Percutaneous Implantable Nuclear Prosthesis

Abstract

A prosthesis for implantation in a de-nucleated intervertebral disc includes a fiber ring-like layer which encloses a polymeric layer to create an annular space. The annular space is inflatable with an in-situ curable liquid polymer and forms an interior cavity. The annular space may be expanded uniformly or differentially to be tailored to the needs of a particular vertebral segment and to achieve optimal disc space width and angle, thereby stabilizing the segment while preserving normal motion of the vertebral segment. The interior cavity provides a void that allows inward deformation of the implant during weight bearing activities and bending. The prosthesis can be elastically deformed through axial elongation to a reduced profile to load into a delivery cannula using pulling techniques.

Claims

1. An implantable prosthetic device comprising: a) an annular hollow inflatable membrane having an inflation port; b) an annular fiber graft enclosing the inflatable membrane; c) an inflation stem coupled to the inflation port for removably receiving an inflation stylet in a substantially leakproof manner; and d) a one way valve assembly in the inflation stem to allow fluid to be injected into the interior of the inflatable membrane, the one way valve assembly comprising a duckbill valve.

2. The implantable prosthetic device of claim 1, wherein the annular hollow inflatable membrane comprises an annular membrane with a first open end and a second open end which are coupled together by a coupling member.

3. The implantable prosthetic device of claim 1, wherein the annular hollow inflatable membrane comprises a unitary member.

4. The implantable prosthetic device of claim 1, wherein the annular hollow inflatable membrane comprises an elastomeric material.

5. The implantable prosthetic device of claim 1, wherein the coupling member further comprises a lumen for fluidly connecting the first and second open ends of the annular hollow inflatable membrane.

6. The implantable prosthetic device of claim 1, wherein the coupling member is substantially solid.

7. The implantable prosthetic device of claim 1, wherein the inflation port further comprises an inflation neck extending from the inflatable membrane.

8. The implantable prosthetic device of claim 7, wherein the inflation stem comprises a single lumen.

9. The implantable prosthetic device of claim 7, wherein the tubular fiber graft comprises an opening for receiving the inflation neck of the inflatable membrane.

10. The implantable prosthetic device of claim 9, further comprising a reinforcing member for reinforcing the opening.

11. The implantable prosthetic device of claim 1, further comprising a push member for engaging the coupling member to deploy the coupling member.

12. The implantable prosthetic device of claim 1, further comprising a deployment device.

13. The implantable prosthetic device of claim 12, wherein the deployment device comprises: a delivery cannula for receiving the inflatable membrane and tubular fiber graft in an uninflated state; an inflation stylet for delivering a curable material to the inflatable membrane; and a release cannula for allowing the inflation stylet to be removed without dislodging the inflatable member.

14. The implantable prosthetic device of claim 1, further comprising a cured material in the inflatable membrane.

15. An implantable prosthetic device comprising: a) an annular inflatable membrane having an inflation port and first and second open ends; b) a tubular fiber graft enclosing the inflatable membrane and having first and second open ends; c) a coupling member coupling the first and second open ends of the inflatable membrane and fiber graft; and d) a one way valve assembly coupled to the inflation port to allow fluid to be injected into the interior of the inflatable membrane, the one way valve assembly comprising a duckbill valve.

16. The implantable prosthetic device of claim 15, wherein the annular inflatable membrane comprises a unitary, elastomeric member.

17. The implantable prosthetic device of claim 15, wherein the coupling member comprises a lumen connecting the first and second open ends of the annular inflatable membrane.

18. The implantable prosthetic device of claim 15, wherein the coupling member is solid.

19. The implantable prosthetic device of claim 15, wherein the coupling member comprises a groove.

20. The implantable prosthetic device of claim 19, further comprising a retaining member for coupling the annular inflatable membrane and fiber graft to the coupling member.

21. The implantable prosthetic device of claim 20, wherein the retaining member comprises a crimpable ring.

22. The implantable prosthetic device of claim 20, wherein the retaining member comprises a radiopaque material.

23. The implantable prosthetic device of claim 15, further comprising an inflation stem detachably coupled to the inflation port for receiving an inflation stylet, wherein the coupling is substantially leakproof during inflation.

