Patent ID: 12232815

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description sets forth numerous specific details to provide a thorough understanding of the invention. However, those skilled in the art will appreciate that the invention may be practiced without these specific details. In other instances, well known methods, procedures, components, instruments or implants have not been described in detail so as not to obscure the invention.

In the Figures like reference numerals are used for like elements unless indicated otherwise or the context requires otherwise.

With reference toFIG.1, there is shown a high level flowchart illustrating various stages of an overall method100of preparing for, planning and conducting total knee replacement surgery100. Aspects of the invention may reside in the overall method100, the individual stages thereof, combinations of the individual stages and combinations of sub-stages of the stages illustrated inFIG.1.

An initial stage102of the overall method involves determining the patient's pre-operative anatomical legal alignment102. This may be carried out in a number of ways, as described in greater detail below, and essentially involves determining the hip, knee, ankle axes of the patient's leg and also the knee joint line. Anatomical information derived from the patient's actual leg are obtained at this initial step102.

After the patients anatomic leg alignment information has been obtained, at step104a planning method according to one aspect of the invention is carried out. The planning method carried out at step104uses the patient's anatomic leg alignment information obtained in step102to determine any angular adjustments to be made to femoral and/or tibial cuts. The result of planning step104are planned tibial and/or femoral cut angles which can then be used in the actual surgical procedure carried out at step106.

Hence, at step106, a total knee replacement surgical procedure is carried out using the tibial and femoral cut positions determined from planning surgery104which may include adjustments to the tibial and/or femoral cut angles compared to those used in conventional procedures. The tibial and/or femoral cut angles planned at stage104implement a surgical philosophy underlying the surgical method step106that the femur is the key kinematic driver of the knee. Therefore, an improved functional restoration of the patient's knee can be achieved by resurfacing the femur within the constraints of maintaining the long leg alignment and tibial cuts angles within safe boundaries.

The pre-operative surgical planning method carried out at step104allows a pre-operative surgical plan to be developed that can restore the natural joint line within predefined safe boundaries of the varus or valgus angle of the tibial cut, restores the long leg alignment of the patient within predefined safe boundaries for the hip-knee-ankle axis and may also restore the natural posterior femoral condyles within predefined boundaries of femoral rotation.

The planning method of step104recognises that the long leg alignment of the patient is dependent on the distal femoral cut angle and tibial cut angle. The method enables calculation of a one of these variables based on second safe target ranges for the other two. The angle of the distal femoral cut and/or tibial cut can be modified to ensure that they stay within predefined ranges of values. Any adjustment to the tibial cut angle can be applied also to the femoral rotation of the knee in flexion.

The planned femoral and tibial cut angles, either of both of which may include adjustments arising from planning method104, are then used as input to the surgical procedure106. Surgical procedure106is then largely conventional and may use largely conventional instrumentation in order to carry out the total knee replacement surgery. However, the angles at which the tibial and/or femoral cutting blocks are set relative to the femur and/or tibia differ to those that would otherwise be used.

The surgical method106can also use any prosthetic knee implant system comprising femoral and tibial components intended for use in anatomy based surgical procedure.

For example, a suitable set of instruments include the Intuition instruments described in the Attune surgical knee system surgical technique document available from DePuySynthes, a Johnson & Johnson company (Intuition is a Trade Mark of DePuySynthes). The knee implants used may be the Attune knee system implants also available from DePuySynthes and also described in the Attune knee system surgical technique document available from DePuySynthes (Attune is a Trade Mark of DePuySynthes which may be registered in some countries). However, the invention is not limited to either very specific implants or the specific instruments of the Attune knee system. Rather, any instrumentation allowing the angle of the distal femoral cut to be adjusted relative to the femur and the tibial cut to be adjusted relative to the tibia can be used.

An exemplary surgical procedure corresponding to step106is described in greater detail below.

With reference toFIGS.2A,2B and2Cthere are illustrated the different leg alignments possible for a human.

FIG.2Ashows a neutral leg alignment200. A neutral leg alignment is characterised by the load bearing axis extending from the centre of the femoral head202to the centre of the ankle204, represented by solid line206, passing through the centre of the patient's knee. Also illustrated inFIG.2A, as a dash dot line208, is the approximate axis of the intramedullary canal of the distal end of the femur, and which represents the distal femoral anatomical axis.FIG.2Ashows the patient's leg alignment in a standing, loaded condition in which the weight of the patient's body passing along the long leg axis206. Also shown inFIG.2Ais the joint line209of the patient's knee. The joint line209is generally parallel to the floor on which the patient is standing and typically approximately 3° offset from perpendicular to the long leg axis206. In other words, the angle subtended between the joint line209and the long leg axis206on the medial side, and inferior to the joint line, and marked δ inFIG.2Ais typically approximately 87°.

