Material and methods

A sample of subjects from various studies on long-term incisor stability after orthodontic treatment was used in this investigation. A detailed description of the methodology is reported elsewhere. The postretention collection in the Department of Orthodontics at the University of Washington in Seattle was screened, and the incisor irregularity (II) index was measured on study models taken at least 10 years out of retention (T3). Depending on the value of the II index, 2 groups were established: relapse (subjects) with II ≥6 mm and stable (controls) with II ≤3 mm.

Patient information and treatment history for all subjects were obtained from the database. Sex, Angle classification, date of birth, dates of end of treatment (T2) and follow-up (T3) records, and treatment alternative (extraction vs nonextraction) were recorded. Length of follow-up time was calculated by subtracting the T2 date from the T3 date. These data allowed matching the relapse and stable groups regarding treatment alternatives, length of follow-up, and II at pretreatment (T1).

The II was measured as the sum of the linear displacements of the anatomic contact points of each mandibular incisor from the adjacent tooth’s anatomic point on the models taken at T1, T2, and T3. Intercanine width was measured as the distance between the cusp tips of the mandibular canines on the T1 and T2 models. The measurements were made to the nearest 0.1 mm by using digital calipers.

After identifying nasion (N), sella (S), point A (A), point B (B), gonion (Go), menton (Me), gnathion (Gn), anterior nasal spine (ANS), posterior nasal spine (PNS), incisal edge and apex of the upper incisor (U1e and U1a, respectively), and incisal edge and apex of the lower incisor (L1e and L1a, respectively) on the cephalograms taken at T2 and T3, we made 10 angular and 5 linear measurements ( Fig ). Overbite (OB) was measured as the distance between U1e and L1e along the vertical plane; overjet (OJ) was measured as the distance between L1e and the maxillary incisor along the horizontal plane (eg, OJ = 0 mm meant that the maxillary incisors contacted the mandibular incisors; OJ = 1 mm meant that there was a 1-mm gap between the maxillary and mandibular incisors). Additionally, 2 ratios were calculated: % of nasal height (N-PP/N-Me ^∗ 100%) and posterior to anterior facial height (SGo/NMe ^∗ 100%).

Angular and linear cephalometric measurements: 1 , SNA; 2 , SNB; 3 , SN/PP; 4 , SN/MP; 5 , NGo/MP; 6 , NSGn; 7 , U1/SN; 8 , U1/PP; 9 , L1/MP; 10 , U1/L1. Ratios: % of nasal height (N-PP/NMe ^∗ 100%) and posterior to anterior facial height (SGo/NMe ^∗ 100%). PP , palatal plane (line through ANS and PNS); MP , mandibular plane (line through Me and Go).

To evaluate the amount of total posttreatment mandibular rotation, the internal structure of mandibular symphysis, the mandibular canal, sella, and nasion were identified on the cephalograms taken at T2 and T3 and traced on acetate paper. After superimposition of the T2 and T3 tracings on the internal structure of the symphysis and the mandibular canal, the total mandibular rotation was calculated as the angle between the SN lines at T2 and T3. The angle was negative in anterior (counterclockwise) and positive in posterior (clockwise) total mandibular rotations. All measurements were made to the nearest half millimeter or degree.

Forty randomly selected cephalograms and 20 study models were reexamined 2 weeks later. Dahlberg’s method was used to assess random error, and paired t tests were used to evaluate the systematic error of the method.

Results

Characteristics of the groups are shown in Tables I and II . Subjects with stable incisor alignment were on average 1 year older at T2 and had a shorter follow-up than the relapsed subjects. However, the differences were not statistically significant. The distribution of the Angle Class I and Class II subjects was balanced in the stable group; in the relapse group, most subjects had Class II malocclusions. The II at T1 was comparable in both groups ( P = 0.163). The II at T2 in the relapse group was greater by 0.85 mm than in the stable group ( P <0.000). Intercanine width change during treatment (T1-T2) was comparable in both groups ( P = 0.284).

