Device for variable fixation of bone fragments

Abstract

Device for bone fixation with a head portion (5), a tapering front portion (8) and a shaft (3) between said head portion (5) and said tapering front portion (8), said shaft (3) having a distal portion (3a) adjacent to said tapering front portion (8) and a proximal portion (3b) adjacent to said head portion (5); whereby said distal portion (3a) being provided with a thread and having a constant outer diameter D.sub.A and an inner core diameter D.sub.I; at least the proximal portion (3b) of said shaft (3) has a core (2) consisting of a biologically non-degradable material with degradation rate BND and having a diameter d≤D.sub.I; a sleeve (9) surrounding said core and consisting of a biologically degradable material with degradation rate BD, whereby BD>BND and said sleeve (9) being fixed to said core in a non-rotatable manner.

Claims

1. A device for bone fixation, comprising: a head portion, a front portion, and a shaft between the head portion and the front portion, the shaft comprising a distal portion, a middle portion and a proximal portion, wherein the distal portion of the shaft comprises a thread and having a maximum outer diameter D.sub.A and an inner core diameter D.sub.I, wherein the proximal portion of the shaft comprises a core comprising a biologically non-degradable material with a degradation rate BND and a diameter d; and a sleeve surrounding the core, the sleeve comprising a biologically degradable material with a degradation rate BD, the sleeve having a diameter Ds≤D.sub.A, whereby BD>BND.

2. The device for bone fixation according to claim 1, wherein the shaft comprises forward cutting elements.

3. The device for bone fixation according to claim 1, wherein the shaft comprises backward cutting elements.

4. The device for bone fixation according to claim 1, wherein the front portion is tapered.

5. The device for bone fixation according to claim 1, wherein the front portion comprises cutting flutes.

6. The device for bone fixation according to claim 1, wherein the sleeve comprises macrostructuring.

7. The device for bone fixation according to claim 1, wherein the device is self-tapping.

8. The device for bone fixation according to claim 1, wherein D.sub.A is between 1.5 mm and 8.0 mm.

9. The device for bone fixation according to claim 1, wherein a length of the device is between 20 mm and 80 mm.

10. The device for bone fixation according to claim 1, wherein the sleeve comprises a short term bioresorbable polymer.

11. The device for bone fixation according to claim 1, wherein the shaft comprises a long term bioresorbable polymer or metal.

12. The device for bone fixation according to claim 1, wherein the device is configured to be used with a plate.

13. The device for bone fixation according to claim 1, wherein the device is configured such that an allowed maximum relative displacement between the core and an inner side of a bone hole, once a degradation of the sleeve is completed, is between 5% and 10% of D.sub.A.

14. A device for bone fixation, comprising: a head portion, a tapered portion, and a shaft between the head portion and the tapered portion, the shaft comprising a distal portion, a middle portion and a proximal portion, wherein the distal portion of the shaft comprises a thread and having a maximum outer diameter D.sub.A and an inner core diameter D.sub.I, wherein the proximal portion of the shaft comprises a core comprising a biologically non-degradable material with a degradation rate BND and a diameter d; and a sleeve surrounding the core, the sleeve comprising a biologically degradable material with a degradation rate BD, the sleeve having a diameter Ds≤D.sub.A, whereby BD>BND.

15. The device for bone fixation according to claim 14, wherein the thread of the distal portion is disconnected due to flutes.

16. The device for bone fixation according to claim 14, wherein the core has a constant outer diameter.

17. The device for bone fixation according to claim 14, wherein the sleeve has a constant outer diameter.

18. The device for bone fixation according to claim 14, wherein the sleeve comprises an outer thread or partial threads.

19. A device for bone fixation, comprising: a head portion and a shaft, the shaft comprising a distal portion, a middle portion and a proximal portion, wherein the distal portion of the shaft comprises a thread and having a maximum outer diameter D.sub.A and an inner core diameter D.sub.I, wherein the proximal portion of the shaft comprises a core comprising a biologically non-degradable material with a degradation rate BND and a diameter d; and a sleeve surrounding the core, the sleeve comprising a biologically degradable material with a degradation rate BD, the sleeve having a diameter Ds≤D.sub.A, whereby BD>BND.

20. The device for bone fixation according to claim 19, further comprising cutting elements.

Description

A BRIEF DESCRIPTION OF THE DRAWINGS

(1) Several embodiments of the invention will be described in the following by way of example and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic view of a cortical screw according to the invention;

(3) FIG. 2 shows a perspective view of a cortical screw according to the invention;

(4) FIG. 3 shows a schematic view of another cortical screw according to the invention;

(5) FIG. 4 shows a perspective view of another cortical screw according to the invention;

(6) FIG. 5 shows a schematic view of a cancellous screw according to the invention;

(7) FIGS. 6a and 6b show the behavior of the screw-plate system according to the present invention in the immediate post-operative period;

(8) FIGS. 7a and 7b show the behavior of the screw-plate system according to the present invention once the resorbable portion of the device has lost its mechanical properties.

