## 4 Discussion  
During the distal femur fracture with bone defect operation treatment, besides using bone grafts to fill the defect, other supporting implants such as lateral site pre-contoured locking plates are normally used to fix the fractures. Postoperative protection is usually required (partially or completely without weight-bearing). After 3-6 months, when the bone starts to heal, weight-bearing is slowly added. Because the locking plate position is close to the knee joint, patients often feel a foreign body sensation and even pain with restricted movement after the operation. Therefore, the locking plate can be removed after the bone is completely mended. Thus, three stages were set up in this study to simulate these three critical treatment time points. Stage 1 was the early surgical stage, with bone stability fixed mainly by the 3 DP patient-specific implant, the locking plate, and screws. The more complete the fixation the better the initial stability. Stage 2 was the period after bone union is completed. The structural stability at this time should be the strongest, with the patient's limb stability reaching  
Figure 6: Overall displacement of the construct can be found at the distal condyle and was about 4.8 mm in the first stage. It was reduced to 2.5 mm with completion of osseointegration in the second stage and can be reduced to 2.1 mm after removing the locking plate in the third stage. Figure 7: Most of the strain values were less than 4000 at the proximal and distal implant-bone contact interfaces and only a small amount was more than 4000 around the proximal stem and the distal bone nail in the stage 1.  
optimum condition. Stage 3 was the time after the locking plate and screws are removed, while the femur and 3 DP implant remained in situ. One of the advantages of using FE analysis is that it is possible to simulate biomechanical performance for different stages. This cannot be achieved using traditional experimental methods. As described in the Introduction, although 3 DP patient-specific titanium implants with good biocompatibility in regard to size and shape are a potential alternative to distal femur fracture with segmental large bone defect treatment, titanium implants are stiffer than natural bone and cause significant stress-shielding effects, leading to bone resorption and eventual implant failure [13; 14]. A lightweight optimized structure that can withstand physiological loads from a biomechanical point of view is the highest guiding principle of 3 DP applications in orthopedics. The proposed novel implant design was fabricated using AM400 titanium alloy printer (Renishaw, Gloucestershire, UK) (powder about 30 \(\upmu\)m) using ISO13485 quality management systems research center (Taiwan Instrument Research Institute, Hsinchu, Taiwan). The implant was treated to delete residual sandblast particles and cleaned using ultrasonic oscillations (Figure 8). The manufactured implant has a lightweight design, with a solid weight of 107 g. The corresponding defect bone weight is 78 g. The implant is only 1.37 times heavier than the original bone, which is closer to the clinical need for replacing the defect bone. Strain is an important indicator for bone healing/remodeling at the implant-bone interface [15]. [PERSON] suggested that bone remodeling is initiated at some critical strain level in his \"mechanostat theory\" [16]. [PERSON] and [PERSON] also pointed out that a newly formed bone area is proportional to the induced strain magnitude [17]. Critical damage strain over thresholds of 4000 \(\upmu\)m could induce microdamage of bone cells and delay interface bone union or nonunion [18]. In stage 1 (immediate postoperative) before bone healing, the simulated results revealed that the implant-bone interface strain was mostly below 4000. This provided an environment conducive to bone healing.