WO2025068171A1 - Screw for arrangement in a vertebral body of a vertebra - Google Patents

Abstract

A screw (1) for arrangement in a vertebral body of a vertebra comprises a screw head (3), directly contiguous to the screw head (3) a first thread portion (6) with a trapezoidal thread and, contiguous to the first thread portion (6), a second thread portion with a metric thread (7) or a a worm thread (9).

Description

Screw for placement in a vertebral body of a spine

The invention comprises a screw for placement in a vertebral body of a spinal column.

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

The population in Germany, Europe, and America is living longer than it did 100 years ago. This statement is supported by statistics. The age pyramid has inverted from a pyramid to a tree. Clinical patterns are also changing. Osteopenia and osteoporosis are now on the rise. And not just among women, who are most commonly affected due to hormone-induced osteoporosis. Men and, increasingly, younger women also suffer from this condition. Furthermore, the incidence of degenerative spinal changes is rising rapidly in Germany and Europe, partly due to increasing age and excellent medical care. Narrowing of the spinal column is most prevalent in the cervical spine and, above all, in the lower thoracic and lumbar spine. The growth of both motorized and non-motorized traffic, such as bicycles, e-bikes, electric scooters, roller skaters, and rollerbladers, has contributed to the fact that accidents in recent decades have also largely involved vertebral fractures.

Overall, the treatment of choice for fractures, whether traumatic or non-traumatic, or for degenerative spinal disorders with spondylolisthesis, is a screw-rod system. This is intended to help immobilize the fracture or spondylolisthesis, for example, to promote the healing process. The composition of the bone and its matrix plays a key role in this. Prior to surgery, many clinics obtain 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 that determine calcium uptake. This is linked to the expanded diagnosis of whether the screws need to be fixed in the vertebral body with or without cement. If this process completed, the operation is carried out using the screw-rod system mentioned above.

Surgical insertion is performed in various ways. Traditionally, if the anatomical conditions are physiological, the screw is inserted using two X-ray image intensifiers. If the anatomical conditions are difficult or even radiographically impossible, but the insertion is medically necessary, navigation is used as an aid.

The screws currently used are based on a special metric thread, with and without cannulation. After the screw is securely inserted, a guide wire is inserted through the cannulation. This wire, which serves as a trajectory for the cannulated screw to ensure a "safe path" through the vertebral body, is used to securely position the screw within the vertebral body through rotational movements.

In addition, the surgical technique of inserting the screw is explained, for example, using the classic and navigation-assisted method:

The classic approach with X-ray image converter in biplanar technique (a.p. and lateral beam path) using the example of the minimally invasive percutaneous variant

The patient is positioned prone. The stab incisions are marked at skin level. The stab incisions are then made under image intensifier control. A Yamshidi-Gosh needle is then positioned so that the entry points are congruent in the AP and lateral views. The needle is then driven in with a hammer, like a nail being driven into the wall. This is done under constant image intensifier control. A thin wire (Kirschner wire) is then pushed over the needle, which has a bore, into the vertebral body. The needle is removed. As an intermediate step, a thread is cut to securely guide the screw in the pedicle. The previously measured screw (length and diameter) is then screwed in over the guide wire. The result is checked biplanarly. The procedure is then complete. The navigation-assisted route

The patient is positioned on their stomach. The skin incision is marked at skin level. The length of the skin incision is limited using the image intensifier, i.e., only as much skin is cut as necessary. This is essential for wound healing and to reduce the risk of infection during and after the operation. The preparation then takes place, including the insertion and secure fixation of the navigation holder to a spinous process. The site is then covered and the navigation scan is performed using CT. The data is transferred from the system itself to the navigation unit, and a virtual 3D image is generated. The instruments required for navigation are then imported into the system and checked to ensure that the accuracy matches the virtual and real world. A channel for the guide wire is then drilled using navigation. The so-called navigated drill sleeve is removed while the wire is fixed. The screw length and diameter are then precisely determined using the visualization on the navigation system. The thread cutter is then used to pave the way for the screw. This is then inserted, and the result is checked and documented using an X-ray image intensifier. It should be noted that this is not done for each screw, but rather when all screws have been inserted.

In all spinal surgeries using this system, at the end of the operation, a rod is inserted into the tulips on the left and right sides of the screw head. The rods are usually pre-bent. The rod is then firmly secured in the screw head in the "tulip" using an internal screw, which is tightened with a torque wrench to prevent the rod from shifting under load. This creates a secure force connection to redirect the energy, allowing the fracture, for example, to begin healing.

When surgically treating degenerative diseases using a screw-rod system from behind, the segmental disc must also be partially removed and replaced with a disc replacement to allow the entire system to heal and prevent loosening or fractures. The disc is removed from behind, from the side, or from the front. Loosening or fractures of the screw or rod and pseudoarthrosis of the spacers in the intervertebral disc spaces are repeatedly observed.

The present invention is therefore based on the object of providing a screw for arrangement in a vertebral body of a spine which does not loosen and in which material fracture does not occur.

