Abstract
A system for restricting spinal flexion includes a compliance member having a body and an elongation limit. The body typically comprises a spring or other tension element which provides elastic constraint to the spinal segment when the compliance member is attached to the spinous processes. The elongation limit prevents overextension of the compliance member, thus reducing the likelihood that the patient will experience over flexion of the spinal segment and reducing the risk of placing excessive mechanical load on the compliance member.
Claims
1. A compliance member for elastically constraining spinous processes, said compliance member comprising: a body having a superior attachment element and an inferior attachment, said body defining a tension spring capable of elastic elongation between said attachment elements, wherein said attachment elements allow the compliance member to be directly or indirectly attached between superior and inferior spinous processes; an elongation limit coupled between the superior attachment element and the inferior attachment element to prevent elongation of the tension spring beyond a maximum elongation length.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic diagram illustrating the lumbar region of the spine including the spinous processes (SP), facet joints (FJ), lamina (L), transverse processes (TP), and sacrum (S).
[0041] FIG. 1A is a schematic illustration illustrating a portion of the lumbar region of the spine taken along a saggital plane.
[0042] FIGS. 1B and 1C illustrate a spinal segment having a center of rotation (COR) both in a neutral position (FIG. 1B) and in a fully flexed position (FIG. 1C).
[0043] FIG. 2 is a schematic illustration of the systems of the present invention comprising superior and inferior tether structures and right and left compliance members.
[0044] FIG. 3 illustrates an exemplary coil spring tension member.
[0045] FIG. 3A illustrates the coil spring tension member of FIG. 4 illustrating the preferred dimensions.
[0046] FIGS. 4A-4C illustrate the use of a locking mechanism incorporated in the tension member of FIG. 3 for removably securing a band member of a tether structure.
[0047] FIGS. 5A and 5B illustrate a constraint assembly similar to that shown in FIGS. 10A and 10B where the sheath contains elements which minimize sheath interaction with the tension element and/or limit the maximum elongation of the assembly under tension.
[0048] FIGS. 6A and 6B illustrate an accordion-type sheath which could potentially also limit maximum elongation.
[0049] FIGS. 7A-7D illustrate different embodiments of internal tethers used to provide the elongation limits of the present invention.
[0050] FIGS. 8A and 8B illustrate external tethers used to provide the elongation limits of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0051] Exemplary spinous process constraints according to the present invention are illustrated schematically in FIG. 2. The systems 10 comprise a superior tether structure 12, and inferior tether structure 14, and right compliance member 16 and a left compliance member 18. The superior tether structure 12 will typically be a continuous band, cable, strap, cord, or other structure which extends between the two compliance members and provides a saddle region 20 which is adapted to lie over and conform to a superior surface of a superior spinous process SSP as described in more detail in the related prior applications which have been incorporated herein by reference. The inferior tether structure 14 will typically comprise a band, cable, or the like which is constructed similarly if not identically to the superior tether structure 12 and has a saddle region 22 adapted to lie over and conform to an inferior surface of an inferior spinous process 22. In certain instances, however, the inferior tether structure 14 may comprise separate bands, cables, straps, cords, or the like, 14a and 14b, shown in broken line, which have anchors 15a and 15b at their lower ends and are adapted to be separately attached to an inferior vertebrae or more commonly to a sacrum. The use of such separate tether structures for inferior attachment are described in more detail in co-pending application Ser. No. 11/827,980 (Attorney Docket No. 026398-000120US), the full disclosure of which has been previously incorporated herein by reference. The tether structures will usually be flexible but effectively non-distensible so that they allow minimum elongation under tensile load.
[0052] The right and left compliance members 16 and 18 will usually have similar or identical constructions and include an adjustable attachment component 32 and a fixed attachment component 34 for securing connecting segments of the superior and inferior tether structures 12 and 14. Usually, each compliance member 16 and 18 will have one of the tether structures 12 and 14 pre-attached to the fixed attachment component 34. The two subassemblies can then be introduced onto opposite sides of the spinous processes, and the tether structures placed over the spinous processes or otherwise attached to the vertebral bodies, as generally described in co-pending application Ser. No. 11/875,674 (Attorney Docket No. 026398-000150US), the full disclosure of which is incorporated herein by reference.
