DE102017120111B4 - Extension device for a bone anchor and release instrument - Google Patents

Abstract

The invention relates to an extension device (2) for a bone anchor (4), in particular for a pedicle screw, and a release instrument (70) for this extension device (2), wherein the extension device comprises at least one shell part (22), characterized in that the release instrument (70) is sleeve-shaped and has a sleeve-shaped housing body (72) with a in the axial longitudinal direction (16) extending through opening (74) viewed in section with an orthogonal to the axial longitudinal direction (16) extending cutting plane circular disc segment-shaped or sickle-shaped, such that the erasing instrument (70) can be pushed onto the shell part (22) releasably fixed to the head (8) of the bone anchor (4) in the axial longitudinal direction (16), and that the erasure instrument (70) has an unlocking means (76) which when pushing the Löerungsstruments on the shell part (20) against the resilient portion (26) Runs and this thereby deflects radially outwards, so that its radially inwardly projecting support region (36) from the head (8) of the bone anchor comes free and the shell part (22) relative to the head (8) of the bone anchor from the hintergreifbaren area (14) can be moved out of the head (8) of the bone anchor.

Description

The invention relates to an extension device for a bone anchor, in particular for a bone screw, in particular for a pedicle screw, in particular in minimally invasive spinal surgery, and a Löheinstrument for this extension device, wherein the extension device is formed with an axial longitudinal direction, a circumferential direction concentric thereto and a radial direction , wherein the extension device is extended in the axial longitudinal direction and by means of a radial projection engages behind an engageable region on the head of the bone anchor in the longitudinal direction and also in the radial direction and in the opposite direction against the head of the bone anchor is supportable, so that the extension device on the Head of the bone anchor detachably, but in the longitudinal direction rigid and also rotatably fixed, the extension device at least one shell part and preferably e comprises two separate, and in the longitudinal direction extended shell parts, wherein the respective shell part comprises a radial projection for engaging behind the graspable region on the head of the bone anchor, the respective shell part is formed substantially substantially dimensionally stable, but has a resilient portion, wherein the resilient region has a radially inwardly projecting support region, by means of which the shell part in the axial longitudinal direction against the head of the bone anchor can be supported, so that the shell part is thereby releasably fixed to the head of the bone anchor when the projection engages behind the reachable behind the back of the bone anchor, and wherein the resilient region for releasing the extension device is deflected radially outwards, so that its support region comes free from the head of the bone anchor and the shell part with respect to the head of the bone anchor from the hintergreifbaren area at the head d it can be moved out of bone anchor. Such an extension device with a release tool for this extension device is known from US 2011/0263945 A1 However, wherein the rearward grip of the extension device on the head of the bone anchor in the longitudinal direction and in the radial direction is achieved by various projections of the extension device. The erasing instrument is sleeve-shaped and can be inserted into an access channel formed by the extension device to the bone anchor, wherein it deflects from the inside, each with an unlocking two opposite resilient portions of the extension device simultaneously outwardly into a release position.

The present invention has for its object to provide a Löheinstrument by means of which an extension device of the aforementioned type can be solved in a user-friendly and reliable manner of the bone anchor.

This object is achieved by an elongation device and a release instrument with the features of claim 1. The invention thus provides that the erasing instrument is sleeve-shaped and has a sleeve-shaped housing body having a longitudinal direction in the axial passage opening, which in section with an orthogonal Viewed in the axial longitudinal direction, the cutting plane is circular-segment-shaped or sickle-shaped, so that the erasing instrument can be pushed onto the shell part releasably fixed to the head of the bone anchor, and the erasing instrument has an unlocking means which, when the erasure instrument is pushed onto the shell part, rests against the resilient member Runs area and this deflects radially outwards, so that its radially inwardly projecting support area of the head of the bone anchor comes free and the shell part opposite de The head of the bone anchor can be moved out of the reachable area on the head of the bone anchor.

