An Occlusal and Cephalometric Analysis of Maxillary First and Second Premolar Extraction Effects

The role of extractions in orthodontic treatment has been historically controversial.1–5 This controversy continues today and, with the exception of whether or not to actually undertake treatment at all, the extraction decision is still the most critical decision made by orthodontists when planning treatment.6 This is further complicated by the fact that the relative efficacy of extraction or nonextraction strategies, in either the short or long term, has yet to be fully established.7 

A number of previous workers have documented that premolars are the most commonly extracted teeth for orthodontic purposes.8–10 Conveniently located between the anterior and posterior segments, premolar extractions would seem to allow for the most straightforward relief of crowding or the correction of an unacceptable interincisor relationship.11 It has been suggested that improvements in techniques for controlling movements of teeth in 3 dimensions and improvements in the correlation of these movements with anticipated facial growth changes have both increased the number of extraction options.12 In fact, different authors, for various reasons, have recommended variations in extraction sequences, including maxillary and mandibular first and/or second premolars.12–16 

It is well accepted that, during orthodontic treatment involving the extraction of teeth, arch dimensional changes occur and that these dimensions continue to change following active treatment.17–21 Quantification of these changes in the maxillary arch, however, has only recently been provided.22 Furthermore, the ability for maxillary extraction spaces to be used in a predictable fashion has not yet been widely presented in the literature.23–25 In one such study, Williams and Hosila24 found that, in cases involving the extraction of 4 first premolars, approximately 66.5% of the available extraction space was taken up by the retraction of the anterior segment. In cases involving extractions of maxillary first and mandibular second premolars, 56.3% of the available extraction space was taken up by the retraction of the anterior segment.

Since it is generally accepted that a strong relationship exists between root surface area and anchorage potential, the choice of teeth to be extracted should have a direct influence on the amount of anterior segment retraction.15,16,26 For instance, Creekmore16 stated, as a rule-of-thumb, that when first premolars are extracted, one can expect the posterior teeth to move forward approximately one-third of the space, leaving two-thirds of the space for the relief of crowding and incisor retraction. When second premolars are extracted, one can expect the posterior teeth to move forward approximately half the extraction space, leaving the remaining half for the relief of crowding and the retraction of anterior teeth.16 

Some authors have reported definite correlations between incisor movements and changes in the overlying soft tissue profile.27–31 Others have shown that changes in tooth position are not necessarily followed by proportional changes in that soft tissue profile.25,32–37 Variations in factors such as lip morphology, the type of treatment, gender, and age have all been held responsible for individual differences in soft tissue response. While it has been claimed by some that extraction treatment is likely to have a detrimental effect on the facial profile, it has now also been shown that the decision to extract teeth in orthodontic treatment does not have to compromise posttreatment esthetics if the decision is based on sound diagnostic criteria.17,18,25,38–43 

Stability following orthodontic treatment continues to challenge all orthodontists.44–46 The ability to maintain long-term alignment following orthodontic treatment involving the extraction of premolars has unfortunately also been unpredictable.20,21,47–50 Controversy still surrounds the question of whether better long-term results are achieved with extraction or nonextraction treatment, with different studies producing conflicting results.17,21,51–53 

With all this in mind, it is obvious that there are some differences in the dental and facial effects of extraction and nonextraction treatment. The influence, however, of various extraction sequences in these areas has been largely derived from anecdotal clinical observations, and there still seems to be little scientific evidence to support the choice of one sequence over another. It was, therefore, the purpose of the present study to investigate the differences in maxillary arch dimensional and positional changes following orthodontic treatment involving the extraction of either maxillary first or second premolars.

Pre- and posttreatment records of 71 premolar extraction cases treated by an experienced orthodontist with preangulated Edgewise appliances (0.018 × 0.028 inches) were obtained for this study. The cases were selected according to the following criteria:

1. All patients had 4 premolar extractions as part of their comprehensive orthodontic treatment plan. Patients with asymmetric premolar extractions within the dental arches were excluded.

2. None of the patients had any adjunctive appliances such as headgears, transpalatal arches, quad helices, functional appliances, or rapid maxillary expanders, as part of their orthodontic treatment. Interarch elastics were used as necessary.

3. All cases included a minimum of pre- and posttreatment lateral cephalograms, study casts, and treatment history records.

The age at commencement, the duration of active treatment, and the numbers of subjects in the various extraction subgroups are shown in Table 1. The average duration of fixed-appliance treatment was 26.4 months, with a range of 14 to 44 months.

