Bone screw and bone screw manufacturing method

Abstract

The present invention can provide a bone screw in which a sliding part, a fastening part, or contact surfaces of different materials or components do not easily slide or break. A bone screw according to the present invention is characterized by comprising a shaft part, a screw part positioned spirally in an axial direction of the shaft part at an outer peripheral part of the shaft part, a first end part positioned at an end part in an insertion direction of the shaft part, a second end part positioned at an end part in a removal direction of the shaft part, a head part positioned so as to enclose the second end part, and a fixing part attached to the head part and capable of fixing the rod part to the head part, wherein the arithmetic surface roughness Ra of at least one of the second end part, a sliding part of the head part slidable with respect to the second end part, and the fixing part is 0.4-1.7 μm.

Description

Bone screw and method for manufacturing bone screw

[0001] The present invention relates to a bone screw and a method for manufacturing a bone screw.

[0002] Bone screws are used in fracture surgery and spinal surgery. For example, pedicle screws inserted into the pedicles are connected to rods or other devices to stabilize the spine.

[0003] Japanese Patent Application Laid-Open No. 2019-76755

[0004] Bone screws are required to have a certain level of durability during surgery and during use. For example, it is preferable that a bone screw inserted into a bone does not deform even when a certain force is applied. In particular, for bone screws equipped with a sliding portion or a fastening portion, it is preferable that the sliding portion or fastening portion does not easily slip even when a certain force is applied during surgery or during use. Furthermore, for bone screws made of multiple materials or parts, it is preferable that the joints between the different materials or parts do not easily slip or break even when a certain force is applied during surgery or during use.

[0005] The bone screw of the present invention comprises a shaft portion, a threaded portion spirally positioned in the axial direction of the shaft portion on the outer periphery of the shaft portion, a first end portion positioned at the end of the shaft portion in the insertion direction, a second end portion positioned at the end of the shaft portion in the removal direction, a head portion positioned to encompass the second end portion, and a fixing portion attached to the head portion and capable of fixing a rod portion to the head portion, wherein the arithmetic surface roughness Ra of at least one of the second end portion, the sliding portion of the head portion slidable relative to the second end portion, and the fixing portion is 0.4 μm to 1.7 μm.

[0006] According to the present invention, it is possible to provide a bone screw in which the sliding portion, fastening portion, or contact surface between different materials or parts does not easily slip or break.

[0007] 1 is a front view of the bone screw of this embodiment; FIG. 2 is a cross-sectional view of the bone screw of this embodiment; FIG. 3 is a perspective view of the bone screw of this embodiment; (a) a top view of the bone screw of this embodiment; (b) a bottom view of the bone screw of this embodiment; FIG. 4 is a diagram showing a flexion / extension moment test; FIG. 5 is a diagram showing the results of a flexion / extension moment test of a bone screw of a comparative example and the bone screw of this embodiment; FIG. 6 is a diagram showing the results of a torsion test of a bone screw of a comparative example and the bone screw of this embodiment;

[0008] The inventors have found a surface roughness that prevents the sliding portion or fastening portion from easily sliding in bone screws having a sliding portion, and also found a surface roughness that prevents the contact surfaces of different materials or parts from easily sliding or being damaged in bone screws made of multiple materials or parts.

[0009] According to this embodiment, it is possible to provide a bone screw in which the sliding portion, fastening portion, or contact surface between different materials or parts does not easily slip or break.

[0010] This embodiment will now be described with reference to the drawings, illustrating a medical bone screw that is fixed to the spine. The bone screw of this embodiment is used to treat spinal disorders (e.g., spinal deformity, spinal fracture, spinal stenosis, spinal dislocation, spinal instability, etc.) in order to improve spinal stability. Furthermore, the bone screw of this embodiment is used to treat spinal disorders by inserting multiple bone screws into the pedicles of the spine and connecting the multiple bone screws with rod portions to provide appropriate support to the spine or restrict spinal movement.

