Semirigid linking piece for stabilizing the spine

[0001] The present invention relates to a connecting member for maintaining the spacing between at least two anchor members which are interconnected by said connecting member.

[0002] Fields of application of the invention include stabilization and arthrodesis of segments of the vertebral column in degenerative pathologies of the spine.

[0003] Systems for stabilizing the vertebral column which brace at least two consecutive vertebrae by means of anchor members fixed into said vertebrae and connected by rigid connecting rods are well known in the art. Systems of this kind are generally coupled systems such that two consecutive vertebrae are interconnected by two substantially parallel rods fixed one on each side of the spinous processes. The anchor members are screwed into the posterior portion of the vertebrae and pass through the pedicles and a substantial portion of the vertebral bodies and therefore provide a fixed and durable connection.

[0004] The above stabilizing systems are routinely used to consolidate several consecutive vertebrae. Thus the vertebrae are interconnected by rigid rods over a substantial length of the vertebral column. Such assemblies hold the vertebrae correctly relative to each other; however, they considerably stiffen the spine in terms of bending. It has been shown that a more flexible stabilizing system, which confers greater relative mobility on the vertebrae, is beneficial in some pathologies.

[0005] Naturally enough, to increase the amplitude in bending of the stabilizing system, it has been proposed to reduce the section of the connecting rods between the anchor members to increase the amount by which said connecting rods deform for the same stress. This increases the mobility of the vertebrae relative to each other and also increases the amplitude of forward/rearward bending of the vertebral column for the same force. However, the vertebral column is not so well stabilized, in particular with regard to lateral stabilization of the vertebrae relative to each other. Also, stresses are higher in connecting rods of smaller section and the connecting rods may deteriorate prematurely.

[0006] A first object of the present invention is to provide a connecting member for maintaining the spacing between existing anchor members and which can bend more than the rods used at present without increasing the internal stresses in said connecting rods.

[0007] To achieve the above object, a connecting member in accordance with the invention for maintaining the spacing between at least two anchor members screwed into vertebrae comprises: a flexible part divided into two spaced continuous branches that are substantially symmetrical about a longitudinal axis of said member, the ends of said branches being interconnected in pairs and defining a first median plane, and two rigid rod-forming parts each having a fixing first portion and a second portion, each of said second portions of said two rigid parts respectively extending said ends of said branches interconnected in pairs in opposite directions, the cross-section of each of said branches being less than the cross-section of said rigid parts so that said connecting member, said fixing portions of which are fixed to respective anchor members, is able to bend elastically about an axis that is perpendicular to the longitudinal axis, said perpendicular axis being contained in said median plane of the connecting member on relative movement of the vertebrae, whereby the vertebrae, which are held spaced relative to each other, are movable relative to each other.

[0008] Thus one characteristic feature of the connecting member is its shape, which is such that the stresses exerted on its rigid parts by bending of the spine cause the two branches to bend in a direction perpendicular to the median plane that they define. This is because the connecting member can bend only in a direction perpendicular to the median plane that the two branches define because the two branches are joined at each end by the rigid parts and bending about another axis would longitudinally elongate one of the branches and longitudinally compress the other one; given the stresses, this would cause little deformation. In this way, the connecting member has a specific bending direction perpendicular to its main axis and is disposed so that the median plane defined by the two branches is substantially perpendicular to the plane in which the spine is able to bend. As explained in more detail below, the section of each of the continuous branches, which is substantially constant, is smaller than the section of the rigid parts, which increases the amplitude by which the member bends for the same force. Also, the bending stresses, representative of the internal forces in the two branches, are lower than the stresses to which a single branch would be subjected when bent by the same amount by the same force. This reduces fatigue of the connecting member of the invention.

[0009] The connecting member is advantageously adapted to interconnect n anchor members and comprises n rigid parts between which there are disposed n1 flexible parts along the longitudinal axis of said member, each rigid part situated between two flexible parts having a first or fixing portion and two second portions, each of said second portions being situated at a respective end of said first or fixing portion, said second portions extending respective ends of the branches of said two flexible parts so that the median planes of all the flexible parts are substantially the same, and the rigid parts situated at the ends of said member advantageously have respective single second portions extending the ends of the branches of the flexible parts.

[0010] Thus, by virtue of this feature, the connecting member maintains the spacing between all the anchor members that it interconnects, each of which is fixed to a vertebra, to align them. Each rigid part is fixed to an anchor member and between the anchor members of each pair there is a flexible part extended by said rigid parts. In this way, a single connecting member stabilizes a plurality of vertebrae, which reduces the time needed to assemble the stabilizing system as a whole, and consequently the operating time. Also, this feature of the connecting member stabilizes a plurality of consecutive vertebrae by connecting them together, while at the same time allowing them great relative flexibility.

[0011] In a particular embodiment of said connecting member, the sum of the surface areas of the sections of said two branches is less than the surface area of the section of said rigid rod-forming parts. Thus the rigid rod-forming parts are more rigid than the two branches which bend more readily, said rigid parts being securely connected to said anchor members.

