Introduction
This study evaluated whether presurgical characteristics, the magnitude of mandibular advancement, and changes in mandibular plane angle are correlated with long-term stability and postsurgical condylar remodeling and adaptations using 3-dimensional imaging.
Methods
Forty-two patients underwent bilateral sagittal split osteotomies for mandibular advancement using rigid fixation. Cone-beam computed tomography (CBCT) scans were acquired before surgery (T1), immediately after surgery (T2), and at long-term follow-up (T3). The average follow-up period was 5.3 ± 1.7 years after surgery. Anatomic landmark identification on the cone-beam computed tomographies and subsequent quantification of the changes from T1 to T2 and T2 to T3 were performed in ITK-SNAP (version 2.4; itksnap.org) and 3DSlicer (version 4.7; http://www.slicer.org ) software. Surgical displacements, mandibular plane angle changes, and skeletal stability were measured relative to cranial base superimposition, whereas condylar remodeling was measured relative to regional condylar registration. Partial correlation coefficients were used to assess relationships between clinical and surgical variables, condylar remodeling, and long-term surgical relapse while controlling for variability in the length of follow-up.
Results
B-point relapsed more than 2 mm posteriorly in 55% of the patients. The only variables strongly associated with the posterior movement of B-point long-term were mesial yaw of the condyle during surgery ( P ≤0.01) and the length of follow-up from T2 to T3 ( P ≤ 0.01). There was no relationship between the magnitude of advancement or presurgical mandibular plane angle and relapse or condylar resorption. Condylar resorption was strongly associated with relapse of B-point in the posterior direction ( P ≤0.01) and clockwise rotation of the mandibular plane long-term ( P ≤0.01). Twenty-nine percent of subjects showed resorption of more than 2 mm in the inferior direction at the lateral pole, and 17% of the subjects showed resorption of more than 2 mm in the inferior direction at condylion. Compared with male subjects, females exhibited significantly greater condylar remodeling ( P ≤0.01) and slightly greater relapse at B-point ( P ≤0.05).
Conclusions
Surgical relapse at B-point may occur slowly over time and is primarily due to condylar resorption in mandibular advancement patients. Mesial yaw of the condyle during surgery may lead to condylar resorption postsurgically. In addition, females are at greater risk of condylar resorption postsurgically.
Highlights
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The length of the follow-up period was associated with the posterior movement of B-point.
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Relapse may occur over time.
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Mesial yaw of condyle during surgery was associated with relapse of B-point.
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Condylar resorption was associated with relapse of B-point long-term.
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Females exhibited greater condylar remodeling and slightly greater relapse at B-point.
Mandibular advancement surgery is a treatment option for patients with mandibular deficiency who have reached skeletal maturity. The bilateral sagittal split osteotomy (BSSO) is the most widely used surgical procedure to advance the mandible. , As with any surgical procedure, it is imperative to understand the surgical risks, stability, and long-term changes associated with the surgery. Few studies have evaluated the long-term stability and remodeling that occurs with mandibular advancement procedures in large sample groups. , Three-dimensional (3D) imaging allows for the quantification of changes in surface models to evaluate bone remodeling and morphologic changes related to surgical treatments. ,
The rates of relapse of mandibular advancement described in the literature are highly variable. , , Reported rates of relapse depend on multiple factors, including preoperative characteristics, the surgical procedure performed (mandibular only or bimaxillary surgery), type of fixation, and length of follow-up. , ,
Late surgical relapse is thought to be primarily due to condylar remodeling, including progressive condylar resorption and muscular influences. The “incidence of condylar resorption after orthognathic surgery reported in the literature ranges from 1% to 31%.” Condylar resorption after surgery is critical to understand because it manifests clinically as a posterior movement of the mandible, anterior open bite, and increased overjet. , Therefore, condylar resorption can diminish the predictability of surgical stability. , The etiology or pathogenesis of condylar resorption after surgery is unknown. , Other factors that may be related to late relapse include gender, steepness of the mandibular plane angle, rotation of the proximal segment, and the magnitude of advancement. , , ,
Currently, the question remains as to whether the amount of mandibular advancement, presurgical condylar morphology, or long-term postsurgical condylar remodeling and adaptations affect the stability of the mandibular advancement. The 3D understanding of complex movements during orthognathic surgery may offer some hints to improve stability. With the “advent of 3D imaging, such as CBCT with voxel-based superimposition methods, clinicians are now provided better understanding of bilateral structures and greater accuracy when compared to other 2D techniques.” This study is particularly relevant to our clinical knowledge of the long-term effects of mandibular advancement on temporomandibular joint changes and the stability of anteroposterior and vertical corrections. These long-term effects may apply to nonsurgical mandibular advancement modalities, including Invisalign precision wings and obstructive sleep apnea appliances.
