WO2018104704A1

WO2018104704A1 - Apparatus for alignment of knee arthroplasty tibial cutting block

Info

Publication number: WO2018104704A1
Application number: PCT/GB2017/053566
Authority: WO
Inventors: James Murray-Brown
Original assignee: Murray Brown James
Priority date: 2016-12-05
Filing date: 2017-11-28
Publication date: 2018-06-14
Also published as: GB201620706D0; GB2558543A

Abstract

Errors can occur at each stage when preparing the two sets of cuts, femoral and tibial, required for knee arthroplasty. Errors can cause deviation from the desired coronal plane alignment. If errors in the two sets of cuts are cumulative, a serious mal-alignment in the coronal plane may result. This apparatus and method allows the tibial cut to be referenced directly from the fully-prepared femoral cuts, with the limb aligned by ligamentous traction, using a closely-fitting component on the distal femur, and an extension arm extending to the tibial cut. Error in varus-valgus (coronal plane) orientation of the distal femoral cut is translated across the joint line as an equal and opposite compensatory adjustment in the varus- valgus orientation of the tibial cut. The chance of cumulative error is reduced.

Description

APPARATUS FOR ALIGNMENT OF KNEE ARTHROPLASTY TIBIAL CUTTING

BLOCK

Field of the invention

Total knee arthroplasty, or replacement. The positioning of a cutting block on the tibial plateau to guide a bone cut with a saw or other cutting device.

Current state of the art

There is a multiplicity of devices that allow different strategies to be used in an attempt to align the components of a total knee replacement. Conventionally, the surgeon attempts to implant the femoral and tibial components such that they lie on the mechanical axis of the lower limb (the straight line in the coronal view - or view from the front - that runs from the centre of rotation of the femoral head to the centre of the ankle), with the joint line perpendicular to this mechanical axis. There exists much controversy about the ideal alignment that the surgeon should strive for. The 'kinematic' alignment argument holds that alignment along the mechanical axis is of secondary importance, or in some cases can be counter, to the task of balancing the knee's native ligaments. Thus, some surgeons are increasingly seeking to implant total knee components such that the overall limb alignment remains in varus (or 'bow-legged'), with the joint line sloped from lateral (high) to medial (low) in order to more closely follow the native knee joint line. The primary aim for this 'kinematic' technique is to balance the knee, maintaining a physiological degree of tension in the knee ligaments throughout the range of movement from full extension to flexion. In practise, some surgeons develop a hybrid mix of techniques.

Other factors in addition to coronal plane alignment include: joint line height, size of the components, rotation of each component along the long axis of the limb, insert thickness (which is closely related to the level of the tibial cut), posterior slope of the tibial component, maintenance of patella-femoral joint mechanics (patella thickness and patella button positioning) and the correct performance of soft tissue releases. This invention relates

I primarily to coronal plane alignment (that is, the varus-valgus positioning of each component relative to the mechanical axes of the femur and tibia, and the overall limb alignment relative to the mechanical axis). A subsidiary claim relates to tibial component posterior slope.

Conventional instrumentation allows the distal femoral cut to be referenced off the femoral canal, and the tibial cut to be referenced either from the tibial canal or from an external jig which locates the centre of the ankle and the centre of the tibial plateau, and so creates a surrogate of the tibial mechanical axis from which to reference from. Computer guided knee replacement employs a 3D camera system to guide cuts with reference to mechanical axes determined by registering anatomical features. Patient specific instrumentation uses a plan predetermined from scans, but this method positions cutting blocks without the need for computer-guidance in theatre. All of these methods can allow errors to occur at one or more of the following steps:

poor surgical plan, for example a lack of appreciation of pre-existing valgus deformity in the femoral shaft, which can lead to significant overall valgus alignment

incorrect insertion of intra-medullary alignment rods, leading to a mal-aligned cutting block

incorrect positioning of an extra-medullary tibial guide, leading to a mal-aligned cutting block

movement of the blocks on the pins, or movement of the pins in the bone, leading to mal-positioning of a cutting block

skiving and bending of sawblades leading to inaccurate cuts

poor technique when cementing the implants in place

A major error is likely to be recognized and corrected by the surgeon. However, minor errors can occur at each step, and may not be recognised. If these errors are cumulative for the distal femoral cut and the tibial cut then a significant error in lower limb overall varus- valgus (coronal plane) alignment can occur, which can be detrimental to the result of the surgery. Of relevance to this invention field, patent US 845461 6 B2 should be read, although this patent relates to unicompartmental knee replacement, rather than total knee replacement. Here, the distal femoral cut and the tibial cut are linked by way of a jig that spans the knee, with the knee distracted in order to tension the collateral ligaments. This current invention shows novelty when compared to US 845461 6 B2, beyond the obvious matter that the current invention is in the field of total knee replacement. Considering US 845461 6 B2:

Firstly: the jig is first applied to both distal femur and proximal tibia, then cuts are made. As a result, errors that accrue during the process of cutting the distal femur can be cumulative with errors accruing from the tibial cut.

