Patent Description:
<CIT> described an anchoring member connected a rod with a bone screw that includes a screw member and a seat part receiving a screw head of the screw member and a rod. The anchoring member further includes a pressure member formed to embrace said screw head. The seat part has a tapering portion and the pressure member has an outer conical surface in a region laterally surrounding said screw head. Said conical surface tapers with a cone angle corresponding to the cone angle of the surface of the seat part. When the conical surfaces engage each other, a movement of the screw head is blocked even if a swivel nut is loosened for readjusting the rod.

A polyaxial bone anchoring device with a function of pre-locking a head of a bone anchor is known, for example, from <CIT>. The polyaxial bone anchoring device comprises a receiving part including a receiving part body with a channel for receiving a rod and an accommodation space for accommodating the head of the bone anchoring element and a pressure element arranged at least partially in the accommodation space. The pressure element has a flexible portion to clamp the head and is can assume a pre-locking position to clamp the head in the receiving part by a pre-stress exerted by the pressure element onto the head. In the pre-locking position an outer surface portion of the pressure element and an inner surface portion of the receiving part body are engaged. In one example, the surfaces can be tapered. Locking of the head is achieved via a locking member.

<CIT> describes a polyaxial bone anchoring device wherein a temporary clamping of a head of an anchoring element in a desired angular position with respect to a receiving part without locking the head can be achieved. The polyaxial bone anchoring device comprises a set screw that is threadable into a through hole of the receiving part and engages a hole in the pressure element such that a force is exerted by the pressure element onto the head that maintains the head at an adjustable angular position.

<CIT> describes a polyaxial bone screw assembly including a threaded shank body having an upper portion receivable in a receiver, the receiver having a channel for receiving a rod and a lower cavity cooperating with a lower opening. A friction fit compression insert, a split retaining ring and a shank upper portion cooperate to provide for snap-on assembly of the shank with the receiver. The receiver and the insert can include apertures for receiving a tool to lock and release the polyaxial mechanism.

<CIT> describes a spinal anchor assembly including a bone screw, a receiver assembly and a closure structure used to capture a rod within the receiver assembly. The receiver assembly includes a receiver and a collet for locking the bone screw relative to the receiver. While the known polyaxial bone anchoring devices exhibit some aspects of locking or provisionally locking the head of the bone anchor while allowing correction steps to be carried out, there is still a need for a polyaxial bone anchoring device that provides more options for carrying out correction steps during surgery more easily and/or repeatedly.

It is therefore the object of the invention to provide an improved coupling device for a polyaxial bone anchoring device, a polyaxial bone anchoring device and a system of such a coupling device or a polyaxial bone anchoring device and an instrument.

The object is solved by a coupling device according to claim <NUM>, a polyaxial bone anchoring device according to claim <NUM> and a system of a coupling device or a polyaxial bone anchoring device and an instrument according to claim <NUM>. Further developments are given in the dependent claims.

According to an embodiment, a coupling device for coupling a rod to a bone anchor comprises a receiving part having a channel for receiving a rod and having a passage defining a central axis and an accommodation space for a head of the bone anchor. The receiving part includes an outer surface with an engagement structure configured to be engaged by an instrument and a pressure member configured to be arranged at least partially in the accommodation space. The pressure member has a flexible portion defining a hollow interior to accommodate and clamp the head therein and an outer surface with an engagement structure configured to be engaged by the instrument. In the receiving part, the pressure member can assume a first position in which the pressure member engages an inner wall portion of the passage such that the flexible portion is compressed to clamp or lock an inserted head with a first locking force and a second position in which an inserted head is clamped or locked with a second locking force smaller than the first locking force or is freely pivotable and wherein the pressure member is movable from the first position towards the second position by engagement with the instrument.

The coupling device permits to easily unlock an inserted head. Also, a clamping and/or locking step and an unlocking step can be carried out repeatedly. The clamping or locking step can be carried out easily, for example using a rod inserted into the channel and/or a locking member. For the unlocking step, the instrument may be used. The instrument works reliably in any situation of provisional locking such as full locking of the head or only frictional clamping of the head.

