It’s Going to Bite You!
I think there is a lot of advancement in dentistry over the last 100 years. We have learned so much on how to prepare teeth, how to restore teeth or how to take impression and more? But somehow, when it comes to taking a jaw relation record, especially in dental school, I was made to believe that it’s something simple. It should take you five minutes at the end of a crown preparation. And somehow, if you squirt enough material between the top and bottom teeth and have the patient bite and then send everything you have to the lab, the lab will miraculously figure it all out and give you a beautiful restoration in return.
After doing a lot of my own lab work during my graduate prosthodontic training, evaluating my models and my jaw relation records, I must say that this process is not as simple as it looks. And even over the years in private practice, I find that I have to plan out my jaw relation record appointment. Sometimes I devote an entire appointment just to do a jaw relation record. There are certain elements that will make this part easy. And there are certain elements that will make it more complicated. What I want to say is there is no one method that will fit all cases. Some cases requiring no planning at all while other cases require a lot of planning. And so today, I want to talk about some factors I have to consider in making a jaw relation record, whether it is for diagnostic purpose only or for treatment purpose.
Just like how you plan out your surgery, or your step by step procedure in your restorative treatment, there should be a system to plan for your jaw relation record.
Type of Record:
First let’s ask yourself what type of record do you want. Conceptually, you should ask yourself do you want to mount your record in Centric Relation (CR) or Maximum Intercuspation (MIP)
So let’s think about three groups of patients
1/The Dentate patient
2/The Partially edentulous patient
3/The Completely edentulous patient
For the completely edentulous patient, with traditional teaching in removable prosthodontics, in order to obtain a jaw relation record, you need a set of occlusion rims. Using these rims, you have to determine aesthetics, phonetics, occlusal plane, arch contour, VDO and finally your CR record. Because you have no more references from teeth, the only predictable way is to guide the patient in CR and record that. I will not go into how to guide the patient in centric. This is not what I want to cover today. But for the completely edentulous patient, it’s pretty clear that this is the only position you can obtain repeatedly.
For the dentate patient, you have to determine if the patient has any occlusal dysfunction. If the patient has no pre-existing occlusal dysfunction, you may want to obtain your record in maximum intercuspation. If the patient has occlusal dysfunction, you may want to record the patient in centric relation diagnostically to evaluate the occlusion. If that is the direction, you will likely need to deprogram the patient with an anterior deprogrammer using leaf gauges or customized resin jig to help obtain this record. The teeth will be slightly separated intentionally in this CR record. This is a position determined by the jaw position and not by the teeth.
For the partially edentulous patient, depending on the occlusal relation and what you are trying to achieve, you may decide to mount the case in CR or in MIP. But in a partially edentulous case, depending on the configuration and the length of the edentulous span, there is usually inadequate occlusal stop and it is likely you will need to have recording bases and occlusion rims to support your jaw relation record.
In the last two group of patients: the fully dentate and the partially edentulous, you will need to perform a proper clinical examination including a detailed occlusal evaluation to plan out your jaw relation record. Some critical information I look at include the following:
- Is it a dental Class I II III?
- Where is incisal position in relation to the smile line?
- What is the overjet and overbite like?
- Is there a severe overjet and overbite or crossbite such that the anterior segment of the dentition is not in contact? Or the teeth are pressing on soft tissue?
- Where are my occlusal contacts? Are they distributed evenly across the arch? (I use shim stock to determine the tightness of these occlusal contacts)
- Are these occlusal contacts cusp fossa contacts or cusp tip to cups tip relationship?
- Are the teeth bulbous in shape or conical in shape
- Any interproximal undercuts?
- Type of occlusal wear
- Any centric slide or a large CR/MIP discrepancy
- Pt’s current VDO: is it acceptable? Do you want to alter it?
- Interocclusal space: is there room for prosthetic replacement?
- Any signs of fremitus
All these evaluation will influence how I will prepare for my jaw relation record. I may decide to mount the case at the current VDO or in CR or at the proposed VDO in CR for prosthetic planning.
So what do I need to prepare? I have already mentioned previously that in some cases, you need to have occlusion rims or anterior deprogrammer fabricated. You may need to have good accurate models in advance to allow you to prepare these deprogrammers or occlusion rims. So obtaining a jaw relation record may need to be done on a separate appointment. Once you have these fabricated, they will need to be tried in and adjusted. Whether it is a deprogrammer prefabricated or made chairside, or the recording base with occlusion rim, do not expect that they will just go in without some form of adjustment. The adjustment may take some time depending on what you are trying to achieve and your experience. Once adjusted, they are additional tools to assist you in obtaining an accurate jaw relation record.
