It is generally acknowledged that persistence of
disease is most commonly due to difficulties that occur
during initial endodontic treatment. Inadequate aseptic
control, poor access cavity design, missed canals,
inadequate instrumentation, and breakdown of
temporary or permanent restorations are examples of
procedural pitfalls that may result in persistence of
endodontic disease (Fig 1).
The reasons for disease persistence in well-treated
root-filled teeth have been poorly characterized until a
series of studies published during the 1990s.
Using block biopsy material from non-healed periapical
tissues including apices of the root-filled teeth, analysis
by correlative light and electron microscopy showed
that there were four factors that may have contributed
to persistence of a periapical radiolucency after treatment (Fig 2). The factors were: (i) intraradicular
infection;
(ii) extraradicular infection by bacteria of
the species Actinomyces israelii and Propionibacterium
propionicum ;
(iii) foreign body reaction;
(iv) cysts, especially those containing cholesterol
crystals.
Rarely, healing may occur by fibrous scar tissue
instead of by bone,
which may be misinterpreted as
disease persistence on follow-up radiographs. Of all
these factors, it is generally acknowledged that the
major cause of post-treatment disease is the persistence
of micro-organisms in the apical part of root-filled
teeth.
Persistent endodontic disease, or apical periodontitis
associated with a root-filled tooth, can continue for
many years and may become apparent only when a
tooth requires a new restoration or is detected on a rou-tine radiograph (Fig 3). The fact that some micro-organisms are capable of survival under harsh, nutri-ent-limited conditions of the root-filled canal for so
long is remarkable. Yet, little information was known
about the micro-organisms involved in persistent intra-canal infection after root filling until 1998, when two
studies revealed that the microbial flora associated with
persistent endodontic disease is quite unlike that found in other oral infections, or that of the untreated root
canal.