This study investigated whether presurgical characteristics, the magnitude of mandibular advancement, and changes in mandibular plane angle are correlated with long-term stability and postsurgical condylar remodeling and adaptations using 3D imaging. This research may provide insight into the predictive capability of pretreatment condylar morphology of patients undergoing mandibular advancement surgery and long-term surgical outcomes and stability.
Material and methods
This investigation involved a secondary analysis of deidentified cone-beam computed tomography (CBCT) scans for patients treated with mandibular advancement surgery at a single private orthodontic practice. The patients were consecutively treated between May 2008 and April 2014. All patients underwent comprehensive orthodontic treatment and a BSSO to advance the mandible. The study was approved by the university institutional review board (HUM00125163). The sample represents a private practice mandibular retrognathic patient population in which patients are considered ideal candidates for 1-jaw surgery (mandibular advancement only).
The surgical technique and rigid fixation protocol were the same for all patients. Rigid fixation hardware consisted of plates and monocortical screws. All patients were recalled to the private practice for clinical purposes to evaluate surgical stability. Forty-nine percent of the recalled patients presented for follow-up resulting in a total sample size of 44 subjects, which was expected considering the participation rate in the long-term recall of patients. In addition to a mandibular advancement, 19 patients received a genioplasty. The records collected included 0.3 × 0.3 × 0.3-mm voxel size CBCT scans (5 mA, 120 kVp, exposure time of 20 seconds, voxel size of 0.3 mm, axial slice thickness of 0.3 mm, and scanning area of 16 × 22 cm) acquired using an iCat machine (Imaging Sciences International, Hatfield, Pa) with a 16 × 22 cm field of view at 3 points: preoperative (T1), immediately postoperative (T2), and long-term follow-up (T3). The average follow-up period was 5.3 ± 1.7 years after surgery.
The records of 44 subjects were evaluated to be included in the retrospective study. The inclusion criteria were presurgical Class II skeletal malocclusion with mandibular deficiency, 6 mm minimum overjet before surgery, cervical vertebral maturation stage 5 or greater before surgery. The exclusion criteria were less than 6 mm overjet before surgery, cervical vertebral maturation stage 5 or lower before surgery, genioplasty procedure affecting B-point, “skeletal deformities from trauma, cleft lip, and palate, syndromic or degenerative conditions such as rheumatoid arthritis”. Two subjects were excluded resulting in a final sample size of 42 subjects. One subject exhibited less than 6 mm overjet before surgery, and another subject presented as cervical vertebral maturation stage 3 before surgery.
Pretreatment 2-dimensional lateral cephalograms were evaluated to determine cervical vertebral maturation stage, Frankfort-mandibular plane angle, overbite, and overjet.
The original Digital Imaging and Communications in Medicine files were converted to deidentified Guys Imaging Processing Laboratory files using open-source software ITK-SNAP (version 2.4; www.itknsnap.org ). Three-dimensional image analysis was performed by a single-blinded examiner (L.E.) in ITK-SNAP and 3DSlicer (version 4.7; www.slicer.org ). Three-dimensional models were generated from the scans and adjusted in a 3D coordinate system to achieve a common head orientation. The scans were analyzed to evaluate the presurgical position of the mandible and condyles as well as changes in the position of the mandible and condyles after surgery and at long-term follow-up. These changes were measured relative to voxel-based superimpositions on the cranial base. Voxel-based superimposition on the cranial base has been previously validated.
Condylar morphology was evaluated before the surgery and at long-term follow-up. Condylar changes were measured relative to regional condylar voxel-based superimpositions on the basis of stable regional anatomic structures. This approach is in contrast to previous studies that used cranial base superimpositions. Regional condylar voxel-based superimposition has been shown to be highly accurate. The voxel-based registration tool is fully automated and registers the T3 condyle to the T1 condyle on the basis of the correspondence of over 1000 voxels to achieve a reliable and reproducible superimposition of the time points. All condyles were cropped within the same 3D plane to standardize the regions of interest to be used for regional registration.