FIG.2Ashows the various axes of the patient's leg projected on to the frontal plane of the body.

FIG.2Bis similar toFIG.2Abut shows a leg having a varus alignment. The long leg axis206extends from the centre of the femoral head202to the centre of the ankle204. A hip-knee axis214, also known as the femoral mechanical axis is defined by the line passing from the centre of the femoral head202to the centre of the knee212. The knee-ankle axis216, also known as the tibial mechanical axis is defined by the line running from the centre of the knee212to the centre of the ankle204. As the leg has a varus alignment, the hip-knee-ankle axis defined by lines214and216is no longer co-linear with the long leg axis206as is the case in the neutral alignment illustrated inFIG.2A. As can be seen inFIG.2B, the angle on the medial side subtended between the hip-knee axis214and knee-ankle axis216is less than 180°. Conversely, the angle on the lateral side between the hip-knee axis214and knee-ankle axis216is greater than 180°. Again, line208represents the alignment of the intramedullary canal at the distal end of the femur.

More specifically, the hip-knee axis, or mechanical axis of the femur,214can be considered to extend from the centre of the femoral head to the mid-condylar point between the cruciate ligaments of the knee. The knee-ankle axis, or the mechanical axis of the tibia,216can be defined by the line from the centre of the tibial plateau to the centre of the tibial plafond.

FIG.2Cshows a valgus leg alignment220. Again, the femoral mechanical axis214and tibial mechanical axis216are no longer co-linear with the long leg axis206. However, the long leg axis206falls laterally of the knee for a valgus alignment whereas the long leg axis206falls medially of the knee for a varus alignment as shown inFIG.2B. The angle subtended by the femoral mechanical axis214and tibial mechanical axis216medially of the knee is greater than 180° and the angle subtended by the femoral mechanical axis214and tibial mechanical axis216on the lateral side is less than 180°.

The joint line of the knee,209, can generally be considered to be the line tangential to the distal most parts of the medial and lateral condyles. Generally, as illustrated inFIGS.2B and2C, the joint line209is substantially parallel to the floor however, the angle of the joint line relative to the mechanical axes of the tibia and femur of the knee varies in the varus and valgus alignments as will be appreciated by a person of ordinary skill in the art.

Returning toFIG.1, at a first anatomical information gathering stage102, information is obtained from the patient's body, either directly or indirectly, sufficient to establish the femoral mechanical axis214, the tibial mechanical axis216and the joint line209.

Indirect approaches typically involve capturing an image of the patient's bones and determining the positions of various anatomical landmarks in the bone images in order to determine the required anatomical alignment information.

For example, a long leg x-ray may be taken of the patient in a standing, loaded position. The x-ray may involve the capture of one or a plurality of x-rays which overlap, sufficient to allow the various anatomical landmark positions to be determined. From the x-ray images, the centre of the femoral head202can be determined as well as the centre of the knee, the centre of the ankle and also the joint line209corresponding to the line tangential to the distal most parts of the medial and lateral condyles.

This may simply involve marking the x-rays, drawing lines, measuring distances and either calculating or measuring angles.

In other more complex approaches, image processing routines may be used on digitised images of the x-rays or digital x-rays themselves in order to manually, automatically or semi automatically determine the required angle information.

In other embodiments, three dimensional modelling and/or computer simulation software may be used and CT scan data may be processed to determine the required anatomical information.

In a direct approach, the positions of various anatomical landmarks on the patient are determined directly on the patient themselves. This may be done by palpating the patient and measuring various distances. Also, computer assisted surgery techniques may be used in which trackable markers are attached to the patient's bones and/or instruments so as to capture the position of various anatomical landmarks by placing a trackable pointer on those landmarks. In this approach, the determination of the patient's anatomical information may be carried out as part of a surgical procedure itself, rather than a purely pre-operative step, as access to the interior of the patient's knee may be required.

Computer assisted surgery methods for determining the centre of rotation of the femoral head are generally known in the art. Examples of these include attaching a trackable marker to the patient's knee, rotating the femur about the hip joint and capturing the locus traced by the trackable marker. From this, the centre of rotation of the hip, corresponding to the centre of the femoral head can be determined.

In instances where the patient has a disease condition, such that, for example, the medial or lateral condyle has worn, then an x-ray may be captured with the lower leg in a stressed position so that the patient's knee adopts an alignment similar to that it would have in the absence of the disease condition and which can be considered more accurately to correspond to the original anatomy of the patient's knee rather than the disease state.

Other ways of obtaining information or data defining the anatomical arrangement and geometry of the patient's knee may be used and will be apparent to a person of ordinary skill in the art and from the discussion herein.

