A prospective evaluation of factors affecting occlusal stability of Class II correction with Twin-block followed by fixed appliances


Modified Twin-block therapy followed by fixed appliances (TBFA) is a standard treatment modality in Class II malocclusion; however, there is little information regarding the stability of this approach. We aimed to assess the stability of Class II correction with TBFA and to evaluate factors that may affect stability.


A prospective evaluation involving convenience sampling of 64 participants treated with TBFA was undertaken over 12 months. Study models and lateral cephalometric radiographs were obtained to record overjet and molar relationship, Peer Assessment Rating score, and skeletal parameters, and a new objective buccal segment interdigitation scoring system was developed. Multivariate logistic regression analysis was used to assess the stability of anteroposterior occlusal correction and the degree of buccal interdigitation, pretreatment skeletal discrepancy, and change in overjet during treatment.


Mean overjet reduction of 6.22 mm arose during treatment, with the canine and molar relationships improving by 3.34 mm and 2.67 mm, respectively. In the 12 months posttreatment, a relapse of 0.67 mm and 0.06 mm in overjet and molar relationship, respectively, was observed, with 25% of subjects having overjet relapse of >1 mm. There is weak evidence that the treatment-induced change in overjet is linked with overjet relapse ( P = 0.05; odds ratio, 0.67; 95% confidence interval, 0.44, 1.01). No significant relationship was observed, however, between anteroposterior stability and buccal segment interdigitation ( P = 0.99), pretreatment skeletal discrepancy ( P = 0.10) or prescribed retention regime ( P = 0.63).


Overall, acceptable levels of stability were observed, although appreciable relapse was noted in 25% of participants. Neither the degree of buccal segment interdigitation nor pretreatment skeletal discrepancy was predictive of anteroposterior occlusal stability.


  • Overjet stability of Class II correction appears acceptable for the majority.

  • A new objective method for assessing buccal segment interdigitation is proposed.

  • Overjet stability is not linked with the quality of the buccal segment interdigitation.

  • Overjet stability is associated with the amount of overjet change during treatment.

Functional appliances are known to effectively reduce increased overjet and decrease the anchorage requirements for subsequent preadjusted edgewise appliance therapy. Class II correction stems predominantly from dental change (60%-70%), with skeletal contribution accounting for the remainder. Skeletal change includes an acceleration in mandibular growth during the pubertal growth spurt, but mandibular length is ultimately believed to be determined genetically. As such, the majority of prolonged benefit appears to reside at the occlusal level.

Although the stability of Class II correction is generally perceived to be satisfactory, there is little evidence to support this assumption and a limited appreciation of the factors that might influence the stability of treatment following comprehensive functional and fixed appliance therapy. The most in-depth analysis of functional appliance stability centered on the Herbst appliance with up to 32 years of retrospective follow-up. Decreases in arch perimeter and lower incisor crowding consistent with maturational changes were noted. Molar and canine correction was stable in 64% and 14%, respectively, whereas 86% of overjet correction was maintained. The majority of relapse occurred in the first 6 years posttreatment, with only minor changes arising after that with poor interdigitation, continuing habits, or inadequate retention predisposing to instability.

The Twin-block (TB) is universally popular because of its versatility and high levels of patient tolerance. Notwithstanding this, a potential limitation is the development of lateral open bites during the functional phase associated with impeded posterior eruption and related to acrylic coverage. The judicious trimming of the blocks allied to phased or complete withdrawal of the appliance mitigates this issue. However, preadjusted edgewise appliances typically follow functional appliance therapy to align the dentition and improve interdigitation purportedly promoting more stable occlusal correction.

There is no prospective research related to the stability of treatment with functional appliances, particularly the TB, followed by contemporary preadjusted edgewise appliances. The attainment of optimal occlusal interdigitation following active treatment is speculated to promote stability; however, the quality of interdigitation was assessed subjectively in previous research. We, therefore, aimed to determine the stability of Class II correction with modified TB followed by fixed appliances and to evaluate factors that may affect the stability of anteroposterior occlusal correction with this approach.

Material and methods

A prospective evaluation was undertaken at the Orthodontic Departments of Barts Health NHS Trust (The Royal London Hospital and Whipps Cross University Hospital) over 12 months following completion of orthodontic treatment with approval from Barts Health NHS Trust Clinical Effectiveness Unit (ID 6274). The inclusion criteria were treated Class II Division 1 malocclusion as defined by the British Standards Institute ; treated with TB appliances followed by preadjusted edgewise appliances; and willingness to participate in the study. Exclusion criteria included failure to complete functional appliance therapy with the subsequent loss to follow-up; single-arch preadjusted edgewise appliance treatment only; and craniofacial syndromes (eg, cleft lip and palate). A convenience sample of participants was recruited before or at debonding as well as at routine posttreatment review clinics.

All participants were treated under consultant supervision with a combination of a TB appliance ( Figure 1 ), followed by preadjusted edgewise appliances. The standard departmental protocol is to undertake functional appliance therapy for 12 months. Following the collation of postfunctional records, preadjusted edgewise appliances are placed after a brief period of either night-only wear or complete withdrawal. Removable retainers were prescribed only for night use.

Fig 1
Standard Twin-block design.