[0053] The present invention is particularly concerned with the nature of the tension elements 30, and a number of specific embodiments will be described hereinbelow. In general, the tension elements 30 will elastically elongate as tension is applied by the superior and inferior tether structures 12 and 14 through the attachments 32 and 34, in the direction shown by arrow 36. As the spinous processes or spinous process and sacrum move apart during flexion of the constrained spinal segment, the superior and inferior tether structures 12 and 14 will also move apart, as shown generally in broken line in FIG. 2. A tension element 30 will elastically resist the spreading with a force determined by the mechanical properties of the tension member. In particular, the tension members will be selected to have a tensile or elastic stiffness, also known as a spring constant, in the relatively low ranges set forth above. Such low elastic constricting forces provide a number of advantages when compared to complete restriction or constriction with a high elastic force as described above.
[0054] The tension elements of the present invention will be positioned over adjacent spinous processes, or over the L5 spinous process and adjacent sacrum, in order to increase the bending stiffness of the spinal segment. Referring to FIGS. 1B and 1C, the bending resistance is the resistance to bending of the spinal segment about a center of rotation (COR) positioned generally within or adjacent to the disk between adjacent vertebral bodies. The center of rotation can be determined from radiographic images, generally as described above, and it can be seen that a point PS on the superior spinous process SPS and a similar point PI on the inferior spinous process SPI will move generally along a curved line or arc A as shown in FIG. 1C. While the center of rotation COR is not fixed during flexion or extension of the spinal segment, and the points will not travel on a true arc, the motion of the spinous processes is nonetheless arcuate in nature as illustrated.
[0055] Thus, the positioning of any of the elastic constraints as described herein at a position on the spinous processes SPS and SPI generally indicated by line L will define a moment arm distance d.sub.m, as illustrated in FIG. 1B. The position L will generally be selected so that the moment arm length d.sub.m will be in the range from 25 mm to 75 mm, preferably from 40 mm to 60 mm. By thus selecting an elastic constraint having a total stiffness in the range from 7.5 N/mm to 40 N/mm, the desired bending stiffness of the spinal segment can be increased by an amount in the range from 0.1 Nm/deg to 2 Nm/deg, preferably from 0.4 Nm/deg to 1 Nm/deg.
[0056] As also shown on FIG. 1C, the spinous processes SPS and SPI will spread to a maximum distance d.sub.s upon full flexion of the spinal segment. In accordance with other aspects of the present invention, it may be desirable to constrain the spreading of the spinous processes to a maximum distance above the distance in the neutral position (as shown in FIG. 1B) in the range from 1 mm to 10 mm, preferably from 2 mm to 8 mm. Certain of the elastic constraints in the present invention can provide for both increased bending stiffness and for a complete stop of flexion. See, for example, the device described in FIGS. 5A and 5B hereinafter.
[0057] A first exemplary tension element 40 constructed in accordance with the principles of the present invention is illustrated in FIG. 3. The tension element 40 comprises a helical spring structure 41 formed from a single piece of material. The tension member 40 includes an adjustable tether connector 42 and a fixed tether connector 44, both of which are preferably formed integrally or monolithically with the helical spring structure 41. Typically, the helical spring structure 41 and both tether connectors 42 and 44 will be formed from one piece of material, usually being a metal such as titanium, but optionally being a polymer, ceramic, reinforced glass or other composite, or other material having desired elastic and mechanical properties and capable of being formed into the desired geometry. In a preferred embodiment, the tension member 40 is machined or laser cut from a titanium rod. Alternatively, a suitable polymeric material will be polyethylene ether ketone (PEEK). Other features may be built into the tension member 40, such as a stress relief hole 46. Components that mate with the adjustable tether connector may potentially include a roller and a lock-nut; such components could be made from the same material as the tension element and adjustable tether connector (e.g. titanium components if the tension member is titanium), or they could be made from a different material (e.g. injection molded PEEK).