FIG.3shows a process flow chart illustrating a total knee replacement planning method300according to an aspect of the invention and corresponding generally to step104ofFIG.1. Planning method300corresponds to a femur based planning approach which aims to maintain the angle of the femoral cut as parallel to the joint line, if possible, by modifying the tibial cut angle, if necessary, to ensure that the tibial cut angle and leg alignment are both within safe boundaries. If this cannot be done by modifying the tibial cut angle alone, then the femoral cut angle can also be modified to ensure compliance with the tibial cut angle and long leg angle safe boundaries. A tibia based planning approach is also possible and is described later with reference toFIG.8.

Method300may be implemented in a number of different ways. For example, with reference toFIG.9, method300may be implemented in software as part of a more general surgical planning computer program940which may be associated with a computer assisted surgery (CAS) system950or may be a standalone computer program dedicated to this purpose only, for example as an application on a smart phone or tablet or other general purpose computing device. In other embodiments, method300may be implemented as a set or rules, or guidelines or a flow chart on printed media to which a surgeon may refer to in order to carry out the method and enter various measurements, values and calculations used in the method. Irrespective of how the method300is implemented in practice its output or results are a planned tibial cut angle, a planned femoral distal cut angle and optionally a planned femoral posterior cut angle. The planning method300does not also plan the height of the cuts as this is not important in realising the benefits of the method, the heights or depths of the cuts may depend on the size of the implants used, the soft tissues of the knee and other factors and the heights or depths of the cuts may be determined as for a conventional total knee replacement surgical procedure.

An initial step302of the planning method300involves setting boundaries for the long leg alignment angle and the angle of the proximal tibial cut. As illustrated inFIG.2A, for a neutral alignment, in which the femoral mechanical axis214and the tibial mechanical axis216are collinear and aligned with the long leg axis206, the angle between the femoral mechanical axis and tibial mechanical axis is substantially 180°. Hence, at step302a range of acceptable leg alignment values is defined. For example, the range of acceptable angles may be 3°, and the boundary values may be 177° and 180°. For the purposes of the examples described below, the magnitude of the angle subtended by the femoral mechanical axis and the tibial mechanical axis on the medial side of the knee will be used and will be referred to herein as α. Hence, α being greater than 180° corresponds to valgus alignment and α being less than 180° corresponds to varus alignment. However, the choice of the definition of the angle used to define the long leg alignment is largely arbitrary.

It is believed that a variation of up to 10° away from neutral alignment will still provide reasonable mechanical performance of the tibial and femoral components of the prosthetic knee. However, in other embodiments, a variation of up to not more than 3° may be used as the boundaries or limits of the long leg alignment of the patient's leg. In this example, at step302the long leg alignment boundaries are set such that 177°≤α≤180°, and hence corresponding to neutral to varus alignments.

At step302, the boundaries for an acceptable range of values of the tibial cut angle is also set.FIG.4shows a schematic diagram of a patient's pre-operative knee joint400and includes the distal part of the femur402and the proximal part of the tibia404. Also shown is the femoral anatomical axis208adjacent the distal part of the femur and corresponding generally to the femoral intramedullary canal. The anatomical axis of the tibia is largely coincidental to the mechanical axis of the tibia216. Using the convention set up above for this example, the medial condyle is406, the lateral condyle is408and the angle α is subtended by the femoral mechanical axis214and the tibial mechanical axis216. The joint line209is tangential to the distal most parts of the medial406and lateral408condyles. The joint line angle can generally be defined as the angle, on the medial side, subtended between the joint line209and tibial mechanical axis216and inferior to the joint line. As shown inFIG.4, the tibial cut angle can be defined as the angle subtended between the tibial cut line410and the tibial mechanical axis216inferior to the joint line and is labelled β inFIG.4. The definition of the tibial cut angle is again largely arbitrary and other definitions are possible, such as the angle subtended between the tibial cut line and tibial mechanical axis superior to tibial cut line410.

A distal femoral cut angle, γ, can also be defined as the angle subtended between the distal femoral cut line412and the femoral mechanical axis214. Again the definition of the distal femoral cut angle γ is largely arbitrary, and other definitions are possible.

As discussed above, for a neutral leg alignment as illustrated inFIG.2A, the joint line209is generally parallel to the floor and approximately 3° offset from perpendicular to the tibial mechanical axis (which is coincident with the long leg axis206for neutral alignment). The purely anatomical surgical philosophy is to simply make the tibial cut line410parallel to the patient's joint line209, thereby replicating the knee geometry of neutral alignment. However, the planning method300instead sets a range of acceptable values for the angle of the tibial cut. In this example, the range of values is not more than 3° less than perpendicular to the tibial mechanical axis, i.e. 87°≤β≤90°, and hence corresponding to neutral to varus alignments. It is purely coincidence that the magnitude of the ranges of values for leg alignment angle and tibial cut angle are both 3°. In other embodiments, the magnitudes of the ranges of values may be different for leg alignment angle and tibial cut angle.

