Patent Description:
A polyaxial bone anchoring device is used in orthopedic surgery, in particular in spinal surgery, for coupling a rod to a bone anchor. Typically, a head of the bone anchor is pivotably received in a receiver which also takes up the rod. The receiver can assume various angular positions with respect to the bone anchor to provide a suitable orientation with respect to the rod to be connected and the receiver and head can be locked in this orientation. Various design concepts have been developed for such polyaxial bone anchoring devices. For example, one concept is to lock the head within the receiver by means of a pressure member placed in the receiver and pressing the head firmly into a seat in the receiver. Another concept is to accommodate the head in a flexible portion of the receiver and to compress the flexible portion by means of a locking ring embracing the flexible portion so as to lock the head therein.

The concept using an outer locking ring is described, for example, in <CIT> which describes a polyaxial colletted locking mechanism for use with orthopedic apparatus and includes a screw or other orthopedic implant element having a curvate head and a coupling element. The coupling element has a tapered and colletted portion having an interior chamber in which the curvate head is initially polyaxially disposed. A locking collar is disposed around the tapered and colletted portion such that translation thereof in the direction of the expanding taper causes the interior volume to contract onto the curvate head and lock it therein.

<CIT> describes a polyaxial bone anchoring device with a receiving part for coupling a rod to a bone anchor wherein the receiving part has a head receiving portion and a rod receiving portion. The head receiving portion is flexible for inserting and clamping the head. A locking ring extends around the head receiving portion. In a pre-locking position the locking ring exerts a first force onto the head receiving portion and the head is prevented from removal.

In a locking position the locking ring exerts a second force greater than the first force onto the head receiving portion to lock the head.

Another polyaxial bone anchoring device is known from <CIT>. It comprises a threaded screw including a head portion and a tulip assembly that selectively "snaps" and locks onto the head portion of the threaded screw in a desired orientation. The threaded screw may be placed in bone before the tulip assembly is engaged.

<CIT> describes a bone anchoring device comprising a receiving part with a head receiving portion for pivotably holding a head and a locking ring mounted around the head receiving portion. Grooves are provided on the outer surface of the head receiving portion and protrusions are formed on the inner surface of the locking ring. A rotational movement of the locking ring around the central axis provides a force which causes the head receiving portion to be compressed.

It is an object underlying the invention to provide an improved polyaxial bone anchoring device that is simple and safe in use and provides a variety of features or applications.

The object is solved by a polyaxial bone anchoring device of claim <NUM>. Further developments are given in the dependent claims.

According to the invention, the polyaxial bone anchoring device includes an anchoring element comprising a shank for anchoring in bone and a head, a receiving part comprising a head receiving portion having a wall defining an accommodation space for accommodating the head of the anchoring element and a central axis extending through the accommodation space. The polyaxial bone anchoring device further includes a locking member mountable to the receiving part such that it embraces at least partially the wall of head receiving portion and is movable between at least a first position and a second position, wherein the head receiving portion is configured to assume at least a first configuration in which the head can pivot in the accommodation space when the locking member is in the first position and a second configuration in which the head is locked in the accommodation space when the locking member is in the second position. The head receiving portion comprises at least one cavity in the wall that is configured such that a thickness of the wall in a radial direction with respect to the central axis between the accommodation space and the locking member is reduced by the cavity.

Since the head receiving portion has a partially reduced wall thickness, the spreading of the head receiving portion may be facilitated. Thus, the receiving part may be mounted more easily onto the head of the bone anchor. On the other hand, another thicker wall portion forming a seat against which the head is pressed by action of the locking member has a sufficient strength and stability against loosening of the clamping.

The polyaxial bone anchoring device includes only few parts which renders it more convenient to handle.

Moreover, the cavity in the head receiving portion enables other applications and/or use. For example, in one embodiment, pharmaceutical substances may be deposited in the cavity. Once the polyaxial bone anchoring device has been implanted into a patient's body, the pharmaceutical substances may be released over time. In another embodiment, the cavity may be used for accommodating a spring portion of a pressure member that is resiliently connected to the receiving part.

According to another aspect of the disclosure, a polyaxial bone anchoring device includes an anchoring element comprising a shank for anchoring in bone and a head; and a receiving part comprising a head receiving portion defining an accommodation space for accommodating the head of the anchoring element and a pressure member which is configured to exert pressure onto the head in the accommodation space wherein the pressure member is configured to assume at least a first position in which the head can pivot in the accommodation space and a second position in which the head is locked in the accommodation space; and wherein the pressure member is monolithic with the receiving part. In a particular embodiment, the pressure member is movable from the first position to the second position by separating the pressure member from the receiving part, preferably at a predefined breaking region. In another particular embodiment, the pressure member is resiliently connected with the receiving part.

