WO2004069061A1

WO2004069061A1 - Device for indicating a location within the body

Info

Publication number: WO2004069061A1
Application number: PCT/GB2004/000421
Authority: WO
Inventors: Robert Michael Wozencroft; Raymond Randle
Original assignee: Robert Michael Wozencroft; Raymond Randle
Priority date: 2003-02-04
Filing date: 2004-02-04
Publication date: 2004-08-19
Also published as: GB0302532D0; GB0516088D0; GB2414411A; GB2398010A; GB2414411B

Abstract

A calibration device for use in locating a locus, such as the axis of a bone (10), within a living body comprises a sheet member (1; 62) which is adapted for attachment to the living body adjacent to the locus. The sheet member (1; 62) has at least one cutout portion (5; 69), which permits indexing of the sheet member (1, 62) relative to the living body. The sheet member (1, 62) is provided with a plurality of calibration markers (2; 68), each having a transmittivity for X-rays that contrasts with the transmittivity of the elongate sheet member (1, 62).

Description

DEVICE FOR INDICATED A LOCATION WITHIN THE BODY

FIELD OF THE INVENTION

This invention relates to a calibration device, more particularly to a calibration device which can be used to locate a locus, such as the axis of a bone, within a living human or animal body. Also described herein is a method for the use of such a calibration device. BACKGROUND TO THE INVENTION

The human knee is a complex joint. It is formed between the femur in the upper leg and the tibia which is the major long bone of the lower leg. The lower end of the femur is provided with a pair of condyles and the tibia has at its upper end a layer of cartilage which provide a lateral meniscus and a medial meniscus to receive the lateral condyle and the medial condyle respectively. Four separate ligaments stabilise the knee joint. On the sides of the joint lie the medial collateral ligament and the lateral collateral ligament which serve as stabilisers for side-to-side stability of the joint. The medial collateral ligament is a broader ligament that is actual made up of two ligament structures, the deep and superficial components, whereas the lateral collateral ligament is a distinct cordlike structure. In the front part of the centre of the joint is an anterior cruciate ligament. This ligament is a very important stabiliser of the femur or the tibia and serves to prevent the tibia from rotating and sliding forward during agility, jumping and deceleration activities. Directly behind the anterior cruciate ligament is its opposite, the posterior cruciate ligament. At the front of the knee the patella bears against the lower end of the femur and is connected to the patella tendon which is connected at its opposite end to the tibia.

The knee is subject to a great deal of stress during normal activity and is readily damaged, for example, as a result of sporting activities. It is also prone to generative disease.

In many patients with knee problems, a potential solution is to implant a knee prosthesis. This may be a complete prosthesis involving resection of the upper end of the tibia and implantation of an implant having menisci formed of a plastics material, such as high molecular weight polyethylene . In addition the lower end of the emur may be resected and a metal femoral prosthesis implanted to replace the natural condyles. It is, however, a disadvantage of such a surgical operation 'that both the anterior and posterior cruciate ligaments have to be sacrificed.

In an alternative surgical technique, the anterior and posterior cruciate ligaments are retained, in which case one or both menisci can be replaced with a corresponding individual tibial component, again with a plastics material meniscus. In this case the corresponding condyle may also be replaced by a metal prosthesis. Such a prosthesis is often termed a unico partmental knee prosthesis.

Current surgical techniques are invasive and require considerable dissection in order to expose the surfaces of the bones to be replaced.

Many knee problems can be investigated and treated without the need for extensive orthopaedic surgery. One such technique is arthroscopy which is frequently used in the treatment of sports injuries in which, for example, damage has occurred to the knee cartilage. According to this procedure a small incision is made sufficient to allow insertion of an endoscope through which a surgeon can insert tools while observing the progress of the operation using an appropriate modified telescope.

In any orthopaedic operation on the knee it is desirable for the surgeon to determine the weight bearing axis of the patient's leg bones and particularly the weight bearing axis of the patient's tibia prior to the beginning of the operation. Thus, for example, United States Patent No. 5,611,353 (Dance et al . ) discloses a method and apparatus for locating functional structures of the lower leg during knee implant surgery by determining the location of the weight-bearing axis. This involves use of a registration clamp which is attached to the patient's femur during the course of the implantation operation. This procedure requires the making of a sufficiently large surgical incision to allow the clamp to be attached by the surgeon to the patient's femur.

