Mounting Cast Models of Hypermobile Teeth

I have noted that clinical instructors at IKO’s student clinic at the University in Tromsø (now called “The Arctic University of Norway”) do not pay too much attention to occlusion and occlusal discrepancies in students’ patients. The reason for that is not really clear to me. Trauma from occlusion does not only affect periodontal tissues but can have deleterious effects on the pulp-dentin complex, the occlusal surfaces of the teeth (usually aggravating the problem with time), the temporomandibular joints and the neuromuscular system. In my lectures I can only address periodontal problems, which had long been considered minor, based on animal experiments. Recent retrospective studies in humans, however, have told us different lessons. In order to get what you should get after non-surgical and surgical periodontal treatment, it may be justified to better remove any balancing contacts and harmonize working side contacts (apart from establishing proper oral hygiene). It is interesting to remember that, without hard evidence, many dentists’s observations, i.e. clinical experience, has suggested that for almost a century. Excesses by so-called gnathologists exempted.

Since the advent of evidence-based medicine in Dentistry about 20 years ago, we know that animal experiments do not provide hard evidence. They may be used in order to formulate a hypothesis which has to be tested in randomized clinical trials in humans. Moreover, if one has a close look again at the mechanical apparatuses which had been installed in beagle dogs in Gothenburg, Sweden, in the 1970s and 1980s in order to simulate occlusal trauma (a supracontact in an artificial crown on a upper premolar which leads to a violent forward-push of a lower premolar whenever the dog closes its mouth, while a lingual bar with a spring wire attached to that particular lower premolar which pulls back the tooth whenever it opens), one instantly doubts whether this actually fits the purpose. In addition, a dog tormented in that way may experience tremendous stress, may even lose any hope and just want to die. This kind of distress has been shown in the meantime to be an important risk factor for periodontal disease in itself. So, claims by the research group in Gothenburg, that all other factors were controlled for while only dental plaque and occlusal forces combined had an influence on the integrity of periodontal tissues may seriously be questioned.

When I had just graduated and joined the new department of Periodontology at the University of Marburg, Germany, in 1980, my new boss had asked me to attend a course by Professor Jan Lindhe from Gothenburg which had been organized by the self-proclaimed dental elite of the Neue Gruppe study club. Lindhe was 45 years old but already world-famous for his astute clinical studies in Periodontology, editor-in-chief of the most important scientific periodontal periodical, the Journal of Clinical Periodontology, and known for his highly entertaining and didactic way of teaching. The course was a true eye-opener and changed my professional life forever. Lindhe explained the clinical studies and animal experiments done in Gothenburg and the United States and presented unbelievable clinical solutions for extremely complex periodontal cases, which, as he put it, were derived from the observations made in the studies. Most of the work had been done by late Professor Sture Nyman in the Perio department in Gothenburg. Lindhe showed bilateral bridges with cantilevers on just a few hypermobile teeth which had lost 50+ per cent of supporting bone. And they were followed up for several years provided patients maintained high levels of oral hygiene. The message was, once you had got control over periodontitis, you may almost do whatever you like with the teeth. The audience, at least half of them “gnathologists”, groaned. So, there was a rather young and talented dentist from Sweden on stage who in fact contradicted everything what had then been taught at any of the numerous German University dental schools. In the discussion, Lindhe was asked by an annoyed dentist, how he would mount model casts in an articulator. “Well,” he said, “I’ll take the upper jaw model cast and the lower and then use plaster to fix it in the device.” They almost fainted.

I had to meticulously report what had happened during the course in our department and my boss told me, I should restore my first successfully treated advanced periodontitis patient accordingly. I did, trusting both Jan Lindhe and her. I became a true believer. It was the beginning of a success story.

When the 3rd edition of my German textbook on Periodontology for undergraduate students was due, I remembered one of the last cases I had treated with circular bridges and cantilevers, way before implants started to revolutionize Dentistry. Part of the series of clinical and radiographic pictures found its way into the book. But the question formulated by an outraged practitioner in Jan Lindhe’s course in 1980 (“How do you mount model casts in an articulator”) has not really been addressed by the profession as far as I know. In the following I would like to explain it a little bit.

opt

Orthopantomogram before treatment

I had completed periodontal surgery in 1994 and was about to refer the patient to the prosthodontic department at Heidelberg University’s dental school. It was one of these cases you rather want to refer. I mentioned to her that the prosthodontist in the upper floor might not be willing to keep the teeth which I had successfully treated but might suggest a complete denture in the upper jaw. The patient was shocked. She begged me to do the restorative treatment myself. I asked her, what she would consider successful treatment and she responded that keeping her teeth for one year would be just fine. Students need to understand that this is an important question to be asked. If the patient had replied with “Ten years at least,” I would certainly have declined.

