INTRODUCTION

The dental follicle of a permanent tooth is connected to the lamina propria of the overlying gum by a structure composed of connective tissue,^1,2 named the gubernacular cord, which originates in the dental lamina. Osteoblastic activity is regulated by the presence of epithelial tissue, which respects the boundaries of the gubernacular cord and forms a canal around it, the gubernacular canal (GC).^1,3–5 The gubernaculum dentis (gubernacular cord and GC) is considered important in the eruption process⁶ since it represents the eruption path of the tooth through the bone.^7,8

Recently, the GC has been revisited by means of panoramic radiography, multidetector computed tomography, and cone-beam computed tomography (CBCT).^6,9 This structure is described as a radiolucent/hypodense corticated canal connected to the dental follicle space.⁹ It has been suggested that the GC forms an eruption pathway, and therefore, its absence may indicate failure in tooth eruption.⁹ However, this structure is also observed in teeth with eruption failure.⁶ The relationship between failure of tooth eruption and GC characteristics remains unclear.

CBCT is not indicated on a routine basis because of the risks of ionizing radiation, especially for children. However, this method is a valuable tool for diagnosis and treatment planning of selected cases in orthodontics, including those in which interceptive orthodontic and/or surgical treatments are considered for teeth with positional or eruption anomalies.^10,11 Volumetric data allow correct identification of tooth morphology and the relationship with adjacent structures, thus contributing to clinical decision making (eg, whether to extract or to consider extrusion of a tooth).¹² Higher radiation doses are applied in CBCT imaging compared with conventional radiographic examination. Thus, when CBCT is indicated, all efforts must be made to obtain the greatest amount of clinically relevant information that potentially contributes to patient outcome. Routine use of CBCT to evaluate the presence of GC is not justified. However, if a CBCT scan has been taken because of clinically justified reasons, assessment of the presence of GC and its characteristics can contribute to treatment planning in orthodontics.

The aim of the present study was to compare detection rates of GC, its imaging characteristics, and features of its corresponding tooth between teeth with normal and abnormal eruption status (ie, delayed and impacted).

MATERIALS AND METHODS

Sample Evaluation

All scans were independently evaluated by two oral and maxillofacial radiologists in a dimly lit and quiet environment. For calibration purposes, 50 cases were evaluated by both radiologists together. After complete evaluation of the sample, the data obtained by the two observers were compared, and in cases of disagreement, a consensus was reached by simultaneous reevaluation with a third oral and maxillofacial radiologist. Patient sex and age were recorded, and each tooth was classified according to dental group (upper incisors, upper canines, upper premolars, upper molars, lower incisors, lower canines, lower premolars, and lower molars). The following parameters were assessed: eruption status (normal, delayed, or impacted), formation status (crown formation, root formation, open apex, or closed apex), angulation (normal, inclined, horizontal, or inverted), and GC detection (detected, not detected, indistinguishable from the periodontal ligament space [PLS] of the predecessor tooth or indistinguishable from an alveolar bone resorption process). If GC was detected, the site of its opening in the alveolar crest (buccal, lingual, or central, or in the PLS of the predecessor tooth) and the site of its attachment to the dental follicle (Figure 1) were further assessed.

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Regarding eruption status, the teeth were classified as impacted when a physical barrier was detected (eg, supernumerary teeth, lack of space in the dental arch, and deviated tooth germ).¹³ Delayed eruption was defined when the unerupted tooth was intraosseous, without any visible mechanical barriers, and the difference between patient age and the mean eruption age of the dental group was greater than twice the standard deviation established for that dental group.^13–15 The mean eruption ages that served as references in our study were obtained from a previous study¹⁴ involving individuals from the same geographical region as those of the current sample because of possible variability among population groups. The mean eruption age of third molars from this geographical region was not available and was therefore adapted from a previous study¹⁵ by calculating the mean age from the 50% probability of complete eruption status for third molars according to sex. Cases that had no visible mechanical barriers and whose patient age was within the mean eruption age were classified as normal eruption.