The instrument is configured to laterally engage the pressure member. Therefore, the unlocking step can be effected when the rod is not in the channel or when the rod is at an elevated position relative to the bottom of the rod channel.

With the instrument and the coupling device, the unlocking step can be precisely carried out as the forces that are necessary for unlocking are automatically generated due to the geometry and position of the pressure member relative to the receiving part.

Engagement structures for the instrument that are provided at the pressure member and the receiving part, respectively, are offset from each other in an axial direction. This leads to an automatic centering and/or aligning of the respective engagement structures when the instrument successively engages the engagement structure of the receiving part and the engagement structure of the pressure member.

The polyaxial bone anchoring device is preferably of the bottom-loading type, which means that the head of the bone anchor is inserted from a bottom opening of the receiving part into the accommodation space. Hence, a modular system can be provided where any type of bone anchor that has a head can be combined with the coupling device to provide a polyaxial bone anchoring device.

A particular application of the coupling device, the polyaxial bone anchoring device and the system may be the field of spinal surgery.

Further features and advantages will become apparent from the description of embodiments by means of the accompanying drawings.

A polyaxial bone anchoring device according an embodiment, which is generally shown in <FIG>, includes a bone anchoring element <NUM> in the form of a screw member having a shank <NUM> which is at least partially threaded and a head <NUM>. The head <NUM> has a spherical outer surface portion. More in detail, in the embodiment, the head <NUM> is shaped as a spherical segment with a substantially flat end surface in which a recess <NUM> for an engagement with a tool may be provided. The bone anchoring device further includes a coupling device for connecting the bone anchoring element <NUM> to a rod <NUM>. The coupling device comprises a receiving part <NUM> and a pressure member <NUM> that is configured to be arranged in the receiving part <NUM> and is further configured to exert pressure onto the head <NUM>. In addition, for securing the rod <NUM> in the receiving part <NUM> and for exerting pressure onto the rod <NUM> and the pressure member <NUM>, a locking member <NUM> in the form of, for example, a set screw, which cooperates with the receiving part <NUM> may be provided. The locking member <NUM> may have a tool engagement recess 7a. The pressure member <NUM> may be secured and/or aligned in the receiving part by means of a securing element, for example by pins <NUM>.

As shown in <FIG>, an embodiment of a system includes a polyaxial bone anchoring device as shown in <FIG> and an instrument <NUM>. The instrument <NUM> is configured to engage the receiving part <NUM> and the pressure member <NUM> to move the pressure member <NUM> relative to the receiving part <NUM>.

As illustrated in particular in <FIG>, the receiving part <NUM> is substantially cylindrical and has a first end or top end 5a, a second end or bottom end 5b and a passage <NUM> extending from the top end 5a towards the bottom end 5b, the passage defining a longitudinal central axis C. The passage <NUM> extends completely from the top end 5a to the bottom end 5b thereby forming an opening <NUM> at the bottom end 5b. Adjacent to the top end 5a, a substantially U-shaped recess <NUM> is provided that has a bottom 53a directed towards the second end 5b. The passage <NUM> may have sections with different diameter. In the embodiment shown, a first section 51a may have a substantially constant diameter and extends from the top end 5a to an axial height above or corresponding approximately to the height of the bottom 53a of the U-shaped recess <NUM>. Following the first section 51a a second section 51c may be provided that narrows towards the bottom end 5b. More in detail, in the embodiment, the second section 51c tapers, preferably conically tapers towards the bottom end 5b. Within the second section 51c a third, widened section 51b is formed that widens towards the bottom end 5b. Hence, the second section 51c is divided in a portion above the widened section 51b and a portion below the widened section 51b. The widened section 51b provides an accommodation space for a portion of the pressure member <NUM> which can expand therein. The diameter change between the widened section 51b and the second section 51c results in an edge 51d protruding into the passage <NUM>. The substantially U-shaped recess <NUM> divides the upper portion of the receiving part <NUM> into two free legs <NUM> and forms a channel for receiving the rod. An internal thread <NUM>, for example, a square thread may be formed on an inner wall portion of the legs <NUM>, preferably adjacent to the top end 5a, for cooperating with the locking member <NUM>. An axial length of the tapered second section 51c may be such that in cooperation with the pressure member <NUM> a locking force may be generated that is configured to compress a portion of the pressure member for frictionally clamping and/or firmly locking of the head <NUM> in the pressure member <NUM>.

