A framework for custom design and fabrication of cranio-maxillofacialprostheses using investment casting
V. Csáky, R.J. Neto, T.P. Duarte & J. Lino AlvesInstitute of Mechanical Engineering and Industrial Management, Faculty of Engineering,University of Porto, Porto, Portugal
M. Couto & M. MachadoInstitute of Mechanical Engineering and Industrial Management, Porto, Portugal
ABSTRACT: This work aims to enhance the design process and to develop a protocol for fabricating customizedcranio-maxillofacial prostheses.The approach entails four tasks: Image processing, biomodelling, fabrication andfinishing. The image processing comprises image segmentation and 3D reconstruction of the patient’s anatomy.The biomodelling consists in designing a custom-fit cranio-maxillofacial prosthesis. Finally, the fabrication isperformed through investment casting.Thin ceramic shells are fabricated using wax models, which are previouslyobtained by wax injection in silicone moulds and wax printing. The wax is eliminated by flash firing and theshell is sintered for 2 hours at 1450◦C. Then, a titanium alloy is melted and casted under a controlled atmosphereusing a copper cold crucible. The final step is the surface finishing using chemical milling that removes thesuperficial α-case layer and reduces the prostheses’ thickness. Ultimately, a 5-axis CNC machining is used tosmooth the surface and to drill the holes for the attachment system.
1 INTRODUCTION
Medical needs for customized prostheses are derivedfrom bone degeneration and related issues, e.g.osteoarthritis, trauma or cancer. Trauma factors arethe major cause for the continuous developmentof custom-fit cranio-maxillofacial (CMF) prostheses(La-Salete et al. 2013, Duarte et al. 2011, Lantada &Morgado (2013), Gassner et al. 2003 & Roccia et al.2010).
Computational advances in medical image acquisi-tion and design methodologies enhanced the processof biomodelling and pre-surgical planning, resultingin benefits in the surgical interventions efficiency andin patients’quality of life. Custom-fit CMF prosthesesincreases the longevity of the implants leading to lessrevision surgeries and cost savings, and so representsan added value when compared with standardizedprostheses (Goh et al. 2010).
Regarding prostheses manufacturing, this workaims to present the fabrication of custom-fit CMFprostheses using investment casting technology for thetitanium bio-alloy Ti-6Al-4V. The investment castingmethodology is preferred to other foundry technolo-gies, due to its ability to reproduce complex geometrieswith low surface roughness and costs, and possibilityto cast a wide variety of metallic materials, which aredifficult to machine or cast, such as titanium alloysand cobalt-chromium alloys that are used in medicalapplications (Swift & Booker 2013).
The optimization of the design and fabrication pro-cess by means of computational tools and numericalsimulation, to enhance process efficiency and patient’scomfort, are the principal motivation for this work(Tsouknidas et al. 2011 & Aquilina et al 2013). Thus,a framework for custom design of CMF prostheses isoutlined. Two distinct case-studies are considered asdemonstrative examples, a craniofacial prosthesis (I)and an orbital prosthesis (II).
2 METHODOLOGY
In this section, a four-step methodology for customdesign and fabrication of CMF prostheses is pre-sented, namely: i) image processing, ii) biomodelling,iii) fabrication and iv) finishing.
2.1 Image processing
Medical images from CT (Computed Tomography)and MRI (Magnetic Resonance Imaging) scans areavailable in DICOM format. Processing these imagesenables a 3D virtual model of the patient’s anatomyand, therefore, evaluation of the damaged region.Within this work, Mimics®16.0 software (MaterialiseNV, Leuven, Belgium) is used. The first step consistsin importing the DICOM files into the software andsegment the region of interest applying an adequatethreshold range to select the grayscale intensities for
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Figure 1. Mimics®16.0 interface with the importedDICOM files and 3D model of the case-study I regardinga craniofacial defect.
the bone. The image tonalities can be visualized ingrayscale or in Hounsfield scale. The second step isto improve the images’ quality by applying specificfilters and tools for CT noise and artifacts reduction.Using the region growing feature, structures that arenot connected to the main anatomical model are elim-inated. Also, artifacts can be eliminated using manualtools (e.g. draw, erase and local threshold paintingoptions) in the image slices or in the 3D reconstructedmodel. Thereafter, the 3D anatomical model’s surfacecan be smoothen and accomplished accordingly, basedon the previously used tools and selected variables.
