WO2012004580A1

WO2012004580A1 - A surgical instrument

Info

Publication number: WO2012004580A1
Application number: PCT/GB2011/051064
Authority: WO
Inventors: Michael Rock
Original assignee: Depuy (Ireland)
Priority date: 2010-07-07
Filing date: 2011-06-07
Publication date: 2012-01-12
Also published as: GB201011448D0

Abstract

A surgical instrument comprising a pair of spacer block portions (14, 18; 16, 20) each having a first surface (22) and a second surface (24) which is inclined relative to the first surface (22). The spacer block portions (14, 18; 16, 20) are coupled together to form a composite spacer block (6; 8) such that the second surfaces (24) are in contact and are arranged to slide relative to one another parallel to a first sliding axis to adjust the thickness of the composite spacer block (6; 8) between the first surfaces (22). The second surfaces (24) of the spacer block portions (14, 18; 16, 20) comprise corresponding structures (38, 40) which when engaged with one another are arranged to resist sliding movement between the spacer block portions (14, 18; 16, 20) parallel to the first sliding axis.

Description

A Surgical Instrument

The present invention relates to a surgical instrument. In particular, embodiments of the present invention relate to variable composite spacer blocks for use in orthopaedic surgery to assess the space between bones in a joint. The present invention also relates to methods of using such a spacer block.

During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one or more of the patient's bones. During a joint replacement procedure it may be necessary to assess the size of the natural gap between the bones of the joint, or to assess the size of a gap formed when portions of the bones within the joint are resected. Furthermore, it may be necessary to assess the tension within soft tissue surrounding the joint for given sizes of gap created between the bones. Embodiments of the present invention relate to surgical instruments for assessing gaps between bones of a joint prior to the final implantation of the prosthetic joint.

An example of a surgical procedure in which it is necessary to assess a gap between bones is a knee replacement procedure. A surgical instrument set for performing a knee replacement procedure is marketed by DePuy Orthopaedics, Inc. under the trade mark Sigma High Performance Instruments.

For implanting a femoral prosthesis using the Sigma HP instrument set, it is first necessary to perform a resection of the distal tip of the femur forming a transverse resected surface to support a femoral prosthesis sizing guide. Similarly, the proximal end of the tibia is resected to create more room in the joint space and to receive a tibial component of the knee prosthesis. Once the femur and the tibia have been resected, the knee is placed in full extension. Laminar spreaders are applied medially and laterally to open up the joint space. The extension gap is preferably rectangular when the knee is fully extended.

The Sigma HP instrument set comprises a set of spacer blocks to assess the joint space. Each spacer block comprises a bar with spacer blocks at each end generally shaped to fit the joint space. The thickness of the block at each end differs. The thicker spacer block is used for assessing the joint space when the knee is in extension. The thinner spacer block is used for assessing the joint space when the knee is in flexion. The thickness of the spacer blocks corresponds to the gap between the bone cuts on the femur and the tibia when the prosthetic knee joint is implanted. Specifically, the thickness of the spacer blocks corresponds to the combined thickness of a posterior or distal portion of femoral prosthesis, a tibial prosthesis and a tibial insert. The difference in thickness between the two spacer blocks is equivalent to the difference in thickness between the distal condyles (facing the tibia when the knee is in extension) and the posterior condyles (facing the tibia when the knee is in flexion) of the femoral prosthesis. The extension gap side of the spacer block is used to determine the appropriate thickness of the tibial insert and to validate the soft tissue balance.

The bar between the spacer blocks includes a series of alignment holes to receive an alignment rod. When the knee is extended, the extension gap spacer block is positioned in the joint space. If the bones have been resected correctly, the alignment rod should extend down parallel to the longitudinal axis of the lower leg, and up parallel to the mechanical axis of the femur (the mechanical axis being an imaginary axis defined from the centre of the femoral head to the centre of the femoral condyles). The spacer block that best fits the available joint space indicates the required size of tibial implant.

While the spacer blocks described above can accurately fit a knee joint space, a large number of spacer blocks are required for variation in the joint space across a population. Specifically, a different spacer block is required for each different size combination of femoral prosthesis, tibial prosthesis and tibial insert. The Sigma HP spacer blocks are required to be double ended owing to the different thickness of the femoral prosthesis at the distal condyles and the posterior condyles (corresponding to the portion of the femoral prosthesis facing the tibia when the knee is in extension and flexion respectively).