Secondly: the use of a distractor, described as being an apparatus with paddles that act on the distal femur and proximal tibia. This is using the principle of joint distraction to produce soft tissue tension: this can tension the collaterals while still retaining an unplanned varus or valgus overall limb alignment.

Thirdly: the jig is applied to the knee joint before the distal femoral cut and the femoral finishing cuts have been made. In the field of total knee replacement, osteophytes and bony impingement will very often not allow the knee joint to be aligned, either by distraction or by ligamentous traction, until sufficient bone has been resected to prevent impingement, and osteophytes that act on soft tissues to increase tension have been removed.

Explanatory note on the principles of ligamentous traction and joint distraction

Traction has been used since ancient times to reduce (align) fractures and dislocated joints. Traction makes use of the principle of ligamentotaxis, or ligamentous traction: in-line pull on a limb will cause tension in the soft tissues that will tend to align joints in such a way that they will lie along the mechanical axis of the limb. For example, in-line traction of the lower limb will tend to align the hip, knee and ankle joints along the mechanical axis, and align fractures in such a way that just such alignment along the mechanical axis is maintained. The use of traction to set the coronal plane alignment during knee replacement surgery has not previously been described: it is a novel aspect of this present invention Distraction of a joint is a technique whereby joint ligaments and capsule are tensioned by a distracting force across the joint: this force is either applied by a device inside the joint such as a laminar spreader or device with paddles, or by a device applied to the bones close by on either side of the joint such as a pin distractor. In contrast to traction, distraction will not necessarily align the limb along the mechanical axis: indeed, the distracting force may introduce deformity.

Distraction is used in some knee replacement systems, with the knee joint flexed 90 degrees, to set femoral component rotation relative to an already-performed tibial cut, and may also be used to select the most appropriate size of femoral component. Distraction has been used in uni-compartmental knee replacement to relate the femoral and tibial cuts to each other, as described in the previous section.

Introduction to the invention According to an aspect of the present invention there is provided apparatus for determining a tibial cut during knee replacement surgery, comprising means for referencing a tibial cut orientation and level from the prepared distal femur, such that the distal femoral cut alignment in the coronal plane can be translated across the joint to the tibial plateau. A further aspect provides apparatus for determining a tibial cut during knee replacement surgery, comprising means for referencing a tibial cut from a prepared distal femur, such that the distal femoral cut can be translated across the joint to the tibial plateau. The tibial cut made possible by the present invention will include any deliberately-introduced joint slope and any joint slope introduced by error when planning and executing the distal femoral cut and femoral finishing cuts. Furthermore, tibial cut orientation and level will not have been referenced directly from anatomical features of the tibia.

The apparatus may comprise a component for closely fitting a prepared distal femur in the manner of a femoral trial component that fits closely around a distal femoral cut and femoral finishing cuts; and an extension arm removably attachable to the component for indicating the level and orientation of a tibial cut that will result when the limb is aligned.

The apparatus may further comprise an ankle halter allowing the application of traction with a point of attachment for an alignment rod over the centre of the ankle and extending proximally to the hip joint; and means of removably locating the alignment rod over the centre of the femoral component, such as parallel alignment lugs or bars.

In some embodiments a tibial cutting block can be removably attached to the extension arm in such a way that the resulting cut is indicated by the orientation of the distal extent of the extension arm.

The tibial cutting block orientation may also include its rotational orientation about the long axis of the limb.

