A systematic review in the Journal of Endodontics reports that, in two randomized trials, enlarging the apex three sizes beyond the initial apical binding file was associated with a higher rate of strict periapical healing in mandibular molars than enlarging it two sizes. Evidence was insufficient to say whether taper independently affects healing, and the authors call taper an exploratory variable.
How large should the apical preparation be? It is a practical question for any general practice in Toronto, Mississauga or Markham that treats molars rather than referring them. As of November 2026, a new review from Hiroshima University offers a narrow, useful data point: it is limited to primary treatment of permanent mandibular molars, and it judges success against a strict definition of healing.
The paper, from researchers at Hiroshima University, appeared online on October 27, 2026. This report is based on the abstract and citation record indexed in Europe PMC. We could not access the full text, so we do not report trial-level details that the abstract leaves out.
What did the review study?
The authors searched PubMed, Embase, Web of Science, the Cochrane Library and trial registries, and used forward citation tracking, through May 18, 2026. Human clinical studies qualified when mandibular molar outcomes and apical preparation data could be extracted.
The outcome was strict periapical healing. The review defined it as no clinical symptoms or signs together with a periapical index (PAI) score of 2 or lower, or an equivalent criterion. Comparative results were pooled as risk ratios, while healing rates from single-arm data were summarized descriptively. Five studies entered the quantitative synthesis.
What did the authors find?
- Apical enlargement: in two randomized trials that based enlargement on the initial apical binding file, +3 enlargement showed a higher strict healing rate than +2 (risk ratio 0.78; 95% CI, 0.67 to 0.91; P < 0.001).
- Taper: evidence was insufficient to determine an independent effect on strict healing (risk ratio 0.90; 95% CI, 0.70 to 1.15; P = 0.39).
- Smaller final sizes: the five single-arm datasets reported final apical preparation sizes of #30 or smaller, with a descriptive pooled strict healing estimate of 72.9% (95% CI, 60.8% to 83.5%).
How should Canadian clinicians read these numbers?
Start with the authors' own wording. They describe the randomized evidence as limited, and the title calls the meta-analysis exploratory. Two trials can point in a direction. They cannot settle a preparation protocol.
The risk ratio needs a moment's care too. The abstract gives 0.78 alongside the statement that +3 healed more often than +2. Our reading is that the ratio most likely expresses healing with +2 relative to +3, although the abstract does not state the direction explicitly.
The single-arm figure carries the most risk of misuse. A pooled 72.9% at #30 or smaller may sound like reassurance about conservative preparation. The authors say the data showed good strict healing at those sizes, "but not equivalence with larger apical preparations." Summarized as single-arm data, that figure cannot tell you what a larger preparation would have achieved in the same teeth.
Taper is a similar story. The authors call it an uncertain independent factor and an exploratory variable, while adding that greater taper "may partly compensate when apical enlargement is conservative." That is a hypothesis worth testing. It is not a licence to trade apical size for taper.
Why does "+3" not mean a fixed file size?
The trial comparison is relative to the initial apical binding file, so the final size depends on the canal you start with. On standard ISO sizing, a canal binding at #15 reaches #25 at +2 and #30 at +3, whereas one binding at #25 reaches #35 and #40.
This is why a binding-file comparison and a dataset of final sizes #30 or smaller answer different questions, and why neither gives you a single target number for every molar canal.
What does an earlier trial with this design show?
A 2021 randomized clinical trial in the Journal of Endodontics by Fatima and colleagues illustrates the design. It assigned 120 patients with asymptomatic mandibular first molars and a PAI score of 3 or higher to enlargement of 2 or 3 sizes beyond the initial apical binding file, each with a 4% or 6% taper. The abstract of the new review does not name its included trials, so we are not stating that this trial is one of the two.
Fatima and colleagues found no significant difference in postoperative pain. Success was lowest, at 57.1%, in the +2 group prepared with a 4% taper, and the authors concluded that this preparation was insufficient compared with larger sizes and tapers. Follow-up ran to 12 months. Its result fits the review's suggestion about conservative apical sizes and taper, though one trial does not confirm it, its success measure may differ from the review's strict definition, and it may itself be among the included trials.
What does the review leave open?
The abstract does not report on irrigation protocols, file systems, dentine removal or root fracture. It also does not extend beyond permanent mandibular molars receiving primary treatment, so applying its results to maxillary molars or retreatment would go beyond the evidence described here. The authors work in Japan, and the abstract says nothing about Canadian practice patterns.
For a dentist in Ontario, the practical value is in measurement more than in protocol change. Your own outcomes are the one dataset you fully control.
Pro Tip: Record the binding file, final apical size and taper for each canal, along with a pre-operative PAI score, then compare them at a 12-month recall radiograph. Over time, consistent notes will tell you more about your own molar outcomes than any single headline.
EBIKO Dental will continue monitoring endodontic outcome research relevant to Canadian dental professionals. More clinical news and supply guides are available at EBIKO Dental.
Sources
- Weihao S, et al. Apical Enlargement, Taper, and Strict Periapical Healing after Primary Endodontic Treatment of Mandibular Molars: A Systematic Review and Exploratory Meta-analysis. Journal of Endodontics, 2026. Europe PMC record (DOI 10.1016/j.joen.2026.10.008)
- Fatima S, et al. Effect of Apical Third Enlargement to Different Preparation Sizes and Tapers on Postoperative Pain and Outcome of Primary Endodontic Treatment. Journal of Endodontics, 2021;47(9). Europe PMC record
Frequently Asked Questions
Q: Does a larger apical preparation improve healing in mandibular molars?
Limited randomized evidence suggests it may. A 2026 review in the Journal of Endodontics found that +3 enlargement from the initial apical binding file was associated with higher strict periapical healing than +2, based on two trials. The authors describe the analysis as exploratory.
Q: Does taper affect periapical healing after root canal treatment?
The review could not determine this. Its pooled estimate for taper had a confidence interval crossing 1 (risk ratio 0.90; 95% CI, 0.70 to 1.15), and the authors regard taper as an exploratory variable.
Q: Is a final apical size of #30 or smaller as good as a larger preparation?
The review does not show that. Single-arm datasets at #30 or smaller had a descriptive pooled strict healing estimate of 72.9%, but the authors state this does not demonstrate equivalence with larger apical preparations.