Figure 1 illustrates the Mimics®16.0 interface afterimage processing for a craniofacial prosthesis. At thispoint, the 3D model can be evaluated and approvedby the surgeon, determining whether there is any areademanding further investigation. Finally, an STL fileconsisting in a triangular mesh is exported from thesoftware and is imported in an additive manufacturingequipment (e.g. stereolithography – SL machine) to bematerialized.
2.2 Biomodelling
The biomodelling task comprises two main steps: anevaluation of the model by means of measurementsand alignment, and the prosthesis design stage itself.This tasks should be performed in a software capa-ble of editing STL files (e.g. Materialise® 3-matic®,Geomatic® Freeform®, Blender™, etc.) due to thespecific triangular mesh surface formats that are noteasily modelled in CAD 3D softwares. For this task,the 3-matic® 8.0 (Materialise NV, Leuven, Belgium) isused.The adopted approach depends on the case-studyand its complexity. For unilateral defects a symme-try plane can be considered in the anatomical modelto use mirror techniques followed by Boolean oper-ations (e.g. intersection and subtraction). In case ofbilateral and/or asymmetric defects, a digital databaseof anatomical models is demanded.
Once the implant’s design is completed, verificationand reparation tools are run to certify that the geometryis valid and no entities are in conflict with each other(e.g. there are no triangles overlapping or missing inthe surface shell of the model).
Figure 2. (A) Anatomical model & (B) Prosthesis materi-alized by SL.
Thereafter, the fixation system of the implant isdesigned. For sake of an appropriate fixation, a bonedensity algorithm is used to identify the more suitableanchorage points and that will provide a reasonablehigher torque in order to ensure that there will be nosignificant prosthesis displacement or distortion dur-ing its lifetime that could compromise the patient’ssafety, comfort and health (Kang et al. 2014). The fix-ation in thick cortical bone is recommended rather thanin thin cortical bone due to better strength and boneintegrity during screw allocation. Furthermore, struc-tural analysis can be performed to validate the previousselection of anchorage points and ensure that the pros-thesis is well designed to withstand the mechanicalstresses present in the defective region (e.g. chewingforces for a mandibular prosthesis). Thus, the fixa-tion system design is a fundamental step to ensurethe implant stability, both during the manufacturingprocesses and in the daily life activities.
The anatomical and prosthetic models are mate-rialized by stereolithography (SL) for pre-validationpurposes (Fig. 2).
The prosthesis assembly and its geometricallowances can then be evaluated and corrected byredesigning whenever imperfections are reported.Thus, the design technique is an iterative process thatis concluded once a multidisciplinary team of doc-tors and engineers agree that the assembly betweenthe defect and the prosthesis is satisfactory. At theend of this process, the surgeon can have a 3D modelof the defective anatomical region of the patient, adesign project of a custom-fit prosthesis and a set of thedetailed information to support the clinical interven-tion, including anatomical distances and the correctpositioning of the implant and fixation systems.
2.3 Fabrication
The investment casting technology entails a set of spe-cific steps to obtain the metallic prosthesis. To initiatethe foundry process, a wax model is necessary. Thismodel can be materialized by two ways: i) wax print-ing, that is the direct printing of the designed implantwax model using adequate additive manufacturing(AM) technology, ii) converting the SL model (can
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Figure 3. (A) Wax pattern for investment casting & (B)Ceramic shell around the wax pattern.
be obtained by other AM process) into a wax modelusing silicone moulds.
The first technique is faster and does not requireso much labour work as the second. Although weused both processes for comparison purposes, the firstmethod is the most recommended and that will be usedin future work (INEGI bought this year a ProJet® 3510CP, 3D Systems® Corporation).
