WO2013179002A1

WO2013179002A1 - A trial instrument for use in orthopaedic surgery

Info

Publication number: WO2013179002A1
Application number: PCT/GB2013/051366
Authority: WO
Inventors: James Anderson; Ivan Green; Gary Moore
Original assignee: Depuy (Ireland)
Priority date: 2012-06-01
Filing date: 2013-05-24
Publication date: 2013-12-05
Also published as: GB201209806D0

Abstract

A trial instrument for use in orthopaedic surgery includes a handle for manipulating the instrument and a body portion for location in a bone cavity. The body portion is shaped as a shallow bowl with a base wall and a side wall such that the depth of the body portion is less than the distance across the body portion in at least one direction. The body portion has first and second openings through its base wall, the first opening being located adjacent to the side wall towards a first side of the body portion and the second opening being located adjacent to the side wall on a second side of the body portion which is generally opposite to the first side. The side wall of the body portion occludes at least one of the openings at least partially when the body portion is viewed from a position on a plane which extends out of the body portion and which includes a line extending between the midpoints of the first and second openings, the position being outside a conical viewing zone having a cone angle of 20°.

Description

A TRIAL INSTRUMENT FOR USE IN ORTHOPAEDIC SURGERY

This invention relates to a trial instrument for use in orthopaedic surgery.

It can be desirable to form a cavity within a bone during treatment of a bone defect. This can arise for example in a patient suffering from avascular necrosis (AVN), which is also known as osteonecrosis (ON), ischemic bone necrosis, or aseptic necrosis. AVN results from the temporary or permanent loss of circulation to the bone tissue, and gives rise to localized death of the bone tissue. The loss of proper blood flow can result from trauma, or compromising conditions such as prolonged steroid use, alcohol use, gout diabetes, pancreatitis, venous occlusion, decompression disease, radiation therapy, chemotherapy, and Gaucher's disease.

WO-A-2010/097632 discloses a support structure implant for location within a bone cavity to support the bone which defines the cavity is formed from wires by braiding in a machine direction from a first end of the structure towards an opposite second end. The wires are formed into loops at the first end of the structure so that two lengths of wire extend away from each loop and are formed into the braid. Each of the wires which is formed into the braid is held at the second end of the structure in a clamp. The clamp is provided by a support ring and a clamp ring which is formed from a shape memory alloy which can shrink to clamp the wires against the support ring when heated. The implant component might be implanted in a cavity in a bone in a procedure to treat AVN in that bone.

Frequently, the portion of a bone which is affected by AVN is concentrated in the superior anterior region of the femoral head. The affected portion can have a generally flat shape when the bone is viewed along the anterior-posterior axis. The width of the portion of a bone that is affected can often be greater than the width of a tunnel required to gain axis to that portion of the bone.

It is known to provide an instrument for forming a cavity in a bone that is arranged to form a cavity which is larger than the access hole into the bone through which the instrument is inserted. US-A-2001/034526 discloses an expandable reamer for forming a space within a vertebral disc. The expandable reamer includes a pair of opposing blades having an expanded position in which they extend from either side of a shaft assembly and a retracted state in which they are substantially retracted into the shaft assembly. When the blades are in the retracted state the shaft assembly may be inserted into a bore extending into a bone. The blades may then progressively be deployed to the expanded position while the shaft assembly is rotated to form a cavity having a greater maximum diameter than that of the shaft assembly or the bore extending into the bone.

WO-A-2008/099187 discloses a tool for forming a cavity within a bone comprises a shaft and a blade which is pivotally mounted on the shaft towards one end thereof. A rod is fastened to the blade, and can slide within the shaft, to change the pivotal position of the blade relative to the shaft. The tool can include an incremental drive to cause the rod to translate along the shaft when the shaft and rod are rotated.

WO-A-2010/013027 discloses an instrument which comprises a shaft, a blade pivotally mounted on the shaft towards a first end of the shaft and a push rod extending within the shaft and coupled to the blade. The push rod is arranged to slide within the shaft to cause the blade to rotate about its pivot. The instrument includes a depth stop coupled to the shaft arranged such that when the shaft is inserted into a bore extending into a bone the depth stop is arranged to engage bone surrounding the bore to limit movement of the shaft into the bore. A spring couples the push rod to the shaft and is arranged to resist axial movement of the push rod within the shaft towards the first end of the shaft. When the depth stop engages bone surrounding a bore, an axial force applied to the push rod which exceeds the spring resistance causes the push rod to slide within the shaft.

