WO2012110783A1

WO2012110783A1 - A surgical instrument

Info

Publication number: WO2012110783A1
Application number: PCT/GB2012/050265
Authority: WO
Inventors: Xiao Hu; Toby Hunt; Andrew Taylor
Original assignee: Finsbury (Development) Ltd
Priority date: 2011-02-16
Filing date: 2012-02-07
Publication date: 2012-08-23
Also published as: GB201102699D0

Abstract

An instrument for forming a slot in an exterior surface of an end of a bone and method of use are described. The instrument comprises a body arranged to contact an end of a bone and shaped such that the position of the body relative to a bone axis extending into the end of the bone is controlled. The body defines at least one drill guide arranged such that a drill bit can extend through the drill guide to cut a slot intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis. A method of forming a slot in an exterior surface of an end of a bone includes coupling the body to an end of a bone such that the body contacts the end of a bone and drilling a slot through the drill guide intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis. The method can be used to prepare the end of the bone to receive a resurfacing implant.

Description

A Surgical Instrument

The present invention relates to a surgical instrument. In particular, but not exclusively, the present invention relates to a surgical instrument for use in preparing an end of a bone prior to implanting a resurfacing implant to replace an articulating surface. The

resurfacing implant may, for instance, be used to replace the articulating surface of the femoral component of a hip joint. More particularly, the instrument is intended to reduce the pressure of bone cement between a prepared end of a bone and a resurfacing implant during implantation.

In a joint replacement procedure it is known to replace only the articulating surface at the end of a bone. Such an implant is known as a resurfacing implant. The bone tissue that provides the ball component of a ball and socket joint (for example the humeral component of a shoulder joint or the femoral component of a hip joint) is prepared to receive a cap- like component having an outer bearing surface which can be received in the socket component of the joint prosthesis, or can bear against the natural bone socket. Preparation of the bone involves shaping the bone tissue of the ball component so that its external shape is approximately the same as the shape of a cavity within the resurfacing implant. Typically, the cavity is rotationally symmetrical. Preparation typically further comprises drilling a bore in the end of the bone in which a central locating implant pin of the resurfacing implant can be inserted during implantation.

The resurfacing implant is implanted over the prepared bone end and secured in position using an impaction force creating a press fit, or using bone cement. Where bone cement is used it is important to carefully control the volume of bone cement between the implant and the bone. Too much bone cement can cause the cement to excessively penetrate into the cut bone. Furthermore, if there is an excessive volume of bone cement then as the cement cures the high temperatures generated can cause damage to the bone, potentially resulting in necrosis, especially if there is excessive penetration of the bone cement into the bone. Excessive penetration is caused by the resurfacing implant creating a sealed volume as the edges of the cap pass over the prepared end of the bone, which pressurises the bone cement as the implant moves further over the end of the bone. A further problem associated with an uncontrolled volume of bone cement between the implant and the bone is that this can cause the implant to be improperly seated on the bone due to the bone cement going off too quickly. The result is that a larger impaction force is required by the surgeon, which can cause micro cracks in the femoral neck and increase the risk of early neck failure after surgery, and / or incorrect final geometry of the implant.

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. In particular, it is an object of embodiments of the present invention to ensure a controlled pressurised volume of bone cement during implantation of a resurfacing implant.

According to a first aspect of the present invention there is provided an instrument for forming a slot in an exterior surface of a bone, the instrument comprising: a body arranged to contact an end of a bone and shaped such that the position of the body relative to a bone axis extending into the end of the bone is controlled; wherein the body defines at least one drill guide arranged such that a drill bit can extend through the drill guide to cut a slot intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis. An advantage of the first aspect of the present invention is that the instrument is operable to form a slot in the exterior surface of an end of a bone. The prepared end of the bone can be received in the cavity of a resurfacing implant. The slot in the bone surface allows excess bone cement to escape when a resurfacing implant is secured to the end of the bone. This prevents excessive pressurisation of the bone cement and ensures a controlled volume of cement is in place between the bone and the implant. This reduces damage to the bone during implantation of the resurfacing implant.