It would be desirable to develop a calibration device enabling a surgeon to ascertain, prior to commencement of a surgical procedure, the position of the axis of a bone within the patient's body, for example the weight bearing axis of the patient's tibia. It would be further desirable to provide such a device which can facilitate a novel surgical procedure for effecting implantation of a unicompartmental knee prosthesis in a patient's knee which utilises incisions of no more than about 1 inch (2.54 cm) in length to provide access for the surgeon to the bone surfaces to be resected and to permit the surgeon to effect resection and to implant the prosthesis . It would further be desirable to develop a calibration device for use in determining the position of the weight bearing axis of a patient's tibia prior to commencement of a knee prosthesis implantation operation, more particularly a unicompartmental knee prosthesis.

BRIEF SUMMARY OF THE INVENTION

The present invention accordingly seeks to provide a calibration device enabling a surgeon to ascertain, prior to commencement of a surgical procedure, the position of the axis of a bone within the patient's body, for example the weight bearing axis of the patient's tibia. It further seeks to provide such a procedure which can facilitate a novel surgical procedure for effecting implantation of a unicompartmental knee prosthesis in a patient's knee which utilises incisions of no more than about 1 inch (2.54 cm) in length to obtain access to the bone surfaces to be resected, to effect resection, and to implant the prosthesis. It would further be desirable to develop a calibration device for use in determining the position of the weight bearing axis of a patient's tibia prior to commencement of a knee prosthesis implantation operation.

According to the present invention there is provided a calibration device for use in locating a locus, for example the axis of a bone, within a living body comprising a sheet member adapted for attachment to the living body adjacent the locus, the sheet member having at least one cutout portion permitting indexing of the sheet member relative to the living body when the sheet member is attached to the living body and a plurality of calibration markers each having a transmittivity for X-rays that contrasts with the transmittivity for X-rays of the sheet member.

In one preferred embodiment the sheet member is formed of a radiolucent plastics material and the calibration markers are formed from a radiopaque material . Preferably the calibration markers include a primary marker which differs in size and/or shape from other calibration markers. Preferably also the markers are arranged in a row. Moreover the markers are desirably substantially equally spaced one from another in the row.

The markers may be arranged in a plurality of substantially parallel rows. In such a calibration device the at least one cutout portion may comprise at least one aperture. Thus the at least one aperture may comprise a plurality of apertures which are arranged in a row substantially parallel to the row of markers. Preferably the spacing between the apertures corresponds substantially to the spacing between the markers .

It is alternatively possible for the calibration markers to comprise areas of radiopaque material each surrounding a corresponding aperture .

In the calibration device of the invention the calibration means may comprise a plurality of markers of circular cross section, for example balls or discs. However, the markers may have any other desired shape, for example squares, rectangles, ellipses, triangles, stars or hexagons .

In a particularly preferred calibration device according to the invention the calibration markers include a primary marker which differs in size and/or shape from other calibration markers and the at least one cutout portion comprises a slit whose axis is aligned with the primary marker .

In an alternative preferred embodiment a calibration device has a sheet member which is formed from a radiopaque materials while the calibration markers comprise secondary cutout portions formed in the sheet member, for example apertures. Such apertures may be circular or of any other shape, for example one of the shapes mentioned above. In such a calibration device the apertures are preferably arranged in a row. The at least one cutout portion can comprise a slit which is larger than the apertures.

Alternatively the calibration markers may include notches in an edge of the sheet member. Preferably the calibration device further includes securing means for securing the sheet member to the living body, such as a skin-compatible adhesive or a strap. Such a strap may be provided with a buckle or it may be arranged to be secured by means of a f stener of the hook and loop type . The calibration device may comprise a sheet member of any convenient shape, for example, it may have substantially the shape of a rectangular strip, a circle, an ellipse, or a square . Also described herein is a method of locating the axis of a bone within a human or animal body comprising:

(a) attaching at least two calibration devices according to the invention to skin of the human or animal body adjacent the bone whose axis is to be located at positions spaced apart along the length of the bone;

(b) marking the skin at positions which correspond to the calibration markers on the calibration devices to provide a plurality of alignment marks on the skin corresponding to the calibration markers; (c) obtaining at least one X-ray image of at least a portion of the human or animal body including the bone and the at least two calibration devices;

(d) determining from the resulting at least one X-ray image the position of the axis of the bone in relation to the respective calibration markers of the at least two calibration devices; and

(e) positioning an alignment device on the human or animal body adjacent the bone in substantially the same relationship to the alignment marks as the axis of the bone exhibits in a corresponding X-ray image to the images of the calibration markers.

Such an alignment device may comprise: an elongate first member for temporary attachment to a lower leg of a patient below a knee requiring orthopaedic surgery, the elongate member having a lower end and an upper end; an elongate second member having a first end with an axial bore therein and a second end, the elongate second member being pivotally attached adjacent its first end to the first elongate member adjacent to the upper end thereof for pivotal movement substantially in an anterior-posterior plane; locking means for locking the elongate second member at a selected angle relative to the first elongate member.

