Spinal distraction system

Abstract

A spinal distraction system and a method of assembly of a spinal distraction system. The spinal distraction system comprises a fixation rod, sliding rod, bearing connector, stop ring, coil spring and one or more bushings. The coil spring is arranged around the sliding rod in between the bearing connector and the stop ring such that one outer end of the coil spring is supported against the stop ring and the other outer end of the coil spring is supported against the bearing connector. As a result of compression of the coil spring a restoring force acts on the fixation rod and sliding rod. One or more bushings are configured to be arranged onto the coil spring as to maintain a spacing between the sliding rod and the coil spring.

Description

[0001] P37308PC00

[0002] SPINAL DISTRACTION SYSTEM

[0003] The invention relates to a spinal distraction system for dynamic correction of spinal deformation of a patient, for example during skeletal growth.

[0004] Spinal deformation during skeletal growth is a disorder with potential devastating consequences. Surgical treatment options are bothersome because correction and fusion of the deformation is incompatible with maintaining growth of the spine. Systems have been used like traditional growing rods, VEPTR (vertical expandable prosthetic titanium rib), and recently magnetically controlled growing rods, also called MAGnetic Expansion Control rods (MAGEC). However, these known growing systems may require repeat operations or repeat elongations at certain intervals. These intervals are not similar to normal gradual growth of the spine, allow the spine to stiffen between lengthenings, lead to unphysiological strains on the tissues at the moment of lengthening, and can be a burden on the patient. In cases where surgical releases of the spine are to be performed, such as congenital deformations, the spine can re-fuse soon after operative release and static fixation with any current growth system and further growth may be impossible even with repetitive distractions.

[0005] EP3406212 discloses a spinal distraction system according to the preamble of claim 1. In this known spinal distraction system, the fixation rod and sliding rod are configured to be secured to a respective bone anchor, or a respective set of multiple bone anchors. Each bone anchor is configured to be connected to a vertebra. Each bone anchor configured to be secured to the fixation rod is to be connected to a vertebra in a first region, and each bone anchor secured to the sliding rod is to be connected to a vertebra in a second region, which is spaced from the first region (in practice by multiple vertebra). The coil spring is to be arranged over the sliding rod, between the bearing connector and the stop ring. The spring is configured to be compressed between the bearing connector and the stop ring in order to provide for a spring force. As a result, a distraction force is exerted between the first and second region of the spine. This spring based distraction force can allow for spinal growth and provide continuous distraction. The magnitude of the distraction force can be set, possibly adjusted, operatively by a surgeon, e.g. based upon tactile feedback or a pre-tension / length table. Additionally, the spinal distraction system can be used in combination with existing posterior spinal fusion systems.In practice, two systems may be mounted side-by-side along the spine to achieve the desired correctional effect.

[0006] As it is envisaged that the installed spinal distraction system(s) can be left in place implanted without being accessed for a long time, it is desired to improve the known system, in particular in view of its long term behavior.

[0007] The invention proposes a spinal distraction system which comprises:

[0008] a fixation rod configured to be connected to a bone anchor;

[0009] a sliding rod configured to be connected to another bone anchor;

[0010] a bearing connector having a slide bearing, which bearing connector is configured to be fastened to the fixation rod, and wherein the sliding rod is configured to extend through the slide bearing of the bearing connector;

[0011] a stop ring, configured to be mounted onto the sliding rod;

[0012] a coil spring having two outer ends, which is configured to be arranged around the sliding rod in between the bearing connector and the stop ring, such that one outer end of the coil spring is supported against the stop ring and the other outer end of the coil spring is supported against the bearing connector.

[0013] The inventive system is characterized in that the system further comprises one or more bushings, preferably multiple bushings, each bushing configured to be arranged or arranged onto the coil spring, wherein each bushing comprises an inner diameter which is larger than an outer diameter of the sliding rod and smaller than an inner diameter of the coil spring, so as to maintain a spacing between the sliding rod and the coil spring when the coil spring is arranged on the sliding rod with the one or more bushings arranged onto the coil spring.