According to an example a method of manufacturing the polyaxial bone anchoring device is provided wherein the method comprises manufacturing the receiving part with the cavity and/or with the pressure member by means of an additive manufacturing method, such as three dimensional printing, for example laser sintering or laser or electron beam melting. With such a method complex shapes can be easily manufactured on the basis of CAD data of the object to be manufactured. Hence, the cavity of the head receiving portion and/or the pressure member can be designed to have any shape regardless of conventional manufacturing restrictions.

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

Referring to <FIG>, an embodiment of the polyaxial bone anchoring device includes an anchoring element <NUM> having a shank <NUM> and a head <NUM>. The anchoring element <NUM> may be in the form of a bone screw with the shank <NUM> being at least partially threaded. The head <NUM> comprises a spherical outer surface portion. More specifically, the head <NUM> may be formed as a spherical segment, preferably including a region of the greatest outer diameter E of the sphere. In the free end surface of the head <NUM> opposite to the shank <NUM>, a recess <NUM> for engagement with a drive tool may be provided. The polyaxial bone anchoring further includes a receiving part <NUM> which is configured to accommodate the head <NUM> of the anchoring element and for receiving a rod <NUM>. The rod <NUM> serves to connect the polyaxial bone anchoring device to at least one further bone anchor which may be an identical or similar polyaxial bone anchoring device. A locking member in the form of a locking ring <NUM> serves for locking the head <NUM> in the receiving part <NUM> in a particular angular position of the shank <NUM>. To fix the rod <NUM> in the receiving part <NUM> a fixation member <NUM> may also be part of the polyaxial bone anchoring device. The fixation member <NUM> comprises an engagement structure <NUM> and a tool engagement recess <NUM>. It may be, for example, a set screw that can be screwed into the receiving part <NUM>.

Referring further to <FIG>, the receiving part <NUM> has a first or upper end 5a and a second or lower end 5b and a substantially cylindrical outer shape around a central axis C. A coaxial passage <NUM> extends from the first end Sa to a distance therefrom. The passage <NUM> narrows in a narrowing portion 51a towards a reduced diameter intermediate portion 51b. The narrowing portion 51a may be spherical but can have also another shape such as, for example, a conical shape. Adjacent to the intermediate portion 51b, an accommodation space <NUM> is formed that is shaped and sized to accommodate the head <NUM> therein. The accommodation space <NUM> has an opening 52a at the second end <NUM>. A substantially U-shaped recess <NUM> extends from the first end 5a to a distance therefrom by means of which two upstanding legs <NUM> are formed. The legs <NUM> delimit a channel for receiving the rod <NUM>. A bottom 53a of the channel may have a substantially V-shaped upper contour which provides a rod support surface. By means of this, rods with different diameters can be inserted into the channel and can be supported on the rod support surface. Any rod with a diameter between a smallest and a greatest diameter that is defined by the size of the recess <NUM> and by the shape of the bottom 53a can be safely rest on the bottom 53a. An internal thread <NUM> is formed on the legs <NUM> which is configured to cooperate with the external thread <NUM> of the fixation member <NUM>. The internal thread <NUM> may have any suitable threadform, preferably however, a threadform that prevents splaying of the legs <NUM> when the fixation member is tightened. For example, the threadform may be a flat thread or a square thread. By means of the U-shaped recess <NUM>, the upper portion of the receiving part defines a rod receiving portion.

A lower portion of the receiving part <NUM>, approximately from the intermediate portion 51b of the passage <NUM> to the second end 5b, which comprises the accommodation space <NUM>, defines a head receiving portion <NUM> of the receiving part <NUM>. The accommodation space <NUM> has a substantially spherical inner contour that matches an outer contour of the head <NUM>. The axial extension of the accommodation space <NUM> is such that it covers the region with the greatest outer diameter E of the head <NUM>. More specifically, the size of the accommodation space <NUM> may be such that the head <NUM> can be held therein by friction. A small outwardly tapering portion 52b that widens towards the second end 5b may be provided for facilitation the insertion of the head and/or for providing a slightly greater range of pivot angles for the shank <NUM>. As can be seen in particular in <FIG> and <FIG>, the accommodation space <NUM> is divided in an upper portion 52c configured to cover the head <NUM> from the top and a lower portion including a seat 52d for the head <NUM> in which the head <NUM> can pivot.

