Titanium alloy rod for spinal fixation

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 that has an α+β phase region 1b (1b1, 1b2) with a difference in strength along the axial direction and an α' phase or α'+β phase region 1a (1a1, 1a2, 1a3), with the α+β phase region 1b existing between the α' phase or α'+β phase regions 1a, where the high-strength α' phase or α'+β phase region 1a is the contact part with a screw and a plug, and the low-strength α+β phase region 1b is 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 in the caudal side and low rigidity is required in 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 is occupied by the second region 12. In this way, as shown in FIG. 6 (see FIG. 7 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 6 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 states that "the α+β type titanium alloy is Ti-6Al-7Nb, Ti-6Al-4V, or Ti-6Al-4V " (Claim 7), "The biomedical titanium alloy processed product according to any one of claims 4 to 7, which is a spinal fixation device or a bone fixation device." (Claim 2), and "α+β type titanium alloys are suitable for medical products to be placed in the body because they are lightweight, have low magnetic susceptibility, low cytotoxicity, and have excellent corrosion resistance and tissue compatibility. Specific examples of α+β type titanium alloys are Ti-6Al-4V, Ti-6Al-4V ELI, Ti-6Al-7Nb, Ti-3Al-2.5V, and Ti-5Al-2.5Fe." (Paragraph

[0014] ).

[0006] It is also known that titanium alloys used in biomaterial products such as artificial bones contain an α' martensite phase, which has high hardness (see Patent Document 3). Patent Document 3 describes "a titanium alloy shaped article having a substrate having a laminated structure of titanium alloy, a second hardened layer composed of ...α phase, and a third hardened layer composed of α' martensite phase and metastable β phase provided on the inner layer of the second hardened layer." (Claim 1) and also describes "a high-frequency induction heating method that improves the yield strength, tensile strength, and fatigue strength of an α+β type titanium alloy by heating an α+β type titanium alloy for a time of less than 1 second to 600 seconds at a temperature of 800°C to 1200°C using a frequency of 1 kHz to 400 kHz for a short time, and then rapidly cooling the alloy to generate the α' phase. "(Paragraph

[0002] ), "Titanium alloys can be used favorably for biomaterial products such as artificial bones that are adapted to the skeletal shape of each user." (Paragraph

[0012] ), "The reason for this high hardness is believed to be that during the above-mentioned high-frequency induction heating, oxygen and nitrogen contained in the air diffuse from the surface 20 of the workpiece toward the inner layer to the third hardened layer 30c, and the rapid cooling effect forms the third hardened layer 30c composed of an α' martensite phase and a metastable β phase." (Paragraph

[0026] ). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-139179 A [Patent Document 2] JP 2016-59951 A [Patent Document 3] JP 2023-69501 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. The titanium alloy rod for spinal fixation described in Patent Document 1 has low strength in the second region consisting of the β phase (see the right side of Figure 6). Therefore, when the second region 12 (see Figure 5) is applied to the head side H shown in Figure 7 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. However, at the head side H, the strength of the rod is also low at the site where the screw and plug are fixed, so that if it is used in combination with a screw and plug with high strength, the durability of that site may be reduced.

[0009] Patent Document 2 describes an α+β type titanium alloy for use as a spinal fixation device, but does not describe further increasing the strength of the α+β type titanium alloy. Patent Document 3 suggests that an α+β type titanium alloy, which is suitable for use in biomaterial products such as artificial bones, can be heated for a short period of time by high-frequency induction heating and then rapidly cooled to convert it to the α'+β phase, thereby increasing its hardness and improving its fatigue strength (durability). However, it does not disclose the formation of an area with a difference in strength in the axial direction of the rod by partially heating a titanium alloy 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, in which an α+β phase region and an α' phase or α'+β phase region with different strengths exist in the axial direction of the titanium alloy rod, and an α+β phase region exists between the α' phase or α'+β phase regions, the α' phase or α'+β phase region with higher strength is the contact portion with the screw and plug, and the α+β phase region with lower strength is the non-contact portion with the screw and plug. (2) A titanium alloy rod for spinal fixation according to (1), in which the α' phase or α' + β phase regions are present in three or more locations and the α + β phase regions are present in two or more locations in the axial direction of the titanium alloy rod. (3) A method for manufacturing a titanium alloy rod for spinal fixation according to (1) or (2), comprising applying high frequency to a contact portion between the screw and the plug in the α+β phase region of a titanium alloy rod having an α+β phase region, thereby subjecting the contact portion to a hardening treatment, thereby converting the contact portion into an α' phase or α'+β phase region. Effect of the Invention

