Material and methods

This study initially involved 31 patients who received MSDO via a custom-made hybrid distractor ( Fig 1 ). To minimize the effect of growth in this study, girls under 12 years of age and boys under 15 years of age at the initiation of treatment (T1) were removed (6 patients). Of the remaining 25, there were 7 male and 18 female patients, with a mean age of 15.8 ± 4.8 years (range, 12.0-30.9 years) at T1.

Custom-fabricated hybrid tooth-borne and bone-borne distractor.

The patients received MSDO to treat at least 1 of the following dental or skeletal problems: mandibular crowding, maxillomandibular transverse deficiency, and unilateral or bilateral posterior scissors-bite (Brodie bite). Traditional orthodontic options and the surgical details of MSDO were presented to each patient. Before distraction, all patients received informed consent. In addition, the MSDO patients were informed that they were part of a research project and that any information related to this study might be used in scientific journals.

Standard orthodontic records were collected at 5 time points: T1, pretreatment; T2, predistraction; T3, postdistraction; T4, posttreatment; and T5, postretention. The T1 records were collected at a mean of 1.05 months before the start of treatment. The records acquired at these time points included clinical and temporomandibular examinations; study models; lateral, posteroanterior, and panoramic radiographs; and facial and intraoral photographs. Submental vertex radiographs were taken at T2 through T4.

On the study models, 8 measurements were made in millimeters with digital calipers by using a total of 11 landmarks ( Fig 2 ): (1) intersecond molar width, the distance between the central fossae of the mandibular second molars; (2) interfirst molar width, the distance between the central fossae of the mandibular first molars; (3) intersecond premolar width, the distance between the mesial fossae of the mandibular second premolars; (4) interfirst premolar width, the distance between the mesial fossae of the mandibular first premolars; (5) intercanine width, the distance between the cusp tips of the mandibular canines; (6) intercentral incisor width, the distance between the mesial contact points of the mandibular central incisors; (7) arch length, the combined measurement of the distances from the mesial marginal ridge of the mandibular right first molar to the contact point of the mandibular central incisors and the mesial marginal ridge of the mandibular left first molar to the contact point of the mandibular central incisors; and (8) irregularity index, the sum of the displacement of the anatomic contact points of the 6 mandibular anterior teeth as described by Little et al.

Study model drawing with the landmarks used in data collection: 1 , Mandibular central incisors; 2 , mandibular lateral incisors; 3 , mandibular canines; 4, mandibular first premolars; 5 , mandibular second premolars; 6 , mandibular first molars; 7 , mandibular second molars; A , mandibular central incisor contact point; B and B’ , mesial contact points of the mandibular first molars.

On each lateral cephalometric radiograph, 1 angular measurement in degrees was made by using 4 landmarks: the mandibular incisor-mandibular plane angle, the angle between the mandibular plane (gonion-menton) and the long axis of the mandibular incisor (incisal tip to the incisal apex).

On the posteroanterior cephalometric radiograph, 8 measurements in millimeters were made by using 16 landmarks ( Fig 3 ): (1) bicondylar width, the distance between the right and left lateral condylar poles; (2) bigonial width, the distance between the right and left gonions; (3) antigonial width, the distance between the right and left antigonions; (4) intersecond molar width, the distance between the lateral cementoenamel junctions of the right and left second molars; (5) intercanine width, the distance between the lateral cementoenamel junctions of the right and left canines; (6) intercentral incisor apical width, the distance between the apices of the right and left central incisors; (7) interbone marker width, the distance between the right and left bone markers; and (8) interdistractor loop width, the distance between the right and left distractor loops.

Posteroanterior cephalometric radiograph with landmarks: LPC , Lateral poles of the condyles; Go , gonion; AG , antigonion; L7 , mandibular second molars; L3 , mandibular canines; IA , mandibular incisor apices; BM , bone markers; DL , inferior distractor loops.

On the submental vertex radiograph, 1 measurement was made by using 2 landmarks between T2 and T4. Bicondylar width was determined by the measurement in millimeters between the right and left lateral poles of the condyles.

At T1, all 25 patients received comprehensive predistraction orthodontic treatment. Initially, full orthodontic appliances were placed in the maxillary arch to achieve an ideal arch shape, thereby eliminating any preexisting maxillary crowding and incisor protrusion. The modified maxillary arch shape served as a template for the treatment of the mandibular arch. Of the 25 patients, 20 received predistraction rapid maxillary expansion, and 3 adult patients received concomitant surgically assisted rapid maxillary expansion. During this phase, orthodontic appliances were placed on most or all of the mandibular teeth. After selecting an osteotomy site, brackets on the teeth adjacent to the osteotomy were angulated distally to create root divergence and thus a safer procedure. In 23 patients (92%), the osteotomy was performed between the central incisors. In 2 patients (8%), severe crowding in the central incisor region prevented adequate root divergence. The osteotomy in these 2 patients was performed between the central and lateral incisors.