At a distance above the bottom 53a of the U-shaped recess two opposite transverse holes <NUM> extend completely through the wall of the receiving part <NUM>. The transverse holes <NUM> are configured to receive the pins <NUM>.

The receiving part <NUM> further comprises at least one, in the embodiment two engagement structures <NUM> for engagement with the instrument <NUM>. The engagement structures <NUM> are formed as narrowing holes or bores, extending completely from an outer surface of the receiving part through a wall of the receiving part into the passage <NUM>. The engagement structures <NUM> may narrow in a conical shape which tapers from the outside towards the passage. Preferably, the shape of the engagement structure is substantially that of a hollow truncated cone. Two such holes may be provided at <NUM>° offset from each other in a circumferential direction around the central axis C. In this embodiment, the engagement structures <NUM> are located and permit engagement in a direction perpendicular to a plane including a channel axis L of the U-shaped recess <NUM> and the central longitudinal axis C. More specifically each engagement structure <NUM> has a central transverse hole axis a that extends perpendicular to the central axis C. Moreover, the engagement structures <NUM> or more specifically the hole axes a are at an axial position Ha with respect to the bottom end 5b. The holes may be symmetrical around the hole axis a. The axial height Ha may be such that the hole axis a is above the bottom 53a of the U-shaped recess <NUM>, preferably it may be at least partially within the second section 51c, i.e. in the narrowing section of the passage <NUM>. An inner size of the engagement structure <NUM> is such that a front portion of the instrument <NUM> as explained below can be received therein in a substantially form-fit manner. Hence an inner wall of the engagement structures <NUM> forms a guiding surface, respectively, for a portion of the instrument.

Finally, at the same circumferential position as the engagement structures <NUM> and in an axial position closer to the top end 5a tool engagement recesses <NUM> may be formed in the outer surface of the receiving part <NUM>. The tool engagement recesses <NUM> may have, for example, a circular contour.

Referring now to <FIG> the pressure member <NUM> will be described more in detail. In the embodiment, the pressure member <NUM> is a monolithic part. The pressure member <NUM> has a first end or upper end 6a in an opposite or lower end 6b. Adjacent to the upper end 6a the pressure member <NUM> has a substantially cylindrical outer shape with an outer diameter only slightly smaller than an inner diameter of the first section 51a of the passage <NUM>, such that the cylindrical portion of a pressure member fits into the first section 51a and can move or slide therein. Adjacent to the upper end 6a a substantially V-shaped recess <NUM> for the rod <NUM> is provided. More specifically, a base 61a of the recess <NUM> has a substantially V-shaped contour. Thereby rods <NUM> of different diameter can selectively be supported. Such rods (not shown) rest on the base 61a along substantially two longitudinal contact lines. A rod that is supported in this manner may be prevented from moving sideward. Sidewalls of the recess <NUM> may be substantially vertical and the deepest portion of the base 61a may be rounded. By means of the V-shaped recess <NUM> upstanding legs <NUM> are formed which legs <NUM> may have such a height, or in other words, the substantially V-shaped recess <NUM> has such a depth, that the top end 6a of the pressure member is below the highest point of the surface of an inserted rod <NUM> when the rod rests on the base 61a.

The lower portion of the pressure member <NUM> comprises a narrowing outer surface <NUM> that narrows towards the bottom end 6b. More specifically, the surface <NUM> is a conical outer surface that tapers towards the bottom end 6b. The outer surface <NUM> is configured to cooperate with the narrowing second section 51c of the passage <NUM> of the receiving part <NUM>. Adjacent to the bottom end 6b, there is a hollow interior section <NUM> for accommodating the head. The hollow interior section <NUM> may comprise substantially spherically-shaped sections 64a that match the spherical shape of the outer surface portion of the head <NUM> and a slightly enlarged section 64b to facilitate pivoting of the head. The hollow interior section <NUM> provides an opening <NUM> at the bottom end 6b. A plurality of slits <NUM> which are open towards the bottom end 6b extend in an axial direction along the wall of the pressure member <NUM> in the region of the hollow interior section <NUM>. A length of the slits <NUM> in the longitudinal direction may be such that when the head <NUM> is inserted the slits end above the region with a greatest outer width of the head <NUM>. Further, the slits <NUM> may have a widened end portion 66a and render the lower portion of the pressure member <NUM> flexible in such a manner that the lower portion can be expanded and compressed. Hence, the lower portion of the pressure member forms a flexible portion. The size of the hollow interior <NUM> may be such that when the head <NUM> is inserted through the opening <NUM>, the flexible portion snaps onto the head and slightly clamps the head <NUM> by a friction.

