WO2006087535A1

WO2006087535A1 - Surgical instrument

Info

Publication number: WO2006087535A1
Application number: PCT/GB2006/000518
Authority: WO
Inventors: Nicholas James Theophilius Metcalfe; Russell Lloyd; David Wycliff Murray
Original assignee: Biomet Uk Limited
Priority date: 2005-02-15
Filing date: 2006-02-15
Publication date: 2006-08-24
Also published as: GB0503137D0; GB2423021A

Abstract

A surgical instrument (2) comprises a jig (4), suitable to be fixed to a bone, a support element (6) mounted within the jig (4) and a first cutting guide (8) mounted on the support element (6), wherein the support element (6) has a substantially spherical head (42), which articulates with the first cutting guide (8) and expands to fix the position of the first cutting guide (8) relative to the support element (6).

Description

SURGICAL INSTRUMENT

BACKGROUND

During all types of joint replacement surgery, it is necessary to make specific cuts into areas of bone. Both the point and angle of entry of each cut is important. An example of this is knee replacement surgery, where it is necessary to resect all or part of the tibial plateau to a specific level and at a specific angle. Guide tools exist to aid surgeons in the placement of bone cuts. Such tools are often difficult and time consuming to operate, and may be inaccurate. Many of the tools of the prior art are also bulky, requiring the removal or displacement of large areas of soft tissue in order to be used.

Guidance as to the point and angle of entry of a cut is conventionally provided by separate functions of a tool or by separate tools altogether. Often, guidance is only provided for one aspect of the cut. For example, the point of entry may be indicated but the angle of entry may be left to the skill and judgement of the surgeon.

SUMMARY OF INVENTION

According to the present invention, there is provided a surgical instrument comprising a jig, suitable to be fixed to a bone, a support element mounted within the jig and a first cutting guide mounted on the support element, wherein the support element has a head with substantially spherical bearing surfaces, which head articulates with the first cutting guide and expands to fix the position of the first cutting guide relative to the support element. The spherical nature of the articulation between the head of the support element and the cutting guide enables the cutting guide to be rotated about multiple axes and/or adjusted to a desired orientation relative to the support element.

The head of the support element may be received within an opening on the cutting . guide. The opening may comprise a chamfered slot that may extend in a substantially anterior/posterior direction. The slot enables the cutting guide to be translated in an anterior/posterior direction relative to the support element as well as rotated about multiple axes. The support element may have a body portion, which may be connected to the head and mounted within the jig.

The head of the support element may comprise two elements, a fixed element, connected to the body portion and a locking element connected to the fixed element. The fixed element and/or the locking element may be substantially hemispherical. The locking element may be connected to the fixed element via a thread such that rotation of the locking element causes the locking element to move away from the fixed element. In this manner, the head may be expanded to frictionally engage an inner surface of the chamfered slot and thus fix the position and/or orientation of the cutting guide relative to the support element.

A planar face of the fixed element may comprise a recess having an internal thread.

An annular protrusion may extend from a face of the locking element towards the fixed element. The annular protrusion may have an external thread, suitable for cooperation with the internal thread on the fixed element.

A surface of the locking element may include a recess. A longitudinal axis of the recess may be coincident with the axis of rotation of the locking element. The recess may have an internal cross section in the form of a polygon. The internal cross section may be in the form of a hexagon. The recess may be of a size and shape to correspond to an appropriately shaped key to enable rotation of the locking element relative to the fixed element.

The jig may comprise a base and a pair of parallel mounting arms. The base may comprise a pair of substantially rectangular, spaced apart flanges defining an inter flange space therebetween. The flanges may extend in a substantially coronal plane when the instrument is mounted adjacent an anterior surface of a tibia. A web may extend between lateral edges of the flanges.

First and second pairs of attachment holes may extend through the superior and inferior regions of the flanges respectively. An attachment pin may be mounted within the web of the jig and may extend in a substantially medial/lateral direction into the inter flange space between the first and second pairs of attachment holes. The pin may be mounted such that it may rotate but not translate relative to the jig. The pin may carry an external thread.