The present invention provides an instrument which can be located within a bone cavity prior to implantation of a support structure implant to assist the surgeon to determine the location of the cavity within the bone.

Accordingly, in one aspect, the invention provides a trial instrument for use in orthopaedic surgery, which comprises a handle for manipulating the instrument and a body portion for location in a bone cavity, the body portion being shaped as a shallow bowl with a base wall and a side wall such that the depth of the body portion is less than the distance across the body portion in at least one direction, the body portion having first and second openings through its base wall, the first opening being located adjacent to the side wall towards a first side of the body portion and the second opening being located adjacent to the side wall on a second side of the body portion which is generally opposite to the first side, such that the side wall of the body portion occludes at least one of the openings at least partially when the body portion is viewed from a position on a plane which extends out of the body portion and which includes a line extending between the midpoints of the first and second openings, the position being outside a conical viewing zone having a cone angle of 20°.

The instrument can be used in conjunction with imaging equipment such as X-ray imaging equipment to locate a cavity within a bone, for example relative to subchondral bone tissue. The radiation used to generate the image is conventionally directed along an axis. The radiation source can be used with the instrument of the invention so that the axis on which the radiation is directed lies in the plane which extends directly out of the body portion and which includes the line extending between the midpoints of the first and second openings. The contrast that is provided between the instrument when located within the cavity and the bone tissue that surrounds the cavity can be significantly greater than the contrast between the bone tissue and the cavity itself. The instrument can therefore provide a reliable indication of the location of the cavity, particularly when the instrument is a snug fit in the cavity so that movement of the instrument within the cavity is minimal. It also assists with confirming the orientation of the instrument within the cavity which helps the surgeon to determine more accurately the location of the periphery of the body portion of the instrument. This can assist with obtaining accurate measurements of the cavity, especially when those measurements are made (a) in the plane which is defined by the outline of the instrument when the instrument is in its optimised orientation plane, or (b) in the plane which is perpendicular to the normal axis of the instrument

The body portion is shaped as a shallow bowl with a base wall and a side wall such that the depth of the body portion is less than the distance across the body portion in at least one direction. The bowl shape is shallow so that it can be positioned in a bone cavity through an access bore having a small transverse dimension (which will be a diameter when the bore has a circular cross-section as when the bore is formed by drilling). The shallow configuration of the body portion will frequently mean that it can be moved within the bone cavity. The provision of the openings in the body portion enable the orientation or location or both of the body portion within the bone cavity to be observed using

appropriate imaging techniques, for example X-ray imaging techniques.

The walls of the body portion of the instrument should provide a visible contrast between the portions in which the openings are formed and the remainder of the walls. This can be accomplished by forming the body portion so that the walls are have no openings formed in them over all of their area other than the first and second openings referred to above.

However, other openings can be provided in the walls, provided that the presence of those openings does not compromise the ability of a user to determine precisely the boundaries of the said first and second openings when viewing the instrument using an appropriate imaging technique.

The base wall of the body portion can be planar. It can be preferred however that the walls of the body portion are rounded when the instrument is viewed in cross-section along an axis which is parallel to the base. The cross-section shape of the walls of the body portion can then be considered to be part of a cylinder. It can be preferred for some applications that the base and side walls are rounded so that there is no discernable boundary between them. When there is no discernable boundary, the openings can still be considered to be in the base of the body portion when they can be observed from a position outside the body portion. The use of generally rounded walls, especially external walls, has the advantage that it can facilitate collection of bone fragments that are present in the bone cavity, for example using a scooping action. This might be accomplished by twisting the body portion within the bone cavity.

Preferably, the first and second openings are shaped and arranged within the body portion so that the open areas of the openings, that are visible from a point on a viewing axis which is perpendicular to a line joining the midpoints of the openings and intersects that line at its midpoint, and perpendicular to the plane defined by the outline of the body portion, are the same. The observed open area of an opening through a wall of the body portion is the area of that opening at the distal face of the body portion when viewed through the opening from a location that is proximal of the opposite proximal surface. The open areas of the openings are not occluded by the side walls of the body portion when the body portion is viewed along a viewing axis which is perpendicular to a line joining the midpoints of the openings and intersects that line at its midpoint, and perpendicular to the plane defined by the outline of the body portion. Twisting the body portion so that the viewing axis is not perpendicular to the plane defined by the outline of the body portion results in the openings becoming occluded so that the open areas of the openings are reduced.

The invention also provides a kit which comprises the instrument of the invention in combination with imaging equipment with which it can be used.

