Insight 64 — Another Way to Look at Ferrule — A Reverse Use of the Famous Ferrule Paper

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Clinical Explanation

When a posterior endodontically treated tooth is referred for crown lengthening, the surgical goal does not have to be a complete 360-degree ferrule; the very criterion Rethinking Ferrule sets for accepting a residual situation — intact buccal and lingual walls, deficient proximal walls — can serve as the goal of the surgery

When we set out to obtain a posterior tooth's ferrule surgically, every millimetre we ask for carries a price that is paid out of the bone. Yet on the referral slip we usually write nothing more than "adequate ferrule", without saying where. This note is about the more precise sentence I write instead, and about the paper behind it — a paper I deliberately read against its own intent.

• Clinical Explanation
  A posterior endodontically treated tooth with a good root but an inadequate ferrule, which has to be referred for crown lengthening. Instead of asking for a complete 360-degree ferrule, I write to the surgeon that the focus should be on the buccal and lingual walls and that a height of about 1 mm is enough interproximally — a deliberate concession that preserves the interproximal bone and the papilla.


• The Situation, and the Question That Is Rarely Asked
  A familiar situation: a posterior endodontically treated tooth, a good root, but an inadequate ferrule, and the patient has to be referred for crown lengthening surgery. So far everything is routine. The question begins where it is rarely asked: now that the surgery is going to happen, exactly how much ferrule should we ask for?


• The Standard, and the Default Answer on a Referral
  Let us first review the standard: an ideal ferrule means a sound wall of dentin all the way around the tooth, roughly 2 mm in height and at least 1 mm thick, which the crown encircles and braces. The default answer on a referral is exactly that: let the surgery go as far as it takes to obtain a complete, uniform 360-degree ferrule — as the saying goes, now that we can, why not?


• What I Write for the Surgeon
  In these teeth I write something else for the surgeon: the focus should be on the buccal and lingual walls, and a height of about 1 mm is enough interproximally. In other words, I deliberately concede the 2 mm standard interproximally and do not go after a 360-degree ferrule.


• The Basis of This Concession, and Reading It in Reverse
  The basis of this concession is the Rethinking Ferrule paper (Jotkowitz and Samet, BDJ 2010) — though not exactly in the form in which the paper was written. The paper is about an imposed situation: a tooth that has broken down and retains only part of its walls. Its central claim is that the efficacy of a ferrule depends on the location of the walls and the direction of the load, not merely on the completeness of the ring. A posterior tooth receives a predominantly vertical load, and the walls that brace that load are the buccal and lingual ones. The conclusion the paper reaches: if the buccal and lingual walls have remained sound and the proximal walls are deficient, the situation is acceptable.
  
  I read that sentence in reverse. The paper says that if these two walls "have remained", it is enough. Well then — when we ourselves are about to obtain the structure surgically, why should our goal be anything more than that? The paper is assessing what is left; I take that same acceptance criterion and set it as the goal of the surgery. If that situation is enough for the tooth to survive, it is enough for building it too.


• Why This Trade Is Not Symmetrical
  And here is the point that makes the whole trade attractive: this trade is not symmetrical. The proximal surfaces are exactly where caries advances further, where boxes get deeper and where structure is lost lower than anywhere else. Which means that bringing the proximal walls up to a complete ferrule is the heaviest part of the surgery. We are trading precisely on this tendency of the proximal surfaces toward caries: the heaviest part of the bone removal would have been required exactly where the tooth's biomechanics needs it least. I let that go and take the two walls that genuinely carry the load.


• What the Patient Gains
  What the patient gains is not small: the interproximal bone is preserved, the papilla and the crown-to-root ratio are less compromised, and no space for food impaction is created. Nor have I made an unsupported decision, because I have taken the acceptance criterion from the very scientific source that redefined the ferrule.


• The Limits of This Decision
  This does, of course, come with conditions. After preparation, the buccal and lingual walls must genuinely remain qualified — that is, they must have adequate height and their thickness must not fall below 1 mm. And the patient's lateral forces must be light; in group function with tall cusps, or in clear parafunction, I too go after more proximal ferrule.


• Reference
  Source: Jotkowitz A, Samet N. Rethinking ferrule, a new approach to an old dilemma. Br Dent J 2010; 209: 25-33.

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