WO2008099176A1

WO2008099176A1 - An instrument assembly for cutting a tunnel in a bone

Info

Publication number: WO2008099176A1
Application number: PCT/GB2008/000507
Authority: WO
Inventors: Alec Paul Birkbeck
Original assignee: Depuy International Limited
Priority date: 2007-02-15
Filing date: 2008-02-14
Publication date: 2008-08-21
Also published as: GB0702947D0

Abstract

An instrument assembly for cutting a tunnel in a bone, which comprises a cutting tool, and a guide for controlling the cutting tool so that the tunnel which it cuts in the bone follows a predetermined curved path.

Description

AN INSTRUMENT ASSEMBLY FOR CUTTING A TUNNEL IN A BONE

This invention relates to an instrument assembly for cutting a tunnel in a bone.

Avascular necrosis (AVN), which is also known as osteonecrosis (ON), ischemic bone necrosis, or aseptic necrosis, results from the temporary or permanent loss of circulation to the bone tissue, and gives rise to localized death of the bone tissue. The loss of proper blood flow can result from trauma, or compromising conditions such as prolonged steroid use, alcohol use, gout diabetes, pancreatitis, venous occlusion, decompression disease, radiation therapy, chemotherapy, and Gaucher's disease.

The condition can give rise to severe pain and limitation of movement within a short period, with a 70 to 80% chance of complete collapse of the bone and surrounding articulating surface after three years if left untreated. Joint replacement can be necessary for many patients.

Treatments for AVN which focus on salvaging the head of the femur or other bone or joint include core decompression, osteomy, bone grafting, and vascularized fibular grafting.

US-6679890 discloses a method and device for treating AVN of the femoral head. The device disclosed in US-6679890 augments the femoral head with bone cement. An open ended and fenestrated tube is inserted through a hole into the femoral neck and uncured bone cement is injected and cured at high pressure. The tube for injection of the cement is straight.

It is found in many patients that the bone tissue which is affected adversely by AVN can tend to be in the superior region of the femoral head. It can be difficult to gain access to this region of the femoral head through a straight access tunnel because of impingement with cortical bone in the region of the femoral neck. The present invention provides an instrument assembly for cutting a tunnel in a bone which includes a guide for controlling a cutting tool so that the tunnel which it cuts in the bone follows a predetermined curved path.

Accordingly, in one aspect, the invention provides an instrument assembly for cutting a tunnel in a bone, which comprises a cutting tool, and a guide for controlling the cutting tool so that the tunnel which it cuts in the bone follows a predetermined curved path.

The assembly of the invention has the advantage that it enables access to the superior region of the femoral head, without risking weakening of the femur in the region of the neck. Access to the superior region of the head is possible through a smaller tunnel than might be necessary when the tunnel is straight. This can help to minimise the trauma suffered by the patient as a result of the procedure.

The curved tunnel can be used to cut away bone tissue which has been affected adversely by the AVN condition. For example, the tunnel can be used to direct a cutting instrument to bone tissue in which blood circulation might be reduced compared with healthy bone tissue.

The curved tunnel can be used to supply material to the region of the femoral head that is affected by the AVN condition in order to treat that condition. The material can be provided to support healthy bone which surrounds a region in which bone tissue has reduced blood circulation or from which bone tissue with reduced blood circulation has been cut away. Material to support bone might include, for example, a support device comprising a plurality of filaments of a resilient material or a sheet of a resilient material, which can be deployed against the healthy bone tissue to support it. Examples of suitable materials for such a support include certain metals and certain polymers, which have suitable biocompatibility. Amongst useful metals, it can be preferred to used certain shape memory alloys which exhibit enhanced elastic characteristics. Suitable alloys include those based on nickel and titanium, sometimes referred to as Nitinol alloys. Other metals which might be used include certain stainless steels, titanium and its alloys. Polymeric materials which might be used can include biocompatible polymers such as polyolefins (especially polyethylenes), polycarbonates, polyesters and so on, optionally reinforced with fillers which might be particulate or fibrous. It can be preferred for the support to be located within a cavity in the bone while in a collapsed configuration, from which it expands once located in the cavity into a deployed configuration.

Material to support bone might include a filler material which can be deployed in a cavity within a bone from which tissue affected by the AVN condition has been removed. Examples of materials which might be deployed in a cavity in this way includes a bone graft material, and curable polymeric materials such as are used in orthopaedic applications including curable acrylate based compositions, optionally containing appropriate inorganic fillers such as calcium phosphates.

