– DBAT: the shortest distance between the crowns of the teeth adjacent to the BES;

– POT: a measure that required the drawing of two lines—one drawn from the cusps of the teeth adjacent to the tooth that directly opposed the BES and a second drawn perpendicularly from the first line to the cusp tip of the tooth opposing the BES, the latter line being measured as the POT;

– ABL: the distance from the cementoenamel junction, or CEJ, to the alveolar crest on the distal aspect of the premolar and the mesial aspect of the molar, measured using previously reported techniques.^11–14 (The crest of the alveolar bone was defined as the most coronal level where the periodontal membrane retained its normal width.) If the CEJ was not visible owing to restoration, the examiners used the gingival border of the restoration. In either case, the same landmarks were used in both the baseline and follow-up radiographs.

At sites where vertical defects were present, the examiners used the bottom of the defect as the apical landmark. Measurements were taken only in instances where reproducible landmarks were visible in both the baseline and the follow-up radiographs. Thus, not each case contributed to each of the four clinical measures.

The two examiners performed all measurements, independently determining landmarks and measuring each case. For each measure—DBAT, POT, PABL and MABL—the examiners calculated the change score by subtracting the distance measured on the baseline radiograph from that taken from the most recent follow-up radiograph. Thus, space loss was reflected by a negative score, while loss of alveolar bone or tooth extrusion was reflected by a positive score. When the change scores of the two examiners differed by more than 0.5 mm or there were differences in perceived "readability" of landmarks (7 percent of all measurements), the two examiners reviewed the cases together and obtained a consensus measurement. For the cases in which the difference between the two examiners’ change scores was 0.5 mm or less, the mean of the two was used in the analysis.

Angular alignment errors that contribute to distortion in radiographic films typically are attributed to film packet placement errors and/or improper tubehead position. In this study, the effect of packet placement errors was considered to be minimal because all clinical films measured were bitewing radiographs exposed using commercially available bitewing tabs attached to conventional periapical films. To evaluate the magnitude of measurement error that could be introduced through improper tubehead positioning, we constructed a jig that allowed for proper film placement parallel to the mesiodistal dimension of the sextants and permitted evaluation of the effect of tubehead angulation on measurements obtained from these radiographs. We determined by trial-and-error exposures of bitewing radiographs that tubehead misalignment in excess of 15 degrees in any plane would generally produce a radiographic image with a degree of distortion that would render the film unacceptable for clinical diagnosis. Accordingly, we set the jig so that it would permit us to make radiographs of a simulated dental sextant at 15 degrees of vertical and/or horizontal tubehead misalignment to gauge the effect of parallax on measurements of nonstandardized radiographic films.

We mounted extracted human teeth in a mixture of dental stone and sawdust to simulate a mandibular posterior sextant composed of a third molar, a second molar, an edentulous space subsequent to the loss of a first molar, a second premolar and a first premolar. Similarly, we created an opposing maxillary posterior sextant and mounted it in the maximum intercuspation position to simulate that used when a bitewing radiograph is made. Indexing slots were placed in the frame to allow accurate orientation of the tubehead.

We made nine exposures using Kodak Ektaspeed Plus film (Eastman Kodak Company), one with zero vertical and horizontal angulation and eight with various combinations of zero and 15 degrees of horizontal and vertical angulation. From the resulting set of films, each of the four clinical parameters was measured by one rater at two different times with the mean used in the analyses. We estimated the extent to which image distortion resulted in measurement error by comparing the distances measured on radiographs exposed from eight angles with the measurement obtained from the radiograph made by the perpendicular central beam. The maximum absolute difference in the measurements from the perpendicular beam and the angulated films was 0.5 mm for the DBAT, 0.9 mm for the POT, and 0.3 mm and 0.7 mm for the PABL and MABL, respectively.

Analysis. We calculated frequency distributions of the change in millimeters between follow-up and baseline measurements for all four clinical parameters. We assessed associations among these change scores and time between radiographs by means of the Pearson product moment correlation. In 15 of the cases, an adjacent tooth was lost during the study period; in these cases, we measured the most recent radiograph in which both adjacent teeth were present was measured. Thus, changes in tooth position in these cases were examined separately. Finally, we compared change in the cases in which baseline radiographs were taken pre- and postextraction to estimate the amount of undermeasurement of movement that could occur.

	RESULTS

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The median time between the baseline and last follow-up radiograph for the 111 cases with no further tooth loss (the no-tooth-loss, or NTL, group) was 6.9 years, and the range was 1.1 to 9.6 years. For the 15 cases in which an adjacent tooth was lost (the tooth-loss, or TL, group), the median time was 2.5 years, with a range of 0.9 to 6.7 years. Table 1 presents the percentage distributions of changes in DBAT, POT, MABL and PABL for both the NTL group and the TL group.

View larger version (43K): [in this window] [in a new window]   Figure 2. Distribution of cases by amount of space lost between teeth adjacent to bounded edentulous spaces in patients who had had no further tooth loss (n = 111).

View larger version (46K): [in this window] [in a new window]   Figure 3. Distribution of cases by amount of extrusion of teeth opposing bounded edentulous spaces in patients who had had no further tooth loss (n = 111).

