NZ520839A

NZ520839A - Device for fixing implants on or in a bone

Info

Publication number: NZ520839A
Application number: NZ520839A
Authority: NZ
Inventors: Jorg Goldhahn; Reto Frei; Jorn Seebeck
Original assignee: Synthes Ag
Priority date: 2000-04-20
Filing date: 2000-04-20
Publication date: 2003-10-31

Abstract

The present invention concerns a fastening element for use in bone, and in particular to a device for fixating implants on or in bone. The device may have a longitudinal axis for anchoring in bone and may comprise a core and at least one wing extending from the core. The wing may have an enlarged peripheral portion adapted to contact bone. A cross-section of the device transverse to the longitudinal axis may have first and second portions, the first portion comprising the core and defining a first cross-sectional area, the second portion comprising the enlarged peripheral portion of the at least one wing and defining a second cross-sectional area, and the second cross-sectional area preferably is greater than the first cross-sectional area. The ratio of the second portion to the first portion preferably being at least about 3:

Description

<div class="application article clearfix" id="description"> 1781/PCT IPONZ <2 JUN2003 19.4.2000 English translation of the International Patent Application No. PCT/CH00/00229 "Device for the fixation of implants on or in a bone" in the name of Synthes AG Chur The invention relates to a device permitting the fixation of implants on, or in, a bone comprising a longitudinal, solid or hollow core (1) with a longitudinal axis (2) and a number of wings (3) extending radially therefrom, the device being contained within an upright, regular cylinder with an outside diameter D = 2S + d and an identical longitudinal axis (2) forming its envelope, d corresponding to the diameter of an optional bore (9) formed in the core (1) and S corresponding to a value between 3 mm and 50 mm. So far, implants have generally been anchored in the bone by means of screws, more recently also by means of monocortical screws anchored in an angularly stable manner. These screws have been designed, as far . as their form is concerned, for use in healthy bone. In fact, the ratio between the cortical screws and the thickness of the corticalis has been optimised for enabling the best possible transmission of the occurring strains. These angularly stable screws no longer have the function of tension screws in the classical sense of the term but rather that of bolts. However, the screws currently in use for these purposes have dimensions which are optimised for use in healthy bone. As a consequence, the angularly stable screws, due to a reduced ratio between core diameter and Device for Fixing Implants on or in a bone outside diameter, are characterised by inferior' strip forces and failure strains. The invention is intended to provide a remedy for this. It is accordingly an object of the invention to create a greater bone-implant contact surface while reducing the implant volume and to create a considerably increased projection surface in the principle directions of strain, or to at least provide the public with a useful alternative. According to the invention, this object is achieved by means of a device comprising a longitudinal, solid or hollow core (1) with a longitudinal axis (2) and a number of wings (3) extending radially therefrom, the device being contained within an upright, regular cylinder with an outside diameter D = 2S + d and an identical longitudinal axis (2) forming its envelope, d corresponding to the diameter of an optional bore (9) formed in the core (1) and S corresponding to a value between 3 mm and 50 mm, wherein considered in a plane (6) extending orthogonally to the longitudinal axis (2) the ratio Aa/Ai existing between the outer surface portion Aa — located within the annulus (7) representing the difference between the areas of the two circles with the respective diameters D = 2S + d and di = S + d extending concentrically to the longitudinal axis (2) -and the inner surface portion Ai - located within the annulus (8) extending concentrically to the longitudinal axis (2) and representing the difference between the areas of the two circles with the respective diameters di and d. -of the IPONZ 12 JUN 2003 cross-sectional surface A = Aa + Ai of the implant located within the portion of the profile serving for the transmission of forces relative to the longitudinal axis (2) of the device, is at least 3:1. The advantages achieved by the present invention consist basically in the fact - that the device according to the invention permits to minimise the damaging of the periost and of the spongiosa structure, thus enabling a better anchoring of the implant within the bone. Further advantageous embodiments of the invention will be characterised in the dependent claims. In a preferred embodiment, a clearance (bore) provided in the main body of a bone plate which corresponds to the cross-sectional profile of the device according to the invention allows an angularly stable anchoring of the device in the bone plate. In another' embodiment, a peripheral enlargement of the wings of the device according to the invention allows to achieve greater flexural strength and torsion resistance with an identical or even smaller