RU2196543C1 - Method for producing stereo lithographic models and biological implants usable in cranio-maxillofacial surgery - Google Patents

Abstract

FIELD: medical engineering. SUBSTANCE: method involves building three-dimensional computer image of a biological object when scanning coordinate table with minimum step in high resolution mode in keeping patient body immovable until complete set of tomograms is produced. Data for restoring bone structures of given density are selected from tomographic data array by defining cut-off level based on intensity gradient coefficient value enclosed between 0 and 225 to enable one to build stereo lithographic model. EFFECT: accelerated operation; satisfactory cosmetic result. 10 dwg, 2 tbl

Description

 The invention relates to medicine, namely to craniofacial surgery.

 A known method of manufacturing models of the maxillofacial region by x-ray tomography of the jaws (1).

 There is also a known method of manufacturing models in the maxillofacial region by tomography (2).

 The disadvantages of the known methods are that they do not provide accuracy in the manufacture of models and the subsequent manufacture of individual implants in maxillofacial surgery.

 As a prototype, a method of manufacturing a transplant was selected to eliminate defects and deformations of supporting facial tissues (3). This method consists in X-ray computed tomography of the necessary area of the facial skeleton, then using mathematical computer processing of the obtained data, volumetric parameters of the studied area are calculated. Further, using standard programs - computer-aided design (CAD) systems - various data transformations (symmetric and others) are performed that allow obtaining volumetric parameters of the endoprosthesis to eliminate an existing defect or deformation. Such mathematical images can then be made in the form of a plastic model on a stereolithographic installation.

 The disadvantage of this method is that standard CAD systems are used to convert computed tomography data, which do not contain algorithms for interpolating data on the surface of bone biological objects and perform reconstruction without taking into account the density gradient of existing structures. As a result of the numerous stages of data conversion (computed tomography, film scan, scanning, loop calculation, volume calculation), the accuracy of the data is lost, in addition, such models contain a characteristic "step", which is associated with layer-by-layer scanning of the object.

 The aim of the invention is to increase the accuracy of the manufacture of stereolithographic models of biological objects by performing high-resolution X-ray computed tomography with a minimum step of the coordinate table and obtaining a set of tomograms in the form of computer files of the original format, which are processed taking into account the density gradient of bone structures at the border of the object, which allows avoid inaccuracies and "stepping" of the mathematical model, which is then made on stereolithographic installation.

 The essence of the method consists in conducting a complete layer-by-layer X-ray computed tomography of the necessary portion of the patient’s skull, which contains a defect or deformation, scanning is performed at complete immobility of the patient in high resolution mode, according to the data obtained in the original computed tomography format, a three-dimensional image of structures of a given density of a biological object bone structures), the data array for reconstruction is selected by setting the gradient coefficient intensively ty (Abramov S.S., Bodyrev N.I., Evseev A.V. et al., 1998) from 0 to 255, which allows one to obtain a mathematical volumetric model of high accuracy and not differing in its geometric parameters from a real object (absent the effect of "stepping").

 At the first stage, it is necessary to obtain computed tomograms of the desired area of the patient's skull. Scanning should be carried out with certain parameters (intensity, step, number of scans), optimal for the further construction of a specific model. So, for example, to obtain comprehensive information in the design and manufacture of a high-quality stereolithographic model, we performed scanning with a minimum step of the coordinate table (in our case 2 mm) in high resolution mode (usually one of the standard modes, for example, for examining the middle ear). When examining the lower jaw, the plane of the slices was chosen parallel to the plane of occlusion of the teeth, scanning began from the chin and ended above the temporomandibular joints. When examining the upper jaw, scanning began from the upper teeth parallel to the plane of occlusion of the teeth and ended when a slice was obtained at the level of the lower wall of the orbit, also using a slice thickness of 2 mm. When examining the skull, scanning began from the upper part of the skull (or from the upper teeth) and continued to the required level in increments of 2 mm. In some cases, when it was planned to obtain a model of the bones of the brain part of the skull (with a small number of small parts), it was allowed to conduct a study with a cut thickness of 4 mm in order to reduce radiation exposure.

