Researchers also concluded the statistical model of a population, guides the design focus of the implant. Nina et al. [94] developed a level set segmentation method to draw the statistical shape model of a target population. Then an implant can be virtually fit onto the statistical model, and determine the population that best fit the implant. The result improved the implant design by highlighting the important region of the implant for a population.
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Parametric approach
The current patient-specific implant design process usually consists of many procedures with a long cycle [140]. To achieve the end design, bioengineers usually need to spend a long time to clean, smooth and patch the 3D bone model converted from CT or MRI scans. The tedious process is time-consuming and laborious, especially in comminuted fractures. Thus, researchers proposed parametric approaches to rapidly generate implant model by adjusting key parameters concluded from a universal bone model.
Van Tongel et al. [159] developed an automatic alignment method to reshape different plates by aligning the centre and principal axes of the contact region to the fracture, as demonstrated in Figure 2-19 Constraint conditions such as the riddance of intersections between plate and bone were applied. The results indicated that the pre-contoured plates of the clavicular diminish significantly hardware prominence. Chen et al. [160] set up parameters onto five surface features defined from an average femur model. The customised fixation plate was designed by adjusting the parameters according to the actual fractures, as shown in Figure 2-20. The author suggested that the parametrical design method effectively saved time, and was a basic tool for patient-specific design and digital restoration of incomplete femurs.
Compared to some researchers designing personalised implant by adjusting parameters defined for an implant, it is more comprehensive to design the customised implant referring to the complete surface geometry of bone [161, 162].
Mirroring reconstruction method
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Figure 2-19 Calculation of desired region of contact for the plate [159].
Figure 2-20 Feature parameterization, (a) head and neck, (b) trochanter, (c) condyle, and (d) shaft [160].
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implant referring to the mirrored unfractured bone, which ignores the geometrical difference between symmetrical bone. For example, Wang et al. [163] designed customised pelvic fixation plate by extracting and thickening the surface of the smoothed pelvic model. The mirrored unfractured hemipelvis was selected for the design process. The implant was then manufactured in Ti6Al4V by SLM method. The intraoperative C-arm and postoperative X-ray results demonstrated good fitness between the plate and fractures. The operation time was reduced to 2 hours. Kozakiewicz et al. [153] converted the CT data of orbital floor fractures into 3D models, and virtually mirrored the models and physically realized the model in photopolymer. As demonstrated in Figure 2-21, the implanted mesh was cut to size by surgeon and bent referring to the normal side. Sensitive anatomical structures were omitted preoperatively, such as the lacrimal sac. The result indicated a reliable and financially affordable method to repair the orbital floor fractures.
Anatomical reconstruction method
Anatomical reconstruction designs are usually applied when the implant replaces the missing bone. The cross-sectional areas of resected bones are virtually connected to be an implant. Moiduddin et al. [72, 73] reconstructed a defect mandible using two geometrical custom implant designs - the mirror reconstruction method and the anatomical reconstruction method, as exhibited in Figure 2-22. The deviation analysis showed that the anatomical design model demonstrated better surface fitness compared to that of mirror reconstruction technique.
In some special conditions, the mirroring image is also useful for extrapolating existing anatomy of the fractured side of the bone. To design an implant of a
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Figure 2-21 Solid model of left orbital floor with formed titanium mesh [153].
Figure 2-22 (a) Mirroring reconstruction design, and (b) anatomical reconstruction design [73].
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Figure 2-23 (a) Reconstructed 3-D pelvic tumour model and prosthesis design, (b) the porous prosthesis and simulated tumour excision, (c) implantation and fixation of a prosthesis, (d and e) 3-D CT reconstruction and X-ray film showed good alignment at 18 months postoperatively [164].
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resection region, Fan et al. [74] used the contralateral anatomical site of the patient to design the implant geometry, as shown in Figure 2-23. The study successfully designed and applied implants for patients with clavicle Ewing’s sarcoma (ES), scapular ES, and pelvic chondrosarcoma (CS).
Fracture registration method
The fracture registration technique repositions fractures to the optimal positions referring to the mirrored bone, and then designs the implant referring to the stitched re-positioned fracture bone. Lee et al. [75] segmented each pelvic fracture to be manipulated individually and then recombined the separated bony region referring to the position of the mirrored semi-pelvis. As demonstrated in Figure 2-24, the broken fragments were supposed to recover to the original positions in space.
Finite element optimization
Finite element analysis is also useful for optimizing the design of the implant. It also validates the functionality of the customised design by simulating different living activities. Grujicic et al. [165] applied a musculoskeletal multi-body inverse- dynamics analysis for a realistic physiological loading condition. The author also proposed applying the analysis to evaluate the implant prior to clinical use, as demonstrated in Figure 2-25.