Posted Apr 28, 2011 in Technical
Q: Occasionally, I'm asked to add a pontic to an orthodontic appliance. No matter what I do, they often break off. What is a good way to attach a pontic?
A: I have tried several techniques from simply cutting an undercut or drilling a hole in the lingual surface of the pontic and filling it with acrylic, to adding ball clasps and wires to further strengthen the area. The technique that has worked best for me is as follows:
Shape the pontic, then cut a large area out of its lingual side so that the entire cutout is beveled, creating an undercut at all edges of the cutout.
Bend a wire (about .025 SS) into a small U shape about 3-5mm wide. About 5-8mm from the bottom of the U, make right angle bends so that the free ends point away from each other.
Cut the free ends close to the right angle bends, leaving enough wire to tuck into the bevel of the cutout of the pontic. This wire ends up looking like an Omega with short barbs pointing out at the ends.
Adjust the wire so that the distance between the barbs is about 3-5mm greater than the mesio-distal measurement of the cutout.
Compress the U with your fingers and insert the barbs into the cutout and release the wire. If adjusted properly, the ends of the wire will fit snugly into the beveled undercut of the pontic cutout, and the wire will remain securely hooked to the pontic. If the wire is loose, remove it, expand the U loop and reinsert it in the cutout.
You should end up with a loop of wire securely attached to the lingual of the pontic. This also makes it easier to place the pontic, since there is a loop of wire to hold onto while adjusting the pontic's position and waxing it into place on the model. You may have to bend the U so that it conforms to the tissue if it protrudes too far. This can be done easily with 3-prong pliers once the wire is placed on the pontic. Now the pontic will have a large area of undercut as well as a loop of wire locked onto the pontic to help hold it to the appliance. Even if the pontic breaks, the chances are excellent that at least one end of the wire will hold onto it so the patient won't swallow it.
Be sure to fill the cutout with the appropriate tooth-shade acrylic so that neither the cutout nor the wire shows through on the labial surface. Then finish applying the standard acrylic.
Answered by Jay Tyler, May 1998
Q: I get orders for removable appliances for pedodontic patients who have little or no retentive undercut. I use ball clasps when I can, but sometimes this isn't possible because of missing teeth. How can I clasp cases like this so the appliance will stay anchored in place?
Posterior retention in pedodontic cases is always a problem. Often the first permanent molars are barely erupted and offer no undercut. Deciduous molars rarely offer adequate undercut for clasping and are often unreliable because they may become loose or be extracted before the appliance has finished serving its purpose.
Some patients have already lost all four deciduous molars, leaving only the erupting first permanent molars with poor or no undercut as the only teeth to clasp in the posterior portion of the appliance. In that case, the appliance must also serve as a space maintainer to hold the position of the molars while the bicuspids erupt. A clasp that accomplishes this—as well as retains the appliance—must be considered.
Here is a technique that I learned several years ago and have passed on to hundreds of technicians and dentists:
Bend a circumferential clasp around the first permanent molar just as you normally would.
Instruct the dentist to seat the appliance and attach a small amount of composite or other bonding material to the buccal surface of the tooth, occlusal to the clasp wire and touching the clasp wire.
After the bonding material has set, the dentist grinds away excess material but leaves a small shelf of material for the clasp wire to lock under, forming an artificial undercut. It requires only a small amount of material and, since the impression of the wire is left on the inferior side of the material, it forms a solid lock position and stabilizes posterior retention.
The same technique can also be used on sound deciduous molars. The feedback I've gotten suggests that the bonding material works best if placed on the mesial third—or closer to the free end of the clasp—where the wire is more flexible and less apt to break the material from the tooth.
Answered by Jay Tyler, May 1998
Q: I sometimes get Schwarz appliances back that no longer fit before the treatment is finished. They don't seat completely and cause soreness in the roof of the mouth. The doctor grinds the acrylic to relieve it, but usually grinds into the screw and ruins the appliance. What can I do?
A: When the maxillary arch is expanded, the palatal vault drops. If this is not accounted for in the construction of the appliance, the situation you have described will occur.
An easy solution is to relieve the palatal vault before making the appliance. This is done by melting a layer of wax in the palate before setting the screw and wires. The thickness of the wax layer depends on how deep the palate is and how much the arch will be expanded.