It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere.

According to a first aspect of the present invention there is provided a surgical instrument comprising: a pair of spacer block portions each having a first surface and a second surface which is inclined relative to the first surface; wherein the spacer block portions are coupled together to form a composite spacer block such that the second surfaces are in contact and are arranged to slide relative to one another parallel to a first sliding axis to adjust the thickness of the composite spacer block between the first surfaces; and wherein the second surfaces of the spacer block portions comprise corresponding structures which when engaged with one another are arranged to resist sliding movement between the spacer block portions parallel to the first sliding axis.

An advantage of embodiments of the present invention is that the thickness of the composite spacer block can be readily adjusted by sliding the first and second spacer block portions relative to one another along the first axis, which causes the first surfaces to move apart. A further advantage is that the corresponding structures allow the thickness of the instrument to be set and resists variation in the thickness of the instrument (that is, sliding movement between the blocks parallel to the first axis) when the instrument is under compression between the first and second surfaces.

The second surface of a first spacer block portion may comprise a detent and the second surface of a second spacer block portion may comprise a recess such that when the detent is received in the recess sliding movement between the spacer block portions is restricted.

The second surface of the second spacer block portion may comprise two or more recesses spaced apart parallel to the sliding axis such that the detent may be received in either recess to restrict sliding movement between the spacer block portions, the thickness of the composite spacer block between the first surfaces of the spacer block portions being different when the detent is received in each recess.

The first and second spacer block portions may be wedge shaped such that when the spacer block portions are coupled together the first surfaces are generally parallel to one another.

A first spacer block portion may comprise a window allowing markings on the other spacer block portion to be visible through the window.

The spacer block portions may each be coupled to respective handle portions which overlie one another to form a bar having a longitudinal axis when the spacer block portions are coupled together. The first sliding axis may be inclined relative to the longitudinal axis of the bar, and the first sliding axis and the longitudinal axis of the bar may lie in a single plane.

The handle portion coupled to one spacer block portion may further comprise an elongate slot extending along the longitudinal axis of the bar and the handle portion coupled to the other spacer block portion may further comprise an elongate lug arranged to be received in the slot when the spacer block portions are coupled together such that the lug can slide within the slot as the spacer block portions slide relative to one another parallel to the first sliding axis.

The lug may further comprise one or more alignment holes each defining an alignment hole axis which is transverse to the longitudinal axis of the bar. Each alignment hole may be arranged to receive an alignment rod extending from the handle portion along the alignment hole axis to indicate the orientation of the spacer block portions about the longitudinal axis of the bar.

The surgical instrument may further comprise a second pair of spacer block portions respectively coupled to the other end of each handle portion, wherein when the first pair of spacer block portions are coupled together the second pair of spacer block portions also couple together to form a second composite spacer block, the second pair of spacer block portions being arranged to slide relative to one another parallel to a second sliding axis parallel to the first sliding axis to adjust the thickness of the second composite spacer block between the first surfaces.

The first surface of one of the spacer block portions may further comprise coupling means for coupling to a shim to increase the thickness of the composite spacer block.

The first surface of one of the spacer block portions may further comprise coupling means for coupling either to further surgical instrument or prosthesis.

The or each composite spacer block may be arranged to assess the size of a gap between bones in a joint.

According to a second aspect of the present invention there is provided a method of assessing the size of a joint space between two bones within a joint, the method comprising: coupling together a pair of spacer block portions to form a composite spacer block, wherein each spacer block portion has a first surface and a second surface which is inclined relative to the first surface, the spacer block portions being couplable together such that the second surfaces are in contact and arranged to slide relative to one another parallel to a first sliding axis, the second surfaces of the spacer block portions comprising corresponding structures which when engaged with one another are arranged to resist sliding movement between the spacer block portions parallel to the first sliding axis; sliding the spacer block portions relative to one another parallel to the first sliding axis to select a first predetermined thickness of the composite spacer block between the first surfaces; forming a joint space between the bones; and inserting the composite spacer block into the joint space to assess the size of the joint space.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompany drawings, in which:

