Introduction

Orthognathic surgery is the treatment of choice for severe skeletal dysgnathia and dentofacial deformities. The surgical discipline, which began in 1849 in the USA, has evolved progressively in central Europe since the 1950s and today is commonly applied [29]. Although most patients benefit from surgery [20, 23], up to 22% are dissatisfied with the esthetic result [10]. The esthetic improvement of the facial profile very much affects the social and psychological well-being of the patient [3, 9, 10]. Prediction of the soft tissue profile is of great importance in treatment planning and patient motivation [16].

The first studies analyzing the soft tissue profile change after orthognathic surgery were published in the 1970s with the purpose to acquire more information that can be used as an esthetic guide in orthognathic surgery [18, 21, 25].

We included 115 maxillary LeFort I osteotomy cases in monognathic and bignathic procedures in this study. Our target was a contribution to the facial profile prediction after maxillary orthognathic surgery by providing profound data of the soft to hard tissue dependencies.

Materials and methods

In this retrospective study, we analyzed 115 patients on whom interdisciplinary orthodontic and orthognathic surgical treatment had been performed at the Department of Orthodontics and the Department of Oral and Maxillofacial Surgery, Ludwig-Maximilians University of Munich, Germany (Table 1).

Table 1 Tab. 1 Descriptive statistics of the 115 patientsDeskriptive Statistik der 115 Patienten
Full size table

The sample consists of 43 class II and 72 class III patients, all interbasal open O1 and N1 types with divergent inclination of the skeletal bases, with posterior inclination of the mandible and anterior inclination of the maxilla in various manifestations.

On all 115 subjects LeFort I osteotomy with posterior maxillary impaction had been performed, 35 in monognathic and 80 in bignathic procedures. All 115 subjects received rigid fixation. For both females and males the minimum age was 18 in order to avoid errors caused by the influence of growth. A history of prior maxillofacial surgery, wire fixation, trauma, clefts and craniofacial syndromes were further exclusion criteria.

The sample size was calculated for a power of 0.8 at a significance level of 0.05 with Altman’s nomogram [2]. As 15 patients were excluded because of unconfident landmark identification caused by insufficient radiograph quality, the study had a power of 0.78 at a significance level of 0.05.

For each subject a preoperative and a postoperative radiograph, taken at least 6 months after surgery, was selected. A Canon EOS 5D digital camera with a Canon compact-macro EF 50 mm, f 1:2.5 lens (Canon Inc., Tokio, Japan) on a “copylizer eVision exe.cutive” camera stand (Kaiser Fototechnik GmbH &Co. KG, Buchen, Germany) was used for the digitization process of the radiographs. One of the authors (S. R.) conducted a cephalometric analysis based on the method of Segner/Hasund [28] with the software DiagnoseFix 12.2006 (Dr. Jörg Wingberg, Buchholz, Germany).

On the basis of the analysis of Lines and Steinhauser [18] and Legan and Burstone [16] our cephalometric analysis was reduced to the examination of these relevant landmarks:

The soft tissue landmarks Sn (subnasale), A′ (soft tissue A) and Ls (labrale superius) and the corresponding hard tissue landmarks A (A point) and Is (incision superius) (Fig. 1). A coordinate system was designed to assess the surgical movement in the sagittal direction. NSL (nasion-sella line) was used as the x‑axis, while the y‑axis was constructed as a line through the landmark sella, perpendicular to the NSL. This y‑axis served as the vertical reference line for the examined landmarks.

Fig. 1 Abb. 1
figure 1

Landmarks and reference lines used in this study: 1 sella; 2 nasion; 3 A point; 4 incision superius; 5 subnasale; 6 soft tissue A point; 7 labrale superius

In der Studie verwendete Referenzpunkte und -linien. 1 Sella; 2 Nasion; 3 A-Punkt; 4 Incision superius; 5 Subnasale; 6 Weichgewebe-A-Punkt; 7 Labrale superius

Full size image

We quantified the distance from the vertical reference line to each soft and hard tissue point and calculated the difference of the postsurgical minus the presurgical values.

The correlations between the shift in soft tissue landmarks (∆Sn, ∆A′ and ∆Ls) and the shift in the corresponding hard tissue landmarks (∆A and ∆Is) were then statistically analyzed. We conducted a bivariate linear regression analysis to determine the soft tissue profile changes related to surgical movement of the underlying hard tissue structures (R 3.1.2, R Foundation for Statistical Computing, Vienna, Austria).