A further aspect provides a method for determining a tibial cut during knee replacement surgery, in which tibial cut orientation and level is referenced from a pre-prepared distal femur, and translated across the joint to the tibial plateau with the limb aligned, including any deliberately-introduced joint slope and including any joint slope introduced by error when planning and executing the distal femoral cut and femoral finishing cuts. According to a further aspect of the present invention there is provided an apparatus for setting the alignment of a tibial cut during total knee replacement with reference to an already- prepared femur, comprising: a component for closely fitting a prepared distal femur in the manner of a femoral trial component that fits closely around a distal femoral cut and femoral finishing cuts; and an extension arm removably attachable to the component for indicating the level and orientation of a tibial cut that will result when the limb is aligned.

A further aspect provides an apparatus for setting and checking the alignment of a limb prior to a tibial cut during knee replacement surgery, comprising: an ankle halter allowing the application of traction with a point of attachment for an alignment rod over the centre of the ankle and extending proximally to the hip joint; and

means of removably locating the alignment rod over the centre of the femoral component described herein, such as parallel alignment lugs or bars.

The component closely fitting the prepared distal femur may be of sufficiently small dimension so as to not extend distally to the level of the joint that would be reached by a standard femoral component closely fitting these similar bone cuts, and so reducing the likelihood of impingement against the tibial plateau.

In some embodiments a tibial cutting block can be removably attached to the extension arm in such a way that the resulting cut is indicated by the orientation of the distal extent of the extension arm. In some embodiments the tibial cutting block can be attached to the tibial plateau, for example by smooth or threaded pins. In some embodiments the extension arm can be unattached from the component closely fitting the prepared distal femur, and unattached from the tibial cutting block, leaving the tibial cutting block in place on the tibial plateau. In some embodiments the extension arm can be of a range of lengths which correspond to a range of tibial component thicknesses.

In some embodiments the distal part of the extension arm that indicates the level and orientation of the tibial cut is of a range of value of slope, corresponding to a range of value of posterior slope of the tibial cut that will result when the limb is aligned.

In some embodiments the component closely fitting the prepared distal femur has locating holes so that it may be removably attached to the distal femur with screws or other suitable means.

In some embodiments the component closely fitting the prepared distal femur has an attachment surface for the extension arm, such that the extension arm may be removably attached using screws, bolts or other suitable means. In some embodiments the component closely fitting the prepared distal femur has lugs removably attached to its anterior surface, or to the extension arm, or both, that locate the alignment rod over the centre of the component closely fitting the prepared distal femur when viewed in the coronal plane. The inventive step in some aspects and embodiments of the present invention is the linking of the tibial cut to the already-prepared distal femoral cut and femoral finishing cuts, with the limb aligned by ligamentous traction. This link is achieved by an extension arm attached to a modified femoral trial component, which acts to align the tibial cutting block. The invention translates the varus-valgus orientation of the distal femoral cut onto the tibial plateau cut, with the limb aligned by ligamentous traction. The chance of a significant cumulative error occurring in coronal plane alignment is reduced by this invention, as any error in the varus-valgus orientation of the already performed distal femoral cut is translated into an equal and compensatory adjustment on the tibial cut with the lower limb as a whole correctly aligned. Importantly, the distal femoral cut does not have to be perpendicular to the mechanical axis of either the femur or of the limb considered as a whole: it can be sloped from lateral (high) to medial (low) by a few degrees (usually up to 3 degrees) as selected by the surgeon, and the invention will translate this slope to the tibial cut. By first carrying out the femoral distal cut, then the femoral finishing cuts, then removing any osteophytes, the limb can be easily and correctly aligned by ligamentous traction (also known as ligamentotaxis). The femoral trial component in this invention is modified such that its distal extent does not reach the joint line: this is of critical importance, as it will allow ligamentous traction to align the limb without impingement between the modified femoral trial component and the tibial plateau.

In some aspects and embodiments a second, subsidiary, inventive step is the apparatus that allows the overall lower limb coronal plane alignment to be set and checked: traction of the limb is applied via an ankle halter which tensions the knee collateral ligaments and aligns the limb. An alignment rod is attached to the ankle halter over the ankle centre, and removably attached to the centre of the modified femoral trial component. The alignment rod extends proximally to lie over the hip joint. The overall lower limb mechanical axis is neutrally-aligned when the alignment rod is over the centre of hip, knee and ankle.

Different aspects and embodiments of the invention may be used separately or together.

Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with the features of the independent claims as appropriate, and in combination other than those explicitly set out in the claims.

Description

An example of the invention is now described; the description refers to the accompanying drawings; here follows the list of drawings and their key.