The instrument can be used to remove bone tissue material from within the cavity which is present in the cavity as a result of forming the cavity. The cavity might be formed using a cutting instrument having one or more features of the cutting instrument disclosed in US-A-2001/034526 or disclosed in WO-A-2010/013027. Such cutting instruments can result in the formation of a cavity which has a generally spherical shape. At least some bone fragments which are generated as the cavity is cut can remain in the cavity after removal of the cutting instrument. It is desirable that fragments within the cavity are removed before an implant is located in the cavity to minimise interference in the positioning of the implant by any such fragments. The bowl shaped body portion of the instrument of the invention can be used to collect such fragments and to remove them from within the cavity. When the instrument of the invention is intended for use in a spherical cavity, it is preferred that the body portion has a generally circular outline.

The invention also provides a kit which comprises the instrument of the invention in combination with a cutter for forming a cavity in a bone. The outline of the body portion of the instrument is selected so that the outline of the body portion matches the cross- sectional shape of a cavity which is prepared using the cutter. It will generally be preferred that the cutter creates a cavity which has a rotational axis of symmetry so that the cutter has an edge which is rotated to create the cavity. The edge will frequently be rounded.

Preferably, the edge is defined by a radius so that the cavity is approximately spherical.

Optionally, the base wall of the body portion is concave on the inside of the bowl when the body portion is viewed in cross-section. Optionally, the base wall of the body portion is convex on the outside of the bowl when the body portion is viewed in cross-section, especially when viewed in cross-section along the length of the instrument. This can facilitate collection of bone fragments that are present in the bone cavity, for example using a scooping action. It can also facilitate insertion of the instrument into a bone cavity through a bore having a circular cross-section.

The concave inside surface of the body portion base wall, or the convex outside surface of the body portion base wall, or each of them, can help to optimise the ability of the body portion to collect bone tissue fragments which is present in the bone cavity. Either or both of these features can help to provide the first and second openings in the wall of the body portion such that the openings are occluded as specified above.

It can be preferred that concave internal base wall of the body portion is approximately cylindrical. Preferably, the first and second openings are spaced apart on opposite sides of the axis which is defined by the cylindrical base wall, towards the side wall of the body portion on opposite sides of that axis.

The openings in the body portion should be shaped so that they facilitate recognition of when either of them is occluded, if only partially, by the side wall of the body portion. It has been found that the incorporation of elongate openings in the body portion can be appropriate. For example, the openings can be slit like, especially with approximately parallel sides. The openings can have rounded ends. Each elongate opening defines an axis. Optionally, the axis which is defined by the first opening being approximately parallel to the axis which is defined by the second opening. When the walls of the body portion are seen as rounded when the instrument is viewed in cross-section along an axis which is parallel to the base, the openings in the body portion can be aligned with that axis. The openings in the instrument enable its orientation to be determined by viewing the instrument from a position in which the openings permit radiation to pass through them so that it can be viewed or otherwise detected. The openings enable the orientation of the instrument to be optimised in a particular plane by minimising the angle through which the instrument is twisted out of that plane. For example, when the body portion has a generally circular outline (which might be provided by two part-circle segments), its outline will appear as circular when the instrument is in the optimised orientation. Twisting the instrument having a generally circular outline out of the optimised orientation means that the outline of the instrument then appears to be elliptical in shape. The likelihood of occlusion of the openings is minimised by viewing the instrument on a plane which includes a line joining the midpoints of the openings. The plane should be perpendicular to the plane which is defined by the outline of the instrument when the instrument is in the optimised orientation. The intersection of the two planes can be considered as defining the normal axis of the instrument. In many embodiments, the normal axis and the viewing axis coincide when the body portion is in its optimised orientation.

Preferably, the first opening in the body portion is arranged so that it starts to become occluded by its adjacent side wall when the viewing angle (which is the angle between the viewing axis and the normal axis) is +a°, and the second opening in the body portion is arranged so that it starts to become occluded by its adjacent side wall when the viewing angle is between -a° and -(a ± 0.1a)°. It is particularly preferred that the angle at which the first opening starts to become occluded has the same size as the angle at which the second opening starts to become occluded. The angle at which an opening starts to become occluded by its adjacent side wall depends on factors which include:

the distance between the side wall and the opening,

• the height of the side wall.

The first opening in the body portion is arranged so that it starts to become occluded by its adjacent side wall when the viewing angle is not more than 10°, so that the cone angle is 20° as specified above. The opening could be occluded at least partially by its adjacent side wall at a smaller viewing angle, for example so that it starts to become occluded when the viewing angle is 5 ° . Preferably, the cone angle is centred on the normal axis of the body portion which is the axis which extends perpendicular to the plane defined by the outline of the body portion.

