DE102013001933A1 - Bone screw for fixation of spine in treatment of e.g. degenerative disc disease, has four threaded outer diameter portions that are parallel to respective threaded core sections in different shapes - Google Patents

Abstract

The bone screw has structured screw head (1) and screw shank (2) consisting of threaded core and threaded outer diameter. The first and third threaded core portions are conical, while second and fourth threaded core portions are cylindrical, such that threaded outer diameter from distal to proximal is subdivided. The first and third threaded outer diameter portions are tapered parallel to respective threaded core sections, while second and fourth threaded outer diameter portion are cylindrical parallel to respective threaded core sections.

Description

Field of the invention

The present invention relates to a bone screw, as used, inter alia, in orthopedics as part of the known bone screw rod systems for the fixation of the spine in various indications (eg degenerative disc disease, spondylolisthesis, traumata, spinal canal stenoses, deformations, bends, tumors, Pseudarthrosis) is used.

Bone screw rod systems are available in static as well as dynamic embodiment. The bone screws serve to anchor the bone screw-rod system in the vertebral body.

Background of the invention

The previously known bone screws are primarily confronted with the following problems:
A major problem of the known bone screws is the prevention of the hypomochlion effect and the maximization of the postoperative hold of the bone screw in the vertebral body.

In both the static and dynamic embodiments of the bone screw-rod systems, the vertebral body is subject to sustained extreme dynamic loading at the point where the screw shaft of the bone screw is inserted in the pedicle.

In this case, forces constantly act perpendicular to the screw axis of the bone screw, wherein the hole in which the bone screw in the pedicle sits (pedicle hole) is anatomically oval and tapered, with an isthmus at the narrowest point (hypomochlion effect). By constantly acting perpendicular to the screw axis of the bone screw forces the pedicle hole is increased over time (long hole). With greatly enlarged pedicle holes can lead to screw loosening and in the worst case to required screw revisions.

In addition, in connection with the optimal retention of the bone screw in the vertebral body must be taken into account that they penetrate through two layers of bone - the hard cortical bone and the soft cancellous bone - and postoperatively there must also provide optimal support.

Another problem of the known bone screws is the creation of a slight Repositionsmöglichkeit at the same time optimal postoperative hold the bone screw.

Another problem of the known bone screws is the most atraumatic possible introduction of the bone screw in the vertebral body. In this context, the minimization of the force required for the surgeon when inserting the bone screw and at the same time reducing the duration of surgery is to be seen.

Another problem is the risk of screw failure and difficult guidance of the bone screw through the two different bone layers, cortical bone and cancellous bone.

Most previously known bone screws have only a single thread on the screw shaft, by which the bone screw is to be anchored in the vertebral body. A single thread is understood to mean a thread with only one thread.

In order to introduce a bone screw with only one thread in the vertebral body, however, it requires in this embodiment of the thread a high number of revolutions and associated with the surgeon also increased force and a longer operation time. To keep the number of revolutions as low as possible, it has already been recognized that it is necessary to design the bone screw with a large pitch. However, this has the disadvantage that such a thread in the relatively thin cortical layer finds only poor grip and so the growth behavior is adversely affected.

Overall, the described disadvantages favor a loosening of the screw, especially with continuous dynamic load. This problem exists in particular in the period in which no or no complete merger has yet occurred.

When the bone screw with only one thread is therefore the risk of early damage and also a high overall risk of revision.

This problem was also recognized early, which is why in EP 0 374 088 also a multi-course external thread has been mentioned and now belongs to the prior art.

A multi-course external thread, as it is used in a variety of marketed bone screws since then, has the advantage that the guidance of the bone screw is facilitated during insertion, while reducing the number of turns to complete screwing the bone screw, which at the same time Strength of the surgeon minimized and the operating time is shortened. At the same time, the thread, compared to the design as a single thread a significantly better grip in the cortex, which also positively influences the growth behavior.

The execution of the external thread as a multiple thread creates an overall external thread with closely spaced thread flanks. However, such external thread finds only limited support in the soft, spongy, interspersed with cavities tissue of cancellous bone.

Although the design of the screw shaft as a multiple thread thus brings advantages in terms of attachment of the bone screw in the cortex and pedicle, but by the construction of the screw shaft, there are disadvantages in terms of attachment in cancellous bone.

Out DE 2807364A1 Finally, a bone screw is known whose external thread is divided into two sections: a distal, in which the distance of the thread flanks is greater than in the proximal and thus the pitch of the two threads is different, wherein the thread core is cylindrical.

These different thread pitches are perhaps helpful for the application of a fracture, especially in tubular bones, but not useful for the application of the bone screw according to the invention in vertebral bodies. By different thread pitches the thread has different feeds in its course at the same speed, whereby depending on the arrangement of the two threads to each other, the thread with the larger pitch pushes the thread with the smaller pitch when screwing in front of him, or pulls behind him. This and because threads are cut with different gradients in the bone or rather pressed or pulled, the hold of the screw is greatly reduced in the bone tissue.

In this solution, however, other problems, such as, for example, a simplified Repositionsmöglichkeit and the reduction of the screw fracture were disregarded.

