The goal of root canal treatment is the prevention or elimination of apical periodontitis [1]. However, in a recent systematic review that included 114 epidemiological studies globally for meta-analysis, it was reported that 39 % of root-filled teeth exhibited apical periodontitis [2].

The healing of apical periodontitis lesions after root canal treatment is dominantly influenced by the nature of prior dynamic host/infection interactions [3,4] and may take a considerable amount of time to manifest on a radiograph. According to the nearly published European Society of Endodontology (ESE) S3-level clinical practice guidelines [5], it was recommended that the radiographic evidence of endodontic treatment outcome for apical periodontitis should be followed up for at least 1 year, longer if possible.

For clinical decision-making, it is meaningful to predict the prognosis of a tooth as early as possible. Knowledge of the time course for radiographic healing of apical periodontitis is helpful for determining whether a root-filled tooth with a periapical lesion should be a candidate for retreatment or whether it should be left without intervention [6]. At present, a few studies have reported the time frame of the healing after primary root canal treatment [[6], [7], [8], [9]]. In Ørstavik's study [6], teeth with/without preoperative periapical radiolucency were treated and followed up yearly for up to 4 years, and the periapical index (PAI) was used to reveal changes in periapical status. Bergenholtz and Kvist [10] pointed out that the scientific evidence describing the natural course and prognosis of root-filled teeth was inadequate and needed further documentation.

Cone-beam computed tomography (CBCT) is a three-dimensional imaging technique that provides details that are not available from periapical radiography (PA). Using histopathology as a reference standard, a consistently higher sensitivity score and negative predictive value for detecting the absence or presence of periapical lesions were reported for CBCT than for PA [11]. Furthermore, by combining CBCT data with the volume rendered by software, accurate measurements of radiolucency volumes have been obtained and validated [12,13], providing a method for quantitatively monitoring changes in periapical lesions in clinical outcome studies on nonsurgical and surgical endodontic treatment [[14], [15], [16]]. In animal study by Paula-Silva et al. [17], they reported that amongst 30 cases where reduction in size of radiolucency was diagnosed by PA, 24 (80 %) appeared as enlarged lesions in CBCT images. And they investigated that lesion expanded in the cancellous bone frequently in the lingual direction, and such enlargement could only be revealed by CBCT. These results indicated that the volumetric assessment could facilitate a more detailed assessment of degree/extent of periapical lesions and reveal additional information on the healing process.

The aim of this study was to use the volume of radiolucency measured based on CBCT data to analyse the tendency towards changes in radiographic outcomes in teeth with apical periodontitis during the 4-year period after root canal treatment. In addition, the factors affecting the radiographic outcomes were analysed.