Fall 1999 New Member Forum

Edited by Dr. Michael A. Sales

In this issue, Dr. Michael Sales asks, "What are your views regarding centric relation articulator mounted models in an orthodontic practice?"

Dr. Paul M. Kasrovi

I remember in dental school, any time we were faced with treatments that required particular attention to occlusion, we had to mount the case using a semi-adjustable articulator. Such cases include fabricating multiple unit crowns and bridges, occlusal rehabilitation or equilibration, any kind of fixed and partial or complete removable prosthesis, etc.

There is undoubtedly a great deal of information that can be obtained from CR-mounted models. These include: relating the position of the maxilla to the cranial base, relating the mandible to the maxilla with the condyles seated in centric relation (CR), treating to a reproducible condylar position, allowing examination of excursive mandibular movements and documenting occlusal interferences.

When I started my residency in orthodontics, I was amazed that we relied solely on hand-articulated casts to arrive at our diagnosis and treatment plans in a majority of the cases. After all, isn't it true that of all specialties in dentistry, orthodontics is the one most concerned with occlusal correction? Then why are we not mounting every single case?

I guess the answer lies in our post-doctoral training. Despite the recent gradual increase in the number of orthodontists (particularly recent graduates) who are mounting cases, the greater majority of my instructors (who I am sure represents the majority of orthodontists practicing today) mounted only select cases.

That is probably why in my practice, I still use mounted models mainly in patients exhibiting a significant CR to MI discrepancy, patients with TMD signs or symptoms, and patients undergoing orthognathic surgery. Many of us have been treating cases this way for years, feel comfortable with it, and feel reluctant to change the way we do things.

There is no question that a lot more information may be revealed if we mounted every case, but this would require a substantial "system" change in many aspects of our practices. Such changes may include but not be limited to: staff training, model storage, lab interactions, cost, etc., which many of us may be unwilling to go through.

Some may argue that by deprogramming the neuromuscular adaptations of patient in search of CR, you may be opening a new can of worms by further complicating the treatment. This may be true in some cases, but still, the values of CR-mounted models substantially outweigh such potential risks.

In summary, I believe that there is more to gain and less to lose when you mount cases. I predict that CR-mounted models will become more prevalent in our specialty, particularly in light of a growing adult population seeking orthodontic treatment.

This would, however, require more emphasis in graduate residency programs and more established orthodontists and orthodontic educators willing to go through a paradigm shift in their own practices.

Dr. John H. Trotter

The articulator-mounted set of study models are only as accurate as the inter-occlusal and face-bow record that was used when mounting the models. The accuracy of the inter-occlusal record is only as good as the ability of the practitioner in registering the patient's "bite." The possibility of correctly registering that bite is directly dependent on whether the patient's muscles of mastication are in a state of relaxation (i.e. without spasm or splinting), and the condyle is in a position of balance or stability, without strain or stress within the articular fossa.

I believe that the vast majority of orthodontic cases do not require the use of articulator-mounted casts, primarily because of the adaptability of the temporo-mandibular joint to accommodate to minor changes within the TMJ complex during treatment and normal growth.

In adolescents and young children, the continuing growth and subsequent changes to the articular eminence, condyle, and its position within the fossa, make articulator-mounted models unnecessary. Of course, no matter what your belief in mounting models, if due diligence is not taken in the initial diagnosis and subsequent treatment plan, any treatment can be compromised.

Aside from the difficulty in registering "Centric Relation," the arbitrary, man-made concept of Centric Relation may or may not be the best position to restore the patient's occlusion. I believe that the patient's individual anatomical requirements (articular eminence, condylar shape, muscular attachment, direction of muscular action, ligament apparatus, etc.) within the TMJ complex should dictate:

1. The position of the condyle relative to the articular fossa and, therefore:

2. That to which the occlusion should be restored orthodontically.

Having said that, there are cases that should be mounted, and possibly re-mounted, either prior to, or during treatment. Some of these include:

1. Orthognathic surgery cases with an extremely complicated occlusal scheme that may mask the patient's true mandibular position or cause the muscles of mastication to artificially place the mandible in a non-physiologic, habitual position (through splinting, spasms, etc.)