For providing access to the tool engagement recess <NUM> of the head <NUM>, the pressure member <NUM> comprises a coaxial bore <NUM>. The coaxial bore <NUM> may extend from the base 61a of the substantially V-shaped recess <NUM> into the hollow interior section <NUM>.

In addition, the pressure member <NUM> comprises at least one, in the embodiment two opposite engagement structures <NUM>. The engagement structures <NUM> are configured to be engaged by a front portion of the instrument <NUM>. More in detail, the engagement structures <NUM> in the embodiment are formed as conical holes or bores that extend through the wall of the pressure member <NUM> from the outside to the coaxial bore <NUM>. A transverse central axis b of the holes may be located at an axial position slightly below the deepest portion of the base 61a of the substantially V-shaped recess <NUM> when seen from the bottom end 6b. More specifically, the axial position of the engagement structures <NUM> is at least partially within the narrowing outer surface <NUM>. The transverse central axes b of the engagement structures <NUM>, respectively, extend substantially perpendicularly to the longitudinal L defined by the recess <NUM>.

A diameter of the holes at the outer surface of the pressure member <NUM> substantially corresponds to a diameter of the holes at the receiving part <NUM> at a location of the passage. Hence, when the pressure member <NUM> is in the receiving part <NUM> at an axial position where the axes a of the holes in the receiving part <NUM> and the axes b of the holes in the pressure member <NUM> are on the same height, the holes of the receiving part <NUM> are flush with the holes of the pressure member <NUM> forming a continuous hollow space, preferably a hollow cone.

Moreover, the pressure member comprises two elongate recesses <NUM> in the center of the legs <NUM> which are opposite to each other and are aligned such that a longer side of the elongate recesses <NUM> is substantially parallel to the central axis C of the receiving part <NUM>. The ends of the elongate recesses <NUM> may be rounded. A width and length of the elongate recesses <NUM> is such that the pins <NUM> can be received therein and moved in the axial direction.

Turning back to <FIG>, the dimensions of the pressure member <NUM> and the dimensions of the receiving part <NUM> are selected such that when the pressure member <NUM> is in the receiving part <NUM> and the head <NUM> is inserted into the hollow interior section <NUM> of the pressure member and the narrowing surfaces of the second section 51c of the receiving part and the narrowing outer surface <NUM> of the pressure member <NUM> are engaged to some extent, the head <NUM> is clamped or firmly locked depending on the extent of engagement. It shall be noted that the narrowing preferably conical shape of the pressure member that extends along at least the axial length of the flexible portion and that cooperates with the narrowing preferably conical second section 51c in the receiving part over an axial length greater than that of the flexible section results in a reliable or strong clamping of the head. As shown in the Figure, the engagement structures <NUM> of the pressure member <NUM> and the engagement structures <NUM> of the receiving part <NUM> are offset from each other in the axial direction. More in detail, the engagement structure <NUM> of the receiving part is at a slightly higher axial position Ha compared to the position Hb of the engagement structure <NUM> of the pressure member <NUM>. This defines a first position of the pressure member relative to the receiving part in which the head <NUM> is clamped so that it can be pivoted only by overcoming the friction force or is even locked.