The curved tunnel can be used to deploy a cutting tool which can cut defective bone tissue, so as to form a cavity within a bone. The cavity can be formed at the end of the curved tunnel which is formed using the assembly of the invention, or at one or more points along the length of the tunnel.

It will generally be preferred for a combination of a support component and a filler material to be provided within a bone in a procedure in which a curved tunnel is made. For example, a support component can be deployed to support bone tissue which surrounds a cavity which is cut using a cutting tool which is deployed through the curved tunnel. A filler material can be provided within such a cavity or within the tunnel or, more preferably, within one or more cavities and the tunnel.

Preferably, the cutting tool comprises a rotating cutting head. Such a cutting head can rotate about the axis along which the cutting tool is advanced along the curved path. The cutting head will preferable have cutting teeth on the face which is directed along the curved path. Preferably, the face which is directed along the curved path is rounded, for example having an overall shape which is part-spherical.

The cutting head should be selected according to the size of the tunnel which is to be cut in the bone. Preferably, the diameter of the cutting head is at least about 3 mm, more preferably at least about 5 mm, for example at least about 7 mm. Preferably, the diameter of the cutting head is not more than about 20 mm, more preferably not more than about 15 mm, for example not more than about 10 mm.

Preferably, the guide tube is curved with an approximately constant radius along at least part of its length. Preferably, the constant radius is at least about 40 mm, more preferably at least about 50 mm, for example at least about 60 mm. Preferably, the constant radius is not more than about 120 mm, more preferably not more than about 100 mm, for example not more than about 80 mm.

The guide tube can have a closed cross-section, along at least part of its length. It can be particularly preferred that the guide tube has a closed cross-section at the end at which the cutting head is located to optimise the support for the cutting head.

The guide tube can have a slot formed in it extending along at least part of its length, especially so that the guide tube is open along that part of its length. A drive shaft can be inserted into the guide tube through such a slot. The faces of the slot can be machined to provide teeth, which can be engaged by a gear component of a drive unit.

A rotating cutting tool can be driven by means of a drive shaft which can rotate about its axis. The drive shaft should be flexible to enable it to be advanced along the curved path. Suitable drive shafts are known, for example which comprise a rotating core within a sheath provided by woven or braided or wound fibres or filaments. Preferably, the assembly of the invention includes a drive device for causing the cutting head to rotate, especially a powered drive device, for example as might be used to drive other rotating cutting tools such as drill bits or reamers.

Preferably, the guide comprises a curved guide tube which can be advanced into the tunnel behind the cutting tool. Such a guide tube can provide support for the drive shaft while it rotates to impart rotational drive to the cutting tool. The guide tube should support the cutting head to control its orientation during operation. For example, when the cutting tool comprises a rotating cutting head, it can be preferred for the cutting head to act against an axially facing surface at or towards the end of the guide tube as the head rotates. It can be preferred for the cutting head to be attached to the end of the cutting tube in such a way that it can rotate relative thereto, for example by means of a circlip or similar device.

The assembly can include a mounting plate which can be fastened to the bone. Preferably, the mounting plate includes at least one adjuster for adjusting the orientation of the guide tube. It can be preferred to provide more than one adjuster, for example to adjust the orientation of the guide tube in two planes, especially planes which are approximately orthogonal to one another. For example, the guide tube can be mounted pivotally on the mounting plate, and the pivotal movement of the guide tube relative to the mounting plate can be controlled by means of an adjustment screw.

The assembly can include an incremental drive unit for advancing the guide tube into the tunnel. The incremental drive unit can be provided on the mounting plate.

The incremental drive unit can comprise a rack and pinion assembly. It will generally be preferred for the rack to be provided on the guide tube, for example in the form of a plurality of spaced apart teeth on the external surface of the tube, which can be engaged by the teeth on the pinion. The pinion can be mounted on the mounted plate for rotation about its axis. It can be preferred in many circumstances for the guide tube can be advanced by manual activation of the drive unit, for example by manually driving the pinion of a rack and pinion assembly.

The instrument of the invention can be made from materials which are conventionally used in the manufacture of surgical instruments. Examples of such materials include metals such as certain stainless steels.

