Beyond the Radiograph: Histological Insights into Human Mature and Immature Teeth in Regenerative Endodontics

By Badr Hefnawi, Raju Gandhi, and Xiaofei Zhu

Many clinicians have ventured into the realm of regenerative procedures, often presenting radiographic evidence of apical closure and canal wall thickening. These observations frequently lead to assertions of successful regeneration. However, it is vital to acknowledge that relying solely on radiographic assessments cannot definitively distinguish between repair and regeneration (Figure 1). A deeper dive must be taken into the histological outcomes of regenerative procedures to attain a better understanding of its success.

Many prospective animal studies have been conducted to examine the histological outcomes to various treatment methods, however results from animal studies may not always be extrapolated to human situations.^1,2,3 This is due to variations in pathophysiology, and also different study protocol conditions that can take place in animal versus human studies. Due to ethical reasons, no randomized control trial has been conducted in humans on histological outcomes. This limits the current review to focus on human histological observational studies as opposed to interventional. Although the number of reports on in vivo teeth that have undergone REP followed by histological analysis is scarce, and the details of each treatment may vary, it does not take away from the importance of evaluating this body of literature.

Literature Search

A comprehensive literature search was conducted across PubMed, Embase, and Web of Science for studies published between 2001 and 2023. Inclusion criteria required that each study report a completed regenerative endodontic procedure with histological evaluation—either of the entire tooth or a relevant section. Out of 389 initially identified papers, 17 human case studies were ultimately included, representing 22 teeth subjected to histological analysis.

Pre-Treatment Diagnoses and Patient Demographics

Of the 22 teeth reviewed, 18 were immature teeth, most diagnosed with pulp necrosis and apical pathology such as acute or chronic apical abscesses, symptomatic or asymptomatic apical periodontitis. These align with the classic indications for REP, where continued root development is desired.^4-20

Two cases involved mature teeth with pulpal necrosis—an area of increasing interest in regenerative endodontics. Notably, two additional cases featured vital, healthy teeth undergoing REP strictly for research purposes before scheduled orthodontic extraction, providing rare control-like conditions.

Clinical Outcomes

Thermal or electric pulp testing was included as part of the evaluations in 19 of the 22 cases reviewed. Five of these teeth recorded a positive reaction to either thermal or electric pulp testing. Notably, two of these teeth cases were mature teeth that belonged to the only case report involving mature teeth with histologic examination. These teeth lasted 3.5 years before requiring extraction due to horizontal crown fractures. These cases demonstrate the potential of regenerative procedures to induce a positive sensibility response in mature teeth, even in necrotic cases.

Percussion testing was reported in 20 of the 22 cases reviewed, with five teeth showing a positive response. Three of these teeth also showed a response to either thermal or electric pulp testing. The remaining two teeth that tested positive to percussion were associated with persistent infection, demonstrated by an acute apical abscess in one case and symptomatic apical periodontitis in the other.

Histological findings

The distribution of tissue responses following regenerative procedures varied according to the underlying etiology (Figure 2). Reparative outcomes were the most frequently observed across all categories, with caries and crown fractures demonstrating the highest number of cases. Dens evaginatus, however, showed a broader spectrum of outcomes, including combined reparative–regenerative and reparative–inflammatory responses, highlighting the complexity of tissue reactions in this condition. In contrast, purely regenerative or inflammatory responses were relatively uncommon overall.

While these findings may, in part, reflect the bacterial nature of certain etiologies, definitive conclusions cannot be drawn due to variability in protocols, including differences in materials, intracanal medicaments, and treatment time frames.

Comparing Immature and Mature Teeth

Although most studies involved immature teeth, the mature tooth cases may be the most groundbreaking.^4,5 Histological analysis of these teeth (combined with clinical testing) demonstrates true regeneration may be possible even without a natural apical blood supply or native stem cells. This could represent a shift in the field’s understanding and a call to expand REP indications, particularly as tissue engineering advances, including the use of platelet-rich/fibrin matrices and allogenic stem cells, continue to emerge.

Conclusion: Regeneration or Repair?

This review provides a nuanced look into the biological realities of regenerative endodontic procedures in humans. While true pulp regeneration remains elusive, the formation of vascularized, functional tissue in some cases, especially in mature teeth, marks an encouraging evolution in endodontic care. As protocols are refined and regenerative materials evolve, a future where true pulp regeneration is routine may not be far off. Until then, continuing to integrate histological insights into clinical decision-making will be key in ensuring REP fulfills its promise.

References

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Badr Hefnawi, Raju Gandhi, and Xiaofei Zhu are with the Department of Endodontics, Boston University Henry M. Goldman School of Dental Medicine.