GB2483531A

GB2483531A - Spinal fixation system

Info

Publication number: GB2483531A
Application number: GB1108901.8A
Authority: GB
Inventors: Shahar Peled; Yaniv Glozman
Original assignee: FACETMED Ltd
Current assignee: FACETMED Ltd
Priority date: 2010-09-13
Filing date: 2011-05-26
Publication date: 2012-03-14
Also published as: WO2012035479A2; GB201108901D0; WO2012035479A3

Abstract

The system comprises a backbone forming part (400, fig 1), at least two bone screws 210, at least one crown element 220, 222 adapted to engage the spherical head 214 of the screw, a head 250 having a portion to accommodate the crown and a portion having a recess to accommodate the backbone forming part and an annular tab 260 which in use holds the crown element within the head. The crown element may be formed a top element 220 and four lateral elements 222, the lateral elements having knurled or bored bone-adjoining portion 224. The portions 224 may be circumferentially confined by annular member 220. The head has a screw thread 252 for a bolt which in use confines the backbone forming part within the recess 254.

Description

SPINAL FIXATION SYSTEM AND PEDICLE SCREW THEREFOR

TECHNICAL FIELD

[001] The present invention relates to posterior spinal fixation systems and methods. In particular, the invention relates to spinal fixation systems that retain some degrees of movement relatively to the vertebra and a pedicle screw therefor.

BACKGROUND ART

[002] It is believed that the pertinent state-of-the-art is represented by US patents Ser. Nos 4805602, 5360431, 5129388, 6032677, 7303563, 5899905, US patent applications Ser. Nos 2005182404, 2008015585, 2009254123, 2002111692, 2005283153, 2005277920, European patent applications Ser. Nos 1059066, 0407332; German patents/applications Ser. Nos DE4110002 and DE4107480, as well as by Chinese patents/applications Ser. Nos 2562737, 2788752, 201168036, 101224133 and international publication W020080451 79.

SUMMARY OF THE INVENTION

[003] Known in the art spinal fixation systems that interconnect several vertebrae are typically affixed during their implementation, forming a rigid interconnecting backbone structure that is anchored in the vertebrae and essentially immovable relatively thereto.

[004] There are spinal fixation systems interconnecting several vertebrae while allowing some degrees of freedom in the movement of one vertebra relatively to the other, such as these disclosed in international patent application W02005087120 entitled "PEDICLE SCREW" and US patent 6402752 entitled "POLYAXIAL PEDICLE SCREW".

However neither of those systems allows a degree of freedom in the movement of the anchoring element and the vertebra itself As will become apparent, a degree of freedom in the movement of the anchoring element and the vertebra itself entails several clinical benefits.

[005] One aspect of the invention is concerned with providing spinal fixation systems, wherein the anchoring element, henceforth referred to as pedicle screw, comprises a part that adjoins the vertebra and retains some degrees of movement relatively thereto. The movement of the part that adjoins the vertebra is characterized by that it is more of an isotropic than polyaxial type. The movements of the part that adjoins the vertebra are characterized by that they are micro-and/or vibratory movements. The micro-and/or vibratory movements, henceforth MVM, between the part of the pedicle screw that adjoins the vertebra and the facet of the vertebra disposed vis-à-vis said part facilitate a prompt ingrowth of the osseous tissue from the latter into the former. The MVM are further distribute and disperse the loads exerted onto pedicle screw and amortize the kinetic energy of the moments within the spinal fixation system.

[006] Another aspect of the invention is concerned with providing spinal fixation systems is concerned with preserving the structural integrity of the vertebral tissue into which the pedicle screw is screwed. The osseous tissue is characterized by heterogeneous density. Cortical osseous tissue is denser, whereas the trabecular or cancellous osseous tissue is relatively more porous and consequently weaker. As a result of the heterogeneous osseous density, the loads exerted onto pedicle screws are unevenly distributed between the cortical and cancellous osseous tissues. Therefore frequently the loads exerted onto pedicle screws known in the art, gradually defect the structural integrity of the cortical osseous tissue surrounding the pedicle screws; whereby the pedicle screws gradually loosening and progressively gain displacement freedom within the osseous tissue.