Once this step is completed, the gating and feed-ing system is assembled to the model, to completethe tree for investment casting (Fig. 3A). Thereafter,thin ceramic layers are deposited around the wax tree.The shell consists in 8 layers, being the first 2 layerscalled facecoat and the other ones backups. For castingreactive metals, such as titanium or cobalt-chromiumalloys, a two-layer facecoat is recommended, other-wise a simple layer is enough.The first facecoat will bein contact with the wax model and then with the metal,therefore its particle size must be very thin to preciselyreproduce all the details. Before the facecoat, a clean-ing step is demanded to promote the slurry adherence.For the first facecoat, the model is immersed in anorganic binder with yttria based flour slurry (AY) andpulverized with yttria sand (125–150µm). The modelis then placed in a controlled atmosphere (temperature24◦C and humidity 40%) with forced convection for 2hours drying. This drying period is applied in-betweenimmersions to promote shell drying and strengthening.The second facecoat and subsequent layers are manu-factured with coarser particles.The model is immersedin an AFAl (fumed alumina with alumina) slurry andpulverized with alumina sand (60 FEPA for the 3rd and4th layer and 36 FEPA for the following layers, beingthe first type of sand thinner than the second). In thefinal step the model is immersed in AFAl and dried for12 hours in the same controlled atmosphere (Fig. 3B).The model is then dewaxed by flash firing (1 hour at1100◦C) and the shell sintered at 1450◦C for 2 hours.
At this point the pre-heated at 1100◦C ceramic shellis ready for metal casting. The alloy is melted andpoured into the shell under a controlled atmosphere
Figure 4. Anatomical model of the case-study I regardingan orbital defect.
using a cold copper crucible, i.e. cold skull induc-tion furnace (Martins, 2008). The ceramic shell is thenknocked out using a vibrating pneumatic hammer, andthe gating and feeding system is removed. The metal-lic prosthesis (net shape) is now ready for finishingoperations.
2.4 Finishing
The prosthesis was designed with an additional thick-ness to compensate models, ceramic and metal shrink-age, and finishing operations. To obtain the finaldesired thickness, chemical milling is performed. Thethin oxygen-contaminated hard and brittle layer (α-case) created during casting is removed in this oper-ation. The final step is a 5-axis CNC machining tosmooth the surface and to drill the fixation system.
3 CASE STUDIES
3.1 Case-study I: Craniofacial prosthesis
This case-study is of a 46 years old male with a cranialdefect above the left orbit. The defect covered both thefrontal zone of the skull and the upper side of the orbit.The origin of this problem was a trauma in the orbitaland frontal bone of the skull. CT scans (Siemens Sen-sation Cardiac 64, Siemens, Germany) were acquiredwith the following parameters: 120 kV and 63 mAs.The DICOM set comprises 82 slices with 3 mm incre-ment and 0.309 mm pixel size. These images providedspecific data of patient’s anatomical trauma defects.The DICOM files were imported using Mimics®16.0,the 3D model was reconstructed and exported as STLfile (Figs. 4, 6A).
The prosthesis design is carried out using 3-matic®8.0 software. For this case, a cranioplastycommand operation of 3-matic®8.0 is used. The firststep is drawing a curve around the defect and theorbital region. An offset of 0.7 mm is applied to obtain
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Figure 5. Design sequence for creating a cranioplasty pros-thesis using 3-matic®: (A) Drawing a curve around thedefect, (B) Applying an offset for the prosthesis’ thick-ness & (C) Trimming the excess material in accordance tothe mirrored healthy side of the cranium.
Figure 6. (A) Reconstruction of the 3D model usingMimics® 16.0 & (B) Custom-fit design prosthesis using3-matic® 8.0.
the prosthesis’ thickness and geometry. A mirror tech-nique reproduces the healthy half skull in the defectiveregion to trim the prosthesis excess material. The trimoperation tailored the orbital curvature in frontal viewand the prosthesis’ main geometry is accomplished.Figure 5 illustrates these sequence design steps.
A bone density algorithm is used to properlychoose the anchorage points. Custom sized flaps(16× 8× 0.5 mm) with rounded edges are designedin accordance to the skull curvature and bone density(Fig. 6B). These flaps are used to substitute standardsurgical plates, which are more expensive and liable tobreak when allocating screws (both in the prosthesisand in the bone). Nevertheless, the use of these platesimplies 2 screws which is a more invasive techniquethan integrated flaps with only one fixating screw.
