Additive Manufacturing (AM) is one of the best techniques to fabricate customized medical models. This technique best suited for the medical industry. The surgical template design and dimensions vary from patient to patient, depending on the anatomy of the patient. AM provides flexibility to design a patient specific surgical template using the patient CT scan data. Medical processing software converts DICOM to 3D CAD data. This 3D CAD data is used to design the patient specific surgical template. This surgical template data is then converted into an STL file format to fabricate medical models using AM machines. The initial surgical template is fabricated using FDM machines, which are easily available and this template is used for pre surgical planning. After satisfactory results are obtained in the pre surgical planning, the same STL file is fabricated using a castable resin. The castable resin model is used for preparation of the mould for casting process. This mould is then used to produce the final surgical template with medical grade SS316. The cost of the final metal surgical template is reduced by 30 percent compared to production of same model fabricated using metal AM system. The major advantage of this process is that we obtain a patient specific template in traditional approved way, but at reduced cost.
Keywords: Additive manufacturing, Computer Aided Design (CAD), Genioplasty, Surgical template, Casting
The 3D printing of medical models have been there for more than 25 years. In the initial days this technology is named as Rapid Prototyping (RP) as the name indicates these are mainly used as prototypes in many industries one of them is medical. When this technology in 1980's there are only very few machines across the globe. The cost of these machines is very high in the those days and the choice of material is also very limited . As progress with technology the machines became affordable for common people, especially the machines with Fused Deposition Modelling (FDM) technique. FDM machines have wide range of materials choice. The beauty of 3D printing technology is each and every model made of 3D printer can be printed customized . This is benefiting most to the medical industry. In case of medical industry almost each and every patient requires customized medical models such as surgical templates or implants .
The first development in the medical industry took place in 1895 with invention of X-ray by Wilhelm. This helped to analyze the bone anatomy of the patient in those days. The X-ray provides only 2D anatomy of the patient that too only bony information of the patient. To overcome this CT scanner has been invented by incorporating multiple X-rays around the gantry tube; depending on the number of emitters and receivers the CT scanner specification varies [4-6]. In the initial days there used to be dual slice CT scanner latter on due to advancement in technology till 256 slices CT scanner is easily available beyond that also there but availability is low. Generally CT scanners are used to acquire bone anatomy of patient whereas MRI is used to acquire soft tissue information of the patient . In the early stages of AM the models are used for preplanning surgery only, but in the current days AM is capable of producing direct implants as per patient specific requirement . Even though the technology is so advanced but still there are issues to identify certified medical implant manufacturer, availability, time and cost of implants or template are still need to be streamlined. To overcome the medical approvals in case of AM medical models the alternate approach is identified in the current work, which utilizes the maximum benefit of AM technology along with conventional casting process.
Under the AM roof there are several processes comes under, the classification is done based up on the technique or material used. In broad way raw material can be liquid, powder or solid. The easily available technique among the all is FDM technique, where solid filament is used as raw material. Since this technology is easily available and economical used largely in fabricating of preplanning surgical models. FDM parts have good strength; these models can be used for functional testing in case of medical these can be used for cutting the medical model in preplanning surgery. Genioplasty is one of the complex surgeries in oral and maxillofacial surgeries; by using the surgical template genioplasty surgery becomes easy. In genioplasty surgery the mandible bone lower portion is need to be cut and move as per patient requirement. To identify the cutting region of bone using CT or X-ray data is very difficult. The designed template provides highest advantage to mark the cutting region on the mandible and mini plates are also bent in prior to surgery using AM medical model.
The procedure followed for casting of patient specific surgical template is shown in the below flowchart (Figure 1).
Acquiring patient CT data
The patient CT scan data is data is acquired using Somatom 128 slice CT scanner with 3D face scan. The acquired DICOM image is reconstructed using optimal reconstruction parameters. The CT scan data of the patient is shown in the Figure 2a below. The DICOM data of the patient is processed using MIMICS, medical processing software to obtain the 3D CAD bone anatomy of the patient. The 3D CAD data of the patient is shown in the Figure 2b below. From the DICOM data the bone anatomy of the patient is segregated using Hounsfield Units value, In the MIMICS the HU value for the bone starts from 226 to 3071. The soft tissue HU value is below 256 in the MIMICS software.