[007] In light of the foregoing, MVM that distribute and disperse the loads exerted onto pedicle screw and hence contribute to the deduction of loads exerted onto cortical osseous tissue. Consequently the distribution and dispersement of the the loads exerted onto pedicle screw by formation of MVM preserve the structural integrity of the cortical osseous tissue.

DEFIN ITION

[008] The terms: facet, vertebral facet and surface of the facet, throughout the specification and claims hereinafter, refer to a portion of the posterior surface of the vertebra, corresponding to a protection of the vertebral pedicle, into which the pedicle screws known in the art are usually screwed. The aforementioned portion of the posterior vertebral surface is colloquially referred to as upper pedicle.

DESCRIPTION OF THE DRAWINGS

[009] The present invention will be understood and appreciated more comprehensively from the following detailed description taken in conjunction with the appended drawings in which: FIG. IA is a posterior perspective view of an inferior portion of a human spine; FIG. lB is an enlarged view of the inferior portion of a human spine shown in FIG. IA, incorporating a exemplar spinal fixation assembly; FIG. IC is an isometric view of the exemplar spinal fixation assembly, shown in FIG. IA; FIG. 2A is an isometric exploded view of an embodiment of a pedicle screw; FIG. 2B is a side view of the pedicle screw shown in FIG. 2A; FIG. 2C is a cross-sectional view of the pedicle screw shown in FIG. 2B; FIG. 2D is a top view of the pedicle screw shown in FIG. 2A; FIG. 2E is a bottom view of the pedicle screw shown in FIG. 2A; FIG. 2F is an enlarged view of the cross-section shown in FIG. 2C; FIG. 3A is an isometric view of another embodiment of a pedicle screw; FIG. 3B is an isometric exploded view of an embodiment of a bone anchoring plate; FIG. 3C to 3E are respectively a side, front and top views of an assembly of the pedicle screw shown in FIG. 3A and the bone anchoring plate shown in FIG. 3B, depicting various degrees of freedom in movements of the former relatively to the latter; FIG. 3F to 31 are respectively a side, isometric, isometric exploded and side exploded views a bone screw and buffering means therefor.

[010] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown merely by way of example in the drawings. The drawings are not necessarily complete and components are not inevitably to scale; emphasis instead being placed upon clearly illustrating the principles underlying the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[011] Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with technology-or business-related constraints, which may vary from one implementation to another. Moreover, it will be appreciated that the effort of such a development might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the

benefit of this disclosure.

[012] In accordance with some embodiments of the present invention, reference is now made to FIGS IA to IC, showing an inferior portion of exemplar human spine 200.

The inferior portion of spine 200 comprises lumbar vertebrae LI to L5, defining the lumbar curve spine 200. Os sacrum OS is disposed inferiorly to vertebra L5 and terminates with Coccyx CO. Lumbar vertebrae LI to L5 are interspaced by intervertebral discs D12, D23, D34, and D34, respectively; whereas disc DOS interspaces between vertebra L5 and Os sacrum OS. Exemplary assemblies 201A and 201A are anchored within vertebrae L4 and L5 to partially sustain the loads exerted onto intervertebral disc D45 and disperse the kinetic energy of the movements between vertebrae L4 and L5 by amortization thereof to MVM; thereby compensating for a performance of defected intervertebral disc D45.

[013] Exemplary assembly 201 comprises vertebra adjoining pedicle screw (henceforth VAPSSS) 205, VAPSS 300 and backbone forming part (thereinafter BFP) 400, interbridging between VAPSS 205 and VAPS 300. VAPS 205 comprises bone screw 210 and polyaxial head 250. Bone screw 210 of VAPS 205 is screwed into to the osseous tissue of vertebral pedicle, substantially coaxially to the longitudinal centerline thereof.

Consequently polyaxial head 250 of VAPS 205 is urged vis-à-vis a facet of the vertebra.

[014] BFP 400 is inserted into arcuate recess at the posterior portion of polyaxial head 250. BFP 400 is tightened within the arcuate recess at the posterior portion of polyaxial head 250 by bolt 405, forming interbridging structure between VAPS 205 and VAPS 300.