The body may comprise a hole arranged to pass over an alignment rod protruding from a bore extending into the end of the bone along the bone axis or an alignment rod extending from the body and arranged to be received in a bore extending into the end of the bone along the bone axis. -J-

The body may define a plurality of drill guides arranged to allow slots to be cut at varying offsets from bone axis.

The or each drill guide may define an axis along which a drill bit is guided such that in use said the or each drill guide axis extends parallel to the bone axis or at a predetermined inclination to the bone axis.

The or each drill guide may comprise a stepped portion defining a narrower diameter than a second portion of the drill guide arranged to allow a first portion of a drill bit to pass through, but to stop an enlarged portion of a drill bit from passing through.

The body may comprise a generally planar plate arranged to rest on the end of the bone or spaced apart from the end of a bone by a block coupled to the plate. The plate may have a knurled edge arranged to be engaged by a surgeon to rotate the plate relative to the end of a bone.

The body may further comprise a second plate spaced apart from the first plate and coupled to the first plate by a neck portion, the second plate defining one or more guide holes corresponding to the or each drill guide and arranged such that a drill bit can extend through a guide hole and a corresponding drill guide.

According to a second aspect of the present invention there is provided a surgical instrument set comprising: a surgical instrument according to any one of the preceding claims; and a drill bit having a cutting part arranged to pass through the or each drill guide and a coupler arranged to couple to a rotary driver.

The drill bit may comprise an enlarged portion arranged to limit the extent of insertion of the cutting part through the or each drill guide. According to a third aspect of the present invention there is provided a method of forming a slot in an exterior surface of an end of a bone, the method comprising: coupling a body to an end of a bone such that the body contacts the end of a bone, the body being shaped such that the position of the body relative to a bone axis extending into the end of the bone is controlled; and drilling a slot through the drill guide intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis. The method may further comprise forming a bore extending into an end of a bone; and inserting an alignment rod into the bore such that it protrudes from the bore, the alignment rod being coupled to or couplable to a body defining a drill guide.

According to a fourth aspect of the present invention there is provided a method of preparing the end of a bone to receive a resurfacing implant, the method comprising: reaming the exterior surface of an end of a bone to remove at least part of an articulating bone surface and to form a first surface generally normal to the axis of the end of the bone and a second surface generally parallel to the axis of the end of the bone and extending about the axis; forming a slot in the second surface according to the method of claim 10; applying bone cement to the first and second surfaces of the bone or to the interior of a resurfacing implant defining an internal cavity; inserting the end of the bone into the cavity of the resurfacing implant; and removing excess bone cement extruded through the slot as the resurfacing implant is driven home over the end of the bone. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates in cross section a conventional resurfacing implant during implantation over a prepared end of a bone forming the ball component of a ball and socket joint;

Figure 2 illustrates a surgical instrument according to a first embodiment of the present invention;

Figure 3 illustrates the instrument of figure 2 in use to form a slot in the side of a prepared end of a bone; Figure 4 illustrates in partial cross section a resurfacing implant part way during implantation over a prepared end of a bone including a slot formed as illustrated in figure

Figure 5 illustrates in partial cross section the resurfacing implant of figure 4 after the implantation is complete;

Figure 6 illustrates a surgical instrument according to a second embodiment of the present invention; and

Figure 7 illustrates in cross section the surgical instrument of figure 6.

Referring to figure 1 , this schematically illustrates a conventional resurfacing implant 2 which forms part of an orthopaedic joint prosthesis. In particular, resurfacing implant 2 forms the femoral component of a hip joint prosthesis and provides a partially spherical articulating surface 4 that is received within an acetabulum implant (not shown) or bears against the natural acetabulum. The resurfacing joint prosthesis 2 provides a hard wearing bearing surface 4 on the head of a femur 6. Figure 1 is illustrated in cross section, and further illustrates the prepared end of the femur 6. The prepared femur 6 is received within an axial cavity 8 formed within the resurfacing implant 2. The axial cavity 8 is generally cylindrical or conical about the axis of the implant 2. The resurfacing implant 2 further comprises an axial implant pin 10 which is received within an axial bore 12 within the prepared end of the femur 6. The bore may be cylindrical or may taper toward its closed end. The implant pin 10 serves to ensure that the resurfacing implant 2 is correctly aligned with the prepared end of the femur 6 before the implant 2 is implanted.

