Nail feeding device capable of being suitable for different pedicle screw threads and thread design method of nail feeding device

Abstract

The invention discloses a nailing device suitable for different pedicle screw threads and a thread design method of the nailing device, and belongs to the technical field of medical instruments. The screw feeding device comprises a main body, the head of the main body is provided with an external thread matched with an internal thread of a pedicle screw seat, the thread form of the axial section of the external thread is a right trapezoid, the long bottom edge is located on the thread tooth bottom side, the short bottom edge is located on the thread tooth top side, the vertical waist side face is a bearing face for bearing screwing torque, and the bearing face is perpendicular to the axis; and the included angle alpha between the guide inclined surface and the bearing surface is more than or equal to 0 degree and less than or equal to 18 degrees. The thread design method comprises the following steps: acquiring tooth form parameters of various pedicle screw internal threads; intersection fitting processing is carried out on the various internal thread profile contours, and a reference fitting contour is determined; and carrying out structure optimization on the reference fitting contour to form the external thread with the axial section being a right trapezoid. By means of the special thread form, the pedicle screw can be matched with pedicle screw threads of different tooth forms, and the pedicle screw implantation device is suitable for implantation operation of pedicle screws in orthopedic surgery.

Description

Technical Field

[0001] This invention belongs to the field of medical device technology, specifically a screw inserter applicable to different pedicle screw threads and its thread design method. Background Technology

[0002] Pedicle screw fixation is an important treatment method in spinal surgery, widely used in the treatment of various conditions such as spinal trauma, degenerative diseases, and deformity correction. During pedicle screw implantation, the screw-installing device is a crucial surgical instrument, and its performance directly affects the success rate and safety of the surgery. The screw-installing device is mainly used to accurately and safely implant the pedicle screw into the pedicle. The external thread of its head engages with the internal thread of the pedicle screw seat to achieve effective torque transmission. For example, publication number CN120770909A discloses a screw-installing device including an implantation mechanism and a torque application mechanism that are connected to the pedicle screw, which can prevent pedicle screw loosening and shorten surgical operation time. Publication number CN217090866U discloses a screw-installing device for universal pedicle screws, which consists of an outer sleeve, a sleeve, and an inner rod arranged sequentially from the outside to the inside, with the front thread of the sleeve threadedly connected to the universal pedicle screw. For example, CN205493991U discloses a universal hollow screw-installing device for pedicle screws, including a strut, a plug, and a hand handle, the outer surface of which has external threads that match the internal threads of the pedicle screw. Although these screw-installing devices have different structures, they all form a threaded connection with the pedicle screw through the external threads of their heads.

[0003] Currently, mainstream spinal internal fixation techniques both domestically and internationally employ a special type of internal thread design for the pedicle screws to achieve the respective advantages of pedicle screws during implantation. While the major and minor diameters and pitches of these internal threads are generally the same across the industry, the special thread shapes manifest in the following three major technical schools: 1. Negative angle internal threads (such as...) Figure 1 As shown): This thread generates axial locking force by setting a negative tooth flank angle to prevent stripping during the nailing process. Its design focuses on enhancing pull-out resistance.

[0004] 2. Hook lock internal thread (such as...) Figure 2 As shown): This thread is modeled after a variant of trapezoidal or sawtooth threads, and adopts a "hook-shaped" tooth profile structure to achieve quick insertion and assembly. Its design focuses on improving loading efficiency.

[0005] 3. Composite internal thread (negative angle + hook lock) (e.g.) Figure 3 As shown): This thread combines the two tooth profile features mentioned above, aiming to balance pull-out resistance and rapid loading capability.

[0006] However, existing screw-attaching devices generally suffer from poor compatibility in clinical applications. In spinal fixation surgery, especially in revision surgeries or emergency situations, surgeons may face situations where different brands of pedicle screws are used interchangeably, or where the tooth profile of the pedicle screws already implanted in the patient is unclear. For a long time, it has been generally accepted in the field that the screw-attaching device thread must maintain a consistent tooth profile with the screw seat thread; otherwise, the two cannot be properly fitted. Based on this understanding, existing screw-attaching devices are all dedicated to a single thread type. Even though screw-attaching devices with interchangeable adapters exist on the market, improving compatibility to some extent, they still require frequent changes of different adapter sizes during use, making the operation cumbersome. This not only prolongs the operation time and increases the risk of anesthesia, but also complicates the surgical procedure and makes it more prone to complications such as screw misplacement, stripping, and connection detachment, posing a potential threat to surgical outcomes and patient safety. Summary of the Invention

[0007] This invention provides a screw-insertion device that can be used with different pedicle screw threads, aiming to solve the technical problems of poor compatibility of existing screw-insertion devices, which require frequent replacement of different specifications of screw-insertion devices or conversion heads in spinal internal fixation surgery, resulting in prolonged anesthesia time, complicated surgical operation, and safety hazards such as screw misplacement, stripping, and falling off.

