## 4 Discussion  
CP is the most common cause of aberrant gait pattern and physical disability in children. The neurological impairment leads to progressive musculoskeletal pathology with muscle-tendon contractures, secondary progressive bony deformities and joint instability. Orthopedic procedures have been designed to address the various components of the progressive musculoskeletal pathology including tendon lengthening, tendon transfers, rotational osteotomies, and joint stabilization procedures. However, planning the surgical strategy in SEMLS is often difficult and no obvious prediction can be made about the effects of surgery on the gait pattern. Currently, the surgical planning of such complex cases is generally accomplished on the basis of the clinical examination and plain radiographs. CT scans are rarely required only to investigate potential torsional deformities. Gait analysis has demonstrated to improve the decision making and possibly alter the surgical indication, but it requires extensive learning curve and experience from surgeons for a correct interpretation of data. This study presents an integrative framework for obtaining a \"virtual movable musculoskeletal model\" of the lower limbs. This virtual musculoskeletal model is obtained by linking data from gait analysis and a CT-based subject-specific model of the lower limbs. The main finding of our study was that it is possible to use our proposed technique in clinical practice as the gait analysis is a non-invasive examination, the software used in our methods are freely available and low-dose CT scans of the lower extremities are possible [42]. The possibility of obtaining a musculoskeletal model of the patient, capable of combining patient-derived anatomic models and data from gait analysis, is the key factor for improving our understanding of such complex pathology. Throughout computer-aided surgical simulation it is possible to predict the effect of surgery on gait pattern, especially when combined and multilevel procedures are planned in a single event operation [43; 44; 45]. In this context, the creation of virtual movable skeletal models, based on the specific patient anatomy, is an appealing option for several purposes, especially for virtual surgical  
Figure 13: Preoperative and postoperative radiographs. planning and simulation. The main advantages of subject-specific models have been already reported by some authors [26; 46]. [PERSON] and colleagues analyzed six children with aberrant gait and torsional deformities of the lower limbs due to CP. They reported that patient-specific MR-based models were more accurate in estimating moment arm length compared to rescaled generic models, confirming that inter-individual variation in musculoskeletal geometry, such as muscle attachment locations and femoral anteversion, play a crucial role in the accuracy of moment arm length calculations [26]. We used CT scans for modeling the skeleton of the lower limbs, applying a very low-dose protocol (VEO, General Electrics). We performed a volumetric-spiral CT acquisition that permits high quality and detailed 2D and 3D imaging reconstructions. This technique offers higher imaging details than EOS. Of course, the effective estimated radiation dose to the patient with this acquisition is significantly lower than a standard CT acquisition (at least 70-80% dose reduction), even if higher than the effective dose obtained with EOS. In this acquisition, the inclusion of iliac wings was required, and this led to an increase of the effective esteemed dose to the patient, due to the inclusion of the pelvis in the scanned volume. In protocols performed in lower limbs only, with this low-dose CT technique, the effective dose to the patient (mSv) would dramatically decrease. The main advantage of this method is the possibility of obtaining very accurate volumetric reconstructions of the bone (and possibly of the soft tissues), even including the reflective markers in the field of view.