For the partially edentulous patient, especially for the patients with a long edentulous span, you will need to contour the occlusion rim to reflect where you envision the prosthetic teeth to be positioned. So you need to think in terms of the buccal lingual and occlusal gingival dimension of the wax rim. Often times, you need to borrow concepts from complete denture to determine your occlusal plane, arch contour and other parameters such as the aesthetics, the phonetics and the lip support. In other words, the occlusion rims are not just for mounting purpose but also a guide for future teeth arrangement. In other times, your occlusal plane may be uneven because it needs to be adjusted to accommodate the existing teeth. But the process of contouring these rims is a diagnostic process in itself that will force you to look at these clinical information. When you record that and mount the case, you will have soft tissue information to relate to. You can also take a pictures of the extra-oral soft tissue information with the contoured rims to record this important information to facilitate future planning.
Once the occlusion rim has been shaped properly, the occlusal surface needs to be adjusted further so it is just shy of the opposing occlusal surface. You also need to make well defined notches on the wax rim so you are creating a space for the bite registration material that will get locked and secured on the wax rim. You will need to use something soft such as softened Aluwax and polyvinyl siloxane registration material so that the material will not interfere with jaw closure. I see students getting the patients to bite directly into the wax leaving an imprint on the pink wax and they use that to mount the case. This is not the most accurate way to record it if you understand jaw movement and the resiliency of soft tissue. The wax needs to be contoured first. You should not use occlusal force to shape the wax rim.
Just like the dentate patient, if your partially edentulous patient has occlusal dysfunction, you may want to mount the case in CR at the proposed VDO that you wish to rehabilitate the patient to. So on top of contouring the occlusal rims based on your complete denture principles, you also need a deprogrammer to assist you in patient’s closing into a predictable stop without influence from the existing teeth. You can see that the more complicated the case it is, the more time you have to prepare for the jaw relation record.
Material: What and How much and Where?
There are many materials available for registering the occlusal relationship. Historically, different forms of wax have been used for registering the jaw relation. There are also more modern materials such as polyvinyl siloxane or self curing resin as well. I used all of them in practice and they all have their advantages and disadvantages. The key is to know their limitation and strength and to develop a strategy so they can perform what you need them to do. Wax distorts easily but they are very forgiving in adapting to the different shapes of teeth. Self curing resin is very rigid and accurate but they can get locked into the mouth for very bulbous teeth or teeth with lots of interproximal undercut. The chemical reaction is exothermic and the material has a certain amount of polymerization shrinkage. This material can also get messy if you have never used the material before. Polyvinyl siloxane is accurate, clean and easy to use but too much of their material can introduce errors if the excess material is not trimmed back enough.
And the decision of where to put these material also lies in how the teeth are related to one another. In some situation, more is not better. In fully dentate patients with cusp to fossa relationship and the bite just locks right into one another in maximum intercuspation, there is no question of how the teeth come together. In such a situation, you may not even need any registration material. Direct apposition of the casts may be the best method.
But there are situations, even for fully dentate patients, where the teeth just do not come together in a way to allow the maxillary cast to be stable over the mandibular cast without having your hands to hold them together. Cases with anterior open bite, anterior crossbite, severe overjet and severe overbite or heavily restored dentition that the teeth don’t come in contact will fall into this category. There is a tendency for the models to be rocking in these cases and I will need some registration material to help support the models during mounting. In these cases, I like to use wax or polyvinyl siloxane bite registration and check how they seat over the models and trim the excess material that is preventing the models to come together. Seeing how your registration material adapt over the model also allows you to see if your models are accurately captured or not. The key is to know how to trim these records so they are there to help mount the case but not there to exaggerate the mounting errors.
At the end of the day, If you mount enough of your cases, you will realize there are many errors during the jaw relation record process. The cases with missing posterior teeth relying on soft tissue supported recording bases will have more errors than cases with well distributed occlusal stops and short edentulous span. What is important is to recognize that these are inherent errors that are unavoidable but there are many prosthetic strategies to counteract these errors.
There are times that I would like to verify the mounting before moving to the next step. For example, I want to try in the occlusal rim after mounting to ensure what I see in the mouth is consistent in what I see on the articulator. Or I do multiple checks of the bite during the fabrication process. For example, in posterior full coverage restoration, it is not unusual for me to ask the lab to put some self-curing resin on the metal coping for a try in to check both the fit of the metal coping as well as to verify the jaw relation. That way, if there is a large discrepancy, I can correct it at the time of metal try before porcelain application.
I have discussed a lot of concepts here with the hope that the dental students and young dentists can appreciate what you have to consider for a jaw relation record. It is not as simple as just asking the patient to bite down. I think it’s important to recognize the more difficult cases and to plan for your jaw relation record carefully. If you don’t do that, the restoration you get back from the lab may not be in good functional relationship and that may end up biting you in the back. Thanks for taking the time to read this post.