An attachment member may be mounted on the attachment pin in the inter flange space. The attachment member may comprise an internally threaded bore in which the threaded attachment pin is received. The height of the attachment member may reduce towards the lateral side of the jig, such that the attachment member partially blocks at least a portion of each of the first and second pairs of attachment holes.

The thickness of the attachment member may be such that it is closely confined by the flanges.

The attachment pin may be formed integrally with an attachment handle that may protrude on the lateral side of the jig, such that rotation of the handle causes rotation of the attachment pin and therefore relative medial/lateral displacement of the attachment member and the jig.

The mounting arms of the jig may extend from a medial side of the base of the jig. The mounting arms may extend in a substantially transverse plane. Aligned openings may extend through each of the mounting arms.

The body portion of the support element may be received within the aligned openings in the mounting arms. There may be cooperating formations on the body portion and the openings that prevent relative rotation between the body portion and the mounting arms. The cooperating formations may comprise planar regions on the internal surface of at least one opening and the external surface of the body portion.

The planar region on the external surface of the body portion may extend the length of the body portion.

An external surface of the body portion may carry a thread.

An adjusting nut may be mounted on the body portion between the mounting arms such that the nut engages the external thread on the body portion. The nut may be closely received between the mounting arms, such that rotation of the nut causes relative proximal/distal translation of the body portion and the mounting arms, as a result of the cooperating threads and formations.

The surgical instrument may further comprise a second cutting guide.

The second cutting guide may be mounted on a proximal guide surface of the first cutting guide. The second cutting guide may comprise an internally threaded bore that extends through the second cutting guide.

The second cutting guide may be mounted over an opening that may extend through the first cutting guide.

A screw may extend through the opening in the first cutting guide and may be received within the threaded bore in the second cutting guide, such that a proximal surface of the head of the screw engages a distal surface of the first cutting guide.

The opening in the first cutting guide may be a slot, which may extend in a substantially medial/lateral direction.

A proximal end of the screw may comprise a recess. The recess may be in the shape of a polygon and suitable to receive a correspondingly shaped key.

According to another aspect of the present invention, there is provided a method of guiding a cutting tool using the instrument of the current invention, the method comprising the steps of:

(a) attaching at least two pins to a surface of a tibia,

(b) inserting the pins into the first and second pairs of attachment holes,

(c) rotating the attachment handle such that the attachment member is translated towards the pins and engages the pins against the internal surfaces of the attachment holes,

(d) rotating the adjusting nut such that the support element and first cutting guide are translated to a desired proximal/distal location

(e) articulating the first cutting guide on the support element to achieve a desired orientation and anterior/posterior position of the first cutting guide, (f) rotating the locking element of the head such that the head expands and locks the first cutting guide in a fixed position relative to the support element, and (f) engaging a surface of the tibia with a cutting tool along the first cutting guide.

According to another aspect of the present invention, there is provided a joint comprising a socket and a ball received within the socket, the ball comprising first and second sections which are connected for relative rotation via a thread, wherein relative rotation between the first and second sections causes the ball to expand and frictionally engage the socket.

The first and second sections may be substantially hemispherical.

A protrusion may extend from a planar face of the second section towards the first section. The protrusion may be received within a corresponding first recess on a planar face of the first section. The first recess and protrusion may carry cooperating threads.

The first recess may comprise a first bore that extends through the first section.

A surface of the second section may include a second recess. A longitudinal axis of the second recess may be coincident with the axis of rotation of the second section.

The second recess may have an internal cross section in the form of a polygon. The internal cross section may be in the form of a hexagon.

The second recess may comprise a second bore that extends through the second section. The second bore may extend through the protrusion, such that the protrusion is an annular protrusion and the second bore is integral with the first bore.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a perspective view of a surgical instrument,

Figure 2 is a perspective view of another embodiment of surgical instrument, Figure 3 is a perspective view of a support element,

Figure 4 is a sectional view of a support element,

Figure 5 is a top plan view of a first cutting guide,

Figure 6 is a side view of the first cutting guide of Figure 5,

Figure 7 is a sectional view of the first cutting guide of Figure 5,

Figure 8 is a sectional view on the line AA of Figure 2,

Figure 9 is a partially exploded, alternative perspective view of the surgical instrument of Figure 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figures 1 and 2, a surgical instrument 2 comprises a jig 4, in which is received a support element 6. A first cutting guide 8 is mounted on the support element 6 and a second cutting guide 10 is mounted on the first cutting guide 8.