A wall of the receiving part <NUM> around the accommodation space <NUM> for the head <NUM> is expandable and compressible in a radial direction with respect to the central axis C. This is achieved by a plurality of axially extending slits <NUM> that are open to the second end 5b and that may extend up to a distance from the second end, preferably up to the intermediate portion 51b of the passage. The size and number of the slits <NUM> may be selected such as to obtain a desired flexibility. Moreover, an end portion 56a of the slits <NUM> may be enlarged, for example with a diamond-shaped, a circular contour or a triangular contour or any other suitable shape.

A cavity <NUM> is formed in the wall circumferentially around the accommodation space <NUM>. The cavity <NUM> may extend in the axial direction from a position below a region accommodating the greatest diameter E of the head up to the end portions 56a of the slits <NUM> as can be seen in particular in <FIG>. Thus, the cavity <NUM> is encircles an inserted head circumferentially and extends along a substantial region in the axial direction along the head <NUM>. Moreover, the cavity <NUM> is in communication with the accommodation space <NUM> through an opening <NUM> which has a smaller width in the axial direction than a radially outer portion of the cavity <NUM>. More specifically, the opening <NUM> has the shape of an annular slit as can be seen in particular in <FIG>. An outwardly directed wall portion 57a defining the cavity <NUM> may be slightly convex and may be formed at a radial position away from the central axis C so that a radially outermost wall portion 50a of the head receiving portion <NUM> is thinner than a lowermost wall portion 50b below the cavity <NUM> at or adjacent to the second end 5b. Also, the end portions 56a of the slits <NUM> are in communication with the cavity <NUM>. An upper wall portion 57b of the wall defining the cavity may be inclined and tapered towards the second end 5b up to the opening <NUM>. A lower wall portion 57c of the wall defining the cavity <NUM> may be substantially flat in a direction substantially perpendicular to the central axis and may be rising in a curved or inclined manner towards the opening <NUM>. It shall be noted that the detailed shape of the cavity is not limited to the embodiment shown but may vary. More generally, the width of the cavity <NUM> is greatest at a radial distance from the central axis C and decreases towards the central axis C.

By the opening <NUM> of the cavity <NUM>, the accommodation space <NUM> is divided into the upper portion 52c and the seat 52d which can be pressed against the head <NUM> by the locking ring <NUM>.

The wall portion 50a is thin compared to the wall portion 50b when seen in the radial direction. Moreover, the wall portion 50b is divided in the circumferential direction by the slits <NUM> in a plurality of sections which behave as flexible wall sections. Due to the cavity <NUM>, the flexible wall sections of the wall portion 50b can be easily spread although they are relatively thick at the second end 5b of the receiving part. When the head receiving portion <NUM> is compressed by the locking ring <NUM>, as described below, the lowermost wall portion 50b is configured to exert pressure onto the head <NUM> from an axial position at or close to the region with the greatest diameter E of the head and from below this region as depicted in <FIG>. As the wall sections of the wall portion 50b are thicker there, a sufficiently strong clamping force can be achieved. Also, the strength against loosening may be enhanced by this design. In other words the wall sections provide a stable seat 52d for the head <NUM> in which the head can be safely clamped.

The outer surface of the wall of the head receiving portion <NUM> widens in a tapered portion 50c close to the second end 5b towards the second end 5b. Thereby, the compression force increases when the locking ring <NUM> is moved along the head receiving portion <NUM> in a direction to the second end 5b. Except for the tapered portion 50c, the outer diameter of the receiving part <NUM> may be substantially the same along the axial length of the receiving part <NUM>. Between the tapered portion 50c and the second end 5b, the outer surface may be convexly rounded and an edge <NUM> may be formed between the rounded surface and the tapered portion 50c.

At an axial position above the bottom 53a of the U-shaped recess <NUM> and at a distance from the first end Sa, a first holding structure for temporarily holding the locking ring <NUM> in an insertion position in which the head is insertable into the accommodation space <NUM> is provided. At the outside of each leg <NUM> the first holding structure may be in the form of a circumferentially extending first rib 59a. The thickness of the first rib 59a in the radial direction is such that when the locking ring <NUM> passes the first rib 59a, it experiences a friction force such that the locking ring <NUM> is held by friction in the respective axial position. As also shown in <FIG>, the position of the first holding structure may be such that the locking ring <NUM> can be held completely above the head receiving portion <NUM>.

