Custom implant design for patients withmandible defects based on rapid prototyping

Qing TAO

，Wen-lei SUN1，Na SU 3，Jin-sheng KANG2

(1 School of Mechanic Engineering，Xinjiang University，Urumqi830046，China)

(2 School of Engineering and Design，Brunel University，Uxbridge，Middlesex，UB8 3PH，UK)

(3 Clinical laboratory，Xinjiang Medical University 1st hospital，Urumqi830046，China)

Custom implant design for patients withmandible defects based on rapid prototyping

Qing TAO1，2*

，Wen-lei SUN1，Na SU3，Jin-sheng KANG2

(1School of Mechanic Engineering，Xinjiang University，Urumqi830046，China)

(2School of Engineering and Design，Brunel University，Uxbridge，Middlesex，UB8 3PH，UK)

(3Clinical laboratory，Xinjiang Medical University 1st hospital，Urumqi830046，China)

This papermainly focuses on themandible surgery application of the rapid prototyping technology and themedical CT technology.The relation of the common approach of CT image data processing was discussed in this paper，and an improved construction method of the rapid prototyping data of jaw bones based on CT image was proposed aswell.A 3Dmodel of themandible defectwas generated after the CT data processing.An individual tray was designed by usingmimics software and fabricated by rapid prototyping(RP)technology.Themethod proposed in this paper could improve both the reconstruction rate and the geometric similarity.

CT image，Image processing，3D reconstruction，Rapid prototyping

*Corresponding author:Qing TAO，Ph.D.，

E-mail:xjutao@qq.com.

1 In troduction

With the rapid development of the three-dimensional digitized technology and the computer graphic，simulation technology hasmore and more applications in themedical field.At the beginning of the 1970s，the computer aided design(CAD)and the computer-aided manufacturing(CAM)technologies were introduced to the fixed design and fabrication process in denture by a famous French oral expert Francois Duvet.In 1978，a digital image processing technology was used for X-ray head piece.In 1980s，a digital image processing X thrum shadow measuring system was developed.Recently，reversing engineering and rapid prototyping technologies have been applied to produce physicalmodels in variousmedical applications［1］.The rapid prototyping technologies and medical CT techniques bring enormous influence on the medical profession.In order to meet the needs of different medicine，it has already become a new hotspot in the rapid prototyping technology to get prosthesis physical model by using the rapid prototyping technology.The computerized processing and analysis ofmedical imaging modalities has become a powerful tool to help physicians to make some important clinical decisions［2］.It has some important applications such as the medical diagnosis，surgical planning and simulation，plastic and artificial limbs surgery，and anatomy teaching［3］.Therefore，the development ofmedical image 3D reconstruction technique has been developed very fast.Mandible surgery as a common surgery and 3D reconstruction technique occurs in this field.Nevertheless，the 3D reconstruction jaw bone，as one of the traditional technique，has been approved that it is not an idealmethod due to the low efficiency.In this paper，amethod of constructing the rapid prototyping data of jaw bones based on CT image is proposed by using the reverse engineering ideas.The rapid prototypingmachine has been developed and used to produce models for clinical applications.The 3D CT data could be exported to produce a precise-fitting prosthesis for a successfulmandible surgery.The rapid prototyping implant production could significantly reduce the operation time by reduction of painstaking surgical procedures such as shaving and casting process.In many critical radiological applications，the multi-dimensional visualization and the quantitative analysis of physiological structures could provide extremely valuable information for diagnosis and treatment.This paper described a method based on surface reconstruction software and rapid prototyping for individual design.A 3D bone of patient could be reconstructed after the acquisition of helical CT data，and then it could bemanufactured based on the rapid prototyping technology.According to the CT image data，the key research focuses on the medical image three-dimensional reconstruction technique.This technique includes the reading of medical image data，the pretreatment of medical image data，the medical image segmentation and the extraction of target tissue.Based on the above mentioned techniques，a rapid prototyping stereo-lithographicmodel and tray could be used to produce a customized implant for the reconstruction of a jaw defect.