In this dissolved state, the shell part can then be removed together with the erasing instrument from the head of the bone anchor. If a so-called "long head" is used, it is also conceivable that in this case a connected via a predetermined breaking line to the head head part is separated together with the shell part. According to the invention caused by the resilient region locking between the shell part and the head of the bone anchor can be achieved that the erasure is pushed towards an axial end position on the relevant shell part, whereby means of the unlocking the resilient portion of the shell part from its locking position is deflected radially outward.

The unlocking means is advantageously tongue or pin-shaped and extends in the axial longitudinal direction. It is expediently positioned so that it starts when pushing the Löerungsstruments of radially inwardly against the resilient portion of the shell part and this area thereby deflects radially outward. The unlocking means has such a material thickness and shape that it displaces the resilient region radially outward. In this case, the unlocking means is supported by radially inward, preferably by the outside of the head of the bone anchor.

It also proves to be advantageous if the unlocking means is arranged in the region of a vertex of the circular disk-segment-shaped or crescent-shaped passage opening of the extinguishing instrument.

Furthermore, it proves to be advantageous if the erasing instrument in the region of a vertex of the circular disk-shaped or crescent-shaped through opening has a first opening extending in the axial longitudinal direction and opening out distally freely of a wall of the sleeve-shaped housing body delimiting the vertex. As a result, the aperture effectively forms a free space in which the resilient region of the shell part can be displaced radially outward.

At the same time, it proves to be advantageous if the unlocking means extends in the region of this first opening.

For pushing the Löfallsstruments it proves to be advantageous if the Löheinstrument has an axial stop, with which it starts axially when pushed onto the shell part against the shell part, whereby a distal end position of the Löerungsstruments is defined.

In a further embodiment of this idea, it proves to be advantageous if this axial stop is formed by a relative to the sleeve-shaped housing body between two longitudinal end positions displaceable in the interior of the passage opening extending stopper body. Due to the displaceability of the stopper body, the removed shell part can then be pushed out of the release instrument. In particular, a mitabgetrennter head portion of the bone anchor can push out, which will be discussed.

In an advantageous embodiment, it is provided that the longitudinal end positions of the stopper body are formed by axial ends of a slot-shaped second opening in the sleeve-shaped housing body and that in the interior of the passage opening extending stopper body on a arranged outside the sleeve-shaped housing body slider element, in particular in the form of an the housing body longitudinally displaceable sleeve is held. When sliding the Löerungsstruments the stopper body is then brought into its proximal longitudinal end position on the sleeve-shaped housing body, whereby the intended distal end position of the Lözeustruments is defined on the relevant shell part or on the bone anchor.

After removing the Lözeustruments velvet shell part, the slider element can be actuated, whereby the shell part and optionally a mitabgetrennter head part in the distal direction pushed out of the tubular housing body and so can be easily removed.

The mentioned formed by the stop body axial stop of the sleeve-shaped housing body could start against a proximal end of the shell part. In contrast, it proves to be advantageous if the shell part in the region of its proximal free end has a recess extending in the axial longitudinal direction in which engages the axial stop of the Lözeustruments when the erasing instrument is pushed onto the shell part in the axial longitudinal direction. This makes it possible to realize a correct self-centering system and further guidance of the components against each other.

The region of the stop body forming the axial stop could be formed by a web projecting through the slot-shaped opening into the interior of the passage opening, a rib or a pin. It is also conceivable that two or more webs, ribs or pins are formed, of which a distal forms the abutment to the distal longitudinal end position and a proximal stop to the proximal longitudinal end position.

As already indicated, it proves to be advantageous that the length of the slot-shaped opening in the sleeve-shaped housing body and thus a displacement of the longitudinally displaceable slide element defines an output or ejection distance to the peeling of the shell part of the head of the bone anchor the shell part and / or in this case a head part of the bone anchor mitabgetrennt intended release from the erasing instrument by the shell part, in particular moved together with the separated head part to this output or ejection path from the erasing instrument in the axial longitudinal direction out.