A second holding structure is provided at a distance from the first holding structure towards the second end 5b. The second holding structure may be also in the form of a circumferential rib 59b at the outer surface of the receiving part <NUM>. The axial position of the second holding structure may be slightly below the bottom 53a of the substantially U-shaped recess <NUM> and above the end portion 56a of the slits. Also the second circumferential rib 59b has such a thickness that the locking ring <NUM> experiences a friction force when it passes along the outer surface of the receiving part. Thereby, the locking ring <NUM> can be held temporarily at a pre-locking position in which the head receiving portion <NUM> is compressed but the head <NUM> is still pivotable without being finally locked. As additionally shown in <FIG>, in the pre-locking position, the locking ring <NUM> still has not fully engaged the tapered outer surface portion 50c of the head receiving portion <NUM>. In the pre-locking position, the head <NUM> is prevented from removal through the lower opening 52a of the accommodation space <NUM>.

At the center of each leg <NUM> in a circumferential direction, a through hole <NUM> or another engagement recess may be provided for engagement with an instrument. Lastly, inclined cutaway portions <NUM> may be formed at either end of the channel for the rod close to the bottom 53a of the substantially U-shaped recess. The cutaway portions <NUM> may extend until below the end portions 56a of the slits and may have a substantially triangular contour with the tip oriented towards the second end 5b. Additional chamfered portions <NUM> may extend from the cutaway portions <NUM> to some extent towards the edges of each of the legs <NUM> on either side of the channel. The cutaway portions <NUM> and chamfered portions may <NUM> facilitate the mounting of the locking ring from the first end 5a of the receiving part <NUM> and/or reduce the size of the receiving part.

Referring additionally to <FIG>, the locking ring <NUM> will be described in greater detail. The locking ring <NUM> is formed as a closed ring with a first or upper end 6a and a second or lower end 6b. The inner surface <NUM> is substantially cylindrical with an inner diameter sized such that the locking ring <NUM> fits around the receiving part <NUM> and is configured to slide along the outer surface of the receiving part <NUM>. Adjacent to the first end 6a a small beveled portion <NUM> may be formed that may serve for sliding the locking ring from the second end 5b of the receiving part along the rounded outer surface of the lower wall 50b and over the tapered portion 50c for mounting. Adjacent to the second end 6b, the locking ring <NUM> may comprise a tapered inner surface portion <NUM> that tapers towards the first end 6a and is configured to cooperate with the tapered outer surface portion 50c of the receiving part <NUM>. The angle of the tapered portions of the locking ring <NUM> and the receiving part <NUM> preferably correspond to each other. Preferably the angle is selected such that a self-locking occurs when the surfaces engage each other. Thus, once the locking ring <NUM> is mounted and the tapered surfaces engage each other, the locking ring is prevented from being inadvertently moved upward and the locking is loosened. However, it may be possible to disengage the cooperating surfaces using an instrument.

The outer surface of the locking ring <NUM> comprises a groove <NUM> that may serve for engagement with an instrument. Adjacent to the first end 6a, the outer surface may be cylindrical and adjacent to the second end 6b the outer surface may be tapered towards the second end 6b. However, the shape is not limited to such a design. The axial length of the locking ring <NUM> may be at least the axial extension of the cavity <NUM>, preferably greater than the axial length of the cavity <NUM>. Thus, the locking ring is configured to exert a radial compression force onto a major part of the head receiving portion <NUM>.

The locking ring <NUM> can also be mounted to the receiving part <NUM> from the first end 5a thereof. In this case, the tapered portion <NUM> can slide along rounded edges of the legs <NUM> at the first end 5a of the receiving part <NUM>.

Parts and portions of the polyaxial bone anchoring device 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, or 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.

A preferred method of manufacturing the receiving part and optionally also the locking ring and/or the bone anchoring element is an additive manufacturing method, more preferably an additive layer manufacturing method, such as three-dimensional printing. More particularly, preferred methods are laser sintering or laser melting or electron beam melting. In such a method, subsequent layers of a powder material, such as a metal or plastic powder, are solidified with an energy beam, particularly a laser or an electron beam, at positions corresponding to the cross-section of the part in the respective layer. By such a method, complex shapes including undercuts etc. can be easily manufactured.

