Comparative structural analysis of samples of transpedicular screws from titanium alloys

The article presents studies of transpedicular screws made of titanium alloy BT6. A metallographic and microstructural analysis of screw blanks, prototypes of transpedicular screws made of BT6 alloy and commercially produced screws made of Ti-6Al-4V was performed. Specimens of transpedicular screws were manufactured using the vortex thread cutting method on a CITIZEN Cincom K16E-VII automatic lathe. The microgeometry of the screw surface was created by sandblasting using quartz sand as an abrasive material. Studies have shown that experimental and foreign samples of transpedicular screws have a similar microstructure, consisting of globular crystals of the β-phase located in a light matrix of the α-phase. The microhardness of the rod part of the screw made of Ti-6Al-4V alloy was 312...338 HV, the microhardness of the material of the prototypes was from 264 to 394 HV. Conclusions have been drawn that determine the feasibility of choosing rods made of titanium alloy BT6 as a blank for the manufacture of transpedicular screws.


Introduction
Pedicle screw fixation is currently one of the most commonly used methods of internal stabilization of the thoracic and lumbar spine.Although segmental fixation with wires, bands, and hooks still plays an important role, the biomechanical advantages of the pedicle screw have led to an increased use of pedicle screw fixation over time.In addition, pedicle screws provide better clinical results compared to other spinal fixation methods.According to the findings of the Future Market Insights report, the pedicle screw market is projected to witness significant growth between 2019 and 2029 due to a number of driving factors such as rising geriatric population with degenerative diseases in different regions [1].According to the analysis of the research, market participants are focused on using new materials with improved biomechanical characteristics in the manufacture of transpedicular screws.Titanium alloy is expected to gain widespread acceptance in the global pedicle screw systems market.The growing demand for titanium alloy can be attributed to its physical endurance characteristics, which make it one of the safest and most reliable materials for spinal surgical implants.
To increase the strength properties of transpedicular screws and medical implants through the development of new designs and the use of new manufacturing technologies, the most famous works are V. Varghese, G. Saravana Kumar, V. Krishnan, M. Hsieh, T. Lam, F. Xu, M. Rusli, H Dahlan and other authors [2,3].
For example, the authors of [4] investigate the effect of severe plastic deformation caused by cold hydrostatic extrusion combined with rotary swaging on process parameters, surface quality, tolerances, material microstructure, grain refinement, mechanical properties, thermal stability and mechanical homogeneity.The authors managed to more than double the strength of CP Ti grand 2. In addition, the surface quality has improved, and the degree of grain refinement has increased.
A group of researchers in [5] conducted quasi-static tests and proved that the developed doublecircuit propellers have a longer fatigue life at all load levels.The double-loop screw design has improved mechanical properties compared to the cylindrical design, except for the pull-out resistance, which showed no significant difference.
In [6], the authors examine the effectiveness of the developed single-plane screw and successfully used uniaxial and polyaxial screws in direct vertebral derotation.As a result, the authors concluded that screw head design played an important role in the efficiency and variability of derotation during direct vertebral derotation.
[7] examine the mechanical properties of titanium alloy pedicle screws produced by additive manufacturing (electron beam melting, EBM) and compare them with the properties of screws produced by traditional methods.As a result, screws produced by additive manufacturing showed lower resistance to pullout tests than screws produced by traditional methods.However, such screws, due to their less sharp thread geometry, provide better load distribution and reduce the notch effect.
In [8], the authors experimentally confirmed that bending fatigue is the main cause of screw fractures.To reduce the failure rate, it is necessary to ensure a minimum roughness of the threaded surface of the screw.
The parameter such as the number of screw fractures as a result of clinical observations is of high significance.For 1000 pcs.pre-installed screws, the number of fractures is: for screws made of VT1-0 alloy -10...15 pcs.; from Ti-6Al-7Wn -4…6 pcs.; from Ti-6Al-4V -1…5 pcs.
The cause of fractures of pedicle screws made of titanium alloys is associated with softening of the material and distortion of the shape due to residual stresses resulting from mechanical processing.The quality of screw manufacturing depends directly on the processing technology and cutting parameters.It is possible to solve these problems by high-performance machining at optimal cutting conditions using progressive cutting tools [9][10].
The purpose of this work is to conduct structural studies of pedicle screws made of titanium alloy ВТ6 and a comparative analysis of screw samples with commercially produced pedicle screws made of Ti-6Al-4V.