For this purpose, it proves to be particularly advantageous if a spring element is provided in a distal end region of the sleeve-shaped housing body in order to keep a captive when detaching the shell part of the head of the bone anchor mitabgetrenntes head part in the erasing instrument so that it does not accidentally slip out in the axial longitudinal direction , This spring element may be formed in any desired manner, wherein it proves to be advantageous and simple if the spring element is formed integrally with the sleeve-shaped housing body by it is cut out of the wall of the housing body to a connection point.

The invention also provides a release instrument having the features of claim 13 and a release member having one or more of the features of the dependent claims, and a method having the features of claims 14-16.

Further features, details and advantages of the invention will become apparent from the appended claims and from the drawings and the following description of the invention.

• 1a, b eine perspektivische Ansicht eines Knochenankers in Form einer Pedikelschraube und einer Verlängerungsvorrichtung mit zwei Schalenteilen;
• 1c, d Ansichten der Schalenteile;
• 2a, b Ansichten des Knochenankers mit erfindungsgemäßer Verlängerungsvorrichtung und des erfindungsgemäßen Löseinstruments;
• 3a - f Ansichten des erfindungsgemäßen Löseinstruments;

In the drawings show:

• 1a, b a perspective view of a bone anchor in the form of a pedicle screw and an extension device with two shell parts;
• 1c, d Views of the shell parts;
• 2a, b Views of the bone anchor with inventive extension device and the Löerungsinstrument invention;
• 3a f views of the erasing instrument according to the invention;

The 1-2 show an inventive embodiment of the extension device according to the invention 2 and their components, partially in a releasable retention connection to a bone anchor 4 in the form of a pedicle screw exemplified. The bone anchor 4 includes a screw shaft 6 which is polyaxially pivotable with respect to a U-shaped head when viewed from the side 8th the pedicle screw 4 is arranged. At the head 8th is an example of a so-called "long head", with longer thighs 10 is formed, wherein a predetermined breaking point 12 is provided, on which the long head can be shortened in the implanted state by a head part 13 is separable. Nevertheless, the head includes 8th of the bone anchor 4 in known manner a tangible area 14 in the form of a grippable groove on the outer circumference of the head 8th is trained. The tangible area 14 of the head 8th can otherwise according to the disclosure in the patent application DE 10 2014 225 327 A1 the applicant whose content is to be included in the disclosure of the present application.

The extension device 2 includes an axial longitudinal direction 16 , a concentric circumferential direction 18 and a radial direction 20 who are equally upside down 8th of the bone anchor 4 Respectively. The extension device 2 includes two separate and in the longitudinal direction 16 extended shell parts 22 , which are exemplary and preferably formed corresponding to each other. The shell parts 22 are substantially rigid and torsionally stable formed in the sense that they are almost completely dimensionally stable, but this does not exclude a slight irrelevant deformability, especially due to the length. The shell parts 22 are detachable on the head in a manner to be described 8th of the bone anchor 4 fastened and then form a limited between them working channel 24 through which the surgeon with other instruments access to the bone anchor 4 and the operating environment can take.

A respective shell part 22 is formed substantially dimensionally stable, but it includes a resilient region 26 in the form of a spring element 28 which is in the axial longitudinal direction 16 is extended and transverse to this approximately radially outwardly deflectable. The spring element 28 is in the illustrated embodiment, for example, in one piece with the shell part 22 trained and goes to his proximal end 30 in one piece in a wall 32 of the shell part 22 over. The spring element 28 is through a material-free slot 34 in the wall 32 of the shell part 22 limited. The material-free slot 34 runs at its proximal ends curved, resulting in a uniform voltage introduction into the shell part 22 causes. The spring element 28 further includes a radially inwardly projecting support area 36 on, by means of which the spring element 28 and thus the shell part 22 in the axial longitudinal direction 16 against the head 8th of the bone anchor 4 is supportable. This support area 36 is in the longitudinal direction 16 for example, proximal to a radial projection 38 of the shell part 22 arranged. The radial projection 38 is in the circumferential direction 18 in the form of an inwardly projecting rib, which also has an extension in the proximal direction, in order to engage behind the area 14 to be able to intervene at the head of the bone anchor. One recognizes best in 1d the extension of the radial projection 38 in the circumferential direction 18 , The radial projection 38 has in the embodiment shown here by way of example according to figures la-d a central interruption 40 on, so that the spring element 28 in the longitudinal direction 16 distally through the radial projection 38 can extend through. It could also be in front of the radial projection 38 end up. In the area distal to the radial projection 38 includes the spring element 28 in the case exemplified a radially inwardly projecting engagement region 42 which is in an intervention recess 44 at the head 8th of the bone anchor 4 can engage and an additional positive connection between the shell part 22 and the bone anchor 4 can produce. The spring element 28 is further developed to be in the Area of the central interruption 40 of the radial projection 38 a rejuvenation or constriction 46 identifies, so there is narrower in the circumferential direction.

