Q: What is the best way to see osseointegration between implant and tissue? Is it possible to find out if the implant is osseo-integrated, histologically, while it's in the patient's mouth?
A: It is possible to verify osseointegration, or the lack thereof, radiographically, before uncovering and fabricating the prosthesis. Once the implant is uncovered and the longer healing cuff is installed, you can check if an implant is loose in the bone by applying a significant amount of torque. Studies have proven that an osseointegrated implant can withstand a counter-clockwise torque of 90ncm, which is notably more torque than will be applied to the implant when seating the restoration.
When an implant begins to fail, you can see a deterioration of the bone around the implant, although the patient may not necessarily feel any pain. I once saw a patient who went to the dental office complaining of a loose screw—but no pain—and when the doctor removed the bridge, the implant came out too.
Answered by Dennis Purinton, CDT, April 1998
Q. Retaining screws that unthread themselves or "back off" in the mouth are a real problem. What are the causes of this phenomenon and what steps could be taken to prevent it?
A: Screw loosening is very annoying and has been a problem since the beginning of implant dentistry. What technicians now know about loosening screws we've learned from the field of mechanical engineering, not dentistry. There are two common reasons for loose screws:
The screw has not been tightened enough . It's easy to tighten a screw on a working cast in the laboratory, however, it becomes next to impossible when reaching into the mouth posteriorly with a tiny intraoral hex driver held by gloved hands covered in mouth fluids.
To properly tighten a titanium or gold screw intraorally, a torque driver must be used because it allows you to reach the necessary 20 to 35ncm torque value. Torque drivers are also an excellent intraoral screw delivery device when using hex drive screws. The dentist can attach the hex drive screw to the driver and carry the screw to the implant. You can purchase torque drivers for as little as $200 or spend up to $2,000 depending on your needs.
The bottom line about screw tightening is this: the fastening screw must be tightened to a point that it will not loosen under the working load (the occlusal forces applied to a restoration). For example, when an implant-supported bridge is being used to chew and grind food, lateral forces applied to the bridge may cause the screws to stretch or gap at the implant interface and then loosen. The tighter the screw, the less likely this problem will occur. Loading and unloading of the restoration, referred to as cyclic load, occurs at a ratio of 500,000 cycles per year.
The bridge doesn't fit passively on the implants. A misfitting bridge is another common reason for loosening screws. Imagine a bridge that doesn't fit passively onto the implants. How can we expect the screws to clamp the restoration to the implant? The clamping force of the torque driver coupled with a tight fit onto the top of the implant is the combination that keeps screws from loosening.
Answered by Dennis Purinton, CDT, April 1998
Q: I would like to hear a definitive answer on how to handle the removal of a damaged screw.
I don't believe that there is a definitive answer for removing a broken screw. Slotting the top of the broken screw probably works best, as long as you're careful not to damage the threads inside the implant. Try cutting a slot using a half-round burr or small fissure burr. Engage the screw using a very small, slotted screw driver. You may need to refine the slot if the screwdriver doesn't work the first few tries or thin the blade on the screwdriver.
Another method I have found helpful is to drill the top of the screw with a small round butt with the handpiece rotating in reverse (counterclockwise). If you apply enough pressure, the friction of the burr will rotate the screw out.
Answered by Dennis Purinton, CDT, April 1998
Q: Can impression posts be interchanged after the impression has been taken?
It's a good idea not to interchange impression posts. There might be slight discrepancies in the overall height of the impression and this would change the depth of the analog on the master cast. The hex may also be different in relation to the flat side and this would change the hex position on the master cast compared to the implant.
Answered by Dennis Purinton, CDT, April 1998
Q: Which is the most popular method of fixation—screw-retained or cemented?
A: This depends on the restoration. A removable appliance such as an overdenture has no advantage in cementing, so a screw-retained bar would be a better choice. A screw-retained restoration is always faster to install and easier to remove.
For porcelain-fused-to-metal restorations, I think a cement-retained prosthesis is best. I prefer fabricating custom abutments for either metal or porcelain, especially if the implants are divergent. When making a custom abutment from scratch, such as a UCLA or IPA abutment, a margin can be added to increase the emergence profile to better resemble the size of a natural tooth and to follow the height of muccusal tissue. There are abutments, newly available, made from ceramic alloys so you can add the collar or margin in porcelain to get esthetic results.
Answered by Dennis Purinton, CDT, April 1998
Q: What should I do to correct a hybrid prosthesis framework that does not fit all abutments properly?
A: First, it's important to determine which part of the framework is causing the problem. This must be done in the mouth or on an accurate master index. However, verifying the error of fit in the mouth can be very time consuming for the doctor and, if he sections the framework, it may not be cut properly for the laboratory to solder. I prefer to fabricate a master index. Here are two techniques:
Technique #1
In the laboratory, section the framework between each implant. If possible, the cuts should be made diagonally to achieve as much area in the cross-section as possible to give maximum strength to the solder joint (diagonal cuts have greater surface area than vertical cuts, thus resulting in a stronger joint). The cut should be made using a very thin (.009"-thick) disk, and kept between .2mm and .3mm.
Send the framework to the doctor so he can place the sectioned framework in the mouth and use pattern resin to connect it one section at a time, allowing each section to fully cure.
After the framework has been indexed in the mouth, remove it, attach analogs and place it in a low expansion stone.
Technique #2
This technique requires more work on the part of your dentist-client.
The doctor uses pick-up type impression copings and seats them on each implant in the mouth.