Conventionally, the preparation of the head of the femur 6 to receive the resurfacing implant 2 involves first forming a bore 12 along or at a desired angle to the axis of the femoral head to receive the implant pin 10. The external surface of the femur 6 is prepared by the formation of three rotationally symmetrical cut surfaces 16, 1 8 and 20 surrounding the bore. A first surface 1 6 is generally aligned parallel or close to parallel to the axis of the bore 12. The second surface extends between the first surface and a third surface. The third surface is generally formed as a plane normal to the axis of the bore 12.

Once the femoral head is prepared, bone cement 22 is inserted into cavity 8 of the implant 2 and / or bone cement is applied to the cut surfaces 16, 1 8, 20. The resurfacing implant 2 is positioned over the femoral head such that the implant pin 10 is aligned with the bore 12 and the implant 2 is driven onto the femur 6. The resurfacing implant 2 is driven along the axis of the implant pin 10 until the implant 2 is fully seated upon the femur 6 as shown in figure 1 .

It will be appreciated that as the edge 24 of the implant 2 passes over the end of the bone 6 the edge 24 will contact cut surface 16 forming a substantially closed volume containing bone cement 22 within the implant cavity 8 between the implant 2 and cut surfaces 18, 20. As the implant 2 is further driven onto the end of the bone the closed volume becomes pressurised. This can cause the bone cement 22 to excessively penetrate into the bone as shown in the cross section of figure 1. As the bone cement cures, the high temperature can lead to necrosis.

Referring now to figure 2, this illustrates a surgical instrument in accordance with a first embodiment of the present invention which is arranged to form a slot in an exterior surface of the end of a bone. Specifically, the instrument is arranged to form a slot in cut surface 16. The slot serves to allow a certain amount of bone cement to escape from cavity 8 during implantation, thereby relieving the build up of pressure, as will be described in greater detail below.

Figure 2 illustrates the instrument comprising a body 30 in the form of a substantially circular plate having a hole 32. Hole 32 is generally in the middle of plate 30, or exactly in the middle of plate 30, and defines an axis extending generally normally to the plate 30. The hole 32 is arranged to pass over an alignment rod 34. As illustrated in figure 3, which illustrates bone 6 as transparent to reveal internal detail, the alignment rod is arranged to be received in bore 12 in the end of the bone such that the rod extends along the axis defined by bore 12. It will be appreciated that in alternative embodiments the alignment rod 34 may be permanently secured to the plate 30.

Plate 30 further defines a series of drill guides 36 extending through the plate and situated about the hole 32 at varying radial distances from the centre of hole 32. The plate 30 is provided with a series of marks 38 for each drill guide 36. The required drill guide 36 is determined according to the size of the resurfacing implant to be used, and hence the prepared size of the end of the bone 6. Figure 2 shows a drill bit 40 extending through a first such drill guide 36. The drill bit 40 has an enlarged portion 42 which cannot pass through the drill guide 36, which thereby serves to limit the depth of insertion of the drill bit 40. Referring to figure 3, this illustrates the instrument of figure 2 mounted on the end of a bone 6 and in use to cut a slot in an exterior surface of the bone. By choosing an appropriate drill guide 36 according to the dimensions of the prepared end of the bone (determined by the chosen resurfacing implant) the trajectory of the drill bit 40 as it extends through drill guide 36 can be chosen to intersect cut surface 16. Specifically, the drill bit 40 engages cut surface 16 to form a slot 44 (not directly visible in figure 3, but visible in the partial cross sections of figures 4 and 5). That is, preferably the slot is open to the exterior of the bone. The length of the slot 44 is determined by the length of the drill bit 40 that can extend through drill guide 36 and the relative dimensions of cut surfaces 16, 18. Figure 3 shows that the plate 30 is coupled to a block 46 which lifts the plate 30 off the end of the bone, however it will be appreciated that this is not essential.

