WO2014108540A1

WO2014108540A1 - Cap for use with acetabular cup

Info

Publication number: WO2014108540A1
Application number: PCT/EP2014/050504
Authority: WO
Inventors: Andrew Clive Taylor
Original assignee: Finsbury (Development) Limited
Priority date: 2013-01-11
Filing date: 2014-01-13
Publication date: 2014-07-17
Also published as: GB201300515D0

Abstract

A cap (10, 110), as part of or for attachment to a tool such as an impaction tool, and for use with an acetabular cup (30, 130) having an inner concave surface (131), part of which articulates with a femur head (136), and an outer surface (32, 132) that implants in a pelvis, comprises (i) an impaction surface, to transmit an impaction force from the tool onto the inner cup surface (131), and (ii) a releasable connector (14, 114) that attaches to the cup (30, 130) via, e.g. onto or through, the inner cup surface (131).

Description

Cap for Use with Acetabular Cup

Field

The present invention relates to caps for use with acetabular cups and methods of attaching and releasing caps to and from cups, e.g. in preparation for and during hip surgery.

Background

In US 2002/0177854 an acetabular cup is attached securely to an impaction cap using a circle of cable that links the cup, introducer and impaction cap together securely by tensioning the cable. This allows the surgeon complete control over the positioning of the tool and acetabular cup. However, wormholes can be needed in the cup, through which the cable travels; these must be formed by casting, which is awkward. During use the wires can be damaged. Wear on the central nut of the introducing tool can lead to weakening and failure of the cables. Lastly, there is a risk of cable failure, and such failure, and the need to cut the cables (using a specialised cutter) in order to release the prosthetic from the cap and introducer, can leave shards of wire in the patient. All metal caps have the problem of the risk of toxic effects caused by metal particles or ions being shed from the prosthesis during use.

WO 2010/146398 discloses a ceramic cup, but these cannot be made with certain features (such as wormholes or fins) on them. The device is complex and therefore expensive. EP 1634552 is known as an alternative to the system of US 2002/0177854, but is more expensive as the introducing cap has to be disposable.

EP 1721586 discloses an impaction cap, having a central spring, that is manipulated by the introducer to retain or release the prosthesis via a multiplicity of lugs which interconnect with sockets or recesses in the cap. The device is also expensive. In addition there are high impaction forces because of the metal-on-metal interaction; therefore there is a lot of wear and this can lead to pins and welds failing. _Λ

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A cup is known from US5609646, which is made of polyether ether ketone (PEEK) and has a 'physiological' Young's modulus. A problem in use is that it is flexible and loses its shape, instead adapting itself to the shape of the surface reamed out of the bone. This also leads to problems with installation by impaction as it has to be inserted in the correct position first time.

Other known cups comprise lugs or other means of attachment to the cap, such as on the outside of the cup, which can interfere with fixation, on the top edge of the cup, e.g. castellations, which can interfere with the range of motion of the joint, or on the internal edge of the cup, which can increase wear and friction. Placement of such cups must be much more accurate as the tolerance for the angle of placement is much less for such cups. All of these systems make the prosthetic cup more expensive. US 2012/184965 describes an orthopaedic impactor device for positioning an orthopaedic prosthetic cup implant during a hip replacement surgery. The impactor device has a threaded end that is designed to engage with a threaded receiving section in the prosthetic cup. In this way the impactor and prosthetic are held in engagement securely, but releasably. Alternative means of achieving the same function are also disclosed: a cup-and-socket type engagement mechanism between the end of the impactor and the cup implant; equipping the impactor with an expandable dome to engage the interior of the cup implant; and equipping the impactor with lugs to engage the inner surface of the cup implant. US 5431657 describes an instrument for installing an acetabular cup assembly. The instrument includes a rod with a threaded portion that matches a threaded hole in the acetabular cup that is being installed such that instrument and prosthesis are reversibly linked together. US 5061270 describes a system for implanting an acetabular cup prosthesis including an implantation instrument with a disposable adapter. The adapter has a convex surface and the prosthesis has a mating concave surface such that the adapter accepts the prosthesis in a snap-fit relationship. US 5540697 describes a prosthetic socket implant installation apparatus and method. The apparatus comprises an implant installation tool that engages a feature around the inner rim of a prosthetic socket implant so that the implant is held firmly on the installation apparatus during the installation process but can be released after installation of the prosthetic socket.