Several pretreatment variables were evaluated so that changes due to any initial group differences could be distinguished from actual treatment effects (Table 2). The 3 extraction subgroups were compared statistically and the differences quantified using a one-way analysis of variance (ANOVA). Four measurements were identified as significantly different among the groups at the 95% confidence level. These variables included 2 study cast measurements (incisal overjet and Class II molar relationship) and 2 cephalometric measurements (incisor angulation and position in relation to the APog reference line). The 4/4 group required treatment of moderate amounts of overjet and Class II molar correction. The 4/5 group had the largest mean overjet (6.1 mm) and a Class II molar relationship. The 5/5 group had a mean overjet identical to the 4/4 group but a mean Class I molar relationship. Relative to the APog line, the mean maxillary incisor position was further forward in all 3 groups than the average reported by Ricketts.54 Among the subgroups, however, the 5/5 group had the least mean incisor protrusion.

The cephalometric measurements used in this study are described in Table 3 and are illustrated in Figures 1 through 3. All lateral cephalograms had been taken using the same calibrated cephalostat and were traced under the same viewing conditions, ie, in a darkened room using a light box with extraneous light blocked out. Measurements were made using the Westcef program (a customized research cephalometric analysis program written for The University of Melbourne by Mr Geoffrey West), which automatically rotates the digitized landmarks so that the pterygomaxillary (PM) vertical line through sphenoethmoidale is in fact vertical (Figure 2). The use of the PM line as a vertical reference plane for anteroposterior changes has previously been suggested.55 Absolute horizontal and vertical distances between landmarks were measured relative to the X and Y coordinates of those landmarks. To evaluate maxillary incisal changes, tracings were superimposed on the palatal plane registered at anterior nasal spine (ANS), as described by Ricketts,56 a method that has previously been shown to be acceptable.57 The PM line was transferred from the pretreatment tracing to the posttreatment tracing to provide a consistent reference plane for evaluating changes. Incisal changes were then measured perpendicular to the PM vertical reference plane, with forward movement of the incisal tip assigned a positive value. Linear cephalometric measurements were multiplied by a factor of 0.92 to take into account the 9% enlargement factor.

The study cast measurements used in this study are described in Table 4. Where appropriate, they are further illustrated in Figures 4 and 5. An electronic digital sliding caliper (Mitutoyo Corporation, Tokyo, Japan) was used to measure the distances between occlusal landmarks to the nearest 0.1 mm.

The amount of crowding was determined using Proffit and Fields'58 segmental method, ie, by subtracting the pretreatment segmental total from the posttreatment segmental total and then adding back in the actual mesiodistal widths of the 2 extracted premolars. The residual space was calculated by subtracting the amount of crowding from the sum of the mesiodistal widths of the extracted maxillary premolars.

Mean changes for the cephalometric and study cast variables were calculated, and analysis of variance was used to identify any statistically significant differences in the changes observed within the 3 subgroups. Pearson's product moment correlation coefficients (r) were also calculated to determine whether any association existed between maxillary incisor changes and any other variables.

To evaluate tracing and measurement error, the records of 10 patients (20 sets of study casts and 20 cephalograms) were selected at random and the experimental procedure repeated. Results of the paired-samples t-test showed no significant differences between the first and second sets of measurements at the 95% confidence interval.

At the end of active treatment, all 3 extraction subgroups showed mean Class I incisor and molar relationships. In relation to the APog line, the mean posttreatment maxillary incisor position and angulation were also similar among the 3 extraction subgroups (Table 5).

The maxillary arch dimensional changes for all groups are summarized in Table 6. Since there was no statistical evidence of sexual dimorphism, each of the extraction subgroups was not further divided into male and female subgroups. As expected, mean reductions in arch depth and chordal arch length were noted in all groups. The mean reductions were, however, similar in all groups. In all groups, there was a mean increase in maxillary arch width across the most anterior premolars. The only statistically significant difference among the groups was for the reduction in intermolar width. Mean reductions of 1.5 mm (±1.7), 2.6 mm (±2.3) and 3.3 mm (±2.2) were observed in the 4/4, 4/5, and 5/5 groups, respectively.

Changes in maxillary incisor position and angulation with treatment are presented in Table 7. A mean reduction in maxillary incisor protrusion and proclination was noted in all maxillary premolar extraction groups. Statistically significant differences for all 5 variables were found among the different extraction groups. There was a significantly larger mean retraction of the maxillary incisors in relation to the APog line in both the maxillary first premolar extraction subgroups than in the second premolar group. The 4/4 and 4/5 subgroups had mean incisor retractions of 4.2 mm and 3.7 mm, respectively, whereas the 5/5 group had a mean retraction of 2.3 mm. Similar results were noted for the changes in maxillary incisor angulation to the APog line, with the overall maxillary first premolar extraction group showing a significantly larger mean reduction in angulation (8.2°) than the second premolar group (3.3°). The mean incisor angulation changes in relation to the palatal plane also were significantly different among the groups, with mean reductions of 5.0° and 1.6°, respectively, in the maxillary first premolar and maxillary second premolar extraction groups. As with all variables, however, there were large ranges of individual variation. Interincisal angulation increased on average as a result of changes in the angulations of both maxillary and mandibular incisors. Large standard deviations were also noted for this measurement.

