Arch width after extraction and nonextraction treatment☆☆☆

Material and methods 

The posttreatment study models of 25 patients who had 4 first-premolar extractions and 25 patients treated without extractions were randomly selected and measured. The only inclusion criterion was that all second molars were present and in alignment. Treatment was performed at a university clinic and directed by various instructors. All patients received fixed appliances; 49 were treated with the edgewise technique, and 1 with the tip-edge technique. In the extraction group, there were 12 Class I and 13 Class II malocclusions. In the nonextraction sample, there were 9 Class I, 13 Class II, and 3 Class III malocclusions.

With an electronic caliper (Digit-Cal, Browne and Sharpe, North Kingston, RI), the widths of the anterior and posterior parts of the maxillary and mandibular dental arches were measured in the canine and the molar regions from the most labial aspect of the buccal surfaces of those teeth. The caliper was placed at the best estimate of a right angle to the palatal suture in the maxillary arch and to a line bisecting the incisor segment in the mandibular arch. The recorded widths between the molars were the widest distances between the first or second molars. The widest part of the posterior part of the arch invariably was in the second molar region. Each distance was measured 3 times, and the average of the 3 values was used as the final measure.

Because arch width is ordinarily established by the mandibular arch, pretreatment widths between the mandibular canines and molars were recorded, as described, to establish that the arch widths of the mandibles of both samples were similar at the start of treatment. The treatment changes in the mandibular intercanine and intermolar dimensions were also determined. The pretreatment maxillary intercanine and intermolar dimensions were not measured because they vary in different malocclusions, and the distribution of malocclusions was not the same in each group.

The reproducibility of the measurements was evaluated by analyzing the differences between 10 double measurements of intercanine and intermolar distances, randomly selected and taken at different times. The error of measurement was assessed by Dahlberg's formula14: where D is the difference between duplicate measurements, and N is the number of double determinations. The errors were 0.22 mm for mandibular intercanine width, 0.50 mm for mandibular intermolar molar width, 0.26 mm for maxillary intercanine width, and 0.27 mm for maxillary intermolar width. Means and standard deviations were calculated, and a 2-tailed t test was used to determine statistically significant differences with P < .05.

Results 

At the start of treatment, the mandibular intercanine and intermolar widths of both groups did not differ statistically (Table I).

Table I. Pretreatment mandibular intercanine and intermolar arch widths: means and SD (mm)

		Extraction (n = 25)	Nonextraction (n = 25)	Significance
	3-3	31.47 ± 2.10	31.24 ± 1.32	NS
	7-7	58.70 ± 3.02	58.51 ± 1.67	NS

3, Canine; 7, second molar; NS, not significant.

Table II. Posttreatment maxillary and mandibular arch intercanine and intermolar widths: means and SD (mm)

		Extraction (n = 25)	Nonextraction (n = 25)	Difference	Significance
	3-3(L)	32.87 ± 1.74	31.93 ± 1.20	0.94	.01
	3-3(U)	39.93 ± 2.02	39.08 ± 1.64	0.83	NS
	7-7(L)	57.54 ± 2.80	58.38 ± 2.37	0.74	NS
	7-7(U)	60.92 ± 2.00	60.88 ± 2.91	0.04	NS

3, Canine; 7, second molar; NS, not significant; L, maxillary; U, mandibular.

Table III. Mandibular intercanine and intermolar width changes: means and SD (mm)

		Extraction (n = 25)				Nonextraction (n = 25)
		Pretreatment	Posttreatment	Difference	Significance	Pretreatment	Posttreatment	Difference	Significance
	3-3	31.48 ± 2.10	32.87 ± 1.74	1.39	.02	31.24 ± 1.74	31.93 ± 1.20	0.69	NS (.06)
	7-7	58.71 ± 3.02	57.17 ± 2.19	1.54	NS	58.51 ± 1.72	58.38 ± 2.37	−0.13	NS

3, Canine; 7, second molar; NS, not significant.

Discussion 

To some advocates of nonextraction therapy, 4 first-premolar extraction treatment is synonymous with narrow dental arches with consequent unesthetic, large black triangles in the buccal corridor.5 This is not an accurate premise when describing treatment results obtained by fixed appliances. A comparison of posttreatment arch widths of patients treated by extraction and nonextraction procedures failed to identify the extraction patients because they had no characteristic narrow dental arches. In fact, the arches were approximately 1 mm wider in the mandibular canine area after extraction therapy, and this region establishes the width of the anterior part of the arches under normal conditions because the maxillary arch is conventionally fit to an idealized mandibular arch.

