# Three-Dimensional Anatomical Analysis of Muscle-Skeletal Districts
## 2 Related Work
### Morphological Characterisation of the Spine  
The spine is a fundamental element of human anatomy that provides support for our body and organs and guarantees a wide range of mobility. Moreover, the spine deals with load transfer and protects the spinal cord from injuries. All these different tasks can be accomplished thanks to the complex anatomical structure of each element composing the spine and to the organisation of the different tissues in the overall district. Bio-mechanical pathologies, in the short term, can lead to pain and disability, which will worsen in the long term [23; 24].  
For the study of the spine, medical imaging supports the evaluation of the status of the different tissues and pathological damage evolution; to this end, CT images are widely used [25]. The analysed image is a grey-scale image, where the tissue density is represented by shades of grey. The _Hounsfield scale_ is a quantitative scale for describing radio density in medical CT and expressing intensity values in _Hounsfield Units_ (HU). On the Hounsfield scale, the air is represented by a value of \(-1000\) HU (black on the grey scale) and bone between \(+700\) HU (cancellous bone) and \(+3000\) HU (dense bone) (white on the grey scale). Bones stand out clearly in CT images since they are much denser than soft tissues. Different studies evaluated HU values in CT for the analysis of the physio-pathological status of the subject to help in disease diagnosis [26; 27]; other works identified a correlation between the HU values and bone mineral densities [28]. This correlation is important since it allows the use of CT HU values for retrieving different information and quantitative analysis, which, in turn, reduces the need for other types of invasive images, thus reducing the overall amount of ionising radiation for the patient. In clinical practice, the evaluation of the HU values is usually conducted on 2D slices and in the ROI manually identified by experts [27; 28; 29], without taking advantage of modern 3D imaging techniques.  
Other works focused on the realisation of 3D models to study the spine's properties, considering the whole spine or a single vertebra. These models are mainly built for Finite Element analysis [30], e.g., [31; 32; 33; 34], but usually are not patient-specific. Indeed, when it comes to assigning the material properties of the tissue, these works rely on the state-of-the-art instead of on the single-subject case. Common techniques for the development of vertebral models include statistical shape models [35; 33] or parametric models [36], which adapt to the subject-specific case with a certain level of accuracy, but usually require the construction of a prior reference.  
For a deeper evaluation of the spine's biomechanics, various studies focused on alignment analysis [37; 38; 39] and the study of the vertebral spine or a single vertebra's morphology [40; 41; 42; 43]. The main drawback is the use of 2D images, which provide a limited visualisation of the spine. Even though rendering techniques have been developed in recent years, the common practice still considers individual slices. However, recent studies have shown that the 3D information of the vertebral spine can make a difference in early diagnosis or subject evaluation [44]. In contrast to 2D image analysis, the use of 3D information allows the inclusion of asymmetries in the evaluations [30], improves the estimation of curvatures [41], and encodes information on the vertebral disc's status [41; 45]. Most of these studies start from CT images, which have the advantage of preserving the real distances between vertebral bodies and the real curvature information [30]. However, previous work on 3D modelling or imaging usually concentrates only on a specific region or motion segment and not on the complete vertebral spine [30], as in [41].