Referring to <FIG>, the steps of mounting the locking ring <NUM> to the receiving part <NUM> and assembling the polyaxial bone anchoring device will be explained. According to a first method of assembling, the locking ring <NUM> is mounted to the receiving part <NUM> from the second end 5b of the receiving part <NUM>. As shown in <FIG>, the locking ring <NUM> is oriented with its first end 6a towards the second end 5b of the receiving part and subsequently pushed on the receiving part <NUM> as shown in <FIG>. During this step, the beveled portion <NUM> slides along the rounded lower surface of the receiving part <NUM> and compresses slightly the head receiving portion <NUM> so that it can move over the edge <NUM> between the rounded portion and the tapered portion 50c. Then, the locking ring <NUM> can be moved further towards the first end 6a. It passes the second rib 59b which is slightly retracted due to the compression of the head receiving portion so that it does not produce an obstacle during mounting. When the locking ring <NUM> reaches the first rib 59a it is held there by the friction hat occurs between the first rib 59a and an upper region of the cylindrical section <NUM> of the locking ring that is close to the beveled portion <NUM> as shown in <FIG>. In this position, which is an insertion position of the locking ring for permitting the head <NUM> to be inserted into the accommodation space <NUM>, the second end 6b of the locking ring is above the head receiving portion <NUM>. Hence, the head receiving portion <NUM> is free to expand to permit the head <NUM> to be inserted, as shown in <FIG>. Due to the flexibility of the wall of the head receiving portion <NUM>, the head <NUM> can be easily inserted and is held in the seat 52d by the friction force of the flexible wall sections as shown in <FIG> e.

Alternatively to the steps shown in <FIG>, the locking ring can also be mounted from the first end Sa of the receiving part.

Referring to <FIG>, the further steps of locking the head <NUM> in the receiving part <NUM> will be explained. In <FIG>, once the head <NUM> has been inserted into the accommodation space <NUM>, the locking ring <NUM> is moved towards the second end 5b of the receiving part whereby the friction force between the first rib 59a and the locking ring <NUM> is overcome, until the locking ring <NUM> passes the second rib 59b where it is held temporarily by friction. This position of the locking ring <NUM> is a pre-locking position in which the head receiving portion <NUM> is compressed to such an extent that the head <NUM> cannot be removed through the opening 52a. Depending on the size of the head and the accommodation space and the flexibility, the head <NUM> may also be held with friction in the pre-locking position. This allows to maintain a preliminary angular position between the receiving part and the head <NUM>. The preliminary angular position can be changed by overcoming the friction force either manually or with an instrument.

For locking the head <NUM> in the head receiving portion <NUM> the locking ring <NUM> is further moved towards the second end 5b until the tapered outer surfaces <NUM>, 50c of the locking ring and the receiving part, respectively, substantially engage each other as depicted in <FIG>. This results in an increased compression of the head receiving portion <NUM> around the head <NUM> so that finally the head <NUM> is locked in the head receiving portion <NUM>. The angle of the tapered surfaces preferably results in a self-locking which means that the friction force between the two surfaces is high enough to prevent a disengagement of the surfaces and thereby upward movement of the locking ring under normal conditions of use. As shown in the enlarged views of <FIG>, the tapered surfaces are in full engagement when the lower end 6b of the locking ring is substantially flush with the lower end of the tapered surface 50c. Hence, the force that compresses the head receiving portion is transmitted mainly at the lower stable wall section 50b in a radial direction to lock the head <NUM> in the seat 52d.

After locking the head <NUM>, the rod <NUM> can be inserted into the substantially U-shaped recess <NUM> and the fixation member <NUM> can be inserted between the legs <NUM>. For final locking, as shown in <FIG>, the fixation member <NUM> is tightened to press the rod <NUM> into the bottom 53a of the substantially U-shaped recess <NUM>. As the rod <NUM> also presses onto the first end 6a of the locking ring <NUM> the compression force exerted by the locking ring increases, whereby finally the entire polyaxial bone anchoring device is locked.

In the clinical use, at least two polyaxial bone anchoring devices are inserted into bone parts or vertebrae and connected through the rod <NUM>. It shall be noted that the polyaxial bone anchoring device can be used in a pre-assembled condition where the head is already inserted into the head receiving portion and with the locking ring being in the pre-locking position as shown in <FIG>. The polyaxial bone anchoring device can be inserted in this pre-assembled condition into bone. In another way of use, the bone anchoring element <NUM> can be inserted first into the bone or a vertebra and the receiving part <NUM> with the locking ring in the insertion position as shown in <FIG> is mounted to the head <NUM>. The cavity may also be used for depositing a pharmaceutical substance therein that is released over a time period.