The object is achieved by a screw for arrangement in a vertebral body of a spinal column having the features of independent claim 1. Advantageous embodiments form the subject matter of the associated subclaims.

The invention comprises a screw for placement in a vertebral body of a spinal column. The screw comprises a screw head, a first threaded section with a trapezoidal thread directly adjacent to the screw head, and a second threaded section with a metric thread or a worm thread adjacent to the first threaded section. The thicker threads are suitable for transmitting larger axial forces before failing by shearing. This ensures that the screw connection does not loosen or cause material fracture due to unfavorable force distribution.

An advantageous aspect provides that the screw further comprises a third threaded section adjoining the second threaded section, with a drill thread for creating a recess in the vertebral body. This eliminates the intraoperative step of pre-drilling, which simultaneously eliminates the time-consuming repositioning of the screw.

It is particularly advantageous if the second threaded section and/or the third threaded section includes at least one opening for the introduction of material for additional fixation of the screw. This allows the screw to be additionally fixed with cement, depending on the diagnosis.

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 inserted through the channel. This wire, which is connected to the cannulated Since the screw serves as a trajectory for the “safe path” through the vertebral body, it is securely positioned in the vertebral body by rotating movements.

According to an advantageous aspect, the screw further comprises a fourth threaded section with a fine pitch thread, located between the first threaded section and the second threaded section. This enables a smooth transition from the trapezoidal thread to the metric thread. It is particularly advantageous if the second threaded section is a worm thread. This allows for rapid insertion of the screw without numerous turns of the screwdriver, saving time and increasing the contact surface inside the vertebral body.

Advantageously, the trapezoidal thread degenerates into a buttress thread in the first thread section. The buttress thread can have a narrowing plateau as viewed from the screw head. This opens up another useful modification of the screw: starting with a threadless drill tip, the buttress thread initially acts as a drill thread with little or no plateau, and then exhibits an increasingly broader plateau toward the screw head.

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

It has proven advantageous if the rod comprises a circular section with an inner core with a strength fi and a sheath arranged around the core with a strength f2, a coupling for a device for arranging the rod, and a conical end section arranged opposite the coupling for guiding the rod through the screw head. This ensures that occurring force peaks are absorbed by the outer core and the inner core provides the necessary bending strength at maximum dynamics of the rod, which overall ensures a longer service life of the rod and prevents the repeatedly observed fractures of the rods. be avoided in order to ultimately save the patient and the health insurance system from further operations.

An advantageous aspect is that the strength fi is greater than the strength f2. This allows for an optimal distribution of tensile and compressive forces.

Furthermore, it is advantageous if the strength f2 is greater than the strength fi. This neutralizes the tensile and compressive forces in zones where fatigue fractures regularly occur, preventing fatigue zones from forming and thus preventing fractures.

It is advantageous if the rod exhibits torsion. This minimizes the peak shear forces and bending moments to such an extent that the risk of the rod breaking is eliminated.

It has proven advantageous if the torsion is directed to the left and/or right. This ensures optimal distribution of the individual force vectors.

An advantageous aspect of the rod is its rectangular base section. This allows for optimal grip of the rod between the screws and very high strength to withstand external forces caused by movement.

Furthermore, it is advantageous if the rod includes a section with a circular base. Such a rod provides the screw-rod system with the necessary dynamic strength under load.

The invention is explained in more detail below with reference to drawings.

They show:

Fig. 1 is a schematic representation of a screw according to the invention with a trapezoidal thread, a metric thread and a drill bit;

Fig. 2 is a schematic representation of a screw according to the invention with a worm thread, a metric thread and a drill bit, Fig. 3 is a schematic representation of a screw according to the invention with a worm thread, a metric thread, a drill bit and a fine thread in the central area;

Fig. 4 is a schematic representation of a screw according to the invention with a trapezoidal thread, a metric thread, a drill bit and a fine thread in the central area;

Fig. 5 is 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 is a schematic representation of a bar and the corresponding cross-section through the layers of different strength;

Fig. 7a is a schematic representation of a conventional screw with a uniform thread over the entire length of the screw;

Fig. 7b is a schematic representation of a screw according to the invention with an increasingly narrow trapezoidal thread which has degenerated into a saw thread, a metric thread section and a drill bit;

Fig. 7c is a schematic representation of a screw according to the invention with a buttress thread in the first thread section, which smoothly transitions into the third thread section up to the thread-free drill tip; and

Fig. 8 schematic representation of a twisted rod with a rectangular base.

Fig. 1 shows a schematic representation of a screw 1 according to the invention for arrangement in a vertebral body of a spinal column. The screw 1 comprises 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. The trapezoidal thread is known from machine and tool construction. Trapezoidal threads are used to transmit movements and forces, for example on lead screws in Lathes or screw presses. The larger pitch is advantageous here. The thicker threads are capable of transmitting greater axial forces before failing due to shearing. In typical applications, the moderate self-locking effect due to the larger pitch is tolerable or insignificant. Furthermore, there is a larger contact surface and a lower pitch. This eliminates the knife-edge effect. Furthermore, the larger contact surface reduces the pressure on the bone matrix, thus potentially reducing or even dissipating the introduced energy to an imperceptible oscillation.

