Increasing Retention of a 3-D Printed Sleep Apnea Appliance
By now most everyone practicing Sleep Dentistry has seen or handled a CAD-CAM sleep apnea appliance; Narval by RESMED and D-SAD by Panthera are common examples. Fabricated by a 3-D printer using a laser sintering process, these appliances are made of type 12 organic polyamide, better known as nylon; a material we simply don’t have much experience with in Dentistry. Despite the initial impression of seeming fragile due to minimal size and thickness, this material has demonstrated itself to be much more resilient then dental materials we have used to date. I have been surprised regarding how much we can grind away at this material when dealing with bite issues or patient comfort, without compromising the appliance.
This nylon polymer has a very unique set of characteristics; rigid at a thickness of 2mm or more and flexible at a thickness of 1mm or less, it can be safely reduced to a thickness of 0.5mm and down to 0.2mm for a small surface area such as a cusp tip, without compromising appliance integrity. It is even possible to pierce through the material over a very small area without affecting the structural integrity of the device, but an uninformed patient may interpret this hole as a defect so this is best avoided or if necessary explained to the patient before performing the adjustment. This nylon polymer is quite remarkable to say the least. However, tightening these nylon appliances in cases of poor retention post fabrication has been an issue.
Attempts have been made to describe tightening the appliance by heating it with a flameless torch, pinching it between your fingers and placing it in cold water to maintain the pinched shape. I have tried this technique on several occasions obtaining only a marginal increase in retention and never enough to actually remedy the problem. That being said, I am told that with practice this technique will allow one to reshape the appliance around a new crown! Perhaps that is possible, but clearly not easily accomplished by the faint-of-heart. After all, we are talking about an expensive appliance that melts when you apply enough heat to it! So, this technique may work better for some than for others, depending on your intestinal fortitude! Another suggestion has been to add some restorative resin to the buccal of the teeth lacking retention, creating more of an undercut. However, this poses the problem of extra cost, extra chair time and the liability of resin that can (and will) fall off. Finally, remakes are costly to everyone: the manufacturer, the patient and to the wet fingered dentist. Any technique facilitating the EASY increase of post fabrication appliance retention would be a very valuable addition to your Sleep Disorders Dentistry protocols. So here goes…
Lionel Dwyer, of Orthodent Dental Laboratory in Oshawa Ontario has come up with the best solution to date. Actually, it turns out that slightly revised, the technique used to increase retention in plastic retainers can also be used to increase retention in nylon sleep apnea appliances. However, before one puts a hot iron on an expensive nylon appliance I would suggest reading the protocol described below and trying the technique on a practice appliance.
This protocol utilizes the following tools: Hilliard Undercut Enhancing Thermoplier, Round Acrylic Burs, Miltex Calipers, Flameless Butane Torch, Digital Thermometer.
What we are talking about is increasing the depth that the nylon protrudes into the interproximal undercuts; thus increasing appliance retention. The Hilliard Undercut Enhancer Thermoplier can be used to do this in the same manner it is used on plastic retainers.
Step 1) Initial Set-up of Thermoplier: Adjust the amount of adaptation the plier will provide by adjusting the hex-screw, which allows you to set exactly how far the plier will push into the nylon. (Once you are set up with a practice appliance you can engage the Thermopliers as described in the text below and set the hex–screw appropriately. Once set, the same depth should work fine from case to case.)
Step 2) Visually survey the lingual and buccal of the teeth lacking retention on the patient’s model, and decide which interproximal areas may be exploited to increase retention. Mark these areas with a pencil inside the appliance. The goal will be to have the marked protrusions extend further into the interproximal undercuts thus increasing retention. Below you see the target retention enhancement areas marked in pencil inside the appliance and at the corresponding spot on the out side of the appliance.
Step 3) If the interproximal area you will be enhancing is too thick, you will need to reduce this thickness with an acrylic bur on the outside of the appliance. Using the pencil mark from the previous step as a guide, reduce the nylon thickness first with a 4mm round acrylic bur, and then with a 2mm round acrylic bur to demarcate exactly where you will be placing the beak of the thermoplier to enhance retention. Avoid grinding the material thinner than 0.5mm. Calipers come in handy to ensure you don’t do this. Below you see the reduction of material by round acrylic bur in the target areas.
Please keep in mind that it is not always necessary to reduce the nylon thickness, or to reduce it to 0.5mm. On your practice model you can experiment with using the thermopliers at various levels of thickness to determine what works best in your hands. Reduction is only necessary to allow the thermopliers to engage the area properly and should be done sparingly.
Step 4) Heat the wedge tip of the plier for 4-5 seconds with a flameless butane torch. This should bring the temperature up to around 30°C. A digital thermometer can be used to confirm that you have not overheated the plier. This nylon has a thermoforming temperature of 50-60°C. However, I find that the plier works well at about 30°C, which can be achieved with about 5 seconds of heating. Remember, too cool and the nylon will not be modified, too hot and you may burn right through the nylon. Melting temperature for this nylon is north of 172°C (341°F); as the temperature you will be working with is far less, there should be no concern. You may be tempted to wing it but an inexpensive digital thermometer can help alleviate any concern regarding irreversibly damaging the appliance, at least while you are becoming accustomed to this technique.
Step 5) Squeeze the handles of the plier together to enhance the undercut ginigival to the contact point. You may have to enhance a number of undercuts but I suggest enhancing one at a time while you are becoming accustomed to this procedure. This material returns to a non-deformable state when the temperature normalizes which takes about 4-5 seconds after applying the pliers to the nylon.
So, first squeeze the pliers, you can observe the undercut area being enhanced inside the appliance as you squeeze, then hold the pliers in place for about 4-5 seconds while the nylon cools, then remove the pliers and check the appliance either on the patients teeth or model to determine if the retention is sufficient. If not, repeat the process at another interproximal area until retention is satisfactory. Of course, if the appliance ends up too tight, a light adjustment with an acrylic bur to the nylon that protrudes into the undercuts should relieve this.
Although these appliances are very accurately manufactured, there are a variety of reasons you may end up with insufficient retention. So far, this is the best technique I have seen to help resolve retention issues without the trouble and expense of a remake. Unlike the “heat and pinch” technique that has not worked for me after several attempts, the “plier trick” worked the very first time and every time since! For those cases not requiring reduction with a round acrylic bur it is has been particularly rewarding to be able to kick the retention up a notch in literally 1-2 minutes. Thanks Lionel.
John Viviano DDS D ABDSM