Reproducible skeletal landmarks were selected ( Table I ). Superior gonion was constructed by bisecting the angle formed by the intersection of the mandibular plane and ramus of the mandible, centered medial-laterally on the angle of the mandible. Superior gonion was used to calculate the roll of each ramus as well as displacement at gonion. Inferior gonion was defined as the most posterior inferior point on the angle of the mandible, centered medial-laterally on the angle of the mandible. Inferior gonion was used to calculate pitch. A modified menton landmark was defined for this study as a point centered medial-laterally on the lower border of the mandible nearest to the mental foramen. This definition of menton was used so that the landmark would not be affected by a surgical cut for a genioplasty.
| Anatomic landmarks |
| Right condylion |
| Right lateral condylar pole |
| Right medial condylar pole |
| Right superior gonion |
| Right inferior gonion |
| Right modified menton |
| Left condylion |
| Left lateral condylar pole |
| Left medial condylar pole |
| Left superior gonion |
| Left inferior gonion |
| Left modified menton |
| B-point |
| Q3DC 3DSlicer calculated anatomic midpoints |
| Midcondyle (between lateral pole and medial pole) |
| Midgonion (between right and left inferior gonion) |
| Midmenton (between right and left modified menton) |
The anatomic landmarks of interest were prelabeled in ITK-SNAP on the registered T1, T2, and T3 segmentations simultaneously. This previously validated method allowed for increased consistency and reproducibility of landmark placement. Linear millimetric measurements were recorded to quantify the changes in 3D distance from T1 to T2 and T2 to T3. The 3D distance component parts included: medial or lateral, superior or inferior, anterior or posterior. Angular measurements recorded the change in pitch of the mandibular plane (clockwise or counterclockwise) defined as a line from midinferior gonion to midmenton, yaw of each condyle (mesial or distal) defined as a line from lateral condylar pole to medial condylar pole, and roll of each ramus (medial or lateral) defined as a line from midcondyle to superior gonion.
Three-dimensional condylar morphology was assessed using reproducible condyle landmarks between T1 and T3 ( Table I ). Individual condyles were compared longitudinally within subjects to detect changes in postsurgical condylar morphology.
Statistical analysis
It was predetermined that B-point would be the primary variable used to assess the clinical stability of the mandibular advancement procedure. The stability of B-point in the anteroposterior dimension followed by the superior-inferior dimension was most clinically relevant. Displacement at a B-point greater than 2 mm was considered unstable. Condylar remodeling changes greater than 2 mm were considered clinically significant.
SPSS software (version 16.0, SPSS, Chicago, Ill) was used for all statistical computations. Normal distribution was assessed with the Shapiro-Wilk test. Nonparametric tests were applied because not all variables followed a normal distribution. However, both parametric and nonparametric tests produced similar results.
The Wilcoxon signed-rank test was used to assess differences between right and left measurements within each patient. No statistically significant differences existed between right and left measurements within each patient. Therefore, the average was computed for the right and left measurements for each patient.
Descriptive statistics were used to report clinical characteristics before surgery (T1), as well as surgical (T1 − T2) and long-term postsurgical changes (T2 − T3).
Spearman’s rank-order correlations were used to determine the relationship between presurgical clinical characteristics and long-term relapse and remodeling. It was discovered that there was a strong positive relationship between the length of the follow-up period (T2 − T3) and posterior movement of B-point. Because the length of the follow-up period was not standardized, subsequent statistical analyses controlled for the variable by using partial correlation coefficients. Partial correlation coefficients were used to determine the relationship between clinical and surgical factors, condylar remodeling, and long-term surgical relapse.
Statistically significant differences were recognized at an alpha level of P <0.05. Age and time between follow-up period (T2 − T3) were measured in years. Linear displacements were measured in millimeters and rotational displacements measured in degrees.
Repeated measures were performed for 10 patients, selected at random, to conduct an intraexaminer reliability test. All landmarks were replicated in ITK-SNAP for T1, T2, and T3 by the same examiner within 1 month of the original landmark placement to test for method error when calculating changes T1 − T2, T2 − T3, and condyle changes T1 − T3. All intraclass correlation coefficients were above 0.95.