Referring now to <FIG>, the instrument <NUM> may be implemented as a hand-held instrument in the form of tongs having a pair of handles <NUM> at one end and a pair of claws <NUM> forming a front portion at an opposite end. Between the handles <NUM> and the claws <NUM> two pairs of levers are arranged. A first pair of levers <NUM> is configured to pivot around a pivot axis 203a and are connected at one side to the handles <NUM>, respectively, and at an opposite side to a second pair of levers <NUM>. The second pair of levers <NUM> is configured to pivot around a pivot axis 204a. The second pair of levers <NUM> is connected to the first pair of levers <NUM> through pivots or hinges <NUM> and is connected at an opposite end to the claws <NUM> in a fixed manner. By means of this, a claw on one side is connected to the handle on the same side and the claw on the other side is connected to the handle on the other side. With this construction, moving of the handles <NUM> towards one another causes the claws <NUM> to move towards each other as well, thereby being aligned to be substantially parallel. The instrument may include a spring <NUM> that holds the claws <NUM> in an open position, the spring force of which must be overcome by the handles <NUM> to close the claws <NUM>.

As shown in more detail in <FIG> the front portion of the instrument <NUM> comprises the claws <NUM>. The claws <NUM> are configured to engage the receiving part <NUM> from an outside thereof. To accomplish this, the claws <NUM> each comprise a cylindrical recess <NUM> adjacent to a front end 200a which is adapted to the outer shape of the receiving part <NUM>. Adjacent to the front end 200a, at an inner wall of the cylindrical recesses <NUM>, an engagement structure <NUM> is provided. The engagement structure <NUM> is configured to engage engagement structure <NUM> at the receiving part and the engagement structure <NUM> at the pressure member <NUM>. Specifically, the engagement structure <NUM> may be a conical peg that is adapted to the hollow conical shape of the conical holes of the receiving part and the pressure member. To accomplish this, the engagement structure <NUM> is located at approximately the center of each claw <NUM> in the circumferential direction. Adjacent to the cylindrical recess <NUM> and facing away from the front end 200a, the claws <NUM> may comprise a further cylindrical recess <NUM>. The cylindrical recess <NUM> may have a smaller diameter than the cylindrical recess <NUM>. During engagement of the claws with the receiving part, the second cylindrical recess <NUM> may provide space for accommodating a portion of the locking member <NUM> therein.

The locking member <NUM> may have a projection 7b at a bottom side thereof that serves for prolonging the locking member <NUM> to permit the fixation of rods of smaller and greater diameter in the rod receiving recess <NUM>. Hence, when a rod of greater diameter is received, the locking member <NUM> may protrude out of the first end 5a of the receiving part <NUM>.

The parts and portions of the bone anchoring device and the instrument may be made of any material, preferably, however, of a bio-compatible material, such as titanium or stainless steel or any other bio-compatible metal or metal alloy, for example or a plastic material. As bio-compatible alloy, a NiTi alloy, for example Nitinol, may be used. Other materials can be magnesium or magnesium alloys. Bio-compatible plastic materials for use may be, for example, polyether ether ketone (PEEK) or poly-L-lactide acid (PLLA). The parts can be made of the same or of different materials from one another.

In use, at least two bone anchoring devices may be anchored in bone and connected through a rod. As the bone anchoring device according to the embodiment described is a bottom-loading bone anchoring device, the head of the bone anchor <NUM> is inserted from the bottom end 5b into the receiving part. This accomplish prior to insertion of the shank into the bone. Alternatively, the shank can be already inserted into bone or a vertebra and the coupling device can be mounted onto the head <NUM>.