[0008] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: Firstly, this invention redesigns the thread profile of the screwdriver head to make it compatible with various mainstream pedicle screw internal thread types, thereby achieving multiple uses of one device, simplifying surgical procedures, shortening surgical time, effectively reducing surgical risks caused by instrument replacement, and improving the overall safety and efficiency of the surgery.

[0009] The technical solution provided by the present invention is as follows: a stapler body, wherein the head of the stapler body is provided with an external thread for engaging with the internal thread of the pedicle screw seat, the external thread being adaptable to at least two different types of pedicle screw internal threads, the at least two different types of pedicle screw internal threads being selected from the following three types: negative angle internal thread, hook-lock internal thread, and composite internal thread; Wherein, the flank angle of the negative angle internal thread is -5°≤β1≤0° and 0°≤β2≤54°; the flank angle of the hook-lock internal thread is -5°≤β1≤0° and 0°≤β2≤5°; the flank angle of the compound internal thread is β1=0° and 0°≤β2≤46°; The axial cross-sectional profile of the external thread is a right-angled trapezoid, with the long base of the right-angled trapezoid located at the root of the thread and the short base located at the crest of the thread. The side of the vertical waist forming the right trapezoid is a bearing surface for bearing the screwing torque, and the bearing surface is perpendicular to the axis of the external thread; The side of the inclined waist of the right trapezoid is a guide inclined surface for guiding the thread to screw in or out, and the angle α between the guide inclined surface and the bearing surface is 0°≤α≤18°; The major diameter of the external thread is less than the minimum of the major diameters of the internal threads of at least two different types of pedicle screws to which the external thread is adapted, and the minor diameter of the external thread is less than the minimum of the minor diameters of the internal threads of at least two different types of pedicle screws.

[0010] The thread design principle of this invention is based on a systematic analysis and parameter extraction of the geometric features of the three major existing pedicle screw internal thread profiles. It proposes a multi-objective compatible reverse fitting design method, enabling a single external thread to simultaneously adapt to multiple internal thread profiles. Specifically, by performing intersection fitting on key parameters such as the flank angle and root width of negative angle internal threads, hook-lock internal threads, and composite internal threads, a benchmark fitting profile capable of geometrically adapting to all three types of internal threads is obtained. Based on this, structural optimization is performed according to functional requirements, ultimately forming an external thread structure with a right-angled trapezoidal axial cross-section.

[0011] The external thread profile of this design method can effectively mesh with negative angle threads, hook-lock threads and compound threads under the same profile, satisfying both geometric compatibility requirements and ensuring the functional requirements of torque transmission and smooth operation.

[0012] Preferably, the included angle α is 15°.

[0013] Preferably, the crest width L1 of the external thread is 0 < L1 ≤ 0.48 mm.

[0014] More preferably, the crest width L1 of the external thread is 0.45 mm.

[0015] Preferably, the bearing surface and the guide slope are connected to the root of the external thread by a circular arc transition.

[0016] Secondly, the present invention also provides a thread design method for a pedicle screw installer. The method, through reverse fitting and structural optimization, can design an external thread for a pedicle screw installer that can simultaneously adapt to the internal threads of various pedicle screws.

[0017] The thread design method provided by this invention includes the following steps: Step 1: Obtain the tooth profile parameters of at least two different types of pedicle screw internal threads; the tooth profile parameters include at least the flank angle, root width, major diameter, and minor diameter; Step 2: Based on the obtained tooth profile parameters, perform intersection fitting on the tooth profile profiles of the at least two different types of internal threads to determine the reference fitting profile that can simultaneously adapt to the at least two types of internal threads. Step 3: Optimize the structure of the reference fitting profile to form an external thread with a right-angled trapezoidal axial cross section on the head of the nailer.