Anteroposterior changes recorded at the maxillary incisor tip when pre- and posttreatment tracings were superimposed on the palatal plane at ANS are shown in Table 7 and are illustrated in Figure 6. There was a statistically significant difference in the mean incisor retraction in the maxillary first premolar group (2.5 mm) compared with the maxillary second premolar group (1.6 mm). Incisors were retracted from their pretreatment positions in all subjects in the 4/4 and 4/5 groups, whereas only 85% of the subjects within the 5/5 group showed any incisor retraction. A wide range of individual changes was evident within each group.

The anteroposterior change in maxillary first molar position was estimated by calculating the difference between the arch-depth change (mm) and incisor position change (mm) measured from superimposition on the palatal plane at ANS. This was made possible by the multiplication of the linear cephalometric measurements by the 0.92 factor. Anteroposterior changes in the maxillary first molar position are shown in Table 8 and are illustrated in Figure 7. Mean changes in the estimated molar movement were not found to be significantly different among the groups. Calculated in this way, mean forward movements of the molars for the groups ranged from 3.7 mm to 4.7 mm. When comparing relative amounts of movement, the incisors had undergone greater movement than the first molars in 20% of the cases in each of the maxillary first premolar extraction subgroups and in 19% of cases in the maxillary second premolar extraction group. Little, if any, incisor retraction was found in 13% of the subjects in both the 4/4 and 4/5 groups, while the same was true in 19% of subjects in the 5/5 group.

Because of the wide ranges of individual variation within each of the groups, it was decided to look for any similarities that might exist between the individuals who had shown extremes of incisor movement within each group. Two individuals were chosen from each extraction subgroup, one in whom there was the greatest incisor retraction and another in whom there was only minimal incisor change. A third individual, in whom there was actual protrusion of the incisors, was chosen in the 5/5 group (Table 9, Figures 8–10). When viewing these cases, there appeared to be a tendency toward greater retraction of the incisors in cases with less crowding or greater amounts of residual space. It was also interesting to note that changes in the position of the maxillary incisors on the underlying bone were not necessarily consistent with maxillary incisor changes in relation to the APog line. For example, in individual B (Table 9), a reduction in the prominence of the incisors in relation to the APog line was accompanied by little, if any, change on the underlying bone. It seems therefore that changes in the anteroposterior and vertical positions of point A and pogonion were occurring at the same time.

When Pearson's coefficients were calculated, changes in interincisal angulation and the initial incisor position in relation to the APog line were both found to be significantly correlated with maxillary incisor movement (Table 10). These findings would not be unexpected since these measurements are all somewhat dependent on each other. No other significant correlations were found.

The presence of significant differences in mean pretreatment overjet, Class II molar relationship, and incisor protrusion among the different extraction subgroups suggests that these factors had somehow influenced the extraction sequence decision. Crowding has consistently been considered the major factor to be taken into account when deciding whether to extract teeth as part of orthodontic treatment,19,59–61 and the choice of particular extraction sequences seems to have been based largely on anecdotal clinical opinions.15,16,26 Recently, Saelens and De Smit25 evaluated pretreatment variables involved in extraction decisions and reported that the amount of crowding had a significant influence on the decision to extract 4 first premolars. In cases with mild crowding and mild dental protrusion, however, the decision was often made to extract 4 second premolars instead. It is interesting to note that, in an earlier study on mandibular premolar extraction effects, it was found that the underlying vertical facial pattern also seemed to significantly influence the mandibular extraction-sequence decision.62 This was not found to be the case in this present maxillary arch study. That may be due to the fact that the final positions of the mandibular incisors are more likely to be chosen on the basis of the vertical facial pattern and the consequential lateral profile effects. It is also interesting to note that, in this randomly selected premolar extraction sample, the 4/4 group accounted for only 21% of the cases, while the 4/5 and 5/5 groups accounted for 37 and 42% of the cases, respectively. This is in contrast with the findings of previous studies8–10 that have reported that 4 first premolars are the most likely teeth to be extracted as part of orthodontic treatment. One limitation of the present study has been this smaller sample size for the 4/4 group. One should realize, however, that the main focus of this study has been on the different maxillary premolar extraction choice.

In the present study, maxillary arch dimensional changes in each group involved, in general, some contraction of the anteroposterior dimension. The fact that the mean reductions in both arch depth and chordal arch length were similar in all groups might have been expected anyway since there were similar mean amounts of crowding in each group, resulting in similar amounts of overall residual space. The mean overall chordal arch-length reduction of 11.3 mm is consistent with those reductions reported by Paquette et al17 and De La Cruz et al20 but is somewhat greater than that reported by Luppanapornlarp and Johnston.18 These latter authors reported a mean chordal arch- length reduction of 8.3 mm during treatment in first premolar extraction cases. The difference is likely to be due to the greater mean crowding found in the Luppanapornlarp and Johnston sample (5.8 mm) compared with the present sample (3.5 mm).