[0058] Referring now to FIG. 3A, preferred dimensions for the tension member 40 are illustrated. In order to accommodate the patient anatomy when the tension members are arranged laterally of and vertically between adjacent spinous processes, as generally shown in FIG. 2, the compliance member will have a length 1 of 38 mm or less, preferably in the range from 20 mm to 30 mm, a depth d in the anterior-posterior direction no greater than 18 mm, preferably in the range from 8 mm to 15 mm, and a width in the direction normal to depth no greater than 15 mm, preferably in the range from 7 mm to 10 mm.
[0059] A free end 53 of the tether structure 52 may be attached to the adjustable tether connector 42, as illustrated in FIG. 4A through 4C. Initially, a barrel locking mechanism 54 is rotationally aligned such that a slot 56 is aligned with an inlet opening 58 on the top of the connector 42 and an outlet opening 60 on the side of the connector. The inlet opening 58 is located centrally and provides a primarily axial load on the compliance member, thereby evenly loading the compliance member and having the advantages described above. The free end 53 of tether 52 is then advanced through the inlet opening 58, slot 56, and outlet opening 60, as illustrated in FIG. 4C. By then rotating the barrel lock 54 approximately 180°, the tether 52 will be locked in place in the connector 42, as shown in FIG. 4A. It will be appreciated that this simple locking mechanism allows tether 52 to be appropriately tensioned for the individual patient before locking the tether in place. A locking feature, e.g. set screw, nut, or pin (not shown) would then be used to lock the tether and roller in place, providing additional resistance to unfurling and opening. The tensioning could be performed separately and/or simultaneously during implantation of the constraint assembly. Additional features of the mechanism such as pins, shoulders, or other features which control the travel of the roller or lock-nut may aid in the alignment and operation of the mechanism.
[0060] Another tether structure (not illustrated) will be attached to the fixed connector 44 at the other end of the tension element 40, typically using a pin (not illustrated). The pin may be anchored in a pair of receiving holes 62, and a free end of the tether wrapped over the pin and firmly attached. Usually, the fixed tether structure will be pre-attached at the time of manufacture so that the treating physician can implant each of the pair of tension members, with one tether structure attached to the fixed tether connector. The remaining free ends of each tether structure 52 may then be deployed around the spinous processes (or attached to a sacrum) in a pattern generally as shown in FIG. 2.
[0061] Referring now to FIGS. 5A and 5B, a flexible restraint system 170 will be described. The flexible restraint system 170 includes a sheath having a plurality of battens or wires 172 which reduce interactions between the sheath and restraint system, as well as provide an axial constraint to limit the maximum axial separation of the fixed and adjustable tether connectors 174 and 176, respectively. As shown in FIG. 5A, the battens 172 are axially compressed so that they bow outwardly, distancing the sheath from the tensile member. In FIG. 5B, the fixed and adjustable tether connectors 174 and 176 have moved to their maximum axial separation, straightening the battens 172 and thus limiting further axial separation of the adjustable tether connectors 174 and 176.
[0062] Referring now to FIGS. 6A and 6B, another flexible restraint system 180 constructed in accordance with the principles of the present invention will be described. The flexible restraint system 180 is similar to system 170, except that the sheath structure has an accordion fold to provide for lengthening and shortening together with the movement of fixed and adjustable tether connectors 182 and 184, respectively. The accordion folds both permit greater gross elongation of the sheath with lower material strains than in a purely cylindrical sheath and potentially reduce interaction between the sheath and tensile member. The sheath with the accordion fold may or may not act as a constraint on maximum elongation of the compliance members. The sheath could also be used with separate tension members for providing the maximum elongation limit.