[015] Bone anchoring plate 450 is fastened into exterior circumferential groove of polyaxial head 350 of VAPS 300 and tightened to the vertebra by screw 470, forming a modular structure characterized by at least two potential degrees of freedom in the movements of various parts thereof relatively to VAPS 300 and capable of further amortization and dispersing of kinetic energy and/or forces exerted onto VAPS 300; thereby forming MVM and facilitating a prompt ingrowth of the osseous tissue into plate 450.

[016] Reference is now made to FIGS 2A to 2F, wherein in accordance with some embodiments of the present invention, VAPS 205 is shown. VAPS 205 comprises bone screw 210, terminating with pointed anterior tip 211 and furnished with screw threading 212, adapted for screwing screw 210 into to the osseous tissue of vertebral pedicle, substantially coaxially to the longitudinal centerline thereof The posterior of bone screw 210 of VAPS 205 comprises terminal spherical head 214, having polygonal recess 216 therein, adapted for operationally connecting a screw driving tool (not shown) thereto.

[017] VAPS 205 further comprises top crown element 220 and lateral crown elements 222. Top crown element 220 and lateral crown elements 222 for an assembly of an essentially spherical shell shape. A plurality of structured explode lines across the spherical shell assembly are formed at the interfaces between crown elements 220 and elements 222 and/or between lateral crown elements 222.

[018] The spherical shell assembly of top crown element 220 and lateral crown elements 222 is adapted to contiguously engage posterior spherical head 214 of bone screw 210, in a spherical joint-like manner; thereby forming a first articulation movement between terminal spherical head 214 of bone screw 210 and crown and crown elements 220 and 222. Lateral crown elements 222 comprise bone adjoining portions 224 that are furnished with a plurality of bores and/or knurling 226, providing for an ingrowth of the osseous tissue of the vertebra into portions 224 of lateral crown elements 222. Bone adjoining portions 224 of lateral crown elements 222 are circumferentially confined by annular member 230.

[019] The aforementioned first articulation movement between terminal spherical head 214 of bone screw 210 and crown elements 220 and 222, inter alia, contributes to a positioning of bone adjoining portions 224 of lateral crown elements 222 and annular member 230 vis-à-vis the vertebra, urged towards the surface of the facet thereof.

[020] Portions 224 of lateral crown elements 222 and annular member 230 as well as any other bone facing and/or adjoining members or parts of the spinal fixation system of the invention can be covered or otherwise furnished with hydroxylapatite Ca5(P04)3(OH), to promote proliferation and adhesion of the osseous tissue.

[021] The anterior portion of polyaxial head 250 of VAPS 205 is adapted to accommodate the spherical assembly of crown elements 220 and 222, in a joint-like manner; thereby forming a second articulation movement between polyaxial head 250 and crown elements 220 and 222. The posterior portion of polyaxial head 250 of VAPS 205 comprises screw threading 252 and arcuate recess 254, adapted for operational connection to one or more BFP/s, interconnecting VAPS 205 with other VAPS/s.

[022] The assembly of VAPS 205 is held by annular tab 260 that is secured by spring washer 262. Annular tab 260 holds the spherical assembly of crown elements 220 and 222 within the anterior portion of polyaxial head 250; whereas spring washer 262 inserted into circumferential groove 264 within annular tab 260 is disposed within anterior polyaxial head 250, thereby securing assembly of VAPS 205.

[023] Upon screwing bone screw 210 of VAPS 205 into to the osseous tissue of vertebral pedicle, bone adjoining portions 224 of lateral crown elements 222 are urged towards the facet of the vertebra; whereby the aforementioned first articulation movement between terminal spherical head 214 of bone screw 210 and crown and crown elements 220 and 222 allows bone adjoining portions 224 to assume the right position vis-à-vis the vertebral tissue, providing inter alia for the formation of MVM therebetween.

[024] The aforementioned second articulation movement between polyaxial head 250 and crown elements 220 and 222 facilitates a more resilient connection between VAPS 205 and a BFP (not shown), which interconnects VAPS 205 with other VAPS (not shown), providing inter alia for the formation of MVM in the spinal fixation system of the invention. The aforementioned second articulation movement additionally provides for attaining a desired position and/or orientation of polyaxial head 250 of VAPS 205, for the installation of the BFP (not shown), prior to the final tightening of VAPS 205 to the vertebra, during the implantation at the surgery room.