implant volume, an identical type of material, and with a considerably increased projection surface in the axial direction. In another embodiment, the profile is spirally twisted, preferably by 100 to 140 degrees on a length of 8 to 12 mm, which tends to prevent axial stripping. IPONZ <2 JUN2003 3 The implant (bone plate) receiving the profile of the device has a clearance which corresponds to the enlarged profile portion and is shaped in such a way as to prevent an inward displacement (in the direction of the bone) of the profile. A closing mechanism secures the profile against being released from its place of reception and absorbs the forces acting in the direction of the longitudinal axis of. the profile. This closing mechanism may consist of any known, positive or non-positive, releasable turning, sliding or click-stop adjustment. In the following, the invention and further developments of the invention will be illustrated in greater detail with reference to the partially diagrammatic representations of several embodiments. In the drawings: Fig. 1 is a cross-section of the device according to the invention; Fig. 2 is a cross-section of a four-wing profile in an x/y coordinate system including the formulae for the calculation of different geometrical moments of inertia; Figs. 3 to 5 show variants of the cross-sectional profile of the device according to the invention; Fig. 6 is a perspective view of a device according to the invention inserted into a bone plate; and 4 Fig. 7 is a perspective local sectional view of a device according to the invention inserted into a bone plate. The diagrammatic profile of the device according to the invention shown in Fig. 1 consists basically of a hollow core 1 with a longitudinal axis 2 (extending vertically to the plane of the drawing) and with three wings 3 extending radially therefrom and including peripheral enlargements 5. The core 1, the wings 3, and the enlargements 5 are contained within an upright regular cylinder (extending vertically to the plane of the drawing) with an outside diameter D = 2S + d and an identical longitudinal axis 2. The diameter d of the bore 9 formed in the core 1 is preferably 2 mm, the value S being between 3 mm and 50 mm, preferably between 5 mm and 30 mm. Considered in a plane 6 extending orthogonally to the longitudinal axis 2, which corresponds to the plane of the drawing, the ratio Aa/Ai existing between the outer surface portion Aa - located within the annulus 7 representing the difference between the areas of the two circles with the respective diameters D = 2S + d and di = S + d extending concentrically to the longitudinal axis 2 - and the inner surface portion Ai - located within the annulus 8 extending concentrically to the longitudinal axis 2 and representing the difference between the areas of the two circles with the respective diameters di and d - of the cross-sectional surface A = Aa + Ai of the implant located within the portion of the profile serving for the transmission of forces relative to the longitudinal axis 2 of the device, is at least 3:1. Typically, said ratio Aa/Ai is between 3.5 : 1 and 3.0 : 1. 5 Fig. 2 shows a four-wing profile in an x/y coordinate system by means of which the calculation of the geometrical moments of inertia Ix/ Iy and Iz is illustrated. The ratio between the maximum and the minimum geometrical moments of inertia Imax/Imin should not exceed a value of 5:1, preferably of 4 : 1. Figs. 3 to 5 show further variants of the cross-sectional profile including enlargements 5. Figs. 3 and 4 show further four-wing profiles, whereas Fig. 5 shows a six-wing profile. In Fig. 3, the enlargements 5 are not circularly cylindrical but shaped in the form of cylindroids. Fig. 4 shows a profile which is similar to that of Fig. 3 but which is not point-symmetric. Depending on the type of application, further profiles may be used, as long as they fulfil the geometrical conditions stated above. Fig. 6 illustrates a device 10 according to the invention including a twisted profile consisting of a core 1 and wings 3, shaped in the form of a bone screw which is inserted into a bone plate 20 comprising a plurality of holes 21 formed therein. The device 10 includes in addition on one of its ends a head 22 which fits the hole 21 of the plate. The head 22 is further provided with a hexagon socket 23 for receiving a corresponding hexagon head screw driver enabling the device 10 to be screwed into the bone. Fig. 7 represents the same device 10 as shown in Fig. 6 inserted into a bone plate 20 which - unlike the one shown in Fig. 6 - is not provided with circularly cylindrical holes but with specially shaped clearances 24 corresponding to the cross-sectional profile of the device 10. In this embodiment, the clearances 24 are shaped in a triradiate form and are arranged in a recess formed in the surface 25 of the bone plate 20, permitting the head 22 of the device 10 to be completely sunk into the circularly cylindrical recess 26 of the bone plate 20 so as to be level with the surface 25. Thus, a positive engagement between the device 10 and the bone plate 20 may be achieved. Unless the context clearly requires otherwise, the word wcomprising'" is used throughout the claims and description of this invention in an inclusive sense. IPONZ 12 JUN 2003 </div>