The range of acceptable angles for the long leg alignment and the proximal tibial cut angle set at step302may be based on a number of approaches either individually or in combination. Theoretical and/or empirical approaches may be used. For example, a more empirical approach would be to analyse survivorship data for implants in different patients and correlate that with the post-operative long leg alignment angles and/or tibial cut angles arising for the patients' implants. A more theoretical approach would be to use computer analysis of computer models of the patient's leg and knee implant to determine the distribution, direction and size of various forces. Another more empirical approach would be to measure the forces arising in a prosthetic knee for different long leg alignment angles to determine the effect of long leg alignment angle and/or proximal tibial angle on the forces in the prosthetic knee joint and/or exerted by the prosthetic knee joint on the patient's resected tibia and/or femur. The results of theoretical and empirical approaches may be combined to help determine the pre-selected ranges of angles used at step302.

As noted above, the definitions of the various angles are arbitrary to an extent. The boundaries of the ranges of values of the long leg angle and tibial cut angle are set at step302. When these ranges are used to determine whether the long leg angle and tibial cut angel requirements are met, this may be a direct comparison, if the ranges and angles are defined in the same way or it may be an indirect comparison, if the ranges and angels are defined in different way. This direct or indirect comparison is covered by determining whether the various angles correspond to an angle falling within or outside the ranges. Hence, this encompasses situations in which the definitions are the same or differ, in which case a transformation may need to be applied to make the angles directly comparable with the ranges, for example adding or subtracting 180° or 90°.

In the following example, the definitions of the angles and values used in the ranges are the same and therefore allow a direct comparison when determining whether various angles correspond to an angle within the first or second ranges of values set at302. Returning toFIG.3, at step304, the patients anatomical data obtained previously at step102, is used to determine whether the patient's long leg alignment is with the boundaries set at step302. The angle between the patient's femoral mechanical axis214and their tibial mechanical axis, α, is compared with the range of acceptable values set at step302. In a first example, α=178, i.e. slightly varus, and so is greater than 177° and so the long leg alignment is determined to be acceptable at step304. The method proceeds to step306at which initial planned femoral cut and tibial cut angles are set. The initially planned tibial cut angle is set to an angle that would restore the joint line209of the patient. Hence, the initially planned tibial cut angle, between the tibial cut line410and the tibial mechanical axis216, is set to make the tibial cut line410parallel to the joint line209, which in this example is 89°. Similarly, the initially planned femoral cut angle γ is set to a value such that the distal femoral cut line412is parallel to the joint line and in this case also to the initially planned tibial cut line410′. Hence, in this example the initially planned femoral cut angle is 89°.

At step308it is determined if the initially planned value for the tibial cut angle is within the boundaries set at step302. Hence, step308determines whether, the initially planned tibial cut angle value of 89° is between 87° and 90° which it is. Hence, at step310the final planned femoral cut angle value is set to the initially planned value of 89° and the final planned tibial cut value is set to the initially planned value of 89° and planning is complete. Hence, the initially planned tibial and distal femoral cut angles have been validated by the planning method as being acceptable final planned cut angles.

A second example illustrates the planning method300further.FIG.5shows a different patient's knee geometry500. In this knee geometry the value of the angle α between the femoral mechanical axis214and the tibial mechanical axis216is again 178° and so the method proceeds at step304to step306. In this knee geometry, the angle between the joint line209and the tibial mechanical axis216is 86°. Hence, at step306, the initially planned tibial cut angle value to restore the patient's joint line209will be 86° (so that the tibial cut line410is parallel to the joint line209). Also, the initially planned femoral cut angle value is set so that the femoral cut line412is also parallel to the joint line209. In this example, the initially planned femoral cut angle value is 92°. At step308it is determined if the initially planned tibial cut angle is within the boundaries set in step302and in this example it is not as 86° is less than the lower limit of 87°. Hence, the method proceeds to step312. At step312, the initially planned tibial cut angle value is set to the boundary value closest to the initially planned value, i.e. to 87° at step314. Hence, an adjustment angle of 1° is applied to the initially planned tibial cut angle to arrive at the final planned tibial cut angle. This is illustrated inFIG.5, by the corresponding final planned tibial cut line414which is no longer parallel to joint line209(and which is exaggerated inFIG.5for clarity of explanation).

