Another major problem with investigations of this topic has been an insufficient number of subjects to conduct rigorous controlled studies. Subjects should have experienced unilateral loss of a primary molar, with an intact symmetrical contralateral molar serving as a control, to enable investigators to assess the differences in measurements from the initial examination to the follow-up examination.¹⁷ Interestingly, patients with premature loss of a primary molar often undergo subsequent extractions of primary molars because of a high caries rate. As a result, it often is difficult for investigators to recruit a sufficient number of subjects to conduct a rigorous scientific investigation.

To our knowledge, limited and inconsistent data have been reported in the literature regarding premature loss of primary maxillary first molars. In 1949, Liu¹⁸ performed the first known extraction-site study (cross-sectional) of dental space loss and found that the loss of a primary maxillary molar resulted in a mean loss of 2.2 millimeters of space for the first molar and 2.49 mm of space for the second molar. In 1952, Jarvis¹⁹ conducted a cross-sectional study and reported that space loss averaged 0.13 mm for the 20 male subjects and 1.37 mm for the three female subjects. In 2000, Northway²⁰ conducted a longitudinal review of 13 cases of premature loss of primary maxillary molars; the results showed a reduction in arch length and a mesial displacement of the permanent canine, which eventually became virtually blocked out.

Lin and Chang¹⁷ used a strict sampling regimen and longitudinal data in their study of the premature loss of mandibular primary first molars. Our study continued this focus and we used this established protocol. We wanted to explore space loss problems with regard to maxillary dentition. The aim of this study, therefore, was to use longitudinal data from dental casts to investigate the space loss problems that may have arisen as a result of unilateral premature loss of a primary maxillary first molar.

	SUBJECTS, MATERIALS AND METHODS

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Nineteen children (13 boys and six girls) with premature loss of a primary maxillary first molar were recruited from the Children’s Dental Clinic of the Chang Gung Memorial Hospital—Kaohsiung Medical Center, Taiwan. All subjects met the inclusion criteria specified by the protocol described in our earlier work,¹⁷ including the following:

– No major craniofacial disease was apparent.

– The permanent first molars were about to erupt or had just erupted.

– The patient was cooperative in finishing dental treatment before impressions were obtained.

– The maxillary dentition featured a unilateral premature loss of a primary first molar due to extensive caries but had intact contralateral primary molars.

– Premature loss of the primary molar was defined as the absence of a permanent tooth after extraction of the primary molar; however, the permanent tooth eventually would erupt into the space.

– Parents or guardians must not have wanted their child to receive dental treatment involving the use of a space maintainer.

– All parents of the children included in this study signed a consent form. Ethical approval for this study was granted by the institutional review board of Chang Gung Memorial Hospital.

A research assistant made maxillary study casts from alginate impressions that were obtained two or three days after one of us (Y.-T.L. or W.-H.L.) extracted the tooth. None of the study subjects received a fixed space maintainer during the entire follow-up period. All patients had a recall appointment six months after their tooth was extracted, and we obtained longitudinal study casts to compare with the initial study casts.

The six reference lines used as test parameters were measured directly from the reference points on the dental casts. One of two experienced researchers (Y.-T.L., W.-H.L.) determined the lines in duplicate using an electronic digital caliper, which was accurate within 0.01 mm.

Cast measurements. The researchers measured six reference lines pertaining to dental-arch development, including the primary molar (D + E) space, arch width, arch length, intercanine width, intercanine length and arch perimeter. We defined these parameters as follows:

– D + E space: the distance between the mesial midpoint of the permanent first molar (or the distal midpoint of the primary second molar if the permanent molar was missing) and the distal midpoint of the primary canine (Figure 1)¹⁶;

– arch width: the distance between the central fossa on the occlusal surface of bilateral primary second molars (Figure 2);

– arch length: the distance from the contact point of the central incisors perpendicular to the arch width (Figure 2);

– intercanine width: the distance between the cusp tips of the two primary canine teeth (Figure 1);

– intercanine length: the distance from the contact point of the central incisors perpendicular to the intercanine width (Figure 1);

– arch perimeter: the arc measured from the mesial midpoint of the permanent first molar (or the distal midpoint of the primary second molar if the permanent molar was missing) through the cusp tip of the canine and the incisal edges of the incisors to the opposite mesial midpoint of the permanent first molar (or the distal midpoint of the primary second molar if the permanent molar was missing), as measured with the aid of brass wire (Figure 1).

