INTRODUCTION

In correcting a dentofacial deformity, the team approach between a surgeon and an orthodontist is essential to obtain a good functional and esthetic outcome.12 As a communicative and diagnostic tool, the surgical treatment objective (STO) can provide the surgeon and orthodontist with information about the amount and direction of the surgical movement of the hard tissue, resulting change of the soft tissue profile, and preoperative and postoperative orthodontic treatment plan.3

Many skeletal Class III malocclusions are known to require two-jaw surgery to get a mandibular setback and various new positions of the maxilla for an esthetic and stable result.45 There have been a number of sophisticated techniques available for orthognathic surgical treatment planning.6–18 However, despite good surgical technique, in cases of complex two-jaw surgery, anatomic obstacles, errors in mounting, model surgery and intermediate splint fabrication, unintended malpositioning of the mandibular condyle, and mistakes in measurement of the external and internal reference points in the operative procedure can make a significant discrepancy in the maxillary repositioning between the STO and surgical result.101719–27 In some instances, inaccurate placement of the maxilla results in an unwanted repositioning of the mandible and less than an ideal functional and esthetic outcome.722232628 Therefore, it is important to compare the actual surgical results of the maxillary repositioning with the STO.

Ong et al26 reported that in advancement of the maxilla, 87% of patients had a difference of 2 mm or less from the prediction in both the vertical and horizontal dimensions. Jacobson and Sarver3 reported that the actual results of 80% of the maxillary impaction and advancement were within 2 mm of the prediction and 43% within 1 mm of the prediction. However, Semaan and Goonewardene,29 in the cases with elongation, impaction, and advancement of the maxilla reported that 66% were within 2 mm of the prediction and only 26% within 1 mm of the prediction. This difference in accuracy seems to occur due to heterogenous samples, various surgical movement types, and multiple operators. To find out the precise accuracy rate according to the surgical movement type of the maxilla, it is important to classify the surgical movement type for the maxillary repositioning and to confine the samples to the same skeletal pattern, surgical technique, and surgeon.

Therefore, the purposes of the present study were to compare the surgical accuracy of the maxillary positioning according to surgical movement type in two-jaw surgery.

MATERIALS AND METHODS

The samples consisted of 52 Korean young adult patients with skeletal Class III malocclusion (26 men and 26 women; mean age: 22.6 years), who were treated with two-jaw orthognathic surgery from January 2006 to July 2007. The lateral cephalograms were taken 1 month before (T0, Figure 1A) and 1 day after surgery (T1, Figure 1B) and traced for superimposition.

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Since the pattern of maxillary positioning could be variable according to the condition of the malocclusion, surgeon, and STO, the samples should be limited as follows:

• —Surgeon: one surgeon to avoid surgeon-related bias;

• —Skeletal pattern: Class III malocclusion to avoid skeletal pattern-related bias;

• —Surgery method: two-jaw surgery (LeFort I osteotomy/bilateral sagittal split ramus osteotomy)

After the maxilla was down-fractured by a LeFort I osteotomy, vertical control of the mobilized maxilla was achieved by a combination of a nasion screw as the external reference point and bony marks above and below the osteotomy line as the internal reference points. Horizontal and transverse movements of the maxilla were controlled with intermediate surgical wafers.

The samples were allocated into maxillary advancement (MA), maxillary total setback (MS), maxillary impaction (MI), and maxillary elongation (ME) according to surgical movement type. If one sample had a combination of the surgical movement types of the maxilla (for example, the maxilla was moved with posterior impaction and advancement), each movement was classified into the corresponding movement type (for example, MI and MA, respectively) (Table 1).

Table 1.  Demographic Data According to the Surgical Movement Types of the Maxilla

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The horizontal reference line (HRL) was a line that angulated 7° clockwise to the sella-nasion line passing through sella and the vertical reference line (VRL), a perpendicular line to the HRL passing through sella (Figure 2A).33031 The distance from the tip of the upper central incisor (U1) to the HRL for ME and to the VRL for MA and MS, and the distance from the mesiobuccal cusp tip of the upper first molar (U6) to HRL for MI at T0 and T1 were measured. Measurements were done using a digitizer (Intuos2 graphic tablet, Wacom Technology Co, Vancouver, Canada) and V-Ceph program (Cybermed, Seoul, Korea) (Figure 2B). These distances were compared with those of the STO.

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A discrepancy of less than 1 mm between the STO and actual surgical result (T1) was regarded as accurate. The accuracy rate ([number of accurate sample/number of sample] ×100) and the surgical achievement ratio ([amount of movement in surgical result/amount of movement] in STO ×100) were calculated. One-way analysis of variance (ANOVA) and crosstab analyses were done for statistical analysis.

RESULTS

Although the MS and MI showed a wider range than the MA and ME, the mean values of discrepancy were less than 1 mm in all groups (Table 2). In addition, there was no statistically significant difference in the mean value of discrepancies among groups (Table 2). Although the MS (69.2%) and MI (69.0%) showed a lower accuracy rate than MA (87.5%) and ME (83.3%), no significant difference existed in the distribution of the accurate (≤1.0 mm) and inaccurate samples (>1.0 mm) among the groups (Table 2).

Table 2.  Comparison of Discrepancy Between the Surgical Treatment Objective (STO) and the Surgical Result^a

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Although ME (93.54%) showed a tendency of undercorrection and the MS (107.10%) and MI (105.42%) a tendency of overcorrection, there was no statistically significant difference in the mean value of surgical achievement ratio among groups (Table 3). In addition, the MI and ME had a wider range of surgical achievement ratios than did the MA and MS (Table 3).

Table 3.  Comparison of Surgical Achievement Ratio^a

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DISCUSSION

The stability and predictability of orthognathic surgical procedures is reported to vary by the direction of the surgical movement, the type of fixation, and the surgical technique employed, largely in that order of importance.2 Therefore, classification of the surgical movement type according to direction would be important to assess the accuracy of repositioning of the maxilla.

CONCLUSIONS

• Although there was no significant difference in accuracy rate and surgical achievement ratio among surgical movement types, a maxillary advancement could be regarded as relatively the most accurate and reliable surgical movement type.

• If the surgical plan and procedure is done with caution, maxillary total setback and maxillary impaction could be regarded as just as accurate a procedure as other types of surgical movement.