Development of patient 3D CAD data
From the complete bone anatomy of the patient required region of intrest is seperated using the operations like region growing, edit mask etc. For the current study in development of srgecial template for genioplasty, mandible is the region of instrest. The 3D CAD mandible of the patient is shown in the Figure 3 below . This 3D CAD data of mandible is used for the development of pateient specific template.
Design of patient specific surgical template
Patient specific surgical template is designed to fit exactly on the mandible of the patient, which is shown in the Figure 4 below. The main purpose of this template is to guide the surgeon while cutting the mandible portion of the bone, to happen this chamfer of 30° angles is provided at the cutting edge of the template. This angle provides flexibility to saw or micro motor drill to operate easily in the surgery. The design of template is made such that to relax inferior alovery nerve when template is placed. The template is designed to take support from teeth of the patient, such that it will be fixed exactly on the mandible and requires minimum area for placing the template. The 3D CAD image of template with patient mandible is shown in Figure 4 below.
Surgical simulation or virtual surgery planning
MIMCS software provides the flexibility of virtual surgery operations like cutting the bone portion and move it to the planned region to analyze the results as shown in the Figure 5 below. After obtaining the expected results the design of the template is finalized else the iteration will be repeated. Since this is only virtual surgery, the same procedure is conducted using the physical AM medical models. To conduct the preplanning surgery FDM models are best suitable and for the current case study to fabricate AM models, the 3D CAD data is of the model is converted in to the STL file format.
Pre planning surgery using FDM template
The STL file format is neutral file format for all the AM machines. The STL file is processed using pre processing softwrae to generate the machine specific code to fabricate AM medical model or template. In the current sudy Flashforge finder of FDM technology is used to fabricate the pateient specific medical model and surgical template for preplanning as shown in the Figure 6 below. Flashprint is the preprocessing software for the flashforge finder machine. PLA (PolyLacticAcid) is the material used for the fabrication of patient's manidible portion, mandible cut portion and patient specific surgical template. FDM provides teh flexibilty of multiple colours of filamnet to fabricate the models. These models will have good strength, which can be used as functional prototype models also. The biggest advantage of FDM is easy of availability and low cost.
Fabrication of patient specific metal SS316 template
For the current study investment casting process is followed to fabricate the patient specific surgical template with SS316 medical grade metal surgical template. The first step to be followed in investment casting is to prepare a pattern which can be evaporated and should be exact replica of the final product. Castable resin pattern of patient specific surgical template for the casting is prepared with SLA process as shown in Figure 7a. This pattern is used to create mould, which can be used only once, since the mold cannot be opened, to remove the final part mould need to be destroyed.
The advantage of this process is any complex model can be fabricated easily. To create the mold the casting pattern is dipped in to slurry of fine grained silica, binders and water. This slurry is forms as a ceramic layer over the casting pattern. When the ceramic coating is thick enough as shown in Figure 7b allows it to dry in air. To evaporate the pattern from mould ceramic mould is heated for 150°C, which forms the cavity for investment casting. The ceramic mold is heated up to 1000°C to strength mold and also to drain out if any castable resign is left over. The molten SS316 is poured in to the mould, when it is still hot, so that the molten metal will flow easily to each minute feature of the mold. This pouring of metal in to hot mould also results in better dimensional accuracy, since the mold and casting will shrink together as they cool. The final SS316 patient specific surgical template is shown in the Figures 7c and 7d, which can be used in surgery. These metal templates can undergo for autoclave or sterilization to use in surgery.
Additive manufacturing technology is best suitable for fabrication of customized low volume models. One of the biggest barriers with the metal AM systems is medical grade certification, cost and choice of material; to overcome these barriers a hybrid approach is followed. The pattern is fabricated with low cost AM systems, which are easily available to everyone. The final required metal surgical template is obtained through traditional investment casting, which is also easily available. The advantage of using this hybrid technique is for the current work SS316 of medical grade is used, but this procedure allows fabricating model with any material. This procedure provides maximum benefit to the customized parts especially in case of medical industry to fabricate the customized surgical templates and implants.