[0014] Since the one or more bushings each have an inner diameter larger than the outer diameter of the sliding rod, they can be positioned around the sliding rod while they are arranged onto the coil spring. Due to the fact that the inner diameter of each bushing is smaller than an inner diameter of the coil spring a spacing is maintained between the coil spring and sliding rod. As a result, even when the sliding rod is curved (e.g. in order to follow the contour of the spine), a distance between the coil spring and the sliding rod is provided at least locally. This enhances long durability and operational effect of the system. It may avoid wear over time of either the coil spring and / or the sliding rod.

[0015] Due to the presence of the one or more bushings, the effective compressible length of the spring may be reduced as each bushing may locally limit / block any spring movement of thecoil spring. So, in embodiments, portions of the coil spring engaged by the bushing may not effectively work as a spring and only the other portions (in between the bushings and / or the outer ends of the spring) work as spring(s). As a result a relatively longer spring may be required compared to the prior art design where no bushings are present.

[0016] In practical embodiments, the portions of the spring which are not engaged by the one or more bushings are devoid of any internal and external sleeve or the like. This minimizes interference with the effect provided by the coil spring. Any tissue growth around and / or into the coil springs appears to have no impact on the functionality in practice.

[0017] In practical embodiments, the spring, for example, has an uncompressed length in the range of 125 mm to 165 mm, e.g. about 145 mm, and a compressed length in the range of 75 mm to 115 mm, e.g. about 95 mm.

[0018] In practical embodiments, the bushings are made of a durable material having a sufficient strength so as to withstand the forces exerted by the coil spring. Examples are discussed herein.

[0019] In an embodiment, the bushings of the system comprise at least one intermediate bushing which is configured to be arranged or is arranged onto an intermediate position on the coil spring in a position in between the two outer ends of the coil spring. Having multiple intermediate bushings allows for maintaining a distance between the coil spring and the sliding rod at multiple locations, while having a minimal effect on the spring action.

[0020] In embodiments, e.g. when the bushings are configured to be screwed onto the coil spring, the position of the one or more intermediate bushings can be easily adjusted to achieve the desired position of each bushing, e.g. taking into account the curvature of the sliding rod. This curvature will in practice be determined by the surgeon who implants the system.

[0021] The provision of one or more intermediate bushings may avoid undue interfering with the spring force, even when the sliding rod is significantly curved.

[0022] In an embodiment, the bushings of the system comprise one or two end-bushings, which endbushings each are configured to be arranged or are arranged and maintained onto an outer end of the coil spring. For example, this may prevent direct contact between the coil spring and the bearing connector and / or the coil spring and the stop ring. This may avoid the wear of materials.In an embodiment, the one or more bushings, for example the intermediate-bushings and / or the end-bushings, are configured to be screwed or are screwed onto the coil spring. This allows for an easy assembly procedure during assembly of the spinal distraction system. For example, the bushings can be screwed onto the spring while it is not arranged around the sliding rod. If the bushings are configured to be screwed onto the coil spring, they also allow for easy adjustment of the relative position of each bushing with respect to the coil spring. This can be advantageous during assembly of the system, e.g. taking into account any curvature of the sliding rod.

[0023] In an embodiment, two-end bushings are provided to be positioned at the outer ends of the coil spring, and multiple, e.g. three, intermediate bushings to be distributed along the coil spring. This allows for a desired distribution of bushings around the coil spring. The two endbushings allow to prevent contact between the coil spring and the bearing connector and the stop ring and provide and maintain a spacing between the sliding rod and the coil spring. The intermediate bearings allow to maintain a spacing between the sliding rod and the coil spring. The combination of both end-bushings and intermediate-bushings allow for a configuration in which long term behavior of the coil spring is optimized.

[0024] In an embodiment, only a single end-bushing is used, for example only on the outer end of the coil spring near the bearing connector or only on the outer end of the coil spring near the stop ring.

[0025] In an embodiment, the end-bushings each have an inner end-stop preventing the coil spring from completely extending through the end-bushing once arranged onto the spring. For example, in an embodiment wherein the bushing comprises a screw thread to be threaded on the coil spring, the inner end-stop can be embodied by a blocking surface along the end portion of the screw thread.

[0026] In an embodiment, the end-bushings each have a flat end-stop face as to have a flat contact surface with the bearing connector or stop ring. A flat contact surface allows for proper transmission of forces from the spring to the bearing connector and from the spring to the stop ring, via the end-bushings. A flat end-stop in the end-bushings prevent the bushing from slowly wearing into the bearing connector or the stop ring, and possibly leaving small pieces of material which have been worn off.