List of Drawings Figure I . Modified femoral trial component, in place on a prepared left distal femur

Figure 2. Left lower limb under traction. Modified femoral trial component, extension arm, alignment rod and alignment rod locating bars are illustrated

Figure 3. Further view of the left lower limb under traction. The alignment rod central section is seen: the alignment rod extends in the coronal view from the centre of the ankle joint to the centre of the femoral head. Tibial cutting block now in place on the extension arm, limb aligned, ready to pin tibial cutting block in place

Figure 4. Long alignment rod at its distal end, illustrating a method for attaching it to an ankle traction halter, with the distal end centred over the centre of the ankle joint in the coronal view

Figure 5. Limb under traction. Alignment rod lying over hip centre (femoral head centre), centre of femoral component and ankle centre, indicating a neutral mechanical axis of the lower limb

Key

I Modified femoral trial component, closely follows the distal femoral cut and femoral finishing cuts in a similar fashion to a standard femoral trial component. Of a range of sizes to match the internal dimensions of the range of femoral components Distal extent of modified femoral trial component, does not extend distally as far as the usual femoral trial component, so does not reach the joint line

Holes to allow the modified femoral trial component to be removably attached to the distal femur, by threaded screws for example

Locating surface for the extension arm, on the modified femoral trial component with suitable attachment method such as threaded bolt holes

Extension arm removably attached to the modified femoral trial component, the distal extent of this arm indicates the level and orientation of the tibial cut which will result. The extension arm can be of a selection of lengths, incrementally raising or lowering the level of the tibial plateau cut

Distal extent of the extension arm, indicates the level and spatial orientation of the tibial plateau cut, can be removably attached to the tibial cutting block, for example by mating with the saw slot

Tibial cutting block removably attaches to the extension arm, and can be attached to the tibial plateau, for example by smooth or threaded pins placed through the holes visible on the front face of the block

Alignment rod of a range of lengths, to reach from the ankle to the hip joint for each patient

Alignment rod locating lugs, orientated over the anterior aspect of the modified femoral trial component such that the alignment rod passes over the centre line of the component when viewed in the coronal plane. These lugs may perform a secondary purpose, to attach the extension arm onto the modified femoral trial component by way of threaded bolts

10 Traction halter outer strap, inner foam

I I Alignment rod attachment, such as a nylon sling, allowing the distal end of the alignment rod to be positioned over the centre of the talus, or the centre of the ankle joint, when viewed in the coronal plane

12 Distal femur

1 3 Tibial plateau

14 Schematic illustration of collateral ligaments. All other soft tissues are omitted for clarity (excepting Figure 4)

1 5 Point for application of Traction

1 6 Hip centre (centre of femoral head)

1 7 Femoral neck

This example of the invention is now described by following through the process in which it can be employed: knee replacement surgery. The description focuses on coronal-plane alignment, and especially on particular aspects pertaining to the positioning of the tibial cutting block.

In the following description, all orientational terms, such as upper, lower, radially and axially, are used in relation to the drawings and should not be interpreted as limiting on the invention.

Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the example set forth herein.

I I Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed and as well as individual embodiments the invention is intended to cover combinations of those embodiments as well. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate. The terminology used herein to describe embodiments is not intended to limit the scope. The articles "a," "an," and "the" are singular in that they have a single referent; however, the use of the singular form in the present document should not preclude the presence of more than one referent. I n other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including," when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof. Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein. After exposure of the knee joint, the distal femur 1 2 is prepared by performing the distal femoral cut and the femoral finishing cuts, using any of the standard techniques, such as measured resection cutting blocks, patient-specific instrumentation, or computer-guided surgery. Peripheral osteophytes are removed from the tibial plateau 1 3, and any residual osteophytes from the distal femur 1 2. Of note, there is no requirement for the distal femoral cut to be perpendicular to the mechanical axis. The surgeon may plan the slope (lateral 'high' to medial 'low') of the distal femoral cut to match the slope of the native joint, and the aim of this invention is to allow the surgeon to accurately translate the slope of the distal femoral cut across to the tibial cut, with the limb aligned.

See Figure I . The modified femoral trial component I is positioned on the distal femur 1 2, and attached by screws via holes 3. This modified femoral trial component I has novel and inventive differences from a standard femoral trial component: firstly, its distal extent does not reach the joint line, allowing the limb to be aligned by traction without impingement between the modified femoral trial component I and the tibial plateau 1 3; secondly it has a locating surface 4 for an extension arm 5, allowing the extension arm 5, and alignment rod locating lugs 9 to be removably attached; thirdly, the screw holes 3 which allow it to be firmly and removably attached to the prepared distal femur 1 2.

