Peek matrisli polimer kompozit temelli insan omurga implantı geliştirilmesi ve granül ekstrüzyon yöntemi ile eklemeli imalatı
Development of human spine implant based on peek matrix polymer composite and additive manufacturing by granule extrusion method
Yazar: MERVE BAĞCI BİLGEN
Danışman: DR. ÖĞR. ÜYESİ ALAEDDİN BURAK İREZ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Katı Cisimlerin Mekaniği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Omurga insan vücudunun önemli parçalarından biridir. İnsan gövdesi ve baş arasında bir köprü görevi görerek insanın ayakta dik şekilde durmasını sağlar. Omurga üzerinde bulunan hareketli eklemler sayesinde öne, arkaya, sağa, sola ve dönme hareketlerini yapmamızı sağlar. Bu hareketlerin yapılabilmesi için omurgada bulunan eklemlerin, bağların, disklerin ve kasların birbiri ile uyumlu çalışması gerekmektedir. İnsan omurgası 33 omurdan meydana gelmektedir. Kafatasının altından başlayarak kuyruk sokumuna kadar devam eder. Omurgada 7 adet servikal omur, 12 adet torasik omur, 5 adet lomber omur ve 9 adet sakrum ve koksal omurdan oluşmaktadır. Omurganın ilk 24 omuru üzerlerinde bulunan eklemler sayesinde hareketli iken geri kalan 9 adet omur hareketsizdir.
Omurga implantları sırt ağrısı, skolyoz, dejeneratif disk bozuklukları ve omur kırıklarının olumsuz etkilerini ortadan kaldırmak için kullanılan yapılardır. Omurga implantlarının ana işlevi, şekil bozukluklarını düzeltmek, omurganın stabilitesini güçlendirmek, geliştirmek ve iki omurun kaynaşmasını kolaylaştırmaktır. Omurganın maruz kaldığı gerilmeler göz önüne alındığında geliştirilecek implant malzemesinin mekanik açıdan yeterli dayanıma sahip olması gerekir. Ek olarak, biyouyumluluk spinal implantlardan beklenen temel bir özelliktir. Bu açıdan titanyum alaşımları ve bazı gelişmiş polimerler implantlarda sıklıkla kullanılmaktadır. Ti ve Ti alaşımları bio-uyumlu ve korozyona dayanıklı olmasından dolayı omurga implantlarında yaygın şekilde kullanılan malzemeler olmasına rağmen, imalat nedenli problemlerden dolayı vücut sıvısı ile Ti alaşımlarının kimyasal reaksiyona girebilme ve dolayısıyla toksik etkiye neden olması riski görülebilmektedir. Toksik etkilerin görülmesinin önüne geçmek için bu çalışmada implant materyalinin matrisi olarak PEEK (Polieter Eter Keton) kullanılmasına karar verilmiştir. Ayrıca, karbon nanotüp (CNT) gibi karbon bazlı takviyeler kullanılarak implant matrisinin kırılma tokluğu ve darbe sönümleme özelliklerinin geliştirilmiştir.
İmplant malzemesi olarak kullanılacak bu kompozit malzemesinde %1 k oranında CNT kullanılmıştır. Ayrıca mikro mekanik malzeme modeli olan Mori Tanaka yöntemi ile implant malzemesi olarak kullanılacak kompozit malzemenin katılık matrisi elde edilmiştir.  PEEK ve CNT malzemelerinin homojen bir şekilde karıştırılabilmesi için İTÜ ARC merkezinde bulunan Kökbir marka çift vidalı bir ekstrüder kullanılmışıtr. Karıştırılan bu malzemelerden filament elde edilmiştir. Daha sonrasında da üretilen filamentler kesici bıçaklar yardımıyla granül hale getirilmiştir. Bu granüller ile granül beslemeli olan Yuzumi markasına ait bir üç boyutlu yazıcı kullanılarak kompozitlere ait deney numuneleri üretilmiştir. Deney numuneleri kullanılarak kompozitlere ait mekanik karakterizasyonlar yapılmıştır. Bu işlemlere paralel olarak, lumbar omurganın L4 omurunun maruz kaldığı yüklemeler yapılan literatür araştırmaları sonucunda belirlenmiştir. omurlardaki gerilme hesabı yapılacaktır.