As described above, the relevant shell part 22 detachable but with respect to all degrees of freedom form-fitting on the head 8th of the bone anchor 4 kept that as best as possible 1a-d it can be seen with its radial projection 38 the reachable area 14 of the head 8th of the bone anchor 4 in the longitudinal direction 16 and in the radial direction 20 back-hooked and in the opposite direction with its support area 36 axially against the head 8th supported. In the above-described embodiment, the spring element engages 28 continue with its distal engagement area 42 into the intervention recess 44 at the head 8th on. Out 1d It can be seen that the respective shell part 22 two inwardly drawn longitudinal edge areas 54 has, with which it with lateral support surfaces 55 from in the circumferential direction 18 opposite sides against the thigh 10 of the head 8th of the bone anchor abuts. The shell part 22 so embraces the thigh 10 essentially at least almost free of play and is thus also in the circumferential direction 18 kept non-rotatable in addition to the action of the engaging elements.

Overall, a form-locking operating connection with respect to all degrees of freedom is thereby created, which can be achieved by the spring element 28 in the radial direction 20 is deflected to the outside. This can be achieved by means of an in 2 and 3 illustrated extinguishing instruments 70 Access to the spring element 28 is taken. This is the case of the respective shell part 22 one in the longitudinal direction 16 extended channel-forming access opening 50 educated. In the exemplary illustrated case, the spring element 28 a run-up slope 52 for the erasing instrument, on the inside on a proximal side of its support area 36 ,

The soldering instrument 70 is sleeve-shaped; it comprises a sleeve-shaped housing body 72 with one in the axial longitudinal direction 16 extended passage opening 74 , The passage opening 74 is sectionally circular or sickle-shaped in section, the sectional plane orthogonal to the axial longitudinal direction 16 runs. The sleeve-shaped housing body 72 and its passage opening 74 are designed so that the soldering instrument 70 in the axial longitudinal direction 16 on a respective at the head of the bone anchor releasably fixed shell part 22 is deferrable. Preferably, the circular segment shape corresponds to the curvature of the shell parts 22 , so that a respective shell part 22 when sliding the Löerungsinstrument 70 slidably sliding and guided substantially free of play against the inside of the sleeve-shaped housing body 74 slides.

The soldering instrument 70 also has an unlocking means 76 on, which when sliding the Löerungsinstrument 70 on the shell part 22 against the resilient area 28 starts and this deflects radially outwards, so that its support area 36 from the head 8th of the bone anchor 4 comes free and the cup part 22 opposite the head 8th out of the reachable area 14 at the head 8th of the bone anchor 4 can be moved out.

The unlocking device 76 is formed tongue or pin-shaped and in the region of a vertex 78 the passage opening 74 arranged. In this case, the sleeve-shaped housing body 72 in the area of the vertex 78 one in the axial longitudinal direction 16 extended and distally freely opening first opening 80 on. The unlocking device 76 extends in the area of this opening 80 , It is therefore deflectable in the radial direction and, if desired, can be integral with the sleeve-shaped housing body 72 be formed. Another task of the first breakthrough 80 in the wall of the sleeve-shaped housing body 72 but is to be seen therein, a sufficient displacement space for the resilient region or the spring element 28 of the shell part 22 ready to put when the unlocking means 76 from radially inward against the spring element 28 starts.