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

Figure 1 is a view along the anterior posterior axis of the head of a femur, with an instrument assembly according to the invention in place relative thereto. Figure 2 is an isometric view, partially in section, of another embodiment of instrument assembly in accordance with the invention.

Figure 3 is an enlarged view of the cutting head of the instrument of the invention, on the end of the guide tube.

Figure 4 is a view along the anterior posterior axis of the head of a femur in an initial stage of a procedure for treatment of avascular necrosis.

Figure 5 is a view along the lateral medial axis of the head of the femur at the same stage in the procedure as is shown in Figure 4.

Figure 6 is an enlarged view of the mounting plate of the instrument assembly shown in Figure 2.

Referring to the drawings, Figure 1 shows the superior end of a femur 2, including the head 4 and the femoral neck 6. The head of the femur is rounded for articulation with the internal rounded surface of the acetabulum.

Bone tissue within the head of a femur which is subject to avascular necrosis becomes subject to reduced blood flow, which can result in weakening of the bone tissue and eventually to the bone tissue dying. This is shown in Figure 1 in a region 8 which is located superiorly. Treatment of the affected bone tissue requires access to this superior region of the femoral head. Access from the lateral side of the femur through a straight tunnel risks weakening the femoral neck, especially on the superior side thereof.

The instrument of the present invention enables a curved tunnel 10 to be formed in the femur, allowing access to the superior region of the femoral head through a tunnel which can be spaced apart from cortical bone in the region of the femoral neck 8.

The instrument of the invention includes a mounting plate 10 which is shaped so that it can be placed stably against the cortical tissue of the femur on the lateral side thereof. The mounting plate has holes formed in it for receiving two or more bone screws 12 by which the mounting plate can be held in place on the femur.

The instrument includes a guide tube 14 with a bore which has a circular cross-section. The guide tube is curved with a radius (measured to the centerline of the bore) of about 75 mm. The guide tube has a series of teeth 16 formed in its outer wall, facing towards the inside of the curve. The teeth are uniformly spaced apart, with each tooth arranged perpendicular to a line which extends along the tube. The guide tube is formed from stainless steel. Its external diameter is about 7.5 mm and its wall thickness (other than where the teeth are provided) is 0.7 mm.

The instrument includes a cutting head 18 which is located at an end of the guide tube so that its position and orientation can be controlled using the guide tube. The cutting head has a rounded external surface, with outwardly directed cutting teeth. The diameter of the cutting head where it contacts the guide tube is 8 mm. Such cutting tools are known for use in orthopaedic and other surgical applications. The cutting head 18 is arranged to rotate about its axis on the end of the guide tube. The instrument includes a drive shaft 19 which extends through the guide tube, by which rotational drive originating with a powered drive device (not shown). The cutting head 18 is retained on the end of the guide tube by means of a circlip 20. The circlip can be received in a slot 21 in wall of the guide tube which extends partially around the wall of the guide tube. The circlip engages a groove 23 in a spigot 25 on the cutting head. This is shown in Figure 3.

The mounting plate 10 carries a support assembly for the guide tube. The support assembly defines a pathway for the guide tube to slide along, between first and second jaws 22, 24 in which the guide tube is a sliding fit. The upper jaw 24 carries a pinion 26 which has a plurality of teeth arranged on its outer periphery. The spacing between the teeth on the pinion 26 corresponds to the spacing between the teeth 16 on the wall of the guide tube. The pinion is located so that the teeth on the pinion can engage the teeth on the wall of the guide tube as the guide tube is advanced between the jaws 22, 24. Figure 2 shows another embodiment of instrument assembly according to the invention. In this embodiment, the guide tube 114 has a slot 140 formed in its side wall. The edges 142, 144 of the slot are profiled to provide an array of teeth.

The mounting plate has a central hole 146 formed in it for receiving a bone screw, by which the mounting plate can be fastened to the patient's bone tissue.

The support assembly for the guide tube comprises a moulded plastic housing 148 which is curved, and sized so that the guide tube 114 is a sliding fit. The housing 148 carries a barrel shaped gear 150 having outwardly facing teeth which are appropriate spaced apart to mesh with the teeth which are provided on the edges 142, 144 of the slot in the guide tube. The support assembly includes a drive handle 152 by which the barrel shaped gear 150 can be made to rotate about its axis.