Next, he ties dental floss to one of the impression copings on either end and zigzags the floss from one side back to the other to create a substrate for the pattern resin.
The doctor places the pattern resin around the impression copings and the floss to form a bar. After the pattern resin bar has fully cured, it is sectioned between each implant.
He reconnects the bar with the pattern resin one connection at a time, allowing each section to fully cure.
Finally, the doctor removes the pattern resin bar from the mouth and sends it to the laboratory.
In the laboratory, connect the analogs and place the pattern resin bar in a low expansion stone.
After the stone has set, remove the pattern resin bar from the master index, place the implant framework on the master index and insert all of the screws. Tighten all of the screws down and back them all out one full turn.
Tighten each screw individually to detect any discrepancies in the framework.
Section the framework until all sections with discrepancies have been cut and are now fitting passively.
Once a master index has been fabricated, you can begin soldering procedures. I like to solder one connector at a time rather than soldering multiple connectors at once; it's more accurate and if there are any discrepancies during the soldering procedure, you can re-section the last joint soldered. Remember to follow the manufacturer's instructions for the soldering investment to achieve the greatest accuracy.
There is a technique new to the dental industry called electrical discharge machining (EDM) which corrects the fit of the framework. Using a specially fabricated master index with wires connected to the EDM machine, the index and framework are submerged in dielectric fluid. The current from the machine flows through the framework to the connection of the analogs and erodes the framework to achieve a passive fit. The process usually takes between five and 10 minutes and can be done on ceramic cases instead of post-soldering. However, this technique doesn't eliminate the need for conventional soldering in cases with large discrepancies. In the February 1997 Journal of Prosthetic Dentistry, there is an article covering the procedures and protocols for this procedure.
Answered by Colin Gibb, CDT, October 1998
Q: What is the best die stone or other material to ensure accuracy and avoid breaking implant abutment impressions? What is less messy and more accurate than epoxy?
A: An article in the March 1997 Journal of Prosthetic Dentistry compares epoxy resin to die stone and shows that they have a similar degree of accuracy. As you mentioned, the drawback to epoxy is the cost and the mess. In my laboratory, I prefer to use a high quality stone. Like any material in the lab, it is important to follow the manufacturer's instructions. When mixing die stone, only use distilled water; tap water contains minerals and deposits that can lead to inaccurate models, poor surface detail and weak models.
Helpful hints
The following are some helpful hints for doctors and laboratories regarding screws:
If possible, use separate screws for fabricating the implant prosthesis and for the case in the mouth. When the case is being made, the screws are placed in and out of the model through all the steps, which can cause the threads of the screws to become worn and loosen in the mouth. If you use separate screws to fabricate the prosthesis and you lose one, the doctor can still set the case without waiting for a new one.
If you are not using separate screws, be careful you don't get wax or metal shavings in the tops of the screws. This can stop the driver from fully seating and cause the head of the screw to strip out. Always steam clean the screws before sending them back.
Many companies have made vast improvements over the conventional slotted screws that were hard to deliver to the mouth without falling off. Some companies now use internal hexes, internal torx (an internal six-pointed star configuration)and internal squares to make delivery to the mouth and torquing easier.
Screws will generally not loosen if they aren't worn out, torqued properly, not overloaded and the prosthesis fits passively.
If a doctor has a problem with screws coming loose and the case has been designed properly, ask him if he torqued the case to the manufacturer's recommendations. Different screws have different torques and may require the use of a different torque driver. Some companies have electronic torque controllers that allow variable settings.
Lately, some companies have made advances in screw technology using solid lubrication technology and gold coatings to achieve a higher yield strength. This results in stronger screws that stay tighter.
When working on any implant case, be sure to use the proper driver. Drivers that exhibit wear may damage the screw.
Author's note: Recently, there have been many advances made in the area of implant prosthetics. Call the manufacturers to receive up-to-date catalogs. Your ability to know and understand the major implant systems as well as new technology is a great service to your dentist-clients. Your know-how will become a valuable resource to them and help build confidence in your professional relationship.
Answered by Colin Gibb, CDT, October 1998
Q: Discuss your technique for waxing and casting the metal framework for a fixed-detachable (hybrid) prosthesis. Also, what are some of your soldering techniques?
A: I think it's helpful to start from the beginning, with the fabrication of the master cast.
After correctly placing the impression post and analog back into the impression, you should always inject a soft tissue material (gingival mask) around the analogs prior to pouring the die stone. This allows you to access the top of the analog to verify fit and is especially helpful when using implant body analogs.
Fabricate a baseplate and occlusion rim, then mount the master cast with the correct vertical dimension and centric relation bite.
Proceed to denture setup and wax try-in. Wax the framework beneath the denture teeth so your framework supports the teeth.
Pick up the denture teeth in a stone matrix that can be repositioned back to the master cast. I prefer using a UCLA-type abutment for hybrid frames. These allow a shorter bar profile that helps to maintain denture teeth by not grinding away as much tooth.
Seat the abutment to the master cast one at a time, reposition the matrix onto the master cast and shorten the abutments needed. Proceed to the next implant. Once you can seat the matrix onto the master cast, with all the abutments in place, begin connecting the abutments together.
There are hybrid patterns available that are much faster to wax than the frame superstructure and are also very inexpensive. I use a burnout plastic pattern that can be shaped with a lathe or a handpiece to the desired shape, using the matrix as a guide.
Begin by drilling holes through the pattern to fit over the UCLA abutments. Seal the pattern to the abutments using wax. Now remove the waxed frame from the cast and shape and contour to the desired size. With the waxup seated on the master cast, return the matrix and adjust the waxup to support the denture teeth.