View larger version (16K): [in this window] [in a new window]   Figure 1. Measurements of the primary molar (D + E) space, arch perimeter, intercanine width and intercanine length.

View larger version (11K): [in this window] [in a new window]   Figure 2. Measurements of the arch width and arch length.

Interexaminer and intraexaminer reliability tests. We used reliability coefficients to compare the consistency and reliability between the intraexaminer and interexaminer groups using an electronic digital caliper for each cast. We defined a statistically significant difference as P < .05. The two examiners (Y.-T.L., W.-H.L.) performed and recorded the measurements for 20 casts obtained from 10 subjects for the purpose of reliability testing; they repeated the measurements three weeks later.

The means and standard deviations of six measurements for each of the 20 casts were the parameters used to compare the interexaminer and intraexaminer groups. Table 1 shows the results of reliability coefficient testing for the interexaminer and intraexaminer groups with regard to the D + E space, arch width, arch length, arch perimeter, intercanine length and intercanine width. The intraclass and interclass reliability coefficients for each measurement differed significantly from zero (P < .01). Indexes of reliability greater than 0.900 for the interexaminer and intraexaminer groups denote measurements that were highly consistent and reliable.

	RESULTS

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The ages of the 19 subjects ranged from 4.1 to 7.1 years, with a mean (± standard deviation) age of 5.9 ± 0.74 years at the time of tooth extraction. Table 2 shows the D + E space changes between the initial examination and the six-month follow-up. Table 3 presents the measurements for the five other parameters at the initial and six-month examinations. As Table 2 shows, the D + E space on the extraction side did not differ from the space on the control side at the initial examination. However, at the six-month examination, the D + E space on the extraction side was significantly smaller than the space on the control side; this space also was smaller than the initial D + E space on the extraction side (Table 2).

About 1 mm of space was lost from the initial examination to the six-month follow-up examination; although this is statistically significant, it is of limited clinical significance.

	DISCUSSION

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Conducting precise studies of spatial losses occurring in the dental arch remains difficult owing to the limited number of qualified subjects and a range of multifactorial issues that might influence the results. These factors include age at the time of tooth loss, facial and dental growth potential, status of dental interdigitation, oral habits and study methodology. Our study focused on subjects who underwent unilateral extraction of a primary maxillary first molar during a certain period of arch development. We excluded potential subjects who had rampant caries or craniofacial anomalies.

Another potential problem with this study’s methodology was the inability to assess the unstable incisor point while attempting to measure the incisal region. Changes in incisor position as a consequence of proximate tooth extraction can induce some variations in arch length and arch perimeter, especially during incisor emergence. In this study, we measured arch width, intercanine width, arch perimeter and D + E space as compensatory surrogates for the influence of the incisor. The D + E space, as reported by Northway and colleagues¹⁶ in 1984, reveals a limited range of factors that are able to influence the spatial changes occurring in the mouth after extraction of the primary maxillary first molar; thus, the D + E space appears to be an appropriate variable for measuring the posterior segment of the primary dentition.

Spatial changes in the mouth on the side of the extracted tooth, as measured by the D + E space six months after tooth extraction, revealed significant spatial loss compared with the D + E space at the time of extraction (and compared with the control side) (Table 2). With regard to arch dimension, the results show a significant reduction in arch length and a significant increase in intercanine width six months after the tooth extraction; however, we found no significant differences for arch width, arch perimeter and intercanine length (Table 3).

For most of the subjects, the primary canines and primary incisors on the extraction side drifted distally during the first six months after the tooth extraction; these changes were responsible for the reduction in D + E space and arch length during this period. In 1998, Cuoghi and colleagues²¹ noted similar results with regard to space loss in the mandibular arch after extraction of the primary first molar, which, they concluded, was due primarily to distal movement of the primary canine and incisors toward the site of the extracted tooth. Our previous study pertaining to the premature loss of the primary mandibular first molar also yielded similar results¹⁷; however, the lingual drift of the anterior incisors on the mandibular arch was not as significant as that found on the maxillary arch in this study.

The results of these studies indicate that closure of the gap resulting from premature tooth loss on either the maxillary or mandibular arch occurs mostly via distal movement of anterior teeth. Furthermore, the results of our study also appear to coincide with those of some published reports^22–25 that suggest a greater level of sagittal alteration in the maxilla than in the mandible after exfoliation of primary teeth.