[0011] Fig. 1 is a front view of the bone screw of this embodiment. Fig. 2 is a cross-sectional view taken along line A-A in Fig. 1. Fig. 3 is a perspective view of the bone screw of this embodiment. Fig. 4(a) is a top view of the bone screw of this embodiment. Fig. 4(b) is a bottom view of the bone screw of this embodiment.

[0012] As shown in FIGS. 1 to 4 , the bone screw 100 includes a shaft portion 1 and a threaded portion 4 spirally arranged on the outer periphery of the shaft portion 1 in the axial direction of the shaft portion 1. The bone screw 100 also includes a first end portion 2 located at the end of the shaft portion 1 in the insertion direction and a second end portion 3 located at the end of the shaft portion 1 in the removal direction. The bone screw 100 also includes a head portion 5 positioned to encompass the second end portion. In this embodiment, the head portion 5 is slidable relative to the second end portion 3, but may also be formed integrally with or fixed to the second end portion 3. The bone screw 100 also includes a fixing portion (set screw) 6, which serves as a fastening portion. The fixing portion 6 is attached to the head portion 5 and can fix a rod portion (not shown) to the head portion 5. The bone screw 100 also includes an abutting portion (pusher) 7 that abuts against the second end portion 3.

[0013] A rod portion (not shown) connecting multiple bone screws is placed in a recess 11 (FIG. 1) of the head 5 so as to contact the abutting portion 7, and the rod portion is compressed and fixed by fastening a fixing portion (male screw) 6 to a female screw 13 (FIG. 2) of the head 5. When the fixing portion (male screw) 6 is fastened to the female screw 13 (FIG. 2) of the head 5, the abutting portion 7 together with the rod portion presses down on the second end 3, thereby compressing and fixing the second end 3 to the sliding portion 12 (FIG. 2) of the head 5.

[0014] At least one of the second end 3, the sliding portion 12 of the head 5 that is slidable relative to the second end 3, and the fixed portion (male screw) 6 has an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm. For example, the arithmetic surface roughness Ra of a portion of the second end 3 that contacts the sliding portion 12 of the head 5 (FIG. 2) is 0.4 μm to 1.7 μm. Furthermore, the arithmetic surface roughness Ra of a portion of the sliding portion 12 of the head 5 (FIG. 2) that contacts the second end 3 is 0.4 μm to 1.7 μm. Furthermore, the arithmetic surface roughness Ra of a portion of the fixed portion 6 that contacts the head 5 when the fixed portion 6 is attached to the head 5 is 0.4 μm to 1.7 μm.

[0015] The arithmetic surface roughness Ra of only one of the second end 3 and the sliding portion 12 of the head 5 that is slidable relative to the second end 3 may be 0.4 μm to 1.7 μm.

[0016] The contact portion 7 has an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm. For example, the arithmetic surface roughness Ra of a part of the contact portion 7 that contacts the second end portion 3 is 0.4 μm to 1.7 μm.

[0017] 2, the shaft portion 1 includes a shaft core portion 8 including the second end portion 3, and a shaft thread portion 9 located on the outer periphery of the shaft core portion 8 and including the thread portion 4. The shaft core portion 8 also includes a through-hole 10 for passing a guide wire or the like therethrough. The arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 0.4 μm to 1.7 μm.

[0018] The shaft core portion 8 (or the second end portion) includes at least one of stainless steel, titanium, iron, nickel, cobalt, and cemented carbide. The shaft thread portion 9 (or the thread portion 4) includes reinforced plastic. The reinforced plastic is at least one of carbon fiber reinforced plastic, glass fiber reinforced plastic, carbon fiber reinforced plastic, boron fiber reinforced plastic, aramid fiber reinforced plastic, Keppra fiber reinforced plastic, Dyneema fiber reinforced plastic, and Zylon fiber reinforced plastic.

[0019] Next, a method for manufacturing a bone screw will be described. The method for manufacturing a bone screw of this embodiment includes a step of performing surface treatment to an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm on at least one surface of the second end 3, the sliding portion 12 of the head that is slidable relative to the second end 3, and the fixing portion 6. The method for manufacturing a bone screw of this embodiment also includes a step of performing surface treatment to an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm on the surface of the shaft core portion 8 that contacts the shaft thread portion 9. The method for manufacturing a bone screw of this embodiment also includes a step of performing surface treatment to an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm on the surface of the abutting portion 7.