In cases with anterior crowding, the osteotomy site was placed as close to the crowding as possible. In all patients, a figure-8 ligation wire was extended from the tooth adjacent to the osteotomy site to the ipsilateral canine. Before the osteotomy, an archwire was placed from the first or second molar to the contralateral molar.

All 25 patients receiving MSDO were treated by the same oral and maxillofacial surgeon (J.C.W.). Intravenous sedation was administered to each patient in a semireclined position. Local anesthesia was also used for hemostasis and postoperative pain management. We used the technique for MSDO as described by Guerrero and Bell. A horizontal incision was made with a number 15 blade approximately 8 mm below the mucogingival reflection extending from the right canine to the left canine. The incision was carried through the mucosa, submucosa, muscle, and periosteum. A full-thickness subperiosteal flap was elevated inferiorly, and the symphysis was completely degloved, including the inferior border of the mandible.

The distractor consisted of a modified expansion screw (SUPERscrew, SUPERspring, Highwood, Ill). The appliance was placed adjacent to the anterior teeth and secured to the bone of the symphysis with 2 bicortical screws (2 × 10 mm) through the loops of the inferior distractor arms. After the distractor was checked for proper positioning, the superior loops were secured to the canine (or other designated tooth) on either side of the planned osteotomy with 24-gauge stainless steel circumdental wire and light-cured composite. A tunnel was made in the mandibular midline beneath the attached gingiva and continued superiorly to the gingival margin. The midline of the mandibular bone was scored vertically with a bone bur from the mandibular incisor apices to the crestal bone. A reciprocating bone saw was then used to create a vertical midline osteotomy cut from the inferior border of the mandible up to the apices of the mandibular incisors. A narrow osteotome and a mallet were used to complete the cut in the scored bone between the roots of the central incisors (or between alternate teeth if a step osteotomy was performed). The 2 halves of the mandible were then mobilized.

After the osteotomy, complete mobilization of the hemi-mandibles was verified by opening the distractor screw to a width of 2.0 mm. The distractor screw was subsequently closed 2.0 mm, and soft-tissue closure was completed. Thus, there was no intentional expansion of the hemi-mandilbes at the time of the midsymphyseal osteotomy. The mean latency period before distraction was 7.7 ± 1.7 days.

Predistraction records (T2) were collected immediately after the latency period. In addition to the previously described records, a digital caliper was used to measure the distractor screw length. The archwire was cut and cinched at the osteotomy site before the distractor screw was activated. The patients were instructed how to properly make 2 turns (0.5 mm each) per day for a total distraction rate of 1.0 mm per day. An instruction card, which included a daily log, was also provided to each patient. Patients were monitored every 2 to 4 days until adequate space had been created to definitively eliminate the crowding and protrusion. At this time, the distractor locknut was secured to prevent back turning of the distractor screw. A denture tooth was then placed in the distraction site and secured to the adjacent teeth with a sectional wire as an esthetic consideration. The mean time between T2 and T3 was 12.1 ± 3.6 days (range, 8-27 days). Postdistraction records (T3) were taken at this time. The distractor was removed at 3.0 ± 1.0 months after distraction, and postdistraction orthodontics were initiated to complete the treatment ( Fig 4 ). The retention protocol included removable maxillary and mandibular Hawley retainers; no patient received a fixed retainer.

Mandibular center and occlusal photographs during the various phases of treatment and follow-up: A, T1, pretreatment; B, T2, predistraction; C, T3, postdistraction; D, T4, posttreatment; E, T5, postretention.

Results

The treatment phases and time points (T1-T5) in this study are illustrated in Figure 5 . The descriptive data measurements for the study models; the posteroanterior, lateral, and submental vertex radiographs; and the distractor screw measurements are presented in Table I . Table II shows the statistical comparisons ( P values) of these variables between each time point.

Patient time points: pretreatment through postretention, T1 to T5.