A number of studies have attributed the reduction in arch length to the palatal erupting path of the permanent maxillary central incisors during the transition of the dental arch from primary to permanent dentition. In 1963, Brant⁶ reported that movement of the anterior teeth to a more upright position, rather than mesial movement of the posterior teeth, closed the extraction-site gaps of the removed primary molars. In the 1940s, Turner^7,26 measured the distal shift of teeth clinically and found that the distal inclination of developing tooth buds in their crypts might explain the distal movement. In 1949, Liu¹⁸ also suggested that closure of the gap remaining after extraction of a primary molar might have occurred as a result of incisors pushing adjacent teeth distally.

In this study, we also observed a significantly greater intercanine width and no apparent change in intercanine length six months after tooth extraction (Table 3). A possible explanation for this is that the increased intercanine width after premature loss of the first molar compensated for the extraction-associated reduction in arch length, the combination of which resulted in no net change to intercanine length. It would be interesting to find out whether the permanent maxillary incisors will erupt in a more labial position, or whether they are likely to remain in the position in which the primary teeth were extracted, a situation that may result in some space deficiency for the anterior teeth.

It appears that clinical dental students are taught that a space maintainer is necessary to prevent space loss immediately after extraction of a primary tooth.^4,8–11 Space maintainers—such as a band and loop, distal shoe, Nance appliance, lingual holding arch, transpalatal arch and removable acrylic saddle appliance—have been recommended for many clinical situations.^14,15 However, the supporting evidence is somewhat weak,¹⁷ on the basis of the results of our study. In addition, clinicians need to consider a variety of clinical situations and factors, as described above, before using space maintainers.

The findings from our study are valuable, but they reveal only part of the scenario pertaining to premature loss of primary teeth and changing dental architecture. The mesial movement of permanent molars and/or the tilting of primary molars after premature removal of primary first molars appears to be of concern to dentists, although these problems do not appear to occur before the six-month follow-up examination.⁴

Space maintainers may be needed when attempting to prevent arch-length loss after eruption of the permanent incisors. With regard to using band-and-loop space maintainers to preserve the extraction space, however, the results of our study indicate that this practice is not necessary for the maxillary arch. However, the results of our study indicate that the use of a palatal arch to prevent palatal movement of incisors may be more valuable than the use of band-and-loop space maintainers with regard to preserving the existing dental arch.

Terlaje and Donly¹⁵ reported similar findings in their review of treatment planning for space maintenance. They suggested that no treatment be administered for unilateral loss of a primary first molar in patients in whom the permanent first molar had erupted, unless leeway space was to be preserved. They reported that space maintenance in the mixed dentition is not as crucial as it is in the primary dentition, because the permanent first molar has already erupted and is passive, thereby not producing a mesial component of eruption force.¹⁵

Northway²⁰ observed cases of premature loss of the primary maxillary first molar and concluded that there was no significant mesial migration of the permanent molar, but that the first premolar erupted in a more mesial direction, causing permanent canine blockout. Furthermore, Northway²⁰ recommended mesial slicing of the primary second molar and use of a space regainer such that drift of the first premolar would not occur in a mesial direction and canine blockout could be prevented.

Although we found some statistically significant differences in our study, we note limited clinical significance in that only about 1 mm of space was lost from the initial examination to the six-month follow-up examination. Therefore, further longitudinal studies are needed to elicit more information regarding space loss after premature extraction of primary molars.

	CONCLUSIONS

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The early space changes in the maxillary dental arch after premature loss of a primary maxillary first molar consist mainly of distal drift of the primary canines toward the site of the extraction and palatal migration of the maxillary incisors. We detected no space changes with regard to arch width or arch perimeter, suggesting that the mesial movement of permanent molars probably did not occur as a consequence of the tooth extraction. The increase in intercanine width is remarkable in that it compensated for the space loss due to palatal movement of the maxillary incisors, resulting in no overall changes to the arch perimeter. Although 1 mm of space loss occurred, which is statistically significant, this is not likely to be of enough clinical significance to warrant use of a space maintainer. If palatal movement seems to be needed, clinicians should consider use of a palatal arch instead of a band-and-loop maintainer.