Claims

1. A device for the fixation of implants on, or in, a bone, comprising a longitudinal, solid or hollow core with a longitudinal axis and a number of wings extending radially therefrom, the device being contained within an upright, regular cylinder with an outside diameter D = 2S + d and an identical longitudinal axis forming its envelope, d corresponding to the diameter of an optional bore formed in the core and S corresponding to a value between 3 mm and 50 mm, wherein considered in a plane extending orthogonally to the longitudinal axis the ratio Aa/Ai existing between the outer surface portion Aa - located within the annulus representing the difference between the areas of the two circles with the respective diameters D = 2S + d and di = S + d extending concentrically to the longitudinal axis - and the inner surface portion Ai -located within the annulus extending concentrically to the longitudinal axis and representing the difference between the areas of the two circles with the respective diameters di and d - of the cross-sectional surface A = Aa + Ai of the implant located within the portion of the profile serving for the transmission of forces relative to the longitudinal axis of the device, is at least 3:1.

2. A device as claimed in claim 1, wherein the ratio Aa/Ai is at least 3.5 : 1.

3. A device as claimed in claim 1 or claim 2, wherein the ratio Aa/Ai is 4 : 1. IPONZ '2 JUN2003

4. A device as claimed in any one of claims 1 to 3, wherein the ratio between the maximal and the minimal geometrical moments of inertia Imax/Imin should not exceed a value of 5 : 1.

5. A device as claimed in claim 4, wherein the ratio between the maximal and the minimal geometrical moments of inertia Imax/Imin should not exceed a value of 4 : 1.

6. A device as claimed in any one of the claims 1 to 5, wherein it has on one of its free ends a coupling.

7. A device as claimed in any one of the claims 1 to 6, wherein the coupling is in the form of an enlarged profile portion which allows a positive or non-positive connection with another implant.

8. A device as claimed in claim 6 or 7, wherein the coupling comprises means permitting the device to be releasably locked within another implant in such a way as to be secured against displacement.

9. A device as claimed in any one of the claims 1 to 8, wherein the number of wings is at least two.

10. A device as claimed in any one of the claims 1 to 8, wherein the number of wings is at least three.

11. A device as claimed in any one of the claims 1 to 10, wherein on their free ends the wings are equipped with peripheral enlargements. IPONZ '2 JUN2003

12. A device as claimed in any one of the claims 1 to 11, wherein on a length of 8 to 12 mm the cross-sectional profile is spirally twisted, relative to the longitudinal axis.

13. A device as claimed in any one of the claims 1 to 11, wherein on a length of 8 to 12 mm the cross-sectional profile is spirally twisted by 100 to 140 degrees, relative to the longitudinal axis.

14. A device as claimed in any one of the claims 1 to 13, wherein the longitudinal core is provided with a bore formed therein.

15. A device as claimed in any one of the claims 1 to 14, wherein the ratio d/D between the core diameter d and the outside diameter D = 2S + d does not exceed a value of 0.5.

16. A device as claimed in claim 15, wherein the ratio d/D between the core diameter d and the outside diameter D = 2S + d does not exceed a value of 0.33.

17. A device as claimed in claim 15, wherein the ratio d/D between the core diameter d and the outside diameter D = 2S + d does not exceed a value of 0.30.

18.-A bone plate including at least two devices as claimed in any one of the claims 6 to 15, wherein it has at least two bores formed therein which are adequately shaped so as to be capable of receiving the couplings of the devices in such a way that they are in positive or non-positive engagement. IPONZ *2 JUN2003

19. A device according to any one of claims 1-17, substantially as herein described with reference to the accompanying figures 1-7, thereof.

20. A bone plate according to claim 18, substantially as herein described with reference to the accompanying figures 1-7, thereof. mo OF CLAIMS IP0N2 >2 mm </div>