Because a significant number of the cases included in this experimental sample involved unerupted, impacted, or buccally displaced maxillary canines, the width across the anterior segment of the arch was measured across the most anterior premolars on both pre- and posttreatment casts. A mean increase in arch width in this region was noted in all groups. This means that the arch form was, on average, rounded out somewhat across the premolars regardless of the extraction sequence. In contrast with this increase in arch width across the premolars, there was a significantly greater mean reduction in intermolar width in the 5/5 group. This would suggest that, in clinical practice, it might be easier to maintain the initial intermolar width if the second premolars have not been extracted.

The mean changes in anteroposterior position of the maxillary incisors found in this study are consistent with those reported in previous studies.18,25 In the present sample, there was a mean maxillary incisor retraction of 2.5 mm (±1.9) and 1.6 mm (±1.6) in the maxillary first premolar and maxillary second premolar extraction groups, respectively. Saelens and De Smit25 reported an average 2.1 mm (±2.5) and 1.9 mm (±2.4) retraction in their 4 first premolar and 4 second premolar extraction groups, respectively. Luppanapornlarp and Johnston18 found an overall mean 2 mm to 3 mm retrusive effect with first premolar extractions. It should be noted that there were wide ranges of individual variation in maxillary incisor changes in both the present study and the study of Saelens and De Smit.25 It would, therefore, seem to be unreasonable to estimate likely incisal position changes in an individual patient by simply using mean values, as has been suggested by some authors.15,18 The mean changes in maxillary incisor position in relation to the APog line are consistent with the incisor changes on the underlying bone in that they do seem to vary according to the chosen extraction sequence. It is important to realize, however, that, as was seen in individual B (Table 9), the changes in the 2 measurements in individual patients may not always be consistent.

It has been generally accepted that greater forward movement of the molars should be expected following the extractions of maxillary second premolars than first premolars.16,23,26,63 This was, however, not necessarily found to be the case in the patients within this experimental sample. When relative maxillary incisor and molar movements were compared, greater molar movements occurred in 73%, 80%, and 81% of cases in the 4/4, 4/5 and 5/5 groups, respectively. If this were true for any maxillary premolar extraction sample, it might suggest that differential extraction alone may not provide sufficient maxillary anchorage control in all cases. Other methods of anchorage control would need to be considered.

Individual variation was also evident when extremes of incisor movement were evaluated (Table 9). It does seem possible for a variety of maxillary incisor changes to accompany each of these premolar extraction sequences, although there do appear to be some definite trends in incisal behavior. For instance, those patients in each of the groups in whom maximum incisor retraction had occurred appeared to have consistently less crowding and, in turn, greater residual space than other individuals in the same groups. A similar tendency has previously been documented for the mandibular arch.62 In both maxillary and mandibular arches, therefore, it seems that both crowding and the residual space following leveling and relief of crowding play significant roles in determining the final incisal positions. Because such wide individual variation has been found in response to orthodontic treatment with any of the investigated premolar extraction sequences, it is important to assess each case on an individual basis when making a detailed treatment plan rather than simply choosing a particular extraction sequence based on published mean incisal changes for different extraction sequences.

From the study results, the conclusions are as follows:

1. Pretreatment characteristics, which may influence the maxillary premolar extraction-sequence decision, include the amount of incisal overjet, the first permanent molar relationship, and the initial amount of incisor protrusion.

2. There are likely to be similar ranges of decreases in maxillary anteroposterior arch dimension regardless of the chosen premolar extraction sequence. Greater reduction in intermolar width is likely to occur following the extractions of maxillary second premolars than first premolars.

3. Although there is some evidence that greater incisor retraction accompanies maxillary first premolar extractions, considerable individual variation in incisor and molar movements is likely to be seen with any premolar extraction sequence. A specific extraction sequence does not necessarily seem to guarantee that certain amounts of incisor retraction or molar protraction will occur.

4. Individual variation in response to growth and treatment is likely to be a result of different treatment mechanics and facial and occlusal objectives and is likely to depend as much on pretreatment characteristics as on the extraction sequence itself.

The authors would like to thank Ms Florence Choo from the Statistical Consulting Center, The University of Melbourne, for her assistance with statistical analysis. They would also like to acknowledge the work of Mr Geoffrey West for the production of the Westcef cephalometric analysis program used in this study. This study was supported in part by a grant from the Australian Society of Orthodontists' Foundation for Research and Education.