The ethics approval of this retrospective clinical study was obtained from the ethics committee of Goethe university Frankfurt under number 181/17.
Study groups and sample size calculation
Our study groups consisted of 2 groups: a group treated with early class II elastics (group IEA) and another group treated with late class II elastics (group IEL).
Calculation of the sample size was done using the Van-Elteren-Test. By assuming an error probability of 5% and a test-relevant probability of P(X1 < X2) = 0.76, which roughly corresponds to one standard deviation in the Gaussian model, a test power of at least 80% would mean a number of at least ten patients per group per stratum. Therefore, the sample size of 10 patients in each group was calculated.
Data collection from archive of the university
The archive of the orthodontic department of Goethe University Frankfurt was searched for the patients with class II malocclusions and the following inclusion and exclusion criteria were applied.
Inclusion criteria:
- Patients, who had completed their entire orthodontic treatment at the orthodontic department of Goethe University Frankfurt between September 2005 and April 2017 with diagnosis of skeletal class II malocclusion (ANB angle > 4°)
- Patients after their peak of growth
- Patients with at least ½ cusp distal occlusion on one side
- Patients in treatment with Damon Q self-ligate multibracket system (Ormco Corporation, Orange, California, USA)
- Begin of the multibracket treatment latest in April 2015
- The use of class II elastics
- Patients with available, intact study models at both pre- and posttreatment (T0 and T1) stages.
- Patients with available T0 lateral cephalograms
Exclusion criteria:
- Severe skeletal class II patients, who required an orthognathic surgery
- Patients with craniofacial syndromes
- Patients with less than ½ cusp distal occlusion on one side
- Patients with skeletal class I and dental class II malocclusions
- Patients with a phase of therapy using removable functional appliances
- Patients with hypoplasia or oligodontia
- Low- and non-compliant patients
- Patients with extraction therapy
- Adults (Patients above 18 years old at the beginning of their treatment)
Data analysis
All of the diagnostic records (study models and lateral cephalograms) were analyzed by a senior attending at the orthodontic department of Goethe University Frankfurt.
- The skeletal maturity of the all patients was determined using the Baccetti cervical vertebrae maturation (CVM) method (10).
- Lateral cephalogram analysis of the available data were performed on T0 images. OnyxCeph³™ software, version 3.2.32 (Image Instruments, Germany) was used for the cephalometric diagnosis.
- Model analysis was made on study models of all of the patients obtained up to 3 months before placement and up to 15 months after removal of multibracket system. The measurements were made using manual caliper to the nearest 0.5 mm. PAR index with the weighting system according to Richmond et al. was assessed using a PAR ruler (11). Occlusal traits of PAR score were measured, summed and multiplied by their weight according to both weighting systems at T0 and T1. Reduction and percentage reduction of the weighted PAR scores between T0 and T1 were also calculated for each patient. Furthermore, numerical and percentage reductions of PAR scores per year were additionally measured for all of the samples.
Statistics
- Nonparametric van Elteren Test was used to measure the sole effect of the different therapy methods on the treatment result.
- Hodges–Lehmann estimator and Rosenthal effect size estimator were used to compare the difference between the median value of 2 groups and assess the superiority of one group against the other.
- The BiAS software version 11.0 was used for all of the statistical measurements.