[0027] In an embodiment, each bushing comprises an inner threading corresponding to the shape of the coil spring, such that the bushing is screwable over the spring, wherein an inner diameterof the threading is larger then the outer diameter of the sliding rod and smaller than the inner diameter of the coil spring. The threading eases the assembling process of the spine distraction system. As the bushing comprises a threading corresponding to the shape of the coil spring it will be easily screwable onto the coil spring. Since the inner diameter of the threading is larger then the outer diameter of the sliding rod and smaller than the inner diameter of the coil spring a distance is proved between the coil spring and the sliding rod. This distance prevents the coil spring and sliding rod from being in contact at least locally around each bushing. This results in less wear between the coil spring and the rod and thus fewer material, or more preferably no material, being worn off of the coil spring or the rod due to small relative movements between the two components.

[0028] In an embodiment, one or more bushings, e.g. each bushing, comprises rounded edges at its axial ends. These rounded edges provide that even during small movement the amount of friction is reduced, and for example that the corners of the bushing would not wear into a surrounding element, for example the bearing connector, the stop ring, or surrounding tissue.

[0029] In an embodiment, each bushing has a length corresponding to two to four coils of the coil spring, preferably about 2.5 coils. This condition provides a sufficient balance between securing a distance between the coil spring and sliding rod and limiting the spring movements. It is undesired to have a long bushing which will interfere with the spring behavior of the coil spring, while it is also a requirement that the bushing provides sufficient support to the coil spring as to not wear into the bushing.

[0030] In an embodiment, the bushings are formed on the spring in a manufacturing process, for example moulded or printed on / around the spring. This omits the requirement of manually placing, for example screwing, the bushings around the coil spring. By forming the bushings around the spring they can become locally integrated with the coil spring, thus further limiting the relative movement between the two. For example, the bushings can be formed on the coil spring by means of injection moulding. This keeps the bushing in a fixed position relative to the coil spring. After injection moulding of the bushing the core of the bushing could be solid. This would require drilling out the core in a diameter corresponding to, or slightly larger to, the diameter of the sliding rod, as to allow the bushing to slide over the sliding rod.

[0031] In an embodiment, each bushing has a length between 5 mm and 15 mm, preferably between 8 mm and 12 mm. This is a length which, preferably, corresponds to two to four coils of the coil spring. A length between 5 mm and 15 mm provides a good balance between providing support to the coil spring around the sliding rod and not interfering with the spring behavior ofthe coil spring. The end-bushings can have a length equal to that of the intermediatebushings

[0032] In an embodiment, each bushing has an inner diameter between 3.5 mm and 7.5 mm, preferably about 5.5 mm, and the sliding rod has an outer diameter (slightly) smaller than the inner diameter of each bushing. The dimensions of the bushing should be chosen depending on the sizes of the sliding rod and the coil spring.

[0033] In an embodiment, the fixation rod, sliding rod, bearing connector, stop ring, coil spring and one or more bushings are made of materials having high wear-resistance, biocompatibility, and corrosion resistance.

[0034] In an embodiment, the bushings are made of plastic, for example polyethylene. Plastics for the bushings may provide enhanced durability. Polyethylene may be preferred, as it provides for low friction characteristics ensuring smooth movement with respect to the sliding rod. Polyethylene does not interact strongly with skin and other tissue.

[0035] In embodiments, the fixation rod and sliding rod are made of a metal alloy, for example cobalt chrome. Cobalt chrome furthermore is also well suited for medical implants such as a spinal distraction system, and is chemically inert as to prevent adverse reactions when implanted.

[0036] In embodiments, the coil spring is made of metal, for example titanium. Titanium as material for the coil spring provides beneficial properties such as durability and biocompatibility to prevent adverse reactions when implanted.

[0037] In embodiments, the bushings are made of IIHMWPE. Ultra-high-molecular-weight polyethylene is a subset of polyethylene and provides improved wear resistance with a relatively low friction coefficient. Furthermore, UHMWPE provides better chemical and corrosion resistance compared to polyethylene which is a desirable property for implants.

