BONE TRANSPLANT

Description

[0001] The invention relates to a tendon anchor for refixing tendons with a cylindrical or frustoconical screw shaft and a screw head for introducing a tightening torque, wherein both the screw shaft and the screw head are provided with an external thread, according to claim 1.

[0002] Screws for surgical procedures are conventionally manufactured from metal or metal alloys. Screws made of resorbable materials, such as polyglycolide and polylactide, are also available. However, these types of screws have several disadvantages in surgical practice. For example, metal or metal alloy screws must be removed via a second operation and are subject to corrosion. This increases costs for the healthcare system and the health risks for each patient due to the need for further surgery. While all resorbable materials are eventually dissolved, their degradation can cause significant osteolysis in the surrounding bone, leading to displacement of the recipient bone from the screw. Osteolytic processes can result in loosening and ultimately failure of the implant.Bioresorption, in turn, is a process that takes years and can be accompanied by inflammation and pain. US 2017 / 0231674 A1 describes a screw for fixing bone fractures. US 2018 / 0028236 A1 describes a device for stabilizing vertebrae.

[0003] Screws made of allogeneic bone (femur and tibial cortex), such as those known from the applicant's PCT / EP2018 / 071619, which is considered the closest prior art for the present invention, offer several advantages. They become vascularized and remodeled without rejection and are suitable, for example, for osteosynthesis procedures where small bone fragments need to be joined, since the screw creates a load-bearing bone bridge during the operation. This bridge improves from the time of surgery onward as it remodels and becomes fully integrated into the living bone. Screws with a diameter of 3-4 mm, for example, become completely vascularized within two months. These bone screws can therefore also be referred to as bone grafts.In contrast, metal screws tend to hinder bone regeneration; in particular, their mere presence reduces the surface area available for bone healing. Biodegradable materials, on the other hand, reach their maximum strength at the time of surgery. They suffer from the same disadvantages as metal screws, and their strength decreases rapidly once the biodegradation process begins, thus weakening the bone area being treated, at least temporarily.

[0004] These disadvantages are particularly evident in the production of tendon anchors. Tendon anchors are used to reattach tendons to bones. A conventionally manufactured metal screw is inserted into a pre-drilled hole using an insertion tool, securing the tendon end through friction between the pre-drilled hole and the inserted screw. Using a tendon anchor made of either metal or plastic reduces the surface area available for tendon ingrowth and causes disruptive artifacts in subsequent imaging procedures such as MRI and CT scans.

[0005] These disadvantages could be avoided by using bone grafts made from allogeneic bone, as the surface of the bone graft also allows the tendon to grow, thus significantly increasing the surface area available for tendon ingrowth. However, when using screws made from allogeneic bone in surgical practice, it must be noted that they differ considerably from metal screws in terms of insertion resistance and strength. Since they are derived from allogeneic human cortical bone, it cannot be expected that knowledge of thread shapes, insertion resistance, rotational stability, or strength, as known from metal screws, can be readily transferred.

[0006] Another challenge when using bone grafts as tendon anchors lies in the stresses exerted by the tendon on the bone graft. Despite these stresses, the bone graft must ensure a secure hold for the tendon and exhibit high rotational stability, meaning it must be highly resistant to unwanted rotation and thus prevent the screw from loosening. By using the smallest possible thread pitch, and therefore as many turns of the same thread per axial unit length as possible, the surface area for tendon ingrowth could be increased, leading to better tendon retention. Furthermore, this approach can enhance rotational stability.On the other hand, the smaller the thread pitch, the greater the stress on the tendon tissue when screwing in the bone graft. This is because the tendon's connective tissue fibers tend to tear in this case, and the tendon loses strength at its attachment points to the bone. It would be advantageous, instead, to minimize stress on the tendon tissue between the tendon anchor and the surrounding bone during insertion to avoid damage. Furthermore, the number of turns required to insert the screw increases with a lower thread pitch. This can prove problematic in surgical practice, especially in hard-to-reach areas. This phenomenon is referred to as the screw's insertion behavior, with good insertion behavior characterized by quick and easy screw insertion.

[0007] The aim of the invention is therefore to optimize bone grafts made from cortical bone for use as tendon anchors. In particular, they should ensure good tendon fixation and be quick to implant.

[0008] These objectives are achieved by the features of claim 1. Claim 1 relates to a tendon anchor for refixing tendons, comprising a cylindrical or frustoconical screw shank and a screw head for applying a tightening torque, wherein both the screw shank and the screw head are provided with an external thread.According to the invention, it is proposed that the tendon anchor is manufactured as a drill-free bone graft from a cortical bone material, wherein at least the external thread of the screw shaft is a multi-start thread and axial recesses are milled into the outer shell of the screw head, which open into the proximal end face of the screw head, wherein the length of the bone graft is at least three times the diameter of the bone graft in the case of a cylindrical design, and at least three times the largest diameter of the bone graft in the case of a frustoconical design of the bone graft.