[0020] In the surface treatment step, a blasting treatment is performed. For example, the blasting treatment is performed using white alundum or zircon beads. Separable components may be blasted while they are separated. For example, if the shaft portion 1, head portion 5, fixing portion 6, and abutting portion 7 are separable, the blasting treatment may be performed while each of them is separated. Furthermore, during the bone screw manufacturing process, the blasting treatment may be performed on the components before they are attached. For example, the shaft core portion 8 may be blasted before the shaft thread portion 9 is attached. In this case, after the blasting treatment is performed on the shaft core portion 8, the shaft thread portion 9 may be injection molded around the shaft core portion 8 as an axis.

[0021] Next, the performance of the bone screw of this embodiment will be described. As shown in FIG. 5 , a flexion / extension moment test was performed with reference to ASTM F1798. The rod portion 50 of the bone screw 100 was placed in the recess 11 ( FIG. 1 ), and the fixing portion 6 was fastened to the head portion 5 with a torque of 8 to 12 Nm. The rod portion 50 was then fixed, and a load (N) was applied at a position 25 mm from the central axis of the rod portion 50. The test was then terminated when the bone screw 100 was significantly deformed. The maximum load (N) up to that point was measured, and this was defined as the bending strength. The second end 3 of the bone screw 100 and the sliding portion of the head portion 5 (including the abutment portion 7) that can slide relative to the second end 3 are primarily composed of titanium.

[0022] 6 is a diagram showing the results of flexion / extension moment tests on the bone screw of Comparative Example 1 and the bone screw of this embodiment. As shown in Fig. 6, the bending strength of the bone screw of Comparative Example 1 (wherein the arithmetic surface roughness Ra of the second end 3 and the sliding portion of the head 5 slidable against the second end 3 is 0.29 µm to 0.38 µm) was 212.4 N.

[0023] In contrast, in Example 1 of this embodiment (where the arithmetic surface roughness Ra of the second end 3 and the sliding portion of the head 5 that can slide against the second end 3 was 0.44 μm to 0.64 μm), the bending strength was 616.2 N. In Example 2 of this embodiment (where the arithmetic surface roughness Ra of the second end 3 and the sliding portion of the head 5 that can slide against the second end 3 was 1.11 μm to 1.22 μm), the bending strength was 387.7 N. Furthermore, in Example 3 of this embodiment (where the arithmetic surface roughness Ra of the second end 3 and the sliding portion of the head 5 that can slide against the second end 3 was 1.22 μm to 1.68 μm), the bending strength was 450.0 N.

[0024] As a result, when the arithmetic surface roughness Ra of at least one of the second end 3 and the sliding portion 12 of the head 5 slidable against the second end 3 was 0.4 μm to 1.7 μm, the bending strength was 380 N to 620 N, which was a sufficient bending strength compared to Comparative Example 1. Preferably, when the arithmetic surface roughness Ra of at least one of the second end 3 and the sliding portion 12 of the head 5 slidable against the second end 3 was 1.2 μm to 1.7 μm, the bending strength was 450 N, which was a sufficient bending strength compared to Comparative Example 1. More preferably, when the arithmetic surface roughness Ra of at least one of the second end 3 and the sliding portion 12 of the head 5 slidable against the second end 3 was 0.4 μm to 0.7 μm, the bending strength was 616.2 N, which was a sufficient bending strength compared to Comparative Example 1.

[0025] Note that loosening of the fixing portion 6 was observed in Comparative Example 1, but no loosening of the fixing portion 6 occurred in Examples 1 to 3. As a result, when the arithmetic surface roughness Ra of the fixing portion 6 was 0.4 μm to 1.7 μm, sufficient fastening strength (fixing strength) could be obtained compared to Comparative Example 1. Preferably, the arithmetic surface roughness Ra of the fixing portion 6 is 1.2 μm to 1.7 μm. More preferably, the arithmetic surface roughness Ra of the fixing portion 6 is 0.4 μm to 0.7 μm.