[0038] In an embodiment, the spring can be embodied as multiple coil springs in series. For example, two coil springs are configured to be mounted onto the sliding rod in series. In an embodiment, each of the coil springs comprises one or more bushings as discussed herein.

[0039] In embodiments, the coil spring comprises between 20 and 40 coils, for example 30 coils. In embodiments, the spring has an inner diameter between 4 mm and 8 mm, preferably 6 mm. In embodiments, the spring has an outer diameter which is larger than the inner diameter,which outer diameter, depending on the inner diameter, ranges from 7 mm to 11 mm, and preferably is 9 mm.

[0040] The present invention also relates to a system in its installed state. Herein, the system comprises:

[0041] a fixation rod connected to a bone anchor;

[0042] a sliding rod connected to another bone anchor;

[0043] a bearing connector having a slide bearing, which bearing connector has been fastened to the fixation rod, and wherein the sliding rod extends trough the slide bearing of the bearing connector;

[0044] a stop ring which has been mounted onto the sliding rod;

[0045] a coil spring having two outer ends, which has been arranged around the sliding rod in between the bearing connector and the stop ring, such that the one outer end of the coil spring is supported against the stop ring and the other outer end of the coil spring is supported against the bearing connector,

[0046] wherein the system further comprises one or more bushings, each bushing having been arranged onto the coil spring, wherein each bushing comprises an inner diameter which is larger than an outer diameter of the sliding rod and smaller than an inner diameter of the coil spring, so as to maintain a spacing between the sliding rod and the coil spring.

[0047] The invention also relates to a method for assembly of a spinal distraction system according to the invention. The method comprises the steps of:

[0048] connecting the fixation rod to a bone anchor;

[0049] connecting the bearing connector to the fixation rod;

[0050] inserting one end of the sliding rod into the slide bearing of the bearing connector; arranging the one or more bushings around the coil spring;

[0051] providing the coil spring around the sliding rod, wherein the one or more bushings maintain a spacing between the sliding rod and the coil spring;

[0052] arranging the stop ring onto the sliding rod

[0053] compressing the coil spring;

[0054] securing the stop ring around the sliding rod as to keep the coil spring in a compressed state;

[0055] connecting another end of the sliding rod to another bone anchor.The fixation rod is connected to the bone anchor by means of fixation, for example by means of a fixation screw or set screw. The sliding rod is connector to its respective bone anchor(s) by a similar means.

[0056] A compressing device can be used for compressing the coil spring to the desired degree. The compressing device can be a medical instrument. The compressing device can be positioned such that the bearing connector, coil spring, and stop ring are situated between the ends of the compressing means, while the stop ring is in an unsecured state on the sliding rod. A force is then applied by the device, for example a force applied to the handle of the instrument, which causes the stop ring to press against the spring, compressing it against the bearing connector. The stop ring can then be secured, for example by means of one or more set screws, in order to secure it in place.

[0057] The steps can be performed in different order, for example, the coil spring can be provided around the sliding rod before the one or more bushings are arranged around the coil spring.

[0058] In a practical use of the system according to the invention each bone anchor configured to be secured to the fixation rod may connected to a respective vertebra in a first region, and each bone anchor configured to be secured to the sliding rod may be connected to a respective vertebra in a second region, which is spaced from the first region (in practice by multiple vertebra). The coil spring is to be arranged over the sliding rod, between the bearing connector and the stop ring. The spring is configured to be compressed between the bearing connector and the stop ring in order to provide for a spring force. As a result, a distraction force is exerted between the first and second regions of the spine. This spring based distraction force can allow for spinal growth and provide continuous distraction. The magnitude of the distraction force can be set, possibly adjusted, operatively by a surgeon, e.g. based upon tactile feedback or a pre-tension / length table. If desired, the spinal distraction system can be used in combination with existing posterior spinal fusion systems.

[0059] The invention will now be discussed with reference to the drawings. In the drawings:

[0060] Fig. 1 shows an isometric view of a spinal distraction system according to the invention; Fig. 2 shows a closeup of the isometric view of the spinal distraction system according to fig.

[0061] 1;

[0062] Fig. 3 shows an isometric view of the bushing according to the invention.

