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Clinical and Radiographic Outcomes of Second Molars with External Root Resorption Following Impacted Third Molar Removal: A Retrospective CBCT-Based Cohort Study

This retrospective CBCT-based cohort study demonstrates that while impacted third molar extraction alone effectively halts external root resorption and preserves pulp sensibility in affected second molars, adjunctive guided bone regeneration significantly enhances distal alveolar bone regeneration compared to extraction alone.

Original authors: Baocheng Yao, Shiyu Qiu, Chengyi Wang, Qiao Ling, Ning Ma

Published 2026-07-07

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CC BY 4.0

Original authors: Baocheng Yao, Shiyu Qiu, Chengyi Wang, Qiao Ling, Ning Ma

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A "Neighborly" Problem

Imagine your mouth is a neighborhood. The Second Molar (M2) is a house, and the Third Molar (wisdom tooth, M3) is a new, unruly neighbor moving in right next door. Because the new neighbor is stuck (impacted) and pushing against the old house, they start damaging the old house's foundation and walls. In dental terms, this is called External Root Resorption (ERR)—the wisdom tooth is literally eating away at the root of the tooth next to it.

This study, conducted by researchers at Peking University Third Hospital, asked two main questions:

If we remove the unruly neighbor (the wisdom tooth), does the damage to the old house stop, and does the house stay safe?
Does the old house's foundation (the bone) fix itself on its own, or do we need to hire a construction crew (a procedure called Guided Bone Regeneration or GBR) to help it heal?

The Experiment: Two Approaches to Fixing the Foundation

The researchers looked back at 55 patients who had this specific problem. They split them into two groups based on how the surgeons treated them:

Group A (The "Clean Up" Crew): The surgeons simply removed the impacted wisdom tooth. They let nature take its course to see if the bone would heal itself.
Group B (The "Renovation" Crew): The surgeons removed the wisdom tooth and immediately performed a "renovation." They filled the gap left behind with a special bone graft material (like a scaffold) and covered it with a protective membrane. This is the GBR procedure.

They used high-tech 3D X-rays (CBCT) to take "before" and "after" pictures of the bone levels, looking at the results at least six months later.

What They Found

1. The Damage Stops Immediately

The Analogy: Think of the wisdom tooth as a leaky pipe flooding a basement.
The Result: As soon as the researchers removed the wisdom tooth (turned off the pipe), the "flooding" (root resorption) stopped completely in 100% of the cases. The damage didn't get any worse.

2. The House Stays Safe (No Need for a "Root Canal")

The Analogy: The "pulp" is the electrical wiring inside the house. Some people worried that because the walls were damaged, the wiring might be fried and need replacing (a root canal).
The Result: The researchers tested the "wiring" (pulp sensibility) in all the teeth. Even though the roots were damaged, the "electricity" was still working perfectly in every single tooth.
The Takeaway: You do not need to do a root canal just because the root is damaged, as long as the tooth doesn't hurt or show signs of infection. The tooth is still alive and well.

3. The Foundation Needs a Helping Hand

The Analogy: Even after the leak is fixed, the basement floor might still be full of holes.
The Result:

Group A (Clean Up only): The bone did grow back, but it was a bit like a patchy lawn. The average bone growth was about 4 mm.
Group B (Renovation/GBR): The bone grew back much more impressively, like a lush, full garden. The average bone growth was about 6.3 mm.

The study found that the group that got the extra "renovation" (GBR) had significantly better bone recovery than the group that just had the tooth pulled.

4. Does the Size of the Hole Matter?

The researchers wondered: Does the GBR only work for tiny holes, or does it help big holes too?
The Result: It helped both. The benefit of the GBR procedure was consistent regardless of how severe the bone loss was before the surgery. Whether the hole was small or large, the "renovation crew" did a better job than the "clean up crew."

The Bottom Line

If a wisdom tooth is eating away at the root of the tooth next to it:

Pull the wisdom tooth: This stops the damage immediately.
Don't panic about the root: You usually don't need a root canal; the tooth stays alive.
Consider the "Renovation": While the bone will try to heal itself, it often leaves a gap. Adding a bone graft (GBR) at the time of surgery helps the bone grow back much fuller and stronger, restoring the foundation of the tooth more effectively.

In short: Removing the cause stops the bleeding, but adding a scaffold helps the wound heal completely.