Placing wax in the palate also allows you to set the screw more easily, since the screw tab can be pushed into the wax and melted to it, eliminating the need for cutting a slot in the model.
Answered by Jay Tyler, May 1998
Q: My solder joints look great, but I still get a lot of breakage, especially lingual wires breaking loose from the bands. What am I doing wrong?
A: The two biggest problems I have seen with solder joints are:
The solder is overheated: When this happens, the solder will bubble and the joint will be weak. Place a wire probe (about .036) in the solder area and heat it along with the wire and band. The brunt of the heat can be directed onto the probe wire, which transfers the heat to the solder area. It also guides solder to areas to which it may not otherwise flow.
The wire is not completely encased in solder: The wire should be completely covered in solder; otherwise, mouth fluids seep between the wire and solder and erode the solder until the wire loosens from the joint. I have seen some beautiful-looking solder joints that were buffed so shiny you could not see that the wire had been exposed, leaving only a small amount of solder on each side of the wire.
I taper the ends of my wires so that there is no blunt end sticking out the distal portion of the solder joint that needs to be beveled. By doing this prior to soldering, and using a probe for heat control, you can build an adequate flow of solder so that after the joint is buffed, the wire is still completely covered.
Answered by Jay Tyler, May 1998
Q. Why do my colored acrylics turn different colors on some models?
A: I'm assuming you're referring to problems with translucent acrylics since opaque acrylics, where the polymer provides the color, are typically more consistent and stable and pose no inconsistencies if sprinkled properly. If cured too quickly, however, the texture of opaque acrylic can be altered, changing the way light refracts off it and, thus, slightly changing the color's appearance.
Therefore, the following problems can occur with translucent acrylics:
High temperature can affect the oil soluble dye with which all colored stones are tinted. Methyl methacrylate produces heat as it cures. This exothermic reaction occurs at about 102 oF. If the water in the pressure pot is significantly above this temperature, it will not only force the acrylic to cure prematurely, but it will possibly affect the model's dye, causing a reaction change in the acrylic's color.
Curing with excessive heat can cause blue and violet tinted acrylics to fade to gray. Another cause of fading could be in the application of model separator. If not adequately covered, the model can absorb some of the dye, giving the appliance a "washed out" color.
Translucent black acrylics may cure to a different color, such as brown, green or purple. Since dye colors available to the dental industry are extremely limited by the FDA and there is no true black dye available to manufacturers, this problem is somewhat unavoidable. Manufacturers must combine high concentrates of the yellow, blue and red dyes, with mixtures varying from one manufacturer to the next. I have found one black concentrate (actually a deep purple) that, when mixed to a high ratio, produces acceptable results. However, do not exceed a 35 to 40% ratio of concentrate to monomer. The concentrates are not a true monomer, and will interfere with the curing process if this ratio is exceeded. The only other solution is to switch to a black polymer.
Crozat appliance tips
When bending cribs, the traditional method is to begin at the center point of the buccal portion of the tooth and continue around for the full length. This method will work, but here's an easier technique:
Begin at the middle of the wire on the lingual aspect.
Mark and bend the wire where you'd like the upward mesial and distal bends to be, then judge and bend to conform to the lingual surface.
Once this is done, hold the crib in place with your thumb and continue the wire one direction at a time. This provides a more stable foundation for the wire and, by splitting the job in two directions, allows you to be more precise by working with shorter lengths.
The standard method of prepping a tooth for a crib is to carve the gingival areas to accommodate the crescent wires before bending the crib wire. I find this to be an invitation for patient discomfort. Once the gingival surface of the model is removed, the tendency is to carry the crib wire too wide before bending up to the marginal ridge. When the doctor delivers the appliance, the crib wire will be far too intrusive to the surrounding tissue. Try this technique:
Carefully mark where you'd like the crib and carve no more than ½ to 1 mm into any of the gingival portions of the model. Only carve when absolutely necessary, as in cases when the tooth is not erupted enough for sufficient retention.
Prep the model for the crescents after you've completed the cribs.
To prep for crescent wires, I was trained to notch out the model at the mesial and distal aspects with a knife, following a horizontal line chosen for the crescents. I now use a quicker and safer method with a handpiece and 557 bur that provides an ideal path to lay the wire for plastering in place:
Carefully cut the path for the crescent wire by laying the bur along the tooth's surface with the tip of the bur facing in the direction of the roots.