[0063] Fig. 4 shows an isometric view of a coil spring comprising multiple bushings according to the invention;Fig. 5 shows an exploded view of a coil spring comprising multiple bushings;

[0064] Fig. 6 shows a sideview of a coil spring;

[0065] Fig. 7 shows a sideview of a coil spring comprising multiple bushings;

[0066] Fig. 8 shows a cross-sectional view of the coil spring comprising multiple bushings of fig. 7; Fig. 9 shows a frontal-view of a bushing according to the invention;

[0067] Fig. 10 shows a cross-sectional view of a bushing according to the invention;

[0068] Fig. 11 shows a cross-sectional view of a coil spring;

[0069] Fig. 12 show a side-view of an end-bushing according to the invention.

[0070] Fig. 13 shows a side-view of an intermediate-bushing according to the invention.

[0071] Figure 1 shows a spinal distraction system 1 according to the invention.

[0072] The embodiment shown relates to a spinal distraction system 1 which is configured to be connected to the vertebrae 2 of a spine 3 via multiple bone anchors 4.

[0073] The system in figure 1 is a dual system and is carried out on both sides of the spine.

[0074] The spinal distraction system 1 shown in figure 1 comprises a fixation 5 rod, which is connected to multiple, here two, bone anchors 4 in a first region of the spine. Each of the bone anchors 4 is connected to a respective vertebra.

[0075] A bearing connector 6 having a slide bearing 7 is connected to the fixation rod 5. For example, a set screw secures the bearing connector 6 on the fixation rod 5.

[0076] In another, second region of the spine 3 a sliding rod 8 is connected to multiple, here three bone anchors 4. Each of the bone anchors 4 is connected to a respective vertebra.

[0077] The sliding rod is at one end thereof connected to the bone anchors 4, and extends trough the slide bearing 7 of the bearing connector 6 at its other end.

[0078] The sliding rod 8 has, preferably, a constant outer diameter over its length.

[0079] A coil spring 9 is arranged around the sliding rod 8 in between the bearing connector 6 and the stop ring 10. The coil spring is configured to be in a compressed state so as to exert a restoring force onto the fixation rod 5 and the sliding rod 8, via the bearing connector 7 and stop ring 10 respectively.The stop ring 10 can be annular with a set screw or multiple set screws, here two, to secure the stop ring 10 on the sliding rod 8. In another embodiment, for example, the stop ring is c-shaped or the like with a tensioning screw to clamp the stop ring on the sliding rod.

[0080] The system 1 further comprises multiple bushings 11.

[0081] In the shown embodiment two end-bushings 12 and three intermediate-bushings 13 are arranged on the coil spring 9, here screwed onto the coil spring. It will be appreciated that a different number can be present.

[0082] Each bushing 12, 13 is used to maintain a spacing between the sliding rod 8 and the coil spring 9. The two end-bushings are used to prevent direct contact between the coil spring 9 and the bearing connector 6 and the stop ring 10.

[0083] The intermediate bushings are arranged in between the two outer ends of the coil spring. The end-bushings are arranged on to the outer ends of the coil spring so as to prevent contact between the coil spring 9 and the bearing connector 6 and between the coil spring 9 and the stop ring 10. As discussed, the number and / or position can be selected, e.g. depending on the curvature of the sliding rod.

[0084] Figure 2 shows a closeup of the embodiment of figure 1. Here the part of the system around the first region of the spine is visible.

[0085] Each fixation rod 5 is connected to two bone anchors 4, which are connected to a respective vertebra 2 of the spine 3. Multiple bone anchors 4 are used for each fixation rod 5 to provide stability to the system.

[0086] The bearing connector 6 is fastened to the fixation rod 5. The bearing connector 6 includes a set screw as to the position of the bearing connector 6 relative to the fixation rod.

[0087] The slide bearing 7 in the shown embodiment comprises two bearing portions 7. These provide enhanced stability and force transfer compared to a single bearing portion. The sliding rod 8 is inserted trough the slide bearing 7 and extends alongside the fixation rod 5 at its outer end 14.

[0088] The coil spring is provided around the other end of the sliding rod 8. The end-bushing 12 prevents contact between the coil spring 9 and the bearing connector 6. Both the intermediatebushing 13 and the end-bushing 12 maintain a spacing between the sliding rod 8 and the coil spring 9.