[0009] In multi-start threads, at least two threads are wound parallel around the screw shank. This increases the pitch of each thread and thus the screw's thread stroke. The space not required by one thread is filled by a second thread or further threads. This allows a specific stroke to be achieved with fewer turns or in a shorter time, improving insertion and protecting the tendon tissue. Furthermore, the use of multiple threads provides a large surface area for tendon tissue to grow onto the tendon anchor, improving tendon fixation and increasing rotational stability. The bone graft according to the invention thus ensures good tendon fixation and can be implanted quickly.

[0010] According to one possible embodiment, it is proposed that the multi-start thread comprises two threads, each with a pitch between 0.8 mm and 3 mm. For this purpose, the bone graft according to the invention is designed to be cone-shaped, thus having a cylindrical section proximally that transitions distally into a conical section. The ratio of thread depth t to thread outer diameter D is preferably between 0.10 and 0.25.

[0011] Preferably, the thread is a pointed thread. The pointed thread not only optimizes the surface area available for tendon tissue growth at the tendon anchor, but also offers practical advantages in the production of the bone graft according to the invention, since the pointed thread can be arranged along the screw circumference with less space requirements.

[0012] According to the invention, the screw head is also provided with an external thread and thus contributes to the strength of the tendon fixation. In particular, the screw head can also be screwed into the bone without having to be cut off. To apply an insertion torque, it is preferably proposed that at least two recesses, arranged distributed around the screw head axis, extending axially in the direction of the screw head axis and opening into the end face of the free end of the screw head, are provided for receiving an insertion tool. The recesses are each formed by lateral surfaces that extend from an outer surface enclosing the external thread of the screw head in the direction of the screw head axis and merge into one another in a surface section near the axis. The external thread of the screw head is thus interrupted only by the axially extending recesses provided for applying an insertion torque.The bone graft is otherwise drill-free and consists entirely of bone material. Particularly in the area of ​​the screw head axis, bone material remains; only axial recesses are milled into the outer surface of the screw head, opening into the proximal end face of the screw head. Axial extensions of an insertion tool can be inserted axially into these recesses from the end face. The insertion torque is then applied to the lateral surfaces of the recesses, thereby being introduced in a kinematically favorable manner in the outer circumferential region of the screw head.

[0013] Furthermore, to increase the insertion torque, it is proposed that four recesses be provided, arranged symmetrically around the screw head axis. During the axial insertion of the insertion tool's projections, with four recesses arranged symmetrically around the screw head axis, the remaining portions of the screw head between the recesses are gripped and effectively "clamped" by the projections of the insertion tool, assuming typical bone screw diameters in the single-digit millimeter range. This prevents bone fragmentation. The proposed recesses also allow the outer diameter of the screw head to align with the outer diameter of the screw shaft and the outer diameter of the insertion tool, thus enabling new surgical applications such as endoscopic or arthroscopic use of the screw.

[0014] The bone graft can be cylindrical or frustoconical. The outer thread diameter of the bone graft is preferably between 7.0 mm and 4.5 mm. According to the invention, the length of the bone graft is at least three times its diameter in the case of a cylindrical design and at least three times its largest diameter in the case of a frustoconical design.

[0015] The invention is explained in more detail below with reference to exemplary embodiments and the accompanying figures. These figures show... Fig. 1 a thread section of a pointed thread to illustrate the relevant thread parameters, and the Fig. 2 an embodiment of a bone graft according to the invention, wherein the Fig. 2a the bone graft according to the invention is shown from above, so that the screw head is visible, Fig. 2b und 2c show the bone graft according to the invention viewed from the side, and the Fig. 2d a perspective view of a bone graft according to the invention is shown.

[0016] Based on the Fig. 1 First, the key thread parameters are explained. These are the thread's outer diameter D and its core diameter d, the difference between which determines the thread depth t, and the thread pitch s, where the pitch s is the distance traveled by one revolution of the screw, i.e., the distance between two thread crests of the same thread, which is specified in millimeters for metric threads. The reciprocal of the pitch s generally corresponds to the number of turns of a thread per unit length, i.e., for metric threads, the number of turns per millimeter. Another parameter is the flank angle γ, which is again derived from the thread depth t and the pitch s.