Conveniently the locking means comprises an arcuate arm mounted on the first elongate member and a locking member for locking the elongate second member to the arcuate arm. The arcuate arm may be provided with a transverse slot while the locking member comprises a locking knob having a threaded shank that extends though the transverse slot and is threadedly received in a corresponding threaded transverse bore formed in the elongate second member adjacent the second end thereof. In addition, the arcuate arm may be provided with a longitudinal slot in which the elongate second member moves. Preferably the arcuate arm is provided with a graduated scale.

It is preferred that the first elongate member is extensible.

In such an alignment device the first elongate member preferably has a notch at each end to permit the axis of the first elongate member to be aligned with respect to markings on the patient's skin so as to align the axial bore of the second elongate member with the weight bearing axis of the patient's tibia.

In a preferred alignment device at least one aperture is provided adjacent the upper end of the first elongate member for passage of a pin or screw for temporary attachment of the device to a patient's tibia. The alignment device may further include means mounted adjacent the lower end of the first elongate member for embracing the patient's leg and securing the lower end of the device to the outside of the patient's leg.

The alignment device can be use as part of a surgical implement for use in resection of an upper surface of a patient's tibia during a surgical operation to implant a unicompartmental knee prosthesis in the patient's knee in combination with a support table mounted at an upper end of a shaft and orthogonal to its axis, the shaft being received in the axial bore of the second elongate member; and a cutting block mounted on the support table provided with at least one slot extending substantially in a plane parallel to the axis of the shaft for passage of a surgical saw for effecting a vertical cut in a patient's tibia during resection thereof and a transverse slot extending substantially orthogonal to the axis of the shaft for making a transverse cut in a patient's tibia at an angle determined by the selected angle.

In such a surgical implement the cutting block can be mounted on the table so as to be pivotable about an axis orthogonal to the upper surface of the table . The cutting block may be removable from the table. In addition the table may be provided with a bore to receive a pin or screw for temporarily securing the table to a patient's tibia during the surgical implantation operation. The surgical implement preferably further includes a stylus mounted on the cutting block having a tip positioned at a selected height above the transverse slot of the cutting block for determining the position of the top of the tibia of the patient prior to effecting the transverse cut during resection of the tibia. Additionally the cutting block may be provided with at least one bore for receipt of a pin or screw to permit temporary attachment of the cutting block to the tibia of the patient . BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood and readily carried into effect, some preferred embodiments thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings wherein: Fig. 1 is a front view of a first calibration device which is useful in the practice of the invention;

Fig. 2 is a perspective view of the device to Fig. 1 illustrating its method of preparation for application to a patient's lower leg; Fig. 3 is a perspective view of two of the calibration devices of Figs. 1 and 2 in place on the lower left leg of a patient;

Fig. 4 shows an anterior posterior (AP) X-ray picture of a patient's foot and lower left leg after two of the calibration devices of Figs. 1 and 2 have been applied to the lower left leg in the manner illustrated in Fig. 3;

Figs . 5 and 6 are enlarged views of upper and lower portions respectively of the X-ray picture of Fig. 4;

Fig. 7 is a further medio lateral (ML) X-ray picture of the patient's lower leg corresponding to the anterior posterior (AP) X-ray picture of Fig. 4;

Figs . 8 and 9 are enlarged views of an upper portion and a lower portion respectively of the X-ray picture of Fig. 7 ; Fig. 10 is a perspective view of an alignment device for use in the course of orthopaedic surgery on a patient's knee;

Fig. 11 shows the alignment device of Fig. 10 temporarily attached to the front of the patient's leg;

Figs . 12 and 13 are enlarged views of an upper portion and a lower portion respectively of Fig. 11;

Fig. 14 is a side view of the patient's left leg showing the use of the alignment device of Figs. 10 to 13 in somewhat extended condition to determine the position of the top surface of the patient's tibia in preparation for resection thereof;

Fig. 15 is a front view of the patient's knee after the preliminary cuts have been made in the tibia as part of resection;

Fig. 16 is a front view of part of a second calibration device useful in the practice of the invention;

Fig. 17 is a perspective view of the calibration device of Fig. 16;

Fig. 18 is a front view of a further form of calibration device;