[0089] Figure 3 shows a perspective view of a bushing 11. The bushing of the shown embodiment comprises an inner threading 15, which corresponds to the shape of the coil spring 9. This allows the bushing to be screwed onto the coil spring 9. The bushing shown furthermore comprises rounded ends, which helps preventing the bushings from wearing into a surrounding element, for example the bearing connector 6, stop ring 10, or surrounding tissue.

[0090] In figure 4 an isometric view of the coil spring 9 comprising multiple bushings 11 is shown. The bushings can be classified into two groups: end-bushings 12 and intermediate-bushings 13. As can be seen in the figure, the end-bushings do not allow the coil spring 9 to extend completely trough the bushing.

[0091] Figure 5 shows an exploded view of the coil spring 9 comprising two end-bushings 12 and three intermediate-bushings 13.

[0092] Figures 6, 7 and 8 show a side view of the coil spring 9. The shown coil spring comprises a consistent pitch and consistent inner diameter, and has a length Lc.

[0093] In figure 7 the end-bushings and intermediate-bushings are shown arranged on the coil spring.

[0094] In figure 8 a cross-sectional view of the coil spring 9 is shown. Here it is shown that the outer ends of the coil spring 9 do not fully extend trough the end-bushings 12. The threading of the end-bushings does not fully extend trough the complete bushing. There is an inner end-stop present which prevents the coil spring from completely extending trough the bushing.

[0095] Figure 9 shows a frontal view of the bushing 11 of figure 4. In the frontal view a starting portion of the threading 15 is visible. The bushing comprises an inner diameter DIB which is larger than the outer diameter of the sliding rod 8.

[0096] Figure 10 shows a cross-sectional view of an end-bushing 12 of the spinal distraction system 1. In the figure the internal threading structure is visible, which corresponds to the shape of the coil spring 9. The bushings comprises multiple turns or coils of the coil spring. The bushing comprises an inner diameter DIB which is larger than the outer diameter of the slidingrod 8. Figure 11 shows a cross-sectional view of a coil spring 9 having a constant inner diameter of Die, which is larger than the inner diameter of the bushing DIB. This allows for the bushing and the coil spring to fit around the sliding rod, while the bushing maintains a distance between the coil spring and sliding rod.

[0097] For example, the inner diameter DIB equals 5.5 mm, and the inner diameter Die of the coil spring 9 equals 6 mm. The outer diameter of the sliding rod 8 for example equals 5 mm.

[0098] The bushing of figure 10 has a length LIB corresponding to around 2.5 coils of the coil spring 9. This, depending on the spring, can for example be 9 mm in length. The threading of the end-bushing 13 does not completely extend trough the end-bushing as is visible in the figure.

[0099] Figure 12 and 13 show a side view of an end-bushing 12 and an intermediate-bushing 13 respectively.

[0100] In figure 12 the flat end-stop face 16 is visible on the right side of the bushing. This flat endstop face 16 is larger than the face 17 at the others side of the bushing. Such a larger face allows for a relatively large contact surface with the bearing connector 6 or the stop ring 10, and is beneficial for transfer of forces between the spring and the bearing connector or stop ring. This is achieved by having a smaller radius for the rounding of the edges. In the figures it is visible that the edges 18 have a smaller radius as compared to the edges 19. This achieves a relatively larger surface 16 over 17.

[0101] The intermediate bushing of figure 13 does not comprise these edges with a smaller radius. For the intermediate-bushings and sides of the end-bushings which do not abut the bearing connector or stop ring, it is preferred to have smooth edges, i.e. edges having a larger radius of curvature, as to minimize the influence on surrounding tissue.

Claims

C LA I M S1. Spinal distraction system (1) comprising:a fixation rod (5) configured to be connected to a bone anchor;a sliding rod (8) configured to be connected to another bone anchor;a bearing connector (6) having a slide bearing (7), which bearing connector is configured to be fastened to the fixation rod (5), and wherein the sliding rod (8) is configured to extend through the slide bearing (7) of the bearing connector (6);a stop ring (10) configured to be mounted onto the sliding rod (8);a coil spring (9) having two outer ends, which is configured to be arranged around the sliding rod (8) in between the bearing connector (6) and the stop ring (10), such that one outer end of the coil spring (9) is supported against the stop ring and the other outer end of the coil spring is supported against the bearing connector, characterized in thatthe system further comprises one or more bushings (12, 13), preferably multiple bushings, each bushing configured to be arranged or arranged onto the coil spring (9), wherein each bushing (12, 13) comprises an inner diameter (DIB) which is larger than an outer diameter of the sliding rod (8) and smaller than an inner diameter (Die) of the coil spring (9), so as to maintain a spacing between the sliding rod (8) and the coil spring (9) when the coil spring is arranged on the sliding rod with the one or more bushings arranged onto the coil spring.