2 Material and m ethods

The patient was presented to the Department of Plastic and Reconstructive Surgery Xinjiang Medical University 1st hospital.His clinical and neurological condition was stable.The spiral CT scanner(GE HIGHSPEED 16 row CT machine)has been used to scan the whole head which gives the detailed data of the entire skull within 50 seconds.The skeletal area of interest for the surgical procedure was scanned in the helicalmode with 1.25 mm table increment.The scanned data were transferred to the Xinjiang University where themodelswere built by a SPS600 solid laser rapid prototyping machine which was steered by Mimics software.The overview of mandible surgery application system was shown in Fig.1.

Fig.1 System overview

2.1 CT data acquisition and format conversion

The data acquisition for 3d modeling was performed with the aid of a helical computed tomography(CT)imaging.Acquisition of the CT data was obtained by using the standard craniofacial protocol.With the protocol，the patientwill get laid in supine position with head supported and immobilized.The spiral CT scanner(GE high-speed 16 row CT machine)will scann the whole head from the top to the chin with adequate table speed.The datawas produced to the consecutive slices with 1-mm thickness，finally about 260 layer data was obtained.The serial CT image DICOM data were written in CD-R.The Digital Imaging and Communications in Medicine(DICOM)include a file format definition and a network communication protocol.The communication protocol is an application protocol that uses TCP/IP to communicate between systems.The first problem needed to be solved is how to convert DICOM format data into the other image data.The Mimics software is used to transfer the DICOM format data，and then to reconstruct the three dimensionalmodel of the patients’head bone.The transferred DICOM format data includes two steps:first，to read the DICOM format data by using the Mimics software;second，to export the data into the general BMP format image file.The Mimics software could read a collection of the DICOM format images，and convert the DICOM format images into an internal general format.The tomography images have to be analyzed bymanually in order to extract some useful information，for example，top，bottom，left，right，anterior and posterior information in the image.As shown in Fig.2，the typical sequences of2D CT image sliceswere presented.

Fig.2 Typical sequence o f 2D CT im age slices

2.2 Image data processing

1)Image pre-processing

There existmany approaches for the image pre-processing，such as filter，interpolation and sharpening，but the filter function wasmainly used in this paper.In order to reduce the noise，it is necessary to enhance image quality and to improve the ratio of signalto-noise.According to the characteristics of CT images，themedian filtermethod was adopted.The goal is to accentuate certain image features for subsequent analysis and display［4］.Practice shows that themedian filter could not only suppress the noise effectively but also keep the useful signal，which could help to obtain a better quality of the CT images.

2)Regional segmentation

In order to analyze the CT images，the bright area and dark area represent the brokenmandible fragments and the soft tissue，respectively.The bones and soft tissue are presented in the different levels of brightness in the CT images.Generally，the CT value of the soft tissue is in the range of0-200 and the CT value of the bone is 300－1 500.The binary was employed to set the threshold for each CT image slice.To choose a threshold T in software，the images of bone and soft tissue are divided into two types of lightand dark，and the binary image segmentation could be obtained by adjusting the value of threshold.When the different values of threshold are chosed，the figure of the bone will be corresponding changed.The choose of different threshold could greatly influence the precision of the three dimensionalmodel.As shown in Fig.3，one of the typical regional segmentation was presented.

Fig.3 Regional segm en tation

3)Contour detection

The contour detection is performed based on the threshold value.The contour detection algorithm requires the user to click on the end points of each fracture contour in the each CT slice［5］.At present，the common medical image contour detection algorithm is the T＆Gmethod［6］.Binarization cannothelp to distinguish between the two fracture fragments.Some undesired artifacts need to be filtered out so thatonly the broken fragments could be remained for the purpose of surfacematching.Thismethod could obtain continuous and closed outline for the further analysis on the images in order to bring a lot of benefits.As shown in Fig.4，it is one of the typical contour extractions.

Fig.4 Contour extraction

4)Dynamic region growing

Automatic seeded region growing algorithm was used for the color image segmentation.In order to automatically seed the selection，the following three criteria must be satisfied.First，the seed pixelmusthave high similarity to its neighbors.Second，in an expected region，at least one seed must be generated in order to produce this region.Third，seeds from different regionsmust be disconnected［7］.In this case，one selects the seeded region first，and then clicks on the automatic regional growth command，the computerwill automatically sift the seeded area and connect the bone area.This could greatly accelerate the speed of image processing.

