Titanium alloy rod for spinal fixation and producing method thereof

Abstract

To provide a titanium alloy rod for spinal fixation that improves a mechanical stimulus to a bone, by making a load on a bone larger, by making smaller a weight applied on an instrument fixed to a bone, and can improve the durability of a rod.SOLUTION: A titanium alloy rod for spinal fixation made of titanium alloy, where there exists an α + β phase region 1a (1a1, 1a2, 1a3) with strength difference in an axial direction of the titanium alloy rod and a β single-phase region 1b (1b1, 1b2) between the α+β phase regions, with the high-strength α+β phase region 1a being in contact with a screw and a plug, and the low-strength β single-phase region 1b being the non-contact part with the screw and the plug.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a titanium alloy rod for spinal fixation and a method for manufacturing the same. [Background technology]

[0002] For the treatment of spinal deformity and other diseases, spinal fixation surgery is performed using titanium alloy rods and screws, which are spinal implants, and titanium alloy rods for spinal fixation have been developed (see, for example, Patent Documents 1 and 2).

[0003] Patent Document 1 describes a titanium alloy rod for spinal fixation, which is composed of "25-37% by mass of Nb, 36-45% by mass of Ta (Nb+0.8Ta), 2-6% by mass of Zr, 0-1.0% by mass of O, and the remainder being Ti and unavoidable impurities, and has a first region in which α phase is precipitated in β phase, and a second region composed of β phase." (Claim 1). It also states that "This configuration makes it possible to achieve a higher level of compatibility between high strength and low rigidity required for a spinal fixation orthodontic rod" (paragraph

[0012] ), and "10% by mass of Ti alloy rod" (paragraph

[0013] ). The positions of the first region 11 and the second region 12 can be determined depending on the usage of the Ti alloy rod. In a typical spinal fixation surgery, since particularly high strength is required on the caudal side and low rigidity on the cranial side, it is preferable that one end of the Ti alloy rod is occupied by the first region 11 and the other end by the second region 12. In this way, referring to FIG. 6 (see FIG. 8 attached to this specification), the first region can be allocated to the caudal side C and the second region can be allocated to the cranial side H." (Paragraph

[0037] )

[0004] Patent Document 1 also states that "For composition G2, in the same manner as in the above experiment, the black scale was removed from a hot-rolled round bar having a diameter of 20 mm, and the bar was held at 800°C for 1 hour for solution treatment, cooled to room temperature by water cooling, reduced in diameter to 5 mm by swaging, and aged at 400°C for 24 hours. Furthermore, a high frequency was applied to about half the length from one end of the obtained bar (length: about 100 mm) to heat it, and high frequency partial hardening was performed to produce a Ti alloy rod of the example." and "The Ti alloy rod of the example was divided into two in the radial direction, and the cross section of the boundary between the first and second regions was observed using an optical microscope. As a result, it was found that the second region was composed of a single phase and the first region was composed of two phases. It was confirmed that the boundary between the first and second regions has a gradient structure in which the structure changes continuously." "Furthermore, the change in Vickers hardness Hv in the longitudinal direction was measured for the cross section of the boundary between the first and second regions. The Vickers hardness Hv was measured with a test force of 4.903 N and a holding time of 10 seconds. The results are shown in Figure 5. The left side of Figure 5 (see Figure 7 attached to this specification) is the first region, and the right side is the second region. It was found that the Vickers hardness Hv gradually decreased from the first region to the second region, and that a gradient structure was formed at the boundary over a distance of 3 to 3.5 mm." (Paragraphs

[0064] ,

[0066] , and

[0067] )