[0018] Preferably, step 2 includes: The tooth profiles of the at least two internal threads are overlapped in a coaxial manner, and the plane perpendicular to the axis is used as the reference plane to determine that the most prominent point of the bearing surface of each internal thread is located on the reference plane. The major diameter, minor diameter, and reference bearing surface position of the external thread are determined by the constraint that the external thread can be screwed into the at least two types of internal threads without interference. The maximum permissible inclination angle of the external thread guide bevel and the maximum permissible value of the tooth crest width are determined by geometric drawing. The geometric construction includes: intersecting the major diameter line of the external thread with each internal thread guide slope, taking the point closest to the reference surface as the starting point P1; at the root of the internal thread, taking the point closest to the bearing surface 21 at the corner of the three internal thread guide slopes as the ending point P2; connecting P1 and P2 to form a slope L, taking the inclination angle of the slope L as the maximum allowable inclination angle, and taking the vertical distance from P1 to the reference surface as the maximum allowable value of the crest width.

[0019] Preferably, step 3 includes: The bearing surface of the external thread is designed to be perpendicular to its axis; The angle α between the external thread guide slope and the external thread bearing surface is equal to the inclination angle θ of the external thread guide slope, and is not greater than the maximum allowable inclination angle; The crest width L1 of the external thread is set to be no greater than the maximum allowable value.

[0020] The external thread designed using the above method has a right-angled trapezoidal axial cross-section, which can simultaneously adapt to at least two of the following: negative angle internal threads, hook-lock internal threads, and composite internal threads. Compared with the prior art, the beneficial effect of the present invention is that by designing the external thread profile of the pedicle screw holder as a right-angled trapezoidal structure, the present invention can simultaneously adapt to three different types of pedicle screw internal threads: negative angle threads, hook-lock threads, and composite threads, effectively solving the problem of poor compatibility of existing pedicle screw holders.

[0021] In clinical use, surgeons no longer need to frequently change between different sizes of screwdrivers or replace adapters on the screwdrivers, simplifying the operation and significantly improving surgical efficiency. Surgeons no longer need to identify the brand or thread type of the screws; one screwdriver can be used for all pedicle screws on the operating table, completely eliminating surgical interruptions caused by using the wrong instrument. Especially in revision or emergency surgeries, it can effectively shorten surgical time and reduce anesthesia risks.

[0022] In addition, this invention reduces the number of different types of staplers to be stocked in the operating room, significantly reducing the sterilization and maintenance costs of instruments, and improving surgical efficiency and safety. Attached Figure Description

[0023] Figure 1 This is an enlarged schematic diagram of a negative angle internal thread; Figure 2 This is an enlarged schematic diagram of the hook lock's internal thread; Figure 3 This is an enlarged schematic diagram of a compound internal thread; Figure 4 This is a partial sectional view of the connection between the screw-attach device of the present invention and the pedicle screw; Figure 5 This is a schematic diagram of the main body of the nailer of the present invention; Figure 6 For the present invention Figure 5 Enlarged sectional view of region I in the middle; Figure 7 The external threads of the nailer of this invention are respectively with Figure 1 , Figure 2 and Figure 3 A diagram illustrating the fit; Figure 8 The external thread of the present invention and Figure 1 , Figure 2 and Figure 3 A schematic diagram showing the overlapping tooth profiles of three different types of threads; In the figure, 1-the main body of the nailer, 2-external thread, 21-bearing surface, 22-guide slope, 3-pedicle nail, 31-internal thread, 32-internal thread bearing surface, 33-internal thread guide slope. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description of the pedicle screw inserter applicable to different pedicle screw threads is provided through embodiments and in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the invention. This application will now be described in detail with reference to the accompanying drawings and embodiments. Example

[0025] This invention provides a screw-installing device that is compatible with different pedicle screw threads, and this screw-installing device is compatible with negative angle internal threads (such as...). Figure 1 As shown), hook lock internal thread (such as Figure 2 (as shown) and compound internal threads (such as) Figure 3 (As shown) Three different internal thread profiles for pedicle screws solve the problem of poor compatibility of existing screw installers.

[0026] like Figures 4 to 7As shown, the screw-attaching device includes a main body 1, the head of which is provided with an external thread 2 for engaging with the internal thread 31 of the pedicle screw 3 seat. This external thread 2 is the core structure of the present invention. The axial cross-section of the external thread 2 is a right-angled trapezoid, as shown below. Figure 6 As shown, the long bottom edge is located on the thread root side, and the short bottom edge is located on the thread crest side; the vertical waist side is the bearing surface 21, which is perpendicular to the axis of the external thread 2 and is used to transmit the screwing torque; the inclined waist side is the guide inclined surface 22, which has an angle α of 15° with the bearing surface 21 and is used to guide the thread to screw in or out smoothly.