Fig. 19 is an enlarged view of part of the calibration device of Fig. 18; and Figs. 20 and 21 are front views of further forms of calibration device. DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings, and to Fig. 1 in particular, a calibration device for use with the surgical tool of the invention comprises a strip 1 of a plastics material which is provided with a line of calibration markers 2 in the form of a small stainless steel or other radiopaque balls or discs (e.g. tantalum or lead balls or discs) which are typically about 1 mm in diameter. In the centre of the line of calibration markers 2 there is a primary calibration marker 3 which is a radiopaque ball or disc of somewhat larger diameter than calibration markers 2. Thus, for example, primary calibration marker 3 can be about 2 mm in diameter. The centres of each of the calibration markers 2 and 3 lie on a single line parallel to the edges of strip l with an equidistant spacing between the centres of adjacent markers. Typically this spacing is about 2.5 mm. Strip 1 is also provided with a number of holes 4 whose centres lie on a line substantially parallel to the line of the calibration markers 2 and 3 with each hole 4 being positioned vertically above a corresponding calibration marker 2. Above primary calibration marker 3 strip 1 is formed with an elongate slot 5 whose axis lies substantially at right angles to the line of the holes 4. The spacing between the centres of adjacent holes 4 thus matches exactly that between adjacent markers 2, while the spacing between marker 3 and its adjacent markers 2 also matches the spacing between slot 5 and the adjacent holes 4.

The reverse side of strip 1 is provided with a layer of a skin-compatible adhesive which is covered with a peelable protective strip 6, as shown in Fig. 2.

The plastics material of strip 1 can be, for example, one of the plastics materials use for the manufacture of conventional aseptic or antiseptic plasters or wound dressings. Similarly the adhesive and the peelable protective strip 6 can be of the kind conventionally used in the manufacture of aseptic or antiseptic plasters or wound dressings.

The markers 2 and 3 can be held in place on strip 1 in any convenient manner, for example by being glued thereto. Alternatively strip 1 can be formed by laminating two layers of plastics material one to another, using any suitable known technique, so as to sandwich the markers 2 and 3 therebetween.

Typically strip 1 is approximately 100 mm long and about 20 mm wide. In use of the device of Figs. 1 and 2, the peelable protective strip 6 is first removed in a manner similar to the manner of preparing a conventional aseptic or antiseptic plaster or wound dressing for application to the skin. Two such strips 1 are attached to a lower leg of a patient requiring orthopaedic knee surgery, as illustrated in Fig. 3, in preparation for taking pre-operative X-ray pictures. As can be seen from Fig. 3, each strip 1 is applied to the patient's leg so as to lie transverse to the weight bearing axis of the tibia and with the primary calibration marker 3 lying approximately in an anterior-posterior plane including the weight bearing axis of the patient's tibia, as judged by the eye of the radiographer or other person supervising the taking of the pre-operative X-ray pictures. One of the two strips 1 is positioned on the patient's leg 7 a short distance below the patient's knee and the other one a short way above the patient's ankle.

After each strip 1 has been attached to the patient's skin, its position is marked on the patient's skin with the aid of a pen 8 or similar device which is used to apply an indelible ink through the holes 4 and slot 5. The resulting marks (see reference numerals 4' and 5' in Figs. 11 to 13) will indicate on the patient's skin a position in the longitudinal direction of the strips 1 corresponding to, but higher on the patient's leg than, a corresponding marker 2 or 3. Since slot 5 is elongated, the resulting ink mark 5 ' on the patient's skin will stand out from the marks 4' corresponding to the holes 4 and will indicate a position on the patient's leg corresponding to, but higher than, the primary marker 3.

The necessary pre-operative X-ray pictures are then taken. Typical X-ray pictures are depicted diagrammatically in Figs. 4 to 9. These show the results of X-ray scans taken on the lower left leg of a patient whose skin has had two strips 1 attached in a similar manner to that illustrated in Fig. 3. In these X-ray pictures the images are typically somewhat magnified compared with the bones and other features corresponding to those images by a scaling factor which is typically in the range of from about 1.01:1 to about 1.20:1. Hence the spacing between the images 2' of adjacent markers 2 will be, for example, from about 1.01 to about 1.20 times larger than the actual spacing between such adjacent markers. Typically the scaling factor is about 1.1:1.

The X-ray picture of Fig. 4 shows the bones of the patient's foot 9, as well as the patient's tibia 10, fibula 11, patella 12, and femur 13. Reference numeral 14 indicates the weight bearing axis of the patient's tibia. Fig. 4 also shows the images 2' and 3' of the calibration markers 2 and 3.

By studying the X-ray picture of Fig. 4, the surgeon can assess where the weight bearing axis 14 of the patient's tibia 10 lies in relation to the images 2' and 3' corresponding to the calibration markers 2 and 3. Although the dimensions of the images of the patient's bones in the X-ray picture of Fig. 4 may not correspond to the actual dimensions of the patient's bones, due to the scaling factor mentioned above, the size of, and the spacing between, the calibration markers 2 and 3 is known with accuracy. Hence the surgeon can estimate how far the weight bearing axis 14 lies from the primary calibration marker 3 of the upper strip 1 and to which side and similarly how far away from the primary calibration marker 3 of the lower strip 1 the weight bearing axis 14 of the patient's tibia actually lies and to which side.