 When scanning, it is important that the patient lies still. That is, if a head is scanned, then throughout the study (until a complete set of tomograms is obtained), one cannot allow changes in the position of the head and lower jaw. If the patient is less than 5-6 years old or his condition does not allow him to lie still for a rather long time (20-40 minutes), then a tomographic examination is recommended under anesthesia, otherwise the stereolithographic model will have an irregular and, therefore, unreliable shape. For fixing the head during the study, you can use a cephalostat, then a three-dimensional computer image of the biological object is formed, at this stage, after obtaining tomography data in the form of digital files, it is necessary to build a three-dimensional computer image of the object. Here, the key point is the selection from the entire array of tomographic data of only the information that should be used when constructing the SL model (for example, only bone structures). To do this, you need to find a certain cutoff level. The 3Dview computer program developed by us (Abramov S.S., Boldyrev N.I., Evseev A.V. et al., 1998) allows importing both original tomographic data and files in the PCX graphic format (version 5). In the first case, the necessary cutoff level is determined empirically by setting the coefficient of the intensity gradient from 0 to 255. Thus, by constructing a three-dimensional model, we can verify the correctness of the selected level. In the second case, when using PCX files, it becomes possible to visually determine those parameters that the solid model will correspond to.

 The developed software (3Dview) has the ability to read the initial data of an X-ray tomograph, generate a brightness representation and bring the image to a three-dimensional brightness matrix (bytes per point). If necessary, smooths a three-dimensional image using a 3x3x3 window. Calculates the real boundary of the object on the image and reduces the representation of the object to a three-dimensional form. Filters a solid-state image (removes small-sized single fragments and cavities). Allows the operator-expert, if necessary, to interactively edit the resulting solid-state model to remove artifacts and internal closed cavities that do not carry useful information and simply cannot be manufactured. Translates solid state representation to STL format.

 Using the capabilities of the CTsoft commercial program, which allows you to work with original tomogram files (the same as you do on a tomograph), and determining visualization parameters (such as the size and center of the window), you can display on the screen based on the density values in Hounsfield units tissue of a certain density (for example, only bone). Moreover, the operator has the ability to directly control the amount of visualized information directly on tomographic sections. Further, the program allows you to get a PCX file for each scan. Since it is not always possible to get rid of “unnecessary” information (for example, the image of slices of a plastic headrest of a computer tomograph comparable in density to bone structures), further files in the PCX graphic format can be processed manually, deleting information that is not needed to obtain the model. This can be done in any graphics editor, since this format is universal for personal IBM-compatible computers. Then, the edited files can be imported by 3DView in order to obtain a three-dimensional file of the STL format, which is then used in a stereolithographic installation to make a plastic copy of the object.

 To create a three-dimensional plastic model of the object under study, the data obtained are sent in the form of a compressed file by e-mail to IPLIT RAS (Shatura). The biomodel is manufactured on stereolithographic installations of the American company "3D-Systems" SLA-250 and its analogue LS-250, developed and manufactured at IPLIT RAS. As the main photopolymerizable composition (FPK), a composition of domestic production OKM-2, synthesized by the Institute of Chemical Physics of the Russian Academy of Sciences, was used. OKM-2 contained a bisalkofen BP inhibitor to prevent spontaneous polymerization. OKM-2 is effectively polymerized by a radical mechanism. As a photoinitiator of radical polymerization, 2,2'-dimethoxy-2-phenylacetophenone, known under the brand name Irgacure 671, was used.

 Stereolithographic models of individual bones and the entire skull of the patient (or its fragments) were used as templates for the production of endoprostheses (bioimplants) by traditional (injection molding and polymerization in gypsum molds) methods when replacing existing or formed during the operation (for example, when the tumor was removed) defect. The technical result consists in expanding the capabilities of operations in maxillofacial surgery, increasing the accuracy of planning operations and improving the cosmetic and functional result of surgical treatment.