[0027] The crest width L1 of the external thread 2 is 0.45mm. The root of the thread is connected to the bearing surface 21 and the guide slope 22 by a circular arc transition. The radii of the circular arcs are R0.15mm and R0.35mm, respectively, to reduce stress concentration and improve the fatigue strength of the thread.

[0028] In this embodiment, the main body 1 of the stapler is made of medical-grade stainless steel (such as 630) with a passivated surface treatment, exhibiting good biocompatibility and corrosion resistance. The external thread portion of its head is formed in one step using a forming tool, ensuring tooth profile accuracy and surface quality; the thread portion can be hardened (such as through heat treatment) to improve wear resistance and service life.

[0029] It should be noted that the external thread of this invention can be matched with the above three types of threads, indicating that the pitch of the external thread is the same as that of the three types of threads by default.

[0030] The design method of the present invention will be described in detail below with reference to specific parameters: The first step is to extract the geometric parameters of the tooth profiles of negative angle internal threads, hook-lock internal threads, and compound internal threads, respectively. In this embodiment, as shown... Figure 1-3 As shown, the tooth profile parameters of the negative angle internal thread are: tooth flank angle β1=-5° and β2=45°, tooth root width 0.42mm, pitch 1.4mm, major diameter φ10 and minor diameter φ8.6; the tooth profile parameters of the hook-lock thread are: tooth flank angle β1=-5° and β2=5°, tooth root width 0.59mm, pitch 1.4mm, major diameter φ9.4 and minor diameter φ7.6; the tooth profile parameters of the compound thread are: tooth flank angle β1=0° and β2=30°, tooth root width 0.46mm, pitch 1.4mm, major diameter φ9.3 and minor diameter φ7.9.

[0031] The second step is to perform intersection fitting on the tooth profiles of the three types of internal threads mentioned above. Based on the tooth profile parameters (including flank angle, root width, pitch, major diameter, and minor diameter) of the negative angle internal thread, hook-lock internal thread, and compound internal thread obtained in the first step, perform the following geometric drawing steps, as follows: Figure 8 The three internal thread profiles shown are overlapped within the tooth groove.

[0032] 1. Establish the datum plane and the overlap of the profile. The axial section profiles of the three internal threads are overlapped in a coaxial manner, and a plane perpendicular to the axis is used as the reference plane (this reference plane passes through the most prominent point of each internal thread bearing surface, i.e., the axial position of each internal thread bearing surface). This reference plane will serve as the design reference plane for the subsequent external thread bearing surface.

[0033] 2. Determine the major and minor diameters of the external thread. The constraint condition is that the external thread can be screwed into three types of internal threads without interference: the major diameter of the external thread is the minimum of the major diameters of the three internal threads; the minor diameter of the external thread is the minimum of the minor diameters of the three internal threads. Thus, the limiting reference lines for the major and minor diameters of the external thread are determined, with the crest of the thread located on the major diameter line (a straight line parallel to the axis) and the root of the thread located on the minor diameter line (a straight line parallel to the axis). Ultimately, the external thread can mate with the three types of internal threads, with the major and minor diameters of the external thread being: the major diameter of the external thread is less than the minimum of the major diameters of the three internal threads, and the minor diameter of the external thread is less than the minimum of the minor diameters of the three internal threads.

[0034] 3. Determine the limit reference line of the guide slope. Determining the starting point P1: At the major diameter line of the external thread (i.e., the horizontal line where the tooth crest is located), draw a straight line parallel to the axis. This line intersects the guide slopes of the three types of internal threads, obtaining three intersection points. Take the point among these three intersection points that is closest to the reference plane (i.e., has the smallest axial coordinate) and denote it as the starting point P1; Determination of endpoint P2: At the root of the internal thread, the point closest to the bearing surface 21 at the corner of the three internal thread guide slopes 33 is taken as endpoint P2; Connect P1 and P2 to form an oblique line L. This oblique line L is the limit reference line of the external thread guide surface 22. The vertical distance from the starting point P1 to the bearing surface 21 is the upper limit value of the tooth crest width L1.

[0035] 4. Calculate the upper limit of the guide ramp inclination angle and the upper limit of the tooth crest width. The angle θ between the oblique line L and the perpendicular line from point P1 to the vertical axis is the maximum allowable value of the inclination angle of the external thread guide surface; the vertical distance from the starting point P1 to the bearing surface is the maximum allowable value of the external thread crest width L1.