Referring to <FIG>, a modified receiving part of the polyaxial bone anchoring device will be explained. The receiving part <NUM>' differs from the receiving part of the previous embodiment mainly in the design of the cavity and the accommodation space. Also, in this modified embodiment, the preliminary holding structures like the first rib and the second rib are omitted. Parts and portions of the receiving part <NUM>' that are identical or highly similar to the parts and portions of the receiving part <NUM> of the previous embodiment are designated with the same reference numerals and the description thereof will not be repeated.

In the embodiment, the accommodation space <NUM>' comprises a seat 52d' that is shorter in the axial direction compared to the seat 52d of the previous embodiment. The widening portion 52b' may have an axial length that is greater than in the receiving part of the previous embodiment. Generally, the widening portion 52b' and the seat 52d' may have approximately the same axial length. Adjacent to the seat 52d' there may be a conical portion 52e' that widens towards the first end 5a of the receiving part <NUM>'. The cavity <NUM>' is elongate in the axial direction. More specifically, the height of the cavity <NUM>' in the axial direction is greater than the width of the cavity in the radial direction. Thus, the shape of the cavity <NUM>' is approximately that of a hollow cylindrical ring or an oblong torus which is located substantially at an axial position around the widening portion 52e' and the upper portion 52c of the accommodation space <NUM>'. The opening <NUM>' that connects the cavity <NUM>' with the accommodation space <NUM>' in this embodiment extends from an upper portion of the cavity <NUM>' closer to the first end 5a of the receiving part into the widening portion 52e'. The lower wall portion 57c' of the cavity is located at the middle or around the middle portion of the cavity <NUM>' in the axial direction. Thus, the cavity <NUM>' is separated to a greater extent from the accommodation space <NUM>'.

The locking ring <NUM> can be temporarily held in the insertion position or in the pre-locking position only by friction with the outer surface of the receiving part <NUM>'.

Referring to <FIG>, a further modified embodiment of the receiving part will be described. The receiving part <NUM>" of the polyaxial bone anchoring device differs from the receiving part <NUM>' of the previous embodiment in the design of the cavity, the accommodation space and the slits. The cavity <NUM>" is similar to the cavity <NUM>' of the previous embodiment. That means, the cavity <NUM>" is substantially hollow cylindrical ring-shaped or torus-shaped and extends around the widening portion 52e" and the upper section 52c of the accommodation space. An inclined circumferential slit <NUM>" provides for the opening and connects the upper portion of the cavity <NUM>" with the accommodation space <NUM>". The accommodation space <NUM>" may have an small widening section 52b". Different from the previous embodiments, the number and length of the slits <NUM>" is reduced. In the embodiment shown, six equidistantly arranged slits <NUM>" are provided compared to more slits in the previous embodiments. However, the number of slits is not restricted thereto. A smaller number of slits may result in greater wall portions in the circumferential direction. Hence, to obtain enough flexibility, the area of the end portions 56a" is increased, for example, the end portions 56a" are elongate in the axial direction. Therefore, the head receiving portion <NUM> has a sufficient flexibility to be easily expanded and compressed around the head <NUM>. With the cavity <NUM>" designed in this manner, the entire axial length of the receiving part and mainly that of the head receiving portion <NUM> can be reduced and contributes to a small size of the receiving part <NUM>".

Referring to <FIG> a further embodiment of the polyaxial bone anchoring device will be described. The polyaxial bone anchoring device differs from the previous embodiment in the design of the receiving part. The receiving part <NUM>‴ is similar to the receiving part <NUM> of the first embodiment with respect to the lower portion of the accommodation space <NUM> and with respect to the shape and position of the cavity <NUM>.

The passage <NUM>‴ accommodates a pressure member <NUM> that is configured to exert pressure onto the head <NUM> from the top of the head <NUM>. The pressure member <NUM> is a substantially cylindrical part that is sized such that it fits into the passage <NUM>‴ but is not tightly guided by the wall of the passage. It has a first or upper end 8a and an opposite second or lower end 8b. At the second end 8b, there is a substantially spherically-shaped recess <NUM> with a radius corresponding to the radius of the head <NUM> that allows to engage the upper surface of the head <NUM> and exert pressure thereto. At the first end 8a, there is an elongate recess <NUM> with a longitudinal axis that is substantially perpendicular to the central axis C. The recess <NUM> comprises a substantially V-shaped cross-section to provide a support surface for rods of different diameter. Close to the first end 8a there are circumferentially extending projections <NUM>, in the embodiment two projections, that are axially spaced from each other and that are configured to engage corresponding circumferentially extending grooves <NUM> provided at the inner surface of the receiving part at an axial position such that when the head is inserted, the inner surface of the spherical recess <NUM> does not yet contact the upper surface of the head <NUM>. The projections <NUM> fit into the grooves <NUM> with an axial and a radial play. The recess <NUM> is aligned with the substantially U-shaped recess <NUM> of the receiving part so that the rod <NUM> can be inserted into the receiving part and rests on the rod support surface of the recess <NUM>. Moreover, the pressure member <NUM> is at such a position in the receiving part <NUM>‴ that the rod support surface of the recess <NUM> is slightly above the rod support surface of the substantially U-shaped recess <NUM> of the receiving part <NUM>‴ as shown in <FIG>.