Fig. 7 is a corresponding medio lateral X-ray picture of the same patient's lower left leg. From the images 2' and 3' of the calibration markers 2 and 3, the surgeon can estimate where a line 15 corresponding to the line 14 shown on Fig. 4 lies. (Line 14 actually represents an anterior posterior plane including the weight bearing axis of the patient's tibia, whereas line 15 indicates a medio lateral plane including that axis) . Reference numeral 16 indicates a line tangential to the right hand side of markers 2, 3 which will correspond approximately to the surface of the patient's skin. With the aid of the X-ray picture of Fig. 7 the surgeon can estimate the angle θ between a plane containing the line 16 and the plane upon which the transverse saw cut required in order to remove the medial side of the top of the patient's tibia 10 should be made as part of a surgical operation to implant a so-called unicompartmental knee prosthesis. That angle θ is not affected by the scaling factor mentioned above and is typically in the range of from about 80° to about 85°. Reference numeral 17 indicates the plane of that saw cut. Prior to the operation to implant a unicompartmental knee prosthesis in the medial side of the patient's knee, the strips 1 are removed and the patient's lower leg is sterilised in conventional manner using an antiseptic solution, such as an iodine solution, which will not remove the ink marks 4', 5' (see Figs. 11 to 13) made by the marker pen 8 on the patient's skin.

Fig. 10 shows an alignment device 20 for use in orthopaedic surgery on a patient's leg. This is illustrated in closed condition in Figs. 10 and 11 but in somewhat extended condition in Fig. 14. Alignment device 20 includes an elongate first member 21 which carries at its upper end a block 22 in which is mounted a pivot 23 for a second elongate member 24 which has a hollow upper end for receipt of a shaft 25 of a table 26 for a cutting block 27 for use in resecting a top end of a patient's tibia 10 during a surgical operation to implant a unicompartmental knee prosthesis. Table 26 is provided with a hole 28 for a purpose which will be described below.

The underside of cutting block 27 is provided with a cleat shaped key (not shown) which includes a transverse bar that is elliptical in plan and is received in a corresponding recess that is provided with an opening in the form of a matching elliptical slot (also not shown) to receive the transverse bar and that has undercut sides so as to provide a space within which the transverse bar can turn. In this way cutting block 27 can be held in place on table 26 and yet can also be swivelled on table 26, while being readily removable when the surgeon so desires by rotation through an angle of about 90° about a vertical axis parallel to that of shaft 25. The forward face of cutting block 27 (which faces to the rear as depicted in Fig. 10) is curved. At its lower end, first elongate member 21 carries a second block 29, the position of which in relation to block 22 can be adjusted as described below in relation to Fig. 14. Above block 29 is a further block 30 having a pair of wings 31 (only one of which is visible in Fig. 10) to one of which is attached a flexible coil spring 32 having at its free end a ring 33 for a purpose to be described hereinbelow.

Immediately above block 30 there is mounted a further arcuate block 34 which is formed with a slot 35 in a vertical plane containing the axis of elongate member 21 and also with a transverse slot 36. Vertical slot 35 receives a flattened end portion 37 of second elongate member 24 while transverse slot 36 receives a threaded shank 38 of a securing member 39, shank 38 being received in a threaded bore adjacent the lower end of flattened end portion 37. This threaded bore extends right through flattened end portion 37 so that securing member 34 can be fitted on either side of the alignment device 20 to suit the surgeon's convenience. Arcuate block 34 also bears a graduated scale 40 on each side marked in degrees (°) for a purpose to be described hereinafter.

Block 22 is formed with a notch 41 and with a pair of ears 42. Similarly block 29 is provided with a notch 43 and with a pair of ears 44.

As can best be seen in Fig. 14, alignment device 20 is extensible, an inner member 45 being slidably received within elongate second member 24 to permit use of the device on patients having different lengths of tibia. In order to increase the accuracy of assessment of the position of the weight bearing axis of the patient's tibia it is desirable to position devices 1 as near as possible to the ends of the patient's tibia. Hence it is convenient to be able to adjust the length of the alignment device. The inner member 45 and elongate second member 24 can be fixed one to another in any convenient manner, for example by means of a locking screw (not shown) .

Cutting block 27 carries a removable block 46 in which is mounted a curved stylus 47 for a purpose to be described hereinbelow. Block 46 is held on cutting block 26 by means of an adjustment knob 48 which comprises the head of a screw whose threaded shank is received in a threaded bore in the cutting block 27.