How best 3f can be seen, points the release instrument 70 an axial stop 82 on, with it when pushing on the shell part 22 axially against the shell part, whereby a distal end position of the Löerungsstruments 70 on the shell part 22 or on the bone anchor 4 is defined. In the case exemplified, this axial stop 82 from one opposite the sleeve-shaped housing body 72 slidable stop body 84 formed, extending into the interior of the passage opening 74 hineinerstreckt. For this purpose, the sleeve-shaped housing body has a slot-shaped second opening 86 on, which with their respective longitudinal ends 88 and 90 the longitudinal end positions for the axial stop 82 or the stop body 84 Are defined. The stopper body 84 is at an outside of the sleeve-shaped housing body 72 arranged slide element 92 held, for example in the form of a longitudinally displaceable sleeve 94 is trained.

To deal with the stop 82 to act together, the shell part 22 in the area of its proximal end 96 one in the axial longitudinal direction 16 extended recess 98 in which the axial stop 82 or stop body 84 of the extinguishing instrument 70 engages when the deletion instrument 70 on the shell part 22 is postponed. The recess 98 is in the longitudinal direction 16 limited and forms there an abutment 100 or a counter-attack for the attack 82 of the sleeve-shaped housing body 70 ,

If the delete instrument 70 with its sleeve-shaped housing body 72 on a bone anchor attached to the shell part 22 is postponed, so passes at initially arbitrary position of the slide element 92 finally, the stopper body 84 in contact with the axial end or counter bearing 100 the recess 98 of the shell part 22 and from there in the proximal direction to the back in the in the 3c to 3f shown longitudinal end position of the housing body 72 postponed. This then also forms the distal end position of the erasing instrument 70 with respect to the shell part 22 , In this end position is then further automatically achieved that the unlocking means 76 the resilient area 26 or the spring element 28 displaced radially outward, thereby unlocking the shell part 22 is effected and the shell part 22 from its rear grip position with the reachable area 14 at the head 8th of the bone anchor 4 can be solved. In the embodiment of a bone anchor with long head shown by way of example, it is intended that during detachment of the shell part 22 at the same time the headboard 13 at the breaking point 12 is separated from the rest of the head. This is possible because the removable headboard 13 together with the shell part 22 in the appropriately sized and formed circular disc segment-shaped or sickle-shaped passage opening 74 of the extinguishing instrument 70 is preferably recorded substantially free of play. The headboard 13 So is by a tilting movement of Löösestruments 70 separated from the remaining head of the bone anchor and together with the shell part 22 decreased. He is then within the passage opening 74 of the sleeve-shaped housing body 72 added. One recognizes 3c to f) that in a distal end region 102 of the sleeve-shaped housing body 72 a spring element 106 is provided, which when pushed onto the shell part 22 from the detachable headboard 13 is easily deflected and thereby the detachable headboard 13 clamped in the passage opening 74 holds, even if the headboard 13 was severed from the remaining head of the bone anchor. So he does not accidentally slip out of the through hole 74 of the extinguishing instrument 70 out. If now starting from this, ie in the detached state of the shell element 22 and the headboard 13 the slider element 92 is moved in the distal direction, so this is the shell part 22 and the severed headboard 13 around an output or ejection path from the erasing instrument 70 moved out. He can then, as well as the shell part 22 , be taken out of the extinguishing instrument or falling by gravity down from alone.