Figure 1 illustrates an exploded perspective view of a surgical instrument in accordance with a first embodiment of the invention and a tibial shim;

Figures 2A and 2B respectively illustrate top and cross sectional views of the surgical instrument of figure 1 when in a first configuration;

Figures 3A and 3B respectively illustrates top and cross sectional views of the surgical instrument of figure 1 when in a second configuration;

Figure 4 illustrates a surgical instrument in accordance with a second embodiment of the invention, a tibial shim and a tibial tray shim;

Figure 5 is a cross sectional view of the surgical instrument of figure 4; and

Figure 6 is a cross sectional view of a surgical instrument in accordance with the first embodiment of the invention inserted into the joint space between a resected femur and a resected tibia.

Referring first to figure 1 , this illustrates a surgical instrument in accordance with a first embodiment of the present invention. The surgical instrument may be referred to herein as a spacer block, however certain embodiments of the present invention comprise first and second spacer blocks (specifically, composite spacer blocks) positioned at either end of a bar. The surgical instrument comprises a first, upper piece 2 and a second, lower piece 4. Collectively, the upper and lower pieces 2, 4 form a double ended surgical instrument having two spacer blocks. The double ended spacer block comprises enlarged composite spacer blocks 6, 8 at either end, coupled together by a bar 10 including a thickened grip portion 12. The upper and lower pieces 2, 4 are sandwiched together to form a single instrument in which the upper and lower pieces 2, 4 can slide relative to one another along the longitudinal axis of the bar 10. The surgical instrument is for use in assessing a knee joint space and is used in a similar manner to the Sigma HP spacer blocks described above. Specifically, after the knee has been opened, one or other composite spacer block 6, 8 is inserted into the knee space to determine the required thickness of the femoral and tibial prostheses and the tibial insert. However, composite spacer block 8 requires a tibial shim 50 to be added before it can be inserted into the knee space, as discussed below.

At each composite spacer block 6, 8 the upper and lower pieces 2, 4 form wedges or spacer block portions. Specifically, the upper piece 2 has a first wedge 14, 16 at each end and the lower piece 4 has a second wedge 18, 20 at each end. Each wedge 14-20 comprises a first, outer surface 22 and a second ramp surface 24 which is inclined relative to the outer surface 22. The wedges 14-20 are brought together such that the ramp surfaces 24 are in sliding contact with one another. As can be seen in figure 1 , the ramp surfaces 24 may include recesses 26 between struts 28 and an outer rim 30; however it will be appreciated that the surfaces of the struts 28 and the outer rim 30 generally lie within the same plane to define the ramp surface. It will be understood that the arrangement of recesses 26 and struts 28 illustrated in figure 1 is only one possible way of manufacturing the spacer block portions, and in alternative embodiments the ramps may extend part or all of the full width of the spacer block portions.

The ramped surfaces 24 are inclined relative to the longitudinal axis of the spacer block defined by the bar. Furthermore, the ramped surfaces 24 in each composite spacer block 6, 8 are generally parallel to a single plane and run in the same direction. The ramped surfaces 24 of the left hand composite spacer block slope downwards as illustrated in figure 1. The ramped surfaces 24 of the right hand composite spacer block 8 slope upwards as illustrated in figure 1. Sliding the upper piece to the right relative to the lower piece 4 causes the upper piece 2 to lift off the lower piece 4 as the ramped surfaces 24 slide over one another. Sliding the upper piece to the left relative to the lower piece 4 causes the upper piece 2 to move towards the lower piece 4 as the ramped surfaces 24 slide over one another.