[0005] Patent Document 2 describes a titanium alloy rod for spinal fixation, which is obtained by using a titanium alloy ingot or titanium alloy powder as a raw material, the raw material containing 8 to 20 mass% of a β-stabilizing element in terms of molybdenum equivalent, 0.05 to 0.25 mass% of oxygen, and the balance being titanium and unavoidable impurities, and by subjecting the rod to a hot forging process when an ingot is used, or a powder metallurgy process when a powder is used, and then subjecting the rod to a solution treatment and a peening treatment. The Young's modulus after the solution treatment is 60 to 110 GPa, the tensile strength is 700 to 1200 GPa, and the surface portion of the rod is " (Claim 1) states, "The inventors have studied ways to increase the fatigue strength at the connection while taking advantage of the characteristic of β-type titanium alloys, namely, their low elastic modulus. They have discovered that by subjecting a β-type titanium alloy rod for spinal fixation to a peening treatment, a deformation-induced ω-phase transformation is induced on the surface of the rod, thereby maintaining the elastic modulus of the entire rod low while hardening only the surface, thereby significantly improving the fatigue strength of the rod at the connection with the screw, and have completed the present invention." (Paragraph

[0008] )

[0006] Furthermore, Patent Document 2 states that "In the titanium alloy rod for spinal fixation of the present invention, the ω phase is induced to deform only in its surface portion, while the other portions maintain the state of the constituent phases after solution treatment. In order to obtain a β-type titanium alloy, in the present invention, titanium and a β-stabilizing element are used as raw materials, and the raw material composition, ingot manufacturing method, and alloy powder composition are selected so that the titanium alloy ingot or titanium alloy powder, which is the material for the rod, contains a predetermined amount of oxygen." and "Pure titanium is in the α phase at room temperature, but transforms into the β phase at 885°C or higher. A metastable β-type titanium alloy can be obtained by mixing titanium with a β-stabilizing element, dissolving it, and then subjecting it to solution treatment. The solid-solution β-stabilizing element used to obtain a β-type titanium alloy in this way Examples of the element include molybdenum (Mo), niobium (Nb), tantalum (Ta), vanadium (V), and rhenium (Re). Examples of the eutectoid β-stabilizing element include chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni). In the present invention, at least one selected from the above β-stabilizing elements is used, and the use of chromium is particularly preferred. "In the present invention, the raw material composition is adjusted so that the ingot or alloy powder contains 8 to 20 mass %, preferably 9 to 13 mass %, of the β-stabilizing element in terms of molybdenum equivalent. The reason for setting the ratio of the β-stabilizing element as described above is to obtain a metastable β-type titanium alloy." (paragraphs

[0023] to

[0025] ). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-139179 A [Patent Document 2] JP 2016-138318 A Summary of the Invention [Problem to be solved by the invention]

[0008] When metal medical devices are fixed to bones, the load on the bone is reduced when the device is subjected to a large load. As a result, the mechanical stimulation to the bone is reduced, the balance between regeneration and destruction is disrupted, and the bone becomes weakened. In the titanium alloy rod for spinal fixation described in Patent Document 1, the strength of the rod is high in the first region where the α phase precipitates in the β phase (see the left side of Figure 7), so if the first region 11 (see Figure 6) is applied to the tail side C shown in Figure 8 as described in paragraph

[0037] , the load on the instrument fixed to the bone will be large in the tail side C, so there is a limit to solving the above problem. Also, since the strength of the rod is low in the second region made of the β phase (see the right side of Figure 7), if the second region 12 (see Figure 6) is applied to the head side H shown in Figure 8 as described in paragraph

[0037] , the load on the instrument fixed to the bone will be small and the load on the bone will be large, but in the head side H, the strength of the rod at the part where the screw and plug are fixed is also low, so if it is used in combination with a screw and plug with high strength, the durability of the part may be reduced.

[0009] Patent Document 2 describes a titanium alloy rod for spinal fixation that uses a metastable β-titanium alloy and improves the fatigue strength (durability) at the connection portion with a high-strength screw. However, since a hard deformation-induced ω-phase transformation is induced on the surface of the rod to improve durability, the strength of the rod is increased, and it cannot be said that the document describes reducing the strength of part of the rod.

