Screw for placement in a vertebral body of a spine

Abstract

Screw (1) for arrangement in a vertebral body of a spine, comprising a screw head (3), a first threaded section (6) immediately adjoining the screw head (3) with a trapezoidal thread and a second threaded section adjoining the first threaded section (6) with a metric thread (7) or a worm thread (9).

Description

[0001] The invention comprises a screw for arrangement in a vertebral body of a spine.

[0002] The invention lies in the technical field of screw systems for spinal surgery and the corresponding accessories.

[0003] The population in Germany, Europe, and America is aging more than it was 100 years ago. This is supported by statistics. The age pyramid has inverted from a pyramid shape to a tree. Disease patterns are also changing. Osteopenia and osteoporosis are now extremely prevalent. This is not only true for women, who are most frequently affected due to hormone-induced osteoporosis, but also for men and increasingly younger women. Furthermore, the number of degenerative spinal changes is rising rapidly, partly due to increasing age and the excellent medical care available in Germany and Europe. Spinal stenosis is most common in the cervical spine and especially in the lower thoracic and lumbar spine.The increasing use of both motorized and non-motorized traffic, such as bicycles, e-bikes, electrically powered scooters, roller skaters and roller skaters, has contributed to a large proportion of accidents involving vertebral fractures in recent decades.

[0004] Overall, the treatment of choice for fractures, whether traumatic or atraumatic, or for degenerative spinal disorders with spondylolisthesis, is a screw-rod system. This aims to immobilize the fracture or spondylolisthesis to promote the healing process. The condition of the bone and its matrix plays a crucial role in this. Prior to surgery, many clinics gather additional information regarding the bone matrix and blood parameters. For example, bone density is determined using DXA (dual-energy X-ray absorptiometry), Q-CT (quantitative CT osteodensitometry), and laboratory parameters related to calcium uptake. This information then informs the diagnosis of whether the screws should be fixed with or without cement in the vertebral body. Once this process is complete, the surgery is performed using the aforementioned screw-rod system.

[0005] Surgical insertion is performed in various ways. Traditionally, if the anatomical conditions are physiological, the screw is inserted using two fluoroscopes. If the anatomical conditions are difficult or even radiographically impossible, but insertion is medically necessary, navigation is used for assistance.

[0006] The screws currently in use are based on a special metric thread, with and without cannulation. After the screw has been securely inserted, a guide wire is introduced through the cannulation. This wire, which serves as a trajectory for the cannulated screw's "safe path" through the vertebral body, is used to precisely position the screw within the vertebral body through rotational movements.

[0007] The following section explains the operative technique for inserting the screw, for example using the classic and navigation-assisted methods: The classic approach using an X-ray image intensifier in biplanar technique (AP and lateral beam path) using the minimally invasive percutaneous variant as an example.

[0008] The patient is positioned prone. The stab incisions are marked at skin level. These incisions are then made under fluoroscopic guidance. A Yamshidi-Gosh needle is positioned so that the entry point is congruent in both the anteroposterior and lateral radiographs. The needle is then driven in with a hammer, like a nail being driven into a wall. This is done under continuous fluoroscopic guidance. A thin wire (Kirschner wire) is then advanced through the needle, which has a hole drilled in it, into the vertebral body. The needle is removed. As an intermediate step, a thread is cut to ensure the screw is securely guided in the pedicle. The previously measured screw (length and diameter) is then screwed in through the guide wire. The result is checked biplane. The procedure is then complete. The navigation-assisted route

[0009] The patient is positioned prone. The skin incision is marked at skin level. The length of the incision is determined using the image intensifier, meaning only as much skin as necessary is incised. This is essential for wound healing and for minimizing the risk of infection during and postoperatively. The dissection is then performed, and the navigation device is inserted and securely fixed to a spinous process. The surgical site is then draped, and a CT scan is performed. The data is transferred to the navigation unit by the system itself, and a virtual 3D image is generated. The instruments required for navigation are then loaded into the system, and their accuracy is verified to match the virtual and real-world representations. A channel for the guide wire is then drilled using the navigation system. The navigated drill sleeve is removed while the wire is secured.The screw length and diameter are then precisely determined using the navigation system's visualization. Next, the thread for the screw is cut. The screw is then inserted, and the result is checked and documented using the X-ray image intensifier. It should be noted that this process is not performed for each screw individually, but only after all screws have been installed.

[0010] In all spinal surgeries using this system, a rod is inserted into the tulip-shaped sockets on the screw head on both the left and right sides at the end of the operation. The rods are usually pre-bent. They are then firmly fixed in the screw head within the socket using an internal screw, and this screw is tightened with a torque wrench to prevent the rod from shifting under load. This ensures a secure connection for energy transfer, allowing, for example, the fracture to begin the healing process.

[0011] In the surgical treatment of degenerative diseases using a screw-rod system from the back, the intervertebral disc of the segment must also be partially removed and replaced with a disc prosthesis to allow the entire system to heal properly and prevent loosening or rod fractures. The disc is removed from the back, the side, or the front.