When the waxup is completed and ready to be invested, section it in half and cast the two halves in separate rings. This reduces the amount of alloy to be cast. Hybrid frames usually weigh at least 12-13 dwt when finished. (This is my personal preference because it reduces the amount of alloy needed to cast a large hybrid framework. You can also easily cast in one piece.)
After casting has been completed, either devest with low-pressure glass beads or chemicals such as stripping acid in the ultrasonic. Remember not to sandblast the interface surface of the abutments, as this may damage marginal fit between the abutments and the implant.
Soldering techniques
Once you have fit the casting back to the master cast, solder the sectional pieces. Pick up the assembled framework from the master cast with a material that will not warp during the time you seat it onto the soldering investment; I like to use an old bur. Lay the bur onto the sectioned area of the frame, then flow baseplate wax to complete assembling the framework into one piece.
Once cooled, place the framework into the soldering investments. I normally do not use analogs for soldering.
Boil off the baseplate wax, trim the solder investment and prepare for the burnout furnace. Heat soak at 1300 oF for 10 minutes, then solder using a low-fusing post solder.
Once the bar has been tried in the mouth to verify fit, proceed with your wax try-in.
Place the finished bar back on the master cast and seat the matrix with the denture teeth luted onto the matrix and transfer the teeth from the matrix to the hybrid bar. Refine the occlusion on the articulator and finish the waxup.
After the intraoral try-in, check the occlusion, phonetics and esthetics. The case is now ready to process and finish.
Answered by Dennis Purinton, CDT, September 1999
Q: I read the remedies for fixing loose screws in the October 1998 issue, but am still having problems. Do you have any other ideas?
Loose screws in implant dentistry are frustrating and can occur for a number of reasons. I think the number one reason for loose screws is simply not tightening them enough. Research has shown that the tiny hand drivers for intra-oral use do not allow the dentist to adequately tighten screws.
Screws must be tightened with a torque driver. I loan my dentist-clients torque drivers to use when seating the case in the mouth.
The second cause of loose screws is sheer force—the lateral force that pushes the crown or abutment sideways, creating a gap at the interface and slightly stretching the screw (a crown functioning in lateral excursions). For example, an implant crown on a maxillary cuspid that creates a cuspid guided disclusion will cause the screw to loosen. Ideally, you would want a group function in lateral excursions.
I have found that we—both technicians and dentists—create most of the problems with implant dentistry ourselves, usually because we fail to follow basic crown and bridge theories. Following fundamental dental theories is especially important when working with implants. And don't forget the basic techniques, such as frameworks that must fit passively, clean castings and abutments that fit perfectly. In addition, always use full size quality articulators, so that you can predict proper occlusal function.
Tips for success with implants
Get all the education you can. Attend programs sponsored by the implant companies; they offer specific details about particular implant systems and are often free.
Join implant organizations such as the International Congress of Oral Implantology and Academy of Osseointegration, and attend their annual meetings to stay abreast of updates in implant technology.
Subscribe to the implant journals. Their articles cover a broad range of implant-related problems.
Answered by Dennis Purinton, CDT, September 1999
Q: I'd like to recommend a technique to my laboratory for fabricating a new denture with Hader/ERA attachments on an implant framework already in the patient's mouth. Can you recommend one?
A: First, the dentist has to provide a good alginate impression of the patient's mouth with the framework in place for the fabrication of a custom tray. After completion of the custom tray, take a final impression of the implant bar in the mouth and all the necessary landmarks. Be sure to block out the underside of the bar so the impression material doesn't lock around it and tear.
Once the impression is sent to the laboratory, the laboratory should order metal analogs that can be placed into the impression to replicate the attachments in the mouth. The laboratory should then pour the impression in die stone, trim the model and fabricate the baseplate and bite rim with the processing attachments in place. This enables the dentist to try-in the base plate and make sure the attachments go properly into place. Then the laboratory should proceed with a set-up try-in and processing procedures.
Answered by Colin Gibb, CDT, October 2000
Q: In your opinion, when making a framework for an implant case, would a predominantly base alloy (i.e., nickel, chromium, beryllium) have a bad reaction when it comes in contact with a titanium implant?
A: There are two classifications of predominantly base alloys: those containing nickel, chromium or beryllium (once referred to as non-precious) and those containing cast or milled titanium.
In my opinion, there could be a bad reaction when Ni, Cr and Be alloys come into contact with a titanium implant. However, cast or milled titanium alloys are acceptable to use with titanium implants. I choose to use high noble alloys for all of the restorations in my laboratory because of the sound evidence that has proven their quality and durability in the oral environment.
I've looked at some studies conducted by Metaux Precieux: Biocompatibility, Allergy and Corrosion Resistance: A Global Scientific Approach and Biocompatibility, Allergy and Corrosion Resistance: A Review of Eight Years of Research. The two studies are available from Metalor USA at 800-554-5504.
According to the studies, Ni, Cr and Be alloys have a low-rest potential, while titanium and high noble alloys have a high-rest potential. The higher an alloy's rest potential, the more stable and corrosion resistant it is.
Galvanic corrosion can occur when two alloys with different rest potentials come in contact. For example, the difference in the rest potential of a high noble alloy and titanium is small, so the galvanic current is small. However, the rest potential difference between Ni, Cr and Be alloys and titanium is much larger, therefore the galvanic current is greater and more likely to result in corrosion.