Materials and methods
A rod made of titanium alloy ВТ6 with a diameter of 14 mm was used as a blank for the manufacture of screw samples.The rod is manufactured in accordance with GOST 26492-85; the chemical composition complies with GOST 19807-91.To improve the workability, the workpieces were annealed at a temperature of 800 °C in an SSHOL-1.1.6/12-M3-U4.2electric furnace.
The production of prototypes was carried out on a lathe with numerical control CITIZEN Cincon K16E-VII.The processing of the screw part was carried out with a whirlwind thread cutting head.
As a result, samples of pedicle screws were manufactured (Figure 1).After mechanical processing, super-finishing of the samples was carried out in an AE&T T06301 sandblasting chamber, using quartz sand (abrasive) for sandblasting machines NEO 12-562 with a particle size of 0.1...0.5 mm.
The preparation of samples for metallographic studies was carried out on a Discotom-10 cutting machine and a Tegramin-25 grinding and polishing machine.
Subsequently, structural studies were carried out on the workpiece, a manufactured sample of a screw made from the ВТ6 alloy, and a commercially produced foreign-made Ti-6Al-4V screw.
The microstructure of the samples was studied using a light metallographic microscope MET-3 in a bright field at a magnification of 100 to 1000 times.Grain size was determined according to ASTM E 112:1988 Standard Test Methods for Determining Average Grain Size.
Microhardness determination was carried out using a Shimadzu HMV-2T microhardness tester (Figure 2) in accordance with GOST 9450-76.The load on the indenter -a diamond pyramid -was 100 g.

Results and discussions
The microstructure of the workpiece material at a magnification of 500 times is shown in Figure 3.The microstructure of the workpiece material is uniform in the transverse and longitudinal directions and is represented by equiaxed grains of the α-solid solution (α-phase).At a magnification of 100 times, no foreign non-metallic inclusions or non-metallic phases are detected in the microsection field.The grain size corresponds to number 8...11, which meets the requirements of GOST R ISO 5832-2-2014.
The microstructure of the material of prototype propellers made of ВТ6 alloy is shown in Figure 4.The microstructure of the ВТ6 alloy can vary significantly depending on the heat treatment mode (temperature and cooling rate).To soften and improve machinability, the alloy is annealed.As a result of annealing, a microstructure is formed consisting of globular crystals of the β-phase (black) located in a light matrix of the α-phase (Figure 4).The presence of a certain amount of β-phase (dark grains) in the structure of the ВТ6 alloy is explained by the low temperature of the end of the martensitic transformation (MC) -below room temperature.This is because vanadium is a β-stabilizer and adding 4% vanadium to a titanium alloy containing 6% aluminum is enough to keep the MC below 25°C.Thus, the microstructure of the test samples corresponds to the microstructure of the annealed ВТ6 alloy.
On microsections of some prototypes, material delamination along the thread contour was detected (Figure 5).Such peeling can be caused by large micro-irregularities on the surface of the thread due to incorrect assignment of processing modes or incorrect selection of the geometry of the cutting tool when cutting threads.This circumstance requires a more detailed study.Figure 6 shows the microstructure of a foreign-made pedicle screw made of Ti-6Al-4V alloy.The microstructure of the screw material is globular, represented by small black grains of the βphase, precipitated in the matrix of the α-solid solution.Along the edge of the threaded part of the rod, an alpha layer with a thickness of 9...20 μm of light color is found with a predominance of the α-phase in the microstructure (Figure 6b). Figure 7 shows the microhardness measurement results of the workpiece material and the foreign screw made of Ti-6Al-4V.The microhardness of the workpiece material was 161…175 HV.The microhardness of the rod part of the screw made of Ti-6Al-4V alloy is 312…338 HV.Measurements of the microhardness of the material of the prototype screws were carried out in the direction from the edge into the depth of the sample in several sections of the microsection.The distance from the edge to the center of the first print was at least 5 print diagonals.
In different areas, the spread of microhardness ranges from 264 to 394 HV.The distribution of microhardness on one sample can be either uniform or uneven depending on the chosen measurement location.Figures 7-8 show the distribution of microhardness in some areas of the prototypes.In test samples of screws 1 and 2 (Figure 8a, b) and in section 1 of sample 3 (Figure 9a) made of BT6 alloy, the microhardness at the surface is locally lower, then an increase and a smooth decrease towards the center of the sample are observed.In section 2 of sample 3 (Figure 9b), a decrease in microhardness from the surface to the center of the sample is observed.

Conclusion
It has been established that the microstructure of the workpiece material from the BT6 alloy is homogeneous, represented by equiaxed grains of the α-phase, without foreign non-metallic inclusions, the grain size corresponds to 8...11.Prototype screws made of BT6 alloy have a microstructure consisting of globular crystals of the β-phase located in a light matrix of the α-phase.A screw made of Ti-6Al-4V alloy has a microstructure similar to the prototypes, with the exception of the presence of an alpha layer along the edge of the threaded part.The microhardness of the BT6 alloy workpiece material was 161…175 HV.The microhardness of the rod part of the screw made of Ti-6Al-4V alloy is 312…338 HV.The microhardness of the material of the prototypes ranges from 264 to 394 HV.The distribution of microhardness on one sample can be either uniform or uneven.On microsections of some prototypes, material delamination along the thread contour was detected; additional research is required to identify the reasons for their occurrence.

Figure 1 .
Figure 1.Sample of a transpedicular screw made of ВТ6 alloy.

Figure 3 .
Figure 3. Microstructure of a workpiece made of ВТ6 alloy as delivered.

Figure 5 .
Figure 5. Delamination of material on a microsection in some samples of experimental screws.