The jig 4 comprises a base 12 and a pair of parallel mounting arms 14, 16. The base 12 comprises two parallel spaced apart flanges 18, 20, that extend in a substantially coronal plane when the jig is mounted adjacent an anterior surface of a tibia. An inter flange space 22 is defined between the flanges 18, 20. As best seen in Figure 2, a web 24 extends between lateral edges of the flanges 28, 20, joining the flanges 18, 20 together and holding them in their parallel orientation. In the embodiment illustrated in Figure 1 , the lateral side is to the right of the instrument 2, as viewed from the anterior approach, as the instrument 2 is oriented to be attached to a left knee of a patient.

However, it will be appreciated that the instrument 2 may be rotated for use on the right knee of a patient, such that the lateral side is to the left of the instrument 2, when viewed from an anterior approach (as illustrated in Figure 2).

First and second pairs of attachment holes 26, 28 extend through each of the flanges 18, 20. The first pair of attachment holes 26 extends through the inferior regions of the flanges 18, 20, and the second pair of attachment holes 28 extends through the superior regions of the flanges 18, 20. In Figure 1 , only the holes extending through the facing flange 20 can be seen.

An attachment pin 30 is mounted in the web 24 of the jig 4, such that the pin 30 extends into the inter flange space 22 along an axis that is substantially equidistant from the first and second pairs of attachment holes 26, 28. A handle 31 is formed integrally with the attachment pin 30 and protrudes from the lateral side of the jig 4. The attachment pin 30 is mounted on the web 24 of the jig 4 such that it may rotate, but not translate, relative to the jig 4. The attachment pin 30 carries an external thread (not shown). An attachment member 32 is mounted on the attachment pin 30 within the inter flange space 22 and is closely received between the two flanges 18, 20. The attachment member 32 comprises an internally threaded bore (not shown) in which the pin 30 is received. The attachment member 32 is constrained by the flanges 18, 20 such that rotation of the pin, 30 via the handle 31 , causes the attachment member to translate in a substantially medial/lateral direction towards or away from the handle 31 , as a result of the cooperating threads on the attachment member bore and the attachment pin 30. The height of the attachment member 32 reduces towards the lateral side of the jig 4 such that the attachment member 32 partially blocks at least a portion of the first and second pairs of attachment holes 26, 28 (as best seen in Figure 1). Translation of the attachment member 32 towards the handle 31 causes a greater portion of the first and second pairs of attachment holes 26, 28 to be obscured.

The parallel mounting arms 14, 16 extend from a medial side of the base 12 of the jig 4 in a substantially transverse plane. Aligned openings (not shown) extend through each of the mounting arms 14, 16 and receive a portion of the support element 6. An adjusting nut 34 is mounted on the support element 6 and is closely received between the mounting arms 14, 16.

Referring also to Figures 3 and 4, the support element 6 comprises a tubular main body 40 and a substantially spherical head 42. A longitudinal bore 44 extends through the main body 40 and head 42 of the support element 6. The head 42 comprises a fixed element 46, which is integral with the main body 40, and a locking element 48, which is connected to the fixed element 46 via a thread. An annular protrusion 50, having an external thread 52, extends from a face 54 of the locking element 48 and is received within the fixed element 46. The longitudinal bore 44 comprises a fixed bore 44a, and a locking element bore 44b. The fixed bore 44a is of circular cross section and substantially constant diameter. The fixed bore 44a receives the annular protrusion 50 of the locking element 48. The fixed bore 44a includes an internal thread 56 that cooperates with the external thread 52 of the annular protrusion 50. The locking element bore 44b is of hexagonal cross section and has a diameter that is smaller than that of the fixed bore 44a.