[025] In some preferred embodiments annular tab 260 comprises a prominent top portion. In such embodiments, upon inserting a BFP into arcuate recess 254 of polyaxial head 250 and tightening thereof therein by a bolt screwed about screw threading 252, a force is exerted by the bottom portion of the BFP onto the prominent top portion of annular tab 260; whereby annular tab 260 is forcefully urges top crown element 220 downwardly.

Consequently the top crown element 220 pushes lateral crown elements 222; thereby distorting the spherical arrangement of crown elements 220 and 222 and contributing to the exploding of the spherical assembly thereof about the structured explode lines formed at the interfaces between crown elements 220 and elements 222 and/or between lateral crown elements 222. The aforementioned distorting of the spherical arrangement of crown elements 220 and 222 and exploding of the spherical assembly thereof about the structured explode lines substantially hampers the aforesaid second articulation movement between polyaxial head 250 and crown elements 220 and 222; whereby the kinetic energy and/or loads associated with VAPS 205 are essentially conveyed for the formation of MVM by the aforesaid first articulation movement between terminal spherical head 214 of bone screw 210 and crown elements 220 and 222. Accordingly, upon tightening the BFP within arcuate recess 254 of polyaxial head 250 by a bolt, the majority of kinetic energy and/or loads associated with VAPS 205 are thereafter dispersed as MVM formed between the bone adjoining portions 224 of lateral crown elements 222 and annular member 230 and the surface of the facet of the vertebra.

[026J In some yet further preferred embodiments annular tab 260 and top crown element 220 comprise an interlocking structural arrangement, such as respective ridge and grove, which interlock and thereby preclude the movement annular tab 260 and top crown element 220 relatively to each other. The preclusion of the movement annular tab 260 and top crown element 220 relatively to each diminishes the aforesaid second articulation movement between polyaxial head 250 and crown elements 220 and 222, upon tightening the BFP within arcuate recess 254 of polyaxial head 250 by a bolt; thereby conveying essentially all the kinetic energy and/or loads associated with VAPS 205 to be dispersed as MVM between the bone adjoining portions 224 of lateral crown elements 222 and annular member 230 and the surface of the facet of the vertebra.

[027] Reference is now made to FIGS 3A to 31, wherein in accordance with some embodiments of the present invention, VAPS 300, bone anchoring plate 450 and screw 470 are shown. Polyaxial head 350 of VAPS 300 comprises exterior circumferential groove 356, used for mounting anchoring plate 450, as will be elaborated infra.

[028] Bone anchoring plate 450 comprises platform 452, pin 454 and yoke 456.

Bone facing surfaces of platform 452 are furnished with a plurality of bores and/or knurling 453, providing for an ingrowth of the osseous tissue of the vertebra into plate 450. Bone anchoring plate 450 comprises oval aperture 455, adapted to receive minor pedicle screw 470, screwed into the osseous tissue of the vertebra thus tightening anchoring plate 450 thereto. Platform 452 further includes apertures 462, adapted to receive pin 454 therein.

[029] Yoke 456 terminates with frusto-bulb-shaped element 458 and comprises aperture 460. Pin 454 comprises screw threading 459, for securing pin 454 within apertures 460 of platform 452. Bone anchoring plate 450 is fastened into exterior circumferential groove 356 of polyaxial head 350, by positioning the frustomed faces of bulb-shaped element 458 essentially in parallel to the circumferential ridges of groove 356 and rotating plate 450 in about 90 degrees.

[030] Frusto-bulb-shaped element 458 of yoke 456 of anchoring plate 450 is slidable along the circumferential ridges of groove 356 of polyaxial head 350 rendering platform 450 capable to assume various angular positions in the directions indicated by arrows 394, 396 and 398, whereas platform 452 is pivotally rotatable relatively to yoke 456 in the directions indicated by arrows 399.

[031] Minor bone screw 470 terminates with pointed anterior tip 471 and is furnished with screw threading 472, adapted for screwing screw 470 into to the osseous tissue of vertebra. The posterior face of having screw 470 comprises polygonal recess 476, adapted for operationally connecting a screw driving tool (not shown) thereto.