Furthermore, the screw 1 comprises a third threaded section 11 adjoining the second threaded section 7, 9, with a drilling thread for creating a recess in the vertebral body. It is also conceivable that the third threaded section 11 comprises a thread with thread-cutting properties.

Fig. 2 shows a schematic representation of a screw 1 according to the invention. 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. In this exemplary embodiment, the second threaded portion is a worm thread 9.

Fig. 3 shows a schematic representation of a screw 1 according to the invention. In this exemplary 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.

Fig. 4 shows a schematic representation of a screw 1 according to the invention, comprising an axial channel 5 for inserting a wire, extending from the screw head 3 in the direction of the third threaded portion 11. For example, the screw head 3 comprises a screw head drive with a hexagon (hexagon socket).

Fig. 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 is in each case engaged on the screw head 3 of the screw 1 The rod 20 comprises a circular section 23 with an internal core 24 having a strength fi and a sheath 25 having a strength f2 arranged around the core 24, 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.

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

Accordingly, a reverse variant is also useful, i.e., the strength f2 is greater than the strength fi. In this case, the elastic dynamic core 24 would be located inside, and the shell 25 would be made of solid, stiffer steel to neutralize the tensile and compressive forces in zones prone to fatigue fractures, preventing fatigue zones from forming and thus preventing fractures.

7a - c show embodiments of the screw 1. Fig. 7a shows a conventional screw 1 in which a metric buttress 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 which has been modified to a buttress thread. The adjoining second thread section has, for example, a metric thread or a worm thread. The flank angle of the buttress thread is preferably 10 to 15 degrees, followed by a plateau, as is the case with the trapezoidal thread. After that there is a falling flank similar to the trapezoidal thread, but which can also be steeper. The pitch of the buttress thread is preferably 6 mm. Fig. 7c shows a further modified form of the screw 1 with a buttress thread. The buttress thread initially has a broad plateau at the screw head, which then becomes increasingly narrower along the screw 1, with the pitch of the buttress thread remaining constant over the thread length. The buttress thread gradually transitions into the third thread section 11 with a further reduced plateau, forming a type of drilling thread, until it reaches a thread-free drill tip. Fig. 8 shows a rod 20 exhibiting torsion. The torsion is directed to the left and/or right. The rod 20 comprises a section 26 with a rectangular base.

An embodiment is also conceivable in which the rod 20 comprises a section 26 with a circular base area.

The torsion describes the twisting of the rod 20, which is caused by the effect of a torsional moment.

There are different forms of torsion, including torsion without curvature and torsion with unrestricted curvature (Saint-Venant).

In closed profiles whose products of wall thickness and distance from the axis of rotation are constant across all sides, shear stresses arise during torsion, but no normal stresses in the longitudinal direction, and thus no warping of the cross-section. This is referred to as torsion without warping. A cylindrical tube of constant wall thickness, for example, meets these conditions. This case of torsion is called a Neuber shell.

Pure torsion, also called Saint-Venant torsion, allows for unhindered displacement of cross-sectional points in the longitudinal direction (Z-direction) of the profile. This is also referred to as unhindered warping of the cross-section. The cross-sectional shape perpendicular to the Z-direction remains intact (minor deformations). It is assumed that the cross-sectional warping is independent of the position of the cross-section and can adjust itself freely. A trick is used to twist profiles that do not have a circular cross-section. These cannot be considered Neuber shells. However, such a profile must not be firmly clamped; it must stand freely in space, and the moment is applied on both sides. This ensures that no normal stresses occur along the profile, even though individual points on the profile may shift longitudinally.

Claims

Patent claims 1. A screw (1) for arrangement in a vertebral body of a spinal column, comprising a screw head (3), a first threaded section (6) having a trapezoidal thread directly adjoining the screw head (3), and a second threaded section having a metric thread (7) or a worm thread (9) adjoining the first threaded section (6). 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 making a recess in 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, further comprising a fourth threaded portion (8) with a fine thread arranged between the first threaded portion (6) and the second threaded portion (7, 9). 6. 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. 7. Arrangement comprising at least two screws (1) according to one of claims 1 to 6 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). 8. Arrangement according to claim 7, wherein the rod (20) comprises a circular section (23) with an internal core (24) with a strength fi and a sleeve (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). 9. Arrangement according to claim 8, wherein the strength fi is greater than the strength f2. 10. Arrangement according to claim 8, wherein the strength f2 is greater than the strength fi. 11. Arrangement according to claim 7, wherein the rod (20) has a torsion. 12. Arrangement according to claim 11, wherein the torsion is directed to the left and/or to the right. 13. Arrangement according to claim 11 to 12, wherein the rod (20) comprises a section (26) with a rectangular base area. 14. Arrangement according to claim 11 to 12, wherein the rod (20) comprises a section (26) with a circular base.