GB2406055A

GB2406055A - Apparatus for inserting a femoral stem into the medulla of a femur

Info

Publication number: GB2406055A
Application number: GB0321632A
Authority: GB
Inventors: Kevin Hardinge
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-09-15
Filing date: 2003-09-15
Publication date: 2005-03-23
Anticipated expiration: 2023-09-15
Also published as: GB2406055B; GB0321632D0

Abstract

An apparatus for inserting a femoral stem 12 into the medulla of a femur 1, the apparatus includes: a tool for releasable attachment to a femoral stem; guide means 8 co-operable with the tool 10 such that the tool is movable in a predetermined direction relative to the guide means; and alignment means for positioning the guide means. The guide means may include a sleeve for receiving a guiding member on the tool. And the guide means may be reversibly attached e.g. by a stirrup 4 to the human thigh. The means for releasable attachment of the tool, may include a key for insertion into a cavity on the shoulder of the femoral stem.

Description

INSERTION APPARATUS

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The present invention relates to surgical apparatus for allowing accurate insertion of a prosthesis into a living body during an operation. For example, the invention can be used accurately to insert a femoral stem into the medulla of a femur during a total hip arthroplasty operation.

Total hip arthroplasty operations are well known. A replacement joint typically comprises two parts: the acetabular cup, located in the acetabulum, and the femoral stem, located in the femur. The stem holds a ball-shaped head, and the two parts join together in a ball and socket joint to imitate the natural arrangement they are replacing.

The femoral stem is located in the medulla of the femur. A cavity is cleared in the medulla, where the stem may he cemented in place or fixed using mechanical

interference. This description describes using a

cemented stem, but it will be appreciated that apparatus of the invention could equally be applied to a cementless situation.

Tools are known (e.g. a brace reamer) which clear a cavity in the centre of the medulla into which cement is placed prior to receiving the femoral stem. The tools have a shape which enables them to create a cavity that lo is coaxial with the femur. A problem arises in aligning the stem centrally in cement which is introduced into the cavity (either pushed in manually or using a cement gun).

At present, this alignment relies on the skill and judgement of the surgeon. It is very difficult, however, to visualize the orientation of the stem as it is inserted fully into the cavity containing the cement.

There is no way of seeing the tip of the stem, so it is possible that the axis of the stem can be skewed away from the axis of the cavity. Figs. la - Id show examples of the types of misalignment that can result. Fig. la shows the values arrangement, where the tip of the stem is tilted to the inside of the femur. Fig. lb shows the varus arrangement, where the tip of the stem is tilted to the outside of the femur. Fig. lc shows the anteverted l position, where the tip of the stem is tilted forwards in the medulla. Fig. Id shows the retroverted position, where the tip of the stem is tilted backwards. In each case, there will be an uneven distribution of cement which results in thin layers Broiled the tip and the top the stem as indicated by arrows A on Fig. Id. Tile uneven distribution of cement means that the thin layers are prone to fracture and fragmentation through weight bearing which would not occur if the stem is accurately lo aligned.

At its most general, the present invention proposes apparatus which allows a surgeon accurately to determine the alignment of a femoral stem as it is inserted into the medulla. The apparatus may allow the surgeon to align a guide parallel to the desired direction of an insertion, and may include a tool which co-operates with the guide to effect insertion in that direction.

According to a first aspect of the invention, there is provided apparatus for inserting a femoral stem into the medulla of a femur, the apparatus including: a tool for releasable attachment to a femoral stems guide means co-operable with the tool such that the tool is movable in a predetermined direction relative to the guide means; and alignment means for positioning the guide means.

Thus, in use, the guide means is alienable with respeGL to the patient's femur, and the tool is movable so as to result in a desired location of the femoral stem.

Preferably, the tool is movable only in the predetermined direction when lt co-operates with the guide means. Thus, once the guide means are positioned, the movement of the tool allowed by said means is clearly defined.

Preferably, the tool is slidably engageable with the guide means. Preferably, the guide means is only co operable with the tool when the tool is positioned in a certain orientation relative to the guide means. Thus, the means for attaching to a femoral stem may only be movable at a known orientation with respect to the guide means when the tool is engaged with it. The femoral stem is preferably fixed in a certain position relative to the tool, therefore the apparatus only allows the femoral stem to be inserted (i.e. moved) in a predetermined direction relative to the guide means. The predetermined direction may be linear, but of course the invention is equally applicable to non-lir-ear (e.g. curved) insertion paths.

The guide means may include a sleeve for receiving a guiding member on the tool. The predetermined direction may be substantially parallel to the axis of the sleeve.