RESULTS

Scans from 159 patients were selected (88 males and 71 females), ranging in age from 5 to 36 years (mean age 17.20 ± 8.65 years), and 762 teeth were evaluated. In 164 teeth, the presence of GC was uncertain because the hypodense area was indistinguishable from the PLS of the deciduous tooth (48 cases, mostly premolars) or from resorption of the alveolar bone ridge (116 cases, mostly molars). These teeth were excluded from the final sample for comparative purposes. To differentiate GC from alveolar bone resorption, the teeth were carefully evaluated on the three reconstruction planes, and GC had to be identified in at least two planes. The final sample for comparative statistical analysis included 598 teeth, divided into eight dental groups: upper incisors (n = 20; 15 central incisors and 5 lateral incisors), upper canines (n = 99), upper premolars (n = 91; 48 first premolars and 43 second premolars), upper molars (n = 145; 26 second molars and 119 third molars), lower incisors (n = 2; two lateral incisors), lower canines (n = 25), lower premolars (n = 62; 30 first premolars and 32 second premolars), and lower molars (n = 154; 36 second molars and 118 third molars). Considering eruption status, there were 423 teeth (70.7%) with normal eruption, 140 impacted teeth (23.4%), and 35 delayed teeth (5.9%).

Table 1 summarizes the sample distribution according to dental group, eruption status, and GC detection. In general, GC was detected in 90.6% of the cases (Figure 2). This rate was significantly lower among teeth with delayed eruption and impacted teeth (62.9% and 87.1%, respectively) compared with those with normal eruption (94.1%). These differences in detection according to eruption status were uneven among dental groups (Table 1). Considering only normal and delayed teeth, delayed eruption was found in 34.2% of teeth in which GC was not detected. In contrast, when GC was detected, delayed eruption was observed in only 5.2% of the teeth.

Table 1 Absolute Frequency and Detection Rate of GC by Dental Group, According to Eruption Status^a

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Table 2 shows the detection of GC among the different eruption statuses according to characteristics of the teeth (formation and angulation). Significant differences in GC detection were found for the formation status of teeth in normal eruption (P = .0001) and in impacted teeth (P = .003), with higher detection rates in the earliest stages of tooth formation. Most teeth with a closed apex had an abnormal eruption status, and GC detection was lower (62.5% for delayed teeth and 75% for impacted teeth).

Table 2 Absolute Frequency and Detection Rate of GC According to Tooth Formation and Angulation Among the Different Eruption Statuses

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The characteristics of GC are shown in Table 3. The anterior and premolar groups were pooled since they represented teeth with primary predecessors. The most common attachment sites of GC to the dental follicle were on the occlusal aspect of the follicle (93.2%) and centrally in the buccal-lingual and mesio-distal directions (96.3% and 90%, respectively). Other locations ranged from 1.1% to 7%. Therefore, the cases in which the attachment was located on the incisal/occlusal aspect of the follicle and centrally in the buccal-lingual and mesio-distal directions were classified as usual location (87.1%), while those at different sites were classified as unusual location (Figure 3).

Table 3 Absolute Frequency and Statistical Significance of CG Characteristics Observed in Teeth With (Anterior/Premolars) and Without (Molars) Predecessor Primary Tooth for Normal, Delayed, and Impacted Teeth^a

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The sites of GC opening in the alveolar crest differed significantly between normally erupting and impacted teeth in the anterior/premolar group (P < .05). Teeth with normal eruption exhibited unusual locations of GC attachment to the dental follicle in 7% of the cases, while the prevalence of unusual locations was significantly higher (29.2%) in teeth with abnormal eruption (P < .05).

DISCUSSION

The detection of GC among the dental groups ranged from 72% for lower canines to 96.6% for upper molars, regardless of eruption status. Detection rates of the GC ranging from 43.7% to 100% have been reported,⁹ and teeth with delayed eruption exhibited the lowest detection rates.⁶ In the present study, the highest detection rates were observed for normally erupting teeth among all dental groups (94.1%), followed by impacted teeth (87.1%) and teeth with delayed eruption (62.9%).