Flat surfaces 60a and 60b are formed on opposite sides of the substantially hemispherical outer surface of the fixed element 46. Consequently, the diameter of fixed element 46 is reduced in the region between the flat surfaces 60a and 60b. Similar flat surfaces 62a and 62b are formed on opposite sides of the outer surface of the locking element 48. Consequently, the diameter of the locking element 48 is similarly reduced in the region between flat surfaces 62a and 62b.

The body portion 40 of the support element 6 carries an external thread 64. A flat surface 66 extends along the length of the body portion 40, as illustrated in Figure 3. The body portion 40 of the support element 6 is received within the openings of the mounting arms 14, 16 of the jig 4. Each of the openings (not shown) in the mounting arms 14, 16 has a flat surface that corresponds to the flat surface 66 on the body portion 40. The body portion 40 is thus prevented from rotating relative to the mounting arms 14, 16 by the interaction of the cooperating flat surfaces. The internal thread on the adjusting nut 34 engages the external thread 64 on the body portion 40 of the support element 6. Rotation of the adjusting nut 34 therefore causes translation of the body portion 40 in a substantially proximal/distal direction relative to the mounting arms14, 16.

Referring also to Figures 5, 6 and 7, the first cutting guide 8 is a substantially planar component having a proximal guide surface 70. A chamfered slot 72 extends in a substantially anterior/posterior direction across a medial portion of the first cutting guide 8. The internal walls 73 of the slot 72 are arcuate, such that the slot 72 tapers radially inwards towards the proximal 70 and distal 76 surfaces of the first cutting guide 8. The radial taper of the internal walls 73 of the slot 72 is such that the diameter of the proximal and distal lips 71 , 75 of the slot 72 is smaller than the diameter of the central region 77 of the slot 72. The diameter of the lips 71 , 75 is also smaller than the diameter of the head 42 of the support element 6. The thickness t of the first cutting guide 8 is increased in the region 74 of the slot 72 to increase the depth of the slot 72. The head 42 of the support element 6 is received within the chamfered slot 72 on the first cutting guide 6.

A plane slot 78 extends in a substantially medial/lateral direction across a lateral portion of the first cutting guide. Referring to Figure 8, the second cutting guide 10 is mounted above the plane slot 78 in the first cutting guide. The second cutting guide 10 comprises a body 80 having a guide surface 82 and a central bore 84 that extends through the body and includes an internal thread 88. The second cutting guide 10 is connected to the first cutting guide 8 via a screw 86. The screw 86 extends through the slot 78 and into the bore 84 on the second cutting guide 10, such that the thread on the screw 86 engages the internal thread 88 on the bore 84. A proximal face 90 of the head 92 of the screw 86 frictionally engages the distal face 76 of the first cutting guide to fix the position of the second cutting guide 10 relative to the first cutting guide 8. A recess 92 is formed in a proximal end of the screw 86. The recess 92 has an internal cross section in the form of a hexagon. The recess 92 may comprise a bore that extends the length of the screw 86.

Referring again to Figures 1 and 2, in order to assemble the instrument 2, the first and second cutting guides 8, 10 must be mounted on the head 42 of the support element 6, with the head 42 of the support element 6 inserted within the chamfered slot 72 on the first cutting guide. In order to insert the head 42 into the chamfered slot 72, the head 42 must be fully assembled, such that the annular protrusion 50 of the locking element 48 of the head 42 is screwed into the fixed bore 44a of the fixed element 46, with the distal face 54 of the locking element 48 in contact with a proximal face 55 of the fixed element 46. In this assembled state, each of the flat surfaces 60a and 60b is adjacent to a corresponding flat surface 62a and 62b, forming two pairs of flat surfaces (60,62)a and (60,62)b, and creating a reduced diameter region of the head 42 between the two pairs of flat surfaces (60,62)a and (60,62)b. The diameter of the distal lip 75 of the chamfered slot 72 is less than the diameter of the head 42. Consequently, in order to pass into the slot 72, the head 42 must be fully assembled and oriented such that the reduced diameter region of the head 42, between the pairs of flat surfaces (60,62)a and (60,62)b, extends across the width (W) of the slot 72. The head 42 may then enter the slot 72 and articulate within the confines of the internal walls 73 of the slot 72. The articulation of the head 42 within the slot 72 may include rotation within the slot 72 and translation along a longitudinal axis of the slot 72. Within the confines of the slot 72, rotation of the locking element 48 relative to the fixed element 46 and main body 40 causes each pair of flat surfaces (60,62)a and (60,62)b to be moved out of alignment, such that the reduced diameter region of the head no longer exists. The head 42 is thus prevented from exiting the slot 72. Relative rotation of the fixed and locking elements 46, 48 also causes the locking element 48 to translate away from the fixed element 46, due to the action of the cooperating threads 52, 56. This expansion of the head 42 causes the external surfaces of the fixed element and locking element 46, 48 to frictionally engage the internal walls 73 of the chamfered slot 72 and thus fix the position of the first cutting guide 8 relative to the support element 6.

