## 1 Introduction  
The pelvic construct is the region of load transition between the upper body and lower limbs and plays a number of roles, such as protecting the organs and vessels in the lower abdomen. It also facilitates the transfer of forces between the lower limbs and the upper body during physical activities such as walking and stair climbing, withstanding loads at the hip joints of up to around six times body weight [1; 2; 3]. Continuous adaptation of bone tissue in response to the mechanical environment surrounding it provides an explanation for the different architecture of bones in the skeletal system [4]. Long bones, such as the femur and tibia, develop thick cortical shafts with trabecular bone concentrated at the joints to transfer contact forces [5; 6], whereas the pelvis is viewed as a sandwich structure where a thin layer of cortical bone surrounds a structure of trabecular bone [7; 8; 9]. Finite element (FE) modelling is a very common approach to investigate the biomechanics of the pelvic ring due to its versatility and low cost compared to clinical investigations, with the added characteristic of enabling the design of bone scaffolds or implants. Computational modelling of bone adaptation to the mechanical environment surrounding it has emerged into an important component in aiding the interpretation of experimental findings and exploring further issues [10; 11]. Bone adaptation models have been previously developed using both continuum elements [12; 13; 14; 15] or structural elements [6; 16; 17; 18]. Early FE models either used simplified geometries [19; 20; 21; 22], were axisymmetric [23; 24] or reduced the three-dimensional pelvis to a two-dimensional slice [25; 26; 27; 28; 29]. The first FE model of the bony pelvis based on subject specific geometry and material properties from medical imaging data was developed by [PERSON] et al. [30] using continuum solid elements, validating it against experimentally obtained data from a cadaveric pelvis loaded at the hip joint. [PERSON] et al. [9] developed an FE model of the pelvic innominate by distinguishing between cortical and trabecular bone using different element types. Cortical bone was modelled using structural shell elements with location dependent thickness derived from medical imaging data. Trabecular bone was modelled using continuum isotropic elements and the model was validated against experimental data by comparing cortical bone strains. The majority of recent FE models representing the pelvic construct were developed from CT imaging data with a tendency of using an isotropic material model to represent bone and distinguishing between trabecular and cortical bone by adding a layer of shell elements [9; 31; 32]. Notably, [PERSON] et al. [9] also developed a purely continuum model in which the surface nodes were assigned the highest Young's modulus they found for cancellous bone (approximately 3.8 GPa) which performed similarly to the model with a shell element layer. Although modelling bone using isotropic material properties has the benefit of reducing computational time, it is insufficient to provide information on directionality of the bone microstructure [33]. Orthotropy has been found to offer a good approximation of bone's anisotropy at the continuum level [34]. The aim of this study was to develop a number of computational models of the pelvic construct using different continuum and structural FE bone adaptation modelling approaches with the purpose of investigating their effects on bone adaptation and predicted mechanical behaviour of the pelvis in comparison to a CT scan derived FE model. The modelling approaches were divided into isotropic and orthotropic material models, followed by a continuum or hybrid approach, where cortical and trabecular bone were distinguished. The motivation behind developing a number of different models was to assess their sensitivity to the approach used. Each model was subjected to an iterative adaptation algorithm based on the Mechanostat principle [4] and tailored for the type of elements used [6; 14].