View larger version (45K): [in this window] [in a new window]   Figure 4. Distribution of cases by amount of loss of alveolar bone level around teeth adjacent to bounded edentulous spaces in patients who had had no further tooth loss (n = 111).

	DISCUSSION

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Information about the consequences of untreated BES is essential if dentists are to make informed treatment decisions. Although we have few good data about this condition, we can begin to examine existing data sources to provide some answers. As we conduct these retrospective analyses, however, we need to keep in mind certain limitations. For example, in this study, the limitations include potential selection bias and the use of unstandardized radiographs. It is likely that selection bias occurred within this sample of cases, as dentists tend to provide fixed partial dentures to patients for whom they believe the prognosis is relatively good, relegating many of those with a poor prognosis to the untreated category. Thus, many of the patients in this sample may represent those whom the treating dentists felt were not good candidates for restorative care. In contrast, if this were a controlled trial and assignment to the untreated category were truly independent of other factors, the consequences likely would be even less severe.

Unstandardized radiographs also can introduce measurement error through the imprecision of selecting "reproducible landmarks" on both baseline and follow-up radiographs. The extent of this error, however, was reduced by using two examiners, with each independently making measurements and requiring a rather strict level of agreement: 0.5 mm. Unstandardized radiographs taken at different angulations also can introduce error in measurements. The average amount of difference between properly oriented and angulated radiographs is less than 0.5 mm. These differences suggest that the amount of error introduced by the use of films exposed at rather divergent angles is similar to the amount of error in the measurement process. Although the requirement for useable radiographs reduced the sample size from an original 239 cases to 126 in these analyses, the median years of follow-up and the percent of cases with subsequent loss of an adjacent tooth changed little, from 6.7 years to 6.9 years and from 13 to 12 percent, respectively. Thus, selecting cases with adequate radiographs did not appear to introduce survival biases.

Arch collapse, or tilting of the teeth adjacent to the BES, did not seem to cause a narrowing of the space to the degree that conventional wisdom would suggest. Across all cases, the mean loss of space was about 1 mm over the study’s six-plus years. Of particular note, however, is that 6 percent of the cases lost more than 3 mm of space between adjacent teeth. This may have practical relevance, as this amount of space loss could signal arch collapse—thus making restoration more difficult or prompting the need for orthodontic treatment.

The association between the time since extraction with the decrease in the width of BES was relatively small but approached statistical significance. This finding suggests that, on average, the space continues to narrow over time. However, the analysis of only the baseline and the single most recent follow-up radiograph did not permit us to comment on how that space loss progresses. For example, there could have been space loss within the first year or two that then was stable for the balance of the time of observation. Alternatively, the space could have been stable for a number of years and then a substantial loss of space could have occurred—or the loss of space could have been linear, a constant loss over time. The follow-up time, which ranged from 1.1 to 9.6 years, may not have been sufficiently long to allow for the detection of more substantial changes, if change indeed continues as time since extraction increases. Thus, future studies need to document the rate at which and the pattern in which movement occurs over time.

A common concern about untreated bounded edentulous space is the adverse consequences for the periodontal health of the adjacent teeth. These results reveal that the average change in PABL was 0.4 mm and in MABL was 0.3 mm. Both of these are well within the ranges of changes in bone levels reported for patients with no BES in other longitudinal studies.^15,16 In addition, there were no differences in the mean amount of alveolar bone loss, regardless of the type of tooth extracted. Eight percent of cases exhibited greater than 2 mm of loss of MABL or PABL, which is greater than what would be expected within this time¹⁶ and is cause for clinical concern. Interestingly, however, only premolar loss of alveolar bone level is associated with time since extraction and with narrowing of the inter-tooth BES distance.

Although we did not analyze differences between the NTL and TL groups statistically because of the small TL sample size, the TL group tended to display more bone loss and somewhat more loss of intra-abutment space. This trend may suggest greater risk of loss of adjacent teeth in cases with a rapid rate of change. Thus, future studies should attempt to quantify the rate of change in space loss, as this may be a predictor of the survival of the adjacent teeth.

Measuring extrusion or supraeruption by noting the change in the position of the opposing tooth was the most difficult measurement to make, and it should be viewed with caution. This measure relied on the examiners’ identifying landmarks on the adjacent teeth and then constructing a perpendicular angle to the tooth of interest. Minor changes in angulation, as well as inherent measurement error, made this a difficult parameter to assess. In any event, the average amount of change was 0.2 mm and was not associated with time since extraction.

	CONCLUSIONS

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Within the limitations imposed by the design of our study, it appears that arch collapse is not as rapid or severe as conventional wisdom would suggest. Similarly, the effect of an untreated BES on adjacent periodontal structures is significant in only a very small number of cases. However, some small number of patients, perhaps 10 percent, experience clinically significant tilting of the teeth adjacent to the BES and loss of the alveolar bone levels of these teeth. These findings suggest that for the large majority of patients in this age range who experience a posterior BES, immediate treatment may not be critical to maintaining arch stability over the course of the first seven years after extraction. These results also suggest that the profession needs to work diligently toward identifying the factors that do predict adverse consequences or that put this small proportion of patients at risk of experiencing arch collapse.