See Figure 4. The limb is aligned by traction applied to the ankle traction halter 1 0. See Figure 2. The surgeon must check that there is no impingement between the modified femoral trial component I and the tibial plateau 1 3. Impingement will interfere with the ability of ligamentous traction to align the limb. If there is bony impingement, then bone can be excised until no impingement exists. An extension arm 5 is selected from a range of incremental lengths that correspond to the available thickness range of tibial inserts, and attached to the locating surface for the extension arm 4, on the modified femoral trial component I . The distal extent of the extension arm 6 now indicates the level and spatial orientation of the tibial cut that will result: if too much or too little tibial bone resection is indicated, then an extension arm 5 of a different incremental length can be selected from the range described above. In this specific example, the alignment rod locating lugs 9 have a subsidiary purpose in that they act as threaded bolts attaching the extension arm 5 to the locating surface 4 on the modified femoral trial component I . Now, it can be seen that the distal femur 1 2 has been prepared, and the limb has been aligned by ligamentous traction. The distal extent of the extension arm 6 indicates the spatial plane of the tibial cut that will result. This tibial cut orientation has been referenced from the already- prepared distal femur 1 2 with the limb aligned. The tibial cut has not been directly referenced from the tibia mechanical axis. Therefore, any error in varus-valgus orientation of the distal femoral cut has been translated across as an equal and compensatory adjustment in the varus- valgus orientation of the tibial cut. See Figure 2. The distal extent of the extension arm 6 indicates the spatial plane of the tibial cut. With the limb aligned by ligamentous traction, this plane can be assessed relative to the slope of the native tibial plateau 1 3 considered in the sagittal plane, sloping from higher at the front (anterior) to lower at the back (posterior). This is commonly known as the 'posterior slope' of the tibial cut, and has important effects on the stability of the knee replacement. The extension arm 5 can be made available with an incremental range of values of posterior slope of its distal extent 6 which can be compared to the slope of the native tibial plateau 1 3, and the most appropriate value selected. Care must be taken in assessing the spatial orientation of the plane of the tibial cut both in the coronal plane, referenced in this present invention from the prepared distal femur 1 2 with the limb aligned by traction, and the sagittal plane (the 'posterior slope').

See Figure 5. The alignment rod 8 is attached distally to the traction halter 10, over the centre of the ankle. See Figures 2 and 3. The alignment rod 8 is located between the alignment rod locating lugs 9 which are positioned such that the alignment rod 8 passes over the centre line of the modified femoral trial component I when viewed in the coronal plane. The surgeon now assesses the proximal end of the alignment rod 8 where it lies over the hip joint. If it lies directly over the hip centre 16 (as for example can be ascertained with use of an image intensifier to determine the location of the centre of the femoral head in the coronal plane) then the lower limb mechanical axis is neutral: the hip centre, prosthetic knee joint centre, and ankle centre, all lie on the coronal plane mechanical axis.

The surgeon can choose to alter the lower limb alignment that traction has created. This can be achieved by soft tissue releases at the knee. It can also be achieved by moving the proximal end of the alignment rod 8 to a desired position: for example, many surgeons aim to introduce a small amount of overall varus into the lower limb, and this can be achieved by placing the proximal end of the alignment rod 8 over the femoral neck 1 7 rather than the hip centre 1 6. The centre part of the alignment rod 8 will act on the knee's coronal alignment via pressure on the alignment rod locating lugs 9, setting the alignment to the surgeon's satisfaction.

See Figure 3. The traction is temporarily released, and the knee flexed sufficiently to slip the tibial cutting block 7 over the distal extent of the extension arm 6. In this specific example, the distal extent of the extension arm 6 mates closely with the saw slot in the cutting block 7. Traction is re-applied, and alignment checked for a final time. Now, the cutting block 7 is attached to the tibial plateau 1 3 using smooth or threaded pins, via the drillholes visible on the anterior surface of the tibial cutting block 7. The alignment rod 8 is removed. The extension arm 5 is now removed by detaching the alignment rod locating lugs 9, and slipping the distal extent of the extension arm 6 free from the saw slot of the tibial cutting block 7. The knee can now be flexed in the usual manner, reducing tension in the posterior soft tissue structures and allowing the tibial cut to be made.

