INTRODUCTION

An understanding of the changes that may occur to the occlusal plane with normal growth and the role and possible manipulation of the occlusal plane as part of orthodontics are important aspects of clinical treatment and esthetics.^1–5 The relationship of the occlusal plane to maxillary–mandibular skeletal patterns,⁶ facial forms,⁷ function,^8–10 and esthetics^11,12 have been reported to influence therapeutic resolution of malocclusions.^13–17

Intermaxillary Class II elastics are an effective method of clinical treatment in resolving aspects of malocclusion and have been part of the orthodontic armamentarium since first described by Maynard in 1843, then further in 1850 by Tucker, still using gum elastics.¹⁸ Henry A. Baker was first to use latex elastics and to combine many of the concepts used by previous dentists into one orthodontic treatment modality referred to as “Baker anchorage.”¹⁸

As part of orthodontic therapy, intermaxillary elastics are used as an active component to correct sagittal (ie, Class II/III) and vertical (open bite) malocclusions. They are composed of natural latex mixed with stabilizers¹⁹ or nonlatex amorphous polymers made of polyurethane material that has the characteristics of rubber and plastic where allergies to the former exclude their use.^20,21 The common use of elastics to correct an angle Class II malocclusion²² has also been reported to cause reciprocal adverse effects.^23–26 Among these is the increase in steepness of the occlusal plane.^16,26–29 This has been reported to be temporary in growing patients and a potential cause of aspects of dental relapse. A return to pretreatment orientation of the occlusal plane has been blamed for this untoward adverse effect in these patients.^30,31 However, these reports were based on small sample sizes,^1,27,31 did not compare extraction to nonextraction treatment modalities,¹⁶ included growing and nongrowing patients,³¹ or evaluated only dental outcomes as a measure of change.³⁰

The purpose of this study was to investigate whether 6-month use of Class II elastics in growing patients would generate changes in the angulation of the functional occlusal plane (FOP) with extraction/nonextraction treatment choice and among skeletal (growth) patterns and compare these changes with changes at least 1 year after treatment. The working hypotheses were that (1) the use of Class II elastics would cause steepening of the FOP, (2) this would be greater in extraction and high-angle patients, and (3) this would tend to relapse after treatment.

MATERIALS AND METHODS

The study sample comprised 50 patients selected from a university postgraduate orthodontic clinic archive. Inclusion in the study sample required that patients were aged between 11 and 16 years at the time of orthodontic treatment, initially presenting with an angle Class II, Division 1 malocclusion (defined as having ½ to full cusp molar distocclusion) after undergoing at least 6 months of Class II intermaxillary elastic use as part of orthodontic therapy using a preadjusted 0.022 × 0.028-inch slot fixed edgewise orthodontic appliance and achieving correction of the molar relationship to Class I. Standardized High-quality lateral cephalometric radiographs taken before treatment (T0) and after treatment (T1) also needed to be available. A total of 411 such patients were identified as meeting the inclusion criteria and were contacted, of which 125 patients attended a posttreatment evaluation at least 1 year after treatment (T2). Of those patients, 50 consented to participate in the present study (n = 28 girls and 22 boys), which required taking a lateral cephalometric radiograph at T2. Approval for this study was granted by the Ethics Committee of Tel Aviv University, and informed consent was provided by each patient or his or her guardian.

Lateral cephalometric radiographs were analyzed using CephX (Orca Dental, Chestnut Hill, Mass) using the following landmarks/planes according to Jacobson³²: SN, MP, FH, and FOP, which bisected the cusps of the first molar and premolar (Figure 1). The angular relationship changes in FOP to SN, MP and FH were measured from T0–T1, T1–T2, and T0–T2.

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Patients were divided into groups according to treatment modality, that is, extraction of four first premolars (E group, n = 12) or nonextraction (NE group, n = 38) and skeletal pattern as normodivergent (n = 26, SN-GoGn [Sella Nasion to Gonion-Gnathion] = 28°–36°), hyperdivergent (n = 15, SN-GoGn >37°), or hypodivergent (n = 9, SN-GoGn <27°).

RESULTS

Longitudinal Changes in the FOP

Angular orientation of the FOP was evaluated relative to the FH, SN, and MP. These were compared between pre-to-post treatment and post treatment to > 1 year post treatment (Table 1).

Table 1. FOP Angles Measured Relative to the FH, SN, and MP at the T0, T1, and T2 Observation Periods^a

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For FOP measured relative to FH (FOP-FH), significant differences were found between the initial and final and between the final and 1-year posttreatment phases (P < .021). At T0, FOP-FH was found to be greater than at T1, whereas FOP-FH at T1 was significantly greater than at T2. Similar results were obtained when measurements were evaluated relative to the cranial base (FOP-SN), yet only the difference between T1 and T2 was found to be statistically significant (P < .045). Significant changes were also found in the FOP measured relative to the MP (P < .035; Table 1). These differences were expressed in a different pattern than that noted above, namely, FOP-MP at T1 was found to be greater than at T0.

FOP and Vertical Skeletal Types

Significant differences were found in the FOP relative to the FH, SN, and MP among the different vertical skeletal types at all of the observation time points except for FOP-FH at T1 (P < .032; Table 2). Patients with the hyperdivergent skeletal type exhibited the greatest FOP angle compared with other types at T0, T1, and T2, whereas patients with the hypodivergent skeletal type had the smallest FOP angle (Table 2, Figure 2). Yet no significant difference was found between the three skeletal pattern groups when the change in FOP during the periods (ΔT0−T1 and ΔT1−T2) was compared (P > .433; Table 3).