US 2010/049257 describes a method of connecting an impactor to an orthopaedic acetabular cup implant wherein the implant includes a threaded hole and the impactor has an expandable threaded pair assembly matching the threaded hole. Locating the threaded pair within the threaded hole and radially expanding the threaded pair engages the threaded hole so that the implant is firmly engaged on the installation apparatus during the installation process but can be released following installation of the implant.

An object of the invention is to provide a cap (an impaction cap) to be used with a cup (an acetabular cup), and to provide a cap and cup system that represents at least an alternative to the above. An object of embodiments of the invention is to provide a cap and cup system such that the cup can be made of different materials, e.g. it does not need to be made of metal.

Invention

Accordingly, the invention provides a cap, comprising:-

(i) an impaction surface, to transmit an impaction force from the impaction tool onto the inner cup surface, and

(ii) a connector that attaches to the cup on or through the inner cup surface.

Hence impaction and attachment of the cap to the cup may both be achieved via a connection to the inner cup surface, e.g. onto or through the inner cup surface.

The cap is suitably part of or for attachment to a tool such as an impaction tool or an introducing tool, and for use with an acetabular cup having an inner surface, which is generally concave and part of which articulates with a femur head, and an outer surface that implants in a pelvis. The cup and cap together may be part of a system whereby the cap grips the cup via its inner surface and impaction force is transmitted also via the inner surface. Therefore, the cup can be made of alternative materials, such as plastics or polymers that hitherto have been avoided because of the previous need to transmit impaction and/or torsion via the outer surface or the cup rim. Thus the cup can be made of a material requiring less or minimal strength and/or stiffness throughout the cup.

The invention hence provides a cap cooperating with a cup that enables the cup to be securely attached to an insertion tool so as to transfer the impaction force to the cup directly via its inner surface and correctly insert the cup into the pelvis.

In embodiments of the invention described herein, the connection between the cap and cup is releasable. This is achieved in one embodiment by a projection from the cap being a tight fit with a bore in the cup body. This fit is sufficiently tight that the cup stays attached onto the cap during pre-surgery manipulations but once the cup is firmly located in the body the cap can then be withdrawn leaving the cup behind. In another embodiment the cap comprises an enlarged aperture through which a connecting rod having a bayonet fitting may pass to engage with a bayonet socket located on the cup - in the specific embodiment the socket is on an internal surface of a bore in the cup surface. Thus, in use, the connector passes through the front of the inner surface of the cup to connect the cup to the cap. More generally, the connector is moveable between a first position in which the cup is held to the cap and a second position in which the cap can be separated or withdrawn, e.g. leaving the cup in situ.

The impaction surface on the cap suitably comprises an outer convex surface that corresponds to and fits into the inner concave surface of the cup to distribute the impaction force evenly thereover. The mutually shaped surfaces mean that the force of impaction needed to seat the cup into a pelvis can be spread, reducing the risk of damage to the cup; again this means the cup can be made of different materials, other than ceramic or metal. The impaction surface may be shaped so that it distributes the impaction force over at least 10% of the inner cup surface, over at least 20% of the inner cup surface, over at least 30% of the inner cup surface area, over at least 50% of the inner cup surface or more. An increased area more evenly distributes the impaction force. Typically the inner cup surface comprises a concave portion that is substantially hemispherical, but which may be truncated slightly. A particular cap of the invention comprises an impaction surface, designed to cooperate with such inner surfaces, and having a portion of its impaction surface in frusto-spherical form. The impaction surface may be interrupted by other features, such as to transmit torsion, but the impaction surface is largely frusto-spherical and furthermore may be of resilient material. Close matching of the impaction surface between the cap and the inner cup surface may also reduce the risk of the cap slipping against the cup at the moment of impaction. In an embodiment of the invention, described below in more detail, with the cap attached to a tool or part thereof the impaction surface is located centrally about a longitudinal axis of the tool. The connector attaching the cup to the cap may also be located substantially along the longitudinal axis of the tool, hence the connector is central to the tool and the impaction surface forms a partial or full ring extending outwardly from the connector. The longitudinal connector axis is generally not aligned with the cup axis and may be inclined from 10 to 40 degrees to the cup axis.