[0012] In another particular embodiment of the invention said branches have portions parallel to each other and to the longitudinal axis of said member and the distance between said branches is at least equal to the length of said portions. This configuration provides a connecting member which can bend much more in a direction perpendicular to the median plane defined by the branches than in any other direction; thus, allowing for the forces exerted on it, the connecting member has a single bending direction.

[0013] In a further particular embodiment of the invention, the distance between the ends of said branches connected in pairs lies in the range 1.5 times to 2.5 times the distance between said branches. This allows increased bending compared to the connecting rods routinely used, while at the same time obtaining a single bending direction perpendicular to the median plane defined by the two branches.

[0014] Also, the section of said rigid rod-forming parts is advantageously circular, which facilitates manufacture of the member, which is preferably made of titanium alloy. Also, if prior art circular section connecting rods are to be replaced by connecting members conforming to the invention without making it necessary to replace the anchor members, it is necessary for said rigid parts to have sections identical to the sections of prior art connecting rods.

[0015] Titanium alloys have mechanical and corrosion resistant properties compatible with the technical specifications required of the connecting member.

[0016] The present invention also provides a vertebral stabilization system for fastening together at least two vertebrae each having a median plane substantially perpendicular to the axis of the spine of which they are part and a posterior wall defining a posterior median plane of said spine, said system comprising at least two anchor members each adapted to be fixed into the posterior wall of a vertebra so that a line which intersects said two anchor members is substantially parallel to said axis of the spine, which system further comprises a connecting member of the invention whose two rigid parts are adapted to interconnect said two anchor members so that said median plane defined by said two branches is substantially parallel to said posterior median plane of said spine, whereby said vertebrae, which are interconnected in their posterior portions, are relatively movable along said axis of said spine.

[0017] Other features and advantages of the invention will emerge on reading the following description of particular embodiments of the invention, which is given by way of non-limiting example and with reference to the accompanying drawings, in which:

[0018] FIG. 1 is a diagrammatic perspective view of a connecting member of the invention,

[0019] FIG. 2 is a diagrammatic perspective view showing anchor members connected by the connecting member, and

[0020] FIG. 3 is a diagrammatic side elevation view of a vertebral column showing two consecutive vertebrae into which there are screwed anchor members interconnected by a connecting member of the invention.

[0021] The various portions of a connecting member of the invention are described initially with reference to FIG. 1.

[0022] The connecting member 8 comprises a flexible part 10 having two parallel branches 12 and 14 interconnected at their ends at two points 16 and 18. Two rigid rod-forming parts 20 and 22 have respective fixing first portions 20, 22 and second portions 20, 22 extending the joined together ends of the two branches 12 and 14 from the two points 16 and 18 in opposite directions along a longitudinal axis A. The two parallel branches 12 and 14 joined at their ends therefore surround a void 24. They are also parallel to the axis A and symmetrical about that axis. The sum of the sections of the two branches 12 and 14 is preferably less than the section of the rigid parts which have substantially the same circular section, with the result that the diameter of the cross-section of a branch is less than 70% of the diameter of the rigid parts. For example, the diameter of the cross-section of a branch lies in the range 50% to 70% of the diameter of the rigid parts.

[0023] Also, the two branches 12 and 14 define a median plane Pm intersecting the two rigid parts 20 and 22 axially.

[0024] Thus the connecting member having the first fixing portions 20 and 22 of its rigid parts 20 and 22 in fixed bearing engagement, is itself able to bend elastically in a direction Dp perpendicular to the median plane Pm when a force F is exerted on each of the branches 12 and 14. This is because, given the section of the branches 12 and 14 relative to the section of the rigid parts 20 and 22, the radius of curvature of the connecting member when it bends is at a minimum at the center of the flexible part 10.

[0025] The sum of the forces F exerted on the branches in a direction perpendicular to the median plane Pm causing the connecting member to bend without breaking would not cause the connecting member to bend in a direction perpendicular to the axis A of the member and contained in the median plane Pm because, in this case, one of the branches would be in tension and the other in compression, which would prevent bending. A preferential bending direction is therefore obtained, which is advantageous in the context of the intended application, as explained in more detail below.

[0026] To obtain a preferential bending direction and an optimum bending amplitude, given the dimensions of the connecting portion, the two branches 12 and 14 are separated by a distance that is not less than the length of their parallel portions. Obviously, the closer together the branches 12,14, the greater the potential for bending about an axis not strictly perpendicular to the median plane Pm, and the force necessary to obtain said bending increases as the total length of the branches 12 and 14 decreases. However, the spacing between the two branches 12 and 14 is limited by the space available for inserting said member, both transversely and along the longitudinal axis, and consequently the distance between the ends of said branches connected in pairs is in the range 1.5 times to 2.5 times the distance between said branches.

[0027] To compare the stress to which a connecting member of the invention is subjected with the stress to which a simple prior art connecting rod is subjected, a rod of diameter 1 was made whose bending amplitude was to be increased by 50%. To achieve this, the diameter of the simple connecting rod had to be reduced by 10%, the consequence of which was a 35% increase in the stress in it. In contrast, to obtain the same amplitude of bending, the diameter of the branches of the connecting member of the invention had to represent 75% of the original diameter of the rod of diameter 1, although the stress in the branches was increased by only 13%.

