What decides whether a tooth can still be saved
- Root canal retreatment is a procedure in which the dentist removes the old filling from the canals, cleans them again and seals them. It is not a repeat of the first procedure, because it begins with the question of what went wrong.
- Root canal retreatment is a procedure in which the dentist removes the old filling from the canals, cleans them again and seals them. It is not a repeat of the first procedure, because it begins with the question of what went wrong.
- In a pooled analysis of twenty-nine studies of root canal retreatment, the lesion at the root disappeared completely in 78.8% of teeth, and shrank or disappeared in 87.5%. In a single university centre, in a series covering only teeth that already had a lesion at the root, success was recorded in 65.5%.
- Healing of the lesion and survival of the tooth are two different outcomes. A tooth counted as a failure in a study can stand in the mouth for years. Before you believe a success rate, check which of those two things was counted.
- The prognosis of root canal retreatment is improved by a small lesion, or none at all, before the procedure, and by a canal filled to the right length.
- Two situations close the door to root canal retreatment: a vertical root fracture and too little healthy tissue for the tooth to be sealed and rebuilt.
On a follow-up radiograph there is a darker spot at the end of the root of a lower first molar, roughly the size of a pea. The tooth was treated with a root canal eleven years ago, it stands under a crown, it has never hurt and it does not hurt now, and the spot on the radiograph is the only sign that anything is wrong with it. The question to consider is the following: if nothing is happening, does anything need to be done at all? What settles it is not whether the lesion is there, but how it behaves on the radiographs that follow.
This is the point at which root canal retreatment enters the conversation, also called repeat root canal treatment, and shortened in Polish surgeries to reendo. The question patients ask alongside it is usually about an implant. That is a separate decision, and it is set out in save the tooth or place an implant.
Why root canal treatment sometimes fails
Root canal treatment removes infected or dead pulp from inside the tooth and seals the space left behind. We speak of failed root canal treatment when bacteria remain in a place nobody reached, or return there years later.
The first route is anatomy, described best in upper molars, and so not in the tooth from the opening scene. The upper first molar most often has a fourth canal. It hides in the mesiobuccal root, that is the one on the cheek side and closer to the front. Where that canal had been missed, tomographic images showed an inflammatory lesion at the root distinctly more often. The figure given in that paper, about five and a half times, is the result of a calculation: it compares the odds of a lesion in the two groups, and does not say how often the lesion occurs. The group was also small, so the size of the difference itself should not be taken literally. The association between a missed canal and a lesion at the root is clear in that paper. For a patient it means one thing: a missed canal is a cause that can be removed.
The second route runs from the crown down. Two Danish groups of adults were followed for ten years each, with complete sets of radiographs every five years, and in both of them the risk of a lesion at the root rose both with poor quality of the root filling and with poor quality of the crown restoration. A sealed restoration is therefore not cosmetic work after the canals have been treated, but a condition of the treatment lasting.
The third group of causes lies outside the canal itself: a lesion that does not heal despite correct treatment, and a vertical root fracture.
Something patients rarely hear: failure does not in itself prove that anyone made a mistake. Part of the anatomy stays out of reach even with careful work. In Danish observations spanning more than two decades, the frequency of lesions at the roots of root filled teeth barely changed.
How failure is recognised: symptoms and the radiographic picture
The symptoms can be unmistakable: pain on biting, tenderness of the tooth to tapping, a sinus tract on the gum, recurrent swelling. The trouble is that often there are none at all. Inflammation at the root can be entirely symptom-free, and that is precisely why such a tooth is sometimes left untreated even though the diagnosis has been made.
So the radiograph settles it.
Apical periodontitis, the inflammatory lesion at the root tip, shows on it as a darker area at the apex of the root, that is at its end, because bone has been lost in that spot. An ordinary radiograph of the tooth, an intraoral film, remains the routine examination. It has recognised limits, though: it shows the tooth flat, so a small lesion can hide behind the root or behind the surrounding bone. The authors of a review of the imaging of such lesions pointed to only one method that detects them early and reproducibly. That method is cone-beam computed tomography, a three-dimensional image of the tooth. A lesion invisible on an ordinary radiograph is sometimes visible on the three-dimensional image. The tooth then receives a different diagnosis and a different plan of management.