Begin at the center of the plane the crescent will follow and continue around the contour of the tooth until you reach the marginal ridge.
Clean up the surface with an Exacto knife. (I use a heavy blade since it is stronger and safer.)
Crozats can utilize many small components, such as springs, hooks, pins and tubes. These are especially prevalent with Universal Crozats. A technician can easily spend a third of the fabrication time placing these "extras" on the model. There is typically very little room to fasten these wires down and trying to avoid contaminating the solder joint with wax and plaster can be both tedious and frustrating. I use the following method:
Spot weld a .020" wire to all of the components that need room and stability for securing in place.
After soldering, simply cut or grind off the extra wire.
Tips for sprinkling acrylic (Salt & Pepper Method):
If you do acrylic design and detail work using the salt & pepper method, proper setup is crucial. Try the following technique to make the tips of all your bottles precisely the same, so that switching from one bottle to another poses no surprises or varying results.
Cut the plastic tips of your polymer bottles exactly the same and bore them out with a hot .028" wire to assure an unobstructed pathway for the powder to flow. The wire size can vary depending on the refinement of the polymer.
Fit all of your monomer bottles with a 22-gauge hypodermic needle, cut to 10mm in length.
If you're having difficulty sprinkling to a uniform thickness, consider this situation in our laboratory: Recently, one of our technicians was having trouble sprinkling acrylic to consistent thicknesses. The problem varied on a daily basis and was proving to be a real frustration for both of us. We finally discovered that since the swing-arm light at her station was frequently moved to access items on a shelf, it was never at the same height. And, after some experimentation, we found that the proximity of the light source and varying acrylic colors affected her depth perception. We now move the light depending on the color of monomer we're using, and have been successful in maintaining uniform acrylic thicknesses.
Edges have rolled or curled away from the model after removing an appliance from the pressure pot. We've ended this frustration by following this simple rule of thumb for both translucent and glow-in-the-dark acrylics:
When sprinkling neons (those with phosphorescent additives), make sure the acrylic is on the dry side going into the pot for curing.
When working with translucent colors (monomer tinted), leave the appliance slightly wet when placing it into the pot.
Tips for cutting acrylic (functional & active appliances):
How a technician segments a functional appliance can make or break the final outcome. It's important for everyone involved in the fabrication process to know exactly where the cuts will be made.
Design each case by marking the models with a .5mm mechanical pencil (any color other than what your dentist-client may use to mark models).
Once you've completed the pumice stage, make the appropriate cuts.
First, mark the acrylic with a fine point permanent marker, following the markings on the model. (With translucent acrylics, we're able to see exactly where the wires and screws are. We do not use opaque acrylics in the fabrication of any appliances that require cutting.)
Begin with a cutting disk wherever possible, being careful not to over-extend the cuts. The disk allows you to lay the groundwork for straight, clean lines, rather than the wavering lines caused by saws and cutting burs.
Once the initial work is done, finish the cuts with a 557 bur. Whenever possible, approach the acrylic at a 45-degree angle. When you lay the bur down at this angle, the initial cuts provide a track or path for the bur to follow. If attempting this cut at a perpendicular angle, the tendency is to stray a bit from side to side, resulting in a ragged final appearance. Do not attempt to cut too quickly, as the bur needs time to cool. Once the bur is hot, the acrylic melts onto the bur.
After the appliance is segmented, bevel the edges by running a polishing cone lightly over the cuts. This will provide a more comfortable surface for the patient's tongue.
Answered by Tim Spaid, June/July 1999
Q: What is the proper technique for fabricating adult flippers? How do we make the pontics look natural?
A: Fabricating adult flippers can be difficult. Following are some guidelines for each fabrication step.
Matching texture and contour Many orthodontic technicians are not schooled in tooth morphology and struggle with matching texture and contour when fabricating adult flippers. Here are some areas to consider:
Shade taking. Though there are conversion charts comparing one tooth manufacturer to another, many shades just do not have a perfect match in other shade guides. Find out what shade guide each of your accounts is using and make sure you are compatible with them.
Stocking plastic teeth. Since our laboratory does not offer dentures or partials, we do not stock racks of plastic teeth. Instead, we stock only the largest size teeth and size down. As with sculpting, we begin with more material than the final piece requires, carving away unwanted material. This allows us to shape the pontic properly, ultimately duplicating the contours of surrounding teeth.