As the planed angle of tibial cut has been changed, the initially planned femoral cut line412will no longer be parallel to the finally planned tibial cut line414and hence, the alignment of the leg would be altered. Hence, at step314, the finally planned femoral cut angle is set to the initially planned femoral cut angle but adjusted by the tibial cut adjustment angle of 1°. Hence, in this example, the value of the finally planned femoral cut angle is set to 91° and gives rise to a corresponding finally planned femoral cut line416which is no longer parallel to joint line209, but is parallel to finally planned tibial cut line414and hence does not change the leg alignment of the patient. So in this example, the finally planned tibial cut angle and distal femoral cut angles no longer provide exact replication of the pre-operative patient anatomy, as the resulting joint line will be 1° rotated compared to original anatomic joint line209, but they are as close as possible within the boundaries set. However, the long leg alignment has not been altered and therefore this aspect of the patient's anatomy will be preserved by these planned cut angles. Hence, in this example, the initially planned femoral cut angle is modified only if the initially planned tibial cut angle is outside of the acceptable range.

A third example illustrates the planning method300further.FIG.6shows a different patient's knee geometry600. In this knee geometry the value of the angle α between the femoral mechanical axis214and the tibial mechanical axis216is 176°. Hence, at step304it is determined that the long leg alignment is not within the long leg alignment boundary set at step302, as 176°<177°. Hence, the method300proceeds to step316. At step316, the initially planned femoral cut angle is set to restore the joint line209. In this example, the initially planned femoral cut angle is set to a value of 87° so that the corresponding initially planned femoral cut line418is parallel to joint line209. Also at step316, an initially planned tibial cut angle is set which will bring the long leg alignment back within the boundary set in step302. Hence, as the lower limit of the long leg alignment range is 177°, a 1° rotation is added to the initially planned tibial cut angle, which is illustrated inFIG.6, by the initially planned tibial cut line420being rotated by 1° relative to a line422parallel to the joint line209. The initially planned tibial cut angle value, including the 1° adjustment, is therefore 90°. At step318it is determined whether the initially planned tibial cut angle is within the tibial cut angle boundaries set in step302, which in this example it is. Hence, the method proceeds to step320at which the value of the finally planned femoral cut angle is set to the initially planned value of 87° and the value of the finally planned tibial cut angle is set to the initially planned value, which includes the 1° adjustment, of 91°. Hence, as the finally planned cut lines418,420are no longer parallel (in this example by 1°) the leg alignment angle α has been increased by 1° and hence the long leg alignment has been brought back within the acceptable range.

A fourth example illustrates the planning method300further.FIG.7shows a different patient's knee geometry700. In this knee geometry the value of the angle α between the femoral mechanical axis214and the tibial mechanical axis216is 176°. Hence, at step304it is determined that the long leg alignment is not within the long leg alignment boundary set at step302, as 176°<177°. Hence, the method300proceeds to step316. At step316, the initially planned femoral cut angle is set to restore the joint line209. In this example, the initially planned femoral cut angle is set to a value of 91° so that the corresponding initially planned femoral cut line424is parallel to joint line209. Also at step316, an initially planned tibial cut angle is set which will bring the long leg alignment back within the boundary set in step302. Hence, as the lower limit of the long leg alignment range is 177°, a 1° rotation is added to the initially planned tibial cut angle, which is illustrated inFIG.7, by the initially planned tibial cut line426being rotated by 1° relative to a line428parallel to the joint line209. The initially planned tibial cut angle value, including the 1° adjustment, is therefore 86°. At step318it is determined whether the initially planned tibial cut angle is within the tibial cut angle boundaries set in step302, which in this example it is not as 86°<87°. Hence, the method proceeds to step322at which the value of the finally planned tibial cut angle is set to the closest tibial cut boundary, being 87°, and therefore including an adjustment angle of 1°. This is illustrated inFIG.7, by corresponding finally planned tibial cut line430, being rotated by a further 1° from the initially planned tibial cut line426. The method proceeds to step324at which the value of the finally planned femoral cut angle is set to the initially planned femoral cut angel but adjusted by the tibial cut angle adjustment of 1° needed to bring the tibial cut angle back within the acceptable range. Hence, the finally planned femoral cut angle is set to the value of 92°. This is illustrated inFIG.7, by corresponding finally planned femoral cut line432, being rotated by a further adjustment angle of 1° from the initially planned femoral cut line424. Hence, as the finally planned cut lines430,432are no longer parallel (in this example by 1°) the leg alignment angle α has been increased by 1° and hence the long leg alignment has been brought back within the acceptable range and also the tibial cut angle has been brought back to within the acceptable range. However, the femoral cut angle has been adjusted away from that which would recreate the joint line.

It will be appreciated thatFIGS.4to7are intended merely to help illustrate the specific angles and geometries described in the text, and the Figures do not themselves necessarily have the same actual angles of the examples described above.

The embodiment of the planning method illustrated inFIG.3prioritises the angle of the distal femoral cut as the method tries to maintain the femoral cut line parallel to the joint line, and preferentially modifies the angle of the tibial cut. For example, the tibial cut angle, rather than the femoral cut angle, is used to modify the long leg alignment axis if that is outside its boundaries. The femoral cut angle is only modified in cases where the tibial cut angle would otherwise be outside its boundaries, in which case the femoral cut angel is adjusted by the same amount of angle as the tibial cut angle is adjusted to bring it back within its boundaries.