In an embodiment of the invention, described below in more detail, a recess in the cup interlocks with a protrusion from the impaction cap so as to lock the positions of the cup and cap with respect to one another. The connector can then insert into a recess located on the inner cup surface. A suitable connector projects forwardly from the impaction surface on the cap.

An assembly of the invention comprises a cap and a cup held together by a clamp, for example a rod which runs through the cap, engages the socket in the cup and keeps the assembly locked together as a single secure unit. The rod for the connector lies along the central axis of an introducing tool. A hole in the cup and the protrusion of part of the connector therethrough is possible because the position of the hole mirrors the position of a natural hole in the pelvis through which a ligament passes.

Preferably, the cap attaches to or via a portion of the inner surface of the cup that after implantation does not articulate with the patient's femur. In this way, normal use of the hip joint after surgery involves fluid film lubrication between the femur head and that separate portion of the inner cup surface that does not overlap with and is not affected by the part of the inner cup surface that has been adapted for connection to a tool, e.g. the introducer or impaction tool.

A further optional feature of the invention is for the cap to so grip the cup via the inner cup surface such that turning the cap turns the cup. Caps of the invention attach not to the cup rim but to or via the inner surface of the cup. The attachment point or points may be central, meaning away from the rim and are preferably sufficiently close to the central axis of the cup to enable the cap to grip and turn the cup. A cap of the invention thus transmits torsional force to the cup via the inner cup surface. An advantage of this embodiment is that the cup is held, force is transmitted to the cup and the cup is turned all through interaction with the inner surface. Attachment to or transmittal of force onto the cup rim or sides is not necessary. Suitably, if the cup is approximated to a hemisphere, the attachment point or points are located within a portion of the inner cup surface that defines a spherical cap centred on the cup axis and 50% or less the height of the cup. The attachment points or points are also generally away from the very central portion of the cup inner surface, for reasons as described above, namely because that portion articulates with a femur head in use. Preferred attachment is thus away from the rim and away from the cup centre and in a ring between the two.

One or more sets of mutually engaging projections and recesses on the outer cap surface and inner cup surface can be provided, whereby a torsional force from the tool is transmitted to the cup by rotating the tool. Each projection can be on the cup with the recess on the cap, or on the cap with the recess on the cup. In embodiments described below in more detail, the recess is on the cup, specifically in the inner cup surface. Caps and cups of the invention are not as limited as to the materials they can be manufactured from as previously caps and cups were. In particular the cups can now be made using different materials including a wide range of polymers known to the skilled person. Generally, the material of the cap is as stiff as or stiffer than the cup. The cups may be made of PEEK (polyether ether ketone) or similar polymer and can be made of PAEK (polyaryl ether ketone) and other polymers regarded as variants of PEEK. The use of other plastics for the cup is also made possible by the invention. Metals and ceramics can now be avoided.

In an example described below, a cup of the invention is made out of PEEK containing carbon fibre. The carbon fibre combines with the PEEK to give a stiffness similar to physiological bone, in the range 5-18 GPa. The material may be more specifically described as CF-PEEK. Both of these materials are injection mouldable so manufacturing costs are lower that for metal; the structures produced are less able to hold a cable and have been regarded until now as unsuitable for making acetabular cups. The invention now enables cups of new materials to be efficiently used in a hip prosthesis. In use, the cap grips the cup so it can be positioned onto and then seated into the pelvis by the application of an impaction force.