When it is desired to remove the head 42 from the slot 72, the head is again fully assembled to create the reduced diameter region between the pairs of flat surfaces (60,62)a and (60,62)b, and is oriented such that the reduced diameter region extends across the width (W) of the slot 72. In this orientation, the head 42 may be removed from the slot 72.

In use, the jig 4, of the instrument 2 of the present invention is attached to an anterior surface of a proximal tibia via two tibial pins (not shown). The tibial pins are attached to the tibia in a spaced arrangement, one atop the other, and the jig 4 is then mounted on the pins by threading each pin through a respective pair of attachment holes 26, 28.

In order to lock the jig 4 onto the pins, the attachment handle 31 is rotated, causing the attachment member 32 to translate towards the handle 31 , locking the pins between an engagement surface 94 of the attachment member 32 and the internal surfaces of the pairs of holes 26, 28. The attachment member 32 and tibial pins thus function in the manner of a dovetail joint.

The guide surfaces 70, 80 of the first and second cutting guides 8, 10 are then placed in the required positions and orientations. Successive adjustments fix the position of each component of the instrument 2 relative to another. Firstly, the support element 6, and thus the assembly of the first and second cutting guides 8, 10, is translated in a proximal/distal direction relative to the jig 4 by rotating the adjusting nut 34. The first cutting guide 8 may then be translated in an anterior/posterior direction relative to the support element 6. The first cutting guide 8 may be translated from a maximum anterior position, where the head 42 is situated in the posterior end of the slot 72, to a maximum posterior position, where the head 42 is situated in the anterior end of the slot 72. The first cutting guide 8 may also be rotated relative to the support element 6 about multiple axes of rotation, due to the spherical nature of the articulation between the head 42 and the slot 72. Once the first cutting guide 8 is in the desired position and orientation relative to the support element 6, the locking element 48 of the head is rotated relative to the fixed element 46 of the head 42, thus expanding the head 42 and fixing the position of the first cutting guide 8 relative to the support element 6. Rotation of the locking element 48 is effected by means of a key (not shown). The key comprises a stem having a hexagonal cross section that engages the hexagonal bore 44b in the locking element 48.

The position of the second cutting guide 10 is then fixed relative to the first cutting guide 8. The frictional engagement between the proximal surface 90 of the head 92 of the screw 86 and the distal surface 76 of the first cutting guide 8 is released by rotating the screw 86 relative to the second cutting guide 10. Rotation of the screw 86 is effected by means of a key (not shown). The key comprises a stem having a hexagonal cross section that engages the hexagonal recess 92 in the screw 86. The key may be the same key that is used to rotate the locking element 48 of the head 42. When the frictional engagement is released, the second cutting guide 10 and screw 86 may translate relative to the first cutting guide 8 in a medial/lateral direction along the slot 78, and may rotate relative to the first cutting guide 8 about a longitudinal axis of the screw 86. Once the second cutting guide 10 is in the desired position and orientation relative to the first cutting guide 8, the screw 86 is rotated via the key such that the surfaces 90, 76 of the head 92 of the screw 86 and the first cutting guide 8 are in frictional engagement, fixing the position of the second cutting guide 10 relative to the first cutting guide 8.