The pressure member <NUM> is monolithically connected to the receiving part <NUM>‴ with a connection portion <NUM> that is arranged orthogonal to the legs <NUM> on both sides of the rod channel as can be seen in particular in <FIG>. The connection portion <NUM> is thin so that it can be easily broken-off when a force is exerted onto the pressure member <NUM>. Thus, the connection portion <NUM> forms a predefined breaking region that is configured to break under load so that the pressure member <NUM> can be separated from the receiving part <NUM>‴. The receiving part <NUM>‴ and the pressure member <NUM> are manufactured preferably as a monolithic part using an additive manufacturing method as described above.

It shall be noted that the pressure member <NUM> lacks a central hole that provides access to the head <NUM>. Hence, the polyaxial bone anchoring device is particularly suitable for in-situ placement of the bone anchoring element <NUM> into bone and mounting the receiving part with assembled locking ring thereafter. In a still further modified embodiment, the pressure member <NUM> may have a central hole providing access to the head <NUM> with a tool.

In use, as shown in <FIG>, the head <NUM> is first inserted into the receiving part <NUM>‴ and clamped by means of the locking ring <NUM> which is moved into the locking position where the tapered surfaces 50c and <NUM> of the receiving part and the locking ring, respectively, are engaged. Then, the rod <NUM> is inserted until it rests on the rod support surface of the recess <NUM> of the pressure member <NUM>. Finally, the fixation member <NUM> is inserted between the legs and tightened. The pressure exerted by the fixation member <NUM> onto the rod <NUM> is transferred onto the pressure member <NUM>. Thereby, the connection portion <NUM> breaks and the pressure member <NUM> can move to some extent in the axial direction so that the pressure is transferred via the pressure member <NUM> to the head <NUM> to finally lock the head in the head receiving portion <NUM>. Due to the axial play, the pressure member can move slightly downward. Due to the radial and the axial play, the pressure member <NUM> can also be tilted slightly or wobble with respect to the head <NUM>. This permits to adjust the pressures more precisely.

Referring to <FIG> a still further embodiment of the polyaxial bone anchoring device is described. The receiving part <NUM>‴' is similar to the receiving part <NUM>‴ of the previous embodiment except with respect to the shape and connection to the pressure member <NUM>' and the shape of the cavity. The pressure member <NUM>' is permanently connected to the receiving part <NUM>"". In order to render the pressure member <NUM>' movable, the pressure member <NUM>' comprises a spring portion <NUM> that connects the pressure member <NUM>' and the receiving part <NUM>‴' monolithically in a resilient manner. The spring portion <NUM> is formed by two portions forming an angle. In greater detail, it comprises a first portion 85a that extends from the outer edge of the second end 8b in an inclined manner upwards towards the first end 5a of the receiving part and a second portion 85b that extends from the first portion 85a in an oppositely inclined manner to a downwardly facing upper wall portion 57b"" of the cavity <NUM>"". The cavity <NUM>‴' has a large opening that opens into the passage so that it is configured to accommodate the spring portion <NUM>. Due to its shape, the spring portion <NUM> provides resiliency to the pressure member <NUM>' in the axial direction and also to some extent in the radial direction. Preferably, the pressure member <NUM>' and the receiving part <NUM>‴' are manufactured monolithically using an additive manufacturing method.

In use, when the head has been inserted into the head receiving portion and the rod has been inserted into the rod channel and rests on the rod support surface of the recess <NUM>, tightening the fixation member <NUM> transfers the pressure via the rod onto the pressure member <NUM>' and finally onto the head <NUM> so that the head is locked in the accommodation space. Loosening the fixation member also loosens the clamping of the head. For example, when the locking ring is in the pre-locking position, several correction steps can be carried out in this manner, before final locking.