Table 2. Comparison of FOP Among the Different Skeletal Patterns at the T0, T1, and T2 Follow-Up Periods

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Table 3. Comparison of FOP ΔT0−T1 and ΔT1−T2 Among Different Skeletal Patterns

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FOP and Treatment Groups

Statistically significant differences were found in the FOP relative to the MP between individuals treated with and without extractions at all three of the observation time points (P < .02; Table 4). Individuals treated in the E group exhibited significantly greater FOP-MP at T0, T1, and T2 than those treated in the NE group. No significant difference was evident between the E and NE groups when FOP-SN and FOP-FH were compared (P > .119). No significant changes in FOP-MP, FOP-SN, and FOP-FH angles during the periods (ΔT0−T1 and ΔT1−T2) were found between the E and NE groups (P > .193; Table 5).

Table 4. Comparison of FOP Between Patients Treated With Extraction and Nonextraction at the T0, T1, and T2 Follow-Up Periods

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Table 5. Comparison of FOP ΔT0−T1 and ΔT1−T2 Between the E and NE Groups

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DISCUSSION

An understanding of the importance of proper control of the occlusal plane is fundamental to clinical treatment success. The present study investigated the effect of fixed appliance orthodontic therapy that included at least 6-month use of Class II intermaxillary elastic wear on the orientation of the FOP in 50 growing patients. It was previously reported that this adjunct to treatment did alter this anatomic relationship; however, the stability of this manipulation has not been established.^26,34 In addition, it was reported that individual trends in craniofacial growth persisted,^35,36 but also that significantly less mandibular growth occurred in Class II patients when this pattern was established during prepubesence.^4,37

The present study found that the FOP-SN and FOP-FH angles decreased from T0 significantly at T1 and at T2 for all participants regardless of how they were subdivided (Table 1). In contrast, the angle FOP-MP was found to have increased from T0 to T1. These findings might likely be associated because the decrease in the angulation of the FOP relative to the SN might engender a reciprocal increase in FOP-MP. Any changes in the orientation of the FOP would inversely effect structures on either side of it. This finding contradicted previous reports that the occlusal plane tended to steepen with the use of Class II elastics. That the use of Class II elastics as part of orthodontic mechanotherapy could alter the orientation of the occlusal plane in a clockwise direction has been previously reported.^34,35,38 Yet, because of the nature of the current sample, namely, growing individuals, the counterclockwise rotation observed could be attributed to compensation as a result of normal growth occurring during the treatment period.^39,40 The findings of the present study indicated that changes in the angulation of the FOP occurring during the reference times may have been associated with normal growth and might not indicate a direct treatment effect. This was in agreement with the findings reported by Creekmore⁴¹ and Schudy,⁴² who attributed such changes to comparatively greater eruption of maxillary molars and mandibular incisors than their occluding counterparts, and Harris et al.,³¹ who reported the same regarding posterior facial height. Thus, these findings could be explained by normal growth changes and were consistent with other studies^15,43–45 that claimed that the treatment effects mirrored those seen in normal growth.

The uniqueness of the current study was related to the investigation of angular changes in the FOP as this occurred with Class II elastic use in different skeletal patterns. A consistent, significant difference in FOP was observed during all three follow-up periods (T0, T1, T2) among the different skeletal groups (Figure 2). Namely, patients in the hyperdivergent group were found to have the largest FOP angles, whereas hypodivergent individuals possessed the lowest. These findings paralleled those derived from growth pattern descriptions relying on the orientation of the MP to the cranial base. Similar findings were previously reported by Schudy⁴² and Isaacson et al.⁴⁵ Of more interest were the findings that there were no significant differences in FOP change during the treatment (ΔT0−T1) and follow-up periods (ΔT1−T2) among the three skeletal pattern groups (Table 3). This suggested that the duration of Class II elastic usage as described produced similar effects on the FOP within each type of skeletal pattern.

In addition, the current study compared differences in FOP between the extraction and nonextraction treatment modalities. It was found that the E group had a larger FOP-MP angle at the initial time point (T0) compared with the NE group (Table 4). This might have been a result of an orthodontic decision preference to extract four premolars in Class II patients presenting with steep initial FOP compared with Class II patients with normal or low FOP. Nevertheless, the changes in FOP relative to the cranial planes were found to be similar between the E and NE groups both during treatment (ΔT0−T1) and during follow-up periods (ΔT1−T2; Table 5). These findings imply that the use of Class II elastics during the treatment of growing individuals did not affect rotation of the FOP differently, whether extractions were or were not decided to be the treatment of choice.

CONCLUSIONS

This aim of this study was to evaluate FOP changes relative to several craniofacial anatomic planes following the use of Class II elastics for at least 6 months in growing patients. The main findings were the following:

• The FOP angle tends to rotate in a clockwise manner during this type of orthodontic treatment as well as during the posttreatment period. This suggests the expression of underlying normal growth patterns and dental compensation.

• Patients initially presenting with a hyperdivergent skeletal pattern exhibit the largest angulation of FOP, and those with hypodivergent patterns the lowest FOP, both during and following orthodontic treatment, yet no difference was found in the effect of Class II elastics on changes in FOP between all skeletal pattern groups.

• Patients treated with premolar extractions presented before treatment with the larger FOP-MP angle, yet no difference was found in the effect of Class II elastic therapy on the changes of FOP found between the extraction and nonextraction treatment modalities.

• The indications for the use of intermaxillary elastics should take into account individual skeletal pattern and projected growth as well as soft tissue morphology.