[0026] Furthermore, the results of the above flexion / extension moment test show that when the arithmetic surface roughness Ra of the abutting portion 7 abutting against the second end 3 (corresponding to a part of the sliding portion of the head 5 that is slidable against the second end 3) is 0.4 μm to 1.7 μm, sufficient bending strength can be obtained compared to Comparative Example 1. Preferably, the arithmetic surface roughness Ra of the abutting portion 7 abutting against the second end 3 is 1.2 μm to 1.7 μm. More preferably, the arithmetic surface roughness Ra of the abutting portion 7 abutting against the second end 3 is 0.4 μm to 0.7 μm.

[0027] Next, a torsion test was performed as shown in Figure 7. The rod portion 50 of the bone screw 100 was placed in the recess 11 (Figure 1), and the fixing portion 6 was fastened to the head portion 5 with a torque of 8 to 12 Nm. The rod portion 50 was then fixed, and a torque (Nm) was applied to the shaft thread portion 9 (or the thread portion 4). The test was then terminated when the bone screw 100 was significantly deformed, and the maximum torque (Nm) up to the end was measured. This maximum torque (Nm) was defined as the torsion strength. The shaft core portion 8 (or the portion of the shaft core portion 8 that contacts the shaft thread portion 9) of the bone screw 100 of this embodiment mainly contains titanium, and the shaft thread portion 9 (or the portion of the shaft thread portion 9 that contacts the shaft core portion 8) mainly contains carbon fiber reinforced plastic.

[0028] 8 is a diagram showing the results of a torsion test on the bone screw of Comparative Example 2 and the bone screw of this embodiment. As shown in Fig. 8, in the case of the bone screw of Comparative Example 2 (wherein the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 0.29 µm to 0.38 µm), the average torsion strength was 4.6 Nm.

[0029] In contrast, in Example 4 of this embodiment (where the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 was 0.44 μm to 0.64 μm), the average torsional strength was 8.628 Nm. Also, in Example 5 of this embodiment (where the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 was 1.11 μm to 1.22 μm), the average torsional strength was 8.912 Nm. Furthermore, in Example 6 of this embodiment (where the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 was 1.22 μm to 1.68 μm), the average torsional strength was 8.956 Nm.

[0030] As a result, when the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 0.4 μm or more, sufficient torsional strength can be obtained compared to Comparative Example 2. Preferably, the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 0.4 μm to 1.7 μm. More preferably, the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 1.1 μm to 1.7 μm. Even more preferably, the arithmetic surface roughness Ra of the shaft core portion 8 in contact with the shaft thread portion 9 is 1.2 μm to 1.7 μm.

[0031] Considering both the bending / extension moment test (FIG. 6) and the torsion test (FIG. 8) of the bone screw of this embodiment described above, when the second end 3 and the shaft core portion 8 of the bone screw are integrally or fixedly formed, the second end 3 and the shaft core portion 8 can be surface-treated simultaneously, thereby reducing the number of steps and increasing production efficiency, and therefore it is preferable to make the arithmetic surface roughness Ra of the second end 3 of the bone screw and that of the shaft core portion 8 match within an appropriate range.

[0032] Therefore, the arithmetic surface roughness Ra of the second end 3 of the bone screw and the shaft core portion 8 is preferably 0.4 μm to 0.7 μm or 1.2 μm to 1.7 μm. Furthermore, the arithmetic surface roughness Ra of the second end 3 of the bone screw and the shaft core portion 8 is preferably 0.4 μm to 0.7 μm.

[0033] This allows the second end 3 and the shaft core 8 of the bone screw to be surface-treated at the same time, thereby increasing production efficiency and providing sufficient bending strength and torsional strength.

[0034] Although the embodiments of the present invention have been described above, the present invention is not limited to these and can be modified and changed within the scope of the claims.

[0035] For example, in addition to blasting, the surface treatment step may include etching, laser treatment, mechanical treatment (such as milling, needling, and sanding), abrasive treatment (such as shot peening), thermal spraying, and electrochemical treatment (such as anodizing).

