## 1 Introduction  
Torsional deformities of the lower limbs, such as femoral neck anteversion and tibial torsion, are frequent in children with infantile cerebral palsy (CP) [1]. These alterations cause important functional problems in walking, also related to the internal deviation of the angle of progression of the foot, with an increased risk of falls, pain, overloading and muscle fatigue [2, 3]. The mechanisms underlying transverse plane gait deviations arise from a combination of dynamic and static factors. The dynamic elements may be due to spasticity, contractures and muscles imbalance, while the static ones may result from excessive femoral neck anteversion which decreases the efficiency of the hip abductors by reducing the muscular lever arms [4]. The orthopedic therapeutic approach has been identified in multi-level functional surgery for the lower limbs, which can include the simultaneous correction of static and dynamic factors [5; 6]. Single-event multi-level functional surgery (SEMLS) has shown favorable effects on the clinical and functional parameters of children suffering from CP with torsional alterations of the lower limbs [7; 8]. In particular, the combination of soft tissue surgery and bony procedures such as derotation osteotomies is providing promising results [9; 10], especially when supported by adequate biomechanical planning [11; 12]. This planning requires an integrated static-dynamic approach of morphological and functional evaluation, based on radiological measurements, physical examination, and instrumented gait analysis [13; 14; 15]. The accuracy of the measurement of anatomical parameters by imaging (MRI and CT) is essential for the design of the femoral and/or tibial derotation surgery [16]. Instrumented gait analysis has been confirmed as essential in patient assessment and surgical decision making for children affected by CP with transverse plane deformities [17; 18]. Gait analysis combined with clinical evaluation can influence the planning of functional surgery, and also improves the outcomes for patients undergoing the procedures themselves [19; 20]. A recent meta-analysis confirmed the positive effects of femoral derotational osteotomy on the kinematics of pelvis and hip in the transverse plane [21]. However, dynamic hip rotation cannot be predicted exclusively by physical examination measures of hip range of motion or from three-dimensional assessment of femoral neck anteversion derived from biplanar radiographs [22]. The combination of morphological data from imaging and functional data from instrumented gait analysis has been revealed to be the most reliable method for supporting surgical decision making. Computational simulations based on musculoskeletal models that integrate patient-specific MR/CT morphological data into gait analysis [23; 24; 25; 26] allow more reliable calculations of kinematic and kinetic parameters. This modelling can also be used for the implementation of a surgical simulation system in pre-operative planning to test the possible effects of the different surgical treatment options in torsional defects of the lower limbs [27; 28]. The digital 3D models obtained from CT scans have been already used for surgical planning in a selected group of pediatric patients suffering from complex multiplanar limb deformities [29]. The implementation of the surgical decision making process, using both a morphological and functional evaluation approach, aims to improve the outcomes of the interventions. The integration of this information into surgical decision making may be useful to optimizing performance of techniques and avoiding pitfalls and complications. The aim of this study is the creation of a patient-specific musculoskeletal model that integrates patient-specific CT morphological data and gait analysis with personalized calculation of kinematic and kinetic parameters.