Coronectomy: when the root is deliberately left behind

A coronectomy removes the crown of a lower wisdom tooth and leaves the roots where they are. The crown carries the enamel, and it is the part associated with pericoronitis and with the decay that can develop on the back of the second molar next to it. The crown is cut off and lifted out. What remains of the roots is then reduced below the level of the surrounding bone, and the gum is closed over them. No attempt is made to retrieve the roots, and that is the purpose of the operation rather than a shortcoming of it.

The operation exists for a narrow situation. In the lower jaw the inferior alveolar nerve travels through a canal in the bone beneath the molar roots, and the roots of a lower third molar sometimes sit directly on that canal — grooved by it, straddling it, or separated from it by no bone at all. Delivering a tooth like that whole means moving roots that are in contact with the nerve. Taking only the crown leaves that contact undisturbed.

This is not the same thing as a fragment of root left behind because it fractured and could not be found. That is an unplanned outcome, recorded as such and followed afterwards. A coronectomy is planned before the flap is raised, consented for specifically, and carried out with a technique whose entire object is keeping the roots still. The term used in the surgical literature is intentional partial odontectomy, which describes it more precisely than the shorter word does.

It applies to lower third molars and to very little else. Upper third molars have no equivalent operation: the structure that constrains them is the maxillary sinus rather than a nerve canal, and the specific concerns there — an opening into the sinus, a fracture of the bone behind the tooth, displacement of a root — are a different conversation with different answers. Whether a coronectomy is the right operation for a particular lower tooth is settled at a consultation, against that tooth's own imaging, and not from a description of the procedure.

The inferior alveolar nerve enters the lower jaw high on the inner surface of the ramus, runs forward through a bony canal beneath the roots of the molars, and emerges at the mental foramen below the premolars. From there it supplies feeling to the lower lip, the chin and the skin over it, the lower teeth on that side, and the gum in front of them.

It is a sensory nerve and only a sensory nerve. Injury to it does not weaken the lip, does not pull the face to one side and does not paralyse anything. What it changes is feeling. The practical consequences people describe are dribbling from that corner of the mouth, biting or burning the lip without registering it, shaving cuts that go unnoticed, and food or drink escaping without warning.

The less common and more difficult version is altered sensation that is uncomfortable in itself: burning, or ordinary light touch registering as pain. That is uncommon, and it is the outcome driving this entire discussion, because it is both hard to live with and hard to treat.

Altered sensation after lower third molar surgery is usually temporary and recovers over weeks to months. Recovery becomes progressively less likely the longer the alteration has been present, which is the reason it is reported promptly rather than watched at home. Permanent alteration is considerably less common than the temporary kind. Where a scan confirms the roots are genuinely in contact with the canal, both become more likely than for a tooth sitting clear of it, and that is the whole reason a coronectomy is raised at all.

One thing a coronectomy does not do is protect the lingual nerve. That is a separate nerve running in the soft tissue on the tongue side of the jaw, supplying sensation and taste to that side of the tongue. Its position varies between people, and it sometimes lies at or above the crest of the bone. It is at risk from flap retraction and from instruments on the lingual side, not from what happens to the roots. A coronectomy addresses one of the two nerves at risk in that operation, not both.

A panoramic radiograph is the usual starting point for any third molar, and it is a two-dimensional picture of a three-dimensional problem. On a flat film the roots and the canal frequently overlap. Overlap is not contact, and a flat film cannot tell the two apart.

What it can do is show signs that raise the question. Seven radiographic signs are conventionally cited, from Rood and Shehab's 1990 review of panoramic predictors of inferior alveolar nerve injury:

• Darkening of the root where the canal crosses it
• Deflection of the root as it meets the canal
• Narrowing of the root at that point
• A dark and bifid root apex
• Interruption of the white line or lines marking the canal
• Diversion of the canal from its expected course
• Narrowing of the canal itself

Interruption of the white lines, diversion of the canal and darkening of the root are the signs most often reported as carrying the strongest association. None of them establishes contact. What they establish is that the flat film has reached the limit of what it can answer, and that a three-dimensional study can answer it.

A cone-beam scan resolves the relationship. It shows whether the cortical outline of the canal is intact along the length of the roots, where the canal sits in relation to them — cheek side, tongue side, directly beneath, or running between two roots — and whether a root is grooved or perforated by it. Those are different anatomical situations with different implications, and they are indistinguishable on a panoramic film.