The invention further provides an acetabular cup for use in hip surgery in combination with a cap comprising an impaction surface, to transmit an impaction force onto an inner cup surface and a releasable connector that attaches the cap to the cup, wherein the cup comprises an inner surface, part of which articulates with a femur head, and an outer surface that implants in a pelvis; and wherein the connector attaches the cap to the cup via, e.g. onto or through, the inner cup surface. As will be apparent, the cap of the invention is for use with the cup of the invention, and vice versa. Features of cups of embodiments of the invention correspond to those described in relation to the caps. Thus, for example, the inner surface of the cup can comprise a concave surface that corresponds to and into which fits the cap impaction surface to distribute the impaction evenly thereover. A recess or bore is suitably approximately centrally located on the inner cup surface to receive the connector.

In an example, the recess or bore is a tight fit for a connector projecting from the cap. The cup can also be for use with a connector moveable between a first position in which it holds the cup onto the cap and a second position in which the cap can be separated from the cup.

In particular embodiments, a portion of the inner surface of the cup that after implantation does not articulate with the patient's femur attaches to the cap. Further, and separately, the cup can be adapted to cooperate with the cap so that it can receives torsional force from the cap via the inner cup surface.

Also provided by the invention is a method of gripping an acetabular cup having an inner concave surface, part of which articulates with a femur head, and an outer surface that implants in a pelvis, with a cap of the invention, comprising bringing the cap impaction surface into contact with the inner surface of the cup and connecting the connector to the inner cup surface so as to grip the cup.

The method can comprise inserting the connector into a recess generally centrally located on the inner cup surface, being away from the cup rim, though preferably also away from the very central cup axis.

The connector for use in the method is as described elsewhere herein.

The method can comprise inserting a portion of the connector into the inner cup surface and moving into a position that holds the cup to the cap.

The method may further comprise turning the cap to transmit torsional force to the cup via the inner cup surface. Also provided by the invention is a kit comprising a cap according to any embodiment of the invention and an acetabular cup having an inner concave surface, part of which articulates with a femur head, and an outer surface that implants in a pelvis.

Still further provided by the invention is an impaction tool comprising a cap according to any embodiment of the invention.

An advantage of the invention is that it allows the cup to be made of material that is much more flexible than the prior art cups, which are made of metal or ceramics such that the introducing force is transferred through the cup rim and wall. In the invention the cup is optionally made of plastic and the flexibility of the cup is accommodated by the cap design that can distribute impaction force evenly over the inner surface of the cup.

To place the cup into the bone, typically a hemisphere is reamed into the bone that is slightly smaller than the cup, and the cup is driven into this gap. In general, a metal cup will not collapse as it is driven in. A plastic cup might collapse slightly but, again, the cap of the invention can accommodate plastic cups by supporting the cup across its inner surface during impaction.

The invention enables the use of cups with a hole, allowing body fluids into the cup from the rear. A specific embodiment of the invention is now described in detail with reference to the accompanying drawings in which:-

Fig. 1 shows a view from the side of an acetabular cup, cap and releasable bayonet connection according to one embodiment of the invention;

Fig. 2 shows a side view of a cap according to another embodiment of the invention;

Fig. 3 shows an alternative view from slightly underneath the cap of Fig. 2 in a different orientation; Fig. 4 shows a top view of a cup for use with the cap of Fig. 2;

Fig. 5 shows a perspective view of the cup of Fig. 4; and

Fig. 6 shows a perspective view of the exterior of a cup of with part of the cap protruding through the bore penetrating the cup and wherein the remainder of the cap is not shown.

Referring to Fig. 1 , a system for use in hip surgery is shown made up of a cap (10), an acetabular cup (30) and a bayonet connector (50). The cap (10) has a body (12) on which there is an impaction surface (16) in substantially frusto-spherical form, designed to mirror an inner surface (not shown) of the cup (30).

A centrally located connector (50) associated with the cap ends in the form of a bayonet connector fitting (14).

In the cup (30) a recess or bore (33) penetrating from the inner cup surface to the outer cup surface (32) comprises a socket (34) into which the corresponding mating surfaces (18) on the ends of the bayonet connector (50) can be inserted and fixed.