It will be appreciated that the relative positions of the components of the instrument 2 may be adjusted in any order and that fine tuning of the position and orientation of the components may involve successive adjustment of all the components until the desired positions and orientations are achieved.

The desired positions and orientations of the first and second cutting guides 8, 10 may be assessed by a surgeon using only skill and experience. Alternatively, the desired positions may be indicated using a computer navigation system, whereby the positions of the cutting guides are recorded and displayed on a screen, to be compared to previously obtained and displayed optimum positions. The positions of the cutting guides may be ascertained using marker arrays attached to the cutting guides and detected by designated reception and recoding equipment. Each marker array may comprise a plurality of marking elements, which may be reflectors, Light Emitting Diodes, or any other marking element known in the art of navigated surgery.

Claims

1 A surgical instrument comprising a jig, suitable to be fixed to a bone, a support element mounted within the jig and a first cutting guide mounted on the support element, wherein the support element has a substantially spherical head, which articulates with the first cutting guide and expands to fix the position of the first cutting guide relative to the support element.

2 A surgical instrument as claimed in claim 1 , wherein the head of the support element is received within an opening on the cutting guide.

3 A surgical instrument as claimed in claim 2, wherein the opening comprises a chamfered slot.

4 A surgical instrument as claimed in claim 3, wherein the slot extends in a substantially anterior/posterior direction.

5 A surgical instrument as claimed in any of the preceding claims, wherein the support element has a body portion, which is connected to the head and mounted within the jig.

6 A surgical instrument as claimed in claim 5, wherein the head of the support element comprises two, substantially hemispherical elements, a fixed element, connected to the body portion and a locking element connected to the fixed element.

7 A surgical instrument as claimed in claim 6, wherein the locking element is connected to the fixed element via a thread, such that rotation of the locking element causes the locking element to move away from the fixed element.

8 A surgical instrument as claimed in claim 6 or 7, wherein a planar face of the fixed element comprises a recess having an internal thread.

9 A surgical instrument as claimed in any one of claims 6 to 8, wherein an annular protrusion extends from a face of the locking element towards the fixed element. 10 A surgical instrument as claimed in claim 9, wherein the annular protrusion has an external thread, suitable for cooperation with the internal thread on the fixed element.

11 A surgical instrument as claimed in claim 10, wherein a surface of the locking element includes a recess.

12 A surgical instrument as claimed in claim 11 , wherein a longitudinal axis of the recess is coincident with the axis of rotation of the locking element.

13 A surgical instrument as claimed in claim 12, wherein the recess has an internal cross section in the form of a polygon.

14 A surgical instrument as claimed in claim 13, wherein the internal cross section is in the form of a hexagon.

15 A surgical instrument as claimed in any of the preceding claims, wherein the jig comprises a base and a pair of parallel mounting arms.

16 A surgical instrument as claimed in claim 15, wherein the base comprises a pair of substantially rectangular, spaced apart flanges defining an inter flange space therebetween.

17 A surgical instrument as claimed in claim 16, wherein the flanges extend in a substantially coronal plane when the instrument is mounted adjacent an anterior surface of a tibia.

18 A surgical instrument as claimed in claim 17, wherein a web extends between lateral edges of the flanges.

19 A surgical instrument as claimed in any one of claims 16 to 18, wherein first and second pairs of attachment holes extend through the superior and inferior regions of the flanges respectively. 20 A surgical instrument as claimed in claim 18 or 19 wherein an attachment pin is mounted within the web and extends in a substantially medial/lateral direction into the inter flange space between the first and second pairs of attachment holes.

21 A surgical instrument as claimed in claim 20, wherein the attachment pin is mounted such that it may rotate but not translate relative to the jig.

22 A surgical instrument as claimed in claim 20 or 21 , wherein the attachment pin carries an external thread.

23 A surgical instrument as claimed in any one of claims 20 to 22 wherein an attachment member is mounted on the attachment pin in the inter flange space.

24 A surgical instrument as claimed in claim 23, wherein the attachment member comprises an internally threaded bore in which the threaded attachment pin is received.