[0010] The objective of the present invention is to provide a titanium alloy rod for spinal fixation that can increase the load on the bone by reducing the load on the instrument fixed to the bone, thereby improving the mechanical stimulation to the bone and improving the durability of the rod. [Means for solving the problem]

[0011] In order to solve the above problems, the present invention employs the following means. (1) A titanium alloy rod for spinal fixation using a titanium alloy, wherein an α+β phase region and a β single-phase region with a difference in strength exist in the axial direction of the titanium alloy rod, and a β single-phase region exists between the α+β phase regions, the α+β phase region with higher strength is the contact portion with a screw and a plug, and the β single-phase region with lower strength is the non-contact portion with the screw and plug. (2) The titanium alloy rod for spinal fixation according to (1), wherein the α+β phase regions are present in three or more locations and the β single phase regions are present in two or more locations. (3) A method for manufacturing a titanium alloy rod for spinal fixation according to (1) or (2), comprising the steps of: subjecting a titanium alloy rod made of a titanium alloy to a first solution treatment; subjecting the first solution-treated portion to an α+β phase region by aging treatment; and subsequently applying a high frequency to a non-contact portion between the screw and the plug in the α+β phase region by a second solution treatment to convert the non-contact portion into a β single-phase region. (4) The method for producing a titanium alloy rod for spinal fixation according to (3) above, wherein the second solution treatment is carried out at 900°C or higher. Effect of the Invention

[0012] According to the titanium alloy rod for spinal fixation of the present invention, the presence of the non-contact portion with the low-strength screw and plug (the portion where the screw and plug are not fixed) reduces the load on the instrument fixed to the bone, which results in a larger load on the bone, improving the mechanical stimulation to the bone, improving the balance between regeneration and destruction, and suppressing the weakening of the bone in the spine. In addition, the presence of the contact portion with the high-strength screw and plug (the portion where the screw and plug are fixed) also improves the durability of the titanium alloy rod in that portion. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a conceptual diagram showing an example of a spinal fixation device using a titanium alloy rod for spinal fixation according to one embodiment of the present invention. [Diagram 2]FIG. 2 is a conceptual diagram showing a titanium alloy rod for spinal fixation according to one embodiment of the present invention. [Diagram 3] FIG. 2 is a conceptual diagram showing a second solution treatment method using high frequency for a titanium alloy rod for spinal fixation according to one embodiment of the present invention. [Figure 4(a)] FIG. 4(a) is a diagram showing the Vickers hardness (HV) of the titanium alloy rod for spinal fixation after the second solution treatment in Example 1. [Figure 4(b)] FIG. 4(b) is a diagram showing the Vickers hardness (HV) of the titanium alloy rod for spinal fixation after the second solution treatment in Example 2. [Diagram 5] FIG. 5 is a diagram showing the state of α-phase and β-phase in the titanium alloy rods for spinal fixation after the second solution treatment in Examples 1 and 2. As shown in FIG. [Figure 6] FIG. 6 is FIG. 1 of Patent Document 1, which is a diagram showing the configuration of the titanium alloy rod for spinal fixation according to the first and second embodiments. [Figure 7] FIG. 7 is FIG. 5 of Patent Document 1, which is "a diagram showing the Vickers hardness at the boundary between the first region and the second region in a titanium alloy rod for spinal fixation according to an embodiment." [Figure 8] FIG. 8 is FIG. 6 of Patent Document 1, and is a "diagram for explaining spinal fixation surgery." DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] One embodiment of the titanium alloy rod for spinal fixation according to the present invention (hereinafter referred to as "this embodiment") will be described with reference to Figs. 1 and 2. Figs.

[0015] FIG. 1 shows an example of a spinal fixation device using a titanium alloy rod 1 for spinal fixation according to the present embodiment. In this example, the rod 1 of the present embodiment is connected to three screws 2, 2, 2. The screw 2 has a threaded portion 21 and a head 22 with a U-shaped cross section at the upper portion of the threaded portion 21. The head 22 serves as a receiving portion for the rod 1, and the inside of this receiving portion is the portion with which the rod 1 comes into contact. In addition, the rod 1 is inserted into the head 22 and covered with a plug 3, so that the rod 1 also comes into contact with the plug 3. The upper portion of the inner surface of the head 22 and the outer surface of the plug 3 are threaded, and the rod 1 is fixed by screwing the plug 3 from above into the inside of the head 22 of the screw 2.