Finally, the skull prosthesis prototype resin modelis materialized by a SL equipment (Viper™ SLA®System, from 3D Systems® Corporation, Rock Hill –SC, USA). The following steps are in accordance withthe described fabrication methodology.
3.2 Case-study II: Orbital prosthesis
The case-study II reports a 68 years old female witha maxillofacial deformation resulting from cancerremoval. CT scans (Toshiba Aquilion, Toshiba Med-ical Systems, Japan), with parameters 120 kV and90 mAs, are acquired to obtain the information aboutthe deformation and surrounding anatomical struc-tures. The DICOM set consists of 215 slices with1 mm slice thickness and 0.351 mm pixel size. Simi-larly to the previous case-study, the 3D reconstruction
Figure 7. (A) 3D model reconstructed with visible defec-tive orbital region, (B) First version of the designed orbitalprosthesis & (C) Second version of the prosthesis’design. Alldesign methodologies were performed using 3-matic®8.0.
is performed and the skull is materialized by SL.After analysing the model, an absence of the orbitalregion and superior maxilla is observed (Fig. 7A).Later, the patient experienced a superior maxilla recon-struction surgery by bone grafting. Considering theunilateral defect, the customized prosthesis is devel-oped using 3-matic®8.0 employing mirroring tech-niques and Boolean operations. The symmetry planewas defined using the middle sagittal plane. Finally,using trimming options the final geometry of theprosthesis and fixation flaps are designed based onthe bone density algorithm (Fig. 7B). A second ver-sion of this prosthesis was designed with a specificcurve flap that attaches on the opposite side of themaxilla bone. This design is demanded due to thechewing forces involved. Also, another specific fix-ation flap is developed to adapt in the zygomatic arch(Fig. 7C). Thereafter, the prostheses are fabricated bythe described investment casting technology. The partsare now in the cast phase.
4 CONCLUSIONS
Custom-fit cranio-maxillofacial prosthesis (CMF)enhances patients’ quality life and increases thelongevity of the prosthesis, resulting in less follow-up interventions. Patient specific implants assuresthe correct positioning and fixation in the corticalbone, enhancing the surgical approach and respectivepreoperative planning. The proposed methodologyfor 3D reconstruction and biomodelling offers sev-eral advantages for orthopaedic surgeons, such aseffective evaluation of the defective region, measure-ments capacity and possibility of studying differentapproaches to the case, including the visualization ofthe final result.
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Investment casting represents the most efficient anddirect method among the foundry industry to fabri-cate custom-sized CMF prostheses. The possibility touse a wide range of materials, including reactive metalsuch as titanium or cobalt-chromium alloys, associatedwith the freeform geometry and thin details that canbe reproduced, turns this process a preferred optionin foundry. Finishing techniques such as chemicalmilling to reduce the prostheses’ thickness and α-case,hence, improve the surface finishing, and 5-axis CNCmachining to smooth the surface and drill the fixationsystem, open great opportunities for more efficient andcost-effective prosthesis.
In this work two distinct case-studies are consid-ered as demonstrative examples of application. Therequired steps to develop a tailored prosthesis arethe evaluation of the defective region, the selectionof the appropriate tools in the biomodelling softwareand the design of the fixation system according tosurrounding bone density and required structural sta-bility to the prosthesis. In both cases, the prosthesesare materialized by waxprinting in order to proceedwith their fabrication by investment casting. The cor-responding skulls are also materialized by SL forpre-validation and final assembly with the metallicprostheses.
As future work, a topology optimization is recom-mended based on finite element methodologies. Thiskind of analysis may lead to an enhancement of thefixation system and also to more lightweight prosthe-ses without compromising their structural stability. Inthe foundry process, a filling and solidification simu-lation is pointed out to optimize the process and reducewastes.
ACKNOWLEDGMENTS
Authors gratefully acknowledge funding of ProjectSAESCTN-PII&DT/1/2011 co-financed by ProgramaOperacional Regional do Norte (ON.2 – O NovoNorte), under Quadro de Referência EstratégicoNacional (QREN), through Fundo Europeu de Desen-volvimento Regional (FEDER).
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