[0017] The larger the flank angle γ at a constant thread depth t, the fewer turns per mm are available for tendon fixation. This reduces the strength of the tendon fixation and the rotational stability of the screw. On the other hand, the thread stroke is increased, thus improving the insertion behavior for surgical practice. Conversely, the smaller the thread pitch, the greater the stress on the tendon tissue when screwing in the bone graft. The features of the invention resolve these conflicting requirements for optimizing the tendon anchor.

[0018] This refers to the Fig. 2 The application describes an embodiment of a bone graft made from cortical bone material for surgical use as a tendon anchor. The bone graft has a cylindrical screw shaft 1 with an external thread and a screw head 2 for applying an insertion torque. This screw head 2 also integrates with the bone and does not need to be cut away like conventional screw heads. The screw head 2 is also externally threaded and has an outer surface enclosing the external thread of the screw head, which is rotationally symmetrical about the screw head axis S. The screw head 2 further has four recesses 3, distributed around the screw head axis S, extending axially in the direction of the screw head axis S and opening into the end face of the free end of the screw head 2, for receiving an insertion tool.The axially extending recesses 3 are each formed by side surfaces 4 extending from the outer surface in the direction of the screw head axis S, which merge into each other in a surface section near the axis (see in particular . Fig. 2a ). The side surfaces 4 of the recesses 3 extending from the outer surface towards the screw head axis S can be convex, and the surface section near the axis can be concave.

[0019] How the Fig. 2a-2d As can be seen, in the illustrated embodiment, the external thread extends over both the screw shank 1 and the screw head 2 with unchanged thread parameters. The screw head 2 thus contributes to the strength of the tendon fixation. In particular, the screw head 2 can also be screwed into the bone without having to be cut off. The external thread is only interrupted by the axially extending recesses 3 provided for introducing a tightening torque. In the area of ​​the screw head axis S, bone material with a core diameter d, as shown in the Fig. 2a This is evident because only axial recesses 3 are milled into the outer surface of the screw head 2, opening into the proximal end face of the screw head 2. Axial extensions of an insertion tool can be inserted axially into these recesses 3 from the end face. The insertion torque is subsequently applied to the side surfaces 4 of the recesses 3.

[0020] According to the invention, the external thread is designed as a multi-start thread. According to the embodiment of Fig. 2 The external thread of the screw shank 1 and the screw head 2 is designed as a multi-start thread with two threads, each with a pitch of 2.5 mm. The overall length of the screw shown is 15 mm with a thread outer diameter D of 5 mm. The axial length of the recesses 3 of the screw head 2 is 4.5 mm, with the remaining core diameter d of the screw head being 1.9 mm.

[0021] By using a multi-start thread, the pitch s of each thread turn, and thus the thread stroke of the screw, can be increased. In this way, a specific stroke can be achieved with fewer turns or in a shorter time, thereby improving insertion and protecting the tendon tissue. Furthermore, the use of multiple thread turns provides a large surface area for tendon tissue to grow onto the tendon anchor, thus improving tendon fixation and increasing rotational stability. The bone graft according to the invention therefore ensures good tendon fixation and can be implanted quickly.

Claims

1. Tendon anchor for tendon refixation, comprising a cylindrical or frustoconical screw shaft (1) and a screw head (2) for applying an insertion torque, wherein both the screw shaft (1) and the screw head (2) are provided with an external thread, wherein the tendon anchor is manufactured as a drill-free bone graft from a cortical bone material, wherein at least the external thread of the screw shaft is a multi-start thread and axial recesses (3) are milled into the outer surface of the screw head, which open into the proximal end face of the screw head, wherein the length of the bone graft is at least three times the diameter of the bone graft in the case of a cylindrical design, and at least three times the largest diameter of the bone graft in the case of a frustoconical design of the bone graft.

2. Tendon anchor according to claim 1, characterized in that the multi-start thread comprises two thread turns, each having a pitch (s) between 0.8 mm and 3 mm.

3. Tendon anchor according to claim 1 or 2, characterized in that the thread is a pointed thread.

4. Tendon anchor according to one of claims 1 to 3, characterized in that at least two recesses (3) for receiving a screwing-in tool are provided, which recesses (3) are distributed around the screw head axis (S), extend axially in the direction of the screw head axis (S) and open into the end face of the free end of the screw head (2), wherein the recesses (3) are each formed by lateral surfaces (4) which extend from an outer lateral surface enveloping the external thread of the screw head in the direction of the screw head axis (S) and merge into one another in a surface section close to the axis.

5. Tendon anchor according to claim 4, characterized in that four recesses (3) distributed symmetrically around the screw head axis (S) are provided.