Material and methods

This retrospective cohort study was approved by the ethics committee of Bern, Nordwest and Central Switzerland (EKNZ 2015-349, HVF, Kat A), and every patient signed informed consent to participate. For the structure of this article, the recommendation of the STROBE Statement were followed.

Two groups were selected: (1) a study group comprising orthodontically treated patients whose treatment was finished 10 to 15 years before this study and (2) a control group of untreated subjects with different types of malocclusions seeking orthodontic treatment.

The study group was selected from patients at a private orthodontic practice in Grenchen, Switzerland. In this practice, it was routine to keep pretreatment (initial, T1) and posttreatment records (final, T2) for at least 10 years after the last retainer follow-up visit, which was usually performed between 1 and 4 years posttreatment. To study patients at least 10 years posttreatment (T3), a recall was established for those who had their last retainer follow-up appointment in 2005, 2006, and 2007 (thus, their T2 records were taken between 2001 and 2006). Three hundred ninety-four consecutive patients who met the following inclusion criteria were eligible to be included in the study: (1) treated with fixed appliances, (2) treated by the same orthodontist, (3) maxillary and mandibular retainers bonded immediately after active orthodontic treatment, and (4) nonsyndromic. They were invited for a recall assessment. No age restriction was applied during the sample selection; 118 patients agreed to participate; of those, 14 were “no shows.” Eventually, 104 patients were evaluated. After the recall appointment, the following additional exclusion criteria were applied: (1) orthodontic retreatment, (2) debonding more or less than 10 to 15 years ago, (3) T1 and T2 casts missing or of poor quality in the region of the gingival margin, (4) periodontal disease other than gingival inflammation, and (5) nonwhite patients. Finally, 88 patients were included. The flow chart ( Fig 1 ) shows in detail the procedure of patient selection.

Flow diagram of the study group (private practice) and the control group (university clinic).

The control group was selected from 969 consecutive adults who came for consultation to the Department of Orthodontics and Dentofacial Orthopedics at the University of Bern in Switzerland between 2006 and 2016. The participants were selected if they met the following inclusion criteria: (1) availability of a full set of pretreatment records (photographs, casts, radiographs), (2) no history of prior orthodontic treatment, and (3) between 22 and 37 years of age at the assessment; the age criterion was applied to match participants from the study cohort at T3. Exclusion criteria were (1) nonwhite, (2) syndromic, (3) casts of poor quality in the region of the gingival margin, (4) periodontal disease other than gingival inflammation; 143 subjects were initially considered for inclusion in the control group ( Fig 1 ). Thirty-three of them had previous orthodontic treatment, and 8 were nonwhite and were subsequently excluded. Finally, 102 subjects were included in the control group.

The primary outcome was the presence of gingival recession in buccal or lingual sites of the teeth, and they were scored as “yes” if the cementoenamel junction in the center of the buccal or labial aspect was clearly exposed; otherwise, it was scored as “no.”

The participants were requested to complete a questionnaire regarding hygiene, habits, and any complications regarding their retainer. Demographic data such as sex, age, hygiene technique, type of toothbrush, and type of retainer were recorded during the clinical assessment at T3.

In the study group, plaster models made at T1, T2, and T3 were scored, using the yes/no scoring system. One rater (M.M.) rated all patients clinically at T3 and scored all plaster models at each time point (T1, T2, and T3). When judgment was difficult on the plaster models, intraoral photographs were additionally used. During the clinical evaluation (T3, 10-15 years after orthodontic treatment), oral hygiene was recorded and rated on a 3-level scale (good, average, or poor). For the hygiene technique, the participants were requested to perform their brushing method and were asked what kind of toothbrush (soft, middle, hard) they generally use. Gingival recession was scored buccally and labially using the 4-point classification system of Miller. If calculus was present around the retainer, it was removed with an ultrasonic scaler about 5 minutes before scoring the recession and taking the impression. The results of this clinical assessment of gingival recession were used to evaluate the kappa level between clinical and cast scores. The condition of fixed retainers was assessed, and any breakage or detachment was noted. Finally, intraoral photographs and alginate impressions were taken. These records made during this appointment were designated as T3.