Sliding movement of the ramped surfaces to the left is limited by the bar portions of the upper and lower pieces 2, 4 coming into contact. Further movement of the upper piece 2 to the left relative to the bottom piece 4 would result in the ramped surfaces 24 separating from one another. However, further left movement is limited because the upper piece 2 comprises an elongate slot 30 and the lower piece 4 comprises an elongate block 32 arranged to extend up through the slot 30. When the block 32 reaches the end of the slot 30 further movement of the upper piece 2 relative to the lower piece 4 parallel to the longitudinal axis of the bar 10 is prevented. The upper piece 2 further comprises a detent 34 extending into the slot 30 which is received in a cut out 36 in the lower piece block 32. Movement between the detent 34 and the cut out 36 further limits sliding movement between the upper and lower pieces 2, 4. Slot 30 and block 32 limit the extending of relative movement between the upper and lower pieces 2, parallel to the longitudinal axis of the bar part of the spacer block. However, the upper piece 2 is freely removable from the lower piece 4 by lifting the slot 30 over the block 32. In use, a surgeon can grip the bar 10, specifically the grip portion 12, which prevents the upper and lower pieces 2, 4 from separating.

Referring now also to figures 2A and 2B, these illustrate the instrument of figure 1 assembled with the block 32 received in the slot 30 and the upper and lower pieces sandwiched together. Figure 2A is a top view and figure 2B is a cross sectional view of the instrument. Figures 2A and 2B illustrate the instrument in a first configuration for use in assessing the flexion gap of the knee joint. In the first configuration each composite spacer block 6, 8 has a minimum thickness.

As can be seen in the cross sectional view of figure 2B and also in figure 1 , the ramped surfaces 24 of wedges 18, 20 of the lower piece 4 each include a pair of recesses 38. The recesses 38 run transverse to the longitudinal axis of the bar 10. With specific reference to figure 1, it can be seen that the recesses comprise notches formed in struts 28. As noted above, the arrangement of struts 28 illustrated in figure 1 is only one possible way of manufacturing the spacer block portions. In alternative embodiments, the notches may extend across the whole ramped surface of each lower spacer block portion. The ramped surfaces 24 of wedges 14, 16 of the upper piece 2 each include a detent 40. The detent 40 may comprise a ridge extending across each ramped surface 24. Alternatively, the ramped surfaces 24 of the upper piece 2 may be similar to the ramped surfaces 24 of the lower piece 4, that is formed from struts and recesses, and the detent 40 may comprise a separate lug on each strut extending in a line across the ramped surface 24. The detents 40 are arranged to be received in a recess 38. The shape of the detents 40 and the recesses 38 means that when the detents 40 are received in recesses 38 further sliding movement of the upper piece 2 relative to the lower piece 4 is inhibit, particularly so if there is a

compressive load applied across either composite spacer block 6, 8. It can be seen from the cross sectional view of figure 2B that the ramped surfaces 24 are generally in contact with one another across a large proportion of their respective areas so long as the detent 40 is received in a recess. It will be appreciated that when the detents 40 do not line up with recesses 38 then the upper and lower pieces 2, 4 are free to slide. The upper and lower pieces 2, 4 are in a stable position when the recesses and detents are engaged. As there are two recesses 38 and one detent 40 for each composite spacer block 6, 8 there are two stable positions for the upper portion 2 to be seated upon the lower portion 4.

The height of the composite spacer blocks differs according to the recess in which each detent is received. In figure 2B, the instrument is in the first configuration and the detents 40 are received in the left hand recesses 38. The composite spacer blocks assume their minimum stable height, which may be the minimum height achievable if the detents were to be removed (i.e. in the central bar portion 10 the upper piece 2 is in contact with the lower piece 4). However, in the embodiment illustrated in figure 2B the minimum stable height is not equal to the minimum height achievable if the detents were removed as the left hand recess 38 is positioned to ensure that there is some clearance between the central bar portions so that it is movement of the detents 40 and not the handle which sets the minimum stable height. Additionally, it can be seen that further movement of the upper piece 2 to the left relative to the lower piece 4 is limited as the elongate block 32 is at the furthest right position within slot 30 in the upper piece 2.