[0036] The present invention is useful as a bone screw in which the sliding parts, fastening parts, or contact surfaces of different materials or parts do not easily slip or break.

[0037] This application claims priority from Japanese Patent Application No. 2023-211736 (filed December 15, 2023), the entire contents of which are incorporated herein by reference.

[0038] DESCRIPTION OF SYMBOLS 1 Shaft portion 2 First end portion 3 Second end portion 4 Thread portion 5 Head portion 6 Fixation portion 7 Contact portion 8 Shaft core portion 9 Shaft thread portion 10 Through hole 11 Recessed portion 12 Sliding portion 13 Female screw 100 Bone screw

Claims

1. A bone screw comprising: a shaft portion; a threaded portion spirally disposed in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end portion disposed at the end of the shaft portion in the insertion direction; a second end portion disposed at the end of the shaft portion in the removal direction; a head portion disposed to encompass the second end portion; and a fixing portion attached to the head portion and capable of fixing a rod portion to the head portion, wherein at least one of the second end portion, the sliding portion of the head portion capable of sliding relative to the second end portion, and the fixing portion has an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm.

2. A bone screw comprising: a shaft portion; a threaded portion located spirally in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end located at an end of the shaft portion in an insertion direction; and a second end located at an end of the shaft portion in a removal direction, wherein the shaft portion includes a shaft core portion including the second end; and a shaft threaded portion located on the outer periphery of the shaft core portion and including the threaded portion, wherein the arithmetic surface roughness Ra of the shaft core portion in contact with the shaft threaded portion is 0.4 μm to 1.7 μm.

3. A bone screw comprising: a shaft portion; a threaded portion spirally disposed in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end portion disposed at the end of the shaft portion in the insertion direction; a second end portion disposed at the end of the shaft portion in the removal direction; and an abutment portion abutting against the second end portion, wherein the arithmetic surface roughness Ra of the abutment portion is 0.4 μm to 1.7 μm.

4. A bone screw according to any one of claims 1 to 3, characterized in that the threaded portion comprises a reinforced plastic.

5. The bone screw of claim 4, wherein the reinforced plastic is at least one of carbon fiber reinforced plastic, glass fiber reinforced plastic, carbon fiber reinforced plastic, boron fiber reinforced plastic, aramid fiber reinforced plastic, Kevlar fiber reinforced plastic, Dyneema fiber reinforced plastic, and Zylon fiber reinforced plastic.

6. The bone screw of any one of claims 1 to 3, wherein the second end comprises at least one of stainless steel, titanium, iron, nickel, cobalt, and cemented carbide.

7. A bone screw comprising: a shaft portion; a threaded portion spirally disposed in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end portion disposed at the end of the shaft portion in the insertion direction; a second end portion disposed at the end of the shaft portion in the removal direction; a head portion disposed to encompass the second end portion; and a fixing portion attached to the head portion and capable of fixing a rod portion to the head portion, the method comprising the step of: applying a surface treatment to at least one surface of the second end portion, the sliding portion of the head capable of sliding against the second end portion, and the fixing portion to have an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm.

8. A bone screw comprising: a shaft portion; a threaded portion spirally disposed in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end portion disposed at an end of the shaft portion in an insertion direction; and a second end portion disposed at an end of the shaft portion in a removal direction, the shaft portion including a shaft core portion including the second end portion; and a shaft threaded portion disposed on the outer periphery of the shaft core portion and including the threaded portion, the method comprising the step of: subjecting a surface of the shaft core portion in contact with the shaft threaded portion to an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm.

9. A method for manufacturing a bone screw comprising: a shaft portion; a threaded portion spirally disposed in the axial direction of the shaft portion on the outer periphery of the shaft portion; a first end portion disposed at an end portion of the shaft portion in an insertion direction; a second end portion disposed at an end portion of the shaft portion in a removal direction; and an abutment portion abutting against the second end portion, comprising the step of: subjecting a surface of the abutment portion to surface treatment to an arithmetic surface roughness Ra of 0.4 μm to 1.7 μm.

Images