24. The implantable prosthetic device of claim 23, further comprising a locking device on the inflation stem configured to engage a mating locking device on the inflation stylet to prevent inadvertent dislodgement of the inflation stylet.

25. The implantable prosthetic device of claim 23, wherein the inflation stem is located outside of a chamber formed by the annular inflatable membrane.

26. The implantable prosthetic device of claim 23, wherein the inflation stem comprises a single lumen.

27. The implantable prosthetic device of claim 23, further comprising an inflation stylet adapted to mate with the inflation stem.

28. The implantable prosthetic device of claim 15, wherein the inflatable membrane comprises an inflation neck extending from the inflation port.

29. The implantable prosthetic device of claim 28, wherein the tubular fiber graft comprises an opening for receiving the inflation neck of the inflatable membrane.

30. The implantable prosthetic device of claim 29, further comprising a reinforcing member for reinforcing the opening.

31. The implantable prosthetic device of claim 15, further comprising a push member for engaging the coupling member to deploy the coupling member.

32. The implantable prosthetic device of claim 15, further comprising a deployment device.

33. The implantable prosthetic device of claim 32, wherein the deployment device comprises: a delivery cannula for receiving the inflatable membrane and tubular fiber graft in an uninflated state; an inflation stylet for delivering a curable material to the inflatable membrane; and a release cannula for allowing the inflation stylet to be removed without dislodging the inflatable member.

34. The implantable prosthetic device of claim 33, further comprising an inflation stem coupled to the inflation port.

35. The implantable prosthetic device of claim 34, wherein the inflation stem is configured to receive the inflation stylet and configured to be placed into the delivery cannula.

36. The implantable prosthetic device of claim 15, further comprising a cured material in the inflatable membrane.

37. A kit for implanting an implantable prosthetic device, comprising: a) a delivery cannula having an internal lumen with an inner diameter; b) a graft comprising an annular inflatable ring with an inflation stem for communicating with an interior of the annular inflatable ring, wherein the inflation stem has a proximal end and an opposed distal end and has an outer diameter and an inner diameter, and wherein the inflation stem is disposed in the internal lumen of the delivery cannula; c) a release cannula slidably disposed in the delivery cannula, the release cannula having a distal portion configured to engage the proximal portion of the inflation port, and d) an inflation stylet with at least one internal lumen, the inflation stylet having a distal portion with an outer diameter configured to be releasably press fit into the inflation stem.

38. The kit of claim 37, wherein the inner diameter of the delivery cannula and the outer diameter of the inflation port are substantially the same.

39. The kit of claim 37, further comprising an introducer cannula sized to receive the delivery cannula.

40. The kit of claim 37, wherein the graft further comprises a tubular fiber graft enclosing the inflatable ring.

41. The kit of claim 40, wherein the tubular fiber graft comprises an opening for receiving the inflation stem of the inflatable ring.

42. The kit of claim 41, further comprising a reinforcing member for reinforcing the opening.

43. The kit of claim 37, wherein the inflatable membrane comprises a tubular membrane with a first open end and a second open end coupled together by a coupling member.

44. The kit of claim 43, wherein the coupling member comprises a lumen connecting the first and second open ends of the inflatable membrane.

45. The kit of claim 43, wherein the coupling member is solid.

46. The kit of claim 43, wherein the coupling member comprises a groove.

47. The kit of claim 46, further comprising a retaining member for coupling the inflatable membrane and fiber graft to the coupling member.

48. The kit of claim 47, wherein the retaining member comprises a crimpable ring.

49. The kit of claim 47, wherein the retaining member comprises a radiopaque material.

50. The kit of claim 43, further comprising a push member for engaging the coupling member to deploy the coupling member.

51. The kit of claim 37, wherein the inflatable membrane comprises a unitary member.

52. The kit of claim 37, wherein the inflatable membrane comprises an elastomeric material.

53. The kit of claim 37, further comprising a one way valve assembly coupled to the inflation stem for allowing fluid to be injected into the interior of the inflatable membrane.

54. The kit of claim 52, wherein the one way valve assembly comprises a duckbill valve.

55. The kit of claim 37, further comprising a cured material in the inflatable membrane.

56. A method of manufacturing an implant, comprising: forming a hollow elastomeric membrane with first and second open ends and an inflation port; enclosing the elastomeric membrane in a tubular fiber graft; and coupling the first and second open ends together.