One caveat belongs with all of this. A cone-beam scan reliably changes what is planned. Whether it lowers the rate of permanent nerve injury is a separate question, and the trial evidence does not establish that it does. The scan improves the accuracy of the conversation; it does not by itself make the operation safer, and that distinction is worth stating plainly.

A scan is taken because a decision depends on it, not as a matter of routine. If one is recommended, it is reasonable to ask what question it is expected to answer. There is a specific answer in every case where one is justified.

Once a scan confirms contact, three plans are on the table, and they are genuinely different propositions rather than gradations of one.

Symptoms and the state of the second molar decide whether the third option is available at all. A tooth producing repeated pericoronitis, or already causing decay on the surface of the second molar behind the gum line, is not a candidate for watchful review; the only remaining question is which operation. A deeply buried, symptomless tooth with a sound neighbour, no cyst and no gum pocket may not need either, and saying so is part of the job.

Age moves the answer in both directions at once. In an older patient the roots are fully formed and the bone is less forgiving, and a fully formed root is the kind that is found in intimate contact with the canal, so the case for a coronectomy is often stronger. In a younger patient with incomplete roots the retained root structure has more capacity to migrate a long way and, occasionally, to work its way through the gum, so it is applied more cautiously. Neither is a rule; both are considerations weighed against the specific scan.

The trade being proposed

Stated plainly, a coronectomy is proposed as a way of reducing the chance of a permanent change in feeling, in exchange for retained root structure, an obligation to be followed up over years, and the possibility of a further operation. It does not remove the possibility of nerve injury. Neither operation removes risk. That exchange, not the technique, is the actual decision, and it is one where one patient's weighting reasonably differs from another's.

Coronectomy is recorded as a distinct procedure from a complete removal, and how a dental or medical plan responds to it is determined by that plan rather than by the name of the operation. Benefits are established with your own insurer before a date is set, and a benefits response is information rather than a determination of payment.

A second opinion before either operation is a reasonable thing to want, and it is a normal request rather than an awkward one. Where there is active infection or uncontrolled pain, the clinical timeline takes precedence and the sequence compresses accordingly.

The lower jaw is numbed with local anaesthetic. Local anaesthetic blocks sensation at the site and nothing else — pressure, vibration and sound are still noticed, and being told that beforehand is more useful than discovering it in the chair. What else is used alongside it, if anything, is agreed at the consultation and depends on the procedure, the number of teeth being treated and your medical history.

A flap of gum is raised and a channel of bone is removed from the cheek side to expose the crown, cut with a surgical handpiece under continuous sterile irrigation, because bone cut without cooling dies. Only as much bone is taken as the access requires. The crown is then divided from the roots with a bur, commonly at or just below the neck of the tooth and often obliquely, so that the cut runs away from the tongue side of the jaw. The crown is lifted out in one piece or in sections.

Two rules define the operation and separate it from an extraction that went unfinished. The first is that all enamel must go. Enamel does not integrate with bone, and enamel left behind is understood to prevent the site healing over properly, so the residual root surface is reduced until none remains — commonly to a level several millimetres below the crest of the surrounding bone. The second is that the roots must not be mobilised.

The pulp inside the retained roots is left alone. Root canal treatment of the retained roots is not part of the standard technique: the cut surface is covered by clot and then by bone, and the pulp in a root treated this way is generally reported to stay healthy. Treating the pulp pre-emptively has been tried and is not standard practice.

The gum is then closed over the site, usually with dissolving sutures, and the aim is to get soft tissue completely over the retained roots. Whether antibiotics are given at the time varies between surgeons and between cases, and it is a decision made against your medical history rather than by a fixed protocol.

One expectation worth correcting: a coronectomy is not a smaller experience than a surgical removal. The same flap is raised and a comparable amount of bone is taken from the cheek side. What is avoided is the part of the operation that moves roots sitting against the nerve. Recovery afterwards resembles recovery from a surgical extraction.

Retained roots usually move. The direction is away from the canal and towards the surface, and most of that migration is reported to happen during the first year, slowing considerably after it. Bone forms over the cut surface as they travel, so a radiograph taken a year later commonly shows the root structure buried in bone and further from the canal than it was on the day of surgery.