Know what you are replacing. How does the original dentition appear? When replacing anteriors, are they crowded? Was the tooth extracted or did it break off? Check with your dentist-client before you make any adjustments such as straightening a crooked tooth. Your dentist-client may have a personal preference and good communication is a must for a satisfying end result. For example, a few of our doctors prefer extraction sites to be "dished out" to allow a tight fit for the acrylic. But not all of our clients accept this. If the tooth has broken off, we leave the area unchanged unless the doctor is planning to extract the remaining root.
Pay attention to where the teeth are located. We try to come as close as possible when matching the pontic to the original root site. However, this is not always possible and sometimes we fabricate the appliance with more or fewer pontics than there were teeth. If the patient waited a significant amount of time to have the flipper made, there may be substantial relapse and it may be esthetically unacceptable to replace tooth for tooth.
Pay close attention to the size and shape of the other natural teeth. For example, if you're replacing a central in a 14mm space and the existing central is 11mm, you probably don't want to fill the edentulous area with a 14mm pontic. Here's what we would normally do in this case:
Size an 11mm pontic and fill the distal diastema with clear acrylic. When fitting the pontic, first grind down to the proper size.
Then match the buccal contours to the surrounding teeth.
Wax completely around the tooth to contain the acrylic when sprinkling the appliance with it. Pay attention to the incisal edge(s) when shaping the pontic. If the existing teeth do not appear worn, be sure to leave enough of the translucent edge on the pontic to match those teeth.
When you feel you've shaped the replacement appropriately, attach it to the model with a drop or two of sticky wax. This allows you to position the tooth without great difficulty. View the placement from all angles to be sure it's set correctly.
Prepping and fabricating tips In general, always wax the undercuts to save model damage and adjustment time. Also, excessive acrylic under the height of contour will have too much "pull" on the tooth when the appliance is being removed.
Designing acrylic. When designing the acrylic for maxillary flippers choose a horseshoe design and keep the acrylic less than 2mm thick. With lowers, fabricate with the same half teardrop shape as your retainers, but try to keep the acrylic at 3mm for the thickest portion. Here are some other tips:
Use clear acrylic around the pontics. Colored acrylic tends to change the tooth shade and overall appearance.
Use stainless steel mesh or wire for support if there won't be enough acrylic to properly secure the pontic.
When grinding, keep the patient's comfort in mind. Too much acrylic will most likely cause speech problems while wearing the flipper.
Clasping. When clasping is left to your discretion:
Always request an opposing model so you can decide the best clasping method, tooth placement and final adjustment of the acrylic. Then follow this procedure:
Use a fairly light wire, not more than .032".
If your case involves recessed gums—as do the majority of our cases—and the surrounding teeth are unstable and insufficiently anchored but you must clasp to those teeth, be careful not to design the appliance with clasps that will risk extracting the teeth when being removed. Arrow clasps are a good choice because they are versatile and can easily be positioned to hold the appliance without grasping it too tightly.
Answered by Tim Spaid, June/July 2000
Q: Can you recommend a technique for achieving a perfect solder joint?
A: There are three essential elements to obtain a perfect solder joint: clean materials, a small torch flame and patience.
1.Clean materials. The number one cause of a porous solder joint is contamination. Even a microscopic spec of dustcan produce enough gas when heated to leave a visible hole in your solder joint. The material used to hold the wire components in place for soldering is usually what contaminates the joints, especially if you're using plaster. To clean the wires, use a pure solvent. Though there are a few others that work, I've found that a few drops of clear monomer applied to the solder site works best. Wick away excess monomer with a piece of tissue and immediately apply the flux to protect the area from floating debris. If repairing an existing solder joint, be sure to grind away any contaminants or solder that appears porous.
2.Flame size. Our lab uses a pencil torch that produces up to 2,400 degrees F. We set the blue flame size at about 5mm—which should be close to 2,000-degree—to keep the heat away from surrounding wires and avoid overheating the joint. When using large torches, it's necessary to cover the surrounding wire for protection. The more plaster or heat screen used, the hotter the torch needs to be to compensate for heat absorption into these materials. If you overheat the wires to an orange glow, you've weakened the metal.
