If the acrylic wings are long enough, this may not require a large amount of compound.
The compound is heated in the hot water and applied to the buccal side of the tray. The compound starts at the mandibular frenum and works distally toward the retro molar triangle pad. The working warm stick should be half the thickness of the cold stick.
Moist glove fingers work the warm compound onto the tray. The tray and compound are passed over the Bunsen burner flame. The compound glazes and adheres to the acrylic tray. Moist glove fingers can mold the compound before it is placed in the hot water to disperse the heat.
The compound is placed against the back of your finger. If the compound feels comfortable, the tray and compound are placed in the patient’s mouth. The cheek and lips are pulled over the compound to mold the attached tissue. The tray is removed and examined. Compound is added or removed as needed.
The lingual portion of the tray is extended into the mandibular posterior undercut (angle) with the compound. The compound is heated in the hot water, and molded to the acrylic tray. It is glazed over the flame to seal the compound to the tray. The hot compound is cooled in the warm water and placed in the patient’s mouth.
The wet gloved finger eases the compound over the mandibular ridge and retracts the tongue to allow the tray and compound to slip into the submandibular fossae (undercut). This extension should not incorporate the undercut. The impression will do this later.
More compound may be needed to take the tray deeper into the submandibular fossae. The tray should extend along the floor of the mouth to the plica sublinualis. The mandibular lingual tray extension should be taken as low as the relaxed tissues will allow.
The tray should be extended on the buccal from the anterior frenum into the retro molar pad, and down into the lingual submandibular fossae. Once the right side of the tray can be inserted comfortably, the compound can be added to the buccal of the left side of the tray.
The same procedure is followed extending the tray into the submandibular fossa. Care should be exercised when the flame is passed over the compound to avoid melting the compound on the right side.
The long extensions in the submandibular fossae need to be free of undercuts. The gloved finger will be needed to guide the soft compound into the fossae. The soft compound has a tendency to catch on the mandibular ridge and distort. Once the tray is in place, the gloved finger can push the compound back into position.
The tray should move in and out freely. The extensions on the lingual will appear as wings on the tray. The tray is sprayed with Spray-on Tray Adhesive and dried with the air syringe. It is now ready for the impression.
The adhesive is poured on the tray, spread with a cotton roll, and air dried. The base and catalyst are mixed and applied to the tray. This is an even thickness over the tray going over the wing extensions.
The trick here is the removal of the impression and tray. You cannot bring the tray straight up without causing injury. The impression material has incorporated the undercut. You must push the tray distally to disengage the undercut. Then the tray can be removed.
The alginate mix is very thin to allow easy flow of the material. The powder is well incorporated with the water until a smooth thin creamy mix is obtained. The compound tray is filled with the alginate impression material.
It will be difficult to keep the impression material on the compound tray without constantly moving it. The patient raises his tongue and the tray is placed in the patient’s mouth and pressed into place. The impression material will flow out the bur holes and over the compound. The cheek, tongue, and lips are pulled free of the impression. The mental and anterior frenum attachments are worked through the impression material.
The fingers should be pulled free of the impression material when it starts to gel. Pressure needs to be continued, but a new position is used. When the impression material sets, the handle is gripped. The impression is removed by lifting and pushing the tray distally.
The tray’s impression material now has undercuts. To avoid distorting the impression or tearing it, the tray is lifted slightly and pushed distally until the impression material is free of the submandibular fossae. The tray is slowly removed from the patient’s mouth, wrapped in a wet paper towel, and placed in a zip-lock bag.
It is very easy to over flame the compound. You are only interested in a glaze. You need to always go back to the warm water before placing the hot compound in the patient’s mouth.
When taking the maxillary alginate impression, the post dam will extrude impression material. This needs to be carefully cut with a scissors to avoid distorting the impression.
The long submandibular soft compound extensions on the mandibular tray tend to catch on the mandibular ridge and distort the material when you try to insert it. This can be avoided by placing your finger on the ridge and guiding the soft extension into place.
If the compound sticks are left in the hot water too long and they will distort into a flat blob. These can be redeemed by rolling the material back into a stick.
Care needs to be exercised when placing more then one stick in the hot water. They may roll together and fuse.
If the alginate is not thin enough on the compound tray when the impression is taken, the alginate will fill the tray and give a poor impression. The alginate needs to be paper thin across the tray and compound. The tray can always be scrapped clean of the alginate, and the impression can be retaken.
The alginate impressions need to be poured immediately to avoid distortions. The Coe-Flex impression can be sent to the laboratory without pouring them.
Material Needed
The lab stone is mixed with water to a creamy consistency to incorporate all of the stone. The creamy mixture is removed from the bowl and placed inside three paper towels. The excess water is removed from the stone mixture by pressing the paper towels over the stone mixture.