In use, the cap is brought into contact with the cup, such that the bayonet connector (50) is inserted into the bore (33) of the cup. The impaction surface (16) engages the inner cup surface. The rod of the bayonet connector (50) can then be inserted through the bore (33) of the cup to link with the socket thereon, thus holding the cup and cap securely together.

Projection (20) on the cap fits tightly into a corresponding recess (not shown) on the cup so that, as well as the cap securely holding the cup, any movement such as turning, or backwards or forwards movement of the cap is transmitted to the cup directly, via the recess and/or the connection (14), to the inner cup surface via the bayonet connector (50). The cup is securely held and can be accurately and firmly moved by movement of the cap now holding it. In assembled form, the bayonet connector (50) penetrates the hole running through the cup. That portion of the inner cup surface that forms the major load-bearing surface that interacts with the femur head of a patient (not shown) is separate from that portion of the inner cup through which the bore passes (33). The cup thickness between the inner cup surface and the outer cup surface is greater around the articulating surface than around the non- articulating surface.

On the outside of the cup, various structures are provided to improve engagement with, and grip of, the cup into the acetabular socket of a patient. The outer surface is coated with hydroxyapatite to improve adhesion to the socket. Both the cap and cup are made of CF-PEEK. The bayonet connector (50) is part of a surgical tool and separately clamped with respect to the cap and cup. Alternatively, the cap is separately attached to a tool and the bayonet connector (50) is operated independently. The edge of the cap has a substantially circumferential projection or indentation (40) to link the cup to the introducer and this supports the edge of the cup and prevents its bending or collapse during impaction.

Referring to Figs 2-6, an alternative system for use in hip surgery is shown made up of a cap (110) and an acetabular cup (130).

The cap (110) has a body (112) on which there is an impaction surface (116) in substantially frusto-spherical form, designed to mirror an inner surface (131 ) of the cup (130).

A centrally located connector on the cap has the form of a protrusion (114) that provides an interference fit with a corresponding a recess or bore (133) in the cup (130)

In the cup (130) a recess or bore (133) penetrating from the inner cup surface to the outer cup surface (132) comprises a socket (134) into which the corresponding mating surfaces of the interference-fit connector protrusion (114) can be inserted and fixed. In use, the cap is brought into contact with the cup, such that the interference-fit connector (1 14) is inserted into the bore (133) of the cup, thus holding the cup and cap securely together owing to the friction between the contacting surfaces of the interference-fit connector (1 14) and its socket. The impaction surface (1 16) engages the inner cup surface (131).

Projection (120) on the cap fits tightly into recess (138) on the cup so that, as well as the cap securely holding the cup, any movement such as turning, or backwards or forwards movement of the cap is transmitted to the cup directly via the recess (138) and/or the connection to the inner cup surface via the interference-fit connector (1 14). The cup is securely held and can be accurately and firmly moved by movement of the cap now holding it. In assembled form, the interference-fit connector (1 14) penetrates the hole running through the cup (133). That portion of the inner cup surface (136) that forms the major load bearing surface that interacts in a patient with the femur head (not shown) is separate from that portion of the inner cup (137) through which the bore passes (133). The cup thickness between the inner cup surface and the outer cup surface is greater around the articulating surface (136) than around the non-articulating surface (137).

On the outside of the cup, various structures are provided to improve engagement with and grip of the cup into the acetabular socket of a patient. The outer surface (132) is coated with hydroxyapatite to improve adhesion to the socket. Both the cap (110) and cup (130) are made of CF-PEEK. The cap (1 10) may separately attached to a tool.

The edge of the cap (1 10) has a substantially circumferential projection or indentation (140) to link the cup (130) to the introducer and this supports the edge of the cup (130) and prevents its bending or collapse during impaction.

The invention thus provides caps for use with acetabular cups that can effectively be made of different materials than were known hitherto.