3.Patience. This last element is perhaps the most difficult. If you try to hurry the process, you will burn off the flux prematurely or overheat the wire. Heat takes time to reach the wire's core (like broiling vs. baking). When the flux turns clear, touch the solder to the wires. Remember to heat the wire, not the solder. When soldering multiple wires at one joint, allow for venting by applying the solder to the center of the joint and working outward. Using solder with a lower melting point helps avoid overheating the wire.
Answered by Tim Spaid, June/July 2000
Maggie Harris offers two tips for fabricating better appliances.
Tip 1. Bending a circumferential clasp. Bending a simple circumferential or C clasp can be time consuming for beginners, but once you've mastered it, you will have all the skills required for more complex wire bending.
Here are the tools you need:
Bird beak and three-pronged pliers
Wire cutters that are spring loaded (they're kind to your hands)
A grease pencil to mark the wire
A waxing tool to remove bubbles and melt wax
A rubber wheel to round and smooth the wire
Here is my technique:
Dip the trimmed working model in water for just a few seconds. (Don't submerge the model for any longer because the water absorbs into the plaster and, if the model is too wet, the wax won't stick.) The water makes the model's surface less smooth and the wire will not slip around on the surface as you contour it. Wetting the model also makes removing bubbles easier.
The C clasp should be placed at the height of contour starting at the mesiobuccal surface of the second molar. (With most orthodontic cases the height of contour on the second molars is usually right at the cervical line). Using the #7 waxing tool carve a small amount of tissue (plaster) around the molar right at that cervical line to ensure that the clasp seats well. However, when making adult partials, the molars often have receded gums, therefore carving away tissue (plaster) isn't necessary.
Draw a line on the molar at the height of contour to help guide you while bending the clasp. I like to use .032 clasp wire, although some doctors request .036 wire which also makes a nice stiff clasp. You may want to use a dead soft wire that can be heated after fabrication to make it more rigid. Regular stainless steel wire is more difficult to bend.
Place the model with the buccal surface facing you. With the wire in one hand, hold the three-pronged pliers so that the tips point upward and you are looking down at them. Maintain this position as often as possible—it gives you much more control over the wire.
Grab the end of the wire while continuing to look down at the pliers. Crimp the wire very gently. Hold it against the mesial of the molar and use the grease pencil to mark where the wire touches the model. This is where you'll make the next bend (i.e., bend the wire where it last contacted the model). As you move around the molar it is always better to make three bends for each grease pencil mark: one right on the mark and one on each side of the mark. This keeps the wire smoothly contouring around the tooth. Remember: keep looking down the nose of the pliers.
As you contour around the distal of the molar and onto the palate, keep the wire slightly off the tissue. This allows the acrylic to flow around the clasp completely.
Finally, either crimp the end of the wire into a zigzag shape with the three-pronged pliers, loop the wire with the bird beaks or bend it in a 45-degree angle to prevent it from rotating and keep it anchored in the acrylic. The appliance will be more esthetic if the length and shape of the wires are consistent.
Smooth the end of the clasp with a rubber wheel, then wax the clasp into place.
Tip 2. Recognizing a distorted model. We require our accounts to pour up alginate impressions in stone within the required time and check for distortion. However, distorted models are still the most frequent problem we face in our laboratory.
Distorted models produce less-than-accurate appliances and recognizing distortion and pinpointing the cause is key to improving your technique. Although some distortions are undetectable, here are some we see every day:
The anteriors look razor thin and sharp and the posteriors may have a trapezoidal shape and sharp cusp tips. This is a dried out, unusable impression. If you make a Hawley retainer on a model like this, the lingual acrylic will be too close to the anterior bow. The doctor will have to grind away the acrylic to make it fit and as the patient removes and inserts the appliance, the bow will have to spring open to fit, fatiguing the wire until it breaks.
The anteriors look fat. Chances are the entire model is distorted and the retainer will not fit at all. We usually get the case back saying the retainer doesn't fit the patient but fits the model.
A molar looks narrow on one side. Use calipers to measure the width of the molar from the mesial cusp tip to the buccal cusp tip, then compare it to the molar on the other side. If they are different, the model is probably distorted and the retainer won't fit properly.
The incisal edges look like they have an extra ridge of plaster. If the tray hits the teeth while the impression is being taken, the alginate may tear, causing a ridge effect.
