Producing hip implants of titanium alloys by additive manufacturing

RESEARCH ARTICLE

Producing hip implants of titanium alloys by additive manufacturing Anatoliy Popovich, Vadim Sufiiarov, Igor Polozov*, Evgenii Borisov and Dmitriy Masaylo Peter the Great St. Petersburg Polytechnic University, Politekhnicheskaya ul., 29, St Petersburg, Russia, 195251 Abstract: Additive manufacturing (AM) technologies, in particular Selective Laser Melting (SLM) allows the production of complex-shaped individual implants from titanium alloys with high biocompatibility, mechanical properties, and improved osseointegration by surface texturing. In this work, the possibility of producing a custom-made hip implant from Ti-6Al-4V powder according to the data acquired via computed tomography of the patient is shown. Different heat treatments were applied in order to achieve better combination of tensile strength and elongation by partial decomposition of the martensitic phase. The implant was installed to the patient, postoperative supervision has shown good results, and the patient is able to move with the installed implant. A successful case of applying AM for producing custom hip implant is demonstrated in the paper. Using AM allowed the production of a custom-made hip implant in a short time and decreases the operation time and lessens the risk of infection ingress. Keywords: selective laser melting, implant, Ti-6Al-4V, biomedical application, prosthesis *Correspondence to: Igor Polozov, Peter the Great St. Petersburg Polytechnic University, Politekhnicheskaya ul., 29, St Petersburg, Russia, 195251; Email: [email protected]

Received: April 11, 2016; Accepted: May 17, 2016; Published Online: June 28, 2016 Citation: Popovich A, Sufiiarov V, Polozov I, et al., 2016, Producing hip implants of titanium alloys by additive manufacturing. International Journal of Bioprinting, vol.2(2): 78–84. http://dx.doi.org/10.18063/IJB.2016.02.004.

1. Introduction

A

dditive manufacturing (AM) technologies, also known as 3D printing, have demonstrated a tremendous growth for the past 30 years from the development of the first polymer machines to the manufacturing of functional metal parts with advanced characteristics and bioprinting[1−3]. AM combines the use of digital design to create a 3D-model of the part and produce the part by adding layers of materials using different techniques. AM allows the production of not only prototypes, but fully functional components for aerospace, the automobile industry, medicine, and et cetera[4,5]. Given the layer-by-layer manufacturing manner, complex-shaped parts can be made without using additional tools and joints. Selective Laser Melting (SLM) is one of the most promising and used methods among metal AM techniques.

SLM consists of forming powder layers, melting them via laser irradiation, and fusing with the previous layer according to the CAD-data. Owing to the fully melted powder particles, the produced parts have a high relative density close to 100% and high cooling rates[6−8] induce fine-dispersed microstructures typical for this method and high mechanical properties[9−11]. Endoprosthesis replacement is one of the most successful techniques for surgical treatment of patients with injuries and hip joint diseases. The demand for endoprosthesis replacement is increasing globally[12]. According to the Hip Arthroplasty Register, about 40,000 arthroplastic surgeries are performed annually in the northern European countries; at the same time more than a million surgeries are performed around the world and it is expected to double in the near 20 years[13]. Despite the high efficiency of endoprosthesis replacement, a high percent of patients require a revi-

Producing hip implants of titanium alloys by additive manufacturing. © 2016 Anatoliy Popovich, et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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sion surgery 10–15 years after the first implantation. Aseptic instability and paraprosthetic osteolysis are considered to be the main reasons for the loosening of implants[14]. The most frequent method of an unsteady implant replacement is the installation of acetabular components with press-fit fixation in the presence of unharmed support bones[15−17]. Three-flank acetabular systems are modeled based on computed tomography (CT) data of the patient, designing a 3D-model of patient’s hip bones considering bone defects, and allowing angles of dimensional orientation of the acetabular component. Titanium alloys—Ti-6Al-4V in particular—are widely used in different industries. One of the application of Ti-6Al-4V alloys is the manufacturing of medical implants due to its high biocompatibility and a combination of mechanical properties[18−20]. Since SLM technology allows the manufacturing of near netshape parts with complex geometry, it is possible to make custom-made implants for each specific patient while also texturing the implant’s surface with a lattice structure for better osseointegration[21,22]. There have been several attempts to manufacture implants via SLM technology from titanium alloys using Ti-6Al-4V, CoCr alloys[23, 24], as well as using Ti-Ta alloy powder, which is promising for medical applications[25]. Another metal additive manufacturing technique for producing complex parts from titanium alloy powders is Electron Beam Melting (EBM), which uses electron beam energy for melting metal powder layer-by-layer in vacuum[23]; a promising technology for manufacturing individual implants from titanium alloys. Usually, patient’s anatomical data is reconstructed in 3D and used for geometric modelling of implants. In this paper, we demonstrate the possibility of producing individual acetabular revision systems to carry out a revised endoprosthesis replacement of a hip implant made of Ti-6Al-4V alloy by using AM and data of the patient’s bone configuration acquired by CT. CT-data is used for 3D printing a polymer model of the patient’s deformed bone, creating a prototype of the implant for modeling the surgery process. The final model of the part is used for manufacturing the hip implant from Ti-6Al-4V alloy powder by SLM.

