Long-term Stability of Treatment With the Twin Block Appliance

Orthodontic treatment during growth is called interceptive orthodontics. The objective is to affect the growth of the jaws and the position of the teeth with the assistance of a removable orthodontic appliance. One such appliance is the Twin block appliance, which is used to treat patients with an Angle class II malocclusion (Clark et al., 2022). The prevalence of Angle class II malocclusion is approximately 25% (Proffit et al., 2013).

There is however a lack of studies on the long term treatment stability of the Twin block appliance (Ehsani et al., 2015). Treatment with removable appliances also carry a great cost for both the individual, the parents and for society as a whole (Permert et al., 1998). The objective of the present study is to present guidelines concerning which patients that would benefit the most from treatment with Twin block appliance over time.

The present study will use a previous patient material consisting of 72 patients from the study by Frilund et al., (2023) "Patient compliance with Twin Block appliance during treatment of Class II malocclusion: a randomized controlled trial on two check-up prescriptions" to review the long term stability of treatment with Twin block appliance. These 72 patients were previously treated with a Twin block appliance. Their records will be reviewed to find patients who have had no additional orthodontic treament, and these patients will be asked to participate in the present study. If they accept, their present bite (T2) will be registered by study casts and with a cephalometric x-ray. This data (T2) will then be compared with their study casts and cephalometric data before treatment start (T0) and after treatment (T1). The main variables that will be investigated are overjet, overbite and molar relationship, as well as cephalometric values. The collected material will also be dichotomized into two groups, one with optimal current status with a neutral molar relation and a horizontal overjet of 4 mm or less, and one with a suboptimal current status presenting an angle class II molar relationship and/or an overjet greater that 4 mm.

The patients that recieved additional orthodontic treatment after their treatment with Twin block appliance will also be investigated to compare the reason for them having additional treatment and to investigate if the primary reason was relapse or other additional factors.

Statistical analysis of collected data will be perfofmed with the appropriate stastical models, depending on if the data has a normal distribution or not.

The study has recieved ethical approval from the ethical board at the University of Linköping, Dnr 2025-08312-02. All patients will have to give their informed consent to participate in the study by signing a consent form.

Results will be published in appropriate medical journals and at interdisciplinary conferences and lectures.