Measurement errors are shown in Table III . Random errors for angular measurements ranged from 1.14° for total mandibular rotation to 3.08° for the interincisal angle; errors for linear measurements were from 0.19 mm for intercanine width to 0.62 mm for OB. No systematic measurement error was found.

Total mandibular rotation relative to the anterior cranial base in the relapse and stable groups was similar ( P = 0.386, Table IV ). On average, anterior (counterclockwise) rotational changes were seen in both groups. However, 7 subjects from the relapse group (21.2%) and 7 from the stable group (19.4%) had posterior (clockwise) total mandibular rotations.

Most angular skeletal variables ( Table V ) showed few posttreatment changes, not exceeding 1°, in subjects with unstable and stable incisor alignment. Only the mandibular plane rotated by more than 1° relative to the cranial base (SN/MP angle) in the relapse and stable groups (–1.72° and –1.40°, respectively). An increase of the ratio of SGo/NMe ^∗ 100% suggested that rotational changes occurred from growth of the posterior face (SGo) rather than a decrease of the anterior face (NMe). Also, small posttreatment changes of the nasion-sella-gnathion angle implied that anterior facial height in both groups was stable. All posttreatment skeletal changes were comparable in the relapse and stable groups ( P >0.05).

Table V

Posttreatment (T2) and long-term out of retention (T3) cephalometric measurements in the relapse and stable groups

	Relapse						Stable						Relapse vs Stable
	T2		T3		Difference		T2		T3		Difference		T2	T3	T2 to T3
	Mean	SD	Mean	SD		P	Mean	SD	Mean	SD		P	P	P	P
SNA (°)	79.52	3.99	79.98	4.4	0.46	0.170	82.17	4.54	81.64	4.54	−0.53	0.154	0.012	0.130	0.051
SNB (°)	76.22	3.93	76.79	4.59	0.57	0.100	79.18	4.20	78.96	4.09	−0.22	0.421	0.006	0.042	0.115
SN/PP (°)	7.33	4.06	7.76	4.28	0.43	0.364	7.48	4.02	7.78	3.93	0.30	0.074	0.764	0.984	0.479
SN/MP (°)	36.3	6.68	34.58	8.44	−1.72	0.009	32.60	5.79	31.20	6.78	−1.40	0.002	0.016	0.053	0.939
NGo/MP (°)	77.95	5.4	76.50	6.42	−1.45	0.004	74.69	4.67	74.37	6.11	−0.32	0.069	0.009	0.166	0.172
NSGn (°)	68.39	4.23	68.03	5.18	−0.36	0.300	65.83	3.33	65.90	4.37	0.07	0.725	0.007	0.071	0.307
% of nasal height (%)	44.58	3.31	44.80	3.4	0.22	0.532	44.3	2.03	44.74	2.45	0.44	0.097	0.675	0.938	0.534
SGo/NMe ∗ 100%	64.89	5.39	66.30	6.81	1.41	0.022	66.88	4.87	68.14	5.11	1.26	0.066	0.113	0.209	0.939
U1/SN (°)	99.39	8.88	98.67	9.02	−0.73	0.525	100.83	8.54	102.97	7.68	2.14	0.072	0.495	0.036	0.082
U1/PP (°)	106.30	8.49	106.24	8.58	−0.06	0.960	107.75	8.54	109.97	6.88	2.22	0.025	0.483	0.050	0.135
L1/MP (°)	92.73	8.03	90.39	7.40	−2.33	0.061	91.88	8.31	90.66	7.44	−0.99	0.285	0.667	0.884	0.370
U1/L1 (°)	131.52	9.51	136.48	10.75	4.97	0.014	134.79	8.38	135.67	10.16	0.88	0.568	0.133	0.746	0.096
OB (mm)	2.12	1.73	3.39	2.09	1.27	0.002	2.24	1.22	3.40	1.69	1.16	0.000	0.749	0.985	0.814
OJ (mm)	0.95	0.95	1.15	1.58	0.20	0.507	0.56	0.63	0.42	0.72	−0.14	0.269	0.043	0.014	0.281

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