2. A patient missing a number of teeth over a significant period of time, which may hide the patient's true mandibular position or cause the muscles of mastication to artificially place the mandible in a non-physiologic position.

3. A patient, who in conjunction with a restorative dentist, is undergoing extensive changes in vertical dimension or inter-occlusal relationship.

4. Any patient who upon clinical examination and palpation/manipulation reveals multiple "bites" which are not repeatable or reproducible upon further examination

5. A patient who in mid-treatment shows a significant, seemingly aberrant change in their occlusion or mandibular position in a relatively short amount of time

6. The patient who presents with signs of TMJ derangement such as pain, limited or anomalous range of motion, or excessive clicking/popping.

To accurately mount any case, one must first place the patient in splint therapy to eliminate the possibility of the muscles of mastication, or the occlusion, unduly dictating the physiologic position of the mandible or the condyle within the fossa.

Only after using splint therapy to achieve a repeatable, comfortable, stable condylar position can one truly assess the occlusion of one of the above patients. This "physiologic" position requires time and multiple adjustment of the splint to finally realize the placement that the condyle and mandibular complex dictates, rather than one that the clinician arbitrarily determines.

Dr. Adrian Vogt

My guess is that orthodontists or all dentists, for that matter, would have great difficulty in coming to any consensus as to what centric relation is and especially how to obtain a wax bite to capture it.

Actually, let's take that line of thinking one step further. Orthodontists cannot come to a consensus as to what good orthodontics is, hence the great extraction vs. non-extraction debate, expansion vs. no expansion, headgears, Herbsts, etc. You get the idea.

On a different note, I still find it curious that all CR wax bites are taken in a supine position, but how does this relate to function in the upright position? Another concept I find curious is the belief that an ideal CRO, anterior guidance occlusion will actually eliminate parafunction.

Back to the question at hand, I now routinely use articulator mounted study models in the initial and ongoing assessment of my cases. I, like most, started my orthodontic career using CO-trimmed hand held models.

The manner in which I obtain the centric relation wax bite is to gently manipulate the mandible (no distal pressure) until I feel that the patient's jaw has relaxed so that it is easily swinging open and closed, and then have them gently squeeze into soft wax, without perforating the wax.

The only way to transfer this CR relationship onto models is to mount them onto an articulator, as the articulator compensates for the thickness of the wax. The only way to capture this CR relationship with hand-held models is to obtain a wax bite, which is perforated at the initial tooth contact, but I have found this technically very difficult to obtain.

I feel articulator-mounted models have the advantage over hand-held models in that they capture more information. Some clear examples would be cases with large shifts, as in large AP shifts, in functional crossbites in pseudo Class III cases, and lateral shifts associated with narrow maxillas.

Another clear example of an advantage of mounted models, is preparation for surgery cases. When using CO trimmed hand held models, information regarding CR-CO discrepancies would instead have to be noted in the chart, which is another way of documenting the occlusion.

Perhaps the more important question would be, "How does mounting study models affect diagnosis and treatment planning and lead to an improved quality of occlusal result?" I have not found mounted casts to be a key element in my diagnosis and treatment. Instead, I rely on a good clinical exam, a TMJ exam and the radiographs.

Do I feel that the future of orthodontics will be to mount all models in centric relation? I think so. The best-case scenario would be a standardized articulator to standardized method of obtaining CR wax bites. One can always dream.

Dr. Gary Stafford

I do not currently mount my study models. At this point in my young career, I don't see the value or benefit of mounting my cases. It is difficult for me to see how it would alter my diagnosis or treatment plan.

My point is not that mounting models is a useless exercise. In fact, someday I may find it very useful in my routine, but right now I do not understand how it would aid the treatment of my patients.

There are a couple of issues that confuse me. One is the implication that the use of mounted models is the only way to treat to centric relation, or at least the best way. I believe that is a fallacy. The key is getting the correct bite, whether using mounted or unmounted models.

Secondly, there seems to be no consensus on the definition of centric relation. It seems odd that while I am attempting to treat to "centric relation," my understanding of it is nebulous.