DETAILED DESCRIPTION OF THE INVENTION

(9) The present invention generally concerns a surgical fixation device and methods used to provide bone fragments or bones having a stability such that the physiological healing process can take place when used in connection with plates featuring a locking mechanism. The device for bone fixation preferably includes a tapering front portion and a shaft between said head portion and said tapering front portion. The shaft has a distal portion being provided with a thread with a constant outer diameter and a proximal portion having a biologically non-degradable core and a biologically degradable sleeve surrounding said core. The biologically degradable material of the sleeve has an outer surface directly engaging the bone to initially fix the bone fragments according to the possible anatomical bone reconstruction. For a given load or moment, the relative displacement of the bone fragments gradually increases with time upon degradation of the sleeve.

(10) There are several possibilities to achieve fixation of the sleeve to the core in a non-rotatable manner, e.g.: The surface of the proximal portion 3b features cross sectional radial extrusions, longitudinally and discontinuously extending along the surface, gripping into the degradable material; The surface of the proximal portion 3b features cross sectional radial grooves, longitudinally and discontinuously extending along the surface, where the degradable material can grip into the non-degradable material; The surface of the proximal portion 3b features a rugosity such that it prevents the sleeve from rotating around the screw axis during screw insertion; A biocompatible adhesive glues the sleeve to the surface of the proximal portion 3b; The manufacturing processes imply the rise of a sleeve controlled radial shrinking.

(11) The invention and the additional embodiments of the invention are explained below with reference to the figures of several embodiments:

(12) FIG. 1 shows a device for bone fixation in a form of a cortical screw with a head portion 5, a tapering front portion 8 and a shaft 3 between the head portion 5 and the tapering front portion 8. The shaft 3 has a distal portion 3a adjacent to the tapering front portion 8 and a proximal portion 3b adjacent to said head portion 5. The distal portion 3a has a threaded surface with a constant outer diameter D.sub.A and an inner core diameter D.sub.I. The proximal portion 3b of the shaft 3 comprises a biologically non-degradable core 2 with the diameter d being smaller than the inner core diameter D.sub.I of the distal portion 3a and a biologically degradable sleeve 9 surrounding the core 2.

(13) FIG. 2 shows a perspective view of the device according to the invention. The distal portion 3a as well as the outer surface of the sleeve 9 in proximal portion 3b of the device as shown in FIG. 2 are provided with outer threads.

(14) The front portions of the embodiments shown in FIGS. 1 and 2 may incorporate self-cutting and self-tapping features which allow the screw insertion in the bone without the respective need for predrilling and for tapping. The distal portion 3a of the shaft 3 is configured to fix the screw in cortical bone in this specific case. The outer diameter D.sub.A of the distal portion 3a ranges between, but not limited to 1.5 mm and 8.0 mm. More specifically the device for bone fixation may have an outer diameter D.sub.A of approximately 1.5, 2.0, 2.4, 2.7, 3.5, 4.0, 4.5, 5.0, 6.5, 7.0, 7.3 and 8 mm). The device for bone fixation is typically 20 to 80 mm long. The relative length of the distal 3a and proximal 3b portions of the shaft 3 are generally similar but better defined according to the intended use accommodating for different bone sizes and anatomical characteristics so that, when the device for bone fixation is implanted, the distal portion 3a of the shaft 3 is engaging the far (trans) cortex, whereby the proximal portion 3b of the shaft 3 is placed within the near (cis) cortex). The biologically degradable material of the sleeve 9 surrounding the biologically non-degradable core of the proximal portion 3b of the shaft 3 is engaging the near (cis) cortex. The compound device stiffness is sufficient to provide the expected bending resistance under expected loading. At the junction between the biologically degradable material of the proximal portion 3b and the biologically non-degradable material of the distal portion 3a there is a middle portion of the shaft 3 being provided with backward cutting elements (e.g. cutting flutes) allowing easier removal of the device at the end of the treatment.

(15) The distal portion 3a of the shaft 3 is provided with a thread having a constant outer diameter D.sub.A and an inner core diameter D.sub.I. The proximal portion 3b of the shaft 3 comprises a core 2 of either fixed or variable diameter (could be used to restrain the sleeve and avoid rotation) and a sleeve 9 surrounding the core 2. The sleeve 9 is provided with a thread having an outer diameter Ds. According to the embodiments of the FIGS. 1 and 2 the outer diameter Ds of the sleeve 9 is identical to the outer diameter D.sub.A of the thread being provided in the distal portion 3a of the shaft 3.