Results
The sample consisted of comparable gender distribution (22 females, 20 males). Nineteen of the 42 patients had a genioplasty procedure in addition to the mandibular advancement. The average follow-up period was 5.3 ± 1.7 years postsurgery. The sample included patients with both high and low mandibular plane angles (Frankfort-mandibular plane angle, 14.0°-34.4°).
The total surgical displacements (T1 − T2) for all patients are shown in Figure 1 . B-point was displaced more than 4 mm anteriorly in 86% of the patients and more than 2 mm inferiorly in 41% of the patients. Gonion was displaced more than 2 mm laterally in 72% of the patients and more than 2 mm anteriorly in 64% of the patients—likely because of torquing of the ramal segment during surgery. The condyles were torqued mesially (yaw) more than 2° in 49% of the patients, and condylion was displaced laterally more than 2 mm in 45% of the patients. Preoperative and postoperative views of an example subject are shown in Figures 2 and 3 .
The total long-term relapse and positional changes (T2 − T3) for all patients are shown in Figure 4 . B-point relapsed more than 2 mm posteriorly in 55% of patients. Gonion relapsed more than 2 mm medially in 69% of the patients, and the role of the ramus relapsed more than 2° medially in 59% of the patients. However, gonion continued to move anteriorly and superiorly at long-term follow-up. The condyles relapsed distally (yaw) more than 2° in 37% of the patients, and condylion relapsed laterally more than 2 mm in 38% of the patients. Postoperative and long-term follow-up views of the example subject are shown in Figures 5 and 6 .
Long-term condyle resorption and remodeling are illustrated in Figure 7 . The greatest amount of remodeling was seen at the lateral poles in the inferior direction (−1.3 ± 1.3 mm). The long-term condyle resorption and remodeling changes for all subjects are shown in Figure 8 . Twenty-nine percent of the subjects showed resorption of more than 2 mm in the inferior direction at the lateral pole.
Spearman and partial correlations determined the relationship between presurgical variables and long-term relapse and remodeling ( Table II ). Posterior movement of B-point long-term was strongly associated with the length of the follow-up period ( P ≤0.01), and increased age of the patient was strongly associated with more anterior movement of gonion long-term ( P ≤0.01). Partial correlations determined the relationship between the number of surgical displacements and long-term relapse and remodeling while controlling for the time between the follow-up period. Posterior movement of B-point long-term was strongly associated with mesial yaw of the condyle during surgery ( P ≤0.01). The greater the surgical displacement of gonion in the lateral and inferior directions and B-point in the inferior direction, the greater the relapse of those landmarks in the opposite direction long-term ( P ≤0.01). In addition, the greater the surgical displacement of the condyles at condylion correlated to greater relapse of the condyles in the opposite direction long-term in all 3 planes of space ( P ≤0.01). Condylar resorption at the lateral pole in the medial direction and both the medial and lateral poles and condylion in the inferior direction was strongly associated with relapse of B-point in the posterior direction long-term ( P ≤0.01). In addition, condylar resorption at the lateral pole in the medial direction and at the lateral pole and condylion in the inferior direction was strongly associated with clockwise rotation of the mandibular plane long-term ( P ≤0.01). Condylar remodeling was strongly associated with the amount of presurgical overjet ( P ≤0.01). Remodeling at both condylion and the medial pole in the lateral direction was associated with older age at the time of surgery ( P ≤0.01).
