Clinicians performing pulp treatment of maxillary first molars in pediatric patients often encounter a wider palatal root than expected. This occurs even when the buccal root apices are nearly closed on panoramic and periapical radiographs. In growing patients, multi-rooted teeth may have different developmental statuses at each root apex. The mesial roots of the mandibular first molars close earlier than the distal roots, and the buccal roots of the maxillary first molars close earlier than the palatal roots[1,2]. This variation in the developmental status of each root may lead to the selection of a different endodontic treatment method for each root; therefore, the practitioners need to determine the developmental status of the tooth before treatment[1,3]. To the best of our knowledge, there have been few studies on variations in apical closure timing.

Determining root development in maxillary first molars using panoramic and periapical radiographs is particularly challenging[4]. Numerous structures are superimposed on the root, making it difficult to determine whether the root is closed, and the palatal root is often observed inside the maxillary sinus[5,6]. This led to the exclusion of maxillary molars from studies measuring dental development[7,8]. Therefore, cone-beam computed tomography (CBCT) can be useful for studying root development in the maxillary first molars.

In the current study, we examined the CBCT scans of patients aged 7 - 11 years who visited the Wonkwang University Daejeon Dental Hospital to determine whether there is a difference in the timing of apical closure for each root of the maxillary first molar and to investigate the chronological age at which the apex of each tooth root closes.

The Wonkwang University Institutional Review Board (IRB) granted ethical approval (No: 202404-018).

Before we can study the variation in the root development of each root in the molars, we should define the criteria for an open apex. The diameter of the apical constriction, which is the criterion for an open apex, is not agreed upon and varies widely from an ISO minimum size of 40 to 100, depending on the authors[12-20]. Moreover, it is unclear why each value was selected as an open apex criterion. One study noted the challenge posed by the absence of criteria for distinguishing between immature and mature root apices[21]. Another author emphasized the need for a precise definition of an open apex[22]. The absence of a clear definition and the lack of a rationale for it also presented challenges in our study. To provide clinical relevance, we first referenced the apical diameter that shows sufficient success rates in non-vital pulp treatment of open apex teeth. In teeth with a wide-open apex, regenerative endodontic treatment (RET) is commonly chosen[23]. In several studies on apical diameters appropriate for RET, the common finding was that RET was successful enough for diameters greater than 0.5 mm[24,25]. In addition, a recent study by Alghofaily et al.[19] has defined the apical closure criterion as 0.5 mm in diameter. For these reasons, we set the apical closure criterion at 0.5 mm.

When using the 0.5 mm criterion to investigate the age of apical closure in the maxillary first molars, differences between genders were observed. At age seven, sex differences were observed in four out of six roots, and at age eight, in three out of six roots. At age 9 years, there was no difference between the sexes; at age 10 years, only the P of UL6 showed a significant difference; and at age 11 years, there was no difference. For all roots with significant differences, males had larger root apex diameters than females, suggesting that root apex completion might occur earlier in girls. In one study, significant sex differences were observed in the root development of maxillary first molars[26]. Further investigation in a larger sample is needed to determine whether there are differences in the timing of root-tip closure between sexes.

A difference exists between the left and right root apical diameters in 10-year-old DB; however, when root development was categorized by 0.5 mm, both left and right DB roots were in the same developmental state with an open apex. Other studies have observed no differences between left and right tooth development[26-29].

Among the stages of tooth development, determining the age at the apical closure stage is particularly challenging. In most studies that investigated the association between tooth developmental stages and chronological age, the age of the apically closed stage could not be examined because, unsurprisingly, there is no upper age limit for this stage[6,29]. The chronological age of apical closure, the final stage of Demirjian tooth development, has been widely studied in third molars[30-33]. Most of these studies set the upper age limit for the apical closed stage at 26 years, but the reason for this upper age limit is not clear. One study stated that the age range of the patients had a large influence on the mean age, but they could not find a reason for this age range[33]. Therefore, other studies took a more indirect approach, examining the median of the developing stage at each age rather than the average age of the apical closure stage[26]. Similarly, in this study, we examined whether the mean apical diameter became smaller than 0.5 mm at each age. To determine the more precise age at which each root apex closes, regularly taking CBCT scans of the same individual would be useful. However, this approach poses significant issues due to high radiation exposure. Therefore, more refined measurement methods and alternative approaches to accurately assess age are necessary.