The teeth simply look funny. This feature doesn't always mean that the impression is distorted. However, if occlusal coverage is required, the appliance will not seat properly because the distortion is precisely on the occlusal surface.
The second molars seem to drop off, causing a reverse Curve of Spee. Usually the second molars are so distorted they don't even look like teeth. When we occlude the upper and lower models, the posterior teeth do not interdigitate. We are often instructed to go ahead with the case, keeping the plastic mesial of these distortions, but we often get the case back because the appliance simply doesn't fit or has broken, indicating that the entire model was compromised.
In all of these cases, we ask for a new impression and model. However, since orthodontic appliances can be a lot more forgiving than other restorations—for example, a crown that must seat perfectly—some doctors tell us to proceed because they are willing to grind the appliance to fit. (However, as mentioned earlier, this can result in an appliance that breaks because it doesn't fit properly.) Other times, our client thanks us and reschedules the patient. The potential causes of distorted models are numerous: the impression popped out of the tray and was glued back in; too much time passed before the model was poured in stone; once poured, the tray isn't left plaster-side up; or the saliva wasn't rinsed out, causing bad tooth definition. Most commonly, the cause is simply bad technique. If you continually receive distorted impressions, suggest that your dentist-client refers to the manufacturer's instructions. Also, be sure that your dentist-clients keep the models in case of a remake. You can refer back to the model if the appliance doesn't fit the patient.
Answered by Maggie Harris, March 2001
Q: Mark Ohlendorf, CDT, offers advice on posterior occlusal coverage.
A: Posterior occlusal coveragevfabricating the appliance so the posterior teeth are covered with acrylic—offers many useful benefits depending on your specific treatment options and goals. When adding posterior occlusal coverage, it's important to communicate with your dentist-client to determine which type of movement is desired.
Bonded appliances, such as the bonded Hyrax or RPE, are the only types of appliances that require posterior occlusal coverage. The acrylic is bonded to the teeth to hold the appliance in place.
While posterior occlusal coverage is not required in any other appliance, it can...
...help removable appliances work faster. Some clinicians recommend that patients wear their appliances while eating because the pressures of chewing help drive the appliance in place which, in turn, helps speed up the action of the appliance. In Schwarz and Sagittal appliances, for example, when the screw is turned 1/4mm to activate the appliance, the appliance is actually 1/4mm larger than the shape of the arch. When posterior occlusal coverage is added to these appliances, the crowns of the posterior teeth are driven into the impressions of the teeth in the acrylic. This further pushes the appliance down onto the teeth and helps expand the arch in the case of the Schwarz appliance, and lengthen the arch with the Sagittal appliance.
...provide some assistance with the direction and movement of an appliance. For example, Sagittal appliances are used to obtain anterior/posterior movement. When posterior occlusal coverage is added to these appliances, the lower posterior teeth bite into the upper acrylic and help engage the appliance.
The acrylic can also have indentations of the opposing teeth. The depth of these indentations also helps enhance direction and movement. For instance, if there are heavy indentations, the cusp tips of the lower posterior teeth act as additional anchors to the upper posterior segment and allow the upper anterior segment to move forward. On the other hand, if there are no indentations of the lower teeth into the acrylic, the posterior sections of the appliance have a tendency to move in a posterior direction.
...help move the teeth bodily rather than the appliance tipping the teeth. The acrylic coverage usually encapsulates the crowns of the upper posterior teeth and, as the expansion screw moves the teeth, the acrylic holds the crowns in place as the entire maxillary bone is moved. This allows the posterior teeth to expand or distalize without tipping. Without the occlusal coverage, the acrylic engages the tooth at the gingival aspect and causes a tipping action. Since, in some cases, tipping is necessary to upright teeth, ask your dentist-client which type of movement is required.
...keep the bite from excessive vertical opening. Most orthodontic treatments have a tendency to open the vertical, and some clinicians believe that posterior occlusal coverage keeps the bite from opening excessively. The acrylic maintains the existing vertical and will not allow the teeth to super erupt.