57. The method of claim 56, wherein the elastomeric membrane is formed in a unitary member by injection molding.

58. The method of claim 56, further comprising coupling an inflation stem to the inflation port.

59. The method of claim 58, further comprising coupling a one-way valve to the inflation stem.

60. A method of forming a graft, comprising: providing an implant according to one of claims 1-36; deploying the implant into a disc cavity; inflating the implant with a curable material; and allowing the curable material to cure.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0047] FIG. 1 is a sectional view of an implant according to an embodiment of the present disclosure after implantation into an intervertebral space;

[0048] FIG. 2 is a sectional view of the inflation port and valve assembly of the implant of FIG. 1;

[0049] FIG. 3 is a sectional view taken along line 3-3 in FIG. 2;

[0050] FIG. 4 is a plan view of an inflatable membrane of the implant of FIG. 1;

[0051] FIG. 5 is a plan view of a mandrel for making the inflatable membrane of the implant of FIG. 1;

[0052] FIG. 6 is a plan view of a tubular fiber graft of the implant of FIG. 1;

[0053] FIG. 7 is a plan view of an access opening in the tubular fiber graft of FIG. 6;

[0054] FIG. 8 is a partial sectional view of a portion of the implant of FIG. 1, prior to installation of a retaining member;

[0055] FIG. 9 is a partial sectional view of a portion of the implant of FIG. 1, after installation of a retaining member;

[0056] FIG. 10 is a sectional view of a delivery cannula with the implant of FIG. 1, prior to deployment;

[0057] FIG. 11 is a sectional view of an alternative embodiment of an inflation stylet;

[0058] FIG. 12 is a sectional view of the implant of FIG. 1 which is partially implanted into the intervertebral space;

[0059] FIG. 13 is a sectional view of the implant of FIG. 1 which is partially implanted into the intervertebral space with a push member;

[0060] FIG. 14 is a diagrammatic view of an implanted implant of FIG. 1 with the adjacent vertebrae substantially aligned;

[0061] FIG. 15 is a diagrammatic view of an implanted implant of FIG. 1 with the adjacent vertebrae bent towards one another; and

[0062] FIG. 16 is a diagrammatic view of an implanted implant of FIG. 1 with the adjacent vertebrae bent towards one another.

DETAILED DESCRIPTION

[0063] In the following detailed description, reference is made to the accompanying drawings, in which are shown exemplary but non-limiting and non-exhaustive embodiments of the invention. These embodiments are described in sufficient detail to enable those having skill in the art to practice the invention, and it is understood that other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims. In the accompanying drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified.

[0064] FIG. 1 illustrates an implant 100 in accordance with an exemplary embodiment of the present disclosure after deployment into a disc cavity 102. The disc cavity 102 is formed by performing a discectomy to remove the natural spinal disc. In some embodiments, the discectomy is performed using minimally invasive techniques, such as percutaneous techniques, so that the annulus fibrosus 104 is left substantially intact, with only a small access opening.

[0065] The implant 100 comprises an annular ring 106 which is filled with a curable elastomeric material 108, such as a curable silicone elastomer. The properties of material 108 may be selected to provide desired properties for the implant 100. For example, curing time, cured durometer, and other physical properties, such as elongation, tear, and tensile strength may be selected to provide implant 100 with desired characteristics.

[0066] Implant 100 forms a interior cavity 110 in the interior of annular ring 106. Interior cavity 110 allows annular ring 106 to deform inwardly to relieve stress and avoid placing excessive pressure on the central region of the vertebral end plates, as will be described in further detail below.

[0067] Annular ring 106 has an inflation port 114 with a one-way valve assembly 116 which allows curable material 108 to be introduced into annular ring 106 while it is still in a flowable state (i.e., prior to curing) while preventing curable material 108 from leaking out. In certain embodiments, annular ring 106 is formed by a tubular inflatable membrane 134 and a tubular fiber graft 136 which encloses the inflatable membrane 134.