Lateral cephalometric radiographs were taken in centric occlusion and hand-traced on cephalometric acetate tracing film (G.R.O.) using a 2HB 0.5-mm mechanical pencil and measurements made using a 3M Unitek Cephalometric Protractor. Sagittal Occlusal analysis was used to assess the discrepancy in the skeletal bases relative to the occlusal line.

Study model analysis involved measurements using digital calipers (Tesa Technology, Renens, Switzerland; resolution 0.01 mm) and Peer Assessment Rating (PAR) ruler. All measurements were performed by the investigator (G.R.O.) who was calibrated in the use of PAR. Study models were blocked randomized in groups of 20. Identifiable information was removed from the models with a unique, random number. Following the measurement of the models, the investigator was unblinded to record participant identity as well as stage of treatment. The investigator was therefore kept blind to the participant identification as well as the time point of assessment.

The primary outcome was the stability of overjet reduction (mm). Secondary outcomes included anteroposterior stability of molar and canine relationship and the PAR score. Independent variables to be assessed were occlusal interdigitation at debonding, pretreatment sagittal skeletal discrepancy, treatment-induced change in overjet, and prescribed retention regime. A novel objective method (Royal London Occlusal Interdigitation Scoring System) was developed to grade occlusal interdigitation accounting both for anteroposterior as well as vertical relationships of the buccal segments. Both left and right buccal segments are considered, and an overall score was given ( Table I ). A maximum score of 16 can be assigned to a set of study models based on the anteroposterior discrepancy; for extraction cases, the maximum score is 12. In the vertical plane, the occlusal contact of the maxillary first molar, premolars and canine were assessed with a maximum score of 4 for each set of study models. The anteroposterior and vertical score is then combined and converted to a percentage based on the maximum possible score. The reliability of this novel approach was assessed on 20 sets of study models measured 2 weeks apart by the investigator (G.R.O.). Statistical analysis included a Bland-Altman plot used to visually determine the random and systematic error and the Pitman-Morgan test to gauge homogeneity of variance. A mean difference of 2.75 (standard deviation [SD] 2.76) and 95% limits (Bland-Altman) of –26.29 and 20.79 indicate no significant difference between the variances for the 2 sets of scores and an acceptable level of random error.

Table I
Scoring system for occlusal interdigitation
Vector Score Description
Anteroposterior 2 Class I or III relationship or <1 mm Class II
Assess the relation of the cusp tip of maxillary canines and premolars relative to the distal contact point of the opposing mandibular tooth. First molars assess the position of maxillary mesiobuccal cusp relative to mandibular buccal groove. 10 >1 mm and <2 mm Class II>2 mm Class II
Vertical 2 Contact of all teeth
Relation of occlusal surfaces of canines, premolars, and first molars. 10 Clear and distinct (>1 mm) absence of occlusal contact on 1 toothClear and distinct (>1 mm) absence of occlusal contact on more than 1 tooth

Intraexaminer reliability for other model-based measures was assessed on 10 randomly selected sets of models and cephalometric radiographs 2 weeks apart. Randomization models were generated in Microsoft Excel (Redmond, Wash). Intraclass correlation estimates and their 95% confidence intervals were at acceptable levels, all above 0.90.

Statistical analysis

Data were analyzed using a statistical package (version 15; StataCorp, College Station, Tex). Statistical analysis included descriptive analysis. Participants were categorized dichotomously as either stable (overjet relapse < 1 mm) or unstable (overjet relapse ≥ 1 mm). Logistic regression analysis was undertaken to assess the potential predictive ability of buccal segment interdigitation at debonding, pretreatment skeletal discrepancy (sagittal occlusal analysis, A/OLp–Pg/OLp), change in overjet during active treatment, and retention regime (dual-arch fixed, single-arch fixed, dual-arch vacuum-formed retainers [VFRs], or other approach) on the stability of overjet reduction.


Sixty-four participants attended for posttreatment review at a 12-month follow-up. Data were unavailable for some participants because of absence or poor-quality study models or cephalometric radiographs. Overall, there were more male (n = 34; 53%; Table II ) than females (n = 30; 47%; Table II ). The mean age of commencement of functional therapy was 12.55 years. The majority (n = 48; 75%) started treatment during the peak pubertal growth.

Table II
Participant demographics
Gender n (%) Age-related to peak growth spurt Age functional fitted (SD), y Age at debonding (SD), y Age at posttreatment review (SD), y Follow-up period, (SD), y
Prepubertal, n (%) Pubertal, n (%) Postpubertal, n (%)
Male 34 (53) 13 (20) 21 (33) 0 (0) 12.52 (1.38) 15.78 (1.77) 16.88 (1.71) 1.10 (0.31)
Female 30 (47) 0 (0) 27 (42) 3 (5) 12.56 (1.38) 15.83 (1.98) 16.93 (2.06) 1.10 (0.23)
Overall 64 (100) 13 (20) 48 (75) 3 (5) 12.55 (1.40) 15.80 (1.86) 16.85 (1.87) 1.10 (0.29)

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Jan 7, 2020 | Posted by in Orthodontics | Comments Off on A prospective evaluation of factors affecting occlusal stability of Class II correction with Twin-block followed by fixed appliances
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