# Curve of Spee and Second Mandibular Premolar Agenesis--Present Knowledge and Future Perspectives
## 2 Materials and Methods  
This study was designed as a bicentric retrospective archive research in which pre-treatment samples were evaluated. The study was approved by the Ethics Committee of the Medical University Innsbruck, Austria (Studies no. 1136/2021), the Ethics Committee of the Medical University Vienna, Austria (2136/2018), and was conducted in accordance with the Declaration of Helsinki of 1975 as revised in 2013.  
Patients' records were collected at the Departments of Orthodontics in Innsbruck and Vienna, Austria.  
Inclusion criteria for the experimental group (n = 100) were agenesis of the second mandibular premolar (unilateral agenesis or bilateral), a ppM2, and a fully erupted dentition. Experimental group included 58 females and 42 males, with a mean age of 18.9 years.  
The control group (n = 100) consisted of age-, sex- and malocclusion-matched patients and inclusion criterion was a fully erupted dentition.  
Exclusion criteria for both groups were previous orthodontic treatment, craniofacial disorders, syndromes, and agenesis of other teeth except the second mandibular premolar.  
The demographic information for the experimental and control group is shown in Table 1.  
All dental casts were available digitally after intraoral scanning during general diagnostics and were analyzed by one experienced investigator using OnyxCeph3 TM (version 3.2.147; Image Instruments, Chemnitz, Germany) software.  
To measure the COS depth, a horizontal reference plane was drawn from the mandibular incisor cusp tip to the distobuccal cusp tip of the second molar, and a perpendicular distance was measured from the buccal cusp tip of the second premolar or the buccal cusp tip of the ppM2, respectively (Figure 1). In case of bilateral agenesis, values were averaged.  
In addition, dental parameters like overjet (OJ), overbite (OB), and angle-classification were recorded.  
Measurements were transferred in an Excel spreadsheet and SPSS (v. 26; IBM SPSS Inc., Armonk, NY, USA) was used for statistical analyses. A Shapiro-Wilk test was used to check for normal distribution and [PERSON]'s test for homogeneity of variance, displaying the variables with means and standard deviation. If variables failed to confirm homogeneity, median and interquartile range were showed. Additionally, method error was evaluated by calculating standard error (SE).  
ANOVA and--in case of non-normality--A Kruskal-Wallis Test were used to analyse differences in the COS depths between the dental casts with and without ppM2, regarding sex, age, and angle-class.  
Bar charts and boxplots were used to display results. The relationship between age and COS depth was visualized by a scatter blot.  
Differences emerging from any of these statistical tests were regarded as significant at \(p<0.05\), or as highly significant at \(p<0.01\).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Demographics** & **Experimental Group (n = 100)** & **Control Group (n = 100)** \\ \hline sex, \% & & \\ female & 58 & 58 \\ male & 42 & 42 \\ age, mean (SD) & 18.9 (6.4) & 19.7 (7.2) \\ angle-class, \% & & \\ class I & 25 & 25 \\ class II & 61 & 61 \\ class III & 14 & 14 \\ \hline \hline \end{tabular} SD, standard deviation.  
\end{table}
Table 1: Descriptive Statistics.  
Figure 1: Digital measurement of the Curve of Spee in a dental arch with a persisting primary second molar.