## 4 Discussion
### Craniomaxillofacial Implants  
The cumulative error pertinent to the process chain of an end-use implant can prevent patient-specific treatment and restoration owing to ill-fitting implants and complications in surgeries [93].  
Though the cumulative error evaluated in the present study can be adequate for most craniomaxillofacial surgical procedures [72; 94; 95], a safety margin of 2.31 mm (4.1% relative) should be respected considering the worst-case error. Practitioners should also consider that the thresholding error and AM magnification error can be corrected through redesign of the implant to significantly improve the results in the future studies.  
For example, the thresholding error (0.602 mm) can be compensated in the implant dimensions during the modeling process. The dominating AM magnification error in the z-axis can be manually compensated in the implant model using the pre-processing software before manufacturing, which may reduce the magnification error by a factor of 15.  
Further, practitioners should avoid placing critical geometries of the implant on the z-axis, particularly to the down-skin surfaces that require addition and removal of support structures.  
In addition to using standard AM process parameters and heat treatments, practitioners need to consider automating the support removal process in case it unavoidably affects critical geometries. This step should be considered before the AM process to ensure that the path of the machining tool is not obstructed by the implant material.  
Through an evaluation of an increasing number of segmented anatomical parts with respect to a calibrated CT system, the thresholding error can be mitigated using regression and neural network models as a function of the HU number and software. Apart from thresholding that was performed in this study to standardize the results, practitioners should also consider manual thresholding alternatives. Though subjective and time consuming, manual thresholding may yield better STL models than the default thresholding values of the segmentation software [39]. Though not yet commercially available, advanced thresholding methods [72] are also expected to decrease the leading significance of the segmentation error. The CT images of the SD head are made available to promote future research [67].  
Practitioners that require a high accuracy for the implants, e.g., the orbital floor implants [96], should respect the recommended safety margin. The prescribed correction measures can be taken if the intended processes and equipment are identical to this study or after implementing the applied methodology with the relevant processes and equipment as a quality control and assurance protocol. A lack of lower uncertainty (higher accuracy) can also be compensated with precise bone resection using additively manufactured cutting guides [97].  
Moreover, it should be considered that it may not always be absolutely imperative to design the implant to fit perfectly at the initial phase. The perforation of the lingual mucosa is one of the complications related to the patient-specific implant. If the lingual foil of the implant scaffold is too high, it is prone to perforate the lingual mucosa during the postoperative healing [98]. For example, the extraction of teeth in the osteotomy line will lead to a resorption and further, remodeling of the marginal alveolar crest, which has to be considered during the planning of the implant.  
A successful implementation of AM from radiologic images is a multidisciplinary task [95]. Achieving the most benefit from this technique requires a very close co-operation between all experts involved in the process.