Table II

Comparisons of the radiographic and dental variables over time with means, standard errors, ANOVA P values, and significance levels

	Change (T2-T1)				Change (T3-T2)				Change (T4-T3)				Change (T5-T4)				Change T4-T1)				Change (T5-T1)
	Mean	SE	P	Sig	Mean	SE	P	Sig	Mean	SE	P	Sig	Mean	SE	P	Sig	Mean	SE	P	Sig	Mean	SE	P	Sig
Study models
Second molar (mm)	−0.63	0.85	0.9475	NS	1.89	0.88	0.2020	NS	0.53	0.77	0.9576	NS	0.36	0.68	0.9837	NS	1.80	0.74	0.1156	NS	2.17	0.71	0.0232	∗
First molar (mm)	0.38	0.74	0.9866	NS	3.14	0.71	0.0002	‡	−0.92	0.62	0.5802	NS	0.73	0.61	0.7515	NS	2.60	0.65	0.0012	†	3.33	0.69	<0.0001	¶
Second premolar (mm)	2.86	0.64	0.0002	‡	4.56	0.57	<0.0001	¶	−3.10	0.52	<0.0001	¶	−0.24	0.48	0.9876	NS	4.32	0.60	<0.0001	¶	4.08	0.64	<0.0001	¶
First premolar (mm)	2.01	0.48	0.0005	‡	5.25	0.40	<0.0001	¶	−3.90	0.35	<0.0001	¶	−0.24	0.44	0.9823	NS	3.44	0.44	<0.0001	¶	3.20	0.55	<0.0001	¶
Canine (mm)	0.67	0.52	0.7028	NS	5.68	0.47	<0.0001	¶	−4.47	0.38	<0.0001	¶	−0.62	0.35	0.3891	NS	1.87	0.44	0.0004	‡	1.26	0.49	0.0861	NS
Central incisor (mm)	0.14	0.16	0.9044	NS	5.75	0.34	<0.0001	¶	−5.60	0.32	<0.0001	¶	0.00	0.00	0.9981	NS	0.28	0.10	0.0327	∗	0.28	0.10	0.0327	∗
Irregularity index (mm)													1.64	0.36	<0.0001	¶	−6.71	0.60	<0.0001	¶	−5.08	0.69	<0.0001	¶
Arch length (mm)	2.05	0.96	0.1481	NS									−1.37	0.63	0.1350	NS	2.52	0.89	0.0289	∗	1.15	0.86	0.5464	NS
PA cephalogram
Bigonion (mm)	1.52	2.63	0.9782	NS	−0.66	2.03	0.9975	NS	−0.72	2.02	0.9965	NS	−1.27	2.33	0.9822	NS	0.14	2.62	1.0000	NS	−1.13	2.84	0.9946	NS
Biantigonion (mm)	0.39	3.18	0.9999	NS	0.30	1.71	0.9998	NS	−0.59	1.65	0.9963	NS	0.65	1.76	0.9961	NS	0.10	3.15	1.0000	NS	0.75	3.24	0.9994	NS
Bicondylar width (mm)	−6.62	4.33	0.5472	NS	1.60	2.54	0.9700	NS	1.89	1.89	0.8535	NS	−1.12	2.00	0.9802	NS	−3.13	3.98	0.9334	NS	−4.25	4.18	0.8463	NS
Bone markers (mm)					5.12	0.43	<0.0001	¶	−0.29	0.52	0.9407	NS	1.11	0.71	0.4054	NS
Distractor loops (mm)					5.66	0.36	<0.0001	¶
Interincisor apices (mm)	3.61	0.65	<0.0001	¶	5.07	0.51	<0.0001	¶	−7.77	0.43	<0.0001	¶	0.88	0.31	0.0438	∗	0.91	0.60	0.5433	NS	1.80	0.60	0.0281	∗
Intercanines (mm)	0.99	0.92	0.8187	NS	5.61	0.40	<0.0001	¶	−3.39	0.39	<0.0001	¶	0.99	0.37	0.0661	NS	3.21	0.91	0.0061	†	4.20	0.94	0.0002	‡
Intersecond molars (mm)	−4.50	1.80	0.0997	NS	2.33	0.88	0.0687	NS	−1.07	0.84	0.7089	NS	0.65	0.83	0.9329	NS	−3.24	1.78	0.3696	NS	−2.59	1.79	0.6013	NS
Distractor screw (mm)					6.33	0.24	<0.0001	¶
Lateral cephalogram
L1-MP (°)	2.53	1.94	0.6870	NS	0.92	1.89	0.9886	NS	−1.05	1.96	0.9834	NS	−2.06	2.47	0.9197	NS	2.40	2.00	0.7527	NS	0.34	2.38	0.9999	NS
Clinical measurement
Distractor screw (mm)					6.37	0.28	<0.0001	¶
SMV cephalogram
Bicondylar width (mm)					−0.26	5.44	0.9987	NS	1.97	5.68	0.9357	NS

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