To fix tile orientation of the tool to the guide means the cross-section of the guiding member may be non- circular, e.g. square, hexagonal or an irregular shape, so that the sleeve only receives It in a certain orientation.

Preferably, the alignment means includes guide lo positioning means for defining the axis of the sleeve.

Preferably, the axis of the sleeve runs parallel to the line where the anteroposterior and mediolateral planes intersect. The anteroposterior and mediolateral planes are two different planes that pass substantially through the axis of the femur; the line of intersection of the planes is substantially the axis of the femur and preferably defines the direction in which the femoral stem travels when it is inserted. Thus, the axis of the sleeve is preferably parallel to the axis of the femur.

Preferably, the guide positioning means includes axis locating means for defining a datum line which is the line of intersection between the anteroposterior and mediolateral planes (i.e. the femoral axis). Preferably, the means for defining a datum line is a guide post for inserting in the medullary canal.

The guide post is preferably long enough to extend into the medulla of the femur so that, as the femur tapers along its length, the guide post abuts the inside walls of bone. The end of the guide post may be shaped to engage the side walls of the interior of the femur so that the axis of the guide post lies along the neutral axis. The guide post may abut the femur at the isthmus.

lo The guide positioning means may further include means (e.g. a jig) for locating the guide means using the datum line as a reference. Preferably, the guide means is mounted on the femur or thigh. It may be adjustably mounted so that it can be aligned with respect to the datum line. The guide means may be reversibly attached e.g. by a stirrup to a human thigh.

Preferably, the stirrup is located on the flare of the femur above the knee, i.e. at the distal end of the femur. The stirrup may be a spring loaded inverted U for gripping the thigh.

Preferably, the guide means is attached to the stirrup via a rod for aligning parallel to the datum line e.g. using a jig. The rod is preferably mounted on the means for reversible attachment (e.g. stirrup) at the distal end of the femur. Preferably, it is adjustably mounted (e.g. on a ball joint) so that, in use, the angle between it and the femur can be varied. The rod may also be attached to a further means (e.g. stirrup) at the proximal end of the femur. Preferably, it is adjustably mounted at one or both stirrups so that, in use, the angle between it and the femur can be varied.

Preferably, it is movable only in a single plane.

Preferably, that plane is the anteroposterior plane as lo determined by the datum line. Thus, the plane in which the rod can be moved is defined by the guide post, such that the ends of the rod may then be heightadjusted until the rod lies parallel to the mediolateral plane.

The mediolateral plane is preferably the horizontal plane through the axis of the femur in the neutral position.

Thus, the rod may be adjusted until it is parallel to the axis of the femur; this may be confirmed visually.

The jig may be mounted on the guide post, and may cooperate with the rod to align it e.g. parallel to the so guide post.

Preferably, the sleeve is mounted on the rod.

Preferably, the sleeve is integral with the rod.

Preferably, the axis of the sleeve is parallel to the rod.

Preferably, the apparatus is used with the direct lateral approach method of hip replacement, but it may also be used in the posterior and anterolateral approaches.

Alternatively, the guide means may be mounted on a plate which is positioned on the femur itself e.g. using a jig from the guide post.

In a second aspect of the invention, there is provided a tool for inserting a femoral stem into the lo medulla of a femur, the tool including: means for releasable attachment to a femoral stem; and a guiding member for co-operation with insertion apparatus such that the tool is only movable in a predetermined direction relative to the femur. Preferably, the insertion apparatus is the guide means and alignment means mentioned above.

Preferably the guiding member is an elongate tube for co-operating with a sleeve on the insertion apparatus. The tool may also include a handle.

Preferably the means for releasable attachment include a key for insertion into a cavity located e.g. in the shoulder of femoral stem, where it can be releasably locked by turning it e.g. by a quarter turn. The key may be T-shaped. To prevent relative movement between the tool and the stem, the key may only be insertable when it is in a certain orientation, and may not be rotatable In the cavity once inserted. Alternatively, the key and cavity may have interlocking features (e.g. projections and corresponding recesses) or may even reversibly snap fit together.

Embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which: lo Figs. la - Id show misaligned femoral stems; Fig. 2a shows apparatus according to a first embodiment of the first aspect of the invention; Fig. 2b shows apparatus according to a second embodiment of the first aspect of the invention; Fig. 3 shows alignment means of the embodiment of Fig. 2a; Figs. 4a-4c show apparatus according to a third embodiment of the first aspect of the invention; Fig. 5 shows the stirrup and alignor of the embodiment of Fig. 2a; Fig. 6 shows a tool which is an embodiment of the second aspect of the invention; Fig. 7 shows the co-operation between the sleeve of the alignment means and the guiding member of the tool of Fig. 6; Fig. 8 shows a femoral stem having a cavity in the shou] der; and Figs. 9a-9c show a tool being reversibly locked to the shoulder of the femoral stem of Fig. 8.