Figure 4 illustrates a preparatory step in a surgical procedure to prepare the head of a femur 200 for treatment of avascular necrosis in a region 202 of the head. The region has the shape of a segment of a sphere, located in the superior region of the head. As is known, the region can be identified and located using appropriate scanning equipment.

The preparatory step of the procedure involves inserting a reference pin 204 so that the head 206 of the pin is located in the region 202 of the head which is affected by the avascular necrosis. The pin is so located by creating a bore in the head which extends from the lateral cortex 208. The pin which is inserted into the femoral head can have cutting teeth so that it can be used to create the bore in the head. Alternatively, the bore can be cut using a conventional drill bit, which is removed from the bore to allow the reference pin 204 to be inserted.

The proper orientation and length of the bore can be monitored during the drilling step by scanning the bone, for example fluoroscopically, to ensure that the end of the bore is located in the region 202 of the head which is to be treated. The reference pin has markings 210 on it, at least in the portion of its length which protrudes from the lateral cortex when the end of the pin is located in the region 202 of the head which is to be treated. The markings enable the length of the pin, measured from the lateral cortex to the region 202 of the head which is to be treated, to be measured.

The reference pin has a thread on its external surface, at least in the portion of its length which protrudes from the lateral cortex when the end of the pin is located in the region 202 of the head which is to be treated.

A mounting plate 212 is positioned on the lateral cortex. The mounting plate has an opening extending through it for receiving the reference pin 204. The mounting plate has a pair of fixation holes for receiving fixation pins 216, by which the mounting plate can be fastened to the lateral cortex of the femur.

In order to create a tunnel which extends accurately to the region 202 of the head which is to be treated, it is important to control the angle relative to the reference pin 204 at which the cutting head initially penetrates the bone. The angle is determined for a guide tube of selected radius with reference to the distance from the lateral cortex to the region 202 of the head which is to be treated which is measured using the reference pin 204. The surgeon can be provided with appropriate reference data to enable this angle to be determined according to the configuration of the head of a particular patient's femur.

As shown in Figure 6, the mounting plate can include an auxiliary support plate 220 for the support assembly for the guide tube. The support plate is mounted on a nut 214 which has a threaded bore for receiving the reference pin, and engaging the thread on external surface of the pin where it protrudes from the lateral cortex. Rotation of the nut causes the support plate 220 to translate along the reference pin to adjust the entry position for the cutting head. The desired position of the support plate along the reference pin can be selected with reference to the markings 210 on the pin.

The support plate includes a fixation screw 222 by which the support assembly for the guide tube can be fastened to the mounting plate. The cutting head is introduced into the bone and is made to rotate by means of an appropriate drive device, connected to the drive shaft. Consequently, the cutting head can be advanced into the bone as it cuts a path ahead of it. The head is advanced on the guide tube, by turning the gear component on the mounting plate, which engages the teeth on the guide tube. The control over the position and orientation of the cutting head by the guide tube ensures control over the path which is cut by the cutting head.

Claims

CLAIMS:

1. An instrument assembly for cutting a tunnel in a bone, which comprises a cutting tool, and a guide for controlling the cutting tool so that the tunnel which it cuts in the bone follows a predetermined curved path.

2. An assembly as claimed in claim 1, in which the guide comprises a curved guide tube which can be advanced into the tunnel behind the cutting tool.

3. An assembly as claimed in claim 2, which includes a mounting plate which can be fastened to the bone.

4. An assembly as claimed in claim 2, which includes an incremental drive unit for advancing the guide tube into the tunnel.

5. An assembly as claimed in claim 4, in which the incremental drive unit comprises a rack and pinion assembly, in which the rack is provided on the guide tube.

6. An assembly as claimed in claim 3, in which the mounting plate includes at least one adjuster for adjusting the orientation of the guide tube.

7. An assembly as claimed in claim 1 , in which the guide tube is curved with an approximately constant radius along at least part of its length.

8. An assembly as claimed in claim 1, in which the cutting tool comprises a rotating cutting head.

9. An assembly as claimed in claim 8, which includes a drive device for causing the cutting head to rotate.

10. An instrument assembly for cutting a tunnel in a bone, which comprises a cutting tool, and a curved guide tube which can be advanced into the tunnel behind the cutting tool to control the cutting tool so that the tunnel which it cuts in the bone follows a predetermined curved path, a mounting plate which can be fastened to the bone, and an incremental drive unit on the mounting plate for incrementally advancing the guide tube into the tunnel.