| Correlation coefficients | T2 − T3 gonion (mm) (+) Ant (−) Pos |
T2 − T3 gonion (mm) (+) Lat (−) Med |
T2 − T3 gonion (mm) (+) Sup (−) Inf |
T2 − T3 B-point (mm) (+) Sup (−) Inf |
T2 − T3 B-point (mm) (+) Ant (−) Pos |
T2 − T3 Pitch (Go-Me) (°) (+) CW (−) CCW |
T1 − T2 B-point (mm) (+) Ant (−) Pos |
T1 − T2 B-point (mm) (+) Sup (−) Inf |
T1 − T2 Pitch (°) (+) CW (−) CCW |
Age at T1 (y) | T1 FMA (°) | T1 OJ (mm) | T2 − T3 condylion (mm) (+) Lat (−) Med |
T2 − T3 condylion (mm) (+) Ant (−) Pos |
T2 − T3 condylion (mm) (+) Sup (−) Inf |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Age at T1 (y) ∗ | 0.54 ¶ | −0.06 | 0.49 ¶ | 0.02 | −0.01 | 0.27 | |||||||||
| Follow−up period (y) ∗ (T2 − T3) |
−0.11 | −0.17 | 0.30 | −0.04 | −0.41 ¶ | 0.08 | |||||||||
| T1 FMA (°) ∗ | −0.07 | −0.08 | −0.05 | −0.04 | 0.00 | 0.08 | |||||||||
| T1 OJ (mm) ∗ | 0.01 | 0.01 | −0.16 | 0.04 | 0.05 | −0.14 | |||||||||
| T1 OB (mm) ∗ | −0.18 | −0.05 | −0.04 | −0.10 | 0.01 | −0.18 | |||||||||
| T1 − T2 condyle yaw (°) † (+) Mes (−) Dis |
−0.25 | −0.36 ‖ | 0.16 | −0.20 | −0.42 ¶ | 0.05 | |||||||||
| T1 − T2 gonion (mm) † (+) Lat (−) Med |
0.06 | −0.50 ¶ | 0.07 | −0.27 | −0.32 ‖ | 0.18 | |||||||||
| T1 − T2 gonion (mm) † (+) Sup (−) Inf |
0.21 | −0.09 | −0.48 ¶ | −0.39 ‖ | 0.15 | −0.15 | |||||||||
| T1 − T2 B-point (mm) † (+) Sup (−) Inf |
0.24 | −0.01 | −0.11 | −0.44 ¶ | −0.01 | 0.26 | |||||||||
| T1 − T2 B-point (mm) † (+) Ant (−) Pos |
0.06 | −0.03 | 0.05 | 0.15 | −0.23 | 0.08 | |||||||||
| T1 − T2 pitch (Go−Me) (°) † (+) CW (−) CCW |
0.14 | 0.11 | −0.36 ‖ | ||||||||||||
| T1 − T3 lateral pole (mm) ‡ (+) Med (−) Lat |
−0.23 | −0.52 ¶ | 0.54 ¶ | −0.16 | −0.05 | −0.17 | −0.07 | 0.06 | −0.33 ‖ | ||||||
| T1 − T3 lateral pole (mm) ‡ (+) Sup (−) Inf |
0.24 | 0.50 ¶ | −0.42 ¶ | 0.14 | 0.03 | −0.10 | 0.12 | 0.18 | 0.27 | ||||||
| T1 − T3 condylion (mm) ‡ (+) Med (−) Lat |
−0.11 | −0.33 ‖ | 0.13 | −0.36 ‖ | −0.26 | 0.28 | −0.52 ¶ | 0.07 | −0.44 ¶ | ||||||
| T1 − T3 condylion (mm) ‡ (+) Ant (−) Pos |
0.01 | 0.27 | −0.27 | 0.27 | 0.17 | 0.16 | 0.12 | −0.20 | 0.44 ¶ | ||||||
| T1 − T3 condylion (mm) ‡ (+) Sup (−) Inf |
0.25 | 0.64 ¶ | −0.56 ¶ | 0.21 | 0.10 | 0.06 | 0.16 | −0.14 | 0.48 ¶ | ||||||
| T1 − T3 medial pole (mm) ‡ (+) Med (−) Lat |
−0.31 | 0.02 | −0.20 | −0.08 | 0.06 | 0.11 | −0.50 ¶ | −0.08 | 0.03 | ||||||
| T1 − T3 medial pole (mm) ‡ (+) Ant (−) Pos |
0.15 | 0.21 | −0.19 | 0.23 | 0.07 | 0.10 | 0.01 | −0.33 ‖ | 0.43 ¶ | ||||||
| T1 − T3 medial pole (mm) ‡ (+) Sup (−) Inf |
0.09 | 0.58 ¶ | −0.36 ‖ | 0.23 | 0.15 | −0.12 | 0.16 | −0.10 | 0.45 ¶ | ||||||
| T1 − T2 condylion (mm) § (+) Lat (−) Med |
−0.79 ¶ | −0.20 | 0.53 ¶ | ||||||||||||
| T1 − T2 condylion (mm) § (+) Ant (−) Pos |
−0.09 | −0.74 ¶ | 0.42 ¶ | ||||||||||||
| T1 − T2 condylion (mm) § (+) Sup (−) Inf |
0.39 ‖ | 0.32 ‖ | −0.82 ¶ |
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