(16) FIG. 3 shows another device for bone fixation in a form of a cortical screw with a head portion 5, a tapering front portion 8 and a shaft 3. The shaft 3 has a distal portion 3a adjacent to the tapering front portion 8 and a proximal portion 3b adjacent to said head portion 5. The distal portion 3a has a threaded surface with a constant outer diameter D.sub.A and an inner core diameter D.sub.I. The proximal portion 3b of the shaft 3 comprises a core 2 having a diameter d=D.sub.I as well as a sleeve 9 surrounding the core 2. The outer surface of the sleeve 9 is not threaded. At the junction between the biologically degradable material of the sleeve 9 of the proximal portion 3b and the biologically non-degradable material of the distal portion 3a, there is a middle portion of the shaft 3 featuring forward and backward cutting elements respectively allowing removing of bone graft arising through insertion of the device for bone fixation and easier removal of the device at the end of the treatment.

(17) In a further embodiment (not shown in the figures) the outer surface of the sleeve 9 of the proximal portion 3b can be partially threaded.

(18) FIG. 4 shows a perspective view of the device according to the invention in a form of a cortical screw with a head portion 5, a tapering front portion 8 and a shaft 3. The shaft 3 has a distal portion 3a adjacent to the tapering front portion 8 and a proximal portion 3b adjacent to said head portion 5. The distal portion 3a has a threaded surface with a constant outer diameter D.sub.A and an inner core diameter D.sub.I. The proximal portion 3b consists of the sleeve 9 consisting of a biologically degradable material and having a constant outer diameter. The sleeve 9 surrounds the core 2 consisting from a biologically non-degradable material.

(19) FIG. 5 shows a schematic view of a device according to the invention. The cancellous screw as shown in FIG. 5 comprises a head portion 5, a tapering front portion 8 and a shaft 3 between the head portion 5 and the tapering front portion 8. The shaft 3 has a distal portion 3a adjacent to the tapering front portion 8 and a proximal portion 3b adjacent to said head portion 5.

(20) This embodiment is indicated to fix bone fragments characterized by a large amount of cancellous bone surrounded by a shell of cortical bone. This device preferably includes a tapering front portion 8 and a shaft 3 between the head portion 5 and the tapering front portion 8. The tapering front portion features cutting flutes and it is configured to fix the screw in cancellous bone in this specific case. The shaft features a threaded, partially threaded or flat sleeve surface consisting of a biologically degradable material and having a constant outer diameter around the core 2 consisting of the biologically non-degradable material. The sleeve 9 features mechanical properties and degradation rate specifically tailored for cancellous bone fixation. This screw aims decreasing the stress at the bone to medical device interface and progressively redistributing the load transmission between the bone fragments through the sleeve degradation. At the junction between the biologically degradable material of the sleeve 9 and biologically non-degradable material of the distal portion 3a there may be a shaft section featuring a backward cutting element allowing easier removal of the device at the end of the treatment.

(21) FIGS. 6a and 6b show respectively the mechanical behavior of a kit for the treatment of bone fractures comprising a plate and a plurality of devices for bone fixation during the very early stages of secondary fracture healing. In FIG. 6b, a long bone critical size defect and the immediate proximal and distal bone fragments are depicted. In the immediate post-operative period the kit behaves like a traditional system providing enough interfragmentary stability and a theoretical asymmetric fracture callus strain. In this case, there is not or a minor relative displacement between the near (cis) cortex and the screw symmetry axis.

(22) FIGS. 7a and 7b show respectively the mechanical behavior of a kit comprising a plate and a plurality of devices for bone fixation as the biologically degradable sleeves substantially lost their mechanical properties and/or are fully resorbed. The lack of support provided by the biologically degradable part of the device for bone fixation leads to an increased interfragmentary displacement. This is evident in particular in that portion of the callus between the near (cis) cortices of the two bone fragments.

(23) For all the devices for bone fixation the allowed maximum relative displacement between the biologically non-degradable part of the device for bone fixation (core, proximal portion 3b) and the inner side of the bone hole in the near (cis) cortex, once the degradation of the material of the biologically degradable sleeve is completed, ranges preferably between 5 and 10% of the outer diameter D.sub.A of the thread of the distal portion 3a. The thickness of the biologically degradable sleeve and the degradation time of the sleeve can be however different according to the indication for use. The screw flexible length is proportional to the outer diameter of the thread of the distal portion of the device for bone fixation and depends on the baricenter of the applied load.

(24) The provided table 1 below reports examples of 5% and 10% allowed displacements for some screws but it is not intended to be an exhaustive description of all the possible characteristics of the device for bone fixation or limit in any way the scope of the invention. The dimensions of this device depends in fact on its intended use, constituent material and effect of the manufacturing processes.

(25) TABLE-US-00001 TABLE 1 Outer diameter of the thread of the Biologically non- Biologically non- distal portion of Device Sleeve 5% max degradable core 10% max degradable core the device length length displacement diameter displacement diameter 3.5 mm 30 mm 12 mm ±.18 mm 3.15 mm ±.35 mm 2.80 mm 4.5 mm 40 mm 17 mm ±.23 mm 4.05 mm ±.45 mm 3.60 mm

(26) Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the scope of the appended claims.

(27) It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.