It will be appreciated that where cut surface comprises a generally cylindrical surface extending about the bore 12 then the slot 44 will comprise a part-cylindrical trench of constant depth along its length. The distal end of the slot 44 may taper to a point corresponding to the shape of the end of the drill bit 40. However, if surface 16 is not parallel to bore 12 then the slot may vary in depth. Indeed in certain embodiments the may form a bore with a closed cross section within the bone for part or all of its length before exiting through the side wall of the bone. As illustrated in figures 2 and 3 the drill guides 36 each define drilling axes which are parallel to the axis of hole 32, but at differing radial distances from hole 32. However, in other embodiments some or all of the drill guides 36 may define drilling axes which are inclined relative to the axis of hole 32 (which may itself be inclined relative to the plane of plate 30).

Referring now to figure 4 this illustrates a resurfacing implant 2 partially positioned over the end of a bone. A slot 44 has been fonned in cut surface 16 using the instrument of figure 2. It can be seen that as the implant 2 is pushed downwards over the end of the bone, as indicated by arrow 48, excess bone cement 22 can flow from cavity 8, through the slot and to the exterior of the implant 2, where it can be removed. Consequently the pressure of the bone cement in cavity 8 is prevented from rising excessively. The pressure may be partly determined by the dimensions of slot 44, and hence the rate at which bone cement 22 is allowed to escape.

Typically, the slot may be arranged to pass along approximately half of cut surface 16, as determined by the length of drill bit 40 passing through drill guide 36. Typically, the slot 44 is fonned anteriorly or posteriorly to allow ready access for the surgeon to remove bone cement 22 extruded through slot 44. The length of slot 44 is chosen such that the slot 44 is sealed by the implant 2 (as shown in figure 5) a few millimetres before the implant 2 is fully seated. This prevents further loss of bone cement 22. By accurate detennination of the position of the slot 44 relative to the final implant position of the implant 2, the implant 2 may be properly seated on the end of the bone with a controlled pressurisation of the bone cement, without requiring the amount of bone cement to be exactly measured.

Consequently the cement volume and pressure may be controlled to prevent excessive penetration into the cut bone.

Refen'ing now to figures 6 and 7, these illustrate a second embodiment of a surgical instrument in accordance with the present invention. The instmment is generally similar to that of figures 2 and 3 except that the body comprises a first plate 30 and a second plate 50 coupled through a neck 52. Hole 32 extends through the neck 52. Plate 50 comprises a series of guide holes 54 aligned with drill guides 36 within plate 30. As guide holes 54 and drill guides 36 are spaced apart the trajectory of drill bit 42 as it extends towards the bone is more accurately controlled owing to the drill bit 40 being guided at two spaced apart points. Markings 38 are provided on the upper plate 50 to indicate the drill guide to be used for each respective resurfacing implant.

Drill guides 36 further comprise a step 56 to limit the extent to which the enlarged portion 42 can extend through the drill guide 36 (the guide holes 54 are sufficiently large to pass the enlarged portion 42). It will be appreciated that the stepped drill guides 36 may also be applied to the plate 30 for the instrument of figures 2 and 3.

For both embodiments of the instrument, plate 30 has a knurled edge 58. The knurled edge 58 makes it easier for the surgeon to rotate the plate 30 when in position on the end of the bone so that the appropriate drill guide 36 can be selected and positioned above the portion of the cut surface 16 where the slot 44 is required.

In alternative embodiments of the invention a surgical instrument may be arranged to position a drill guide relative to a prepared end of a bone without using an alignment rod inserted into a central bore. If an end of a bone is prepared to receive a resurfacing implant which does not have a central implant pin then there may be no need to form a bore in the end of the bone. Regardless of whether there is a bore in the end of the bone, the claims of the present specification are intended to encompass any instrument for forming a slot in a surface of an end of a bone and including a mechanism for locating a drill guide relative to the axis of the end of the bone. As an example, a prepared end of a bone may be machined to form a rotationally symmetrical shape (and in particular a cylindrically machined bone). A surgical instrument may comprise three spaced apart rods that extend from the body of the instrument to contact the sides of the bone ensuring that the instrument is placed concentrically on the end of the bone and the or each drill guide is positioned correctly relative to the surface of the bone where the slot is required. While the present invention has been predominantly described herein in the context of the preparation of the end of a femur in order to receive an implanted resurfacing implant onto the head of a femur, it will be readily apparent the invention is not limited to this. The present invention may be readily applied for preparing a bone in any situation in which an implant is to be applied to the end of a bone, for instance a resurfacing humeral implant forming part of a prosthetic shoulder joint. Furthermore, the present invention has been described above in connection with forming a slot in a particular cut surface extending about the end of a bone. However, more generally the present invention is arranged to form a slot in any exterior surface of an end of a bone, whether a natural surface or otherwise. Further advantages and applications of the present invention will be readily apparent to the appropriately skilled person without departing from the scope of the appended claims.