Preferably, the first opening in the body portion is arranged so that it is fully occluded by its adjacent side wall when the viewing angle is +β°, and the second opening in the body portion is arranged so that it is fully occluded by its adjacent side wall when the viewing angle is between -β° and -(β ± 0.1β)°. It is particularly preferred that the angle at which the first opening becomes fully occluded has the same size as the angle at which the second opening becomes fully occluded. The angle at which an opening becomes fully occluded by its adjacent side wall also depends on factors which include:

the distance between the side wall and the opening,

• the height of the side wall,

• the size of the slot.

The first opening in the body portion can be arranged so that it is completely occluded by its adjacent side wall when the viewing angle is not more than 35 ° (so that the cone angle is 70°), for example not more than 30°, or not more than 28°. The opening could be completely occluded by its adjacent side wall at a smaller viewing angle, for example so that it is completely occluded when the viewing angle is 15 ° or more, for example 20° or more.

Optionally, the body portion comprises first and second bowl shaped pieces, each of the pieces having a base wall and a side wall, with at least one of the first and second openings being provided in the base wall of each of the pieces. It can be preferred that each of the first and second pieces of the body portion has first and second openings formed in its base wall, the first opening being located adjacent to the side wall towards a first side of the body portion piece and the second opening being located adjacent to the side wall on a second side of the body portion piece which is generally opposite to the first side, such that the side wall of the body portion piece occludes at least one of the openings at least partially when the body portion piece is viewed from a position outside a conical viewing zone having a cone angle of 20°. Optionally, the first bowl shaped piece of the body portion can be inverted relative to the second bowl shaped piece. An instrument in which first and second bowl shaped pieces are arranged with one inverted relative to the other can be used to collect bone fragment material in each of the scoops by twisting the body portion within the bone cavity.

Material which is collected in first and second bowl shaped pieces of a body portion can be retained in the body portion by sliding one of the pieces relative to the other piece so that one of the bowl shaped pieces effectively functions as a lid for the other bowl shaped piece, at least partially covering the other bowl shaped piece.

Accordingly, an instrument which has first and second bowl shaped pieces can be used particularly effectively to remove bone fragments from within the bone cavity. Bone fragments can be scooped into the bowl shaped pieces while they are separated angularly. They can be retained within the bowl shaped pieces by moving one of the pieces so that it is positioned as a lid for the other piece.

Optionally, the first and second pieces of the body portion are mounted on the handle so that one of the first and second pieces can move relative to the other of the first and second pieces between a deployed configuration and a collapsed configuration, in which the extent of overlap between the first and second pieces is greater in the collapsed configuration than in the deployed configuration. For example, the instrument can be arranged so that each of the first and second pieces moves between the deployed and collapsed configurations. The deployed configuration of the body portion can have dimensions which correspond more closely to those of an implant component which is subsequently to be implanted in the bone cavity, consistent with the instrument functioning as a trial implant corresponding to the implant component.

It can be preferred that at least one of the first and second pieces of the body portion, preferably each of the first and second pieces of the body portion, is mounted for pivotal movement relative to the handle between the deployed and collapsed configurations of the body portion. When each of the first and second body portions is mounted for pivotal movement, they can be connected to one another and to the handle by means of a pivot pin. A cam mechanism can be used to control the movement of the or each movable piece of the body portion.

It can be preferred that the first and second pieces of the body portion are biassed resiliently towards the deployed configuration so that they can be moved towards the collapsed configuration by application of a collapsing force. A collapsing force might be applied to cause the configuration of the body portion of the instrument to change from the deployed configuration towards the collapsed configuration to facilitate movement of the body portion through an access bore in a bone, into or out of a bone cavity. The instrument can be used in its partially collapsed configuration when collecting bone fragments within the bone cavity. The resilient bias towards the deployed configuration can help to ensure that the configuration of the body portion when no collapsing force is applied is the deployed configuration or close to it. This facilitates use of the instrument as a trial for an implant component which is subsequently to be implanted in the bone cavity. It can also facilitate the last stages of collecting bone fragments and other material which might be in the bone cavity in the bowl shaped body portion of the instrument. If it is apparent that the body portion of the instrument is blocked against expansion to the deployed configuration while in the bone cavity, the surgeon should consider whether the cavity should be cut further, or whether the blockage against expansion is as a result of bone fragments which are positioned between the body portion and the cavity wall. In this latter event, it is appropriate to use the instrument to collect the bone fragments for removal from the cavity.

