Results.

Initial observations.The radiographic findings, originally recorded by observers O.M. and A.H. and later at the last follow-up, are given in Table 1. The percentage of roots with periapical radiolucencies was reduced from 49.8% at the time of root filling to 16.6% observed 10-17 years later, and further to 6.4% 10 years later. Corresponding increases were observed in the percentage of roots with normal periapical findings, whilst the number of roots with widening of the periodontal ligament space had reduced slightly on both follow-up occasions.

**Table 1.** Diagnostic grouping of periapical findings in 265 roots treated endodontically by undergraduate students as evaluated by the two initial observers (percentage distribution).

Final diagnostic grouping.All of the 72 roots recorded by the original observers (O.M. and A.H.) to be without periapical radiolucencies, either at the first or second follow-up, were classified in the same category by the new observers (I.F. and D.M.). Furthermore, the findings of the latter observers indicated lower numbers of roots with radiolucencies on both follow-up occasions. Successive and final joint evaluations by all four observers are presented in Fig. 4 and Table 2.

There were 1.5% of the cases classified as having developed radiolucencies after 20-27 years (Fig. 5). The percentage of roots diagnosed as having radiolucencies on both follow-up occasions was 3.4% (Fig. 6), adding up to a failure frequency of 4.9%. There were 6.4% of cases recorded as having radiographic evidence of periapical repair after 20-27 years (Fig.7). The total recorded percentage of successful cases was 95.1% including roots with no periapical radiolucencies both at the final follow-up and 10 years earlier and 5.3% of the material finally classified with an increased width of the periodontal space. Typical cases are illustrated in Figs 5-7.

**Figure 4.** The origin of failures after root-canal treatment. The periapical findings recorded after separate and joint evaluation of 72 roots by four observers are given to the left. The origin of the13 roots with radiolucencies recorded by the same observers at the 20-27-year follow-up is indicated by the numbers in the arrows. The number and origin of roots with late changes, classified as late failures* (in arrows) and late successes** (arrow), are given separately.

**Figure 5.** Late development of a radiolucency, failure in a mesial root of a mandibular molar with technically deficient root-canal filling.

(a) No lesion11 years after treatment.

(b) Periapical lesion after 21 years.

**Table 2.** The final results after re-evaluation and joint discussion of 72 critical cases (Fig.1) by all four observers.

**Figure 6.** Tooth with a radiolucency classified as a failure on two follow-up occasions in a maxillary lateral incisor.

(a) Periapical lesion after 13.5 years.

(b) Periapical lesion after 24 years.

**Figure 7.** A mandibular premolar that originally had root-filling material extended through the apical foramen.

(a) Periapical radiolucency 13 years after treatment.

(b) Normal periapical condition after 23 years. Note: Disappearance of filling material within the root canal.

Additional analysis - treatment variables.The 17 cases with late signs of healing were characterized by a high number (fourteen) of over-extended root fillings in cases recorded with periapical radiolucencies when they were root filled (Fig.7). The rest (three) consisted of two pulpectomy cases with root fillings ending at a substantial distance from the apex and one retreatment case filled just short of the radiographic apex.

The four cases with late signs of failure consisted of two pulpectomy cases, one with over-extension of the root filling and one with a deviating preparation in the apical part of the root (Fig. 5). The remaining two cases were an over-extended root filling after treatment of a necrotic pulp and a re-treatment case with a root filling ending 7.5 mm short of the apex.