When root canal retreatment is indicated
A lesion shrinking across successive radiographs is healing in progress. A lesion that is growing, that has newly appeared, or one alongside which symptoms are developing, is an indication for treatment, most often root canal retreatment. A tooth due to receive a new crown stands apart. Any doubt about the earlier treatment of the canals is resolved before the restoration, not after it.
What happens if I do nothing
That leaves the intermediate case, the one from the opening scene: a stable lesion, a tooth without symptoms. What happens to such a tooth when nobody treats it has not been well studied. What is known is only that in the Danish observations, teeth with a lesion visible at the start more often still had one ten years later, or had already been extracted.
Waiting therefore has its price: a smaller lesion before the procedure is associated with a better outcome of root canal retreatment, and by how much exactly a delay of one year worsens the prognosis is not known, because there are no data on it.
Observation without follow-up radiographs is not observation. It is putting the matter off.
What root canal retreatment actually involves
Root canal retreatment is a different procedure from the first one, longer and technically harder. It runs in several stages, in which the dentist in turn:
- reaches the canals through the crown of the tooth, or removes the prosthetic crown and the post and core;
- removes the old root filling, the gutta-percha and the sealer;
- finds the canal missed at the first treatment, if there is one;
- removes an obstacle, for example a fragment of a fractured instrument, which has to be retrieved or bypassed; this stage is carried out under magnification, in treatment under a microscope;
- cleans and disinfects the canals, usually over more than one appointment;
- fills the canals and rebuilds the crown of the tooth with a sealed restoration.
Not every canal has to be opened in the process. In a study covering seventy-five multi-rooted teeth, only the root alongside which a lesion could be seen was retreated, and a favourable outcome was obtained in close to nine cases out of ten; at the roots left without repeat treatment, a new lesion appeared rarely. This is one small group and a short retrospective observation, that is a look back at treatment already carried out. The treatment was done exclusively by endodontists working under a microscope, and the authors themselves caution that the result may be transferred only to a practice of that kind.
The last point on that list tends to be put off, and it should not be. The time after which the definitive restoration is placed has been described as a factor affecting survival of the tooth, and a temporary filling is a stage of retreatment, not an end state.
Prognosis: what the chance of success depends on
The pooled analysis of twenty-nine studies of root canal retreatment gives two figures rather than one. The result depends on what is counted as cure. Under the strict criterion, which requires the lesion to disappear completely on the radiograph, the lesion healed in 78.8% of teeth. Under the loose criterion, which accepts a reduction in the lesion alone, in 87.5%. A better result was associated with the absence of a lesion before the procedure or with its small size, with a canal filled to the right length, and with a longer period of observation.
One centre reports a distinctly lower figure. At the university clinic in Oslo, root canal retreatment carried out on three hundred and fifty-one teeth with apical periodontitis gave success in 65.5%, and older patients and teeth with a larger lesion before the procedure fared worse. The treatment there was done by dentists in postgraduate training. The discrepancy is not an error in either paper. The pooled analysis also included teeth treated without a lesion at the root, that is in an easier situation. The same analysis reports a success rate under the strict criterion as well: 78.0%. Comparing success with success therefore gives 78.0% against 65.5%, and the difference remains.
How much is a "small lesion"? There is no generally accepted cut-off. What is known is only that a larger lesion is associated with a worse prognosis. The size of your own lesion has to be asked about in the surgery, at the radiograph, before the decision on a repeat procedure is taken.
A healed lesion is not the same thing as a saved tooth
Healing of the lesion at the root and survival of the tooth in the mouth are two different outcomes, which merge into one in conversation: a tooth with a lesion that has shrunk but not disappeared is sometimes counted as a treatment failure and still serves the patient for years. In the study of selective retreatment described above, observation lasted about fifteen months on average. More than nine teeth in ten were still in place at that point.
The success rate of a repeat procedure means nothing until it is known which of those two things was counted.