[0012] Loosening or breakage of the screw or rod and pseudoarthrosis of the spacers in the intervertebral disc spaces are repeatedly observed.

[0013] The present invention is therefore based on the objective of providing a screw for arrangement in a vertebral body of a spine that does not loosen and in which material breakage does not occur.

[0014] The problem is solved by a screw for arrangement in a vertebral body of a spine with the features of independent claim 1. Advantageous embodiments are the subject of the dependent claims.

[0015] The invention comprises a screw for installation in a vertebral body of a spine. The screw includes a screw head, a first threaded section immediately adjoining the screw head with a trapezoidal thread, and a second threaded section adjoining the first threaded section with a metric thread or a worm thread. The thicker threads are suitable for transmitting larger axial forces before shearing failure, thus ensuring that the screw connection does not loosen or that material fracture occurs due to unfavorable force distribution.

[0016] One advantageous aspect is that the screw also includes a third threaded section adjoining the second threaded section, featuring a drill thread for creating a recess in the vertebral body. This eliminates the intraoperative step of pre-drilling, thereby also eliminating the time-consuming repositioning of the screw.

[0017] It is particularly advantageous if the second threaded section and / or the third threaded section includes at least one opening for introducing material to further secure the screw. This allows the screw to be additionally fixed with cement, depending on the diagnosis.

[0018] According to a preferred aspect, the screw further comprises an axial channel extending from the screw head towards the third threaded section for the insertion of a wire. After the screw has been securely inserted, a guide wire is introduced through the channel. This wire, which serves as a trajectory for the cannulated screw's "safe path" through the vertebral body, is used to securely position the screw within the vertebral body by rotary movements.

[0019] One advantageous feature of the screw is a fourth threaded section with a fine thread, positioned between the first and second threaded sections. This allows for a smooth transition from trapezoidal to metric threads. It is particularly advantageous if the second threaded section is a worm thread. This enables rapid insertion of the screw without numerous screwdriver turns, saving time and increasing the bearing surface inside the swivel body.

[0020] The advantage of this design is that the trapezoidal thread in the first thread section degenerates into a saw thread. Viewed from the screw head, the saw thread can have a plateau that narrows. This allows for a further practical modification of the screw: starting after a threadless drill tip, the saw thread initially functions as a drill thread with little or no plateau, and then develops an increasingly wider plateau towards the screw head.

[0021] Advantageously, an arrangement for connecting and fixing the screws to the vertebral body comprises at least two screws and at least one rod, the rod being arranged to engage the screw head. This allows for optimal immobilization of a fracture to promote the healing process.

[0022] It has proven advantageous for the rod to comprise a circular section with an inner core of strength f1 and a sheath surrounding the core of strength f2, a coupling for a device to position the rod, and a conical end section opposite the coupling for guiding the rod through the screw head. This ensures that peak forces are absorbed by the outer core and that the inner core provides the necessary bending strength under maximum rod dynamics, thus ensuring a longer service life for the rod and preventing the frequently observed rod fractures, ultimately sparing the patient and the health insurance system further operations.

[0023] One advantageous aspect is that the strength f1 is greater than the strength f2. This allows for an optimal distribution of tensile and compressive forces.

[0024] Furthermore, it is advantageous if the strength f2 is greater than the strength f1. This neutralizes the tensile and compressive forces in zones where fatigue fractures regularly occur, preventing the formation of fatigue zones and thus avoiding fractures.

[0025] The invention will be explained in more detail below with reference to drawings.

[0026] They show: Fig. 1 a schematic representation of a screw according to the invention with a trapezoidal thread, a metric thread and a drill tip; Fig. 2 a schematic representation of a screw according to the invention with a worm thread, a metric thread and a drill tip, Fig. 3 a schematic representation of a screw according to the invention with a worm thread, a metric thread, a drill tip and a fine thread in the central area; Fig. 4 a schematic representation of a screw according to the invention with a trapezoidal thread, a metric thread, a drill tip and a fine thread in the central area; Fig. 5 a schematic representation of a rod with a coupling and a guide tip for an arrangement with two screws according to the invention; Fig. 6 a schematic representation of a rod and the corresponding cross-section through the layers of different strength; Fig. 7a a schematic representation of a conventional screw with a uniform thread over the entire length of the screw; Fig. 7b a schematic representation of a screw according to the invention with an increasingly narrow trapezoidal thread which degenerates into a saw thread, a metric thread section and a drill tip; and Fig. 7c a schematic representation of a screw according to the invention with a saw thread in the first thread section, which transitions smoothly into the third thread section up to the threadless drill tip.