In the control group, scoring was performed retrospectively on plaster or digital models made from scanned plaster models (approximately in 50% of the subjects). The rater (M.M.) scored the plaster models manually and the digital models on the computer monitor using Onyx Ceph 3 software (Chemnitz, Germany). When the judgment was difficult to make, intraoral photographs were viewed.

In both groups, the pretreatment malocclusion in the sagittal, vertical, and transverse planes and the amount of crowding were assessed. In the sagittal plane, the Angle classification was used; in the vertical plane, the amount of overbite was registered, and 3 subgroups were created: (1) large overbite, greater than 4 mm; (2) normal overbite, between 1 and 4 mm; and (3) reduced overbite, less than 1 mm. In the transverse plane, the presence of unilateral or bilateral crossbite was registered. Crowding was categorized in each dental arch as mild (space deficiency, <3 mm), moderate (space deficiency, 3-5 mm), or severe (space deficiency, >5 mm). When the extent of crowding differed in the maxillary and mandibular dental arches, the arch with more severe crowding was used to categorize the participant.

Information on smoking habits, plaque accumulation, and bleeding on probing of gingival pockets was not available for any participant in either group.

For the clinical assessment, 2 raters (M.M. and M.G.) evaluated together 10 participants from the study group to familiarize themselves with the scoring system. Subsequently, both raters evaluated 26 participants twice and independently from each other.

For the assessment on plaster models, 4 months after the clinical assessment, both raters calibrated themselves by scoring recession sites on the plaster models of 7 subjects who did not participate in this investigation. Any disagreements between the raters were discussed and resolved after inspection of the corresponding intraoral photographs. Subsequently, both raters scored 25 plaster models independently from each other to assess interrater agreement.

To evaluate the agreement between scores of plaster and digital models, 35 plaster and 35 digital models of the same participants were scored independently by the 2 raters.

For the assessment on digital models only, 3 months after scoring the 35 digital models, both raters independently rescored the same models to assess interrater agreement within and between raters.

Results

The proportion of women was slightly higher, but not statistically significant, in the treated group than in the untreated group (71.6% vs 61.7%, respectively; Table I ). Both groups were well matched regarding age: the mean ages at T3 were 27.9 years (SD, 2.5 years; range, 23.8-37.1 years) in the study group, and 28.7 years (SD, 3.1 years; range, 22.6-37.1 years) in the control group. The mean time since debonding was 12.9 years (SD, 1.2; range, 10.4-15.9 years). All orthodontically treated participants received bonded retainers in the maxilla (bonded to all incisors) and mandible (bonded to either canines only or all 6 anterior teeth). In participants who had a mandibular retainer bonded only to the canines (44.3%), a round 0.027-in beta-titanium alloy wire was used; the remaining participants (55.6%) had a 0.016 × 0.022-in braided steel wire (D-Rect; Ormco, Glendora, Calif).

In both groups, the malocclusion at T1 was analyzed, and the distribution of the malocclusion characteristics is shown in Table II .

Table III gives intrarater and interrater agreement for the clinical examinations, the plaster and digital model scoring in the different tooth groups (incisors, canines, premolars, and molars), and the different locations of recession (buccal and lingual). In general, the agreement in scoring recession was good to very good (kappa, >0.6). Only the interrater agreement for clinical assessment of lingual recession in the region of incisors and canines was fair to moderate (kappa, 0.27 and 0.48, respectively), and agreement for clinical and cast assessments of buccal recession in the molars was moderate (kappa, 0.59).

The prevalence of labial/buccal gingival recession increased during orthodontic treatment: 54.5% of the participants had at least 1 recession site at T1, and 85.2% had at least 1 recession site at T2 ( Table IV ). After treatment, there was a further increase of the prevalence of recession: 98.9% of the orthodontically treated participants had at least 1 recession site at T3. From T1 to T3, also the proportion of patients with multiple recession sites (>5) increased from 10.2% (T1), to 27.3% (T2), and to 86.3% (T3).

The prevalence values of recession in the control subjects and the orthodontically treated participants at T3 were similar: almost no one was recession-free. Approximately 10% of the participants had 1 to 4 recession sites, and 53% had 5 to 14 recession sites. However, 37.2% of the controls had 15 recessions or more, whereas only 26.1% of the orthodontically treated participants had 15 or more recession sites ( Table IV ; Fig 2 ).

