A retrospective study of Class II mixed-dentition treatment

Orthodontic intervention during the mixed dentition has been widely practiced for more than a century.^1,2  However, over the past two decades, some investigators have questioned its effectiveness in treating Class II malocclusions. Their concerns are based primarily on the interpretation of data acquired in three randomized clinical trials (RCTs).^3–6  In two of these, the investigators reported that skeletal correction of Class II malocclusions during phase 1 treatment was not sustained during subsequent phase 2 treatment.^3–5  They concluded that early Class II correction did not provide any additional benefits and was less efficient than LateTx treatment and that it placed more burdens on the patients and clinicians.^3–5  However, some of the generalizations from these RCTs may not be applicable for clinicians who use more comprehensive phase 1 treatment approaches that employs the use of partial fixed appliances (2 × 4), headgear, and mandibular lingual arches. Though these modalities have been utilized extensively by some clinicians, treatment outcomes have not yet been rigorously studied.

The present study investigates the outcome of one particular comprehensive mixed dentition treatment approach in correcting moderate and severe Class II malocclusions. It also examines differences in outcome between Class II treatment started in the mixed dentition between the ages of 7 and 9.5 years and that started in the permanent dentition between 12 and 15.

This study attempts to answer the following questions:

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  How much Class II correction was achieved through early treatment?

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  At the end of treatment (EndTx), what differences were noted in molar relationship and other cephalometric measurements between early treatment patients and patients who began treatment in the permanent dentition?

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  How did treatment duration and extraction rate compare between early- and later-treated patients?

The present study is part of a larger investigation of a specific method of “comprehensive mixed dentition treatment” developed in the mixed dentition clinic at the University of the Pacific School of Dentistry.⁷  The sample was drawn retrospectively from the clinical practices of three clinicians highly experienced in this mixed dentition treatment approach (Table 1). For the purposes of this paper, samples from the three practices were combined. Two iterations were required to reduce the sample for the present paper (Figure 1). In the first iteration, representative samples of patients who had presented at different ages were assembled. The first consisted of 305 patients between the ages of 7 and 9.5, the second, 182 patients between 12 and 15. The reasons for exclusion are fully recorded (Table 2). In the second iteration, the sample from each of the two age groups was reexamined to identify all patients in each who had a malocclusion greater than end-on Class II molar relationship regardless of incisor relationship.

For the early-presenting patients, there were 156 Class II patients, of whom 112 received comprehensive early treatment (54 had greater than end-on relationships [EarlyTx group]), 18 underwent various limited appliance therapies (5 had greater than end-on relationships), and 26 were deferred for treatment at a later time (only 3 of these patients had T1 records, 2 of whom had greater than end-on relationships). The Class II molar severity and the reasons for deferring treatment of those who did not have records at initial presentation (23 deferred patients) were not recorded. However, we can estimate that the 54 EarlyTx Class II patients comprised between 64% and 88% (54/[54 + 5 + 25]) and (54/[54 + 5 +2]) of early-presenting Class II patients with greater than end-on molar relationships. For the late-presenting patients, this yielded 58 of the 89 Class II patients (LateTx group).

Fifty-one untreated Class II subjects were selected from the AAOF Craniofacial Legacy Growth Collection (http://www.aaoflegacycollection.org) to serve as an untreated comparison Class II group (UnTx). To qualify for inclusion in this group, lateral cephalograms of prospective candidates from the Web site were matched by age and Class II severity by visual comparison with lateral cephalograms of the previously selected EarlyTx group.

Figure 2 illustrates data distribution within the sample. The sample includes three groups of Class II subjects: EarlyTx, LateTx, and UnTx. In the EarlyTx and UnTx groups, data are available at T1, T2, and T3. In the LateTx group, data collection did not start until T2. Note further that the EarlyTx group was composed of two subgroups—a two-phase subgroup of 33 patients who received a second phase of comprehensive orthodontic treatment in the permanent dentition, and a phase 1 only subgroup of 21 patients who were either deemed by their clinicians not to require a second phase of treatment or who declined further treatment.

An initial phase of comprehensive mixed dentition treatment (phase 1) for Class II malocclusions typically employed headgear, a maxillary 2 × 4 appliance, and a mandibular lingual arch (Figure 3). The aim of phase 1 treatment was complete correction of the malocclusion, which included obtaining a Class I molar relationship, reduction in skeletal jaw discrepancy, ideal overjet and overbite, proper incisor alignment, and adequate arch length and width. Following phase 1 treatment, retainers were delivered and the patients were seen regularly every 2–4 months during a supervision phase, which was designed to monitor the patient's growth and occlusal development to preserve the gains made in phase 1 treatment.^7,8  Upon eruption of the permanent second molars, the patients who underwent phase 1 treatment were reexamined, and a decision was made as to further treatment. Phase 2 treatment ranged from partial fixed appliances in a single arch to full fixed appliances in both arches with or without extractions. Table 3 shows the distribution of various auxiliary appliances used in the EarlyTx and LateTx groups.