Although this finding might dissatisfy some nonextraction enthusiasts, particularly those who view extraction treatment negatively,5, 6 it is not a surprising observation if one examines the results of studies on posttreatment long-term stability in which mandibular incisor stability was acceptable. Specifically, the mandibular intercanine width increased 1.07 mm in an extraction sample.9 In contrast, in the nonextraction subjects, the increase in mandibular intercanine dimension was less than 1 mm in both Class I13 and Class II patients.10, 11, 12

In borderline patients, the long-term increase of the mandibular intercanine width in those treated by extraction therapy was 1.0 mm, whereas the increase in the nonextraction sample was only 0.5 mm.7 At the end of treatment, the intercanine widths of both groups were the same. Luppanapornlarp and Johnston8 evaluated the posttreatment and long-term results of treatment in clear-cut extraction and nonextraction patients and noted that the mandibular intercanine dimension of the extraction subjects was greater at all stages examined than the same parameter in the nonextraction patients. These data indicate that there is no systematic narrowing of the dental arches as a result of 4 first-premolar extraction treatment.

The increase in the mandibular intercanine width in this study in both extraction (1.4 mm) and nonextraction (0.69 mm) subjects compares with the previously cited increases recorded by others who evaluated patients treated by both strategies.7, 8, 15, 16 In these reports, a slightly larger increase occurred in those treated with extractions, possibly reflecting minor lateral movement as the canines are moved distally into the premolar sites. As an example, BeGole et al15 observed that the mandibular intercanine widths increased 1.58 mm in an extraction sample and 0.95 mm in nonextraction subjects. Uhde et al16 illustrated treatment changes in graph form in extraction and nonextraction subjects and demonstrated that the pretreatment mandibular intercanine widths were comparable in both groups, and, during treatment, a larger increase occurred in the extraction sample.

An unanticipated and surprising observation was that the intermolar widths of both groups of subjects were the same after treatment, because the usual expectation is that the posterior part of the arch becomes narrower as the molars move mesially into a narrower part of the arch during space closure. When including second molars in the assessment, this narrowing apparently does not occur. This observation also supports the view of Johnson and Smith2 that “typically, transverse arch width at any particular location in the buccal segments is maintained or slightly enlarged after extraction.” They also opined that the logic of those who believe that premolar extraction results in a reduction of the radius of the curve of the dental arch is faulty because the dental arch is not a circle and does not behave as a circle. Therefore, a reduction of the radius is not a necessary outcome of extracting a premolar on each side of the arch.

To some, maxillary arch width is a determinant of smile esthetics.4, 5, 6 Because maxillary arch width in the extraction and nonextraction subjects was the same, the effect of the 2 treatment strategies on smile esthetics would be similar. This conclusion is consistent with the results of a study that evaluated smile esthetics after both extraction and nonextraction treatment.2 There were no differences in esthetic scores between the groups. Interestingly, the investigators found that intercanine width, relative to the visible dentition, was wider in the extraction patients.

The labial surfaces of the canines and the molars were chosen as the measuring landmarks instead of the more customary cusp tips for several reasons: (1) to determine the widest possible widths of the arches, (2) to prevent confusion when selected cusps tips were not distinct, and (3) to avoid identifying a specific molar for measurement because in extraction treatment, as noted by Johnson and Smith,2 the first molars might have moved forward into a narrower part of the arch. For this reason, arch width measured between first molars would not adequately represent the width of the posterior aspect of the dental arches.

Changes in maxillary canine and molar widths were not recorded because the maxillary dentition does not customarily establish the widths of the dental arches. Also, these measurements can produce confusing information unless the groups compared are essentially identical at the outset. For instance, a pretreatment comparison of Class II Division 1 and Division 2 malocclusions with well-aligned mandibular arches and identical mandibular intercanine widths might show that the maxillary intercanine dimension is narrower in the Class II Division 1 malocclusion, particularly if the arch form in the overjet region is tapered. Under these conditions, the dimensional change resulting from treatment would be greater for the Class II Division 1 malocclusion even though the posttreatment intercanine distance in both examples would be the same because the mandibular intercanine distance is the same.

No patient in this study was treated with appliances specifically designed to increase mandibular arch width. The instability of excessive intercanine expansion (> 1-1.5 mm) in the mandibular arch17 is 1 reason that our customary treatment does not include these appliances.

In a more global sense, the contention that extraction treatment results in less esthetic smiles because of narrowing of the dental arches is, to an extent, similar to a once-presumed damaging effect of extraction therapy on the temporomandibular joint.18 In both instances, the consequences of extraction treatment are considered to be decidedly negative. The imputed injurious relationship between extraction treatment and temporomandibular disease has largely been discredited.19 The available data indicate that narrow dental arches are not the expected consequence of extraction treatment. Therefore, the esthetically compromising effect of narrow dental arches on smiles is not a systematic outcome of extraction treatment.