[0068] Referring to FIG. 2, inflatable membrane 134 forms an annular balloon 138 with inflation port 114. A one-way valve assembly 140 is coupled to the inflation port 114. One-way valve assembly 140 allows curable material to be injected into annular balloon 138 while preventing substantially any material from escaping. In some embodiments, inflation port 114 comprises an inflation neck 208 which is formed integrally with inflatable membrane 134. In certain embodiments, an inflation stem 142 with a lumen 148 extending from a proximal end 144 to a distal end 146 is inserted into inflation neck 208 and coupled to inflation neck 208 by adhesive, welding, or the like. One way valve assembly 140 may comprise a duckbill valve (i.e., a flutter-type or Heimlich valve) 150 comprising a thin elastomeric material extending from inflation stem 142. The thin elastomeric material of duckbill valve 150 stretches to allow curable material 108 to flow through it when pressure is applied to curable material 108. When pressure is removed, the thin elastomeric material constricts to prevent back-flow. Duckbill valve 150 may be coupled to inflation stem 142 prior to assembly with inflatable membrane 134, or may be coupled to inflation stem 142 after inflation stem 142 is coupled to inflatable membrane 134. One-way valve assembly 140 and inflation stem 142 may be placed substantially in the inflation neck 208. That is, they may be placed so that they are outside of the annular portion of inflation membrane 134. Placing the inflation and valve componentry outside of the annular portion of inflation membrane 134 eases manufacturing, improves the function of implant 100 during deployment, and improves the functionality and durability of implant 100 after deployment.

[0069] In some embodiments, inflatable membrane 134 is formed of an elastic material, such as silicone, so that it is compliant (i.e., it expands as the internal pressure increases). A compliant balloon reduces the need for precise sizing of the membrane. In other embodiments, inflatable membrane 134 is semi-compliant. That is, inflatable membrane 134 expands to a given diameter under a certain amount of pressure, and only expands moderately from this diameter as the internal pressure increases beyond that pressure. Semi-compliant inflatable membranes 134 may be advantageous in some circumstances.

[0070] Inflatable membrane 134 may be formed by conventional techniques, such as extrusion, injection molding or dip casting. In some embodiments, inflatable membrane 134 is formed by injection molding. Referring to FIGS. 4 and 5, a core mandrel 152 is used in conjunction with corresponding injection molding dies (not shown). In some embodiments, mandrel 152 comprises three pieces 154, 156, 158 which may be removably attached to one another by interlocking joints, such as threads or keys. Mandrel 152 is placed into the molding dies, and uncured silicone is injected into the die and allowed to cure to form inflatable membrane 134. After inflatable membrane 134 is cured, the molding dies are opened, and the mandrel with the inflatable membrane 134 is removed. The mandrel may then be removed through inflation port 114 and inflation neck 208. In some embodiments, mandrel pieces 154, 156 and 158 are disassembled and removed through inflation port 114 and inflation neck 208. In other embodiments, inflatable membrane 134 may be cut to form first and second legs 160, 162 with open ends 120, 122 through which mandrel pieces 156 and 158 are removed. In certain embodiments, first and second legs 160,162 of inflatable membrane 134 are approximately the same length, although they may be unequal lengths.

[0071] Cutting inflatable membrane 134 to form legs 160, 162 allows tubular fiber graft 136 to be formed separately and then installed on inflatable membrane 134. Referring to FIGS. 6 and 7, in some embodiments, tubular fiber graft 136 comprises a textile formed of a biocompatible material. The textile material may be a woven, braided or knitted durable biocompatible material. In some embodiments, tubular graft 136 comprises a first layer comprising a plurality of semi-elastic or substantially inelastic fibers extending longitudinally and circumferentially along graft 136. In certain embodiments, a second layer of semi-compliant fibers are layered over the first layer. In other embodiments, circumferential fibers are formed from substantially inelastic materials, and hoop fibers are formed from semi-elastic materials. This allows the graft 100 to expand moderately in the cross-sectional plane while constraining radial or equatorial expansion. In this manner, graft 100 mostly deforms inward toward interior cavity 110 and in the axial or craniocaudal plane. In some embodiments, tubular fiber graft 136 incorporates radiopaque markers at one or more locations to enable clinicians to visualize graft 100 during implantation. In certain embodiments, the radiopaque markers comprise radiopaque fibers.