Technical Summary: Clinical and Radiographic Outcomes of Second Molars with External Root Resorption Following Impacted Third Molar Removal

Problem Statement
External root resorption (ERR) of the second molar (M2) associated with impacted third molars (M3) represents a significant, yet historically underestimated, clinical threat. While conventional 2D radiography suggested a low prevalence (0.3–13.3%), recent Cone-beam computed tomography (CBCT) analyses indicate incidence rates ranging from 11.0% to 54.3%. Although the removal of the impacted M3 is known to arrest the resorptive process, the periodontal consequences—specifically the healing of distal alveolar bone defects and the necessity of prophylactic endodontic treatment—remain insufficiently characterized. Furthermore, it is unclear whether adjunctive Guided Bone Regeneration (GBR) offers a measurable benefit in restoring distal bone levels in the presence of ERR, as existing literature largely focuses on non-resorptive defects.

Methodology
This single-center, retrospective cohort study analyzed 65 M2s in 55 patients diagnosed with ERR associated with impacted M3s between 2022 and 2024. The study adhered to STROBE guidelines and was approved by the Ethics Committee of Peking University Third Hospital.

Inclusion Criteria: Radiographic evidence of ERR due to direct M3 contact, clinically asymptomatic M2s with normal pulp sensibility (positive cold test), M3 extraction performed, a minimum 6-month follow-up, and high-quality pre- and post-operative CBCT scans.
Exclusion Criteria: Caries in the resorption area, apical periodontitis, mobility ≥ Grade II, resorption extending to the pulp chamber floor, or other etiologies (cysts, tumors, trauma).
Grouping: Patients were stratified into two non-randomized groups based on surgical intervention:
1. Extraction-Only Group (n=22): M3 removal alone.
2. GBR Group (n=43): M3 removal with concurrent GBR using deproteinized bovine bone mineral (Bio-Oss) and a resorbable collagen membrane (Bio-Gide).
Radiographic Assessment: CBCT scans were used to measure five parameters: Root Length, Upper/Lower Boundary Distances of the defect, Distal Alveolar Bone Height (h), and Comparison Root Length. The primary outcome was the change in distal bone height ( $\Delta h$ ) between baseline and follow-up.
Statistical Analysis: Generalized Estimating Equations (GEE) were employed to evaluate the association between treatment modality and bone gain, adjusting for baseline defect severity, age, tooth position, and potential confounders. Interaction analysis was performed to determine if GBR efficacy varied by baseline defect severity.

Key Results

Cessation of Resorption and Pulp Vitality: Following M3 removal, ERR progression ceased in all 65 cases. Crucially, 100% of the teeth (64/64) that had not undergone preoperative endodontic treatment maintained positive pulp sensibility at the final follow-up. No signs of periapical pathology or increased mobility were observed.
Bone Regeneration Outcomes:
- The GBR group demonstrated significantly greater distal bone gain compared to the Extraction-Only group ( $6.32 \pm 2.67$ mm vs. $4.08 \pm 3.50$ mm; $P = 0.007$ ).
- After adjusting for baseline defect severity and other covariates, GBR remained a significant predictor of increased bone gain ( $\beta = 1.202$ , $P = 0.004$ ).
- Baseline distal bone height was significantly associated with $\Delta h$ , indicating that sites with more severe initial bone loss exhibited greater absolute bone gain.
Interaction Analysis: No significant interaction was observed between treatment modality and baseline defect severity ( $P = 0.945$ ), suggesting that the relative benefit of GBR was consistent across the range of defect severities included in the study.

Significance and Claims
The study concludes that the removal of the impacted third molar is sufficient to arrest external root resorption, rendering prophylactic root canal treatment clinically unwarranted as pulp sensibility is robustly maintained. However, the authors note that spontaneous periodontal recovery of the associated distal defects appears incomplete, likely due to the altered root surface morphology compromising spontaneous reattachment.

The primary contribution of this work is the evidence that adjunctive GBR is associated with enhanced distal bone regeneration in M2s affected by ERR. The authors suggest that the management of ERR-associated defects may require a different decision-making framework than conventional M3 surgery, where the structural integrity of the root surface should be considered alongside the extent of bone loss. They propose a lower threshold for considering regenerative interventions in these cases to improve periodontal healing predictability.

Limitations
The authors acknowledge the retrospective nature of the study, which introduces potential selection bias as treatment allocation was based on clinical judgment rather than randomization. Additionally, the absence of clinical periodontal parameters (e.g., probing depth) and the reliance on radiographic bone gain as a surrogate outcome are noted. The sample size, while sufficient for primary outcomes, may have limited the statistical power to detect interaction effects, and the 6-month follow-up does not address long-term stability.