Lateral cephalograms were traced independently by two or more calibrated judges (orthodontic residents and faculty), and the average values were recorded in a numerical database; outliers were excluded based on landmark-specific envelopes of error.⁸  Anterior cranial base superimposition was performed, and the Frankfort horizontal and occlusal planes were transferred as frames of reference for subsequent comparisons of changes through time. Thirteen conventional cephalometric measurements were reported to describe changes in dental and skeletal relationships (Figure 4).

Class II molar severity was measured directly on study casts using electronic digital calipers with a stated accuracy of 0.1 mm. Class II severity was defined as the distance between the mesiobuccal cusp of the maxillary first molar and the buccal groove of the mandibular first molar when the casts were in centric occlusion (Figure 5). The value reported for each study cast pair was the average of two judges' estimates for the side of the mouth with the more severe Class II molar relationship.

Descriptive statistics were generated to report the mean, standard deviation (SD), and range of the demographic information for each of the three groups. Unpaired t-tests were used to compare mean differences, and chi-square tests were used to compare proportions.

Since this study was an exploratory investigation of a particular treatment modality that had not been rigorously studied before, we considered more variables than would be appropriate in a definitive hypothesis test. Rather than perform a formal multiple-comparisons adjustment, we considered only measurements that had P values less than .01, using the SAS statistical package (version 9.1, SAS, Cary, NC).

This study was approved by the Institutional Review Board of the University of the Pacific and conducted at the Craniofacial Research Instrumentation Laboratory in the Department of Orthodontics.

At T1, EarlyTx group Class II molar severity averaged 0.7 ± 1.4 mm, overjet 6.3 ± 2.3 mm, and ANB angle 5.3 ± 1.7°. During phase 1, mean Class II molar severity was reduced to −1.6 ± 1.4mm, overjet to 3.1 ± 1.1 mm, and ANB angle to 2.5 ± 2.3°. Thus on average, successful Class II correction was achieved during early treatment. In patients who had a second phase of treatment, there was a further reduction in Class II molar severity (T3 minus T2 = −0.8 ± 1.4) but no further average improvement in overjet or ANB angle. In contrast, SNB and SN-Pog angles increased and MPA decreased continuously throughout the observation period. We conclude that substantial dental and skeletal Class II correction occurred before the second phase of treatment was initiated and was maintained through the supervision period.

Since the EarlyTx and the LateTx groups started treatment at different ages, it was appropriate to compare each group with an untreated group of the same age. Therefore, at StartTx, the EarlyTx group was compared with the UnTx at T1 and the LateTx group was compared with the UnTx group at T2. The differences at the StartTx between the treated groups and the UnTx group were small and nonsignificant, suggesting that there is no gross violation of the assumption that the patients seeking treatment early are similar to the patients seeking treatment later.

There were no statistically significant differences in any of the 13 cephalometric measurements between the EarlyTx and LateTx groups at EndTx. We infer that the magnitude of Class II correction was similar in the EarlyTx and LateTx groups.

Figure 6 shows box plots for seven key lateral cephalometric measures at each available time point for the EarlyTx, LateTx, and UnTx groups. At T1, the plots for the EarlyTx and UnTx groups are generally similar. At T2, the values of the still-untreated LateTx group were generally similar to those of the UnTx group. At T3, all seven cephalometric measurements were similar for the EarlyTx and LateTx groups, but significantly different from the UnTx groups, suggesting treatment effects.

Extractions were employed in the permanent dentition in 22 of the 58 (37.9%) LateTx cases, but in only 3 of the 54 (5.6%) EarlyTx cases (P < .001). Twelve of the 22 extraction cases in the LateTx group involved only the maxillary first premolars and were finished in a Class II molar relationship (Table 7). All three EarlyTx extraction cases had four premolar extractions. All the maxillary first premolar extraction cases in the LateTx groups presented a relatively well-aligned mandibular arch with minimal crowing.

At StartTx, Class II severity averaged 3.9 ± 1.3 mm for the EarlyTx group and 3.4 ± 1.1 mm for the LateTx group (Table 8). At EndTx, patients in the EarlyTx group had mean residual Class II severity of less than 1 mm on the more severely affected side (0.6 ± 0.8 mm for the Phase 1 only and 0.1 ± 0.7 for the two-phase groups). Since molar correction was effectively achieved at the end of the initial treatment phase in both the phase 1 only and the two-phase groups, molar severity did not appear to be a major contributing factor in the decision for phase 2 treatment.

We considered treatment successful if residual Class II severity was less than 1 mm and overjet was less than 4 mm. Table 9 shows that by this standard, success was achieved in 78% of all EarlyTx cases and 75% of all LateTx cases.