Enough water is left in the mixture to allow the stone to flow easily into the impressions. Using the vibrator, the stone starts at the post dam and flows across the palate and into recesses. More stone is added until the stone protrudes above the impression.
A four-by-four inch tile is placed over the protruding stone. The tile and the stone filled impression are turned over and placed on the bench.
The excess stone is pushed up around the impression with the spatula to cover the compound. The stone base should be at least a quarter of an inch thick.
When the stone begins to set, the stone is very pliable. The stone is shaped and trimmed from the impression with a spatula. The stone model needs to fit into the flask to process the denture. This should be kept in mind as the stone is trimmed from the impression. The stone is allowed to harden.
The cured stone model is placed in the hot water pan. The compound softens and the acrylic tray is removed. The model is lifted from the hot water and the soft compound is picked off the stone model.
The lab stone is mixed to a creamy texture, and the excess water is removed with the paper towels. This technique removes the air bubbles incorporated in the stone while mixing. It leaves the stone pliable and gives it enough texture to hold its shape once it is poured into the impression.
The stone mixture is placed on one side of the impression and vibrated around the impression to the other side. To avoid injury to the impression only the handle of the tray is vibrated.
Once the impression is covered, more stone can be added until it overflows. The impression filled with stone is covered with a four-by-four inch tile. The impression, stone and tile are turned over and placed on the bench. The spatula is used to pull the tray stone material up around the impression. Attention needs to be paid to the lingual extensions to insure stone remains in the impression. Stone is added to the space between the extensions.
This impression is the most difficult one to pour because of the lingual extensions. There is a tendency to allow the extensions to push through the stone base. The peripheral compound border of the impression needs to be in the stone.
Once the stone begins to set, the spatula is used to trim the model. Repairs can still be made and the excess stone is removed. The model needs to fit inside a flask later. This needs to be kept in mind in the trimming process.
The lingual extensions are worked free of the stone on the lingual side, and the stone is smoothed between the two sides. This will save you digging the extensions out when the stone is hard. When the trimming and smoothing are completed, the stone is allowed to harden.
The cured stone model is placed in the hot water bath until the compound softens enough to remove the acrylic tray. The stone model is lifted out and the remaining red compound is picked off. The stone model can be reinserted into the hot water to soften the compound more if necessary.
The stone models may need to be trimmed on the stone grinder to correct areas missed in the trimming. Usually a utility knife can make the minor corrections by smoothing over the sharp edges.
The mandibular extensions may protrude out the bottom of the base or be very thin in this area. More stone can be added to the base once the stone starts to set. The lab stone is mixed and drained of excess water with the paper towels. The tile and impression full of stone is lifted off the bench. Taking the impression full of stone in your hand, the tile and impression full of stone are turned over. The tile is removed and more stone is added. The four-by-four tile is replaced. The impression full of stone and tile are turned over, and placed on the bench. The spatula is used to remove the excess stone and correct the voids.
It is important to remove the stone on the lingual side of the extensions. Stone should be removed all the way to the bottom of the compound. This will save time digging them out later in hard stone.
The size of the flask should be kept in mind during the trimming process. The base does not need to extend more then a quarter of an inch below the lingual extensions.
A person gags because he believes he will die from the lack of air to his lungs. He must clear his mouth immediately to breathe. This is a panic situation brought on by his inability to breathe.
To correct the situation the patient must be aware he can breathe through his nose any time he wishes. Have the patient hold his breath for a few seconds to calm himself, and then tell the patient to breathe slowly through his nose.
I usually go through this with an apprehensive patient. I tell him exactly what I will do if he panics and start to gag. I will yell at him: "Hold your breath and count to five! One, Two, Three, Four, Five, Now breathe slowly through your nose!"
This usually works very well even when the patient is in an immediate problem. Sometimes when the impression material and tray are inserted, the patient will take a deep breath. This allows the impression material to flow into the patient’s throat. The patient will begin a gagging reflex attempting to remove the impression material.
The patient can still breathe through his nose. If you can keep the patient calm with the above technique, the impression material will harden in place. Then the hardened impression material and tray can be withdrawn.
The setting time of the impression material in the patient’s mouth can be controlled by:
The warmer the water used to mix the alginate material, the faster the alginate material will set.
The tray filled with the impression material can be held outside off the patient’s mouth until it is ready to set. Then the tray filled with impression material is inserted. It will set in seconds instead of a minute or more. One has to be aware of the setting time of the material to make this an effective method.
There are many opinion and techniques in taking impressions. This course has given you a basic technique that works very well and will give you consistently good impressions. The crown impression techniques works especially well with the compression technique if care is taken when the patient closes his mouth.
The technique for taking denture impression will give you consistent dentures that do not require adhesives to remain in the patient’s mouth. The retention of a denture depends on its extensions and the bite. The more area the denture can cover, the less destructive force the alveolar bone will take, and the more surface the denture will have for retention.