In the previously known bone screws, there are none that combines solutions for all the aforementioned problems in itself.

Summary of the invention

The present invention now united by their protection claim 1 and the dependent claims features the solution to all the above problems and results in comparison to the previously known bone screws to a significant further reduction of Hypomochlioneffekts and an increase in the maintenance of the bone screw in the vertebral body.

The invention will be explained below with reference to the illustrations of some embodiments.

It shows:

1 : Bone screw in top view;

2 : Bone screw in cross section;

3 : Bone screw after insertion into the bone

1 shows the bone screw consisting of the structured screw head ( 1 ) and the here in distal, facing away from the user direction screw shank ( 2 ). The screw shaft can be subdivided from distal to proximal into two different thread forming sections. The first thread forming section ( 3 ) is zweigängig, the second thread forming section ( 4 ) is four-shaped. The individual threads of the two-start thread forming section start each offset by 180 °. The two additional threads of the four-start thread forming section start each offset by 90 ° to the two threads of the two-start thread forming section.

2 shows the bone screw in cross section and the special interaction of the thread core ( 5 ) with the outer thread diameter ( 6 ). The thread core is divided into four thread core sections, viewed distally to proximally, which merge into one another, whereby the first thread core section (FIG. 7 ) conical, the second threaded core section ( 8th ) cylindrical, the third threaded core section ( 9 ) conical and the fourth thread core section ( 10 ) is cylindrical and the outer thread diameter is divided into four outer thread diameter sections, wherein the first outer thread diameter section ( 11 ) conically parallel to the first thread core portion, the second thread outer diameter portion ( 12 ) Cylindrical parallel to the second thread core portion, the third outer thread diameter section ( 13 ) conically parallel to the third thread core portion and the fourth thread outer diameter portion ( 14 ) is cylindrical parallel to the fourth thread core section.

3 shows the bone screw after complete screwing into the vertebral body with the cylindrical fourth threaded core section ( 10 ) and the four-pass thread forming section ( 4 ) in the cortical bone and in the pedicle and the double-flighted tapping section ( 3 ) in cancellous bone.

The bone screw has several beneficial effects:
The special course of the thread core and the outer thread diameter in conjunction with the two thread form sections allow improved Repositionsmöglichkeit with maximum support, optimal screw guide, reduced operating time and atraumatic insertion, at the same time the risk of bolt breakage is significantly reduced.

The Repositionsmöglichkeit is significantly improved in comparison to the previously known bone screws, characterized in that the fourth threaded core portion and the fourth thread outer diameter portion is cylindrical, instead of conical as in the known bone screws. As a result, more guidance is available to the bone screw during repositioning, which ensures the secure seating of the screw in the bone even after repositioning.

In addition, the postoperative support is considerably improved by the combination of the specially designed threaded core and the outer thread diameter of the bone screw according to the invention with the two adapted to the particular circumstances of the cortical bone and cancellous bone thread forming sections.

The diameters of the thread core and of the thread outer diameter run parallel to one another over the entire length of the screw shank. The largest diameter at the proximal and the smallest diameter at the distal end of the screw shaft.

The screw shaft is divided into two sections in the region of the second thread forming section with respect to the thread core and the thread outer diameter. In a first section, which extends from the distal starting point of the second thread forming section or from the proximal end point of the first thread forming section in the proximal direction, the thread core and thread outer diameter geometries are designed conically. In a second, subsequent to this section, which extends in the proximal direction to the end of the second thread-forming portion, the two geometries are cylindrical.

In the proximal region of the screw shaft, which connects directly to the screw head and is subjected to particularly high loads by the dynamic forces acting on it, the screw shaft accordingly has a relatively large thread core diameter. The forces acting on this area are thus distributed over a large area, which considerably increases the postoperative hold and minimizes the risk of a screw breakage.

Due to the configuration of the screw shaft such that the distal section of the outer thread diameter has a four-start external thread and a proximal section of the screw shaft, the external thread meets the requirements of the various bone structures of the vertebral body.

In this case, the four-start external thread (that is, the second thread-forming section) in the proximal portion of the screw shaft is designed to be adapted to the cortical layer of the bone or the length of the pedicle in the length of the section of the screw shaft. The double-threaded external thread (i.e., the first thread forming portion) in the distal portion of the screw shaft is accordingly designed in its length so that it is in the spongy part of the vertebra after screwing in the screw.

This ensures that the bone screw finds the best possible support both in the cortical and spongious bone tissue.

In this case, the external thread of the bone screw can be performed both self-tapping and not self-tapping. The screw tip can be made both pointed and dull.

The individual threads have a large pitch in relation to bone screws with a single thread.

This causes a quick screwing in the screw with low number of revolutions, which leads to a shortening of the operation time and reduces the force required by the surgeon.

At the same time, the configuration of the external thread as a four-pass external thread in the proximal region leads, in addition to the large thread pitch of the individual threads and the resulting advantages, the spacing of the individual thread flanks to be tight overall. This creates a large area of contact with the bone tissue of the cortex and the pedicle, which causes the bone screw in the thin cortical layer and in the pedicle finds a firm grip, which in addition to the Encouraged growth behavior and the osseointegration time is shortened.

