Narval CC & D-SAD Strap Failures: Where would you place the Fuse?
Although 3D printed nylon appliances such as the Narval CC by ResMed and D-Sad by Panthera have demonstrated themselves to be extremely robust, it turns out that they do have an “Achilles Heel”; the advancement strap. Even though this only applies to those patients that adequately challenge their appliance through extreme bruxism, it would be prudent, to have a plan in place to manage this issue.
Let's start by understanding the “raison d’etre”. The easiest way to explain this is by comparing the advancement strap to an electrical fuse. Patented in 1880 by Thomas Edison, a fuse protects from catastrophic surges of electricity. By placing a 'Fuse" between the electrical source and the electronic circuitry, the integrity of the circuitry is preserved in the event of an electrical surge, protecting from catastrophic events such as damage to Hard Drives, Mother Boards, Circuit Boards etc.
In the oral cavity, “extreme bruxism events” can be likened to these “electrical surges”. According to the Oxford Handbook of Applied Dental Sciences (2002), maximum biting forces generate around 110 to 160 lbs/sf. Typically, for bruxers that sufficiently challenge their appliances, catastrophic events such as appliance breakage is common. In 3D printed nylon appliances, the advancement straps actually act as a “Fuse”. When these straps stretch or fail, they protect from more serious "Catastrophic" events such as breakage of the appliance body, restorative work or teeth, and straining of the periodontium, musculature or Temporo-mandibular Joint.
When the D-SAD first arrived, it came with a much beefier strap than the Narval CC. Which at first appeared to be an enhancement, until they demonstrated to be problematic with heavy bruxers; the attachment nubs (Figure 3) simply distorted, which allowed the advancement strap to pull through the engagement hole. At this point the strap required replacement. The solution was simple. Panthera created a "b" version of their straps that had beefier nubs, meant to stand up to the forces anticipated in these heavy bruxing situations. When I inquired as to why all the straps were not made beefier I was told that making the straps beefier reduces comfort, so it is better to use the beefier nub only when indicated. This made sense to me and all was fine until the same patient had the same problem with his new "b" strap. That is when I was told that for very extreme bruxers they have an even beefier strap referred to as a "B" strap. Interestingly, to date, the patients I have transitioned to either a "b" or "B" strap have not reported any reduction in comfort at all.
The original Narval CC advancement strap had a few shortcomings; the numbers were too small for most to read without a visual aide, no 0.5 mm increment and they stretched over time when challenged by a heavy bruxer. So, a beefier strap, with larger numbers available in 0.5 mm increments was introduced. Everyone was happy, until the first time a nub responsible for holding the advancement strap in place distorted, allowing the strap to dis-engage in the mouth, similar to what happens with the beefier D-SAD straps. So, it appears that by making the strap beefier, the role of the "Fuse" was transferred from the strap, to the engagement nubs. ResMed may need to revisit their strap design, perhaps going the Panthera route providing different "Brux" versions of their advancement straps for those bruxing patients that sufficiently challenge their structural integrity. In the meantime we need a way to manage this issue for that subset of patients.
Before we get into the solution I would like to clarify a few things. When patients are first becoming accustomed to mandibular advancement they challenge their appliance much more. So, for some patients that tear through the new straps in a short time, this issue may subside with time. In addition, we now better understand that reducing AHI often leads to a reduction in Sleep Bruxism so this could also reduce breakage with time. I found this to be the case in the very early days when I made a lot of Silencer appliances. The Silencer's “Fuse” is the titanium pin holding the upper and lower component together. I found that in a subset of patients, breakage went from the category "Often" to "Never" with simply the passage of time. Some very heavy bruxers that continued to experience pin breakage even after being jumped to the alternative hinge that sported a much beefier pin, simply stopped breaking their pin with passage of time! This of course dates back to the '90's demonstrating that this is not a new issue.
What you have to ask yourself is this, for your appliance of choice, where is the “Fuse” located? They all have one, even if it is the main body of the appliance itself, or the teeth or the musculature or the TMJ! I find that when appliances break, there is usually a common theme for that particular appliance, for example, Dorsal Style appliances have their advancement mechanism sheer off, Silencer appliances have their titanium pin break, Klearway appliances have a wire dis-engage from the acrylic, Herbst appliances experience breakage at the metal-acrylic interface, and EMA appliances stretch out their weaker elastic straps prematurely or sheer off the strap attachment nub when using their stiffest strap (indicating that the stiffer strap transfers the “Fuse” to the attachment nub). Maybe thinking about this "Fuse" concept will help you better understand some of the breakage issues you have experienced with your appliance of choice.
So, how does one deal with the beefier Narval CC advancement straps breaking? Simply go back to the original version of the strap for that patient. Thankfully, I did not throw them away! I would rather have a strap that I monitor for stretching and replace as needed than deal with a disgruntled patient coming into the office with what he perceives to be a "Broken" appliance! The good news is that I have only experienced the original version of the strap breaking into two pieces once. Indicating to me that this is a very rare occurrence and stretching of the strap is the most likely problem you will deal with.
Let's collectively give RESMED and Panthera our feedback so they can go back to their computers and continue making these already exceptional appliances even better. These types of modification are relatively easy to accomplish due to the CAD-CAM process. It really is a New World and these appliances continue to differentiate themselves as New World appliances.
For those of you that are having a problem with this “fuse concept”, think about the breakage issues you have experienced with your appliance of choice and try to determine where the "Fuse" for that particular appliance is; if you do this sincerely, you may not be too pleased with the answer. Finally, the electronics people figured this out a long time ago; if I were designing a Sleep Apnea appliance, I would place the "Fuse" in the most easily and in-expensively replaced part of the appliance that caused no harm when it failed.
Where would you place the Fuse?