Referring now to figures 3A and 3B, these illustrate the instrument of figure 1 assembled in a second configuration in which each composite spacer block 6, 8 has a maximum thickness. Figure 3A is a top view and figure 3B is a cross sectional view of the instrument. The second configuration may be for use in assessing the extension gap of the knee joint for implant systems in which the femoral condyle is thicker distally than posteriorly, for instance a PFC Sigma implant commercially available from DePuy

Orthopaedics, Inc. Other implants may have a reverse situation or the thickness of the femoral condyles may be matched distally and posteriorly, as discussed below. Figures 3 A and 3B are generally the same as figures 2A and 2B except that the upper piece has shifted to the right relative to the lower piece 4 such that the detents 40 are received in the right hand recesses 38 so that the composite spacer blocks 6, 8 assume their maximum stable height. In the first configuration of figures 2A and 2B it can be seen that wedge 14 overhangs wedge 18 at the left hand composite spacer block 6. Similarly, wedge 20 juts out from underneath wedge 16 at the right hand composite spacer block 8. In the second configuration of figures 3 A and 3B it can be seen that wedge 18 juts out from underneath wedge 14 at the left hand composite spacer block 6. Similarly, wedge 16 overhangs wedge 20 at the right hand composite spacer block 8.

Figures 2A to 3B further show that the upper piece 2 comprises windows 42, 44 allowing portions of the lower piece 4 to be visible. The windows 42, 44 are also visible in figure 1. The instrument of the first embodiment of the present invention illustrated in figures 1 to 3 B is intended for use for assessing the size of a joint space in a knee joint. Consequently, the composite spacer block 6, 8 are generally shaped to conform to the shape of the upper tibia, and specifically are shaped similarly to tibial inserts for the prosthetic joint. The first, thinner, configuration of figures 2A and 2B is intended for assessing the size of the gap when the knee is in flexion. As noted above, implants where the distal condyle is thicker than the posterior condyle the joint space is smaller when the knee is in flexion. The difference in thickness may be reversed or absent for other implants. To make clear to the surgeon which configuration the instrument is in, printed upon the lower piece 4 and visible through windows 42, 44 is the word "flexion" as shown in figure 2A. Similarly, the second, thicker, configuration of figures 3A and 3B is intended for assessing the size of the gap when the knee is in extension. The word "extension" is printed upon the lower piece 4 and visible through windows 38, 40 as shown in figure 3 A.

The difference in thickness between a femoral prosthesis, and hence the difference in the joint space between flexion and extension may be 1mm. Consequently, the difference in height of the composite spacer blocks 6, 8 between the first (flexion) and second (extension) configurations must also be 1mm. The ramped surfaces 24 have a gradient of 6:1 relative to the longitudinal axis of the bar 10 and also relative to the outer surfaces 22. That is, 6mm of longitudinal sliding movement between the upper piece 2 and the lower piece 4 results in a 1mm height difference in the composite spacer blocks. The recesses 34 are positioned upon the lower ramped surfaces 24 6mm apart so that the height difference between the flexion and extension configurations is 1mm. The words "flexion" and "extension" may be printed upon the lower piece 4 in text which is 6mm high and still be separately visible through windows 38, 40 (which are also 6mm wide). It will be appreciated that in alternative embodiments of the present invention the gradient may vary. A steeper gradient would allow a greater difference in height for the composite spacer blocks 6, 8 to be achieved for the same longitudinal sliding movement of the upper and lower pieces 2, 4. Alternatively, a smaller longitudinal movement may be required. However, if the gradient is steeper then the size of the printing on the lower piece and the width of the windows 42, 44 must be reduced in order for the words "flexion" and "extension to remain separately visible, which may make the printed marks harder to read. Additionally, the composite spacer block 6, 8 which is inserted into the knee joint may be under tension. The steeper the gradient, the more likely it is that the detent mechanism can be overcome when the composite spacer blocks 6, 8 are compressed resulting in undesired sliding movement between the upper piece and the lower piece. It will be appreciated that the shape and size of the detents 40 and recesses 38 also significantly affect the degree to which sliding movement between the upper and lower pieces 2, 4 can occur when the composite spacer blocks 6, 8 are under compression.

A shallower gradient reduces the risk of uncontrolled sliding movement between the upper and lower pieces 2, 4 and also increases the maximum possible text height of the markings and also the width of the windows. However, if the gradient is reduced, then the upper and lower pieces 2, 4 must slide further relative to one another to achieve the same difference in height. Consequently, the amount of under or overhang of wedges 14 - 20 increases. This can result in the composite spacer blocks 6, 8 being unstable within the knee space when under compression. As an example, when the instrument is in the first configuration the word "flexion" is visible through the window and the composite spacer block is 17mm thick (tibial tray 4mm, insert 5mm, posterior condyle 8mm). Sliding the upper piece relative to the lower piece along the gradient 6mm changes the height by 1mm to 18mm, (tibial tray 4mm, insert 5mm, distal condyle 9mm) and the window moves to reveal the word "extension".