Editor's Comments

Mounting models has become a fiercely debated issue in orthodontics. An increasing number of orthodontists feel that orthodontic cases should be evaluated with the help of CR articulator mounted models.

The 19th century French painter, Thomas Coultre, noted, "Science prevents this doctor from seeing that which is obvious to everyone else." Could we here be dealing with such an issue? A technology in search of a justification? Or, do we have a useful tool that is rejected because it is unfamiliar and uncomfortable?

There are so many contested aspects surrounding this debate that it can be difficult to determine just what the difference of opinion is about. Mounted models, the proponents say, are a way of representing in three dimensions the relations between the TMJ, the dental occlusion, the hinge axis, and the cranial base.

But in order for the articulated models to provide meaningful information, there must be a hinge axis, a centric relation, and a method of accurately and consistently recording these variables. There also must be a device capable of accepting the information and replicating the patient's actual masticatory physiology. Unfortunately, there is no agreement on any of this.

Furthermore, even amongst those who do believe that a CR-CO discrepancy can cause TMD, undermine occlusal stability, and bring about dental attrition, there are differences of opinion as to how to document CR and at what level of discrepancy a threat exists.

They also disagree about which articulator system is appropriately anointed to perform such precise work. Throw into the fray some very strong-willed advocates on both sides of the aisle and a paucity of any outcome-based studies to validate the various claims, and it becomes obvious why there is no clear-cut rationale or protocol when it comes to the use of articulated CR-mounted models. Dare I mention the differences of opinion as regards what constitutes an appropriate overbite, overjet, cupid disclussion, and incisal guidance?

Given this background, one might be surprised to learn that for a number of years I have obtained what I consider to be the "centric relation" bite registration and, using a face bow transfer, have articulated the casts of almost every case entering my practice. This came about after many more years of not doing so.

I reconsidered for a number of reasons. I believe that properly articulated models represent a more accurate record of the functional occlusion. The transverse and AP occlusal plane cants are visualized with greater accuracy than with hand held models. Lateral and protrusive movements, even if not 100% accurate, will be represented closer to reality than with any hand held attempt. Communication with patients and colleagues sharing an interest or responsibility for the case is made easier. Mock articulator mounted model setups allow a three-dimensional visualization of surgical as well as orthodontic movements.

The fact that we do not agree on concepts such as hinge axis and centric relation does not diminish the value of documentation to the highest available standard. We have taken two-dimensioned photographs of our patients for years knowing that these photos can sometimes be misleading. Certainly, what we see visually in three dimensions and in motion from an infinite number of observation points is not the same as what we learn from eight or nine still photos. How many times have you heard yourself or a colleague state that "she actually looks great in person, but somehow the profile or smile photo is just not that flattering"? Or, "the camera angle makes this look more class II than it is." We take the pictures because a word description is not as revealing as a set of photos even with their limitations.

Similarly, no written description of a functional slide is likely to be as accurate as the documentation achieved with properly articulated models. I accept the imperfections in our current systems because perfection is not the point. Providing the best overall service and documentation is! The better our documentation the better our ability to accurately diagnose, treatment plan, and communicate.

Also, accurate documentation provides a meaningful way for us to evaluate the treatments we have rendered, thus enhancing our effort to advance the knowledge of orthodontics through outcome based analysis.

Centric Relation mounted models are just one small piece of an information gathering process that can provide useful data as part of an entire spectrum of orthodontic care. Band and bracket placement, attention to the detail of finished tooth positions, accurate perception of the skeletal, dental, and soft tissue structures, quality of x-rays, treatment timing, recognition of periodontal and restorative implications, TMJ health, and a host of other factors must all be considered in an effort to deliver excellent orthodontic treatment.

Mounting models in and of itself means little, but they do provide a potentially useful tool which may help us to raise our overall standard of care. Outcome based studies are necessary (although difficult to control in regard to this subject) if we are to resolve this debate.

Perhaps three-dimensional computerized simulations of masticatory function will someday obviate the need for mounted models. When this type of software is available, I suspect that the high tech lure of computerization will stimulate renewed interest in the study and documentation of centric relation and functional occlusion.

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