[0016] The titanium alloy rod 1 for spinal fixation of the present embodiment has two regions in the axial direction of the titanium alloy rod, an α + β phase region and a β single phase region, which have a difference in strength. 1 , 1a 2 , 1a 3 (hereinafter, collectively referred to as 1a) is the contact area with the screw 2 and the plug 3. On the other hand, the β single phase region 1b 1 , 1b 2 (hereinafter collectively referred to as 1b) is a non-contact area with the screw 2 and the plug 3. 1 , 1b 2 is α+β phase region 1a 1 , 1a 2 , 1a 3 It exists between each of the above. More specifically, the α+β phase region 1a is a portion of the rod 1 that contacts the inside of the receiving portion of the head 22 of the screw 2 and the lower surface of the plug 3. The β single-phase region 1b is a portion that does not contact the inside of the receiving portion of the head 22 of the screw 2 and the lower surface of the plug 3 and protrudes from the receiving portion of the head 22 and the lower surface of the plug 3. In this embodiment, as shown in FIG. 2, the α+β phase region is 1a 1 , 1a 2 , 1a 3 There are three regions, and the β single phase region is 1b 1 , 1b 2 There are two places: When fixing a spinal fixation device using a titanium alloy rod 1 for spinal fixation to the spine, at least two α+β phase regions 1a are required, but the effects of the present invention can be obtained by having a β single phase region 1b between the α+β phase regions 1a. That is, due to the presence of the low-strength non-contact portion between the α+β phase regions 1a and the screw and plug (parts where the screw and plug are not fixed: β single-phase region 1b), the load on the instrument fixed to the bone is partially absorbed by the low-strength β single-phase region 1b, reducing the load, and as a result, the load on the bone increases, improving the mechanical stimulation to the bone, improving the balance between regeneration and destruction, and suppressing the weakening of the bone in the spine. In addition, due to the presence of the high-strength contact portion between the screw and plug (parts where the screw and plug are fixed: α+β phase region 1a), the durability of the titanium alloy rod in that part is also improved. It is preferable that the α+β phase regions 1a are present in three or more locations, and the β single phase regions 1b are present in two or more locations between the respective α+β phase regions 1a. This allows the titanium alloy rod for spinal fixation according to the present invention to be used in a wider range of applications as a spinal fixation device.

[0017] For the screw and plug, titanium alloys such as Ti-6Al-4V, which have high strength, are usually used. Therefore, by making the α+β phase region 1a, which has a large Vickers hardness HV (high strength) of the titanium alloy rod 1, the contact portion with the screw 2 and plug 3, the wear resistance is increased and durability is improved. Also, by making the β single phase region 1b, which has a small Vickers hardness HV (low strength), the non-contact portion with the screw 2 and plug 3, the load on the instrument fixed to the bone by the screw 2 is reduced, and as a result, the load on the bone is increased, and the mechanical stimulation to the bone is improved. In other words, there is a possibility that the durability of the titanium alloy rod 1 and the stress blocking suppression to the bone as the instrument fixed to the bone by the screw 2 can be achieved at the same time. The strength difference between the α+β phase region 1a and the β single phase region 1b is, for example, 50 HV or more in Vickers hardness.

[0018] The titanium alloy used for the titanium alloy rod 1 may be a metastable β-type titanium alloy. Examples of β-stabilizing elements in metastable β-titanium alloys include V, Mo, Nb, Ta, Fe, Cr, Mn, etc. That is, examples of metastable β-titanium alloys that can be used include Ti-15V-3Cr-3Sn-3Al, Ti-12Mo-6Zr-2Fe, Ti-15Mo-5Zr-3Al, Ti-45Nb, Ti-35Nb-7Zr-5Ta, Ti-29Nb-13Ta-4.6Zr, Ti-12Cr, etc. The titanium alloy used in the titanium alloy rod 1 is not particularly limited as long as it can form the above-mentioned α + β phase region 1a and the β single phase region 1b between the α + β phase region 1a by heat treatment such as solution treatment or aging treatment described below. In addition, when a metastable β-type titanium alloy is used as the titanium alloy for a titanium alloy rod for spinal fixation, the metastable β-type titanium alloy is preferable because it has age hardenability and therefore easily forms the above-mentioned α+β phase region 1a.