after-perio-surgery

Clinical and radiographic situation after periodontal surgery in 1994

Well, we did it. Restoration of the lower jaw with inlays, onlays and a crown on hemisectioned tooth #36 was easy. It was done in a first step while the patient still wore a temporary removable dental prosthesis in the upper jaw. The remaining five teeth in the upper jaw were very mobile, however. If the missing teeth ##13 to 21 were replaced in a fixed dental prosthesis, it would mean another cantilever rendering the bridge hypermobile as well. Distal cantilevers were necessary to stabilize the bridge and a balanced occlusion (as in a complete denture) was the aim.

after-restoration

Oral rehabilitation with 10-yr follow-up

I will turn to the technical aspects (how to mount the master model casts in the articulator) below but at this point it should be clear that a balanced occlusion had to be checked and established in the patient’s mouth after the bridge had definitively been cemented. Due to  severe loss of bone, cementing the bridge temporarily is not possible because any attempt to remove the bridge would comprise an extreme risk for inadvertent extraction of one or more teeth.

balanced-occlusion

Balanced occlusion established after definitive cementing

How to mount master model casts in a semiadjustable articulator?

(1) Mount the upper jaw model in nearly correct relation to cranium and TMJ in the articulator with an arbitrary face bow in the usual way.

The main problem is to mount the lower jaw model in absolutely correct relation to the upper jaw model. Note that the position of hypermobile teeth in a model represents only one of indefinite possibilities. Thus, crown tips can never fit into an index which has been done in the patient’s mouth. Therefore, the index should be made in the laboratory.

(2) Mount the lower jaw model in a nearly correct relation to the upper jaw model.

Plate1

The lower model is mounted against the upper model in a nearly correct relation in order to manufacture a registration plate

An acrylic registration plate rather than one of wax should be made.

(3) Rubber dam is wrapped around the upper model to indicate only the tips of the crowns.

Plate2

Rubber dam is used to indicate just the crown tips

The registration plate has to be as thin as possible in order not to increase the vertical dimension.

(4) When applying the not-yet-set resin registration plate, use a piece of sponge packing material to avoid any lower jaw cusp impressions in the resin.

Plate3

The not-yet-set resin registration plate is about 1-2 mm thin and has been placed between a piece of sponge packing material and the rubber dam. The articulator is then maximally closed

(5) Check and adjust the registration plate after setting in the patient’s mouth.

(6) Place the watered upper jaw model into Temp Bond on the registration plate.

registration-plate-4

Temp Bond is applied to the maxillary part of the registration plate and the watered upper model is placed into the registration paste

(7) After setting, reduce resulting impressions with a scalpel. Note (and check) that the plate will fit precisely in the patient’s mouth.

(8) Apply some heated-up impression compound in the central incisor area of the lower jaw side of the registration plate.

(9) Place the registration plate in the upper jaw.

(10) Guide the patient in central relation into the soft (still hot) impression compound until it has set. Check whether the patient can find the position without any problems. Make sure that this will lead to only a small increase in the vertical dimension.

(11) Apply Temp Bond on the registration plate and let the patient close into the incisal jig while holding the plate.

Plate8

Heated impression compound is applied in the central incisor area of the lower jaw side of the registration plate. The patient is guided in central relation to make impressions with his/her central incisors in the still hot compound. Then Temp Bond is applied on the registration plate and the patient closes into the incisal jig

(12) Remove the incisal jig and reduce the Temp Bond impressions with a scalpel.

(13) Mount the lower jaw model cast (which hopefully fits into the impressions).

(14) Lower the incisal pin until a contact between natural teeth occurs. Check the situation with that in the patient’s mouth.

mounted

The mounted lower jaw model in the articulator

The patient was periodontally treated and successfully restored in 1994/95. Until I definitely moved away from Heidelberg in 2004, I had scheduled her for quarterly and later biannual recalls. Few 5-mm pockets, in particular distally of teeth ##25 and 36 where root concavities were present, were frequently re-treated by subgingival scaling. The patient was advised to buy a scaler and remove any supragingival calculus herself. In 2011, I identified her telephone number, called her and asked her whether the teeth were still in place. “Not in the upper jaw”, she said. A new professor in the department of Prosthetic Dentistry in Heidelberg had decided to remove the bridge together with the teeth and manufacture a complete denture in 2005. He felt more comfortable with the traditional solution.

20 November 2013 @ 2:02 pm.

Last modified October 3, 2014.

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