The cone angle which defines the conical viewing zone determines the extent of twisting movement of the instrument before one or more of the openings in the body portion becomes occluded by the side wall of the body portion. The body portion considered as a whole might frequently have a circular outline. For example it might include two or more body portion pieces, each of which is shaped as a segment of a circle. The conical viewing zone will then normally be assessed about an axis which extends perpendicular to the plane of that circle.

When the trial instrument is to be used to determine the location of a bone cavity which is approximately circular in at least one plane, especially approximately spherical, it can be appropriate for the trial instrument to define the outline of at least part of a circle. When the body portion comprises first and second body portion pieces, each of them can have an outline shape which corresponds to a segments of a circle, comprising a circular wall and a plane wall. The first and second pieces of the body portion are fastened to one another so that, when the body portion is in its deployed portion, the first and second pieces define separate segments of the same circle, the diameter of the circle corresponding approximately to the diameter of an implant component which is to be implanted in the bone cavity. When the relative movement between the first and second pieces of the body portion is pivotal movement, it will generally be appropriate for the included angle between the plane walls of the body portion pieces when the body portion is in its deployed configuration to be an acute angle. Changing the configuration of the body portion towards the collapsed configuration causes the size of that angle to be reduced.

When each of the body portion pieces is shaped as a segment of a circle, it can be preferred that the height of the segment (which is the maximum value of the distance from the straight wall to the circular wall) is not greater than the transverse diameter of the portion of the handle which is intended to fit through an access bore (it being realised that there might be local reductions in the transverse diameter of the handle due to for example fixing holes, features for deploying the body portion etc).

The instrument can include an actuator to cause the or each movable piece of the body portion to move between the collapsed and deployed configurations of the body portion. The actuator can comprises a tube and an actuator rod which is a sliding fit in the tube. The tube can provide the handle for manipulating the instrument. One of the tube and the actuator rod can be connected directly or indirectly to a movable piece of the body portion. Movement of the actuator rod within the tube can cause movement of that piece of the body portion relative to the other parts of the body portion which are not so connected. When the body portion comprises first and second pieces which are both movable relative to the instrument handle, each of them can be connected (directly or indirectly) to one of the tube and the actuator rod. The connection between a movable body portion piece and a component of the actuator can be by means of a cam in the form of a pin which slides in a slot on an extension of the body portion piece.

The body portion of the instrument should be configured so that its size corresponds in at least some dimensions to that of an implant component which is subsequently to be fitted within a bone cavity. It is appropriate for some applications to provide a plurality of implant components which differ from one another in terms of their size. It will then be preferred to provide a plurality of trial instruments which differ from one another in terms of their size, each trial instrument corresponding to an implant component. For example, when the an implant component is for use in the treatment of AVN, it can have an approximately spherical shape for fitting into a bone cavity having a correspondingly spherical shape. It can then be appropriate to provide a plurality of implant components which differ from one another in terms of their transverse dimension (which will be their diameter when the components are spherical). A plurality of cutting instruments and of trial instruments can be provided having sizes which correspond to respective ones of the implant components.

When the implant component is for use in the treatment of AVN in the femoral head, it can be provided in a plurality of sizes which differ from one another in terms of their diameters. A set of implant components which the surgeon can choose from might have diameters of 15, 16, 18, 20, 22, 24, 26 and 28 mm. Corresponding sets of cutting and trial instruments might have effective diameters of 13, 15, 17, 19, 21, 24, 25 and 27 mm.

Embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which:

Figure 1 is a sectional view of a femur showing the formation of a spherical cavity in the femoral head in which a support implant can be implanted.

Figure 2 is a plan view of a trial instrument for location in a bone cavity such as might be formed in a femur as shown in Figure 1.

Figure 3 is a side view of the trial instrument shown in Figure 2. Figure 4 and 5 are enlarged plan views of the body portion of the instrument shown in Figures 2 and 3, in collapsed and deployed configurations respectively.

Figures 6 and 7 are isometric views, from above and from below respectively, of one of the pieces of the body section.

Figures 8 and 9 are a view from above, and a sectional elevation on the line B-B respectively, of the body portion piece shown in Figures 6 and 7.

Figures 10 to 12 are schematic views illustrating the occlusion of the openings in a body portion as a result of rotation of the body portion.

The invention is described in the drawings in rleation to treatment of a bone defect in the femoral head. The invention is applicable to the treatment of defects in bones other than the femur, for example the humeral head.