Alternatively, the invention may also be implemented as a tibial cut prioritised plan using a similar approach but in which the proximal tibial cut angle is preserved as an anatomical cut (i.e. to recreate the joint line) unless needed in order to keep the tibial cut angle within the acceptable boundaries. The tibial based planning method800is illustrated by the flow chart shown inFIG.8. The planning method800illustrated inFIG.8uses the same overall approach as the planning method300illustrated inFIG.3of setting leg alignment and tibial cut boundaries and then checking that the tibial cut angle and leg length alignment fall within those boundaries, and if not then adjusting the tibial cut and/or femoral cut angles so that the finally planned tibial and femoral cut angles result in a knee joint geometry that does fall within those boundaries. However, the approach of the method800differs in that it initially sets the tibial cut angle as the finally planned tibial cut angle and then only the femoral cut angle is subsequently adjusted, if necessary to bring the leg length alignment within its acceptable boundaries.

At step802, the boundaries of the leg length alignment are set, e.g. 177°≤α≤180°, and the boundaries of the tibial cut angle relative to the tibial mechanical axis are set, e.g. 87°≤β≤90°. At step804, the angle between the joint line and the tibial mechanical axis is compared to the tibial cut angle boundaries, to see if an anatomical approach, i.e. the tibial cut replicating the patient's joint line, is acceptable. If so, then at step806, the planned tibial cut angle is set to the angle resulting in the tibial cut line being parallel to the joint line and also the initially planned femoral cut angle is set so that the distal femoral cut line will also be parallel to the joint line. At808, it is determined if the angle between the femoral mechanical axis and the tibial mechanical axis is within the boundaries set at802. If the long leg alignment is determined to be within the boundaries at808, then the finally planned femoral cut angle is set to the initially planned femoral cut angle at810and the planning is complete. The result is hence a planned distal femoral cut and a planned tibial cut each being parallel to the patient's joint line and hence reproducing the patient's original anatomy, while also ensuring appropriate mechanical operation of the prosthetic knee joint.

Returning to step808, if the long leg alignment is determined not to be within the boundaries at808, then the finally planned femoral cut angle is set to the initially planned femoral cut angle but adjusted to bring the long leg alignment back within the boundaries at812. For example, if the angle between the patient's femoral mechanical axis and tibial mechanical axis is 175°, then a 2° adjustment at least is needed to bring the long leg alignment back within the boundaries. Hence, at step812, a 2° adjustment is made to the initially planned femoral cut angle and that value is then used as the finally planned femoral cut angle. Hence, planning is complete and results in an anatomy preserving tibial cut, but a slight change in the long leg alignment of the patient.

Returning to step804, if it is determined that the angle between the joint line and tibial mechanical axis is outside the tibial cut boundaries, e.g. is 85°, then at step814, the tibial cut angle is planned as being the closest boundary value, i.e. 87°. It will be appreciated that by choosing the closest boundary value, as is also done in method300, the adjustments made away from the patient's anatomy are minimised, thereby helping to preserve the benefits arising from anatomy based surgical philosophies. Also at814, the initially planned femoral cut angle is set to that needed to restore the joint line, but also including the tibia cut adjustment angle. Hence, the initially planned femoral cut angle is set as including the 2° tibial adjustment angle at814. Hence, the planned tibial cut line and initially planned femoral cut line are parallel at this stage of the method. However, the long leg alignment angle has now been adjusted, by 2° in this example. At816it is determined if the angle between the tibial mechanical axis and the femoral mechanical axis that would arise from the planned tibial cut angle and the initially planned femoral cut angle is within the acceptable boundary. If originally, the long leg angle was 175°, then the adjusted long leg angle at this stage would be 177° and therefore within the boundaries. Hence, at816, the method proceeds to818and the planned femoral cut angle is set to the initially planned femoral cut angle, which includes the 2° adjustment. Hence, the planned tibial cut line is left as close as the boundaries allow to the anatomical cut line, and a minimal adjustment to the long leg alignment so as to be within the acceptable boundaries has been introduced.

Returning to step816, if originally, the long leg angle was 174°, then the adjusted long leg angle at this stage would be 176° and therefore outside the boundaries. Hence, at816, the method proceeds to820and the planned femoral cut angle is set to the initially planned femoral cut angle but including an angular adjustment to bring the long leg alignment back within the boundaries. Hence, a further 1 adjustment is added to the initially planned femoral cut angle, to bring the corresponding long leg alignment axis to 177° and therefore within the boundary. Hence, the planned tibial cut line is left as close as the boundaries allow to the anatomical cut line, and a minimal adjustment to the long leg alignment so as to be within the acceptable boundaries has been introduced, but which is slightly greater than that of the preceding example.