Referring to <FIG> a still further embodiment of the polyaxial bone anchoring device is shown that is modified with respect to the cooperation of the receiving part with the locking ring. Parts and portions that are identical or highly similar to those of the embodiment according to <FIG> are designated with the same reference numerals and the description thereof will not be repeated.

The locking ring <NUM>' has at a particular position in the circumferential direction an extension portion <NUM> that extends from the first end 6a in a direction above the first end. The extension portion <NUM> may be inclined towards the central axis and may resilient to some extent. Exemplary, the extension member <NUM> has a substantially rectangular cross-section.

The receiving part <NUM> is different from the receiving part <NUM> of <FIG> in the design of the holding structures at the outer surface of the receiving part. A first holding structure includes a rib <NUM> that is located above the bottom 53a of the substantially U-shaped recess <NUM> and at a distance from the first end 5a. The rib <NUM> extends only over about half a leg <NUM> from one edge 54a of one leg <NUM> in a circumferential direction up to the through hole <NUM>. Adjacent to the rib <NUM> at the side towards the first end 5a is a groove <NUM> that runs concentric with the rib <NUM>. The groove <NUM> provides space for the free end of the extension portion <NUM> of the locking ring. Further, a guiding groove <NUM> is formed that extends from the through hole <NUM> in the axial direction downward up to the second end 5b of the receiving part. At a distance from the through hole <NUM> a downwardly inclined ramp or obstacle <NUM> is provided in the groove <NUM>. The ramp <NUM> permits the extension portion <NUM> to snap over it when moving downward but prevents the extension portion <NUM> to move upward again. The ramp <NUM> is located at such an axial position that the locking ring is in the pre-locking position when the extension portion <NUM> has passed the ramp <NUM>.

A mounting groove <NUM> is formed at the receiving part <NUM> at the side of the rod channel. In greater detail, the mounting groove <NUM> extends from the second end 5b up to a small distance from the bottom 53a of the substantially U-shaped recess <NUM>. The mounting groove <NUM> permits the extension portion <NUM> to extend therein when the locking ring <NUM>' is mounted to the receiving part from the second end 5b.

As shown in <FIG>, the locking ring <NUM>' is mounted to the receiving part <NUM> from the second end 5b of the receiving part in an orientation in which the extension portion <NUM> is aligned with the mounting groove <NUM>. The extension portion <NUM> engages the mounting groove <NUM> and the locking ring <NUM>' is moved upwards as depicted in <FIG>. When the extension portion <NUM> has reached the circumferentially extending groove <NUM>, the locking ring <NUM>' is rotated until the extension portion <NUM> snaps into the through hole <NUM>. In this position, the extension portion <NUM> engages the bottom of the edge of the through hole <NUM>, as also shown in <FIG> and <FIG>, and is temporarily held there in the insertion position where the head can be inserted.

Once the head is inserted into the accommodation space <NUM> of the head receiving portion, the locking ring <NUM>' can be moved downward until it snaps over the ramp <NUM> into the pre-locking position as shown in <FIG> and <FIG>. In the pre-locking position the head cannot be removed from the receiving part. For Thereafter, the locking ring can be moved further downward until the head <NUM> is locked.

Claim 1:
A a polyaxial bone anchoring device including
an anchoring element (<NUM>) comprising a shank (<NUM>) for anchoring in bone and a head (<NUM>),
a receiving part (<NUM>, <NUM>', <NUM>", <NUM>‴, <NUM>"", <NUM>) comprising a head receiving portion (<NUM>) having a wall defining an accommodation space (<NUM>, <NUM>', <NUM>", <NUM>‴) for accommodating the head of the anchoring element and a central axis (C) extending through the accommodation space; and
a locking member (<NUM>, <NUM>') mountable to the receiving part such that it embraces at least partially the wall of head receiving portion and is movable between at least a first position and a second position;
wherein the head receiving portion (<NUM>) is configured to assume at least a first configuration in which the head can pivot in the accommodation space when the locking member is in the first position and a second configuration in which the head is locked in the accommodation space when the locking member is in the second position;
characterized in that head receiving portion (<NUM>) comprises at least one cavity (<NUM>, <NUM>', <NUM>", <NUM>‴) in the wall that is configured such that a thickness of the wall in a radial direction with respect to the central axis (C) between the accommodation space (<NUM>, <NUM>', <NUM>", <NUM>‴) and the locking member (<NUM>, <NUM>') is reduced by the cavity.