How far they move varies widely between people. Some travel several millimetres, some barely shift, and the pre-operative scan does not predict which. Migration is not something the patient can influence, and nothing about diet, rinsing or activity changes it.

What the roots do not do is become a tooth again. They do not erupt into the bite, they do not decay the way the crown decayed, because the enamel surface that decays has been removed, and they do not regrow anything. On a later film they look like a rounded fragment sitting in bone, which is what they are.

Follow-up is radiographic and periodic. The purpose is to confirm two things — that bone has healed over the site, and that the roots are moving away from the canal rather than towards the surface of the gum — and to identify early the small number of cases that behave differently. The interval is set at the time and it extends over years rather than months, which is one of the obligations that comes with choosing this operation.

Once the site has healed it usually feels like any other healed extraction site. There is no ordinary sensation of the roots being present, and no long-term restriction on what can be eaten. A patient who does feel something at the site, or who notices a hard point coming through the gum, has a reason to be seen rather than a reason to wait.

A proportion of retained roots eventually have to come out. That proportion is small, and it is not zero. A coronectomy is offered on the explicit understanding that a further operation is possible, and a patient who cannot accept that possibility should have the complete removal instead — that is a legitimate preference, not a failure to understand the explanation.

The reasons a retrieval becomes necessary are a short list:

• The roots migrate far enough to become superficial, or to work through the gum into the mouth
• Infection develops around the retained root structure, usually presenting as swelling, tenderness or discharge at the site
• Persistent pain at the site with no other identifiable cause
• A root that was disturbed at the original operation, though that is normally dealt with at the time rather than later

When it happens it is usually a year or more after the first operation, and sometimes much longer. A site that has healed over and been radiographically stable across several reviews is the reassuring picture, though stability at one review does not settle the question for the next one, which is why the follow-up continues rather than stopping as soon as things look quiet.

Where the roots have migrated away from the canal in the intervening period, the retrieval is often less demanding than the original complete removal would have been, because the anatomy that caused the concern is no longer the anatomy present. Often, not always. A root that has not migrated is retrieved from where it started, with the same proximity to the nerve that the first operation avoided, and that has to be part of how the original decision is framed rather than a discovery made later.

If a coronectomy site becomes swollen, tender or starts discharging months or years afterwards, that warrants an appointment rather than a wait-and-see. It is straightforward to assess with a film, and it is the presentation that most often precedes a retrieval.

Coronectomy has a narrow indication, and a considerable number of teeth that look like candidates on a panoramic film turn out not to be once the scan and the clinical findings are put together.

There is also a category that has nothing to do with the tooth. A coronectomy commits you to periodic radiographs over years. Someone who is moving away, who does not intend to attend follow-up, or for whom returning is impractical is a poor fit for an operation that carries an open-ended follow-up obligation, and an operation that finishes when it finishes suits that situation better. That is a scheduling reality rather than a judgement, and it is worth saying at the consultation rather than discovering at the two-year review.

One further point, because it gets lost: a coronectomy is not a way of avoiding a decision about a tooth that does not need operating on at all. If the honest answer for a particular tooth is radiographic review, then the answer is radiographic review, and a partial operation is not a compromise between removing it and leaving it. The indication for a coronectomy starts from the position that this tooth is coming out, and asks only how much of it.

Recovery follows the same curve as any surgical lower third molar removal. Oozing for several hours is expected and usually settles on the day of surgery with firm, continuous pressure on gauze. Swelling builds for roughly 48 to 72 hours before it turns, so day two or day three is commonly the worst of it. Jaw stiffness peaks alongside the swelling and eases over the following week. Ice through the first day, warm compresses and gentle warm saline rinses from day two, cold soft food to begin with and a widening diet through the first week.

Nothing about the retained roots changes any of that, and there is no additional restriction on eating, brushing or activity beyond the post-operative instructions you are given. Recovery varies between people more than any description predicts, and the timings above are common patterns rather than a schedule to hold yourself to.

One more, repeated deliberately. Any numbness, tingling, burning or altered feeling in the lower lip, the chin or the tongue is reported as soon as you notice it, by telephone, on the day. It is not an emergency in the sense of needing a hospital, and it is time-sensitive in a way that ordinary post-operative problems are not.