Referring to the drawings, Figure 1 shows a femur 2 having a shaft 4, a neck 6 extending proximally from the shaft, and a head 8 at the proximal end of the neck.

The femur has a region 10 in the head, located superiorly within the head, in which the cancellous bone has degraded. This could be due to avascular necrosis.

Figure 1 shows the femur with a cutting tool 12 extending into the femoral head 8 through a bore which is open at the anterior cortex 14. The cutting tool has cutting teeth 16 at its proximal end. The cutting tool can have features of the tools which are disclosed in one or both of WO-A-2008/099187 and WO-A-2010/013027. The tool can be deployed through a bore which is prepared in the femoral neck using a drill. The drill and the cutting tool can be used with a depth stop controller which limits the depth of the drill bit and cutter when in use.

An implant can be positioned in the cavity in the femoral head to provide support for the head. Material which promotes bone growth and revascularisation can be located within the bone cavity. For example, morcelised bone tissue can be used. The cavity can be accessed for location of the implant and the bone growth promotion material through the bore which was used to create the cavity. Details of implants which can be used in this way are disclosed in WO-A-2010/097632.

It is important to ensure that the cavity is formed in the femoral head in the correct location, for example so that (i) the formation of the cavity removes an appropriate proportion of the degraded bone tissue, and (ii) the cavity is appropriately located relative to the sub-chondral bone of the femoral head. The degraded bone tissue itself can be located on X-ray images of the femoral head before the procedure is started. This information can be used by the surgeon in planning the procedure. The contrast between the cancellous bone tissue in the femoral head and the cavity that is formed in the head is not always sufficiently clear for the surgeon to be able to determine the location of the head using X-ray images that are created during the procedure.

The present invention provides an instrument which can be positioned in the cavity in the femoral head to assist the surgeon in determining the location of the cavity.

Figures 2 and 3 are plan and side views of an instrument 20 having proximal and distal ends 22, 24 which can be positioned in a cavity in the femoral head. The instrument comprises an elongate shaft 26 which has first and second body portion pieces 28, 30 at its distal end 24. The body portion pieces are mounted on the shaft so that they can pivot about a pivot pin 32 between deployed and collapsed configurations. The instrument is shown in its deployed configuration in Figures 2 and 3. The deployed and collapsed configurations are discussed in more detail below with reference to Figures 4 and 5.

The shaft 26 has a trigger plate 34 at its proximal end 22 with a recess 36, 38 on the distal facing face of the trigger plate on each side of the shaft.

The shaft 26 is hollow with a bore extending along its length. A control rod 40 is provided in the bore extending along the length of the shaft so that it can slide in the bore. The control rod protrudes from the shaft at its proximal end 22. The proximal end of the control rod has a cap 42 fitted to it. A helical spring 44 acts between the cap 42 on the control rod and the trigger plate on the shaft and urges the control rod in the proximal -Indirection relative to the shaft. The spring can be compressed by the application of a compressive force to the cap on the control rod, to urge the control rod in the direction towards the distal end of the shaft.

The body portion pieces 28, 30 are connected to the control rod 42 at its distal end by means of a pair of connector plates 46 which are located on opposite sides of the body portion pieces. A deployment pin 48 connects the connector plates and passes through the body portion pieces. The deployment pin 48 and the pivot pin 32 are parallel to one another.

Referring to Figures 6 to 9, each of the body portion pieces 28, 30 has a shallow bowl portion 60 at its end. The bowl is shallow in the sense that its depth "d" is less than its length "1". The bowl has a base wall 62 and a side wall 64. The base wall has a curved cross-section when the bowl is viewed along the length of the body portion piece (in the direction of the arrow 66 in Figures 6 and 7, and as shown in Figure 9) so that the base wall is cylindrical in shape. Each of the bowl portion pieces has the shape of a segment of a circle, having a rounded side wall portion 68 and a straight edged side wall portion 70.

Each of the body portion pieces has a first opening 72 and a second opening 74 formed in it. Each of the openings is elongate and has rounded ends. The openings are arranged so that they extend in a direction which is parallel to the straight edged straight edged side wall portion 70.

Each of the body portion pieces has a control arm 76 which has a hole 78 formed in it in which the pivot pin 32 can be received. Each of the body portion pieces also has a slot 80 formed in it, extending at an angle of about 25 ° to the axis of the body portion piece (defined by the straight edged side wall portion 70) in which the deployment pin 48 is received.