When the GC was detected, only 5.2% of the teeth had delayed eruption. However, when the GC was not detected, delayed eruption of the teeth occurred in 34.2% of the cases. This corresponded to a six-fold higher chance of teeth having delayed eruption when the GC was not detected.

There is no consensus in the literature about the existence of GC in permanent teeth without primary predecessors (ie, permanent molars). It is believed that their dental germs originate directly from a posterior extension of the dental lamina.¹⁶ Some early reports suggested the presence of GC in permanent molars and named them molar gubernacular cord, while others¹ stated that this structure was exclusively found in permanent teeth with primary predecessors. The current results confirmed the presence of GC in molars. In fact, the highest rates of GC detection were observed among molars. However, the presence of this anatomical structure in primary teeth is still unclear, and further studies are required.

Among teeth with abnormal eruption, those with a horizontal position exhibited slightly higher GC detection rates than those with more favorable positions (normal and angulated). It may be speculated that angular deviation in a tooth germ with abnormal eruption impairs or delays resorption of the gubernacular cord. Thus, this structure remains detectable even though the tooth is unlikely to erupt. Further studies of delayed eruption are necessary to clarify this finding.

In teeth with predecessors, the GC opening was found mainly on the lingual aspect of the alveolar crest. In molars, the GC opening was usually located in the center of the alveolar crest. These findings were consistent with previous knowledge about the origin of the GC in anterior and premolar teeth, where it arises from the predecessor tooth germ, and in molar teeth, where its origin is a posterior extension of the dental lamina.¹⁶ Unusual attachment sites of the GC to the dental follicle were significantly more common among teeth with abnormal eruption status. These results suggest that, if the GC is seen connected to the follicular space in less typical locations on CBCT, the teeth are more likely to have abnormal eruption.

The process of tooth eruption is not fully understood. Studies have demonstrated the involvement of alveolar bone, dental follicle, osteoclasts, osteoblasts, and cytokines.¹⁷ The dental follicle seems to play an essential role in triggering bone remodeling of the alveolar process, which is required for tooth eruption.¹⁷ The gubernacular cord, as an extension of the dental follicle, may also have some influence on this complex process. The results suggested that observing the presentation of the GC on CBCT may help anticipate tooth eruption problems. However, numerous other factors also affect the eruption process, such as hormonal, systemic, and morphological factors related to jaw growth and development.^18,19

Detection of the GC might not be straightforward in some cases because of variations in trabecular bone microarchitecture or proximity to primary teeth. Particularly in premolar regions, the hypodense band corresponding to the periodontal ligament of primary teeth may be indistinguishable from a GC of the permanent successor germ. Also, in the molar region, a large hypodense area is commonly seen above the tooth germ. In this case, distinction between the GC and the resorption process requires careful evaluation.

Limitations of this study were inherent to the limitations of cross-sectional studies. The data collected in this study corresponded to the imaging features at a specific time point and did not show the sequences of events during tooth eruption and variations in the GC that may have occurred over time. Longitudinal studies should be conducted to further understand the changes that the GC undergoes during tooth development and eruption, as well as the possible effects of GC features on the eruption process. However, the use of ionizing radiation for this purpose would not be justified.

The present study adds information about the presentation of the GC, which may contribute to clinical decision making in cases in which CBCT is available. Requesting a CBCT to assess the presence and imaging characteristics of the GC remains unjustified. Principles of radiation protection such “as low as reasonably achievable” (ALARA) and “as low as diagnostically acceptable” (ALADA)^20,21 must be followed, especially because patients with unerupted teeth are usually children or young adults who are more vulnerable to the potential biological effects of ionizing radiation.

CONCLUSIONS

• GC detection rates are lower among teeth with abnormal eruption status, and careful evaluation is therefore recommended when this structure is not detected.

• When the GC is attached to the dental follicle at an unusual site, the corresponding tooth is more likely to have an abnormal eruption process.

• Further investigation of these parameters can contribute to treatment planning involving unerupted teeth, whether to consider tooth extraction, extrusion, or monitoring.