Referring to <FIG> steps of mounting the coupling device to the bone anchor are shown. The bone anchor <NUM> has been inserted into bone <NUM> that the head <NUM> protrudes out of the bone surface. The coupling device may be preassembled. In this configuration, the pressure member has been inserted into the passage to such an extent that the flexible portion is located in the widened section 51b of the passage <NUM> that forms an accommodation space. The pins <NUM> are placed in the holes <NUM> such that its front end extends into the recesses <NUM> of the pressure member <NUM>. Thus, the recess <NUM> of the receiving part and the recess <NUM> of the pressure member are aligned. The rear end of the pins may be flush with the outer surface of the receiving part. As depicted in <FIG>, the pressure member <NUM> is at a highest position relative to the bottom end 5b of the receiving part <NUM>. This highest position defines an insertion position where the head <NUM> can be inserted. In the insertion position the lower ends of the recesses <NUM> of the pressure member <NUM> abut against the inserted pins <NUM>. The engagement structure <NUM> of the pressure member is at an axial height Hb that is relative to the bottom end 5b above an axial height Ha of the engagement structure <NUM> of the receiving part <NUM>. As depicted in <FIG>, the coupling device is placed onto the head <NUM>. The pins <NUM> serve as abutment for the pressure member <NUM>. When the head <NUM> enters into the hollow interior <NUM>, the flexible portion can expand in the accommodation space 51b. Finally, as shown in <FIG> the coupling device is pulled upwards relative to the head <NUM>. As the head <NUM> is clamped in the flexible portion of the pressure member, the receiving part <NUM> is moved slightly upwards so that the narrowing surface in the second section 51c of the passage <NUM> and the narrowing surface <NUM> at the outside of the pressure member engage. Thereby, the head <NUM> is clamped by friction so that it can pivot only by overcoming the friction force. Simultaneously, the head <NUM> can no longer escape through the lower opening 52b of the receiving part <NUM> as the opening is reduced by the pressure member.

Referring to <FIG>, steps of locking the polyaxial bone anchoring device are shown. In <FIG>, the rod <NUM> is inserted and the locking member <NUM> is screwed in-between the legs <NUM> of the receiving part <NUM>. Tightening the locking member <NUM> results in a pressure exerted through the rod onto the pressure member <NUM>. Thereby, the pressure member <NUM> is pushed deeper with the narrowing outer surface <NUM> of the flexible portion into the narrowing second section 51c of the passage. This results in a compression of the flexible portion of the pressure member around the head <NUM> which locks the head <NUM> in the receiving part <NUM> at a specific angular position of the shank <NUM> relative to the central axis C. As shown in <FIG>, unscrewing or removing the locking member <NUM> allows the rod <NUM> to move in axial direction or to be removed at all. For example, the rod <NUM> can assume an axial position above the base 61a, i.e. the rod may no longer rest on the base 61a. Hence, various correction steps can be carried out with the rod being not present at all or the rod being at an elevated position in the channel. When the head <NUM> is locked or provisionally locked, the engagement structure <NUM> of the pressure member <NUM> is moved downward relative to the engagement structure <NUM> of the receiving part <NUM>. In the embodiment, in the locked configuration, the axial position of the engagement structure <NUM> of the receiving part <NUM> is higher than the axial position of the engagement structure <NUM> of the pressure member <NUM>. Hence, in the locked or provisionally locked configuration, the engagement structures of receiving part and pressure member are offset from each other in such a way that bringing them to alignment with each other results in unlocking the head <NUM>.

Next, as shown in <FIG>, the claws <NUM> are moved in an open state towards the receiving part <NUM> and oriented such that the pegs <NUM> are circumferentially aligned with the engagement structure <NUM> of the receiving part.

<FIG> shows that the pegs <NUM> of the claws <NUM> engage the engagement structure <NUM> of the receiving part. The rod <NUM> may be at an elevated position and a locking member <NUM> may protrude into the cylindrical recess <NUM>. As shown in <FIG> actuating the handles <NUM> of the instrument <NUM> moves the claws from both sides in a parallel manner towards the central axis. The pegs <NUM> extend through the holes of the receiving part <NUM> and enter into the holes of the pressure member <NUM>. Because of the offset of the axial position of the hole axes an upper front edge portion 222a of the pegs <NUM> abuts against the upper wall of the hole of the pressure member <NUM> as depicted in the enlarged portion of <FIG>. Thus, the upper wall of the engagement structure <NUM> of the pressure member <NUM> forms a guiding surface for guiding the engagement portion <NUM> of the instrument.

Finally, as shown in <FIG>, the engagement of the engagement structure <NUM> at the pressure member with the engagement portion <NUM> of the instrument tries to align the engagement structures of the receiving part and the pressure member as depicted in more detail in the enlarged portion of the Figure. Thereby, a force is generated that moves the pressure member <NUM> upward relative to the receiving part <NUM> to a second position in which the engagement structures are aligned. When the pressure member is moved upwards, the compression of the head <NUM> is decreased and the head is unlocked. It shall be noted that depending on the dimensions of the pressure member and the receiving part the head can be completely freely pivotable when unlocked or still held by friction so that the shank can be maintained provisionally at an angular position. This position can be changed by overcoming the friction force.