[0012] According to the titanium alloy rod for spinal fixation of the present invention, the durability of the titanium alloy rod at the contact portion with the high-strength screw and plug (the portion where the screw and plug are fixed) is improved. Also, the load on the instrument fixed to the bone is reduced due to the presence of the non-contact portion with the low-strength screw and plug (the portion where the screw and plug are not fixed), resulting 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. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a conceptual diagram showing an example of an intraspinal fixation device using a titanium alloy rod for spinal fixation according to one embodiment of the present invention. [Figure 1] 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. 1 is a conceptual diagram showing a high-frequency hardening method for a titanium alloy rod for spinal fixation according to one embodiment of the present invention. [Figure 4] FIG. 4 is a diagram showing the Vickers hardness (HV) of the titanium alloy rod for spinal fixation in the embodiment. [Diagram 5] FIG. 5 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 6] FIG. 6 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 7] FIG. 7 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 according to 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] In the titanium alloy rod 1 for spinal fixation of this embodiment, in the axial direction of the titanium alloy rod 1, there are α+β phase regions with strength differences and α' phase or α'+β phase regions between the α+β phase regions, and the α' phase or α'+β phase regions 1a1, 1a2, 1a3 (hereinafter collectively referred to as 1a) with high strength are the contact parts with the screw 2 and the plug 3. On the other hand, the α+β phase regions 1b1, 1b2 (hereinafter collectively referred to as 1b) having low strength are non-contact portions with the screw 2 and the plug 3. In addition, the α+β phase regions 1b1, 1b2 exist between the α' phase or α'+β phase regions 1a1, 1a2, and 1a3, respectively.

[0017] More specifically, the α' phase or 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 α + β 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 that protrudes from the receiving portion of the head 22 and the lower surface of the plug 3. Within the scope of the present invention, the contact portion with the screw 2 and the plug 3 may include a portion of the α' phase or α' + β phase region 1a having high strength, and the non-contact portion with the screw 2 and the plug 3 may include a portion of the α' phase or α' + β phase region 1a having high strength.

[0018] In this embodiment, as shown in FIG. 2, there are three α' phase or α'+β phase regions 1a, 1a1, 1a2, and 1a3, and two α+β phase regions 1b, 1b1 and 1b2, in the axial direction of the titanium alloy rod 1. When fixing a spinal fixation device using a titanium alloy rod 1 for spinal fixation to the spine, at least two α' phase or α' + β phase regions 1a are required, but the effects of the present invention can be obtained by having an α + β phase region 1b between the α' phase or α' + β phase regions 1a. It is preferable that the α' phase or α'+β phase regions exist in three or more locations, and the α+β phase regions exist in two or more locations between each of the α' phase or α'+β phase regions. 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.

[0019] Since titanium alloys such as Ti-6Al-4V, which is an α+β type, are usually used for the screw and plug, the α' phase or α'+β phase region 1a of the titanium alloy rod 1, which has a higher Vickers hardness HV and higher strength than Ti-6Al-4V, is used as the contact portion between the screw 2 and plug 3, thereby improving wear resistance and durability. By using the α+β phase region 1b, which has a lower Vickers hardness HV (low strength), as the non-contact portion between the screw 2 and plug 3, the load on the instrument fixed to the bone is reduced, and as a result, the load on the bone is increased, improving the mechanical stimulation to the bone. In other words, there is a possibility that durability and stress blocking suppression for the bone as an instrument fixed to the bone by the screw 2 can be achieved at the same time. The strength difference between the α' phase or α'+β phase region 1a and the α+β phase region 1b is, for example, 50HV or more in Vickers hardness.

[0020] An α+β type titanium alloy can be used as the titanium alloy used in the titanium alloy rod 1. The α+β type titanium alloy includes alloys containing V, Nb, Fe, Mo, Cr, Mn, etc., and specifically, these components are generally contained in Ti together with Al, specifically, Ti-6Al-4V, Ti-3Al-2.5V, Ti-6Al-7Nb, Ti-5Al-2.5Fe, etc., but α+β type alloys that do not contain Al, such as Ti-8Mn and Ti-2Fe-2Cr-2Mo, are also known.

[0021] The titanium alloy rod of this embodiment is obtained by quenching the α+β type titanium alloy having the above-mentioned composition by the method described below. A titanium alloy rod having an α+β phase region is prepared, and a high frequency is applied to the portion within the α+β phase region that will be in contact with the screw and plug, thereby subjecting the portion to hardening treatment, thereby converting the portion into an α' phase or α'+β phase region.

[0022] Specifically, in this embodiment, a part of the α+β phase titanium alloy rod is partially quenched by high-frequency induction heating to form an α' phase region, thereby achieving partial hardening. This partial quenching produces a titanium alloy rod for spinal fixation that has an α' phase with strength difference and an α+β phase region between the α' phase regions. Note that β phase may remain in the α' phase, and the α'+β phase is also included in this embodiment. In other words, by carrying out the partial hardening process described above, a titanium alloy rod for spinal fixation is produced in which an α+β phase region and an α' phase or α'+β phase region with a difference in strength are formed in the axial direction of the titanium alloy rod, an α+β phase region exists between the α' phase or α'+β phase regions, the high strength α' phase or α'+β region is the contact area with the screw and plug, and the low strength α+β phase region is the non-contact area with the screw and plug.