Localized corrosion between alloys can cause them to break down even faster due to crevices created by screws and screw holes or the spaces in-between frameworks. The stagnant conditions of these spaces cause the acidity of the saliva to greatly increase and the corrosion process to be accelerated.
Another aspect to consider is the biocompatibility of the chosen alloy. Ni, Cr and Be alloys have a high ion release and some of the alloy's components may be considered toxic, mutagenic or allergenic. High noble alloys and titanium have a low ion release and, therefore, are more biocompatible.
Finally, here's yet another reason why I wouldn't use Ni, Cr and Be alloys for an implant case. My first concern is to achieve an accurate and precise fit to the implants and I think the best way to achieve this is to use machined gold cylinders and cast a high noble alloy to them. Since Ni, Cr and Be alloys can't be cast to gold cylinders, you have to use plastic ones. Some labs like to use plastic cylinders because they're less expensive and they claim to have success with them.
In addition, if you have to solder the framework to achieve a passive fit, there is no question that soldering to a high noble alloy framework is far more predictable than soldering to a Ni, Cr and Be one.
TECH TIPS
Tip No. 1: When fabricating a custom abutment, shape, size and emergence are critical to creating the appearance of a natural tooth. Let's say that you're fabricating an implant to restore tooth #8; looking at #9 can give you an idea of what the shape of #8 should be.
However, the shape of #9 at the level of emergence from the tissue is difficult to see when looking down on it from an incisal position due to the tooth's height of contour. To make this size and shape easier to identify, pour an extra model in stone, trim #9 down to the tissue level and outline it with a red pencil. If the tissue has receded, the shape will still be similar, only smaller.
Tip No. 2: Most technicians agree that using machined gold cylinders helps achieve a better interface with the implant and therefore a more accurate fit. Since the implant interface is the area we don't want to violate, we use abutment holders and polishing protectors to preserve it. Not much consideration is given to the surface that truly holds the abutment to the implant: the small shoulder inside the cylinder that the screw shoulder rests on.
When investing, this area can easily have a small bubble trapped in it. After the cylinder has been cast, the bubble may be overlooked, causing the screw to loosen, break or be ground away, taking away some of the surface onto which the screw shoulder clamps down. For these reasons, it's best to invest implant restorations and place them in a pressure vessel while the investment is in a liquid state at a pressure of 90psi to 110psi until the investment is cured. Due to the compression, small bubbles become virtually non-existent, while large bubbles become smaller.
Since allowing the investment to cure under such high pressure results in a denser surface on the investment, you will have to alter the water/liquid ratio. The investment will have more expansion than normal, so reduce the amount of liquid (compared to water) to decrease the expansion. Once the cylinders or frameworks are cast, it's best to chemically divest the units and avoid sandblasting with any material. Any precision attachment or implant should never be sandblasted for any reason as it will alter the precise fit.
Answered by Colin Gibb, CDT, October 2000
Colin Gibb, CDT, covers custom and pre-fabricated abutments.
Cases with properly placed implants, as well as good tissue thickness and tissue depth, can often be restored with pre-fabricated titanium or ceramic abutments. Pre-fabricated abutments usually don't require a lot of modification or additional materials, saving time and labor. However, pre-fabricated abutments don't give you as much control of the size, shape, angulation and emergence profile as custom abutments.
Pre-fabricated titanium abutments can usually be ordered in a variety of sizes, shapes, cuff heights (distance from the implant interface to the margin) and angles. If you're using a lot of pre-fabricated abutments for a given implant system, it's best to buy an abutment trial kit. Since the kit has replicas of all the different abutments available, it will help you select the correct abutment and ultimately save you money.
If you're not working with a trial kit, here's how to select and modify a pre-fabricated titanium abutment:
Identify the diameter of the implant(s) being restored.
Look at the tooth or teeth being replaced and determine what shape the abutment needs to be (i.e., an upper central would be more triangular shaped, a bicuspid more round or elliptical, etc.).
Measure the tissue height and select an appropriate abutment from the manufacturer's catalog or chart. Most pre-fabricated abutments can be modified slightly to correct divergent implants, but too much divergence may require the use of an angled abutment.
Once you receive the abutment(s), minor modifications to the occlusal clearance, margin height, etc., may be necessary. Use a straight handpiece with a carbide that is designed for cutting titanium; it works better than traditional carbides.
Use an old diamond bur—it's less coarse—on the crown portion of the abutment to produce a uniform-looking finish. Be careful to stay away from the margin.
On the abutment's buccal surface, use a No. 1 carbide bur to engrave the number of the tooth it is replacing.
Polish the abutment using polishing protectors to prevent any damage to the implant interface. You can use normal crown and bridge polishes, but there are polishes designed specifically for use on titanium.
Pre-fabricated ceramic abutments aren't available in as many shapes and sizes as titanium ones, so a ceramic abutment may not always be an option for your case. In addition, it's important to understand the limits to which a ceramic abutment can be modified; an over-modified abutment will lead to failure. Most manufacturers have guidelines that you can follow; some recommend the use of a high-speed handpiece with a diamond bur irrigated with water at a speed above 200,000rpm. This helps reduce the possibility of fracturing the abutment. There are also new stones designed for modifying hard ceramic materials.
Here's my technique for selecting and modifying pre-fabricated ceramic abutments:
Determine the implant's diameter.
Look at the tissue depth and see what different cuff heights are available.
Once your selected abutment arrives, modify as necessary, keeping in mind its limitations.
Use a polishing protector and polish the tissue side of the abutment with ceramic pre-polishers and polishing paste.