Although an illustrative embodiment of the invention has been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

1 . Apparatus for determining a tibial cut during knee replacement surgery, comprising means for referencing a tibial cut orientation and level from a prepared distal femur, such that the distal femoral cut spatial orientation can be translated across the joint to the tibial plateau.

2. Apparatus as claimed in claim I , comprising a component for closely fitting a prepared distal femur in the manner of a femoral trial component that fits closely around a distal femoral cut and femoral finishing cuts; and an extension arm removably attachable to the component for indicating the level and orientation of a tibial cut that will result when the limb is aligned.

3. Apparatus as claimed in claim 2, further comprising an ankle halter allowing the application of traction with a point of attachment for an alignment rod over the centre of the ankle and extending proximally to the hip joint; and means of removably locating the alignment rod over the centre of the femoral component, such as parallel alignment lugs or bars.

4. Apparatus as claimed in claim 2 or claim 3, in which a tibial cutting block can be removably attached to the extension arm in such a way that the resulting cut is indicated by the orientation of the distal extent of the extension arm.

5. Apparatus as claimed in claim 4, in which the tibial cutting block orientation also includes its rotational orientation about the long axis of the limb.

6. A method for determining a tibial cut during knee replacement surgery, in which tibial cut orientation and level is referenced from a pre-prepared distal femur, and translated across the joint to the tibial plateau with the limb aligned, including any deliberately-introduced joint slope and including any joint slope introduced by error when planning and executing the distal femoral cut and femoral finishing cuts.

7. An apparatus for setting the alignment of a tibial cut during total knee replacement with reference to an already-prepared femur, comprising:

a component for closely fitting a prepared distal femur in the manner of a femoral trial component that fits closely around a distal femoral cut and femoral finishing cuts; and an extension arm removably attachable to the component for indicating the level and orientation of a tibial cut that will result when the limb is aligned.

8. An apparatus for setting and checking the alignment of a limb prior to a tibial cut during knee replacement surgery, comprising:

an ankle halter allowing the application of traction with a point of attachment for an alignment rod over the centre of the ankle and extending proximally to the hip joint; and

means of removably locating the alignment rod over the centre of the femoral component described in Claim 7, such as parallel alignment lugs or bars.

9. An apparatus as claimed in Claim 7, where the component closely fitting the prepared distal femur is of sufficiently small dimension so as to not extend distally to the level of the joint that would be reached by a standard femoral component closely fitting these similar bone cuts, and so reducing the likelihood of impingement against the tibial plateau.

1 0. An apparatus as claimed in Claim 7, where a tibial cutting block can be removably attached to the extension arm in such a way that the resulting cut is indicated by the orientation of the distal extent of the extension arm. I I . An apparatus as claimed in Claims I and 4, where the tibial cutting block can be attached to the tibial plateau, for example by smooth or threaded pins.

1 2. An apparatus as claimed in Claims I , 4 and 5, where the extension arm can be unattached from the component closely fitting the prepared distal femur, and unattached from the tibial cutting block, leaving the tibial cutting block in place on the tibial plateau.

I 3. An apparatus as claimed in Claim I , where the extension arm can be of a range of lengths which correspond to a range of tibial component thicknesses.

1 4. An apparatus as claimed in Claim I , where the distal part of the extension arm that indicates the level and orientation of the tibial cut is of a range of value of slope, corresponding to a range of value of posterior slope of the tibial cut that will result when the limb is aligned.

1 5. An apparatus as claimed in Claim I , where the component closely fitting the prepared distal femur has locating holes so that it may be removably attached to the distal femur with screws or other suitable means.

1 6. An apparatus as claimed in Claim I , where the component closely fitting the prepared distal femur has an attachment surface for the extension arm, such that the extension arm may be removably attached using screws, bolts or other suitable means.

1 7. An apparatus as claimed in Claim I and Claim 2, where the component closely fitting the prepared distal femur has lugs removably attached to its anterior surface, or to the extension arm, or both, that locate the alignment rod over the centre of the component closely fitting the prepared distal femur when viewed in the coronal plane.

1 8. A method for aligning femoral and tibial cuts during knee replacement surgery, comprising the steps of:

- conducting a distal femoral cut and femoral finishing cuts;

- applying ligamentous traction to the limb; and - translating the coronal plane alignment of the distal femoral cut to a tibial plateau cut.