[0063] Referring now to FIGS. 7A through 7D, four different embodiments of elongation limit tethers for limiting the maximum elongation of compliance members 40 are illustrated. The compliance members 40 are shown in section with an open chamber 400 shown within the spring structure 41. The open chamber 400 extends between the superior tether attachment element 42 and the inferior tether attachment element 44. As described thus far, the compliance members 40 are identical to those described in FIGS. 3 and 3A above.
[0064] A first exemplary tether structure in form of a single cord 402 is illustrated in FIG. 7A. The tether 402 is typically formed from a relatively non-distensible material, such as ultra high molecular weight polyethylene, commercial available under the trade name Dyneema Purity® from suppliers such as DSM®. The cord will be formed so that it is essentially non-distensible, typically with tensile stiffness twice as stiff as the compliance members and preferably ten times as stiff; or typically greater than 20 N/mm, preferably greater than 100 N/mm. The non-distensible cord 402 may have plate, washer or T-shaped anchors 404 at each end and may be held between anchor plates 406 as illustrated. Other mechanisms to fasten a cord or tether to a rigid component may be employed, such as knots, crimps, splices, welds, etc. The inelastic cord 402 will have a certain length of “slack” in it when the compliance member 40 is in its shelf or non-elongated configuration, as shown in FIG. 7A. Thus, the inelastic cord 402 will thus have room to lengthen as the compliance member 40 is stretched when exposed to flexion during use after implantation. The amount of slack will determine the maximum elongation length of the compliance member, typically being in the ranges set forth above.
[0065] Referring to FIG. 7B, an alternative non-distensible cord 410 is illustrated, where the cable has loops 412 formed at each end, where the loops may be placed over anchor plates 414. Loops 412 may be preformed or alternatively could be formed using crimps, ties, or other attachments which allow attachment and/or adjustment of length in the field.
[0066] Referring now to FIG. 7C, an inelastic cord 420 may comprise a continuous loop which is disposed about upper and lower anchor plates 422. While the continuous loop 420 is illustrated without splices or connection, it would also be possible to provide a connector to allow for adjustment of the length of the loop.
[0067] Referring now to FIG. 7D, the inelastic cord may be formed in a single loop 430 attached to a base anchor 432 which is received in an anchor plate 434 in the interior tether attachment 44.
[0068] In addition to the internal “cord” tethers of FIG. 7A-7D, the tethers or inelastic constraints may be formed externally over the elastic constraints 40, as illustrated in FIGS. 8A and 8B. For example, the inelastic tether could be a mesh sheath or jacket 440 disposed about the spring structure 41 of the elastic constraint, as shown in FIG. 8A. The mesh sheath 440 would be attached to both the superior tether attachment 42 and inferior tether attachment 44 and would have sufficient slack to allow the desired elongation and maximum elongation limit as the elastic constraint is elongated. Alternatively, as shown in FIG. 8B, the external constraint could be a cable or cord 450 which is threaded through passages in the superior tether attachment 42 and the inferior attachment 44, as illustrated. As with all prior embodiments, the cable or cord 450 would have sufficient slack to allow the desired elongation while providing the hard stop once the desired maximum elongation is reached.
[0069] The length of the elongation limit may be set either during fabrication or immediately prior to use. In a first fabrication protocol, the compliance member will be adjusted in a jig or other apparatus to the desired maximum elongation. The tether or inelastic cable or cord which will be used as the elongation limit may then be introduced into or over the elastic constraint and pulled until it is taut. Once it is taut, it can be attached to the anchors, exterior, or otherwise to the body of the compliance member. By attaching when the compliance member is in its desired elongated configuration, the proper relative adjustment of the elongation limit can be assured.
[0070] In other instances, however, it may be desirable to adjust the elongation limit in situ after the compliance member has been initially implanted. In such cases, the spinal segment can be manipulated to the desired maximum flexion and the elongation limit fixed to the compliance member while the spinal segment remains in the desired flexion.
[0071] While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.