 For the development and clinical application of the method of stereolithographic biomodeling, patients of the Moscow Center for Pediatric Maxillofacial Surgery with pathology of the maxillofacial region requiring complex reconstructive surgery were selected.

 In total, between February and November 2000, 19 patients were selected. The average age of the patients was 11 years (5 to 16 years). Of these, girls 62.1% (11 people) of middle age 11.5 years (5 to 16 years) and boys 57.9% (8 people) middle age 10.4 years (4 to 14 years). All patients had a complex pathology of the maxillofacial region. Patients were divided into three groups. The first group included 5 patients (26.3%) with tumor diseases of the maxillofacial and craniofacial regions. The second group included 8 patients (42.1%) with congenital pathology (congenital malformations and complex syndromes). The third group included 6 patients (31.6%) with defects and underdevelopment of the jaw. Moreover, diseases in the groups were distributed as indicated in Table 1.

 A total of 20 SL models were made for 15 of these patients. For the rest, the study was limited to obtaining a three-dimensional computer image, which, in turn, also provided additional information for the surgeon (which can be compared in terms of information content to spiral computed tomography). The technology of stereolithographic biomodeling has been applied for various purposes. The main ones are listed in Table 2.

 Example 1

 Patient G., 12 years old. Dz: Syndrome I-II of branchial arches. Frontal plagiocephaly on the right.

 This patient has a complex combination of several syndromes, expressed in significant deformation and asymmetry of the supporting and soft tissues of the face (deformation and underdevelopment of the front orbital complex on the right, deformation of the nasal bones, underdevelopment of the lower jaw on the right, and microtia on the right). The patient underwent repeated operations, including reconstructive surgery for frontal plagiocephaly, lower jaw distraction, the first stage of otoplasty on the right. 6 months after surgery for frontal plagiocephaly, a relapse occurred (Fig. 1).

 At this stage of treatment, it was decided to correct the deformation of the frontal bone. The patient’s characteristics are associated with a deficiency of bone tissue in the frontal area on the right, which is associated with a previous operation, the frontal bone contained defects in this area, which would significantly complicate the repeated reconstructive surgery in this area and would lead to an unsatisfactory cosmetic effect. Therefore, it was decided to use contour plasty to eliminate the existing deformity using a subperiosteal implant made of PolyGAP based on hydroxyapatite. For the manufacture of an individual implant, it was decided to use the stereolithographic biomodeling technique.

 A layer-by-layer scan of the patient’s skull was carried out using a Siemens Somatom CR X-ray tomograph with a layer thickness of 2 mm. Digital tomogram files in the original format were transferred from the hard disk of the tomograph to a personal computer. Thus, it was possible to visualize the obtained tomograms on a personal computer. To convert tomograms to the STL format (a three-dimensional file for a stereolithographic installation), a mathematical model of the object as a solid was constructed. Using the developed software (3Dview), a set of planar tomograms was imported and converted into a three-dimensional file of a special STL format, which was used to obtain a plastic copy (stereolithographic model) of a three-dimensional object on a LS-250 stereolithographic installation (Fig. 2). This computer program allows you to use automatic interpolation algorithms to create a realistic model, in contrast to standard CAD applications that behave inappropriately when pulling a smooth surface on the contours of layers with different connectivity. Such algorithms make it possible to avoid the effect of "stepping" of the model, which exactly repeats the "steps" with a step of the tomograph of 2 mm Thus, the resulting model has some contours and surfaces that are more consistent with real bone structures.

The developed software performs the following functions:
- reads the initial data of an x-ray tomograph;
- forms a luminance representation and brings the image to a three-dimensional brightness matrix (bytes per point);
- if necessary, makes smoothing a three-dimensional image using a 3x3x3 window;
- calculates the real boundary of the object on the image and brings the representation of the object to solid form;
- filters a solid-state image (removes small-sized single fragments and cavities);
- allows the expert operator, if necessary, to interactively edit the resulting solid-state model to remove artifacts and internal closed cavities that do not carry useful information and simply cannot be manufactured;
- translates solid-state representation in STL format.