Claims (16)

An extension device (2) for a bone anchor (4), in particular for a bone screw, in particular for a pedicle screw, in particular in minimally invasive spinal surgery, and a release instrument (70) for this extension device (2), wherein the extension device is formed with an axial longitudinal direction (16 ), a concentric circumferential direction (18) and a radial direction (20), wherein the extension device (2) in the axial longitudinal direction (16) is extended and by means of a radial projection (38) an engageable region (14) on the head (8 ) of the bone anchor (4) in the longitudinal direction (16) and also in the radial direction (20) engages behind and in the opposite direction against the head (8) of the bone anchor (4) is supportable, so that the extension device thereby by the head (8) the bone anchor solvable, but in the longitudinal direction (16) rigid and also rotatably fixable, wherein the extension Device (2) comprises at least one shell part and preferably two separate, and in the longitudinal direction (16) extending shell parts (22), wherein the respective shell part has a radial projection (38) for engaging behind the engageable region (14) on the head (8). the bone shell (4), wherein the respective shell part (22) is formed substantially substantially dimensionally stable, however, a resilient region (26), wherein the resilient region (26) has a radially inwardly projecting support region (36), by means of which the shell part (22) can be supported against the head (8) of the bone anchor in the axial longitudinal direction (16) so that the shell part can be detachably fixed to the head of the bone anchor when the projection (38) engages behind the reachable area (14) on the head (14). 8) of the bone anchor engages behind, and wherein the resilient region (26) for releasing the extension device (2) is deflected radially outwardly, so that its Abstützber eich (36) from the head (8) of the bone anchor comes free and the shell part (22) relative to the head (8) of the bone anchor from the hintergreifbaren area (14) on the head (8) of the bone anchor can be moved out, wherein the erasure instrument (70 ) is sleeve-shaped and has a sleeve-shaped housing body (72) with a in the axial longitudinal direction (16) extending through opening (74) viewed in section with an orthogonal to the axial longitudinal direction (16) extending cutting plane circular segment or sickle-shaped, so that the erasing tool (70) in the axial longitudinal direction (16) on the head (8) of the Bone anchor (4) releasably fixed shell part (22) can be pushed, and wherein the release instrument (70) has an unlocking means (76) which starts when sliding the Löerungsstruments on the shell part (20) against the resilient region (26) and this after radially outwardly deflects so that its radially inwardly projecting support region (36) from the head (8) of the bone anchor comes free and the shell part (22) relative to the head (8) of the bone anchor from the hintergreifbaren area (14) on the head (8) of the bone anchor can be moved out. Extension device and Löschungstrument after Claim 1 , characterized in that the unlocking means (76) is tongue-shaped or pin-shaped and is extended in the axial longitudinal direction. Extension device and Löschungstrument after Claim 1 or 2 , characterized in that the unlocking means (76) in the region of a vertex (78) of the circular disc segment-shaped or crescent-shaped through hole (74) of the erasing instrument is arranged. Extension device and Löschungstrument after Claim 1 . 2 or 3 , characterized in that the erasing instrument (70) in the region of a vertex (78) of the circular disc-shaped or sickle-shaped passage opening (74) extending in the axial longitudinal direction (16) and distally freely opening out first opening (80) of the apex (78) limiting Wall of the sleeve-shaped housing body (72). Extension device and Löschungstrument after Claim 4 , characterized in that the unlocking means (76) in the region of this first opening (80) extends. Extension device and release device according to one of the preceding claims, characterized in that the release instrument (70) has an axial stop (82), with which it axially against the shell part when pushed onto the shell part (22), thereby defining a distal end position of the Löerungsstruments is. Extension device and Löschungstrument after Claim 6 , characterized in that this axial stop (82) of a relative to the sleeve-shaped housing body (72) between two longitudinal end positions displaceable in the interior of the passage opening (74) extending stop body (84) is formed. Extension device and Löschungstrument after Claim 7 , characterized in that the longitudinal end positions of the stopper body (84) of axial ends (88,90) of a slot-shaped second opening (86) in the sleeve-shaped housing body (72) are formed and that in the interior of the passage opening (74) extending stop body (84) is held on a sleeve element (92) arranged outside the sleeve-shaped housing body, in particular in the form of a sleeve (94) which is longitudinally displaceable on the housing body. Extension device and Löschungstrument after Claim 6 . 