Bite registration for occlusal splints

When making an occlusal splint, we need to provide the lab with an upper and lower impression (digital or analogue) and a bite registration. One arch record is used to make the appliance, the opposing is used to gain the correct occlusion and the bite record is used to mount the models so that the occlusion can be adjusted.

The most common bite record that I see taken by my colleagues is either a bite registration in maximum intercuspal position or no bite record at all. This can work out sometimes but from my perspective it is not an accurate bite for splint construction. Essentially, I see a splint as a removeable tool to reestablish the patient's occlusion at an open vertical dimension. When the patient wears a splint, their vertical dimension is opened, and the teeth should ideally contact evenly all around the splint and dynamic movements should be controlled as well. This is what we are aiming for with tooth-tooth occlusion as well. Therefore, the bite records for a splint should be of the same quality and type as those taken for a single arch or full mouth rehabilitation where we are recreating the patient's occlusion in tooth borne fixed or removeable restorations.

Following that concept, Taking a bite record in MIP or hand articulation and using that to mount the models in the articulator requires the pin on the articulator to be opened up to create space for the splint material. This is inherently an in accurate alteration as we are relying on the relationship between the teeth on the model and the hinge on the articulator to replicate the patient's teeth-joint relationship. This is not the case. The accuracy of the reproduction of this relationship would be a bit better if we tooth a facebow record and mounted the models on a semi adjustable articulator but this still relies of average values and isn't a perfect reproduction. Furthermore, most labs won't go to this effort to use the facebow or aren't familiar with how to use the record and will just go to their default method and use a simple hinge articulator. The image below shows this discrepancy perfectly.

An evidence-based guide to occlusion and articulation. Part 6: Artificial jaws: articulators real and imagined  S. Afr. dent. j. vol.77 n.6 Johannesburg Jul. 2022

In my experience, what usually happens with this method of lab work is that the pin opening causes a more rapid opening of the posterior occlusal space compared to the patient's opening. This is because the hinge of the articulator is much closer to the occlusal surfaces of the model than the patient's TMJ is to the occlusal surfaces of the teeth. Imagine the pages of a book opened 1cm measured at the edge of the book. Now imagine a door opened 1cm from it's closed position. The amount of opening near the hinge of the door and the book is very different. The book will be much more open near the hinge position than the door. This is because the distance from the "business end" of the door and the hinge is much larger than that of the book. Regardless of this concept, it only comes into play if you are expecting the articulator to mimic mandibular movements away from the static position the models were mounted at. Provided the models are mounted correctly at the bite record that is taken, the relationship of the models are accurate at that position. If you attempt to open or close the vertical dimension (i.e the articulator pin) or move the articulator into protrusive or lateral excursion, all accuracy goes out the window as the articulator cannot perfectly mimic the human TMJ movements. Then, the type and quality of the articulator comes into play. A semi adjustable articulator will be more true to life movement compared to a hinge articulator purely because of the hinge position. Better yet, a facebow record mounted model will position the models at a more realistic distance from the hinge and produce a more accurate occlusal plane position so opening and closing the pin will be more accurate. 

When I took MIP bites for splint construction, the most common problem I would get was a premature contact at the back of the splint around the second molars. This makes sense as the models would be placed on a hinge articulator, the pin opened which opens the anterior occlusal space too much. The lab would open the pin to the minimum thickness required to make the splint ~2mm at the posterior area and then construct the splint. This means that when the splint was inserted into the patient's mouth, the posterior teeth would hit first and I would need a large amount of reduction to get the anterior teeth to touch. What happened most of the time was that I would perforate the splint at the second molar position because there was a minimum thickness there to begin with and the space discrepancy was too much and required a lot of adjustment. Since changing my approach, I have had a lot less adjustments and straightforward splint adjustment appointents.

With regards to splint construction, we need the models mounted in an open position. My preferred method of taking the bite record is to take it at the vertical position that the splint will be constructed. This removes the need to open the articulator pin and therefore removes that unpredictability.  Splints can be constructed on hinge articulators this way with a good level of accuracy. Excursions still can't be properly replicated properly but that can be dialed in at the insert stage.

Generally I use a leaf gauge to achieve this position. I put in a good amount of leaves in to achieve an appropriate vertical dimension. I am looking for a bit over minimum thickness of space in the posterior area ~2mm so will probably put about 35 leaves (~3.5mm) at the anterior to begin with. I will get the patient to attempt to bite down on the back teeth and hold that position. The idea is that the elevator muscles will overpower the lateral pterygoid and force the joint to seat into centric relation. Hinging the patient's jaw open past the first contact allows the joint to seat further. After a minute or two, I recheck the minimum thickness at the posterior and can either add or subtract leaves as required. Ideally I have the splint as thin as possible but thick enough to tolerate bruxism forces. Some patients have flatter cuspal inclines and there will be more space for acrylic at the molar region, some will have tall cuspal inclines, a deep "posterior overbite" if you will. These patients will need to be opened more as the overlap of the molar cusps mean that the same amount of opening at the anterior will result in less thickness of acrylic compared to those with flatter cusps.

Below are some pictures of my bite records that are used to construct the occlusal splint.