Walk this way

We’ve all seen them, those open contacts that occur in the posterior, trapping food and driving patients crazy. At a patient’s check-up, you ask them how that crown – often a second molar – you cemented six months ago is doing, and they tell you they get everything stuck between it and the teeth next to it every time they eat. The patient is not happy.

You’re almost afraid to look. You know what’s happened. That distal molar has distalised, opening up a gap between the first and second molars. Sure enough, there’s an open contact you could drive a bus through.

So now what? Do you tell the patient the contact was nice and tight six months ago, and that you just don’t know why the tooth moved? Or, with some embarrassment, do you tell them that you’ll book them in to replace the crown? But then you remember a previous occasion where you replaced a crown before, only to have it distalise again months later too. Maybe you’ll prep up a filling on the first molar to close the space. Or how about just telling the patient the space is supposed to be there to make flossing their teeth easier (that is a joke, by the way).

But what if you could tell them this happens sometimes, that it’s very easy to fix, here and now, and it will take only a minute or two? Well you can if you use my ‘walking forward’ technique, a non-invasive method that easily corrects the open contact and food trap by influencing the distalised tooth to migrate mesially again.

We know teeth move in response to orthodontic treatment. And we know that when maxillary and mandibular teeth come into contact, the topography of the occlusal surfaces – the cuspal inclines – cause vectors of force in various directions (Figure 1). The sum of these vectors of force will determine if a tooth migrates or not (Figures 1 to 3).

Regardless of your attention to detail in creating an ideal occlusal scheme for a new crown, there will be at least some movement of the tooth after the crown has been installed. In the surgery, we can’t always duplicate all the excursive movements the tooth will have in the patient’s daily life, although usually any movement is minimal and insignificant, and the tooth appears to maintain its position without problems.

There are times, however, when a ‘domino effect’ occurs. The tooth moves a few microns, bringing it into contact with another cuspal incline. This creates a vector of force, causing the tooth to drift in another direction, which in turn causes contact against another cuspal incline, and so on (Figure 1).

Before you know it, a gap has developed between the molars (Figure 2) and a food trap is born. We see this even with virgin teeth, simply because of normal wear on the teeth that creates changes in vectors of force, so the ‘domino effect’ can happen here too. But it’s when it occurs after a restoration, especially of a crown, that we’re concerned with here.

So how do we fix it? Well, if vectors of force can cause unwanted movement to the distal, it stands to reason that opposite vectors of force would cause active drifting to the mesial. This would cause the interproximal contact to close and the problem would be corrected.

Let’s assume that the second molar has drifted distally, opening up a food trap between the first and second molars (Figures 1 and 2). Please note in figure 1 the intense gingival inflammation and edema in the open interproximal space as a direct result of food getting trapped there (Figure 1).

Mark the area with articulating paper and inspect the marks on the distalised second molar (Figure 2). You may see marks on cuspal inclines (Figure 2) and on flat areas such as marginal ridges, cusp tips, and fossae. Look for vertical marks on the mesial marginal ridge of the distalised molar and for marks on the distal inclines of the cusps on the mesial half of the tooth. These are the marks that will cause forces that move and tilt the tooth mesially – to ‘walk forward’.

These marks are our friends. But the marks on the mesial inclines and the vertical marks on the distal marginal ridge, as well as other vertical marks on the distal half of the tooth, are our enemies, and cause the tooth to move distally. Marks on mesial inclines (Figure 1) and the distal marginal ridge cause vector forces that push and tilt the tooth distally – the opposite of what we want. The vertical stops on the distal marginal ridge and other vertical stops on the distal half of the tooth would tend to prevent the distal half of the tooth from moving in an occlusal (eruptive) direction, which is what happens when a tooth tilts (rotates) to the mesial (Figure 4).

To correct the problem, adjust the occlusion of the distalised tooth in the following way. Leave vertical stop contacts on the mesial half of the tooth, especially the mesial marginal ridge (Figure 4). If there is no contact on the mesial marginal ridge, create one by bonding a thin layer of composite on the ridge. Also, leave any contacts on distal inclines found on the mesial half of the tooth (Figure 3). Minimally remove vertical contacts on the distal half of the tooth (Figure 4) and all contacts on mesial inclines (Figure 3). Tell the patient that correction may take more than one adjustment visit.