[0072] The cross-sectional diameter of tubular fiber graft 136 is selected to allow inflatable membrane 134 to be inflated to full size while preventing over-expansion of inflatable membrane 134. The materials are selected so that inflatable membrane 134 does not bond to fiber graft 136 and is free to move within fiber graft 136 to a limited extent.

[0073] In some embodiments, tubular fiber graft 136 is a split annular ring with first and second open ends 164, 166. An opening 168 is provided in tubular fiber graft 136 to provide access to inflation port 114. Opening 168 may be reinforced by stitching (e.g., a buttonhole stitch). Further, a reinforcing member 170 may be provided to reinforce the opening. Fiber graft 136 is installed over inflatable member 134 by placing inflatable member 134 through either first and second open end 164, 166 and threading it through fiber graft 136. Inflation neck 208 is placed through opening 168.

[0074] Open ends 164, 166 of fiber graft 136 and open ends 120, 122 of inflatable membrane 134 are coupled to one another to form implant annular ring 106. In one embodiment, a coupling member 124 is provided to couple open ends 164, 166 and open ends 120, 122. Referring to FIGS. 8 and 9, coupling member 124 comprises a cylindrical member with a groove 172. In FIGS. 8 and 9, inflatable member 134 and fiber graft 136 form first and second tubular legs 174, 176 with open distal ends 178, 180, respectively. For clarity, inflatable membrane 134 and fiber graft 136 are shown as a single line in FIGS. 8 and 9. Coupling member 124 is placed into the interior of open distal end 180 of second tubular leg 176, and open distal end 178 of first tubular leg 174 is placed over coupling member 124 and distal end 180 of second tubular leg 176. A retaining member 182 is placed over coupling member 124 to couple distal ends 178, 180 to coupling member 124. In some embodiments, retaining member 182 is a permanently crimpable member which is crimped into groove 172. Retaining member 182 may comprise a radiopaque material to serve as a radiopaque marker.

[0075] In some embodiments, coupling member 124 may be a solid member which forms a partition to prevent fluid communication between first and second tubular legs 174, 176. In other embodiments, coupling member may have a lumen which connects first and second tubular legs 174, 176.

[0076] FIG. 10 illustrates implant 100 loaded into percutaneous deployment device 112. Percutaneous deployment device 112 comprises a delivery cannula 128, a release cannula 130 and an inflation stylet 132. Deployment device 112 may be placed in an introducer or access cannula 126. Access cannula 126 extends through annulus fibrosus 104 to provide access to disc cavity 102. Access cannula 126 is deployed using conventional percutaneous access techniques. Access cannula 126 may be a conventional cannula. In some embodiments, access cannula 126 comprises an access cannula used to remove the nucleus pulposus, such as the access cannula described in US Patent Publication No. 2014/0276832, entitled Surgical Device, which is hereby incorporated by reference in its entirety. Implant 100 is stretched out in a deflated state and placed into delivery cannula 128. The inner diameter of delivery cannula 128 is substantially the same as the outer diameter of inflation stem 142 so that inflation stem 142 fits snugly into delivery cannula 128. The outer diameter of release cannula 130 is selected to fit snugly into delivery cannula 128. The distal end 184 of release cannula 130 engages the proximal end 144 of inflation stem 142 so that release cannula 130 can be used to push inflation stem 142 and thus implant 100 out of the end of delivery cannula 128 to deploy implant 100 into disc cavity 106. Release cannula 130 can also be used to hold inflation stem 142 into place while withdrawing inflation stylet 132 after deployment (as will be described in further detail below).

[0077] Inflation stylet 132 is placed into inflation stem 142. Inflation stem 142 is elastic and stretched to fit over the outer diameter of inflation style 132 so that the two pieces fit together snugly. The snug fit of inflation stylet 132 into inflation stem 142 together with the snug fit of inflation stem 142 into delivery cannula 128 form a tight seal to substantially prevent leakage during deployment and inflation of implant 100. Furthermore, the snug fit prevents inadvertent dislodgment of the inflation stylet prior before completion of the inflation, despite the relatively high pressures which may be used to inflate inflatable membrane 134.