Selective Laser Sintering (SLS) from polyamide powder using a 3D Systems Sinterstation HiQ+HS machine. After making a polyamide model of the patient’s bone, the design of the implant configuration was carried out using CAD-software. Owing to severe deformations of the hip bone, the physical model of the implant was made out of polymer clay, taking into account anatomical features of the patient. The implant was 3D-scanned using a Faro Platinum Arm scanner to obtain a CAD-file of the implant. The configuration of the implant was further improved using CAD-software; in particular, a partial texturing of the implant surface has been done. Ti-6Al-4V Grade 5 powder was used as the initial material for manufacturing the metal implant, produced by plasma atomization. The particles have a spherical form without any defects in the form of satellites (Figure 1) with the following particle size distribution: d10 = 27 µm; d50 = 47 µm; d90 = 76 µm. The metal implant was manufactured using SLM Solutions SLM 280HL machine with the parameters set providing the relative density of about 99.9% and described in other works[26,27]. The build accuracy of the manufactured implant is about 200 µm. A schematic sequence of operations used to produce a titanium hip implant by AM is presented in Figure 2. Microstructure studies were performed using a Leica DMI 5000 light microscope. Mechanical tests were carried out on Zwick/Roell – Z100 machine using standard samples manufactured by SLM and then machined to the specific size according to ASTM E8/E8M. Three specimens were used for each test point. Annealing was carried out using a vacuum furnace, ALD MonoTherm in a vacuum with a pressure of 10−3–10−4 mbar.

2. Materials and Methods (A)

CT-data of the patient’s hip bone structure in DICOM file-format was used to make a physical model of the patient’s hip bone, which was then manufactured via

(B)

Figure 1. SEM images of Ti-6Al-4V powder particles, produced by plasma atomization with (A) X200 and (B) X1000 magnification.

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Producing hip implants of titanium alloys by additive manufacturing

Figure 2. Sequence of operations used to produce an implant via additive manufacturing.

3. Results and Discussion Using CT-data of the patient’s bone structure, a 3Dmodel of the hip bone was formed by matching the size and form of the patient’s bone. The polyamide model of the hip bone was then 3D-printed using a SLS machine (Figure 3). The build accuracy of the SLS model is around 100 µm.

Figure 3. The polyamide model of the patient’s hip bone. 80

In order to take into account severe deformations of the hip bone, a physical model of the acetabular hip implant was made out of polymer clay, which considers the deformations and future points of bearing. After that, the polymer implant model was 3D-scanned and the implant configuration was further designed considering the implant’s rotation center. Using the CAD-model of the implant, its surface was partially texturized to increase its roughness and create areas with high specific surface in order to increase contact surface with the human bone and improve osseointegration[28]. The obtained CAD-model was used to produce a polyamide implant model, simulating endoprosthesis replacement with the made models. The CAD-model of the implant was positioned relative to the building plate of the SLM machine using Materialise Magics software—support structures were also created to preserve the implant geometry during the building process. The implant orientation was chosen in such a way as to minimize thermal stresses during SLM and minimize a number of supports. Supports were placed on the surfaces that do not have special patterns in order to simplify the removal process and prevent residuals of the supports from remaining on the implant’s surface. The chosen orientation


Anatoliy Popovich, Vadim Sufiiarov, Igor Polozov, et al.

