## 4 Other Methods
### AI-Based Approaches  
In 2018, the applicability of the Sauvegrain method through DL was investigated by [PERSON] et al. [112]; as elbow X-rays are less commonly applied for age assessment, they first applied the data augmentation to expand the number of images needed to for DL algorithms and trained a CNN on 576 images, then compared the automatic age estimation results with those performed by experts. The MAE of the automatic version of the Sauvegrain method was 2.8 months and the Mean Absolute Percentage Error was 0.018, demonstrating a performance similar to that of experienced radiologists [112]. Automatic methods for bone age assessment in the knee are mainly based on MRI, through the assessment of growth plate ossification; some authors developed a DL system trained on 185 coronal and 404 sagittal studies of Caucasian males aged 13 to 21, involving image pre-processing, bone segmentation, and age estimation. The combined use of CNN and three ML-based algorithms allowed an accuracy of 90.9%, a sensitivity of 88.6%, and a specificity of 94.2% in classification and a MAE of 0.49 years in age regression [113]. In their proposal, [PERSON] et al. combined two CNN models: the first one selected the most useful images from the MRI study, which were then analyzed by a second module \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Authors** & **Anatomical Region** & **Imaging Technique** & **Process Description** & **Age Range of Subjects** & **Disadvantages** \\ \hline [PERSON] et al. [95] & Elbow & Radiography (AP and LL projections) & Evaluation of 4 elbow ossification centers, basing on a 27-point scoring system & 0–18 & The double projection exposes to a higher radiation dose \\ \hline [PERSON] et al. [99] & Medial clavide epiphysis & Radiography & Evaluation of ossification degree of the cartilage, basing on a 5-stage classification system & 18–22 & Conventional clavide X-ray can be hindered by overlapping images related to the mediastinal structures, vertebrae, or ribs \\ \hline [PERSON] et al. [101] & Proximal humeral physical & Radiography & Evaluation of the humeral head epiphysis and fusion of the external portion of the physics, based on a 5-stage scale & 10–15 & Historical collection of radiographs (dated 1926–1942) \\ \hline [PERSON] et al. [103] & Iliac crest & CT & Evaluation of the apophysical ossification of the iliac crest, according to Risser’s sign, a 5-score classification & 7–25 & The ossification of iliac crest apoptosis is not uniform, thus it can create some discrepancies \\ \hline [PERSON] et al. [104] & Iliac crest & US & Apophyseal ossification of the iliac crest, according to Risser’s sign, a 5-score classification & 11–20 & The ossification of iliac crest apoptosis is not uniform, thus it can create some discrepancies \\ \hline [PERSON] et al. [105] & Cervical vertebrae & Radiography (lateral cephalometric) & Evaluation of C2, C3 and C4, basing on a 6-stage maturation scale & 8–17 & The method originally was developed more than 5 decades ago, without a fair description of the classification system, until a few years ago. \\ \hline [PERSON] et al. [108] & Calcaneal apophyss, (lateral foot projection) & Radiography calcaneal apophyss based on a 5-stage scale & 7–16 & Ethnical differences \\ \hline [PERSON] et al. [109] & Knee & Radiography (AP and LL knee projections) & Evaluation of the stage of the epiphyseal union at the knee joint, basing on a 5-stage scale & 9–19 & The study was applied to highly selected population (lrish), thus the results are difficult to generalize \\ \hline [PERSON] et al. [110] & Distal femur & 3T MRI & Evaluation of the ossification stage of the distal femoral epiphysis, based on a 5-stage scale & 10–30 & Unbalanced age distribution of subjects, particularly in the lower age groups; only one sectional plane and only one MRI weighting were considered. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The different methods for bone age estimation based on the analysis of different skeletal segments.