Claims

1. A cap (10, 110), as part of or for attachment to a tool such as an impaction tool, and for use with an acetabular cup (30, 130) having an inner concave surface (131), part of which articulates with a femur head (136), and an outer surface

(32, 132) that implants in a pelvis, comprising:- an impaction surface, to transmit an impaction force from the tool onto the inner cup surface (131); and

a releasable connector (14, 1 14) that attaches to the cup (30, 130) via, e.g. onto or through, the inner cup surface (131).

2. A cap according to claim 1 , wherein the impaction surface comprises an outer convex surface that corresponds to and fits into the inner concave surface of the cup to distribute the impaction evenly thereover.

3. A cap according to claim 2, wherein the impaction surface distributes the impaction over at least 10% of the inner cup surface.

4. A cap according to any of claims 1 to 3, wherein the impaction surface is located centrally about a longitudinal axis of the tool.

5. A cap according to any of claims 1 to 4, comprising a resilient frusto-spherical impaction surface.

6. A cap according to any previous claim, wherein the connector inserts into a recess or bore centrally located on the inner cup surface.

7. A cap according to claim 6, wherein the connector projects forwardly from the impaction surface.

8. A cap according to either of claims 6 or 7, wherein the connector comprises a projection from the cap which is a tight fit with a bore in the cup body.

9. A cap according to either of claims 6 or 7, wherein the connector comprises a bayonet fitting that engages with a bayonet socket located on the cup.

A cap according to any previous claim, wherein the connector is moveable between a first position in which it holds the cup onto the cap and a second position in which the cap can be separated from the cup.

A cap according to any previous claim, wherein the cap attaches to a portion of the inner surface of the cup that after implantation does not articulate with the patient's femur.

A cap according to any previous claim, which transmits torsional force to the cup via the inner cup surface.

A cap according to claim 12, comprising one or more mutually engaging projections and recesses on the outer cap surface and inner cup surface whereby a torsional force from the tool is transmitted to the cup by rotating the tool. 14. An acetabular cup (30, 130) for use in hip surgery in combination with a cap (10, 1 10) comprising an impaction surface, to transmit an impaction force onto an inner cup surface (131) and a releasable connector (14,

1 4) that attaches the cap to the cup (30, 130),

wherein the cup comprises an inner surface (131), part of which articulates with a femur head (136), and an outer surface that implants in a pelvis (32,

132); and

wherein the connector attaches the cap to the cup via, e.g. onto or through, the inner cup surface (131).

15. A cup according to claim 14, wherein the inner surface of the cup comprises a concave surface that corresponds to and into which fits the cap impaction surface to distribute the impaction evenly thereover. WO 2014/108540 . ,_ PCT/EP2014/050504

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16. A cup according to claim 14 or 15, comprising a recess or bore centrally located on the inner cup surface to receive the connector.

17. A cup according to claim 16, wherein the recess or bore is a tight fit for a connector projecting from the cap.

18. A cup according to any of claims 14 to 17, for use with a connector moveable between a first position in which it holds the cup onto the cap and a second position in which the cap can be separated from the cup.

19. A cup according to any of claims 14 to 18, wherein a portion of the inner surface of the cup that after implantation does not articulate with the patient's femur attaches to the cap.

20. A cup according to any of claims 14 to 19, which receives torsional force from the cap via the inner cup surface.

21. A method of gripping an acetabular cup according to any of claims 14 to 20 with a cap according to any of claims 1 to 13, comprising bringing the cap impaction surface into contact with the inner surface of the cup and connecting the connector to the inner cup surface so as to grip the cup.

22. A method according to claim 21 , comprising inserting the connector into a recess centrally located on the inner cup surface.

A method according to either claim 21 or claim 22, comprising turning the cap to transmit torsional force to the cup via the inner cup surface.

24. A kit comprising a cap according to any of claims 1 to 13 and an acetabular cup according to any of claims 14 to 20.

25. An impaction tool comprising a cap according to any of claims 1 to 13.

26. A cap substantially as hereinbefore described with reference to Fig. 1 or Figs 2 to 6.

27. A cup substantially as hereinbefore described with reference to Fig. 1 or Figs 2 to 6.