The fact that the screw shaft is equipped only in the area with a four-course external thread, which remains in the cortical layer and in the pedicle of the vertebral body after complete screwing, causes the two newly added threads are introduced very late in the cortical bone layer. As a result, the impact on the bone and the injury of the bone tissue is kept as low as possible, which in turn positively influences the growth behavior.

The hold of the external thread in the cortical layer is promoted by the configuration of the screw shaft with respect to the Gewindekern- and the outer thread diameter of the screw shaft.

Overall, the configuration of the proximal portion of the screw shaft leads to a higher primary stability of the bone screw in the region of the pedicle. This greatly reduces the risk of early complications.

In contrast to the proximal portion of the screw shaft, the screw shaft in the distal section, which is located in the cancellous bone tissue after screwing in the bone screw, has only two external threads. The thread pitch corresponds to the thread pitch in the proximal section.

This embodiment of the external thread is the soft, sponge-like, interspersed with cavities spongiosa tissue fair and here also achieves a firm grip.

Overall, the execution of the screw shaft in the manner shown achieves the best possible retention of the bone screw both in the cortical bone and in the cancellous bone tissue. As a result, the risk of screw loosening, especially in the period between the introduction of the bone screw and the complete merger, greatly reduced and minimizes the risk of a revision.

In summary, it can be stated that the conical configuration of the first threaded core section and the thread outer diameter allows for atraumatic insertion, the cylindrical configuration of the second threaded core section and the thread outer diameter improves the screw guide, the conical configuration of the third threaded core section and the thread outer diameter also promotes atraumatic as possible introduction and the cylindrical configuration of the fourth thread core portion and the outer thread diameter allows, as already shown, an improved reduction of the bone screw.

The first thread-forming section in the proximal region of the bone screw also promotes as atraumatic an insertion of the bone screw through the hard cancellous bone with little effort.

In other embodiments, the screw shaft can be cannulated and / or fenestrated several times, cannulation along the longitudinal axis of the screw shaft serving as a guided introduction of the bone screw. The Fenestrierungen can be designed as running perpendicular to the longitudinal axis of the screw shaft elongated holes, which follow in its formation of the thread pitch and extend in the thread root in the region of the first thread form portion. These fenestrations serve to apply bone cement through the screw shaft in order to improve the retention of the bone screw, especially in osteoporotic bone conditions.

The bone screw can be either cannulated or fenestrated or performed as a combination of both cannulated and fenestrated.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0374088 [0014]
• DE 2807364 A1 [0018]

Claims (7)

Bone screw consisting of a structured screw head ( 1 ) and a subsequent thereto in distal, facing away from the user direction screw shank ( 2 ), which consists of a threaded core ( 5 ) and a thread outer diameter ( 6 ), wherein the screw shaft in the distal, user-facing region, designed as a cancellous bone thread and in the proximal, the user-facing area as a cortical thread is characterized in that the threaded core is viewed from distal to proximal divided into four threaded core sections, which merge into one another the first thread core section ( 7 ) conical, the second threaded core section ( 8th ) cylindrical, the third threaded core section ( 9 ) conical and the fourth thread core section ( 10 ) is cylindrical and the outer diameter of the thread from distal to proximal view is also divided into four outer thread diameter sections, wherein the first outer thread diameter section ( 11 ) runs conically parallel to the first thread core portion, the second thread outer diameter portion ( 12 ) is cylindrical parallel to the second thread core portion, the third thread outer diameter portion ( 13 ) runs conically parallel to the third threaded core section and the fourth threaded outer diameter section ( 14 ) is cylindrical parallel to the fourth thread core section. Bone screw according to claim 1, characterized in that the screw shaft consists of two thread-forming sections, wherein the distal thread-forming section facing away from the screw head (FIG. 3 ) a zweigängiges and the proximal, the screw head facing thread forming section ( 4 ) has a four-start thread. Bone screw according to claim 1, characterized in that the individual threads of the double-threaded section start each offset by 180 ° to each other and start the two additional threads of the four-start thread forming portion offset by 90 ° to the two threads of the double-threaded section. Bone screw according to claim 1, characterized in that the screw shaft in its length and in the design of the two thread form sections is designed so that after screwing into the vertebral body of the thread forming section with the four-start thread in the cortical bone and in the pedicle and the threaded portion with the zweigängigen thread in the area of cancellous bone remains. Bone screw according to one of the preceding claims, characterized in that the threaded core has a diameter which is greater in the proximal region than in the distal region. Bone screw according to claim 1, characterized in that the bone screw can be made self-tapping or non-self-tapping. Bone screw according to one of the preceding claims, characterized in that the bone screw can be cannulated and / or fenestrated several times, wherein the cannulation along the longitudinal axis of the screw shaft can extend and the fenestrations as perpendicular to the longitudinal axis of the screw shaft extending slots, which in their formation of the Follow thread pitch and run in the thread root in the first thread forming section, can be formed.

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