A method of using instruments in accordance with embodiments of the present invention will now be described. The method is generally the same as for the Sigma HP spacer blocks described above when used to implant a PFC Sigma implant. The Sigma HP spacer blocks require two ends for each combination of femoral prosthesis, tibial prosthesis and tibial insert to account for the difference in size of the femoral prosthesis between flexion and extension. However, as discussed above, for embodiments of the present invention each composite spacer block can be adjusted in height by an amount chosen to be equal to the difference in thickness of the femoral prosthesis between flexion and extension. The shape of the composite spacer blocks 6, 8 as seen in the top views of figures 2A and 3 A is chosen to correspond to the shape of the knee joint in cross section, and to be comfortably received in the smallest possible knee joint.

A first composite spacer block 6 is chosen to correspond to the flexion and extension thicknesses required for the smallest possible combination of femoral prosthesis, tibial prosthesis and tibial insert (for instance 5mm). The knee joint is exposed and the distal femur and proximal tibia resected. The joint space is then exposed and the instrument inserted to assess the size of the gap and the tension in soft tissue surrounding the knee. Figure 6 illustrates an instrument in accordance with the embodiment of figures 1 to 4B inserted into a joint space between a resected femur 200 and a resected tibia 202. The knee is in extension and the instrument is in the second, extension configuration as illustrates in figure 3B in which the detents 40 are seated in the right hand recesses 38 and the thickness of each composite spacer block 6, 8 is at the maximum setting. The composite spacer block 6 is used initially as this represents the minimum possible thickness. The composite spacer block 6 may then be used in the first configuration for assessing the joint space when the knee is in flexion. Advantageously, only a single composite spacer block, that is a single ended instrument, is needed to assess the size of the joint space in flexion and in extension.

While the first composite spacer block 6 is arranged to correspond to the smallest combination of femoral prosthesis, tibial prosthesis and tibial insert, the second composite spacer block 8 is arranged to receive tibial shims to allow it to have a variable thickness. Consequently, the first composite spacer block 6 may be referred to as a fixed spacer composite spacer block and the second composite spacer block 8 may be referred to as a variable composite spacer block. Figure 1 shows a tibial shim 50 arranged to couple to the variable composite spacer block 8. Figures 2A to 3B show the first embodiment of the instrument without a coupled tibial shim. The tibial shim 50 is shaped generally the same as the second composite spacer block 8 and comprises first and second holes 52 arranged to receive spigots 54 upon the upper surface 22 of wedge 16 of the second composite spacer block 8. By coupling a tibial shim 50 to the second composite spacer block 8 the height of the second composite spacer block 8 is increased in both the first and second configuration. A range of tibial shims 50 may be provided having different thicknesses in order to allow the variable composite spacer block 8 to be used to assess each different required combined thickness of femoral prosthesis, tibial prosthesis and tibial insert. It will be appreciated that regardless of the thickness of the tibial shim, the difference in height for the variable composite spacer block 8 between the first configuration and the second configuration will be the same as this is determined by the gradient of the ramped surfaces 24 and the longitudinal distance that the first and second pieces slide relative to one another as the detents 40 move between recesses 38. By using tibial shims, only a single instrument is required to assess all possible combinations of femoral prosthesis, tibial prosthesis and tibial insert.

The bar 10 between the composite spacer blocks includes a series of holes 60 to receive an alignment rod 204 (illustrated in figure 6). When the knee is extended, either the first or the second composite spacer block 6, 8 is positioned in the joint space when the instrument is in the second configuration as illustrated in figure 6. An alignment rod 204 is inserted through one of the holes 60. If the bones have been resected correctly, the alignment rod 204 should extend downwards parallel to the longitudinal axis of the lower leg and upwards parallel to the mechanical axis of the femur.