Another embodiment of the coupling device and the polyaxial bone anchoring device is shown in <FIG>. The coupling device differs from the coupling device described in the previous embodiment by the design of the receiving part and the pressure member. The receiving part <NUM>' and the pressure member <NUM>' comprise engagement structures <NUM>' for the receiving part <NUM>' and engagement structures <NUM>' for the pressure member <NUM>' which are at an angle different from <NUM>° relative to the longitudinal axis L of the rod receiving recess <NUM>. As shown in <FIG>, the engagement structures <NUM>', <NUM>' are formed at an angle of <NUM>° or less relative to the channel axis which is also the rod axis of a rod to be inserted. As in the first embodiment, in each of the receiving part <NUM>' and the pressure member <NUM>' two engagement structures <NUM>', <NUM>' are provided that are offset by <NUM>° in the circumferential direction. Thus, the engagement is effected closer to an inserted rod. Thereby, less space is used in a direction perpendicular to the rod axis which may have advantages in certain cases, such as a reduced spreading of the wound during surgery. All other features of the polyaxial bone anchoring device are identical or similar to those of the previous embodiment.

Modifications of the embodiments described above are conceivable. Parts are not limited to their detailed shape as depicted in the embodiments. In particular, the engagement structures at the receiving part and the pressure member may have a different shape and the shape of the engagement structure and the instrument may also be different and adapted to another shape. One engagement structure on one side of each of the receiving part and the pressure member may be sufficient. An instrument with only one engagement portion can be also sufficient. For the compression of the flexible portion of the pressure member around the head the narrowing surface may narrow in another shape. For example, a combination of a spherical or bulged shape that abuts against a tapered shape or a spherical or bulged shape that engages a hollow spherical or hollow bulged shape may also be possible, even a combination of two cylindrical surfaces.

For the locking member other known locking members may be used, for example a two-part locking member that permits to lock the head and the rod separately. The pressure member in this case may have legs that extend above the surface of the rod. The base forming a rod support surface may be different and may be also designed to receive only a rod of a single diameter.

Claim 1:
A coupling device for coupling a rod to a bone anchor comprising
a receiving part (<NUM>) having a channel (<NUM>) for receiving a rod and having a passage (<NUM>) defining a central axis (C) and an accommodation space (51b) for a head (<NUM>) of the bone anchor (<NUM>), the receiving part (<NUM>) comprising an outer surface with an engagement structure (<NUM>) configured to be engaged by an instrument (<NUM>); and
a pressure member (<NUM>) configured to be arranged at least partially in the accommodation space (51b), the pressure member (<NUM>) comprising a flexible portion defining a hollow interior (<NUM>) to accommodate and clamp the head (<NUM>) therein and an outer surface with an engagement structure (<NUM>) configured to be engaged by the instrument (<NUM>),
wherein the pressure member (<NUM>) can assume a first position in which the pressure member (<NUM>) engages an inner wall portion (51c) of the passage (<NUM>) such that the flexible portion is compressed to clamp or lock an inserted head (<NUM>) with a first locking force and a second position in which an inserted head (<NUM>) is clamped or locked with a second locking force smaller than the first locking force or is freely pivotable;
wherein the pressure member (<NUM>) is movable from the first position towards the second position by engagement with the instrument (<NUM>);
wherein the engagement structure (<NUM>) of the receiving part (<NUM>) and the engagement structure (<NUM>) of the pressure member (<NUM>) are offset relative to each other in the axial direction defined by the central axis (C) when the pressure member is in the first position such that when the engagement structure (<NUM>) of the receiving part (<NUM>) is first engaged by the instrument (<NUM>) and the engagement structure (<NUM>) of the pressure member (<NUM>) is subsequently engaged by the instrument (<NUM>), the pressure member (<NUM>) is moved from the first position into the second position and
wherein an outer surface of the flexible portion of the pressure member (<NUM>) comprises a narrowing section (<NUM>) and wherein the narrowing section is tapered and extends along substantially the whole length of the flexible portion.