25 A surgical instrument as claimed in claim 23 or 24 wherein the height of the attachment member reduces towards the lateral side of the jig, such that the attachment member partially blocks at least a portion of each of the first and second pairs of attachment holes.

26 A surgical instrument as claimed in any one of claims 22 to 25, wherein the thickness of the attachment member is such that it is closely confined by the flanges.

27 A surgical instrument as claimed in any one of claims 20 to 26, wherein the attachment pin is formed integrally with an attachment handle that protrudes on the lateral side of the jig, such that rotation of the handle causes relative medial/lateral displacement of the attachment member and the jig.

28 A surgical instrument as claimed in any one of claims 15 to 27, wherein the mounting arms extend from a medial side of the base.

30 A surgical instrument as claimed in claim 29, wherein the mounting arms extend in a substantially transverse plane. 31 A surgical instrument as claimed in any one of claims 15 to 30, wherein aligned openings extend through each of the mounting arms.

32 A surgical instrument as claimed in claim 31 , wherein the body portion is received within the aligned openings in the mounting arms.

33 A surgical instrument as claimed in claim 32, wherein cooperating formations on the body portion and the openings prevent relative rotation between the body portion and the mounting arms.

34 A surgical instrument as claimed in claim 33, wherein the cooperating formations comprise planar regions on the internal surface of at least one opening and the external surface of the body portion.

35 A surgical instrument as claimed in claim 34, wherein the planar region on the external surface of the body portion extends the length of the body portion.

36 A surgical instrument as claimed in any one of claims 5 to 35, wherein an external surface of the body portion carries a thread.

37 A surgical instrument as claimed in claim 36, wherein an adjusting nut is mounted on the threaded external surface of the body portion between the mounting arms.

38 A surgical instrument as claimed in claim 37, wherein the nut is closely received between the mounting arms, such that rotation of the nut causes relative proximal/distal translation of the body portion and the mounting arms.

39 A surgical instrument as claimed in any of the preceding claims, further comprising a second cutting guide.

40 A surgical instrument as claimed in claim 39, wherein the second cutting guide is mounted on a proximal guide surface of the first cutting guide.

41 A surgical instrument as claimed in claim 39 or 40, wherein the second cutting guide comprises an internally threaded bore that extends through the second cutting guide. 42 A surgical instrument as claimed in any one of claims 39 to 41 , wherein the second cutting guide is mounted over an opening that extends through the first cutting guide.

43 A surgical instrument as claimed in claim 42, wherein a screw extends through the opening in the first cutting guide and is received within the threaded bore in the second cutting guide such that a proximal surface of the head of the screw engages a distal surface of the first cutting guide.

44 A surgical instrument as claimed in claim 42 or 43, wherein the opening in the first cutting guide is a slot.

45 A surgical instrument as claimed in claim 44, wherein the slot extends in a substantially medial/lateral direction.

46 A surgical instrument as claimed in any one of claims 43 to 45, wherein a proximal end of the screw comprises a recess.

47 A surgical instrument as claimed in claim 46, wherein the recess is in the shape of a polygon.

48 A surgical instrument as claimed in any one of the preceding claims, wherein each of the locking element and the fixed element comprises at least one partially flat surface.

49 A surgical instrument as claimed in claim 48, wherein the opening on the cutting guide is shaped such that the head of the support element can only be admitted to the opening when the flat surfaces are disposed adjacent one another.

50 A surgical instrument as claimed in any one of the preceding claims, wherein a marker is removably attached to at least one of the first and second cutting guides.

51 A surgical instrument as claimed in claim 50, wherein the marker comprises an array of a plurality of marking elements. 52 A surgical instrument as claimed in claim 51 , wherein the marking elements comprise reflective elements or light emitting diodes.

53. A joint comprising a socket and a ball received within the socket, the ball comprising first and second sections which are connected for relative rotation via a thread, wherein relative rotation between the first and second sections causes the ball to expand and frictionally engage the socket.

54 A surgical instrument substantially as described herein with reference to, and as shown in, the accompanying drawings.