Referring no to figures 4 and 5, these respectively illustrate a perspective view and a cross sectional view of an instrument in accordance with a second embodiment of the present invention. The second embodiment is generally the same as the first embodiment and so corresponding features are referred to using reference numerals which have been incremented by 100.

As for the first embodiment, the second embodiment generally comprises an instrument formed from an upper piece 102 and a lower piece 104. The spacer block comprises a fixed composite spacer block 106 and a variable composite spacer block 108 couple together via a bar 110. The variable composite spacer block 108 is arranged to receive tibial shims 150 and the composite spacer blocks 106, 108 are arranged to switch between the first configuration and the second configuration using the same mechanism as described above. Where the second embodiment differs from the first embodiment is that the lower wedge 120 forming the variable composite spacer block 108 comprises a socket 170 arranged to receive a tibial tray shim 180 in order to complete the variable composite spacer block 108. However, if the tibial tray shim 180 is removed, the socket 170 is arranged to couple directly to other components. For instance, the variable composite spacer block 108 may be arranged to couple to a force sensor for measuring the force within the knee joint when the variable composite spacer block 108 is compressed within the knee joint. Alternatively, it may be desirable to couple the variable composite spacer block 108 to a trial tibial tray prosthesis which has already been coupled to the tibia. It will be appreciated that the fixed composite spacer block 106 may also have a socket to receive a tibial tray shim or to couple to another component.

It will be appreciated that instruments in accordance with embodiments of the present invention may be provided in both single use and reusable forms. The principle difference between the two forms is in terms of materials. For instance, reusable instruments may be manufactured from a metallic material such as stainless steel for durability and ability to be sterilised. A single use instrument may be formed from a plastics material to reduce the cost.

While instruments in accordance with the embodiments of the invention described above have a thickness adjustment mechanism to adjust the thickness of the composite spacer blocks to allow assessment of both a flexion gap and an extension gap for a knee joint, the adjustment mechanism may have other uses. For instance, for a knee prosthesis in which the femoral component has the same thickness in extension and in flexion, an instrument may be provided to assess the required gap for multiple different combinations of femoral prosthesis, tibial prosthesis and tibial insert using a single composite spacer block, thereby reducing the required number of instruments. For instance, the height difference when the detent is received in each recess may correspond to the difference between two different combinations of femoral prosthesis, tibial prosthesis and tibial insert. Furthermore, a larger number of recesses may be provided so that by sliding the upper piece further relative to the lower piece the thickness of the composite spacer block may be adjusted between three or more predetermined thicknesses. In the latter case, the window may reveal the thickness of the composite spacer block, e.g. "5mm", "6mm" or "8mm". The same principle can be applied to implant systems where the thickness of a mobile bearing tray differs from that of a fixed bearing tray (in this case the marking would be "mobile" and "fixed"). At its most general, the present invention provides a method of changing the thickness of a surgical instrument by a predetermined amount. While the present invention is particularly suited to variable thickness composite spacer blocks, such as for use in knee surgery, it is not limited to this and may be applied where ever there is a need to a rapid adjustment of the dimensions of a surgical instrument. For example when balancing a knee prosthesis, femoral positioners and tensioners often reference the thickness of the insert and tibial tray. Such a mechanism could account for differences in insert or tray thicknesses

It is particularly advantageous that embodiments of the present invention provide a window mechanism to allow rapid identification of the configuration of the device.

As a further option, there may be two or more detents arranged to be received within two or more pairs or groups of recesses to provide greater stability as the thickness of the composite spacer block varies. In a further option, the instrument may have only a single composite spacer block. That is the instrument may be single rather than double ended. In such an embodiment the bar may also be dispensed with, or replaced with some other form of handle.

In the method of using spacer blocks in accordance with embodiments of the present invention described above the femur and the tibia are resected before the spacer block is inserted into the joint space. The spacer block indicates the extension and flexion gaps between the resected femur and resected tibia for both fixed bearing and mobile bearing implants. However, the instrument can also be used to assess the gap between a resected tibia and a non-resected femur using the posterior face of an open cutting block coupled to the femur.

It will be readily apparent to the appropriately skilled person that further modifications may be made to the present invention and further applications may be found for the present invention from the teaching herein, without departing from the scope of the appended claims.