[0036] Using the geometric drawing method described above, the maximum allowable tilt angle θ and the maximum allowable value of the tooth crest width L1 of the external thread guide can be determined uniquely and unambiguously, providing clear numerical limits for subsequent structural optimization.

[0037] The third step, based on the contour fitting in the second step, is to optimize the structure according to functional requirements, as follows: 1. Load-bearing surface design To ensure that the external thread can effectively transmit the screwing torque, the bearing surface 21 is designed to be perpendicular to the axis of the external thread 2. This bearing surface coincides with the reference surface established in the second step, ensuring effective contact with the bearing surfaces of each internal thread.

[0038] 2. Guide slope design To ensure sufficient clearance between the guide slope 22 of the external thread and the guide slope 33 of the three types of internal threads, allowing the external thread to smoothly screw into or out of the rivet seat with different tooth profiles, the inclination angle θ of the guide slope 22 shall not exceed the inclination angle of the limit reference line L determined in the second step. In actual design, θ can be selected between 0° and this upper limit value.

[0039] 3. Tooth crest width design The tooth crest width L1 should not be greater than the vertical distance from the starting point P1 determined in the second step to the bearing surface (i.e., the maximum allowable value) to ensure that the external thread tooth crest does not interfere with the bottom of any internal thread tooth groove.

[0040] After the above optimization, the final thread axial section profile is a right-angled trapezoidal structure: the long base is located on the root side of the thread, the short base is located on the crest side of the thread, the bearing surface is the vertical waist, and the guide slope is the inclined waist.

[0041] In this embodiment, because the thread profile is a right-angled trapezoid, the angle α between the inclined line L and the bearing surface 21 is equal to the inclination angle θ, and the upper limit of the angle α is measured to be 18°; the vertical distance from the starting point P1 to the bearing surface 21 is the upper limit of the thread crest width L1, which is measured to be 0.48 mm. After further optimization, when the guide inclined surface angle α is 15° and the thread crest width L1 is 0.45 mm, the external thread has the best fit with the three types of internal threads. Actual fit tests verify that the external thread designed with the above parameters can effectively fit with negative angle threads, hook-lock threads, and compound threads, ensuring reliable torque transmission while ensuring smooth screwing in and out.

[0042] To further verify the feasibility and adaptability of the technical solution of this invention, the applicant used computer-aided design software to conduct a simulation analysis of the fit between the above-mentioned right-angled trapezoidal external thread and three different types of internal threads, and conducted fit tests with actual samples for verification. The results show that when the angle α between the guide slope and the bearing surface of the external thread satisfies 0°≤α≤18°, the external thread can simultaneously adapt to the following three types of pedicle screw internal threads: Negative angle internal threads: flank angle β1 is -5°≤β1≤0°, β2 is 0°≤β2≤54°; Hook-lock internal thread: flank angle β1 is -5°≤β1≤0°, β2 is 0°≤β2≤5°; Composite internal thread: flank angle β1=0°, β2 is 0°≤β2≤46°.

[0043] Furthermore, when the tooth flank angles β1 and β2 of the above three types of internal threads are both 0°, their tooth profiles all evolve into rectangular threads, and the external threads of the present invention can also achieve effective mating.

[0044] Simulation analysis and experimental verification results both show that the right-angled trapezoidal external thread designed in this invention has good versatility and adaptability, and can achieve smooth engagement with various types of pedicle screw internal threads while ensuring the reliability of torque transmission.

[0045] like Figure 4 and Figure 7 As shown, the external thread 2 of the pedicle screw head mates with the internal thread 31 of the pedicle screw 3. The internal thread 31 can be the aforementioned negative angle internal thread, hook-lock internal thread, or compound internal thread, or it can be a rectangular thread evolved when both the tooth flank angles β1 and β2 are 0°. The external thread 2 of this invention can achieve a good fit with all the above-mentioned types of internal threads 31, which can transmit sufficient torque and avoid stripping or jamming.

[0046] In actual use, the doctor connects the screwdriver to the pedicle screw base, and by rotating the screwdriver body, the external and internal threads gradually engage. The bearing surface 21 bears the rotational torque, ensuring stable power transmission, so that the screwdriver can screw the pedicle screw into the bone tissue; the guide ramp 22 guides the threads to screw in smoothly, avoiding thread jamming.

