The ‘T’ Stop
In this month’s tips I should like to consider the case of providing a posterior bridge where the last standing molar tooth has been used as a bridge abutment. The classic case is the provision of a 3 unit bridge to replace a first permanent molar, using crowns on the second premolar and second molar. An implant retained bridge will present similar technical challenges.
If we imagine that there are no obvious complications such as tipped or rotated teeth, no active caries, but moderately large sound cores, for example in amalgam and no periodontal disease or over-eruption of teeth from the opposite arch, (or ideally aligned implants) then the case looks to be reasonably straightforward. But is it? What are the pitfalls and what causes unexpected complications, and with it the need to spend more time for fitting the bridge than we had allowed? The following ideas result from a medico-legal case that I reported on over 20 years ago when a substantial claim was made by the patient against a colleague for inducing temporomandibular joint dysfunction syndrome following the fitting of just such a bridge in the mandible. It cost a protection organisation a 5 figure sum to settle the claim.
If we consider the details of our preparations, or the implant abutments, then once the occlusal surface of the posterior tooth has been reduced, the normal centric stop has been lost, and in order to restore the case we have to make some record for our technician to be able to articulate the models in the correct horizontal and vertical dimensions. If our record is incorrect, then the occlusion on the bridge will be incorrect, unless we have in place some form of check. All errors result in increasing the vertical dimension of occlusion.
When I ask the question on my courses, most colleagues take a bite and then fit the bridge on the next appointment without having a second, try in appointment. Again, it is quite clear from their comments, that in some cases the bridge fits and the occlusion is correct or nearly so, but in too many, and an unpredictable too many cases, significant occlusal equilibration is required at what should be the fit appointment.
So What Happens?
We remove the temporary bridge and fit the finished article, say ‘bite together’ and is that all right. Answer no, its high. What follows is a series of testing the occlusion with articulating paper, removing the bridge, grinding away where you think it might be high, refitting, and so the cycle is repeated again and again and again. In my medico-legal case above the notes, read ‘ground in bite, end of appointment, bridge fitted’ No follow up was offered, and in such circumstances it proved to be impossible to defend a colleague. I suggest when repeated adjustments are required all you will achieve is to raise questions in your patient’s mind about your clinical ability. Is this bite ever going to be right? In these circumstances confidence in you can drain away. I had a similar bridge fitted during my student days and was told ‘the bite would settle down’ – it never did, and yes I also had episodes of TMJ dysfunction syndrome before the bridge was replaced by a colleague who understood more about occlusion.
Is There An Answer?
I think there is, and this depends on a second try in appointment. The usual try in appointment is limited to checking the fit, but there is a way to check both fit and occlusion, and have a reduced and therefore easy area to adjust to correct the occlusion. I call the method the ‘T’ stop. I leave you to decide if the ‘T’ is for temporary or is eponymous.
As mentioned, every stage may increase the occlusal vertical dimension; it is never reduced. Blows on the occlusal surfaces (see Model Practice) will give a restoration high on the bite. Whatever method of recording the occlusion, whether wax or special bite pastes has the potential to be incorrectly mounted on an articulator in the laboratory.
- When you make your record place the material of your choice only on the side where the teeth have been prepared. Placing material all round the mouth increases errors and prevents your technician from seeing that the teeth, on the non-prepared side are interdigitating properly.
- Accept that however good you are, there will be cases when the occlusion will come back from the laboratory incorrect. If your technique is to go from impressions to finish, then there is no opportunity to have a check on the accuracy of both your and your technician’s work. Even in the best of hands errors occur!
- Accept that this is one case where a try in of the bridge framework is not a waste of time. However, do not make the standard framework, whether for a porcelain fused to metal (PFM) or a zirconium oxide reinforced framework. In the case of PFM bridges, ask your technician to either incorporate a small stop area in the wax pattern prior to casting the framework, or add a small, 3 – 4 square millimetre stop in porcelain fused to the metal. For ZO crowns abutments, a small area of porcelain can also be added (Fig 1).
- As you remove the temporary bridge identify how the teeth interdigitate anteriorly and identify at least two places, one on each side of the mouth, where the teeth meet. These are your reference points. Fit your try in, have the patient close together and both look at the reference points and test the contacts with shimstock. If there is a space, then the occlusion cannot be correct. Now, using articulating paper, held by your nurse, mark any high spots on the ‘T’ stop and correct the occlusion on this small area until the reference points are identical with the framework both in and out of the mouth (Fig 2).
- Now re-record the occlusion. I found the best material was ‘duralay’, a very accurate self polymerising acrylic resin placed around the ‘T’ stop. Make sure your patient is advised not to open their mouth whilst the acrylic hardens, and just to be on the safe side, place your fingers under their chin, when you can detect any attempts at mouth opening, which would, of course, give yet another incorrect vertical dimension. If you have followed previous tips and purchased your own articulator for these cases, then break off the mounting plaster between the working model and the mounting ring. This ensures your technician has to remount the case before finishing the porcelain build up. If you cannot place the models in the correct position neither can your technician, so have a trial before returning the case to the laboratory, and if in doubt, seal the casts together with sticky wax.
My personal preference was to keep the metal ‘T’ stop intact, and I always tried to advise patients that it would be better to bite onto a metal surface. Some patients will insist that no metal shows. In this case, once the models have been re-articulated, ask your technician to grind away the stop and replace it with porcelain.
When the bridge is returned for the fit appointment, minimal adjustment should be required. Again, use the reference point method to check the occlusion. Minor adjustment may be required, particularly in border movements especially if a face bow has not been used to mount the casts (Fig 3).
In summary, use the’ try in’ appointment to make the majority of the adjustments required, after all it is a ‘trial’ and should be explained as such to your patient. Patients expect the fit appointment to be just that with minor, and never major, adjustments.
I found over the years that this method served me well. At least you will then be able to demonstrate in the clinical notes that you have taken every precaution to prevent the development of TMJ dysfunction syndrome following fitting the bridge, and given another expert appointed to review the case on your behalf, a real opportunity to prepare a defence.