[0078] Referring to FIG. 11, in certain embodiments, inflation stem 186 is tapered from a proximal end 194 to a distal end 196. A distal end 198 of the inflation stylet 198 forms a complementary shape. A lock 200 releasably holds inflation stylet 198 in inflation stem 186. In certain embodiments, lock 200 comprises a protuberance 192 which engages a groove 190. In certain embodiments, protuberance 192 comprises a ridge molded around inflation stylet 198. The size, shape and number of ridges and grooves can be selected to provide a desired force required to detach the inflation stylet from the inflation stem.

[0079] Referring to FIG. 12, to deploy implant 100, the existing nucleus pulposus at the target site is removed by inserting access cannula 126 through a small access opening through the annulus fibrosus 104. The existing nucleus pulposus is removed through access cannula 126 by performing a discectomy. The annulus fibrosus 104 is left substantially intact to form disc cavity 102.

[0080] Implant 100, which has been loaded into percutaneous deployment device 112, is inserted into the disc space. Release cannula 130 and inflation stylet 132 are pressed toward disc cavity 102 to engage proximal end 144 of inflation stem 142 and begin to deploy implant 100. In certain embodiments, a push member 210 extends from inflation stylet 132 and engages coupling member 124. During deployment, push member 210 pushes coupling member 124 toward the far end of the disc cavity 102 to help ensure proper placement of implant 100.

[0081] Alternatively, release cannula 130 may be advanced so that implant 100 is partially deployed, and curable material 108 may be delivered through inflation stylet 132 to partially inflate inflatable membrane 134. Release cannula 130 may then be advanced again, and additional curable material 108 may be delivered. This process is repeated until inflation stem 142 has been advanced to the distal end of delivery cannula 128, thereby fully deploying implant 100.

[0082] After implant 100 is fully deployed into disc cavity 102, curable 108 is deployed into the inflatable membrane 134 to press implant 100 firmly against annulus fibrosus 104 and distract adjacent vertebral segments. A pressurized syringe may be used to deploy curable material 108 and supply sufficient pressure achieve the desired intervertebral distraction. After sufficient curable material 108 is deployed, inflation stylet 132 may be removed. Release cannula 130 holds inflation stem 142 into place, thereby preventing inadvertent withdrawal of implant 100 when inflation style 132 is removed. One-way valve 116 prevents curable material 108 from leaking out of implant 100. Thus, inflation stylet 132 may be removed prior to curing of curable material 108. When inflation stylet 132 is removed, inflation stem 142, which is formed of an elastomeric material, collapses so that it only leaves a small amount of material in the opening of the annulus fibrosus (i.e., the annulotomy). The collapsed inflation stem 142 reinforces the function of duckbill valve 150 by preventing duckbill valve 150 from inverting due to the pressure of the curable material (prior to curing).

[0083] Curable material 108 may be a silicone elastomer. The properties of the material, such as the curing time, uncured viscosity, cured durometer, etc. may be selected as desired to provide the desired properties for graft 100, which are dependent upon the patient under treatment. The curable material 108 may be compatible with the material of inflatable membrane 134 so that they fuse together and form a single component.

[0084] Referring to FIGS. 13-15, once implant 100 has cured, it forms a substantially non-compressible ring 106 which is contained within annulus fibrosus 104. Implant 100 distracts adjacent vertebral segments 202 by pressing against vertebral end plates 204. Interior cavity 110 is formed in the center of annular ring 106. When vertebral segments 202 are moved with respect to one another, annular ring 106 deforms into interior cavity 110. This prevents implant 100 from being subjected to too high of pressures, and prevents implant 100 from applying too high of pressures to vertebral end plates 204. Furthermore, the peripheral location of implant 100 distributes weight to the peripheral portions of the vertebral end plates, and interior cavity 110 prevents excessive force from being applied to the central region 206 of vertebral end plates 204. The peripheral portions of the end plate are typically stronger than the central regions. Further, interior cavity 110 provides space for shock absorption and for inward deformation during loading and sudden increases in intradiscal pressure.

[0085] The above specification and examples provide a complete description of the structure and use of exemplary embodiments. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the various illustrative embodiments of the present devices are not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, components may be combined as a unitary structure, and/or connections may be substituted (e.g., threads may be substituted with press-fittings or welds). Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments.

[0086] The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) means for or step for, respectively.