[0047] The thread design of this invention eliminates the need for surgeons to change to different sizes of screwdrivers or adapters based on the brand or thread type of the pedicle screw during surgery. Surgeons can directly use the screwdriver of this invention to complete the pedicle screw implantation, significantly improving surgical efficiency and safety. Especially in complex scenarios such as revision surgeries, emergency trauma, or the use of multiple brands of instruments, surgeons can achieve reliable connections without needing to identify the specific thread type of the pedicle screw, fundamentally eliminating medical risks such as stripping and dislodgement caused by mismatched thread profiles. Furthermore, hospitals can reduce the number of screwdrivers stocked with different thread types, significantly lowering instrument sterilization and maintenance costs.

[0048] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A screw inserter adaptable to different pedicle screw threads, comprising: The main body of the pedicle screw holder has an external thread at its head for engaging with the internal thread of the pedicle screw seat. The external thread is characterized in that it can be adapted to at least two different types of pedicle screw internal threads, and the at least two different types of pedicle screw internal threads are selected from the following three types: negative angle internal thread, hook-lock internal thread, and compound internal thread. Wherein, the flank angle of the negative angle internal thread is -5°≤β1≤0° and 0°≤β2≤54°; the flank angle of the hook-lock internal thread is -5°≤β1≤0° and 0°≤β2≤5°; the flank angle of the compound internal thread is β1=0° and 0°≤β2≤46°; The axial cross-sectional profile of the external thread is a right-angled trapezoid, with the long base of the right-angled trapezoid located at the root of the thread and the short base located at the crest of the thread. The side of the vertical waist forming the right trapezoid is a bearing surface for bearing the screwing torque, and the bearing surface is perpendicular to the axis of the external thread; The side of the inclined waist of the right trapezoid is a guide inclined surface for guiding the thread to screw in or out, and the angle α between the guide inclined surface and the bearing surface is 0°≤α≤18°; The major diameter of the external thread is less than the minimum of the major diameters of the internal threads of at least two different types of pedicle screws to which the external thread is adapted, and the minor diameter of the external thread is less than the minimum of the minor diameters of the internal threads of at least two different types of pedicle screws.

2. The nailer according to claim 1, characterized in that, The included angle α is 15°.

3. The nailer according to claim 1, characterized in that, The crest width L1 of the external thread is 0 < L1 ≤ 0.48 mm.

4. The nailer according to claim 3, characterized in that, The crest width L1 of the external thread is 0.45 mm.

5. The nailer according to claim 1, characterized in that, The bearing surface and the guide slope are connected to the root of the external thread by a circular arc transition.

6. A thread design method for the nailer according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Obtain the tooth profile parameters of the internal threads of at least two different types of pedicle screws; Step 2: Based on the obtained tooth profile parameters, perform intersection fitting on the tooth profile profiles of the at least two different types of internal threads to determine the reference fitting profile that can simultaneously adapt to the at least two types of internal threads. Step 3: Optimize the structure of the reference fitting profile to form an external thread with a right-angled trapezoidal axial cross section on the head of the nailer.

7. The thread design method according to claim 6, characterized in that, Step 2 includes: The tooth profiles of the at least two internal threads are overlapped in a coaxial manner, and the plane perpendicular to the axis is used as the reference plane to determine that the most prominent point of the bearing surface of each internal thread is located on the reference plane. The major diameter, minor diameter, and reference bearing surface position of the external thread are determined by the constraint that the external thread can be screwed into the at least two types of internal threads without interference. The maximum permissible inclination angle of the external thread guide slope and the maximum permissible value of the tooth crest width are determined by geometric drawing. The geometric construction includes: intersecting the major diameter line of the external thread with each internal thread guide slope, taking the point closest to the reference surface as the starting point P1; at the root of the internal thread, taking the point closest to the bearing surface 21 at the corner of the three internal thread guide slopes as the ending point P2; connecting P1 and P2 to form a slope L, taking the inclination angle of the slope L as the maximum allowable inclination angle, and taking the vertical distance from P1 to the reference surface as the maximum allowable value of the crest width.

8. The thread design method according to claim 7, characterized in that, Step 3 includes: The bearing surface of the external thread is designed to be perpendicular to its axis; The angle α between the external thread guide slope and the external thread bearing surface is equal to the inclination angle θ of the external thread guide slope, and is not greater than the maximum allowable inclination angle; The crest width L1 of the external thread is set to be no greater than the maximum allowable value.

9. The thread design method according to any one of claims 6-8, characterized in that, The at least two different types of pedicle screw internal threads include at least two of the following: negative angle internal threads, hook-lock internal threads, and compound internal threads.

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