[0023] The heating temperature for the partial quenching treatment is preferably 900°C or higher, and more preferably 1000°C or higher. In addition, since there is a risk of coarsening of the crystal grain size and melting, the heating temperature is preferably 1300°C or lower, and more preferably 1200°C or lower. In order to prevent excessive heating due to thermal conduction, the heating time is preferably 10 seconds or less, more preferably 3 seconds or less, and even more preferably 1 second or less. After heating, the material is rapidly cooled at a cooling rate faster than water cooling to perform partial quenching, and the β phase is transformed into the α' phase (martensite). The heating device and cooling device in the quenching treatment, as well as the manufacturing conditions thereof, i.e., the heating means, heating temperature, heating / cooling rate, holding time at the heating temperature, cooling means (e.g., air cooling or water cooling), and cooling rate, can be set appropriately.

[0024] FIG. 3 is a conceptual diagram showing a high-frequency quenching method for a titanium alloy rod for spinal fixation according to one embodiment of the present invention. Specifically, the quenching treatment is performed by arranging the part (part forming the α' phase or α' + β phase region) of the titanium alloy rod 1 having the α + β phase region 1b that will be the contact part with the screw and the plug in the α + β phase region 1b in the heating and cooling region 5c of the high-frequency induction heating device having the heating and cooling part 5 (heating coil 5a, cooling jacket 5b) that serves as both a heating coil and a cooling jacket, and performing high-frequency heating and rapid cooling (i.e., quenching treatment). Note that, when forming a plurality of parts that will be the contact parts, after performing the quenching treatment of the first part (symbol 1a1) (forming the α' phase or α' + β phase region), the direction of the titanium alloy rod 1 is changed or moved, or the heating and cooling part 5 is moved, and the quenching treatment of the second part (symbol 1a2) is performed (forming the α' phase or α' + β phase region). Furthermore, although not shown, when forming a third portion (symbol 1a3 shown in FIG. 2), after performing a hardening treatment on the second portion (symbol 1a2) (forming an α' phase or α' + β phase region), the orientation of the titanium alloy rod 1 is changed or moved, or the heating and cooling section 5 is moved, etc., to perform a hardening treatment on the third portion (symbol 1a3) (forming an α' phase or α' + β phase region). EXAMPLES

[0025] As the titanium alloy rod, a rod having a composition of Ti-6Al-4V and a diameter of 5.2 mm and a length of 30 mm was prepared. As shown in FIG. 3, a predetermined portion (first portion 1a1 in FIG. 3) of one end of the titanium alloy rod having a Vickers hardness of about 300 to 330 HV was placed in the heating and cooling area 5c (length 15 mm, diameter 11 mm) of the heating and cooling unit 5. A high-frequency current of 200 kHz was applied to the heating coil 5a for 1 second, and the output of the high-frequency power source was adjusted to heat the rod to 1100°C. After that, the rod was water-cooled (rapidly cooled) in the cooling jacket 5b, thereby performing a quenching treatment (partial quenching) of the first portion 1a1.

[0026] The result of the partial quenching of the first portion 1a1 is shown in FIG. As shown in Figure 4, the partially quenched area had a higher hardness than the unquenched area. When heated to 1100°C, the Vickers hardness increased to approximately 380HV at the highest point in the range of 10mm to 20mm from the bottom end of the titanium alloy rod, and the maximum difference in hardness (ΔHV) was approximately 50 to 80HV. The Vickers hardness was measured under the following conditions: test force 2.94N, holding time 10 seconds. It was found that this quenching (local heating) caused the β phase to transform into α', thereby achieving partial hardening with a strength difference of 50HV or more.

[0027] From the above, it was confirmed that the titanium alloy rod having the α' phase or α'+β phase region was obtained in the embodiment. Therefore, by forming the high-strength α' phase or α'+β phase region in two or more places as the contact portion with the screw and the plug, the titanium alloy rod for spinal fixation of the present embodiment can be obtained. [Industrial Applicability]

[0028] The titanium alloy rod for spinal fixation of the present invention, in which the α' phase or α' + β phase region is the contact area with the screw and plug, and the low-strength α + β phase region is the non-contact area with the screw and plug, can improve the mechanical stimulation to the bone and improve the durability of the rod, and therefore can be effectively used for the treatment of spinal deformity diseases, etc. [Explanation of symbols]

[0029] 1: Titanium alloy rod 1a (1a1, 1a2, 1a3): α' phase or α' + β phase (contact area with screw and plug) 1b (1b1, 1b2): α+β phase region (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, comprising an α+β phase region and an α' phase or α'+β phase region with different strengths in the axial direction of the titanium alloy rod, an α+β phase region between the α' phase or α'+β phase regions, the α' phase or α'+β phase region with higher strength being the contact portion with a screw and a plug, and the α+β phase region with lower strength being the non-contact portion with the screw and plug.

2. 2. The titanium alloy rod for spinal fixation according to claim 1, wherein the α' phase or α' + β phase regions are present in three or more locations, and the α + β phase regions are present in two or more locations in the axial direction of the titanium alloy rod.

3. The method for manufacturing a titanium alloy rod for spinal fixation according to claim 1 or 2, wherein a high frequency is applied to the contact portion between the screw and the plug in the α+β phase region of the titanium alloy rod having the α+β phase region, and the contact portion is hardened to make the α' phase or α'+β phase region.

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