Geliştirilen implantın bu gerilmeleri karşılamaya yönelik tasarımı omur gövdesi rekonstrüksiyon ameliyatlarında kullanılan LFS (Lateral Fiksasyon Sistemi) uygulamasına uygun olacak şekilde yapılmıştır. Sonlu elemanlar ortamında bir insanın günlük aktivesi olarak yaptığı ayakta durma, ayakta duruken sağa dönme, ayakta dururken arkaya esneme ve ayakta dururken sağa doğru eğilme hareketlerinin sınır koşulları tanımlanarak  sonlu elemanlar yöntemiyle yapısal analize tabii tutulmuştur. Yapılan analiz sonucunda elde edilen maksimum Von Mises değerleri, ayakta duruken 21.61 MPa , ayakta dururken sağa doğru dönme durumunda 22.89 MPa, ayakta dururken arkaya doğru esneme durumunda 30.78 MPa ve ayakta duruken sağa doğru eğilme durumunda 30.52 MPa'dır. Ayrıca 2020 yılında yapılan benzer bir çalışma ile karşılaştırma yapılabilmesi için 400N basma kuvveti altında dört durum için tekrar analizler yapılmıştır. Bu karşılaştırma sonucunda tez kapsamında tasarımı yapılan implant geometrisi ve geliştirilmiş malzeme formülasyonunun daha başarılı olduğu ortaya çıkmıştır.
Eklemeli imalat ile üretilen numunelere çekme testi, basma testi ve üç nokta eğme testi uygulanmıştır. Basma testi sonuçları dışındaki tüm testlerde CNT takviyesinin saf PEEK malzemesinin çekme ve gerinim gibi mekanik özelliklerini beklenenin aksine iyileştirmediği görülmüştür. Ayrıca üç nokta eğme testinde filamanın akış yönünün etkisi de incelenmiştir. Her iki malzemede de filamentin akış yönü numune boyunca olduğunda gerilme ve birim deformasyon gibi mekanik özellikleri iyileştirdiği gözlenmiştir.

Abstract:
The spine is one of the important parts of the human body. By acting as a bridge between the human body and the head, it allows the person to stand upright. Thanks to the movable joints on the spine, it allows us to make forward, backward, right, left and rotational movements. In order to perform these movements, the joints, ligaments, discs and muscles in the spine must work in harmony with each other. The human spine consists of 33 vertebrae. It starts under the skull and continues to the coccyx. The spine consists of 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, and 9 sacrum and coccal vertebrae. While the first 24 vertebrae of the spine are mobile thanks to the joints on them, the remaining 9 vertebrae are immobile.
Spine implants are structures used to eliminate the negative effects of back pain, scoliosis, degenerative disc disorders and vertebral fractures. The main function of spinal implants is to correct deformities, strengthen and improve the stability of the spine, and facilitate the fusion of the two vertebrae. There are four types of spinal implants. These are cage implants, plate implants, screw implants and rod implants. Within the scope of this thesis, a cage implant design was made to replace the damaged vertebral body. Two screws and one rod were used to position this designed implant in the spine.
Considering the stresses that the spine is exposed to, the implant material to be developed must have sufficient mechanical strength. In addition, biocompatibility is a key feature expected from spinal implants. In this respect, titanium alloys and some advanced polymers are frequently used in implants. Although Ti and Ti alloys are widely used in spine implants due to their biocompatible and corrosion resistant properties, there is a risk that Ti alloys may react with body fluids and cause toxic effects due to manufacturing problems. In order to avoid toxic effects, it was decided to use PEEK (Polyether Ether Ketone) as the matrix of the implant material in this study. In addition, the fracture toughness and impact damping properties of the implant matrix have been improved by using carbon-based reinforcements such as carbon nanotube (CNT). In this composite material to be used as implant material, CNT at the rate of %1 k was used.
In addition, the stiffness matrix of the composite material to be used as implant material was obtained with the Mori Tanaka method, which is a micro-mechanical material model. In order to mix PEEK and CNT materials homogeneously, a Kökbir brand twin screw extruder located at the ITU ARC center was used. Filament was obtained from these mixed materials. Afterwards, the produced filaments were granulated with the help of cutting blades. With these granules, test samples of composites were produced by using a three-dimensional printer of Yuzumi brand, which is granule-fed.
Mechanical characterizations of composites were made using test samples.
In parallel with these procedures, the loadings to which the L4 vertebra of the lumbar spine is exposed were determined as a result of the literature research. Calculation of the stress in the vertebrae will be made. The material properties obtained by the Mori Tanaka method were imported to Abaqus CAE for finite element analysis. These identified material properties are assigned to the vertebral body implant geometry. As a result of the literature research, it was deemed appropriate to assign the properties of Ti-6Al-4V material for the two screw and rod implants on the model. The modulus of elasticity of the Ti alloy used was 110000 MPa and the Poisson ratio was 0.3. The material properties of the cortical bone tissue to the L3 and L5 vertebrae are described. The modulus of elasticity of the cortical bone was 12000 MPa and the Poisson ratio was 0.3. There are 7 different ligaments in the model to be analyzed, namely ALL, PLL, TL, CL, LF, ISL, and SSL. These ligaments are entered into hyperelastic material properties. Within the scope of this thesis, a static/general analysis was made. The types of interaction between the parts have been determined. Connection (tie) restrictions were made for the screws on the left surfaces of the L3 and L5 vertebrae, the rod between the screws, the places where the ligaments attach to the vertebrae, and the surfaces where the implant contacts the L3 and L5 vertebrae. Thus, these parts will be able to move together. A coupling is formed around the reference point formed in the middle of the upper surface of the L3 vertebra.