On the abutment's buccal surface, use a fine permanent marker to indicate the number of the tooth it's replacing. After the abutment has been placed in the mouth, have your dentist-client remove the number with some alcohol before placing the crown on the abutment.
Custom abutments I use custom abutments for the majority of my cases. Here's my technique:
Determine the diameter of the implant to be restored.
Measure the tissue depth; some manufacturers offer different cuff heights.
Use a machined gold cylinder with an attached plastic sleeve, not an all-plastic one. The machined cylinder not only provides a precision implant interface, but also gives an internal precision seating surface for the screw. Start by placing the cylinder on a soft tissue model and reduce the cylinder for proper occlusal clearance.
Remove the cylinder from the model.
Seal the junction of the plastic cylinder and the machined base with sticky wax, staying at least .25mm away from the interface of the cylinder.
Place the cylinder back on the model and, looking from an occlusal view, begin to wax the proper tooth shape for the tooth being restored (i.e. triangular, round, ovoid, etc.). I prefer to use a milling wax because it doesn't gum up on wax cutting burs.
Wax the remainder of the abutment shape as if it were a prepared tooth.
Remove the waxup from the model and fill in the tissue side of the abutment to provide a good emergence profile. Place the waxup back on the model.
Use a 2-degree round-end wax cutter in your handpiece and cut the buccal margin on the model even with the tissue and the lingual margin about .5mm above the tissue. This produces a heavy chamfer margin or a chamfer/shoulder.
Remove the abutment and drop the buccal margin about 1mm.
Finish the interproximal margins if you couldn't get to them on the model. Placing the lingual margin slightly above the tissue allows a place for the cement to escape when the doctor is seating the restoration and gives him a good visual indicator that the restoration is fully seated.
Use a cotton swab saturated with denatured alcohol and smooth the tissue side of the waxup. Spin the swab over the interface to remove any wax or residue.
Sprue the abutment, invest the ring and place the ring in a pressure vessel with at least 90psi. The pressure vessel provides a very dense surface, increases the strength of the investment and helps prevent alloy from flashing over the implant interface. It's necessary to modify the water/liquid ratio by using more water and less liquid because of the added expansion of the investment from curing under pressure.
Proceed with recommended burnout procedures and cast. I prefer to use a yellow crown and bridge type alloy instead of a ceramic alloy.
Use a hydrofluoric substitute to chemically divest the abutments; this doesn't cause any damage to the machined surfaces. Avoid blasting with aluminous oxide or glass beads.
Finish the abutment following the last four steps described for pre-fabricated titanium abutments, but use a straight round-end bur designed for gold alloys.
Colin Gibb, CDT, June/July 2001
Colin Gibb, CDT, covers techniques for fabricating a custom ceramic abutment.
Many of today's implant patients have very high esthetic demands. Custom ceramic abutments are beneficial in many of the same ways as custom cast abutments—they allow more control of the size, shape, angulation and emergence profile--and have the additional benefit of being more esthetic and allowing the use of all-ceramic restorations. New CAD/CAM systems show much promise in fabricating machined custom abutments in a number of materials, but the technology may currently be cost-prohibitive to many labs.
The following is the conventional laboratory technique for fabricating a custom ceramic abutment. Many of the steps are the same as for fabricating cast custom abutments.
Use a soft tissue model with removable soft tissue. Identify which implant system the dentist is using and order a machined gold cylinder that engages the anti-rotational feature of the implant and has the shortest available cuff height. The shortest cuff minimizes the exposed metal.
Place the gold cylinder on the model and make sure its cuff or collar is well below the top of the soft tissue so it won't be exposed.
Look at the thickness and depth of the tissue on the labial or buccal surface. If this tissue is very thin—the collar of the cylinder will cause the tissue to appear gray—or if the collar is close to the top of the tissue, you'll need to reduce the collar.
To maintain the correct orientation of the cylinder so that you reduce it in the proper area, remove the soft tissue from the model, place the cylinder back on the model and cut the collar down where necessary. The collar can be reduced almost completely as long as you don't take away from the implant interface. The implant companies' alloys for machined gold cylinders are non-oxidizing so don't place ceramic on them. Reduce the collar where you want to apply ceramic, so you can wax and cast a ceramic alloy to these areas.
After the collar has been reduced, proceed with waxup:
Seal the junction of the cylinder and the machined base with sticky wax, staying at least .25mm away from the interface of the cylinder.
Place the cylinder back on the model and, looking from an occlusal view, begin to wax the proper tooth shape for the tooth being restored (i.e. triangular, round, ovoid, etc.). I prefer to use a milling wax because it doesn't gum up on wax cutting burs.
Wax the remainder of the abutment shape as if it were a prepared tooth.
Remove the waxup from the model and fill in the tissue side of the abutment to provide a good emergence profile. Place the waxup back on the model.
Use a 2-degree round-end wax cutter in your handpiece and cut the buccal margin on the model even with the tissue and the lingual margin about .5mm above the tissue. This produces a heavy chamfer margin or a chamfer/shoulder.
Remove the abutment and drop the buccal margin about 1mm.
Finish the interproximal margins if you couldn't get to them on the model. Placing the lingual margin slightly above the tissue allows a place for the cement to escape when the doctor is seating the restoration and gives him a good visual indicator that the restoration is fully seated.
Once the custom abutment has been waxed, remove it from the model, get another analog and fasten the custom abutment to it. It's important to use an analog that is an implant replica—not a polishing protector—so that after the modifications have been made to the custom abutment it can be properly re-oriented.