Cutting block 27 is further provided with a pair of vertical saw guide slots 49, 50 and with a horizontal saw guide slot 51 for making a transverse cut during resection of the patient's tibia. A pair of holes 52 are also provided in cutting block 27 through which pins (not shown) can be passed to secure cutting block 27 to the top of the patient's tibia 10. Block 22 is also formed with a bore 53. Block 22 is provided with a pair of holes 54 (only one of which is visible in Fig. 10) for passage of corresponding screws or pins (not shown) by means of which the surgeon can secure block 22 to the patient's tibia 10 during the planned surgery on the patient's knee.

As described above in relation to Figs. 1 to 9, a pair of calibration devices 1 are attached to the patient's lower leg prior to pre-operative X-ray scanning. In addition the position of the calibration devices 1 are marked on the patient's leg (as shown at 4' and 5' in Fig. 11) in indelible ink using the marker pen 8 to record on the patient's skin the position of the holes 4 and slot 5. Before the surgeon makes the first incisions, the calibration devices 1 are removed and the patient's skin is sterilised using an antiseptic solution in which the ink of the marker pen 8 is essentially insoluble.

The surgeon can utilise the X-ray pictures of Figs. 4 to 9 to assess where the weight bearing axis of the patient's tibia 10 lies. Thus, for example, in Fig. 4 the plane of the weight bearing axis 14 passes to the left of image 3' of primary marker 3, between the images 2' of the first and second calibration markers 2 of the upper calibration device 1. The plane of axis 14 also passes to the right of the image 3' of the primary calibration marker 3, between the images 2' of the first and second calibration markers 2 to the right of primary calibration marker 3 of that lower calibration device 1. Although both of the calibration devices 1 have at this stage now been removed, there remain the markings 4', 5' in indelible ink on the patient's skin showing the positions of the holes 4 and of slot 5 of each calibration device 1 which were vertically aligned with its calibration markers 2 and with its primary marker 3 respectively. By adjusting the length of the alignment device 20 as necessary and aligning the notches 41 and 43 in positions relative to the indelible ink markings 4', 5' on the patient's skin corresponding to the positions of the images 2' and 3' on the X-ray pictures of Fig. 4, the surgeon can ensure that the axis of first elongate member 21 is substantially aligned with the weight bearing axis of the patient's tibia, as illustrated in Fig^'s. 11 to 13.

Fig. 11 illustrates diagrammatically the surgical opening 55 made by the surgeon in the patient's knee 56. Typically, opening 55 is formed by making a substantially vertical cut approximately 1 inch (2.5 cm) in length or less (e.g. about 2.2 cm in length) through the tissue overlying the top of the patient's tibia 10 and the bottom of the patient's femur 13. As illustrated, the operation to be performed on the patient's knee is insertion of a unicompartmental or unicondylar knee prosthesis comprising a tibial component (not shown in Fig. 11) and a femoral component (also not shown in Fig. 11) on the medial side of the patient's knee 56. The surgeon can observe the progress of the operation by means of an arthroscope (not shown) inserted through a portal incision into the lateral side of the patient's knee.

Fig. 11 also shows how spring 32 is used to secure the lower end of alignment device 20 to the patient's lower leg. As can be seen from Fig. 11, spring 32 passes around the patient's leg and ring 33 is secured over a pin 57 provided for that purpose on the other wing 31 (which is not visible in Fig. 10) . In addition the surgeon can temporarily secure block 22 directly to the patient's tibia 10 by means of screws or pins passed through holes 54. Having secured first elongate member 21 in position on the patient's leg, the surgeon can then pivot the elongate second member 24 about the pivot 23 until an inscribed line 58 (see Fig. 10) on flattened end 37 of second elongate member 24 is aligned with the appropriate indicium on the scale 40 corresponding to the angle θ which the surgeon has selected and which is indicated in Fig. 7. Having made this adjustment the surgeon then secures second elongate member 24 at the required angle by tightening securing member 39. This will then set the horizontal saw guide slot 51 at the correct angle to make the lateral cut along plane 17 of Fig. 7.