7 or 8th , characterized in that the shell part (22) in the region of its proximal free end (96) in the axial longitudinal direction (16) extending recess (98), in which engages the axial stop (82) of the Löhesstruments when the Löheinstrument ( 70) is pushed onto the shell part in the axial longitudinal direction. Extension device and Löschungstrument after Claim 8 or 9 , characterized in that the length of the slot-shaped second aperture (86) in the sleeve-shaped housing body (72) and thus a displacement of the longitudinally displaceable slide member (92) defining an output or ejection path to after the detachment of the shell part (22) from the head (8) of the bone anchor (4) to release the shell part (22) and / or in this case from the top of the bone anchor mitabgetrenntes head part of the erasing instrument (70) by the shell part (22) in particular together with the separated head part (13) around this Output or ejection path from the erasing instrument (70) in the axial longitudinal direction (16) is moved out. Extension device and release device according to one of the preceding claims, characterized in that a spring element (104) is provided in a distal end region (102) of the sleeve-shaped housing body (72) in order to be separated during the detachment of the shell part (22) from the head of the bone anchor as intended To keep the head part (13) in the erasing tool (70) so that it does not unintentionally slide out in the axial longitudinal direction (16). Extension device and Löschungstrument after Claim 11 , characterized in that the spring element (104) is formed integrally with the sleeve-shaped housing body (70) by being cut free except for a connection point from the wall of the housing body. Release device for an extension device according to the preamble of Claim 1 , characterized in that the erasing instrument (70) is sleeve-shaped and has a sleeve-shaped housing body (72) with an in the axial longitudinal direction (16) extending through hole (74) viewed in section with a orthogonal to the axial longitudinal direction (16) extending cutting plane circular segment or sickle-shaped, so that the erasing instrument (70) in the axial longitudinal direction (16) on the Head (8) of the bone anchor (4) releasably fixed shell part (22) can be pushed, and that the release instrument (70) has an unlocking means (76) which upon insertion of the Löerungsstruments on the shell part (20) against the resilient region (26) starts and this thereby deflects radially outwards, so that its radially inwardly projecting support region (36) from the head (8) of the bone anchor comes free and the shell part (22) relative to the head (8) of the bone anchor from the hintergreifbaren area (14) can be moved out of the head (8) of the bone anchor. Method for releasing the extension device (2) according to one or more of the preceding claims from the detachable connection on the head (8) of the bone anchor (4) using the release instrument (70) according to one or more of the preceding claims, characterized in that the sleeve-shaped Housing body (72) of the erasing instrument (70) with a shell part (22) in the axial longitudinal direction (16) is aligned and pushed from proximally in the distal direction on the shell part (22) and in that case the unlocking means (76) of the erasing instrument ( 70) against the resilient portion (26) starts and this deflects radially outwards, so that its radially inwardly projecting support portion (36) from the head (8) of the bone anchor (4) comes free, and that the shell part (22) then moved out of the gripping area (14) on the head (8) of the bone anchor (4) by means of the erasing instrument (70) and removed from the head of the bone anchor will be. Method according to Claim 14 , characterized in that when pushing the sleeve-shaped housing body (72) of the erasing instrument (70) on the shell part (22) and a head part (13) of the bone anchor (4) in the through hole (74) is received and that when removing the shell part ( 22) of the head (8) of the bone anchor (4) at the same time the recorded in the through hole head part (13) at a predetermined breaking point (12) is broken off the head of the bone anchor and thereby in the through hole (74) of the sleeve-shaped housing body (72) of the Löerungsstruments (70) remains and then in the axial longitudinal direction (16) from the passage opening (74) of the sleeve-shaped housing body (72) is output. Method according to Claim 15 , characterized in that in the passage opening (74) of the sleeve-shaped housing body (72) of the erasing instrument received and broken off at a predetermined breaking point from the head of the bone anchor head part (13) by means of a radially biased spring element (104) in the through hole (74) frictionally held is so that it does not accidentally slip out of the through hole (74).

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