[032] Bone screw 470 comprises circumferential groove 474, adapted to accommodate buffering means 480. Buffering means 480 is an annular element comprising slanted slot 482. Bone anchoring plate 450 is fastened into exterior circumferential groove 356 of polyaxial head 350 and screwed to the vertebra by screw 470, forming a modular structure characterized by at least two potential degrees of freedom in angulations of the former relatively to the latter; capable of amortization and dispersing of kinetic energy and/or forces exerted onto VAPS 350, thereby forming MVM and facilitating a prompt ingrowth of the osseous tissue.

[033] It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims which follow:

Claims

CLAIMS1. A posterior spinal fixation system 201, comprising: [a] at least one backbone forming part; [b] at least two vertebra adjoining pedicle screws 205, said pedicle screws comprising: [1] a bone screw 210, said bone screw 210 comprises: [ii a screw threading 212, adapted for screwing screw 210 into the osseous tissue of vertebral pedicle; [ii] a pointed anterior tip 211; [iii] a posterior spherical head 214, having a recess 216 therein, adapted for operationally connecting a driving tool thereto; [2] at least one crown element, said at least one crown element is adapted to contiguously engage said posterior spherical head 214 of said bone screw 210, in a spherical joint-like manner; [3] a polyaxial head 250, said polyaxial head 250 comprises: [i] an anterior portion, said anterior portion is adapted to accommodate said at least one crown element, in a joint-like manner; [ii] a posterior portion, said posterior portion comprises a recess 254 adapted to accommodate said backbone forming part, said recess 254 comprises a screw threading 252 for tightening said backbone forming part therein by a bolt 405; [4] an annular tab 260, said annular tab 260 holds said at least one crown element within said anterior portion of said polyaxial head 250; said system is characterized by: [c] a first articulation movement formed between said spherical head 214 of said bone screw 210 and said at least one crown element; [d] a second articulation movement formed between said polyaxial head 250 and said at least one crown element.
2. The posterior spinal fixation system as in claim 1, wherein said pedicle screws further comprising a spring washer 262 inserted into circumferential groove 264 within said annular tab 260 and disposed within polyaxial head 250, thereby securing assembly of said pedicle screws 205.
3. The posterior spinal fixation system as in claim 1, wherein said annular tab 260 is affixable within said polyaxial head 250. -11 -
4. The posterior spinal fixation system as in claim 1, wherein said crown is an assembly of an essentially spherical shell shape, comprising a plurality of individual elements, said elements form structured explode lines across said spherical shell.
5. The posterior spinal fixation system as in claim 1, wherein said crown comprises a top element 220 and approximately four lateral elements 222, forming an assembly of an essentially spherical shell shape, wherein a plurality of structured explode lines are across said spherical shell, at the interfaces between said top element 220 and/or said lateral elements 222.
6. The posterior spinal fixation system as in any one of the claims 1, 4 or 5, wherein said crown or crown elements comprise/s bone adjoining portion/s 224.
7. The posterior spinal fixation system as in any one of the claims 1 to 6, wherein said pedicle screws further comprising an annular member 230 circumferentially confining said bone adjoining portions 224 of said crown.
8. The posterior spinal fixation system as in any one of the claims 6 or 7, wherein any surface to adjoin a vertebral tissue, selected from group consisting of: a surface of said bone adjoining portions 224 and a surface of said bone adjoining portions 224, are furnished with a plurality of bores and/or knurling 226, allowing an ingrowth of the osseous tissue thereto.
9. The posterior spinal fixation system as in any one of the claims 1 to 8, wherein any surface that adjoins the vertebral tissue is furnished with hydroxylapatite Ca5(P04)3(OH), to promote proliferation and adhesion of the osseous tissue.
i0.The posterior spinal fixation system as in any one of the claims ito 9, wherein upon screwing said bone screw 210 into the osseous tissue of vertebral pedicle, said bone adjoining portion/s 224 of said crown is/are urged towards the facet of the vertebra.
ii. The posterior spinal fixation system as in any one of the claims 1 to 10, wherein said first articulation movement between said terminal spherical head 214 of said bone screw 210 and said crown and allows said bone adjoining portions 224 to -12-assume a position vis-à-vis the vertebral tissue.