Kronfeld[34] found that the age of root completion in maxillary first molars is 9 - 10 years, which is still widely accepted. However, this study used two-dimensional radiographs and did not address the differences between the buccal and palatal roots. In another study examining the association between tooth development and chronological age, the minimum age at which the maxillary first molar root apex closed was 9 years. This study also used two-dimensional panoramic radiographs[26].

However, the use of two-dimensional radiographs for the observation of maxillary first molars has had limitations. The roots of the maxillary first molars are located close to the maxillary sinus floor and are prone to overlap, distortion, malposition, rotation, and crowding; therefore, maxillary teeth have been excluded from dental development studies, and studies have been conducted primarily in the mandible[5,35]. Moreover, maxillary teeth were less reliable than mandibular teeth in a study of root developmental stage assessment of permanent teeth, and the maxillary first molars were the least reliable of all permanent teeth, except the third molars, in the dental age estimation of Demirjian, Nolla, and Moorrees[4]. Owing to these limitations, using panoramic and periapical radiographs alone makes it difficult to observe the differences in the developmental stages of the buccal and palatal roots of the maxillary first molars. Therefore, unlike previous studies, this study used CBCT to show a 2-year difference in the timing of buccal and palatal root completion.

The apical diameter is important in non-vital pulp therapy for immature permanent teeth for several reasons. Forming an apical stop is challenging, which makes it difficult to achieve an apical seal[22,36]. Electronic apex locators and radiographs should be used in combination for working length determination (WLD)[22,37]. Additionally, if RET is required, the apical diameter needs to be at least 0.5 mm[24,25]. Moreover, the likelihood of irrigant extrusion increases in immature teeth with an open apex[38-40]. Clinicians unfamiliar with the pulp treatment of immature permanent teeth may not anticipate these considerations and may experience unexpected challenges. If the clinician sees only the buccal root in the panoramic view, determines that all the roots are closed, and plans to treat all the roots with conventional root canal therapy, the clinician may have to change the treatment plan abruptly after treatment has begun.

The limitations of this study include the fact that the study population was comprised of patients presenting with malocclusion; therefore, it may not be representative of the general population of children. In addition, CBCT studies lack precision compared with studies in adults. Studies in adults on the apical root morphology and diameter of maxillary first molars have been much more precise, using micro-CT and scanning electron microscopy observations of extracted teeth[41-43]. However, it is difficult to precisely study immature permanent teeth. The age range of immature permanent teeth is smaller than that of adults, and it is difficult to obtain specimens of immature permanent teeth extracted in the absence of apical lesions. Methods that are more precise than those used in the current study would require further investigation.

Another limitation is that this CBCT study used a threshold of 0.5 mm for apical closure but included periapical radiograph studies about RET as the basis for this threshold. We reviewed studies that compared linear measurements between periapical radiographs and CBCT images. In a study related to WLD measurements, there was no significant difference between the linear measurements on periapical radiographs and CBCT[44]. In a study comparing linear measurements from CBCT, periapical radiographs, and intrasurgery in maxillary molars with furcation defects, CBCT showed no significant difference from intrasurgical measurements in both height (vertical furcation bone loss) and width (furcation width) of furcation defects, whereas periapical radiographs showed no difference from intrasurgical measurements in width, but did show a height difference[45]. Further studies are needed to compare the linear measurements on periapical radiographs and CBCT axial views of the maxillary molars.

In this study, we suggested 0.5 mm as a criterion for the AD of the open apex. Using CBCT, we investigated the age at apical closure in the maxillary first molars. The age at apical closure of the buccal roots of the maxillary first molars was 9 years, and the age at the closure of the palatal roots was 11 years, indicating a difference of 2 years.