Results

Soft tissue A point

Soft tissue A point changed minimum −5.5 mm and maximum 9.7 mm, while hard tissue A point changed minimum −6.6 mm and maximum 10.7 mm. The Shapiro–Wilk test showed normal distribution for displacements both in soft tissue A point (p = 0.53) and in hard tissue A point (p = 0.35). The Pearson correlation coefficient for soft tissue A point and hard tissue A point was 0.83 (Fig. 2a). The model to predict the change of soft tissue A point was soft tissue A point = 0.73 × hard tissue A point with a coefficient of determination of r2 = 0.69 and a standard error of the estimate of 1.7 mm.

Fig. 2 Abb. 2
figure 2

Scatterplot and prediction model of sagittal changes for soft tissue A point (a), subnasale (b), and labrale superius (c)

Streudiagramm und Vorhersagemodell der sagittalen Veränderungen für Weichgewebe-A-Punkt (a), Subnasale (b) und Labrale superius (c)

Full size image

Subnasale

Displacements of subnasale ranged from minimum −5.2 mm to maximum 8.9 mm and were normally distributed according to the Shapiro–Wilk test (p = 0.42). The Pearson correlation coefficient for subnasale and hard tissue A point was 0.76 (Fig. 2b). Displacements in subnasale were predicted by the model Subnasale = 0.57 × hard tissue A point. The coefficient of determination was r2 = 0.58 with a standard error of the estimate of 1.8 mm.

Labrale superius

Labrale superius changed minimum −7.2 mm and maximum 8.9 mm, whereas incision superius changed minimum −7.5 mm and maximum 10.7 mm. Normal distribution of the displacements was indicated by a Shapiro–Wilk test for labrale superius (p = 0.47) and incision superius (p = 0.73). Pearson’s correlation coefficient for labrale superius and incision superius was 0.81 (Fig. 2c). The prediction model of displacements for labrale superius was Labrale superius = 0.73 × Incision superius with a coefficient of determination of r2 = 0.66 and a standard error of the estimate of 1.9 mm.

The scatterplots (Fig. 2) show a linear correlation between each pair of landmarks, but with a wide spread for all three pairs. The residuals of the prediction models were symmetrically distributed without any recognizable pattern that would indicate another additional prediction variable (Fig. 3).

Fig. 3 Abb. 3
figure 3

Residual plots for predicted sagittal changes in soft tissue A point (a), subnasale (b), and labrale superius (c)

Residuenplots für die prognostizierten sagittalen Veränderungen von Weichgewebe-A-Punkt (a), Subnasale (b) und Labrale superius (c)

Full size image

Discussion

Our results in this study for A′/A (73%), Sn/A (57%) and Ls/Is (73%) correspond to many other studies [4,5,6, 17, 18, 24]. Some authors found results equal to ours by using slightly different methods, like for example different reference lines [4, 24, 30]. But there also exists a high variation of different results in literature [7, 8, 14, 15, 27]. The reasons that could be responsible for this high variance might be tonicity, posture, muscle pull and difficulty adopting a relaxed lip position during cephalogram exposure [22]. Another reason might as well be that the soft tissues follow the maxillary hard tissue structures in a relationship not as close as in the mandible [26] because the soft tissue of the upper lip is firmly connected to the base of the nose [18]. Furthermore, the lower lip also has an influence on the position of the upper lip. A surgical change of the position of the maxilla leads to a change of the position of the mandible and of the lower lip, both in bignathic and monognathic surgery.

It is apparent that maxillary soft tissue depends on a complexity of functional and anatomical influences, which might be the reason for our rather high prediction error of about 2 mm, as well as the high variety of results in literature.

Although landmark localization on lateral cephalograms may be impaired by distortion or magnification [11], this two-dimensional method offers high reliability [1, 12].

Three-dimensional technologies can improve diagnostics providing a highly accurate reproduction of the facial morphology and even a very precise automatic cephalometry with exact landmark detection [13, 19]. However, our objective was to evaluate the ratio of the soft to hard tissue changes. The lateral cephalogram displays both hard and soft tissue structures in just one image, and that at a very low radiation exposure. Therefore, it was the medium of choice for our purpose. Furthermore, most clinicians simply do not have three-dimensional equipment, which still makes radiographs a widespread and valuable technique that should not be underestimated [26].

Conclusions

In this retrospective study we revealed a linear correlation between each pair of soft and hard tissue landmarks. But at the same time all three measurements demonstrated a wide distribution of measurement values. This outcome and also the mean prediction error of about 2 mm prompts a cautious use of postsurgical predictions of the maxillary-related soft tissue profile changes.