## 4 Discussion  
Osseeintegration serves as a long-term anchoring method in diverse applications, including limb amputation. Primary stability, achieved through mechanical connection, is evaluated via the assessment of bone-implant micromotions. The magnitude of these micromotions is a key factor in successful osseointegration. While micromotions exceeding 50 \(\upmu\)m may promote fibrous tissue formation, a range of 50-150 \(\upmu\)m is generally considered acceptable. Consequently, precise morphological matching between the implant stem and the medullary canal is crucial for ensuring adequate stability. The purpose of this research is to establish if a customized implant design, based on the medullary canal, increases immediate post-operative primary stability. In particular, two curved patient-specific intramedullary stems and a straight one, used as a reference model, were designed. To assess the primary stability, micromotions at the bone-implant interface were calculated in a virtual model of the residual bone under an axial load of 1000 N.  
Different performances in terms of micromotion values and distributions were calculated for each stem. Micromotions are higher in the distal part (Figure 10), having all the implants at their maximum micromotion value in the first portion of the contact area. In detail, the PSD prosthesis' maximum micromotion just overcame 50 \(\upmu\)m with a value of 51.4 \(\upmu\)m that was registered in a small region of the distal part of the contact surface, and a no-slip area was found in the proximal part of the contact area. On the other hand, the micromotion distributions of the SD showed a wide area, about half of the contact length, where the micromotions exceed 50 \(\upmu\)m, with values that range from 56 to a maximum of 73.9 \(\upmu\)m. The PPSD implant showed a maximum value of 58.6 \(\upmu\)m in a reduced area similar to the PSD, a strong asymmetric distribution of the micromotions, unlike the PSD. This suggests that the patient-specific approach could have positive implications in long-term stability, suggesting that the PSD implant, as well as the PPSD one, can prevent the formation of fibrous tissue related to an excessive relative motion between the bone and the implant, as well as the PPSD. On the other hand, due to higher micromotions,  
Figure 13: Maps of von Mises stress distribution along the sagittal plane for the analyzed implants. the SD implant can more probably lead to the formation of fibrous tissue and consequent incomplete bone ingrowth [13; 15; 20; 21; 22]. By the examination of the contact pressure at different sections (Figure 12), it can be stated that the SD implant causes the highest maximum contact pressure among the evaluated stems, but the average contact pressure is the lowest, suggesting that for the PSD and PPSD, more bone is involved in the interference, leading to a better press fit and a more homogeneous distribution of the contact pressure. Taking a closer look at the von Mises distribution (Figures 11 and 13), useful indications could be extrapolated about the stress distribution. Specifically, the von Mises stress maps appear more homogeneously distributed in the whole contact zone for the PSD and PPSD, engaging a thicker portion of cortical bone in the load transferred from the implant to the bone. Also, stress distribution could provide some information for long-term stability; in fact, the stress distribution and the micromotion of the patient-specific implants suggest that a homogenous bone remodeling could occur with no fibrous tissue formation, preventing possible adverse events such as the risk of implant loosening [6; 13; 18; 21; 28; 30; 35; 36; 48]. A close comparison of the results reported in this study can be carried out with the works of [PERSON] et al., 2019 [6], in which the primary stability was investigated using the same load condition adopted in our work, finding similar values compared to our results for the micromotions at 1000 N (76.02 um [6]).