Loading types and boundary conditions were determined. As a result of literature research, 1000N compression force was applied. This determined compressive loading was applied over a reference point created in the middle of the upper surface of the L3 vertebral body. In addition to the pressing force, a moment of 10000 Nmm was applied in each axis to simulate the movements that occur as a result of a person's daily activities. In addition, the lower surface of the L5 vertebral body is fixedly supported.
In this step, the mesh structure to be used in the parts is defined. T3D2 mesh structure, with all ligaments being a one-dimensional two-node lattice element, is defined. For the meshwork of other parts (L3, L4 geometry outside the vertebral body, L5, vertebral implant, rod and two screws), the tetrahedral element C3D4 is defined. The mesh size is targeted to be 1mm in the whole model. Since the geometries of the parts are complex, deviations from the targeted mesh size have occurred. The network structure was created with the lowest mesh size of 0.3 mm and the highest mesh size of 3.31 mm. The design of the developed implant to meet these stresses was made in accordance with the LFS (Lateral Fixation System) application used in vertebral body reconstruction surgeries.
In spinal reconstruction surgeries using LFS, the soft tissue is removed and the vertebral implant is placed on the vertebral body in the upper and lower regions. The geometric dimensions of the vertebral body implant designed within the scope of this thesis are given in this way. The design was made with Solidworks 2020 software. The lower and upper surfaces of the designed implant are not flat surfaces. These surfaces are the anatomical surface to which they are attached. In the finite element environment, the boundary conditions of the movements of standing, turning to the right while standing, stretching backward while standing and bending to the right while standing, which are the daily activities of a person, were defined and subjected to structural analysis using the finite element method.
The maximum Von Mises values obtained as a result of the analysis are 21.61 MPa while standing, 22.89 MPa for right rotation while standing, 30.78 MPa for standing backward flexion and 30.52 MPa for standing right bending. In addition, in order to be able to compare with a similar study conducted in 2020, repeated analyzes were made for four cases under 400N compression force. As a result of this comparison, it was revealed that the implant geometry designed within the scope of the thesis and the improved material formulation were more successful.
An analytical solution was made for the analyzed implant. The system has been simplified while performing analytical analysis. The cross-sectional geometry of the designed implant was taken. A compressive force of 1000 N was applied along the y-axis for the standing position on this section geometry. As a result of the applied compressive force, maximum and minimum planar stresses were obtained. The maximum planar stress was 0 MPa, the minimum planar stress was -7.89 MPa, the maximum shear stress was 3.95 MPa. A compression force of 1000 N and a moment of 10000 Nmm were applied along the y-axis for the right rotation movement while standing. As a result of these applied loads, the maximum planar stress was 21.33 MPa, the minimum planar stress was -29.12 MPa, the maximum shear stress was 35.38 MPa. A compressive force of 1000 N along the y-axis and a moment of 10000 Nmm along the x-axis were applied for the backward expansion movement while standing. This situation was examined separately for the A and B points on the anterior and posterior surfaces of the implant. For point A, the maximum planar stress was 41.83MPa, the minimum planar stress was 0 MPa, and the maximum shear stress was 20.92MPa. For point B, the maximum planar stress is 0 MPa, the minimum planar stress is -57.62 MPa, the maximum shear stress is 28.81 MPa.
For the bending movement to the right while standing, a compressive force of 1000 N along the y-axis and a moment of 10000 Nmm along the z-axis were applied. This situation was examined separately for the C and D points on the anterior and posterior surfaces of the implant. For point C, the maximum planar stress is 0 MPa, the minimum planar stress is -57.62 MPa, the maximum shear stress is 28.81 MPa. For point D, the maximum planar stress is 41.83MPa, the minimum planar stress is 0 MPa, and the maximum shear stress is 20.92MPa. In order to compare the accuracy of this analytical analysis, a simple analysis was made in the Abaqus CAE program. The results of the analysis and the results obtained by the analytical analysis were almost the same.
Tensile test, compression test and three-point bending test were performed on the samples produced by additive manufacturing. In all tests except the compression test results, it was observed that the CNT reinforcement did not improve the mechanical properties of pure PEEK material, such as tensile and strain, contrary to expectations. In addition, the effect of the flow direction of the filament in the three-point bending test was also investigated. In both materials, it has been observed that when the flow direction of the filament is along the sample, it improves the mechanical properties such as stress and unit deformation.