 Further, on the basis of the obtained model, which was an exact copy of the patient’s skull, a wax template of the future implant was made, restoring the symmetry of the upper third of the face (Fig. 3). Since the SL model accurately repeats the surface relief of the patient’s bones, the inner surface of the implant turned out to be completely congruent. Next, on the basis of a wax template, an endoprosthesis was made of PolyGAP material, which was a purely technical task (Fig. 4).

 During the operation to eliminate the deformation of the frontal bone on the right, additional implant correction was not required. The whole operation was reduced to providing access (skeletalization of the frontal bone) and fixing the implant with titanium screws. The skin is sutured with interrupted sutures. The postoperative period was uneventful. Primary healing. Sutures were removed 10 days after surgery. The patient was discharged home in satisfactory condition (figure 5).

 Thus, the manufacture of an individual implant according to the described method significantly reduces the time of the operation, which reduces the time of exposure to anesthesia on the body, and also allows to obtain a predictable satisfactory cosmetic result.

 Example 2

 Patient B., 3 years old. Dz: Desmoid fibroma of the lower jaw on the right. A patient with a benign tumor of the lower jaw on the right was admitted to the clinic. About 3 months ago, parents noticed a swelling in the lower jaw on the right, which gradually increased. A biopsy was performed at the place of residence, which revealed a benign nature of the formation (Fig.6).

 The patient was comprehensively examined. According to X-ray examination and computed tomography, it was revealed that the tumor affects only the body and part of the lower jaw branch on the right. Therefore, it was decided about the possibility of an economical resection of the lower jaw with preservation of the alveolar process and the lower dentition from the affected side. It was planned to eliminate the postoperative defect of the lower jaw simultaneously with an endoprosthesis from the PolyGAP material based on hydroxyapatite.

 For the manufacture of an individualized precision endoprosthesis, it was decided to use the stereolithographic biomodeling technique. On a Siemens Sometom CR computer tomograph, a layer-by-layer (2 mm pitch) scan of the patient's lower jaw was performed. According to the method described above, a stereolithographic model was obtained (Fig. 7), and a mirror model of the lower jaw was obtained using symmetric transformations of digital data.

 The model simulated resection of the lower jaw. Using a mirror model, a mock endoprosthesis was made, completely symmetrical to the healthy side, eliminating the resulting defect. Based on the layout, a wax template was made (Fig. 8), which was used in the standard technological process for the manufacture of a biocompatible endoprosthesis from the material PolyGAP (Fig. 9).

 An operation was performed to remove the tumor, an economical resection of the lower jaw on the right. The resulting defect is eliminated by a prefabricated endoprosthesis that fits exactly. Exarticulation was not performed. The endoprosthesis is fixed with titanium miniplates and screws. The installation of the endoprosthesis did not require additional correction, which significantly reduced the time of the operation.

 After surgery, the contour of the lower jaw on the affected side is symmetrical to the healthy side, the function of the lower jaw is normal. Thus, a good cosmetic and functional result was obtained (Fig. 10).

Literature
1. The technique of x-ray cephalometric analysis in the examination of patients with dentoalveolar anomalies and deformities. Guidelines. Kalinin, 1988, p. 3-10.

 2. Actual issues of maxillofacial surgery. St. Petersburg, 1995, p. 51-57.

 3. Shalumov A.Z., Lyuschanov M.A., Bazhanov N.N. and others. A method of manufacturing a transplant to eliminate defects and deformations of the supporting tissues of the face. Dentistry - 1998 .-- 77 (5): 26-29.

Claims (1)

 A method of manufacturing stereolithographic models and bioimplants in the cranial-maxillofacial region by performing X-ray computed tomography, forming a three-dimensional computer image of the biological object and constructing a three-dimensional plastic model of the biological object from it, characterized in that the scanning during X-ray computed tomography is carried out with a minimum coordinate step table in high resolution with the patient stationary while receiving complete set of tomograms of an array of tomographic data are selected by setting the cutoff level, setting the intensity gradient ratio from 0 to 225, information for restoring the image of the bone structures to construct certain density stereolithographic model.

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