In some embodiments, the planning methods300and800may be embodied or implemented in a printed medium which bears instructions readable by a user and guiding the user through the steps of the planning methods300and800illustrated inFIGS.3and8, or other steps which ultimately implement the planning method of the invention as illustrated by the specific methods ofFIGS.3and8. The instructions may include written instructions as well as instructions in graphical form, such as one or more diagrams of a knee joint showing the various axes and angles used in the planning methods300,800.

The printed medium may also bear an indication of the first pre-selected range of values, e.g. 177°≤α≤180°, and the second pre-selected range of values, 87°≤β≤90°. The medium may also include one or more first fields where a user can record a first type of data. The first type of data may be anatomical data derived from the patient and may include the angle of the femoral mechanical axis, the angle of the tibial mechanical axis, the angle between the femoral mechanical axis and the tibial mechanical axis, the angle of the joint line, the angle between the joint line and the femoral mechanical axis and/or the angle between the joint line and the tibial mechanical axis. The anatomical data should include at least enough data to allow the angles of the joint line relative to the tibial mechanical axis to be determined and also the angle between the tibial and femoral mechanical axes. The medium may include one or more second fields where a user can record the result of a calculation. The result of the calculation may be a value of an angle. Hence, at various places on the medium, fields may be provided near instructions to add or subtract various angles to enter the result of that calculation so as to maintain a record of the initially planned angles, any angular adjustments applied by following the planning methods and also the resulting finally planned tibial and femoral cut angles.

Referring back toFIG.1, having described the surgical planning carried out at104, a method of carrying out total knee replacement surgery using the planned tibial and femoral cut angles, corresponding to step101, will now be described with reference toFIG.9. As mentioned above the planning method104and surgical method106can be used for any total knee replacement system suitable for mechanical axis alignment. Also, the surgical method is largely conventional other than the angular adjustments to the planned tibial and femoral cut angles and hence largely conventional surgical instrumentation can be used. A number of conventional surgical steps are therefore not described in order not to obscure the invention. As an example only, the surgical method106may be carried out using the Attune Knee System and Intuition instruments as provided by DePuySynthes and as described in the Attune Knee System Surgical Technique document also provided by DePuySynthes (ATTUNE is a Trade Mark of DePuySynthes, which is registered in some countries and INTUITION is a Trade Mark of DePuySynthes).

In some embodiments the surgical method may be carried out using a computer assisted surgery (CAS) system950, for example as illustrated inFIG.9. CAS systems are generally known and typically including a computer system952, tracking system954and a display956which provides visual and other guidance to the surgeon to guide them through the various steps of the workflow of the surgical procedure and also provides guidance as to the relative positions of tools, instruments, implants and body parts, represented by958,960,962, to asset in the carrying out of various acts, such as positing instrumentation, making cuts, and placing trial components and prosthetic components. The body parts and the tools, instruments and implants used in the CAS system may include one or more markers964which are trackable by the tracking system954which provides positional information or data966to the computer system952. Various different types of tracking technology can be used such as wired or wireless, including infra-red, optical, acoustic or magnetic. Hence, a CAS system may including software942which configures the CAS system to assist the surgeon to carry out the surgical steps illustrated inFIG.9. The CAS system may also include planning software940which configures the CAS system to implement the planning method used at step104. Hence, the tibial and femoral cut planning data generated by the planning software940may be passed to the surgery workflow software942and used by the surgery workflow software942to provide an indication of the planned position of the tibial and femoral cuts and any angular adjustments or settings to be used with the femoral and tibial cutting block so as to reproduce the planned tibial and femoral cut positions on the display956. Although the tracking system954is shown separately inFIG.9for the sake of explanation, it will be appreciated that the tracking system954can be integrated into the computer system952to provide a unified CAS system950in other embodiments.

The surgical method900begins at902by opening the patent's knee with the patient's leg generally extended. After any preparation of the surgical site, at904a femoral cutting block may be attached to the patient's femur. This may involve drilling a hole into the distal end of the femur to access the femoral intramedullary canal which defines the local anatomical axis of the femur. An intramedullary rod with an angle adjustable jig is connected to a distal femoral cutting block inserted into the intramedullary canal. A suitable arrangement is the distal femoral jig assembly as shown in the Attune Surgical Technique document.

As noted inFIGS.4to7above, the anatomical axis of the femur208is offset from the mechanical axis of the femur214by an angle that varies from patient to patient, depending largely on the length of their femur. For a shorter femur the angle is typically about 3°-4°, for a medium femur typically about 5°, and for a longer femur, typically about 6°-7°. As the femoral cut angle is defined in the planning stage104relative to the femoral mechanical axis214, the angular offset between the anatomical axis208and the mechanical axis is taken into account in determining the angel to be set using the adjustable jig so as to set the correct planned position of the distal cutting block. This is because the angular position of the cutting block is typically defined relative to the anatomical axis (along which the intramedullary rod passes), such that a zero degree angle of the adjustment jig corresponds to the distal cut line being perpendicular to the femur's anatomical axis.