GoGn to SN (Gonion Gnathion plane to Sella Nasion plane) is one of several cephalometric measurements that helps determine the vertical and growth direction of the patient. Even if your dentist-client determines this angle—known as cephalometric tracing—you should still understand how it's determined. To determine the GoGn to SN angle, draw a line tangent to the posterior border of the mandible and a line tangent to the inferior border of the mandible. The bisection of this angle gives you GoGN. To determine SN, draw a line from the center of Sella to Nasion. Then draw both lines to the posterior until they intersect and measure the interim angle. This angle, formed by the two planes, gives you the correct degree of the skeletal vertical; the normal range is 32° ± 5°. As cases approach an angle of 37° or more, some clinicians automatically request posterior occlusal coverage.
...help open the vertical and bring the mandible down and forward in TMJ/TMD patients. These patients are often in pain, and adding posterior occlusal coverage to properly increase the opening of the bite helps relieve many of the symptoms. Most Sagittal appliances used as both orthodontic and TMJ splints have posterior occlusal coverage. Dentist-clients usually request deep indentations of the lower posterior teeth in the upper acrylic coverage to provide the patient with an index for the proper positioning of the mandible as he closes.
...help in cross bite cases. In some anterior cross bite cases, it's also necessary to hold the vertical open to allow the teeth to move out of crossbite. If the bite is not opened, the opposing teeth won't allow the teeth in cross bite to move across the occlusion into the correct position.
Some cases of posterior cross bite also require posterior occlusal coverage, because the cusp tips have a tendency to lock the occlusion. In these cases, as the maxillary arch is expanded with either a fixed or removable appliance, the lower posterior teeth follow the expansion of the upper arch. This can be unwanted movement that hinders the proper expansion needed for the case. Posterior occlusal coverage with no indentations of the lower posterior teeth in the upper acrylic allows the maxillary teeth to be expanded without any movement in the lower arch.
The standard thickness of the occlusal coverage is 2mm, however, it can vary depending on the particular case. For example, if the appliance is being used for a TMJ patient, the thickness may need to be more or less than 2mm so it's comfortable and won't aggravate the TMJ. In these cases, the dentist determines the thickness and sets the proper opening to help relieve pain.
Also, in cross bite cases, the acrylic coverage must be built up as necessary—usually 3 to 5mm thick—to allow movement of the tooth or teeth out of cross bite without interference of the lower anterior teeth. Since cross bite appliances are normally worn for a short period of time, there's usually little problem with making the acrylic so thick, except that it may be slightly uncomfortable for the patient.
When fabricating posterior occlusal coverage, ask your dentist-client for both the upper and lower models and a construction bite. Here's my technique:
To ensure the appliance is constructed to the proper thickness, it's critical to articulate the models to the construction bite when you first receive the case:
Place the construction bite between the articulated models.
Put one mark on both the upper and lower models at any point.
Measure the distance between the two marks with the construction bite in place.
Fabricate the appliance as usual.
Create the occlusal coverage:
When adding the acrylic, apply more acrylic than you need—about an additional 1-2mm of thickness.
Place the appliance on the articulated models and close the bite to the proper vertical. The excess acrylic will be displaced.
Trim away the excess to leave the required amount of acrylic.
Before returning the appliance, double check the thickness of the occlusal coverage:
Place the finished appliance on the articulated models. You may want to use an articulator with an anterior pin to be sure you obtain the same result.
Measure the marks again to be sure the distance is the same with the appliance in place as it was with the construction bite.
When fabricating posterior occlusal coverage on bonded appliances, the technique is a bit different. Follow the technique above, but carry the acrylic down to the gingival on both the lingual and buccal surfaces; coverage over the occlusal is there by default. Then the dentist bonds the appliance in place using a light-cure adhesive.
Answered by Mark Ohlendorf, CDT, November/December 2002
Tom Magill discusses sagittal appliances.
Sagittal appliances have been used in their current form in the U.S. since 1973, when Dr. John Rosberg of Moose Lake, MN, clipped photographs of Dr. Hermann Voss' design from a German orthodontic journal and sent them to an American laboratory. Since the appliance offered effective sagittal (anteroposterior) movement, it caught on very rapidly. While the appliance is made with great frequency, too often it is fabricated with little understanding of the dynamics necessary to control movement.
Since many clinicians have little knowledge of the technical aspects of sagittal appliance construction, the laboratory must construct the appliances correctly to ensure successful treatment. By following the principles outlined here, you can produce sagittal appliances that achieve predictable clinical results, benefiting your laboratory, the clinician and, most importantly, the patient.