[0019] The α+β phase region 1a has a structure in which the α phase precipitates in the β phase, and therefore has a higher strength than the β single phase region 1b with the same composition. The β single phase region 1b has a structure consisting of the β phase. As a result, it has lower strength than the α + β phase region 1a with the same composition. The β single phase region 1b may contain a phase other than the β phase as long as it is substantially composed of the β phase. The β single phase region 1b being substantially composed of the β phase means that other phases may be present in the β single phase region 1b within the scope of the present invention. More preferably, the proportion of phases other than the β phase in the β single phase region 1b is 5% or less in volume fraction, and even more preferably, 0.5% or less in volume fraction. Within the scope of the present invention, the contact portion between the screw 2 and the plug 3 may contain a portion of a β single-phase region 1b having low strength, and the non-contact portion between the screw 2 and the plug 3 may contain a portion of an α + β phase region 1a having high strength.

[0020] The titanium alloy rod for spinal fixation of this embodiment is obtained by the manufacturing method described below. First, a first solution treatment is performed on a titanium alloy rod made of a metastable β-titanium alloy. The first solution treatment is performed on the titanium alloy rod made of a metastable β-type titanium alloy by holding it at 700°C to 1000°C for a certain time (preferably 0.5 hours to 3 hours) and then cooling it. The first solution treatment is preferably performed on the entire titanium alloy rod. Thereafter, the portion that has been subjected to the first solution treatment is subjected to an aging treatment. The aging treatment is carried out by holding the titanium alloy rod at 400°C to 650°C for a certain period of time (preferably 0.5 hours to 168 hours) and then cooling the rod. The aging treatment is preferably carried out on the entire titanium alloy rod when the first solution treatment is carried out on the entire titanium alloy rod. This aging treatment causes the α phase to precipitate in the β phase of the portion that has been subjected to the first solution treatment (preferably the entire titanium alloy rod), resulting in a titanium alloy rod having a high-strength α+β phase region.

[0021] Next, the titanium alloy rod having an α+β phase region that has been subjected to the first solution treatment and aging treatment is subjected to a second solution treatment. The second solution treatment is further performed by applying high frequency to the non-contact portion with the screw and plug in the α+β phase region. By performing the second solution treatment, the non-contact portion becomes the β single phase region. In the second solution treatment, the titanium alloy rod with α+β phase region that has been subjected to the first solution treatment and aging treatment is partially heated in the α+β phase region at a temperature above the β transformation point. As the temperature increases, the β phase gradually increases, and when heated above a certain temperature, it transforms into a single β phase. The temperature at which the phase transforms into a β single phase varies depending on the composition. In this embodiment, a titanium alloy rod having an α+β phase region that has been subjected to a first solution treatment and aging treatment is subjected to partial solution treatment by applying high frequency to a portion of the α+β phase region, thereby forming a β single phase region between the α+β phase regions. The second solution treatment is carried out by holding the material at 700° C. to 1100° C. for a certain period of time, followed by cooling. The time for holding the material at the certain period of time is preferably 10 seconds or less, more preferably 3 seconds or less, and even more preferably 1 second or less, in order to prevent the α+β phase region from being heated more than necessary by thermal conduction and becoming a β single-layer region. The second solution treatment is preferably carried out at a temperature of 900° C. or higher. By setting the heating temperature in this range, partial softening can be more effectively achieved.

[0022] By carrying out the above-mentioned first solution treatment, aging treatment and second solution treatment, a titanium alloy rod for spinal fixation is manufactured in which an α+β phase region with a difference in strength in the axial direction of the titanium alloy rod and a β single-phase region between the α+β phase regions are present, the high-strength α+β phase region being the contact area with the screw and plug, and the low-strength β single-phase region being the non-contact area with the screw and plug. The heating and cooling devices in the first solution treatment, aging treatment and second solution treatment, as well as the manufacturing conditions thereof, i.e., the heating means (furnace heating or high-frequency heating, etc.), heating temperature, heating and cooling rates, holding time at the heating temperature, cooling means (air cooling or water cooling, etc.), and cooling rate, can be set appropriately unless otherwise specified.