Take the analog and abutment, mix some silicone lab putty, form it into a patty and place it so that it comes up to or just past the margin on the abutment.
After the putty has set, remove the custom abutment and cut the putty mold down so that it's even with the margin of the abutment.
If the case is going to be restored with ceramo-metal units with porcelain margins, cut the margin area of the waxup back from approximately 2mm above the margin down to the collar of the abutment. Make sure to keep the areas of the cylinder that will have ceramic baked to them covered with wax so that the ceramic alloy will be cast in its place after burnout procedure.
If the case is for an all-ceramic restoration, cut the waxup back in all areas to allow for opaque and ceramic application.
Use the silicone mold of the custom abutment and the implant model as a guide for reducing the waxup. The purpose of the silicone mold is to create a form from the custom abutment into which porcelain can be applied. It's easier to create the emergence profile with the custom abutment on a soft tissue model.
Remember to design the waxup to give proper support to areas receiving porcelain. To make steps easier later, place a small gauge piece of wax on the lingual surface. This allows you to hold the abutment with hemostats after it has been cast. It's also important to cover the entire gold cylinder that will receive porcelain with wax; if the ceramic is applied directly to the cylinder, it may crack. With the exception of using a ceramic alloy with which to cast, proceed with spruing, investing and divesting:
Use a cotton swab saturated with denatured alcohol and smooth the tissue side of the waxup. Spin the swab over the interface to remove any wax or residue.
Sprue the abutment, invest the ring and place the ring in a pressure vessel with at least 90psi. The pressure vessel provides a very dense surface, increases the strength of the investment and helps prevent alloy from flashing over the implant interface. It's necessary to modify the water/liquid ratio by using more water and less liquid because of the added expansion of the investment from curing under pressure.
Use a hydrofluoric substitute to chemically divest the abutments; this doesn't cause any damage to the machined surfaces. Avoid blasting with aluminous oxide or glass beads.
Once the abutment has been cast and divested, place the unit on the silicone mold and on the model. Reduce any necessary areas and finish the areas to receive porcelain according to the alloy manufacturer's recommendations.
- When blasting the unit with aluminum oxide, place a polishing protector on the abutment to protect it from the abrasive. Follow the recommended procedures for metal heat treatment and opaque application for the correct shade.
The abutment is now ready for ceramic buildup.
Start by applying a ceramic margin separator to the silicone mold and blow out any excess.
Hold the abutment with a pair of hemostats, apply a bead of ceramic material around its base and place the abutment carefully onto the implant analog in the mold. I like to apply a ceramic designed for porcelain margins to the abutments. Choose whatever type of ceramic you wish, but use a shade that will work with the restoration being fabricated on top of it.
Once the abutment is placed on the mold, add ceramic material to fill up the mold to your previously determined margin height.
If the abutment is to be fully covered with ceramic, complete the rest of the contour.
Dry and remove. After each ceramic application, be sure to wipe any particles off the machined interface of the gold cylinder and fire according to the ceramic manufacturer's instructions.
It will be necessary to do at least one correction bake to compensate for shrinkage after firing. Repeat the procedure as necessary.
If the unit is all-ceramic, remove it from the mold and place it on the working model to check any areas that may need additions; the mold only establishes the emergence profile and the margin.
Once the corrections have been made, place the abutment on the implant model with the soft tissue in place and define the margin with a fine diamond in a high speed handpiece. I also use the diamond and handpiece to make any adjustments to the contours.
Finish the tissue side of the abutment on an analog with a diamond impregnated pre-polishing wheel.
Apply glaze to the ceramic portions of the abutment and fire it.
After glazing, cut off the lingual metal holder, lightly blast the crown portion with aluminum oxide and polish the tissue side with ceramic polishing wheels and diamond paste. The crown portion can also be etched with acid for all-ceramic restorations that will be bonded. Any metal on the tissue side of the abutment needs to be polished while on a polishing protector.
Answered by Colin Gibb, CDT, September 2001
Q: Is it better to splint multi-unit implants or to make them single units?
A: This is an excellent question that seems to come up frequently and should be addressed during treatment planning. The short answer is "it depends on the situation." Let's take a more in-depth look at some of the considerations that come into play.
Maintaining oral hygiene is very important in maximizing the life of the implant and restoration. Poor periodontal health can lead to bone loss and, ultimately, loss of the implant and surrounding dentition. Flossing around a splinted restoration is difficult and since it requires extra effort, patients are often less motivated to do their part in maintaining oral hygiene. However, individual units can be flossed as easily as natural teeth and therefore are generally preferred over splinting, provided that the following conditions are satisfied:
Quality and location of suitable bone always dictates whether or not implants can be used and where they are placed. If a patient has suitable bone present, individual crowns supported by implants will likely be indicated. However, patients without suitable bone—especially common in those who have been edentulous for an extended time—don't have the necessary support for several implants and individual units. A suitable option for them would be a bridge supported by fewer implants that may require multiple splinted abutments.
Another option for these patients is to undergo a bone grafting procedure that would allow them to regain the lost bone and obtain a foundation on which to support more implants and, therefore, individual units. However, if an acceptable alternative is available, the patient may refuse to undergo a grafting procedure.
Load distribution. If the density and quantity of bone is sufficient, longer implants can be used and greater loads can be placed onto the restoration and implant. In this situation, single units are usually the preferred treatment choice.