Sagittal appliances can move anteriorly or posteriorly and certain basic design elements determine in which direction the appliance moves:
If posterior movement is desired, the sagittal screws must be oriented parallel to a line drawn through the central fossae of the bicuspids and molars (or the deciduous molars and permanent molars in a younger patient). With this orientation, the teeth are carried distally in the mediallary trough, with the least possible resistance to movement.
If the appliance is constructed with a midline screw as well as the sagittal screws, activation of the midline screw must be delayed until distalization of buccal segments is completed. This maintains molar positions in the trough, so that anchorage isn't adversely affected. Distalization gains anchorage from the remainder of the arch, so it is most effective if it is carried out on one side at a time.
When the appliance is constructed with occlusal acrylic bite planes—as is usually the case—those bite planes cannot be deeply indented; the indentations offer resistance to desired movements.
Fixed sagittal designs
In the last decade, several fixed sagittal designs have emerged to overcome patient compliance problems. The most commonly used and effective designs are the Ching Distalizer (C-D Distalizer) and the Magill Sagittal, which I initially designed in 1990. For technical reasons, fixed sagittal appliances are seldom constructed with occlusal bite planes, but all of the other dynamic considerations that apply to sagittal construction also apply to fixed sagittal appliances. In addition:
In all fixed sagittal designs, the screws must be parallel to the occlusal plane so the appliance will not lift away from the tissues or teeth as it is activated.
In fixed sagittal designs that use a buccal power source coupled with a lingual guide tube, the power device and the guide tube must be parallel in all planes to prevent binding that would limit or redirect movement.
When anterior movement is desired, it's necessary to reorient the screws; they must now be parallel to the palatine raphe. Many published articles show sagittal appliances with the screws pointed toward one other anteriorly with acrylic between them; this is incorrect. In this situation, the screws would have to move through the acrylic to move anteriorly, breaking the appliance. While this would seem obvious, thousands of sagittal appliances have been made in this manner with aberrant and random results.
Anterior drive sagittal designs
Anterior drive sagittal appliances are used to correct retruded maxillary incisors, through either proclination of the incisors or superior rotation of the premaxilla. Here are some tips to remember:
When a midline screw is added to an anterior drive sagittal, it should be activated at least six times before the sagittal screws are activated, to improve anchorage. Activate the sagittal screws simultaneously with one another and indent occlusal acrylic to add a degree of anchorage in support of anterior movement.
While it is rarely appropriate, occasionally an anterior drive sagittal is requested for the lower arch. In order to obtain proper orientation of the screws, they often need to be placed more distally in the appliance, usually in the area of the second bicuspids.
Certain wire elements often help in directional control of sagittal appliances; the most common of these are cuspid hooks. If cuspid hooks are added to a properly constructed anterior drive sagittal appliance, they can be beneficial because they add anchorage to the posterior section of the appliance. However, cuspid hooks are a waste of effort in distal drive appliances.
Forgotten appliances: The Stomatopedic Special
There are some classic designs, that although effective, have fallen out of use over the years as other designs have emerged. One example is the Stomatopedic Special appliance that is used to close anterior open bites in patients with narrowed maxillae. It widens the maxillary arch while controlling tongue thrust and has been in use since the late 1960s with excellent results. It is indicated for anterior open bites with bilateral maxillary dental or skeletal crossbites, accompanied by tongue thrust. Contraindications include skeletal open bites, sufficient maxillary width or a lack of the indications above.
This appliance is essentially a Schwarz plate with posterior occlusal acrylic and an acrylic-free premaxillary area. Here's my technique:
Form wire loops in horizontal elongated U patterns, extending from each side and overlapping at the midline, with insertion just lingual to the cuspids.
Cover the loops in latex tubing or soft plastic tubing to lessen tongue irritation.
Include sufficient recurve near the point of insertion to allow easy adjustment of the wires to block tongue thrust. Final positioning of the wires is done by the clinician.
Enhance the efficacy of the appliance by placing a tongue bead that is 5 to 6 millimeters in diameter on a telescoping wire at the mid-first molar point. Place it high in the vault to attract the dorsum of the tongue. And, finally, offer your clients extra tubing to give to their patients to help keep their appliances hygienic.
Answered by Tom Magill, September 2003
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