12. The posterior spinal fixation system as in any one of the claims 1 to 11, wherein said first articulation movement between said terminal spherical head 214 of said bone screw 210 and said crown contributes for the formation of micro-and/or vibratory movements between said bone adjoining portions 224 and a facet of the vertebral tissue.
13.The posterior spinal fixation system as in any one of the claims ito 12, wherein said second articulation movement between said polyaxial head 250 and said crown facilitates attaining a desired position and/or orientation of said polyaxial head 250 for the installation of said BFP.
14.The posterior spinal fixation system as in any one of the claims ito 13, wherein said annular tab 260 comprises a prominent top portion.
15.The posterior spinal fixation system as in any one of the claims 1 to 14, wherein upon tightening of said BFP within said arcuate recess 254 of said polyaxial head 250 by said bolt, said annular tab 260 is forcefully urged against siad crown
16. The posterior spinal fixation system as in any one of the claims I to 15, wherein upon tightening of said BFP within said arcuate recess 254 of said polyaxial head 250 by said bolt, siad top crown element 220 pushes said lateral crown elements 222; thereby distorting the spherical arrangement of said crown elements 220 and 222 and contributing to the exploding of the spherical assembly thereof about said structured explode lines.
17. The posterior spinal fixation system as in any one of the claims 1 to 16, wherein said second articulation movement between said polyaxial head 250 and said crown is controllably hamperable upon tightening of said BFP within said arcuate recess 254 of said polyaxial head 250 by said bolt.
18.The posterior spinal fixation system as in any one of the claims 1 to 17, wherein upon tightening of said BFP within said arcuate recess 254 of said polyaxial head 250 by said bolt, the majority of kinetic energy and/or loads associated with said vertebra adjoining pedicle screws 205 are dispersed as micro-and/or vibratory movements between said bone adjoining portions 224 and a facet of the vertebral tissue.
19. The posterior spinal fixation system as in any one of the claims 1 to 18, wherein said annular tab 260 and said crown comprise an interlocking structural arrangement, which precludes the movement of said annular tab 260 relatively to said crown.
20. The posterior spinal fixation system as in any one of the claims Ito 19, wherein said second articulation movement between said polyaxial head 250 and said crown is controllably precluded upon tightening of said BFP within said arcuate recess 254 of said polyaxial head 250 by said bolt.
21.The posterior spinal fixation system as in any one of the claims 1 to 19, further comprising a modular bone anchoring plate 450.
22.The posterior spinal fixation system as in any one of the claims 1 to 21, further comprising a platform 452, pin 454 and yoke 456.
23.The posterior spinal fixation system as in any one of the claims 21 or 22, wherein any surface to adjoin a vertebral tissue is furnished with a plurality of bores and/or knurling 453, allowing an ingrowth of the osseous tissue thereto.
24.The posterior spinal fixation system as in any one of the claims 21 to 23, wherein any surface that adjoins the vertebral tissue is furnished with hydroxylapatite Ca5(P04)3(OH), to promote proliferation and adhesion of the osseous tissue.
25.The posterior spinal fixation system as in any one of the claims 21 to 24, wherein said one anchoring plate 450 comprises oval aperture 455, adapted to receive minor pedicle screw 470.
26. The posterior spinal fixation system as in any one of the claims 21 to 25, further comprising a minor pedicle screw 470 adapted to be screwed into the osseous tissue of the vertebra thus tightening anchoring plate 450 thereto.
27.The posterior spinal fixation system as in any one of the claims 21 to 26, wherein -14 -said yoke 456 terminates with frusto-buib-shaped element 458.
28.The posterior spinal fixation system as in any one of the claims 21 to 27, further comprising a vertebra adjoining pedicle screw 300, said pedicle screw 300 comprises a polyaxial head 350 characterized by an exterior circumferential groove 356 therein.
29.The posterior spinal fixation system as in any one of the claims 21 to 28, wherein said bone anchoring plate 450 is fastenable within said exterior circumferential groove 356 of said polyaxial head 350.
30.The posterior spinal fixation system as in any one of the claims 21 to 29, wherein said minor pedicle screw 470 is further furnished with a buffering means 480.
31.The posterior spinal fixation system as in claim 30, wherein said buffering means 480 is an annular element comprising slanted slot 482.