Claims

CLAIMS:

1. A surgical instrument comprising:

a pair of spacer block portions each having a first surface and a second surface which is inclined relative to the first surface;

wherein the spacer block portions are coupled together to form a composite spacer block such that the second surfaces are in contact and are arranged to slide relative to one another parallel to a first sliding axis to adjust the thickness of the composite spacer block between the first surfaces; and

wherein the second surfaces of the spacer block portions comprise corresponding structures which when engaged with one another are arranged to resist sliding movement between the spacer block portions parallel to the first sliding axis.

2. A surgical instrument according to claim 1, wherein the second surface of a first spacer block portion comprising a detent and the second surface of a second spacer block portion comprising a recess such that when the detent is received in the recess sliding movement between the spacer block portions is restricted.

3. A surgical instrument according to claim 2, wherein the second surface of the second spacer block portion comprises two or more recesses spaced apart parallel to the sliding axis such that the detent may be received in either recess to restrict sliding movement between the spacer block portions, the thickness of the composite spacer block between the first surfaces of the spacer block portions being different when the detent is received in each recess.

4. A surgical instrument according to any one of the preceding claims, wherein the first and second spacer block portions are wedge shaped such that when the spacer block portions are coupled together the first surfaces are generally parallel to one another.

5. A surgical instrument according to any one of the preceding claims, wherein a first spacer block portion comprises a window allowing markings on the other spacer block portion to be visible through the window.

6. A surgical instrument according to any one of the preceding claims, wherein the spacer block portions are each coupled to respective handle portions which overlie one another to form a bar having a longitudinal axis when the spacer block portions are coupled together.

7. A surgical instrument according to claim 6, wherein the first sliding axis is inclined relative to the longitudinal axis of the bar, the first sliding axis and the

longitudinal axis of the bar lying in a single plane.

8. A surgical instrument according to claim 6 or claim 7, wherein the handle portion coupled to one spacer block portion further comprises an elongate slot extending along the longitudinal axis of the bar and the handle portion coupled to the other spacer block portion further comprises an elongate lug arranged to be received in the slot when the spacer block portions are coupled together such that the lug can slide within the slot as the spacer block portions slide relative to one another parallel to the first sliding axis.

9. A surgical instrument according to claim 8, wherein the lug further comprises one or more alignment holes each defining an alignment hole axis which is transverse to the longitudinal axis of the bar, each alignment hole being arranged to receive an alignment rod extending from the handle portion along the alignment hole axis to indicate the orientation of the spacer block portions about the longitudinal axis of the bar.

10. A surgical instrument according to any one of claims 6 to 9, further comprising a second pair of spacer block portions respectively coupled to the other end of each handle portion, wherein when the first pair of spacer block portions are coupled together the second pair of spacer block portions also couple together to form a second composite spacer block, the second pair of spacer block portions being arranged to slide relative to one another parallel to a second sliding axis parallel to the first sliding axis to adjust the thickness of the second composite spacer block between the first surfaces.

1 1. A surgical instrument according to any one of the preceding claims, wherein the first surface of one of the spacer block portions further comprises coupling means for coupling to a shim to increase the thickness of the composite spacer block.

12. A surgical instrument according to any one of the preceding claims, wherein the first surface of one of the spacer block portions further comprises coupling means for coupling either to further surgical instrument or prosthesis.

13. A surgical instrument according to any one of the proceeding claims, wherein the or each composite spacer block is arranged to assess the size of a gap between bones in a joint.

14. A method of assessing the size of a joint space between two bones within a joint, the method comprising:

coupling together a pair of spacer block portions to form a composite spacer block, wherein each spacer block portion has a first surface and a second surface which is inclined relative to the first surface, the spacer block portions being couplable together such that the second surfaces are in contact and arranged to slide relative to one another parallel to a first sliding axis, the second surfaces of the spacer block portions comprising corresponding structures which when engaged with one another are arranged to resist sliding movement between the spacer block portions parallel to the first sliding axis; sliding the spacer block portions relative to one another parallel to the first sliding axis to select a first predetermined thickness of the composite spacer block between the first surfaces;

forming a joint space between the bones; and

inserting the composite spacer block into the joint space to assess the size of the joint space.