[0027] In Fig. Figure 1 is a schematic representation of a screw 1 according to the invention for arrangement in a vertebral body of a spine. The screw 1 comprises a screw head 3, a first threaded section 6 with a trapezoidal thread immediately adjoining the screw head 3, and a second threaded section adjoining the first threaded section 6 with a metric thread 7 or a worm thread 9. The trapezoidal thread is known from machine and tool construction. Trapezoidal threads are used for transmitting movements and forces, for example, on leadscrews in lathes or in spindle presses. The larger pitch is advantageous in this context. The thicker threads are suitable for transmitting larger axial forces before they fail due to shearing. In typical applications, the moderate self-locking effect resulting from the larger pitch is acceptable or insignificant.Furthermore, a larger contact area and a shallower slope are observed. This eliminates the cutting edge effect. Additionally, the larger contact area reduces the pressure on the bone matrix, potentially reducing or even dissipating the energy introduced to an imperceptible vibration.

[0028] Furthermore, screw 1 includes a third threaded section 11 adjoining the second threaded section 7, 9, with a drill thread for creating a recess in the vortex body. It is also conceivable that the third threaded section 11 includes a thread with thread-cutting properties.

[0029] Fig. Figure 2 shows a schematic representation of a screw 1 according to the invention. The second threaded section 7, 9 and / or the third threaded section 11 comprises at least one opening 4 for introducing material for additional fixing of the screw 1. In this embodiment, the second threaded section is a worm thread 9.

[0030] In Fig. Figure 3 shows a schematic representation of a screw 1 according to the invention. In this embodiment, the screw 1 comprises a fourth threaded section 8 with a fine thread arranged between the first threaded section 6 and the second threaded section 7, 9.

[0031] In Fig. Figure 4 shows a schematic representation of a screw 1 according to the invention, comprising an axial channel 5 extending from the screw head 3 towards the third threaded section 11 for the insertion of a wire. For example, the screw head 3 comprises a screw head drive with a hexagon (internal hexagon).

[0032] Fig. Figure 5 shows a schematic representation of a rod 20 for an arrangement on two screws 1 according to the invention. Such an arrangement comprises at least two screws 1 and at least one rod 20 for connecting and fixing the screws 1. The rod 20 can be arranged to engage the screw head 3 of each screw 1. The rod 20 comprises a circular section 23 with an inner core 24 having a strength f1 and a sheath 25 arranged around the core 24 with a strength f2, a coupling 21 for a device for arranging the rod 20, and a conical end section 22 arranged opposite the coupling 21 for guiding the rod 20 through the screw head 3.

[0033] Fig. Figure 6 shows a schematic representation of a rod 20 and the corresponding cross-section through the layers of different strengths. It is advantageous for the strength f1 to be greater than the strength f2 to achieve an optimal distribution of tensile and compressive forces in the elastic dynamic shell 25.

[0034] Accordingly, a reverse variant is also useful, i.e., the strength f2 is greater than the strength f1. In this case, the elastic dynamic core 24 would be in the inside and the shell 25 would consist of solid, stiffer steel in order to neutralize the tensile and compressive forces in such zones that are prone to fatigue fractures, so that no fatigue zones form and fractures are thus avoided.

[0035] In Fig. Figures 7a - c show embodiments of screw 1. Fig. Figure 7a shows a conventional screw 1 in which a metric saw thread extends over the entire length of the screw. The pitch of the thread form is 3 mm. In Fig. 7b, the first thread section 6 is a trapezoidal thread modified to a saw thread. The subsequent second thread section has, for example, a metric thread or a worm thread. The flank angle of the saw thread is preferably 10 to 15 degrees, followed by a plateau, as is the case with the trapezoidal thread. After this, there is a sloping flank similar to the trapezoidal thread, which can also be steeper. The pitch of the saw thread is preferably 6 mm. Fig.Figure 7c shows another modified form of screw 1 with a saw thread, in which the saw thread at the screw head initially has a wide plateau, which becomes progressively narrower along the screw 1, with the pitch of the saw thread remaining constant over the thread length. The saw thread gradually transitions into the third thread section 11 with a further reduced plateau, acting as a kind of drill thread, until it reaches a threadless drill tip.

Claims

[1] Screw (1) for arrangement in a vertebral body of a spine, comprising a screw head (3), a first threaded section (6) with a trapezoidal thread directly adjoining the screw head (3) and a second threaded section with a metric thread (7) or a worm thread (9) adjoining the first threaded section (6), further comprising a fourth threaded section (8) with a fine thread arranged between the first threaded section (6) and the second threaded section (7, 9). [2] Screw (1) according to claim 1, further comprising a third threaded portion (11) adjoining the second threaded portion (7, 9) with a drilling thread for introducing a recess into the vertebral body. [3] Screw (1) according to one of the preceding claims, wherein the second threaded portion (7, 9) and / or the third threaded portion (11) comprises at least one opening (4) for introducing material for additional fixation of the screw (1). [4] Screw (1) according to one of the preceding claims, further comprising an axial channel (5) extending from the screw head (3) in the direction of the third threaded portion (11) for inserting a wire. [5] Screw (1) according to one of the preceding claims, wherein in the first threaded section (6) the trapezoidal thread is degenerated into a saw thread. [6] Arrangement comprising at least two screws (1) according to one of claims 1 to 5 and at least one rod (20) for connecting and fixing the screws (1) to a vertebral body, wherein the rod (20) can be arranged in an engaging manner on the screw head (3) of the screw (1).

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