For the EarlyTx group, phase1 treatment time was on average 1.6 ± 0.5 years, and active supervision lasted an average of 3.2 ± 1.1 years (Table 10). Thirty-nine percent (21/54) of the EarlyTx group did not undergo a second phase of treatment. Of these 21 Phase 1 only patients, 6 declined clinician's recommendations for further treatment and 15 were deemed by their clinicians not to require a second phase. The remaining 33 EarlyTx patients constituted the two-phase group. They received a second phase of treatment that lasted an average of 1.7 ± 0.8 years. The average treatment time for the LateTx group was 2.6 ± 0.7 years; phase 2 treatment time in the EarlyTx, two-phase group was 10.8 months shorter (P < .0001). When phase 1 and phase 2 are summed, the total active treatment time for the EarlyTx group was very close to that of the LateTx group (2.6 ± 1.3 vs 2.6 ± 0.7 years, respectively). However, when supervision time is included, total treatment time was significantly longer in the EarlyTx group than in the LateTx group (5.8 ± 1.6 vs 2.6 ± 0.7 years, P < .0001) with more total office visits for the EarlyTx group than the LateTx group (53.1 ± 18.8 vs 33.7 ± 8.3, P < .0001).

The central thesis of this paper is not that the early treatment protocol reported here is the best possible method for treating patients with early mixed dentition Class II malocclusions. Rather, we have sought to demonstrate that it is one feasible method that experienced clinicians have successfully used to correct such malocclusions.

The cited RCTs^3–5  concluded that early treatment did not reduce the complexity of phase 2 treatment, since there were no significant differences in extraction rate and treatment time for full fixed appliance therapy.⁴  Yet disagreement exists among the RCTs with regard to some of the outcome measurements such as extraction rate,^4,9  treatment time,⁶  and psychosocial effect.^10,11  The present authors believe that these discrepancies in findings among the RCTs and the findings between our study and the Class II RCTs can be attributed to the different approaches in phase 1 treatment.

Our finding of a higher extraction rate in the LateTx group than in the EarlyTx (38% vs 5.6%), is supported by the findings from one of the RCTs.⁹  From the findings of a high proportion of the maxillary first premolar–only extractions in all extraction cases reported by two of the RCTs and our LateTx group, we infer that an extraction regimen was frequently utilized in order to correct Class II malocclusions with tooth movement alone.^4,9  Besides fewer extractions, our results report additional benefits of early treatment, such as shorter treatment times in full fixed appliances for phase 2 therapy by about 11 months (1.7 ± 0.8 years vs 2.6 ± 0.7 years) and elimination of phase 2 treatment for some patients. Approximately 16 out of 54 (28%) of the early treatment patients did not require a full-bonded second phase of treatment and 5/54 (11%) elected to not pursue further treatment. Similar findings were reported by the RCTs^8,9,13  but involved fewer patients than did our study.

On the other hand, total treatment time and number of visits for two-phase treatment was greater than that for LateTx treatment when all stages of treatment—Phase 1, Phase 2, and supervision—are taken into account. This may be an important factor for children, parents, and clinicians considering early treatment. However, many clinicians consider shortening the time spent in full-bonded fixed appliance therapy at a later age when patients are frequently less compliant as a worthwhile goal for early treatment.¹² 

It was not surprising to many advocates of mixed dentition treatment that the RCTs reported no consequential differences in extraction rate and treatment time between the EarlyTx and LateTx groups, since no active intervention was employed for leeway space preservation in their studies. Other investigators have reported that preserving leeway space can prevent extractions in up to 73% of patients.¹³ 

Based on cephalometric and study cast measurements, Class II correction in the EarlyTx and LateTx groups were comparable. The present authors believe with Bishara¹⁴  that there may be a window of opportunity to correct Class II malocclusions during the growth period and that experienced clinicians can frequently produce similar outcomes for Class II malocclusions whether they treat early or late. On the other hand, our study suggests that delaying treatment until the permanent dentition may require more extractions and longer treatment times to correct Class II buccal malocclusions. In addition, correcting overjet early may reduce the risk of incisor trauma.¹⁵ 

Like every clinical study of which we are aware, the present study has imperfections. Retrospective studies are particularly subject to missing charts and incomplete records. We have tried to document our study's deficiencies of this type in Table 2. In addition, this study has two additional limitations: (1) The subjects being treated with early comprehensive treatment were not defined by specified eligibility criteria and only three clinician practices were sampled, limiting the generalizability of results; (2) although we have assessed gross differences by the use of an untreated comparison group, EarlyTx patients may be intrinsically different from LateTx patients, making comparisons between the EarlyTx and LateTx groups less definitive. In spite of these deficiencies, we believe that our results are sufficient to establish that comprehensive early treatment can be an effective treatment modality for the correction of Class II malocclusions in many patients.

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  Successful Class II correction during early treatment was achieved for three quarters of the EarlyTx patients. Sixteen of the 54 EarlyTx patients (28%) were deemed by their clinicians not to require a second phase of treatment.

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  EarlyTx patients had fewer extractions in the permanent dentition than did LateTx patients(3/54 vs 22/58)

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  EarlyTx patients spent less time in full-bonded fixed appliance therapy in the permanent dentition. On the other hand, the LateTx patients spent less time in total treatment and had fewer total visits.

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  EndTx cephalometric measurements not involving molar relationship were generally similar for both the EarlyTx and LateTx groups

This study was supported in part by a grant of Orthodontic Faculty Development Fellowship Award from the American Association of Orthodontists Foundation and a three-year, Full-Time Faculty Teaching (FFT) Fellowship Award from the American Association of Orthodontists.