Frequencies (%) of labial/buccal and lingual/palatal gingival recessions per tooth at 3 time points for the study group and at 1 time point for the control group.

The prevalence of lingual/palatal gingival recession increased during the orthodontic treatment: 18.2% of the participants had at least 1 recession site at T1, and 59.1% had at least 1 recession site at T2 ( Table IV ). After treatment, there was a further increase of the prevalence of recession: 85.2% of the orthodontically treated participants had at least 1 recession site at T3. From T1 to T3, the proportion of patients with multiple recession sites (>5) increased from 0% (T1), to 9.1% (T2), and to 38.6% (T3).

The prevalence values of lingual/palatal recession in the control subjects and the orthodontically treated participants at T3 were different. In general, the treated participants had fewer recession sites than did the controls; 14.8% of the treated participants were recession-free, whereas all controls had at least 1 recession site; 46.6% of the treated and 39.3% of the untreated participants had 1 to 4 recession sites; 34.1% of the treated and 52.0% of the untreated participants had 5 to 9 recession sites ( Table IV ; Fig 2 ).

The regression analysis assessing the effect of malocclusion on the sum of recessions is shown in Table V . In the study group, the participants with a crossbite at T1 showed 2.73 (95% CI, 0.28-5.17; P = 0.029) more buccal recessions at T3 than did those with no discrepancy. There was a weak association between recession and crowding and vertical malocclusion at T1 ( Table V ). For the palatal/lingual recession, patients with a reduced overbite showed 2.19 (95% CI, 0.15-4.22; P = 0.035) recessions more than did patients with a deep or normal overbite before treatment. There was a weak association between recession and Class III malocclusion at T1 ( Table V ).

Table V

Regression analyzing the effect of type of malocclusion (for the study group at T1) on the sum of buccal and lingual gingival recessions

Predictor	Study group (n = 88)			Control group (n = 102)
	β coefficient	95% CI	P value	β coefficient	95% CI	P value
Recession labial/buccal
Malocclusion Sagittal-plane
Class I	Reference
Class II	−0.53	−3.43, 2.37	0.718	−0.12	−2.61, 2.36	0.922
Class III	1.60	−3.65, 6.85	0.718	−1.59	−6.21, 3.03	0.497
Malocclusion vertical plane
Overbite 1-4 mm	Reference
Overbite >4 mm	−2.28	−4.85, 0.28	0.080	−3.53	−6.05, −1.01	0.007
Overbite <1 mm	−0.21	−3.85, 3.42	0.907	−1.66	−5.05, 1.72	0.331
Malocclusion transverse plane
No discrepancy	Reference
Crossbite	2.73	0.28, 5.17	0.029	0.65	−2.32, 3.62	0.663
Crowding
Mild 0-3 mm	Reference
Moderate 3-5 mm	2.51	−0.11, 5.13	0.060	3.29	0.73, 5.68	0.012
Severe >5 mm	1.17	−2.17, 4.53	0.487	4.92	1.70, 8.15	0.003
Recession palatal/lingual
Malocclusion Sagittal-plane
Class I	Reference
Class II	0.00	−1.60, 1.59	0.990	1.06	−0.22, 2.33	0.103
Class III	2.86	−0.03, 5.76	0.052	−0.5	−2.87, 1.87	0.676
Malocclusion vertical plane
Overbite 1-4 mm	Reference
Overbite >4 mm	0.12	−1.31, 1.55	0.866	−0.59	−1.94, 0.77	0.393
Overbite <1 mm	2.19	0.15, 4.22	0.035	0.26	−1.56, 2.08	0.777
Malocclusion transverse plane
No discrepancy	Reference
Crossbite	0.57	−0.84, 1.98	0.428	0.98	−0.55, 2.52	0.207
Crowding
Mild 0-3 mm	Reference
Moderate 3-5 mm	0.92	−0.56, 2.42	0.219	0.91	−0.48, 2.30	0.198
Severe >5 mm	0.09	−1.81, 2.01	0.918	1.151	−0.60, 2.90	0.194

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