5)3D reconstruction of the mandible and output STL files

Themirrored image was used to match the missing segment of themandible［8］.The tray was designed from themirrored image which covers the defect area to restore the mandible continuity［9］.As shown in Fig.5，the reconstruction of the mandible is presented.The CT image data were translated into a STL format data by using Mimics software，and were constructed by rapid prototyping manufacture to the 3d STL digitalmodels.The output STL data could choose a binary or text form.Text form of STL files for the skull data could take up about90 M storage space.In order to avoid themistakes during the reading process of rapid prototyping machine，the best way is to save this data as a binary format，and it only occupies20M storage space.

The raw CT data of the patientwas translated into a STL format by using Mimics software.The data were read by a LPS600 rapid prototyping machine to produce stereo-lithographic skullmodels.Stereo-lithography uses a solid free-form fabrication method to build a physical model through a layer by layer building process.The solid model was converted to STL file format［10］.STL is a facet based representation that approximates surface and solid entities only.Implant will use the light sensitive resin.With this skullmodel，the jaw defect could be clearly shown and evaluated.As shown in Fig.6，the 3Dmodel of patient skull was presented.

Fig.5 Reconstruction o f the m andib le

Fig.6 3D m odel o f the patient sku ll

2.3 Supporting parts design

The mandible prosthesis consists of two parts:the tray and the cortical bone used as lid.For mandible prosthesis reconstruction，a healthymandible datawill be used to construct the defect part.The corresponding points are also seton the patient facialmodel.The healthy mandible model will be manually fitted their position onto the patient facial image.By using the reverse engineering software，the two mandible models could bemerged.Then，the STL file of the solidmodel is generated and exported to RPmachine to build the prosthesis.

3 Conclusion and discussion

Traditionalmethod to replace a jaw bone is the surgeon harvests a pelvic bone，sculpts it into a desired shape and inserts it into a patient’s mandible.This method will extend the surgery time and require a relatively larger volume of the bone，moreover an exact facial contour is usually difficult to be obtained.The method presented above was successfully applied in mandible substitute.The prosthesis geometry has precisely fitted the defects，leading to smooth and harmonic reconstruction of the deficient contours aswell.In this case，through the median filter pretreatment，regional segmentation，T＆G edge extraction method，by usingmimics software processing image，generation of rapid prototyping physicalmodels，which could get the bones of the wheel profile data are clear and complete，concise and accurate.This method is simple and easy，reconstruction model and realmeshing degree are high and precisely fit.CT imaging，rapid prototyping and computermodeling have improved the surgery planning and reduced the operating time.It has a good application prospect in orthopedics，maxillofacial surgery and other fields［11］.Due to the limited conditions，the CT scan was chosen in this paper，if MRIscan could be used in clinical application，cartilage boundary will be clearer and the outline of the figure will becomemore accurate.

Acknow ledgem ents

This paper is supported by National Natural Science Foundation of China(No.51465056)and Xinjiang autonomous region of natural science fund(No.20140718367，2013911032).

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基于快速成型技术的下颌骨缺陷患者植入体设计

陶 庆1，2*，孙文磊1，苏 娜3，康金胜2

1.新疆大学机械工程学院，乌鲁木齐 830046
2.布鲁内尔大学，英国伦敦 UB8 3PH
3.新疆医科大学第一附属医院，乌鲁木齐 830046

阐述了利用快速成型技术和医学CT扫描技术的病患者下颌骨缺陷植入体设计方法，探讨了CT图像数据处理的常用方法，并提出了一种改善的方法。该方法基于CT图像数据构建出患者下颚骨的快速成型数据，对CT数据进行处理后，可以生成下颌骨缺陷的三维模型。根据下颌骨缺陷的三维模型，使用MIMICS软件和快速成型(RP)技术制作一个特制的下颌骨缺陷托盘植入体，提出的方法可以提高重建速度和几何相似。

CT图像;图像处理;三维重构;快速成型

10.3969/j.issn.1001－3881.2015.06.005 Document code:A

TB476

16 July 2014;revised 29 September 2014;accepted 10 November 2014