Fig. 2a shows a first embodiment of the present invention as applied to a human leg resting in the lo neutral position as determined by a vertical tibia with no internal or external rotation. The neutral position is an accepted medical term describing the position of bones in a leg when the tibia hangs vertically from the knee joint (without twisting) whilst the femur is in a substantially horizontal position. Thus, the femur 1 is positioned laterally so that the tibia 2 hangs without twisting substantially vertically from the knee joint.

The proximal end of the femur is exposed for the insertion of a femoral stem 12.

The medullary canal of the femur is developed with a taper reamer to ensure that the neutral axis of the femur is obtained. The femur is reamed using taper reamers of increasing size until a firm cortico- cancellous fit is obtained.

A stirrup 4 is located over the thigh at the distal end of the femur. As shown in Fig. 5, the stirrup is e.g. an inverted IJ-shaped member haying e.g. elastic arms which may deform to grip the thigh. The position of the stirrup 4 is e.g. determined by the flare of the femur at its distal end. The stirrup 4 has attachment means 5 mounted on it. When the stirrup 4 is mounted on the thigh, the surgeon can use the position of the patella 3 to align roughly the attachment means 5 so that it lies lo in the anteroposterior plane of the femur, i.e. the vertical plane extending up through the axis of the femur. The attachment means 5 includes a ring 31 mounted on a ball joint 2] and height adjuster 20. The ring 31 includes an e.g. locking nut 22 for fixing a rod 6 into the ring 31. The surgeon may use the patella 3 to align roughly the ring 31 such that the rod 6 lies in the anteroposterior plane of the femur. The alignment need not be exact any plane including the axis of the femur will do. The ball joint 21 is adjustable so that the orientation of the rod 6 can be varied.

To align the rod accurately, the neutral axis of the femur (i.e. the line of intersection of the anteroposterior and mediolateral planes of the femur) can be found by inserting a guide post (e.g. a brace reamer) into the medulla of the femur until it abuts the inner walls e.g. at the isthmus. The guide post may have a size matching the final taper reamer used to develop the medulla. It is tapped firmly in place to establish the datum line. This IS the axis of the guide post, and it will correspond to the neutral axis; the rod 6 can be aligned with respect to It, e.g. using a jig.

Fig. 2b shows a second embodiment of the invention where the other end of the rod 6 is attached to the femur lo using another stirrup 19. Again, the rod 6 is attached to the stirrup 19 via adjustable connector 20. Once the datum line is established, the stirrups 4, 19 and attachment means 5 can be adjusted until the rod 6 is e.g. parallel with the axis of the femur. The above method effectively fixes the orientation of the rod 6 with respect to the datum line. Thus, the relationship between a sleeve 7 mounted on the rod and the axis of the femur is known and can be exploited e.g. using a further jig to insert a femoral stem accurately. Fig. 3 shows how the ring 5 and rod 6 can be moved.

Once the alignment has been observed to be neutral In the anteroposterior and lateral planes, the jig for aligning the rod 6 is disconnected from the intra- medullary guide post, and the guide post is removed.

An insertion tool 10 IS then mounted on a trial femoral stem. The guiding member 8 of the ln;ertion tool is inserted into the sleeve 7.

A trial range of motion of the stem is observed to ensure that the stem is stable in a normal range of movement.

The trial stem is removed to allow a plug to be inserted (if cement is to be used) and the cement is added to the medullary canal with a cement gun. The lo definitive femoral stem 12 is then inserted using the insertion tool 10.

The alignment of the definitive femoral stem 12 is assured by passing the guiding means 8 into the sleeve 7.

The stem 12 is thus inserted into the femur in a pre-determined axis that is neutral in the anteroposterior and mediolateral planes.

The sleeve 7 is integral with the rod 6. As shown in Fig. 7 the sleeve 7 is shaped to slidably receive the guiding member 8 of the insertion tool 10 (e.g. shown in Fig.6). The sleeve 7 is shaped such that it will only accept the guiding member 8 in a particular orientation.

The cross-section of the guiding member 8 and/or the sleeve 7 may be noncircular, e.g. square, hexagonal or simply irregular.

The orientation is such that the key portion 9 connected to the guiding member by a length adjustable handle 11 only moves In a fixed relati<'nshlp relative to the sleeve 7. Since the relationship between the sleeve 7 and the datum line is also fixed, it is possible to arrange for the key portion 9 to move parallel to (or even along) the axis of the femur. The handle is length adjustable to deal with the different sizes of patient, i.e. different distances between the sleeve 7 and femur lo axis. The key portion 9 is for releasably attaching to a femoral stem 12. The connection between the stem 12 and tool 10 is such that the femoral stem will be inserted into the medulla through hole 13 in a straight direction along the axis of the femur. Since the hole may be filled with cement, it would be of great benefit to the surgeon to know the orientation of the stem as it moves through the cement.