However, if the supporting bone height is less than ideal, shorter and possibly wider implants must be used and additional implants are sometimes required. Splinting is necessary to more evenly distribute the occlusal forces throughout the restoration, supporting implants and the surrounding bone, resulting in decreased load levels for individual implants.
For cases that are cemented instead of screw retained, the additional benefit of splinting is that it's much easier to attain a passively fitting restoration. Screw-retained splinted restorations require a perfect fit in order to achieve passivity while cemented cases are more forgiving.
Evaluating a patient's occlusion and para-functional habits is an essential part of treatment planning. If a patient exhibits bruxism or has a history of fracturing restorations, you must make provisions for additional strength in the restoration. (In fact, we sometimes receive cases with a prescription that reads "large strong man; provide extra bulk for strength.") Splinting is one method of increasing strength.
In the posterior region, the occlusal forces are usually transmitted along the long axes of the implants. This is due to the fact that the opposing teeth are directly in line with those forces. In the maxillary anterior, however, occlusal forces can occur in directions that may increase the leveraging forces being transmitted to the implant.
For example, I recently had a case where maintaining the original overbite and overjet was crucial to the success of treatment. The location of bone was significantly lingual to the intended location of the incisal edge, potentially causing higher leveraging forces being transmitted to the implants during incising. The presence of pontics made splinting the only choice for this patient.
Actual position of implants/esthetics . Because the precise location of the implant is decided during surgery, the actual position can vary from its originally intended location. A slight change in implant location may affect the intended design and, therefore, the function and/or esthetics. In these situations, communication with your dentist-client is critical.
For example, a dentist-client intended to place two implants to support two individual crowns in positions 12 and 13 and to have custom abutments and cemented crowns. However, after the implants were placed, their actual location was slightly distal to the position that was initially planned.
After impressions were taken, I determined that the intended design would result in the teeth being too wide mesio-distally. I created an alternate design—one that used a splinted restoration rather than individual units—to maintain symmetry with the contra-lateral side. The interproximals were located directly in the middle of the implant; three crowns were necessary instead of two.
Cost is also a factor when determining whether to splint or use single units, especially in larger cases. Since single units are usually more expensive, sometimes the dentist-client will present the option of adding pontics to eliminate the expense of additional surgery and components. This can be an acceptable form of treatment as long as sufficient bone exists.
Answered by Dennis DeMarchi, RDT, March 2002
Colin Gibb, CDT, describes his techniques for fabricating a surgical guide or stent.
Implant laboratories are often asked to be magicians and restore cases in which the implants are positioned improperly, aren't deep enough or there aren't enough implants to support the restoration. One way to combat this problem is to fabricate a surgical guide or stent. While some periodontists or surgeons request surgical guides that only include the labial or lingual contours or occlusal, guides designed in this way exclude important information and often result in an improperly placed implant. The guide must contain all tooth dimensions as well as the depth of the implant.
There are many ways to fabricate surgical guides and each may work equally as well as the next. Regardless of which technique you use, the surgical guide must include all of the necessary information so it can be used for both pre-surgical planning (i.e., CT scans, tomograms, etc.) and as a guide for surgical placement. A properly designed guide can tell doctors whether or not ridge augmentation or grafting procedures are necessary prior to placement and show the exact tooth size and shape as well as how deep the implant must be placed.
Fabricating a surgical guide for fixed crown and bridge work: Since this type of guide gives the surgeon the exact tooth dimensions in the X-rays or CT scans, it's much more valuable than other guides that use indicators like ball bearings, metal pins or holes filled with barium, etc. This guide also gives the surgeon a minimum depth measurement for placing the implant as well as a guide to follow if it's necessary to build up the ridge.
Here's my technique:
Start with a properly mounted set of upper and lower models and a prescription describing the teeth to be restored.
Duplicate the models and mount the duplicated set. This will be the set on which you fabricate the guide.
Select the teeth that most closely fit the case. I recommend using some of the new radiopaque denture teeth because they show up in X-rays and CT scans, giving the doctor valuable information for implant size and placement. If you don't use radiopaque teeth, they will need to be coated with barium so that they show up in the scans.
Adjust the teeth to fit mesial-distally.
This step is critical: each tooth needs to be contoured to the CEJ (cervical enamel junction) of the adjacent teeth, if present, or a natural-looking crown length to fit the case. The surgeon should place the implant at least 3mm below this point; placing the implant any less than 3mm from this point will hinder the esthetics of the final result.
Cut some mechanical retention into the lingual side of the denture teeth.
Using setup wax, attach the teeth to the model on the lingual side—not the occlusal—and adjust the occlusion. In some cases, the denture teeth will not touch the model on the tissue side; this tells the surgeon and restorative doctor that the ridge needs to be built up prior to implant placement.
Using baseplate wax, wax up onto the linguals and occlusals of the adjacent teeth, approximately 3mm thick. If there are no adjacent teeth or not enough teeth present, it's necessary to cover the palate or ridges with wax to give the guide sufficient support in the mouth.
Flask the model and follow normal processing procedures and pack or inject with clear acrylic. If you don't have this capability in your lab, have a removable lab process it for you.
Check the guide for fit on the original set of models and adjust as necessary.
Finish and polish.
I prefer to let the surgeon drill the holes because he may use a system with a different size pilot drill than the one I use to drill the hole in the guide. However, if the doctor requests that you drill the holes, wait until after the pre-surgical planning is complete because it may find that the angle is better in one direction than another.
Fabricating a surgical guide for removable cases:
Follow steps 1 and 2 above.
