An Important Point About the Opening of the Contact Between the First and Second Molars (6 and 7)

One of the occasionally common problems in delivering crowns on second molars (tooth 7) is the opening of the mesial contact after delivery — that is, at first everything looks perfect, but after a while the patient returns with an open contact and the pain it causes.

Is occlusion and contact on the cusp incline the only factor?

The cause of this problem is most often attributed to occlusion and the contact of the opposing teeth on the cusp inclines, which creates a force vector and movement of the teeth. But a different experience shows that the matter is not this simple:

• Suppose we have two crowns side by side — that is, crowns on the 6 and 7 — and after a while the contacts open
• The crown on the 6 is removed and sent to the laboratory to correct the contact.
• In this state we are certain that the position of the 7 is now stable, because the forces on the incline have already had their effect and the 7 has reached its final position.

But when the crown of the 6 comes back from the laboratory, it often needs adjustment in order to seat in place; it is adjusted, seated in place, and again after a while the contact opens.

Question: If the only factor in the displacement is occlusion and contact on the cusp inclines, why does the 7 shift again after the 6 is present?

• The difference between the presence and absence of the 6 is the wedging force exerted at the contact point of the two marginal ridges.

It is recommended that when the 6 is sent for correction, it «definitely» be replaced with a provisional crown. In this case the wedging force is not removed, the 7 will not move mesially in the absence of this force, the crown of the 6 — after it returns from the laboratory — seats easily and without adjustment, the combination of forces does not change, and the contact does not open again.

• So the point is to take into account and preserve the wedging force between the correction sessions. Even in the case of a single crown on the 7, first cement it temporarily; if the contact opens at follow-up, then during the interval while the crown is back from the laboratory, the position of the 7 must definitely be maintained with a provisional crown — and, of course, with a correct contact — so that the effect of this wedging force on the marginal ridges of the 6 and 7 is preserved, and the position of the 7, which is influenced by it, does not change.
• Sometimes it is necessary to delay the correction of the definitive crown so that, by intermittently correcting the contact of the provisional crown at defined intervals, the position of the 7 is finalized.

In explaining the opening of the contact between the 6 and 7,
limiting the matter only to contact on the cusp incline is simplistic and ineffective.

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