Over the subsequent four weeks, the distalised tooth will migrate mesially. It does this by tilting mesially. The distal half of the tooth moves in the occlusal direction, due to rotation (Figure 4), and will often come into contact with the opposing tooth, which will prevent further mesial migration. Other cuspal inclines will come into contact. When the mesial inclines again come into contact, this will also prevent further mesial rotation.

See the patient again in four weeks to evaluate the movement. If the open interproximal contact was originally minimal, the contact will often be fully closed by then (Figure 2 and 3). Evaluate the occlusion again with articulating paper (Figure 2).

Whether or not the space appears to have closed fully, make appropriate extra adjustments (Figure 3). If the space has fully closed, the adjustments will ensure the space remains firmly closed. If the space has not closed fully, the adjustments will cause continued mesial migration and rotation of the distalised tooth. Most contacts will be closed after one or two adjustments (Figures 2 and 3). However, in some rare cases more may be needed.

We all know that teeth do not drift in any direction – instead, they tilt (Figure 4). Usually we show diagrams of this to our patients when discussing the need to replace missing posterior teeth with bridges or implants to prevent such movement. Leaving – or creating – the vertical contact on the mesial marginal ridge will help cause the distalised tooth to move mesially (Figures 1 and 4). And, of course, removing any vertical contact on the distal marginal ridge will allow mesial movement of the tooth (Figure 4).

If there is no contact on the mesial marginal ridge, it is simply a matter of acid-etching the ridge and adding a thin layer of flowable composite to bring it into occlusion. If the tooth has a porcelain crown, consider using hydrofluoric acid to etch the porcelain, followed by silane, bonding agent and flowable composite. Even though the flowable composite may not be as wear-resistant as we’d like, it will serve to help the tooth finish its mesial tilting, and the other forces you have created should serve to hold it in place. Just be sure that when you place this composite, you do not allow it to get into the open interproximal space.

Now some of you may already have asked, ‘What about the physical law that says every force produces an equal but opposite force?’ In other words, ‘Won’t these vectors we are creating to influence the tooth to move mesially cause the opposing tooth to move distally?’ This was my first question too, but in 25 years of performing this procedure, on at least 100 patients, this has never happened.

I think there may be three reasons for this. First, the opposing tooth has most likely been in a stable position for a many years, and the tooth we are trying to move mesially has only recently – within a year or so – moved distally, opening up the interproximal contact. The bone may not be as dense around this tooth so it, rather than the opposing tooth, may be the one to ‘yield’.

Second, we’ve all been told of the phenomenon of ‘mesial drift’, which indicates that posterior teeth ‘want’ to move mesially due to certain curvatures of the jaw. And third, the vertical forces on the mesial marginal ridge typically occlude in a vertical fashion against the opposing tooth, striking it on a mesial cusp tip, also causing it not to drift distally.

So, the vertical stops will be your most important adjustments. If possible, make sure there is a nice, heavy contact on the mesial marginal ridge of the distalised tooth that you want to ‘walk forward’. And remove all vertical stops on the distal half of this tooth (Figures 1 and 3).

These vertical forces will cause mesial movement of the tooth with minimal or no effect on the opposing teeth. And remember, when you make these adjustments, tell the patient to be prepared to come back for further adjustments after four weeks. I find that, depending on the size of the interproximal space, no more than three adjustments are usually required.

However, I have had experience of one or two cases with such large interproximal spaces that four adjustments were needed. Each time you make an adjustment, adjust only enough tooth structure to remove the articulating paper mark on the mesial inclines of the cusps, and all other markings on the distal half of the tooth.

As I said, I’ve used this method for many years, and it has helped me and my patients avoid the need to replace existing crowns. Also, by knowing which occlusal contacts to avoid when placing new restorations, I have been able to avoid distalisation whenever possible. I hope you find the technique just as useful.

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