Fabricate a baseplate and bite rim as you normally would for an edentulous case and send it to the doctor to record the necessary information.
Set up the case using radiopaque denture teeth selected to fit the case.
If necessary, send out the setup for a patient try-in.
Process the setup in clear acrylic.
Discuss the placement of the implants with the restorative doctor and mark the implant sites on the model with a permanent marker. Again, I prefer to let the surgeon drill the holes for the best angulation and to fit the pilot drills he uses.
Developments in the implant market There are many developments in the implant industry and some new ones on the horizon. A few of the cad/cam systems on the market currently offer all-ceramic abutments that can be designed from a waxup or totally designed on the computer with 3D software. I have recently fabricated all-ceramic abutments both ways and they are fantastic. There are limitations for minimum thickness in order to maintain the strength, but I plan to use the systems whenever possible on cases that are going to be restored with an all-ceramic restoration.
Another development I saw at a recent American Academy of Implant Dentistry meeting is the new scalloped implant. Instead of the implant being flat on top or even in height, the interproximal surface is higher due to the scallop shape that is cut down on the labial and lingual surfaces. This shape allows for more esthetic placement and hopefully a more predictable result as far as maintaining the interdental papilla and the bone that supports it.
Many of the implant organizations and associations are very surgically driven, but it's amazing how much information I've picked up to help me understand and design my implant cases differently. More and more of these groups are offering lectures for technicians, so contact implant manufacturers or check their websites. Remember, one of the best ways to attract new clients and keep them is to have a good understanding of all phases of implant dentistry and the various systems. Be a resource for your accounts; this will place you a step above your competition and make you more difficult to replace.
Answered by Colin Gibb, CDT, January 2003
Q: Can you give me simple, step-by- step procedures for fabricating overdentures with some of the most commonly used implants and attachments?
A: Given the increase in demand for these types of restorations, this is a very good question. However, it's a broad topic, so I'll highlight some key considerations:
Combining attachments with implants can generally be designed in three ways, depending on the case:
Screwing an attachment directly into an implant. Most implant manufacturers provide components, such as ball attachments, that can be screwed directly into their implants to retain a denture. These parts are economical, simple and work extremely well, provided that the angulation of the implant is not severe and does not interfere with the denture's path of insertion. If it does interfere, a UCLA-type component and a castable attachment allow you to correct the angulation and conform to the ideal path.
Connecting multiple implants with a bar attachment or using the milled bar technique distributes the load and increases strength. Some bar attachments are ideal for situations in which vertical clearance is limited; however, some of their connectors are smaller and require the use of a stronger alloy.
Adding attachments to a connecting bar. You need ample clearance for this design, but making a bar that has distal extension attachments placed on the sides of the bar, or stud-type attachments placed on top of the bar, can offer tremendous strength and retention for overdent ures. Attachments with a self-paralleling feature are very helpful to doctors who like to set their own attachments intraorally because they don't have to worry about rotating the housing to the correct path of insertion--the attachment does it for them. Attachments with thick nylon clips are durable and work extremely well in situations that have enough clearance for a secondary casting or superstructure with a cast housing.
Once you and the dentist have decided which method/design to use, you can start fabricating the case. Here's my technique:
First, be sure to get an accurate impression of the mucosal areas, natural teeth and implants. Dentists often overlook the accuracy of the mucosal areas in impressions for implant cases, but this is important since you'll use this model to fabricate your denture as well as the bar. Also be sure the casts have adequate stone around the base so that you can index them for a silicone matrix.
It's best to do a setup in wax for the doctor to try in—especially if it's a large case. All the components in the world won't help if the final prosthesis is too bulky or doesn't allow enough clearance for the prosthetic teeth to look natural. Starting with an approved setup allows you to tailor the design of the hardware to the desired result. If you skip this step, you may find yourself in the unfortunate position of having to alter the bar and/or attachments to accommodate a changed setup.
Once the doctor has accepted the setup, make a silicone matrix along the entire buccal and incisal of the prosthesis, as well as a separate lingual matrix. These matrices will help you design and place the bar in the best position.
Remove the teeth from the waxup and secure them into the buccal matrix using sticky wax or glue.
Place the matrix with the teeth back onto the working cast. This allows you to see how much space you have to build the bar and components.
Decide what type of bar and attachments to use, based on available space and any preferences to a particular attachment type or brand. In many cases, more than one attachment type will work successfully; however, many of them offer benefits that are unique to their own particular design. If you have any doubt in choosing the correct parts for the case, consult the implant company's technical support staff. In addition, manufacturers' product literature or procedure manuals provide instructions and diagrams for proper fabrication steps for each type of attachment.
Wax up the bar so that it fits within the space that exists with the silicone matrices in place. You should allow space for a cast superstructure and acrylic resin as well. If additional space is needed, check to see if the denture teeth can be ground further. Make sure that the connectors are at least 3mm x 3mm and also have an embrasure space around each implant abutment for hygiene.
Finish the bar and send it to the dentist for a try-in.
Once the fit has been verified, proceed to making the cast superstructure or RPD casting. Set the attachments along the path of insertion and block out any undercuts around the bar. Use metal housings and spacers if provided. Silicone duplicating material produces a very accurate refractory model.
Use the same mold to pour a stone duplicate model for acrylic processing.
Cast and finish the superstructure or RPD, then use the matrix to set the teeth in position, including the superstructure
You can either send the case for a final try-in, or go ahead and finish it. Again, be sure to use the duplicate model for processing acrylic, not the master model.
Answered by Dennis DeMarchi, RDT, October 2003