Fig. 14 illustrates the alignment device 20 positioned on the patient's left leg (but with spring 32 omitted for the sake of simplicity) . Indicium 58 is set at angle θ of approximately 82°. The tip of stylus 47 is inserted through surgical incision 55 and is observed by the surgeon through an arthroscope (not shown) inserted into the lateral side of the patient's knee. As shown, the tip of stylus 47 is resting on the top of the patient's tibia 10. Once the surgeon is satisfied that stylus 47 is in the correct position, table 26 is temporarily fixed to the patient's tibia with the aid of a pin or screw passed through hole 28. The position of the saw guide slot 51 below the tip of stylus 47 is predetermined and is equivalent to the thickness of the tibial implant (not shown) to be implanted. Thus the position of the resection level is now fixed. Typically this distance is 7.5 mm. Then the surgeon can turn cutting block 27 about its vertical axis until he has aligned the appropriate vertical saw guide slot 49 or 50 to his satisfaction so that, when he eventually makes a vertical saw cut it will not damage either of the cruciate ligaments. In the case of a medial prosthesis for the left knee 56, the surgeon will use vertical saw guide slot 50. Cutting block 27 can then be temporarily secured to the patient's tibia 10 by means of a screw or pin passed through one or both of holes 52. The surgeon can then utilise a conventional reciprocating surgical saw (not shown) to make the vertical cut 59, using the appropriate one of the guide slots 49, 50, and to make the lateral cut 60, as shown in Fig. 15. After removing cutting guide 27 the surgeon can then complete the cuts needed to free the resulting resected piece 10' of the patient's tibia 10 and remove this as well as any knee fragments through incision 55. Alignment device 20 can then be removed after first having removed any pins or screws which have been used to hold it temporarily in position on the patient's tibia 10.

For further details of suitable surgical tools for completion of the unicompartmental knee prosthesis implantation and their manner of use, the attention of the reader is directed to our copending British patent application No. (Case 503616 - F037) filed simultaneously herewith, the entire disclosure of which is herein incorporated by reference.

The calibration device 1 of Figs. 1 to 9 is provided with a single row of markers 2, 3 and with a single row of holes 4 and slot 5. Alternatively the markers 2, 3 could be arranged in two parallel rows with the markers 2 in one row offset transversely with respect to the markers 2 in the other row. Similarly two parallel rows of holes 5 could be provided with the holes in one row being offset transversely with respect to the holes 4 in the other row.

An alternative form of calibration device 61 is shown in Figs. 16 and 17. This comprises a flexible strip 62 of radiopaque material, such as stainless steel, tantalum or lead, the ends of which are passed through a pair of links 63 and then bent back upon the strip 62. A pair of strap members 64, 65 are secured to respective links 63. At its free end, strap member 64 carries one part 66 of a hook and loop fastener (for example, a hook and loop fastener sold under the registered trade mark VELCRO) , while the other strap member 65 carries the other part 67 of the hook and loop fastener. In this way the calibration device 61 can be secured around the lower leg of a patient requiring knee surgery.

Flexible strip 62 is provided with a series of holes 68 arranged in a line and with a central slot 69. Holes 68 constitute calibration markers for the device 61, while slot 69 performs the function of a primary calibration marker. The use of calibration device 61 is similar to that of calibration device 1, of Figs. 1 to 9. Thus after application to the patient's leg in preparation for pre- operative X-ray scanning, a marker pen is used to mark in indelible ink the positions of the holes 68 and of the slot 69 upon the patient's skin. As in Fig. 3, two calibration devices 61 are positioned on the patient's lower leg before X-ray pictures are taken.

The resulting X-ray pictures will in this case show an opaque transverse band corresponding to the position in each of the strips 62 of the respective calibration devices 61, while the positions of the holes 68 and of the slot 69 will show up as clearer patches on the X-ray pictures. As with the calibration device 1 of Figs. 1 to 9, the surgeon can assess the position of the weight bearing axis of the tibia on the X-ray pictures relative to the positions of the holes 68 and slot 69 and can then use the alignment device 20 in conjunction with the ink marks on the patient's skin to set up the alignment device 20 in alignment with the axis of the patient's tibia 10.

Instead of utilising a fastener of the hook and loop type, the strap members 64, 65 could be fastened using a conventional buckle. Yet another possibility is to replace the strap members 64, 65 with an elastomeric strip which is fixed at one end to one of the links 63 and is provided at its other end with a hook-shaped catch for engagement with the other link 63. Figs. 18 and 19 show a further form of calibration device 70 which consists of a strip of radiolucent material, for example a plastics material of the type described above in relation to calibration device 1. As with calibration device 1, calibration device 70 can be provided on its rear side with a skin-compatible adhesive which, prior to use, is covered with a peelable protective strip (not shown) .

Device 70 is formed with a series of holes 71, each surrounded by calibration marker in the form of a ring 72 of a radiopaque material, as can be best seen from Fig. 19, which is an enlarged view of one of the calibration markers 72. It also has a primary marker 73 also made of a radiopaque material which is provided with a cross-shaped slot 74.