2. Spinal distraction system according to claim 1, wherein the system comprises at least one, preferably multiple, intermediate bushing which is configured to be arranged or is arranged in an intermediate position on the coil spring in between the two outer ends of the coil spring.

3. Spinal distraction system according to any one or more of the preceding claims, wherein the system comprises one or two end-bushings, which end-bushing(s) is / are configured to be arranged or is arranged and maintained onto the outer end of the coil spring as to prevent contact between the coil spring and the bearing connector and / or between the coil spring and the stop ring.

4. Spinal distraction system according to any one or more of the preceding claims, wherein the one or more bushings, for example the one or more intermediate-bushings and / or the end-bushings, are configured to be screwed onto the coil spring.

5. Spinal distraction system according to any one or more of the preceding claims, wherein the system comprises two-end bushings arranged or to be arranged at the outer ends of the coil spring, and one or more, e.g. three, intermediate-bushings arranged or to be arranged at intermediate position(s) distributed along the coil spring.

6. Spinal distraction system according to any one or more of the preceding claims, wherein the end-bushings each have an inner end-stop preventing the coil spring from completely extending trough the end-bushing once arranged onto the spring.

7. Spinal distraction system according to any one or more of the preceding claims, wherein the end-bushings each have a flat end-stop face as to have a flat contact surface with the bearing connector or stop ring.

8. Spinal distraction system according to any one or more of the preceding claims, wherein the one or more bushings comprise(s) an inner threading such that the bushing is screwable onto the coil spring, wherein an inner diameter of the threading is larger then the outer diameter of the sliding rod and smaller than the inner diameter of the coil spring.

9. Spinal distraction system according to any one or more of the preceding claims, wherein the one or more bushings comprise(s) rounded edges at its axial ends.

10. Spinal distraction system according to any one or more of the preceding claims, wherein each bushing has a length corresponding to two to four coils of the coil spring, preferably about 2.5 coils.

11. Spinal distraction system according to any one or more of the preceding claims, wherein the bushings are formed on the coil spring, for example moulded on the coil spring.

12. Spinal distraction system according to any one or more of the preceding claims, wherein each bushing has a length between 5 mm and 15 mm, preferably between 8 mm and 12 mm.

13. Spinal distraction system according to any one or more of the preceding claims, wherein each bushing has an inner diameter between 3.5 mm and 7.5 mm, preferably about 5.5 mm, and the sliding rod has an outer diameter smaller than the inner diameter of each bushing.- 15-14. Spinal distraction system according to any one or more of the preceding claims, wherein the one or more bushings are made of plastic, for example polyethylene, the fixation rod and sliding rod are made of a metal alloy, for example cobalt chrome, and the coil spring is of made metal, for example titanium.

15. Spinal distraction system according to any one or more of the preceding claims, wherein the one or more bushings are made of polyethylene, preferably LIHMWPE.

16. Spinal distraction system according to any one or more of the preceding claims, wherein the coil spring is made of titanium, and the fixation rod and / or the sliding rod is / are made of cobalt chrome.

17. Spinal distraction system according to any of the preceding claims, wherein:the fixation rod has been connected to a bone anchor;the sliding rod has been connected to another bone anchor;the bearing connector has been fastened to the fixation rod, and wherein the sliding rod extends trough the slide bearing of the bearing connector;the stop ring has been mounted onto the sliding rod;the coil spring has been arranged around the sliding rod in between the bearing connector and the stop ring, such that the one outer end of the coil spring is supported against the stop ring and the other outer end of the coil spring is supported against the bearing connector,wherein the one or more bushings each have been arranged onto the coil spring, wherein each bushing comprises an inner diameter which is larger than an outer diameter of the sliding rod and smaller than an inner diameter of the coil spring, so as to maintain a spacing between the sliding rod and the coil spring.

Images