[0023] FIG. 3 is a conceptual diagram showing the second solution treatment method by high frequency for the titanium alloy rod for spinal fixation of this embodiment. Specifically, the second solution treatment is performed by arranging a portion (a portion forming a β single layer) of the α+β phase region 1a of the titanium alloy rod 1 having the α+β phase region 1a that has been subjected to the first solution treatment and aging treatment, which will be a non-contact portion with the screw and plug, in the heating and cooling region 5c of a high-frequency induction heating device having a heating and cooling unit 5 (heating coil 5a, cooling jacket 5b) that serves both as a heating coil and a cooling jacket, and applying high frequency to heat and cool (i.e., the second solution treatment). Note that, when multiple portions that will be non-contact portions are formed, the first portion (symbol 1b 1After the second solution treatment (forming a β single layer) of the titanium alloy rod 1, the direction of the titanium alloy rod 1 is changed or moved, or the heating and cooling unit 5 is moved, etc., to form the second portion (reference symbol 1b 2 ) is subjected to a second solution treatment (to form a β single layer). EXAMPLES

[0024] Example 1 As a titanium alloy rod, a rod with a diameter of 5.2 mm and a length of 60 mm shown in FIG. 3, whose composition is Ti-15V-3Cr-3Sn-3Al, was prepared. The entire rod was subjected to a first solution treatment in which the rod was heated in a furnace at 800°C for 1 hour and then cooled in water. Thereafter, the entire rod was heated in a furnace at 450°C for 8 hours and then air-cooled in the same manner, and then aging treatment was performed. Through these treatments, a titanium alloy rod with a Vickers hardness of about 400HV was obtained. The Vickers hardness was measured under the conditions of a test force of 2.94N and a holding time of 10 seconds. In addition, phase identification was performed by X-ray diffraction, and it was confirmed that the α phase and β phase (i.e., the α+β phase region) existed throughout the entire rod. The X-ray diffraction was measured using a Cu tube under the conditions of a tube voltage of 40kV and a tube current of 40mA.

[0025] Next, as shown in Fig. 3, a predetermined portion (first portion in Fig. 3) of one end of the titanium alloy rod having a Vickers hardness of about 400HV was placed in the heating and cooling area 5c (length 15mm, diameter 11mm) of the heating and cooling unit 5, a high-frequency current of 200kHz was applied to the heating coil 5a for 1 second to heat it to 900°C, and then water-cooled in the cooling jacket 5b, thereby performing a second solution treatment on the first portion. Thereafter, the titanium alloy rod was turned upside down, and a predetermined portion (second portion in Fig. 3) of the other end was placed in the heating and cooling area 5c of the heating and cooling unit 5, a high-frequency current of 200kHz was applied to the heating coil 5a for 1 second to change the output of the high-frequency power source to heat it to 1000°C, and then water-cooled in the cooling jacket 5b, thereby performing a second solution treatment on the second portion.

[0026] Example 2 A predetermined portion (first portion in FIG. 3) of one end of the titanium alloy rod having a Vickers hardness of about 400HV was placed in the heating and cooling area 5c of the heating and cooling section 5, and a high-frequency current of 200kHz was applied to the heating coil 5a for 1 second, and the output of the high-frequency power source was changed to heat the rod to 700°C. Then, the rod was water-cooled in the cooling jacket 5b to perform a second solution treatment on the first portion. Then, the titanium alloy rod was turned upside down, and a predetermined portion (second portion in FIG. 3) of the other end was placed in the heating and cooling area 5c of the heating and cooling section 5, and a high-frequency current of 200kHz was applied to the heating coil 5a for 1 second, and the output of the high-frequency power source was changed to heat the rod to 800°C. Then, the rod was water-cooled in the cooling jacket 5b to perform a second solution treatment on the second portion. The rest is the same as in the first embodiment, so the explanation will be omitted.