The calibration device 70 can be used in a similar manner to that described above in relation to calibration device 1, the holes 71 and the cross-shaped slot 74 permitting marking of the patient's skin after application of the calibration device 70 thereto. In this case the images of the calibration markers 72 will take the form of rings, while the image of the primary calibration marker 73 will show up as a larger dark patch with a lighter patch corresponding to the cross-shaped slot 73.

Fig. 20 shows a still further form of calibration device 75 which is circular in plan but is otherwise similar in construction to the calibration device 70 of Figs. 18 and 19. This is provided with calibration markers 76 and primary calibration markers 77 which are similar to markers 72 and 73 respectively of device 70. Calibration device 75 may also find application in other forms of pre-operative X- ray imaging, for example to locate the position of the hip centre, the top of the tibia, a fracture, a tumour, kidney stones or gallstones, or a foreign metal body, such as a bullet . Fig. 21 shows yet another calibration device 78 which is square in plan and is similar in construction to that of Fig. 20, the primary calibration marker 78 being similar to the primary calibration marker 73 of Fig. 18 and the corresponding primary calibration marker 77 of Fig. 20. Calibration markers 80 are similar to calibration markers 72 and 76.

Claims

CLAIMS :

1. A calibration device for use in locating a locus within a living body comprising a sheet member adapted for attachment to the living body adjacent the locus, the sheet member having at least one cutout portion permitting indexing of the sheet member relative to the living body when the sheet member is attached to the living body and a plurality of calibration markers each having a transmittivity for X-rays that contrasts with the transmittivity for X-rays of the sheet member.

2. A calibration device according to claim 1, wherein the sheet member is for'med of a radiolucent plastics material and the calibration markers are formed from a radiopaque material .

3. A calibration device according to claim 2, wherein the calibration markers include a primary marker which differs in size and/or shape from other calibration markers.

4. A calibration device according to claim 2 or claim 3, wherein the markers are arranged in a row.

5. A calibration device according to claim 4, wherein the markers are substantially equally spaced one from another in the row.

6. A calibration device according to claim 4 or claim 5, wherein the markers are arranged in a plurality of substantially parallel rows.

7. A calibration device according to any one of claims 2 to 6, wherein the at least one cutout portion comprises at least one aperture.

8. A calibration device sheet member according to claim 7, wherein the at least one aperture comprises a plurality of apertures which are arranged in a row substantially parallel to the row of markers.

9. A calibration device according to claim 8, wherein the spacing between the apertures corresponds substantially to the spacing between the markers .

10. A calibration device according to claim 7, wherein the calibration markers comprise areas of radiopaque material each surrounding a corresponding aperture.

11. A calibration device according to any one of claims 2 to 10, wherein the calibration means comprise a plurality of markers of circular cross section.

12. A calibration device according to any one of claims 2 to 9, wherein the markers comprise a plurality of balls.

13. A calibration device according to any one of claims 2 to 12, wherein the calibration markers include a primary marker which differs in size and/or shape from other calibration markers and wherein the at least one cutout portion comprises a slit whose axis is aligned with the primary marker.

14. A calibration device according to claim 1, wherein the sheet member is formed from a radiopaque materials and _* wherein the calibration markers comprise secondary cutout portions formed in the sheet member.

15. A calibration device according to claim 14, wherein the cutout portions are apertures.

16. A calibration device according to claim 15, wherein the apertures are arranged in a row.

17. A calibration device according to claim 15 or claim 16, wherein the at least one cutout portion comprises a slit which is larger than the apertures.

18. A calibration device according to any one of claims 14 to 17, wherein the calibration markers include notches in an edge of the sheet member.

19. A calibration device according to any one of claims 1 to 18 , further including securing means for securing the sheet member to the living body.

20. A calibration device according to claim 19, wherein the securing means comprises a skin-compatible adhesive.

21. A calibration device according to claim 19, wherein the securing means comprises a strap.

22. A calibration device according to claim 21, wherein the strap is provided with a buckle.

23. A calibration device according to claim 21, wherein the strap is arranged to be secured by means of a fastener of the hook and loop type.

24. A calibration device according to any one of claims 1 to 23, wherein the sheet member comprises a substantially rectangular strip.

25. A calibration device according to any one of claims 1 to 23 , wherein the sheet member comprises a substantially square member.

26. A method of locating an axis of a bone within a human or animal body comprising:

(a) attaching at least two calibration devices according to any one of claims 1 to 25 to skin of the human or animal body adjacent the bone whose axis is to be located at positions spaced apart along the length of the bone;

(b) marking the skin at positions which correspond to the calibration markers on the calibration devices to provide a plurality of alignment marks on the skin corresponding to the calibration markers;

(c) obtaining at least one X-ray image of at least a portion of the human or animal body including the bone and ' the at least two calibration devices;