## 4 Discussion
Interestingly, the hybrid orthotropic model and structural model were found to be very similar (Tables 6 and 7), suggesting that both approaches result in similar directionality dependent behaviour and both can be used in the design of bone scaffold structures. In addition, a structural representation of the trabecular bone provides a clear image of bone architecture, suggesting that this approach is capable of picking up trabecular motifs present in bone. The total bone volume of the CT scan derived model was much greater than any of the adapted models (Table 8). A reason for this discrepancy could be attributed to the adaptation algorithm being set up to generate the minimum amount of bone required to sustain the load cases tested. In addition, the selected load cases do not capture the full loading environment that a pelvis is subjected to and develops in throughout its lifetime, including strenuous activities such as running, which would result in higher muscle and joint contact forces. The disappearance of bone in the dead zone was modelled as instantaneous in the algorithm, whereas, in reality, this process is time dependent. In addition, the grayscale values defining the material distribution of the CT scan model might overestimate the true stiffness of local regions. At the scale of clinical CT imaging, directional stiffnesses are not clearly defined which can lead to inaccuracies when quantifying the stiffness using a single isotropic value. The modelling framework presented in this study has a number of limitations that must be acknowledged. In the musculoskeletal model used to derive the loading environment applied on the pelvis, muscle actuators did not take into account contraction dynamics and force-length-velocity relationships [49]. Furthermore, the muscle actuators were not spread over bone attachment areas and compressive forces of muscles applied on bone were not taken into account. This limitation was partially overcome by having a large number of muscle actuators in the musculoskeletal model. The FE model was developed to contain only bone, ligaments and cartilage. The lack of internal organs and soft tissue associated with the pelvic region could have an impact on the resulting bone architecture in some regions. A potential limitation of the FE models is the scale at which they were developed. The average edge length of the continuum elements was 3.76 mm, closer to the macroscale rather than the microscale, as discussed in the supplementary material in [PERSON] and [PERSON] [39]. In addition, this scale would not allow for bone voids to be modelled. However, the adapted models could potentially allow for assessment of bone voids and bone microarchitecture using macro-to-micro conversion relationships [54]. The lack of separate cortical and trabecular bone representations in the continuum models can be a hindrance in the cases when bone architecture needs to be assessed locally. Similarly, the resolution of the clinical CT scan was not suitable to allow cortical bone to be distinguished from trabecular bone throughout the pelvis. Although the hybrid continuum models overcome this limitation, they do not provide sufficient information on bone architecture. The hybrid orthotropic model provides trabecular bone directionality in addition to the hybrid isotropic model, but it does not allow for an assessment of trabecular bone architecture locally. This capability is provided by the structural model. On the other hand, the high stiffness of the shell elements in the hybrid and structural models followed immediately by elements with lower stiffness can pose a limitation as moments that may be present in the cortex might not be transferred to the trabecular bone in its vicinity. All models were found to have run times of approximately three minutes for a single loadcase on a workstation PC with one Intel Xeon E5-2630 v2 2.60 GHz (Dell Computer Corporation, Round Rock, TX, USA) and 64 GB of RAM.