[0027] The results of Examples 1 and 2 after the second solution treatment are shown in FIGS. 4(a), 4(b) and 5. Figure 4(a) and Figure 4(b) are diagrams showing the Vickers hardness (HV) of the titanium alloy rod for spinal fixation after the second solution treatment in Example 1 and Example 2. Figure 5 is a diagram showing the state of α-phase and β-phase of the titanium alloy rod for spinal fixation after the second solution treatment in Example 1 and Example 2. As shown in Figures 4(a) and 4(b), the part that had been subjected to the second solution treatment had a lower hardness than the part that had not been subjected to the second solution treatment, and the difference in hardness tended to increase with increasing heating temperature. In Example 1, which was heated to 900°C or higher, the Vickers hardness in the lowest portion of the second solution treatment area dropped to about 260 to 270 HV, and the maximum difference in hardness (ΔHV) was about 140 HV at 1000°C. It was found that the second solution treatment can achieve partial softening with a strength difference of 100 HV or more in the axial direction of the titanium alloy rod. In Example 2, in which the rod was heated to 700°C and 800°C, the Vickers hardness was reduced to about 355HV and about 305HV at the lowest points in the range of the second solution treatment, and the maximum difference in hardness (ΔHV) was about 50HV at 700°C and about 100HV at 800°C. It was found that the second solution treatment at 700°C and 800°C could achieve partial softening with a strength difference of 50HV to 100HV in the axial direction of the titanium alloy rod.

[0028] 5 showing the results of X-ray diffraction, it was found that the α phase peak attenuates while the β phase peak intensity increases with increasing heating temperature, and the β single phase is formed by heating at or above 900° C. Therefore, the heating temperature of the second solution treatment is preferably 900° C. or higher, from the viewpoint of forming a β single phase region, and more preferably 900° C. to 1000° C. From the above, it was confirmed that in Examples 1 and 2, by performing the second solution treatment on the first portion and the second solution treatment on the second portion as described above, a titanium alloy rod having three α+β phase regions (areas not subjected to the second solution treatment) and two β single phase regions (areas subjected to the second solution treatment) was obtained. Therefore, by making this α+β phase region the contact area with the screw and plug, the titanium alloy rod for spinal fixation of this embodiment can be obtained. [Industrial Applicability]

[0029] The titanium alloy rod for spinal fixation, which has the α+β phase region of the present invention as the contact area with the screw and plug, can improve the mechanical stimulation to the bone and the durability of the rod, so that it can be effectively used for the treatment of spinal deformity diseases, etc. [Explanation of symbols]

[0030] 1: Titanium alloy rod 1a(1a 1 , 1a 2 , 1a 3 ): α+β phase region (contact area with screw and plug) 1b(1b 1 , 1b2 ): β single phase area (non-contact area with the screw and plug) 2: Screw 21: Screw part 22: Head 3:Plug 5: Heating and cooling section 5a: Heating coil 5b: Cooling jacket 5c: Heating and cooling area

Claims

1. A titanium alloy rod for spinal fixation using a titanium alloy, wherein an α+β phase region and a β single-phase region having a difference in strength in the axial direction of the titanium alloy rod exist, and a β single-phase region exists between the α+β phase regions, the α+β phase region having higher strength is a contact portion with a screw and a plug, and the β single-phase region having lower strength is a non-contact portion with the screw and plug.

2. The titanium alloy rod for spinal fixation according to claim 1, wherein the α + β phase region is present in three or more places, and the β single phase region is present in two or more places.

3. The method for manufacturing a titanium alloy rod for spinal fixation according to claim 1 or 2, comprising the steps of: performing a first solution treatment on a titanium alloy rod made of a titanium alloy; performing aging treatment on the portion that has been subjected to the first solution treatment to make the portion into an α+β phase region; and then applying high frequency to the non-contact portion between the screw and the plug in the α+β phase region to further perform a second solution treatment to make the non-contact portion into a β single phase region.

4. The method for producing a titanium alloy rod for spinal fixation according to claim 3, wherein the second solution treatment is carried out at 900°C or higher.

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