What the course of treatment itself depends on, by contrast, is what went wrong.
| Cause of failure | What is done in retreatment | What to ask before deciding |
|---|---|---|
| Canal missed at the first treatment | finding and preparing the missing canal | whether all the canals can be seen, and whether a three-dimensional image is needed |
| Root filling too short or not sealed | removal of the old material, filling to the right length | whether the canal can be filled to the right length |
| Leaking restoration, decay under the crown | treatment of the canals plus a new, sealed restoration | when the definitive restoration will be made |
| Fractured instrument in the canal | retrieval of the fragment, or bypassing it | whether the canal can be filled along its whole length despite the obstacle |
| Persisting infection with a large lesion | the same procedure, a longer period of observation | when the follow-up radiograph is scheduled for |
| Vertical root fracture | usually extraction; in a multi-rooted tooth, sometimes removal of the fractured root | what the diagnosis rests on |
When root canal retreatment does not make sense
The answer arranges itself into a set of traffic lights: red closes the road, amber makes it worse, green allows you to wait.
A red light comes on in two situations, and each of them settles the matter. The first is a vertical root fracture. The crack runs along the root and cannot be sealed from the inside. The three-dimensional image does not show the crack itself reliably; what it does show is the pattern of bone loss around the root that is typical of a fracture, and that is what raises the suspicion. What confirms it is not the radiograph but the examination: a deep, narrow, isolated pocket at one root, a sinus tract closer to the crown than to the root tip, and finally inspection of the root surface under magnification. The prognosis of such a tooth is poor and it most often ends with extraction.
The second is too little healthy tissue for a ferrule to be obtained after treatment, that is a ring of the tooth's own wall for the crown to hold on to. Without it, a tooth whose canals have been treated will fracture under load anyway. How much wall is left becomes visible once the old crown has been taken off and the decay removed, and it is settled before the canals are prepared, not after.
An amber light belongs to a tooth with extensive bone loss around the root caused by periodontal disease, and to a tooth with a perforation, that is an opening connecting the inside of the tooth with the tissues around the root, located close to the gum margin. Treatment is possible here, but curing the canals does not on its own remove the problem that puts the tooth at risk.
A green light belongs to a tooth without symptoms, with a small stable lesion and a sealed restoration: here it is permissible to observe and monitor with radiographs rather than treat straight away.
Outside this scale, and outside any delay: swelling of the face, fever, difficulty swallowing or difficulty opening the mouth are not matters to be scheduled for the next appointment. This is an urgent presentation, the same day. Difficulty swallowing or difficulty opening the mouth means going at once to hospital emergency care, which in Poland means the izba przyjęć, the hospital admissions unit, or the SOR, the hospital emergency department. If there is difficulty breathing, call the emergency number 112.
Retreatment, apical surgery or extraction: what the conversation about the choice looks like
Drugą drogą jest resekcja wierzchołka korzenia: zabieg chirurgiczny, w którym lekarz dochodzi do końca korzenia od strony dziąsła, usuwa jego wierzchołek razem ze zmianą i zamyka kanał od tej strony. Porównano ją bezpośrednio z leczeniem nieoperacyjnym, pierwszym albo powtórnym. Do przeglądu weszło pięć prac i nieco ponad pięćset zębów, obserwowanych od pół roku do niespełna dziewięciu lat. Wyniki wskazywały na lepsze gojenie zmiany po resekcji i rzadszą potrzebę kolejnych interwencji. Przeżycie samego zęba było za to wyższe w grupie leczonej nieoperacyjnie. Autorzy nie mogli przeprowadzić metaanalizy, czyli zsumować wyników w jedną liczbę, a w czterech pracach na pięć sposób, w jaki je przeprowadzono, mógł zniekształcić wynik. Żadna z metod nie okazała się w tym przeglądzie wyraźnie lepsza. W serii z Oslo zabieg chirurgiczny wypadł lepiej niż leczenie nieoperacyjne. Trafiały do niego jednak inne zęby.
In everyday clinical practice at Modern Dental & Orthodontics (Klinika MDO) we meet the expectation that one of these routes will turn out to be objectively better. The usual starting point is the nonsurgical route, because it does not close off the possibility of a surgical procedure later, whereas the reverse order, apical surgery first and root canal retreatment in the same tooth afterwards, can be impossible to undo. What root canal treatment looks like at the clinic is described on the page about endodontic treatment.
Frequently asked questions
How many times can the same tooth have root canal treatment?