Claims

CLAIMS:

1 . An instrument for forming a slot in an exterior surface of an end of a bone, the instrument comprising:

a body arranged to contact an end of a bone and shaped such that the position of the body relative to a bone axis extending into the end of the bone is controlled;

wherein the body defines at least one drill guide arranged such that a drill bit can extend through the drill guide to cut a slot intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis.

2. An instrument according to claim 1 , wherein the body comprises a hole arranged to pass over an alignment rod protruding from a bore extending into the end of the bone along the bone axis or an alignment rod extending from the body and arranged to be received in a bore extending into the end of the bone along the bone axis.

3. An instrument according to claim 1 or claim 2, wherein the body defines a plurality of drill guides arranged to allow slots to be cut at varying offsets from the bone axis.

4. An instrument according to any one of the preceding claims, wherein the or each drill guide defines an axis along which a drill bit is guided, such that in use said the or each drill guide axis extends parallel to the bone axis or at a predetermined inclination to the bone axis.

5. An instrument according to any one of the preceding claims, wherein the or each drill guide comprises a stepped portion defining a narrower diameter than a second portion of the drill guide arranged to allow a first portion of a drill bit to pass through, but to stop an enlarged portion of a drill bit from passing through.

6. An instrument according to any one of the preceding claims, wherein the body comprises a generally planar plate arranged to rest on the end of the bone or spaced apart from the end of a bone by a block coupled to the plate.

7. An instrument according to claim 6, wherein the plate has a knurled edge arranged to be engaged by a surgeon to rotate the plate relative to the end of a bone.

8. An instrument according to claim 6 or claim 7, wherein the body further comprises a second plate spaced apart from the first plate and coupled to the first plate by a neck portion, the second plate defining one or more guide holes corresponding to the or each drill guide and arranged such that a drill bit can extend through a guide hole and a corresponding drill guide.

9. A surgical instrument set comprising:

a surgical instrument according to any one of the preceding claims; and a drill bit having a cutting part arranged to pass through the or each drill guide and a coupler arranged to couple to a rotary driver.

10. A surgical instrument set according to claim 9, wherein the drill bit comprises an enlarged portion arranged to limit the extent of insertion of the cutting part through the or each drill guide.

1 1. A method of forming a slot in an exterior surface of an end of a bone, the method comprising:

coupling a body to an end of a bone such that the body contacts the end of a bone, the body being shaped such that the position of the body relative to a bone axis extending into the end of the bone is controlled; and

drilling a slot through the drill guide intersecting an exterior surface of the bone at a predetermined trajectory and offset from the bone axis.

12. The method of claim 1 1 , further comprising:

forming a bore extending into the end of the bone along the bone axis; and inserting an alignment rod into the bore such that it protrudes from the bore, the alignment rod being coupled to or couplable to the body.

13. A method of preparing the end of a bone to receive a resurfacing implant, the method comprising:

reaming the exterior surface of an end of a bone to remove at least part of an articulating bone surface and to form a first surface generally normal to the axis of the end of the bone and a second surface generally parallel to the axis of the end of the bone and extending about the axis;

forming a slot in the second surface according to the method of claim 1 1 ;

applying bone cement to the first and second surfaces of the bone or to the interior of a resurfacing implant defining an internal cavity;

inserting the end of the bone into the cavity of the resurfacing implant; and removing excess bone cement extiuded through the slot as the resurfacing implant is driven home over the end of the bone.