of the implant is shown in Figure 4A, while the other possible orientation is demonstrated in Figure 4B. Then the acetabular custom-made implant was produced by SLM out of Ti-6Al-4V powder (Figure 5). The build accuracy of the manufactured titanium implant is around 200 µm. After heat treatment, the supports were removed from the implant, it was shot peened, additionally cleaned, and prepared by medical staff—the implant was then installed into the patient during surgery. Postoperative supervision has shown good results; the patient can move with the installed implant. Two types of heat treatment were applied to the Ti6Al-4V material after SLM. Annealing was carried out in vacuum with the following parameters: 1) 950 °С for 1.5 hours; 2) 800 °С for 4 hours; both with furnace

cooling. The microstructures of the bulk material after SLM and after the second type of heat treatment are shown in Figure 6. Before annealing, the microstructure consists of α′-phase (Figures 6A and B), which is the result of high cooling rates during the SLM process[29] and similar to the microstructures in literature[30,31]. Metastable martensitic α′-phase and the α-phase have HCP crystalline structure. The main difference between them is the greater amount of vanadium in the martensitic phase compared to the α-phase[6]. The material after SLM shows high tensile strength, but low elongation at break (Table 1). High temperature gradients during SLM result in increased thermal stresses in the material[29] and high residual stresses in the produced part[32,33]; wherefore annealing is needed for achieving better mechanical properties and stress

(A)

(B)

Figure 4. Possible variations of the implant orientation on the build platform: (A) supports are placed on the surfaces without texturing, this one was chosen for manufacturing the implant by SLM, (B) supports are placed on the texture surfaces of the implant.

(A)

(B)

Figure 5. The Ti-6Al-4V hip implant produced by SLM: (A) general view with supports; (B) a closer view of the texturized surface. International Journal of Bioprinting (2016)–Volume 2, Issue 2

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Producing hip implants of titanium alloys by additive manufacturing

(A)

(B)

(C)

(D)

Figure 6. Microstructures of the Ti-6Al-4V alloy produced by SLM before (A, B) and after annealing at 800°С (C, D): α-phase (gray lamellae), β-phase (black lines).

relieving. After annealing (Figures 6C and D), a partial decomposition of the α ′-phase into the α- and β-phases occurred with enlargement of the acicular α′phase and formation of the β-phase on grain boundaries and needles of the martensitic phase. Heat treatment of the produced material leads to increased elongation at break with a slight decrease in tensile strength. Different annealing parameters used in this work did not noticeably affect the mechanical properties of the material. Compared to the properties of the Ti6Al-4V alloy obtained by EBM, SLM materials shows higher tensile strength, but significantly lower elongation at break. This might be due to higher cooling rates during the SLM process which result in finer microstructures and higher residual stresses of the SLMmaterial. The overall mechanical properties (Table 1) of the produced material meet the requirements of ASTM F2924–14 (Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with 82

Powder Bed Fusion) and ISO 5832-3 (Implants for surgery – Metallic materials – Part 3: Wrought titanium 6-aluminium 4-vanadium alloy).

4. Conclusion The capabilities of additive manufacturing technologies were shown to successfully produce a custommade component of a hip implant endoprosthesis from Ti-6Al-4V alloy. Since the configuration of the implant matches the anatomical features of the patient, the risk of early instability development is decreased, the surgery time is reduced together with blood loss and risk of infectious complication development. The possibility of creating a texturized surface of the implant by SLM technology allows to potentially improve the osseointegration process by creating areas with high specific surface. Microstructures of the produced material after annealing consist of partially decomposed martensitic


Anatoliy Popovich, Vadim Sufiiarov, Igor Polozov, et al. Table 1. Mechanical properties of Ti-6Al-4V after SLM and heat treatment Condition

Tensile Yield strength, strength, MPa MPa

Elongation at break, %

SLM, as fabricated

1220 ± 60

1140 ± 60

SLM, 800 °С, 4 h

1080 ± 10

983 ± 25

9.9 ± 1

SLM, 950 °С, 1.5 h

1083 ± 10

977 ± 35

10.6 ± 1

EBM, as fabricated[23] ASTM F2924–14 ISO 5832-3

915 – 1200

830 – 1150

6.

3.2 ± 1,5

13 – 25

≥825

≥895

6–10

860

780

8–10

α′-phase into the α- and β-phases. Mechanical properties after annealing show a good combination of tensile strength and elongation at break, meeting the requirements of ASTM F2924 for additively manufactured Ti-6Al-4V alloy and ISO 5832-3 Implants for surgery from titanium 6-aluminium 4-vanadium alloy. The tensile strength of the annealed material is about 1080 ± 10 MPa with the elongation at break about 10%. Future work will be focused on studying osseointegration processes and improving mechanical properties by applying different lattice structures along with computer simulation of the implant and material characteristics.

7.

8.

9.

10.

Conflict of Interest and Funding The authors declare no conflict of interest.

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