Figs. 4a-4c show a third embodiment of the invention, which is an alternative juxta-cortical method JO of aligning the femoral stem. As before, the datum line is defined using the intra-medullary post 23. Here, a jig is used to position an e.g. curved plate 25 (convex anteriorly, concave posteriorly) on the anterior surface of the femur, just below the anterior intertrochanterlc line 24. The dimensions of the plate may be 4 x dams.

The plate has e.g. two apertures 26 on Its proximal end face (i.e. the face closer to the proximal end of the femur) e.g. for the insertion cuff the mounting guide, 29 of a jig.

The plate ?5 is aligned using a jlg from the intramedullary post 23 so that the plate lies on the neutral axis of the femur. The plate is secured in position with a clamp 27 which may be fixed closed using lo locking tool 30. The sleeve 7 is then put in position using a jig. The jig includes guides 29 which are received by the two apertures 26 in the proximal end face of the plate 25. The sleeve is mounted on e.g. two support members 28 extending from the guides. The relationship between the plate 25 and the intramedullary post 23 is known, therefore the position of the sleeve 7 with respect to the post 23 (i.e. femur axis) is known, therefore accurate stem insertion is possible.

Fig. 8 shows a possible stem 12 for use with the apparatus. The stem 12 has a cavity 14 in its shoulder for co-operating with a T-shaped key. Figs. 9a-9c show how a T-shaped key would be locked in place. Cavity 14 has a charmer 16 located beneath protrusions 18 so that when the crossbar 15 of the T-shaped key is passed l6 through the entrance 17 to the chamber 16 and is rotated by a quarter turn, the key is prevented from being pulled out of tile cavity 14 by the protrusions 18. The crcss- bar 15 may have protrusions that engage accesses in the chamber 16, so that tile key is maintained in the locked position.

The apparatus of the invention enables a surgeon to determine a direction parallel to the axis of the femur and by the nature of the co-operation between the tool lo and the alignment means and the tool and the femoral stem, the stem can be accurately inserted in a consistent direction, thereby avoiding the problems associated with misalignment.

A person skilled in the art will appreciate that the ideas expressed in the embodiment can be achieved in ways other than those specifically described; the scope of the present invention is not intended to be limited by the

examples.

Claims

1. Apparatus for inserting a femoral stem into the medulla of a femur, the apparatus including: a tool for releasable atlacT-ment to a femoral stem; guide means co-operable with the tool such that the tool is movable in a predetermined direction relative to the guide means; and alignment means for positioning the guide means.

2. Apparatus according to Claim 1, wherein the tool is slidably engageable with the guide means.

3. Apparatus according to either one of Claims 1 or 2, wherein the guide means is only co-operable with the tool when the tool is positioned in a certain orientation relative to the guide means.

4. Apparatus according to any one of the preceding claims, wherein the guide means includes a sleeve for receiving a guiding member on the tool.

5. Apparatus according to Claim 4, wherein the predetermined di.reGtion is substantial].y parallel to the sleeve.

6. Apparatus accordir-g to cipher one of c]aims 4 or 5, where.n the alignment means includes means fo determining a datum line to represent the axis of the femur.

lo

7. Apparatus according to Claim 6, wherein the guide means is attached to a rod, the rod being alignab]e with respect to the datum line.

8. Apparatus according to Claim 7, wherein the rod is attachable to a human thigh by a stirrup.

9. Apparatus according to claim 8, wherein the rod is adjustably mounted on the stirrup.

10. Apparatus according to any one of claims 1 to 6, wherein the guide means is attached to a plate, the plate being locatable in a predetermined position with respect to the datum line.

11. Apparatus according to any one of the preceding claims, wherein the tool includes a key portion for reversible insertion into a cavity In a femoral stem.

12. Apparatus as -herein dcsGribed with reference to, and as illustrated in, Figs. 2a, 2b or 4a to 4c.

13. A tool t-or inserting a femoral stem into the medulla of a femur, the tool including: means for releasable attachment to a femoral stem; and a guiding member for co-operation with insertion apparatus such that the tool is only movable in a predetermined direction relative to the femur.

14. A tool according to Claim 13, wherein the means for releasable attachment to a femoral stem includes a key portion for reversible insertion into a cavity in a femoral stem.

15. A tool as herein described with reference to, and as illustrated in, Fig. 6.