Referring to Figures 4 and 5, the body portion pieces are fastened to the shaft 26 by means of the pivot pin 32 so that they can pivot between the collapsed configuration as shown in Figures 3 and 4 and the deployed configuration as shown in Figures 2 and 5. The body portion pieces are arranged so that the open faces of the bowl portions face one another when the body portion pieces are in the collapsed configuration. The shallow depth of the bowl portions and the cylindrical cross-section shape of the base walls means that the cross-section shape of the body portion provided by the body portion pieces 28, 30 is similar to that of the shaft 26. This allows the instrument to be inserted into the bone cavity through a bore in the bone.

The configuration of the instrument can be changed from the deployed configuration shown in Figures 2 and 5 to the collapsed configuration shown in Figures 3 and 4 by applying pressure to the cap 42 on the control rod 40 to move the control rod relative to the shaft 26. This can be done by the user gripping the trigger plate 34 with the fingers of one hand, and pressing on the cap 42 with the palm of the hand. This causes the control rod to be displaced in a direction towards the distal end 24 of the shaft and towards the body portion pieces 28, 30. The deployment pin 48 extends through the slots in the control arms 76 of the body portion pieces. The angle between the slots 80 and the axis of the shaft means that the body portion pieces are angularly about the pivot pin 32 as the deployment pin is displaced along the slots 80.

Figures 10 to 12 show the body portion pieces 28, 30 in the deployed configuration in three orientations relative to the direction in which X-ray radiation is directed at the instrument. The direction of the radiation is indicated by the parallel lines 100 which aligned with the long edge of the drawings page.

In Figure 10, each of the body portion pieces is arranged so that the base wall 62 is perpendicular to the radiation direction and the side wall 64 is parallel to the radiation direction. The radiation which is directed at the openings 72, 74 in the body portion pieces is not masked by the side walls 64. The radiation can therefore pass through all four of the openings in the first and second body portion pieces which are therefore visible on images which are created using the radiation which is detected after passing the instrument.

The instrument is depicted in Figures 11 and 12 first and second twisted orientations. In each of the twisted orientations, radiation is masked by the side walls from passing through two of the openings (which are marked with the symbol "^x") so that only two of the openings are therefore visible. It is therefore apparent from image data generated using the radiation that the instrument is twisted out of the plane which is perpendicular to the orientation. This information is useful for the surgeon because it enables the position of the instrument and therefore of the cavity in which the instrument is positioned to be estimated accurately.

The dimensions of six sizes of body portion pieces having a rounded cross-section are set out below in Table 1 (in millimetres unless otherwise indicated). The dimensions that are set out in the table are depicted in Figures 8 and 9, and are:

In each of the body portion pieces referred to in Table 1 ,the effective height of the rounded wall 68 (to which the closer opening is the one labelled 72) is 4.4 mm, and the effective height of the straight edged wall 70 (to which the closer opening is the one labelled 74) is 3.3 mm.

Table 1

The dimensions of six sizes of body portion pieces having a square cross-section (in which the angle between the base wall 62 and the side wall 64 is 90°) are set out below in Table 2. The height of the rounded wall 68 (to which the closer opening is the one labelled 72) is 5.0 mm, and the height of the straight edged wall 70 (to which the closer opening is the one labelled 74) is 4.25 mm.

Table 2

The width of the opening 74 which is closer to the straight edged wall 70 is smaller than the width of the opening 72 which is closer to the rounded wall 68. This it to take account of the smaller height of the straight edged wall compared with that of the rounded wall. The smaller width of the opening ensures that the angle through which the instrument is twisted at which one or other of the openings becomes completely occluded is the same, irrespective of the direction of the twist (comparing Figures 11 and 12).

The instrument of the invention can be used in a method of treatment of avascular necrosis in the femoral head, which involves:

1. Performing pre-operative planning by using X-ray images of the femoral head to locate the degraded tissue, including the extent of the degraded tissue within the femoral head.

2. Identify size of implant component to be located in the femoral head to fill a cavity which results from removal of degraded tissue, taking account of the location of the cavity and the need to ensure that the tunnel to access the cavity should not weaken the femoral neck.

3. Use a reference drill to define the path for the access tunnel, using fluoroscopic imaging to guide the drill.

4. Using a bone harvesting drill to create the access tunnel, using a depth limiter.

5. Finishing the access tunnel using a tunnel drill, ensuring that the depth to the drill can be inserted is limited using a depth stop.

6. Using a cavity cutter to create a spherical cavity in the femoral head, ensuring that the depth to the cavity cutter can be inserted is limited using a depth stop.