[0028] The above measurements showed that, for the same bending, the connecting member of the invention was subjected to lower stresses than a conventional connecting rod, thereby reducing metal fatigue, so that its service life should be longer than prior art rods. Also, the lower the stresses in the connecting member, the better it retains its elastic properties.

[0029] Deformation of the connecting member on relative movement of the anchor members 26 and 28 is described below with reference to FIG. 2.

[0030] FIG. 2 shows the two rigid parts 20 and 22 of the connecting member interconnecting two anchor members 26 and 28. The two anchor members 26 and 28 are parallel to each other in a common axial plane Pa. The connecting member is fixed to the anchor members 26 and 28 so that the median plane Pm defined by the branches 12 and 14 is substantially perpendicular to the common axial plane Pa.

[0031] Each anchor member 26, 28 has a threaded shank 30 with a U-shaped head 32 at the top whose inside wall is threaded so that a screw-forming member 34 can be screwed into it. Thus the first or fixing portions 20 and 22 of the rigid parts 20 and 22 are accommodated in the heads 32 of the respective anchor members 26 and 28 and are locked to them by tightening the screw-forming members 34.

[0032] In this way, when the threaded shanks 30 of the anchor members tend to move towards each other due to the effect of opposite forces T and T in the plane Pa and substantially parallel to the axis A, the anchor members 26 and 28 deform the connecting member to an arcuate shape in the plane Pa. The stress induces bending of the branches 12 and 14 of the connecting member in a direction perpendicular to the median plane Pm. When the stress is removed, the connecting member reverts to its original rectilinear shape and the threaded shanks of the anchor members 30 return to their former relative position.

[0033] The mechanism of elastic bending of the connecting member and the anchor members described above is the same if the threaded shanks 30 of the anchor members 26 and 28 move away from each other, the connecting member forming a reversed arcuate shape.

[0034] The use of the connection member 8 in a vertebral stabilizing system for fastening together at least two vertebrae V1 and V2 is described below with reference to FIG. 3.

[0035] Each vertebra V1, V2 has a respective median plane PV1, PV2 substantially perpendicular to the axis Ar of the spine of which it is a part and a respective posterior wall PPV1, PPV2 defining a posterior median plane PPr of said spine.

[0036] The stabilizing system includes at least two anchor members 26 and 28 respectively screwed into the posterior walls PPV1 and PPV2 of the vertebrae V1 and V2 so that a line L that intersects the two anchor members 26 and 28 is substantially parallel to said axis Ar of the spine. The two first or fixing portions 20 and 22 of a connecting member 8 interconnect the two anchor members 26 and 28 so that said median plane Pm defined by said branches 12 and 14 is substantially parallel to said posterior median plane PPr of said spine. In this way, the vertebrae V1 and V2, which are interconnected in their posterior portions, are relatively movable along the axis Ar of the spine.

[0037] Thus when the spine is stretched, the vertebrae V1 and V2 move away from each other in respective directions E and E, which causes the threaded shanks 30 to move away from each other, deforming the connecting member 8, and in particular its flexible part 10. The deformed connecting member has its concave side facing away from the spine.

[0038] When the spine is bent, the inverse effect occurs and the vertebrae V1 and V2 move towards each other, which induces deformation of the connecting member with its concave side facing toward the spine.

[0039] Obviously, because it can bend more than prior art connecting rods, the connecting member 8 of the invention increases the mobility of the vertebrae relative to each other. Also, because of its construction, with two parallel branches 12, 14 connected together at their ends and extended by the rigid parts 20, 22, the connecting member 8 allows the spine to bend and extend in the plane Pa while at the same time limiting bending in the plane Pm perpendicular to the plane Pa. Thus the spine is stabilized laterally and relative movement of the vertebrae is limited.

[0040] In a particular embodiment, not shown, the connecting member has three rigid rod-forming parts interconnected by flexible parts. To this end, the central rigid part has two second portions extending respective ends of said fixing portion, said second portions respectively extending the ends of the two interconnected branches of the two flexible parts. The rigid parts extend the ends of the interconnected branches so that the median planes of both flexible parts are substantially the same. A longitudinal member is thus obtained having two rigid first portions, one at each end, and a central rigid part between the two rigid first portions, the rigid parts being interconnected in pairs by flexible parts.

[0041] The connecting member therefore maintains the spacing between the three anchor members that it interconnects, which members are fixed to three substantially equidistant vertebrae, to align them. Each rigid part of the connecting member is fixed to an anchor member so that there is a flexible part between pairs of vertebrae. In this way a single connecting member stabilizes three vertebrae, which reduces the time needed to assemble the stabilizing system as a whole and consequently the operating time. Also, because the three vertebrae are interconnected by a single connecting member, their mobility relative to one another is better controlled.

[0042] It goes without saying that providing connecting members having more than three rigid parts separated by flexible parts would not depart from the scope of the invention.