There is no limit on the number. Every further opening takes away a little healthy tissue, so what decides is not a count of procedures but how much of the tooth wall is left and whether it can be rebuilt with a sealed restoration; after a further failure the conversation usually moves towards apical surgery.
Does retreatment hurt more than the first treatment?
That cannot be predicted in advance. The procedure is carried out under anaesthetic, and in canals that have already been prepared there is no pulp left, so the pain does not come from there; it is different in a canal missed at the first treatment. Discomfort after the procedure comes from the tissues around the root and depends on their state rather than on which treatment in the sequence this is.
How long does root canal retreatment take?
Usually longer than the first treatment of the same tooth, because removal of the old filling is added, and sometimes of the crown and the post as well; repeat root canal treatment is most often planned over two appointments with an interval for disinfecting the canals. The length of that interval is not fixed in advance: it is set by the state of the tooth, not by the calendar.
What are the realistic chances of success?
It depends which chance you are asking about. Healing of the lesion, or the tooth staying in place? The pooled analysis of studies reports 78.8% healing under the strict criterion and 87.5% under the loose one, and one university centre reports success in 65.5%; survival of the tooth was measured by a single study: after about fifteen months on average, more than nine teeth in ten were still in place. The prognosis is improved by a small lesion and worsened by a large one.
Can I do nothing and just watch it?
With a tooth without symptoms, with a small and stable lesion and a sealed restoration, yes, but only on condition that watching means further follow-up radiographs. Putting the matter off is not observation. The decision is changed by growth of the lesion, by symptoms appearing, by a leaking restoration or by a new crown being planned. Absence of pain alone is not enough, because a lesion at the root can be symptom-free.
When will I know whether the treatment worked?
Not straight away. Bone rebuilds over months, so the outcome is assessed on a radiograph no earlier than a year after the procedure, and sometimes later. Absence of pain is not proof of healing, because inflammation at the root can run its course without symptoms. What settles it is the comparison of successive radiographs.
Do the crown and the post have to be removed?
Not always. Sometimes a small opening in the crown is enough, and it is closed again after treatment. When the crown is not sealed and there is decay underneath it, removing it is necessary in any case. A post and core is removed with care, because doing so risks damaging the wall of the root.
The tooth started hurting years after root canal treatment — what does that mean?
Tooth pain years after root canal treatment most often comes from the tissues around the root rather than from inside it, because the pulp is no longer there; it usually means an infection that has persisted or returned. Less often, a root fracture lies behind it. What settles the question is tapping and a radiograph, not the character of the pain.
Would it not be better to go straight for an implant?
These are two different decisions, not two versions of the same one. Root canal retreatment preserves your own tooth together with the ligament and the bone around it, while an implant requires the tooth to be extracted, which is an irreversible step; the sensible order is to assess first whether the tooth can still be rebuilt. We set that decision out on its own in save the tooth or place an implant.
The fate of the tooth is settled before the canals are opened
The whole decision is framed by questions, none of which concerns the technique of the procedure: what went wrong, how much healthy tooth wall is left, and whether the root is fractured. The last two can close the matter; the first shifts the prognosis. The figure worth remembering is a different one from the one usually assumed: not the success rate, but the size of the lesion at the root before the procedure. There is no generally accepted boundary between a small one and a large one, and what settles it is the comparison of successive radiographs.
Read more:
- Root Canal Treatment in Warsaw
- Save the tooth or place an implant — how to make a decision you will not regret
- Root canal treatment under a microscope — what magnification really changes
- Post and core — when it rescues a tooth for crown restoration
Content and liability disclaimer
This article is informational and educational in nature and does not constitute medical advice, a diagnosis or a therapeutic recommendation — it does not replace a consultation with a specialist. If you are experiencing symptoms, have doubts or are facing a decision about treatment, consult a dentist. The methods described and the data cited are general in nature. The results of scientific studies relate to populations, not to an individual patient. The course and outcome of therapy depend on the individual clinical situation and may vary between patients. No information contained in this article constitutes a guarantee of result. The content has been prepared with due care, based on publicly available medical knowledge and the scientific publications indicated in the Sources section. We do not advise taking or refraining from any health-related action solely on the basis of the content of this article, without prior consultation with a doctor.
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