## 1 Introduction  
The individual growth variations, observed at patients with the same chronological age, for both sexes, within the same family and even at dizygotic twins, led to the developing of the concept of biological age, which defines the path followed until reaching the individual maturity.  
Out of all the indicators of the biological age, the skeletal age is considered by most authors as having the best accuracy. Determining the skeletal age using a hand X-ray is the consecrated method and in the specialty literature there are numerous studies done upon different population groups, which shows it value and its applicability at a large scale [1, 2, 3, 4, 5].  
On the other hand, estimating the skeletal age by using the development of the cervical vertebrae is a rather recent method, compared to the use of the hand and wrist X-rays. This method gained many supporters, the motive of its popularity being the fact that the interpretation is being done on the lateral cephalogram, a routine investigation within the orthodontic treatment, thus requiring no additional exposure of the patients to any other X-rays [6, 7].  
During the last decades, due to the popularization of the evaluation methods of the skeletal age through the analysis of the cervical vertebrae on the lateral cephalograms, the doctors' interest in the analysis of the cervical region on lateral X-rays increased. There were authors who showed that the lateral cephalograms could be useful for evaluating the natural position of the head [8]. Furthermore, some associations were found between -cervical vertebrae anomalies, such as the appearance of fusion in the cranio-facial syndromes and dental malocclusions [9]. Given the context of the existing correlation between malocclusion development at the level of the dento-maximillary apparatus and the development of the cervical vertebrae, there rises the legitimate question whether the CVMS indexes keep their validity during the orthodontic treatments that aim to correct the dento-maximillary anomalies.  
There exist in the specialty literature a series of studies that confirm the existing correspondence between the CVMS and SMI indexes during the stages of the pubertal growth [1; 2; 3; 4; 5; 6; 7; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30].  
Objectives:  
Prior to this study, a transversal study was done upon a group of 150 subjects (89 female and 61 male), aged between 8-18 y. Given the existing data in the specialty literature and the average age for reaching the skeletal maturity index SMI and cervical vertebral maturation stage CVMS within the researched group, a hypothesis was formulated, regarding the correspondence between the SMI and CVMS stages during the pubertal growth (Table 1) [23].  
This hypothesis was statistically checked using the Cohen's kappa coefficient and the results (k = 0.82 for the female and k = 0.67 for the male) confirmed the concordance of the SMI and CVMS indexes within the analyzed group and validated the correspondence hypothesis that has been formulated.  
Starting with this hypothesis, the present study aimed to verify this hypothesis within a longitudinal retrospective study, done upon a selected group, for which serial investigations were done along the orthodontic treatment. The results of the two studies were subsequently compared.  
Proving the correspondence of the SMI/CVMS indexes for a group of patients undergoing an orthodontic treatment would confirm the clinical applicability of these indexes along the treatment.  
For realizing this study, the approval of the Committee for Research Ethics was received, as the study used existing investigations from previously created data bases [23].