7. Inserting the instrument of the invention, sized to match the size of the cavity cutter, through the access tunnel into the bone cavity while in its collapsed configuration.

8. Using the instrument of the invention, sized to match the size of the cavity cutter, to collect bone debris within the cavity. This involves twisting the instrument clockwise so that the open faces of the body portion pieces are driven towards debris in the cavity by the twisting action. Obstruction to the twisting action caused by uncut bone on the cavity wall requires withdrawal of the instrument and additional use of the cavity cutter. Obstruction to the twisting action caused by a bone fragment requires collection of the fragment, possibly by collapsing the instrument slightly.

9. Aligning the instrument to the plane of a fiuoroscope by twisting the instrument until the fiuoroscope image shows the four openings in the body portion. The circular outline of the body portion then represents the internal wall of the bone cavity. The surgeon can proceed to implant an appropriate support in the bone cavity once the position of the cavity has been verified in this way.

Claims

CLAIMS:

1. A trial instrument for use in orthopaedic surgery, which comprises a handle for manipulating the instrument and a body portion for location in a bone cavity, the body portion being shaped as a shallow bowl with a base wall and a side wall such that the depth of the body portion is less than the distance across the body portion in at least one direction, the body portion having first and second openings through its base wall, the first opening being located adjacent to the side wall towards a first side of the body portion and the second opening being located adjacent to the side wall on a second side of the body portion which is generally opposite to the first side, such that the side wall of the body portion occludes at least one of the openings at least partially when the body portion is viewed from a position on a plane which extends out of the body portion and which includes a line extending between the midpoints of the first and second openings, the position being outside a conical viewing zone having a cone angle of 20°.

2. A trial instrument as claimed in claim 1, in which the base wall of the body portion is concave on the inside of the bowl when the body portion is viewed in cross- section.

3. A trial instrument as claimed in claim 2, in which the concave base wall of the body portion is approximately cylindrical.

4. A trial instrument as claimed in claim 3, in which the first and second openings are spaced apart on opposite sides of the axis which is defined by the cylindrical base wall.

5. A trial instrument as claimed in any one of claims 1 to 4, in which the first and second openings are elongate, the axis which is defined by the first opening being approximately parallel to the axis which is defined by the second opening.

6. A trial instrument as claimed in any one of claims 1 to 5, in which the body portion comprises first and second bowl shaped pieces, each of the pieces having a base wall and a side wall, with at least one of the first and second openings being provided in the base wall of each of the pieces.

7. A trial instrument as claimed in claim 6, in which each of the first and second pieces of the body portion has first and second openings formed in its base wall, the first opening being located adjacent to the side wall towards a first side of the body portion piece and the second opening being located adjacent to the side wall on a second side of the body portion piece which is generally opposite to the first side, such that the side wall of the body portion piece occludes at least one of the openings at least partially when the body portion piece is viewed from a position outside a conical viewing zone having a cone angle of 20°.

8. A trial instrument as claimed in claim 6 or claim 7, in which the first bowl shaped piece of the body portion is inverted relative to the second bowl shaped piece.

9. A trial instrument as claimed in any one of claims 6 to 8, in which the first and second pieces of the body portion are mounted on the handle so that one of the first and second pieces can move relative to the other of the first and second pieces between a deployed configuration and a collapsed configuration in which the extent of overlap between the first and second pieces is greater in the collapsed configuration than in the deployed configuration.

10. A trial instrument as claimed in claim 9, in which at least one of the first and second pieces of the body portion is mounted for pivotal movement relative to the handle between the deployed and collapsed configurations of the body portion.

11. A trial instrument as claimed in any one of claims 1 to 10, in which the size of the cone angle at which the first opening becomes partially occluded by the portion of the side wall adjacent to it is approximately the same as the size of the cone angle at which the second opening becomes partially occluded by the portion of the side wall adjacent to it.

12. A trial instrument as claimed in any one of claims 1 to 11, in which the size of the cone angle at which the first opening becomes completely occluded by the portion of the side wall adjacent to it is approximately the same as the size of the cone angle at which the second opening becomes completely occluded by the portion of the side wall adjacent to it.

5 13. A trial instrument as claimed in any one of claims 1 to 12, in which at least one of the first and second openings appears to be completely occluded by the portion of the side wall adjacent to it when the body portion is viewed from a position on a plane which extends out of the body portion and which includes a line extending between the midpoints of the first and second openings, the position being outside a conical viewing zone having a l o cone angle of 70 ° .