Hence, at step906, the angle of the jig is adjusted so as to place the distal femoral cutting block so that the distal femoral cut line corresponds to the planned distal femoral cut angle, taking into account the offset between the mechanical axis and anatomical axis of the femur. The depth of the femoral cut will depend on the size of the implant being used and is often in the range of 4 to 16 mm, with 8-11 mm being typical. The distal femoral cutting block is then pinned in position, the femoral jig and IM rod removed and then the distal femoral cut is made at908.

At910, the tibial cutting block is attached to the tibia. The tibial mechanical axis and anatomical axis are usually coincidental and so usually there is no angular off set to be taken in to account for the tibial cutting block adjustment. A tibial alignment guide can be used to position and attach the tibial cutting block. A stable tibial alignment guide is described in the Attune Surgical Technique document. The tibial alignment guide is attached to the lower leg of the patient by attaching a first end to the patient's ankle and aligned with the second toe of the patient's foot and the mechanical axis of the tibia. A tibial cutting block is attached to the second end of the alignment guide and includes a central aperture through which a bone pin is placed aligned with the centre or midpoint of the knee. The medial-lateral position of the first end of the alignment guide is adjustable and when varied causes the tibial cutting block to pivot about the bone pin. Hence, at912the angle of the tibial cutting block can be adjusted by changing the medial-lateral position of the first end of the tibial alignment guide, until the cutting block angle corresponds to the planned tibial cut line. The tibial cutting block is then pinned in place and the proximal cut is made at914.

It will be appreciated that in other embodiments, the order of making the distal femoral cut and the proximal tibial cut can be reversed, and steps904to908may be carried out after steps910to914, with any appropriate modifications to the surgical steps resulting therefrom, and which modifications will be apparent to a person of ordinary skill in the art.

Hence, the distal femoral cut and proximal tibial cuts have now been made, but using the planned cut orientations obtained from the planning step104rather than conventionally planned positions. The planed tibial an femoral cut orientations help to ensure proper mechanical operation of the prosthetic knee while also maintaining the long leg alignment and anatomy of the patient as much as possible.

At916a spacer instrument may be inserted into the extension gap between the resected tibia and femur to assess the gap and any soft tissue release may be carried out to provide balance with the knee in extension. Soft tissue release may be more likely to be required in instances where the long leg alignment has been altered.

At918, the knee may be articulated into flexion to allow femoral sizing and rotation to be assessed. Two different approaches may be used. A measured femoral sizing and rotation approach may be used as indicated by step920. A measured femoral sizing and rotation guide as described in the Attune Surgical Technique document may be used. The size of the femur may be determined and the position of the femoral cutting block used to make the rest of the femoral cuts can be determined. A stylus attached to the guide can be used to determine the size of the femur. The guide is angle adjustable to allow the angular position of the femoral cutting block to be set on the resected distal femoral surface. If the angle of the tibial cut was not adjusted during planning and corresponds to the joint angle, then no change to the rotation of the femur may be introduced at this stage. The rotation of the femur is defined by the line tangential to the posterior most parts of the lateral and medial condyles. It is generally desirable for the angular relationship between this line and the plane of the resected proximal tibia to be kept the same. Hence, if an angular adjustment of the tibial cut was introduce during planning, e.g. 3°, to bring the tibial cut angle and/or the long leg alignment back within their boundaries, then the same angular adjustment is added to the femoral rotation. Hence, the femoral cutting block is rotated by an extra 3° to maintain the angular relationship between the posterior part of the condyles and the plane of the proximal tibial surface before being pinned to the resected distal femur.

Hence, a feature of the planning method may also include planning an angle of a posterior femoral cut to set the femoral rotation and which includes any angular adjustments made to the tibial cut angle during the planning method104.

An alternative to step920is to use a balanced approach, rather than a measured approach, to femoral sizing and rotation at step922. A balancing device, for example including a pair of spreaders, is used and introduced into the flexion gap to apply an equal force to the posterior parts of the medial and lateral condyles. The surgeon then positions a cutting block at an angle such that, when the posterior condyles are under load, the posterior cut is generally parallel to the plane of the resected tibial surface. Hence, in this approach, the soft tissue structure define the femoral rotation rather than the angle of the posterior femoral cut.

After the femoral cutting block has been positioned, using either the measured approach920, which may include applying an angular adjustment to the cutting block position, or using the balanced approach of922, at924, the posterior femoral cut is made at924.

After the posterior femoral cut has been made at924, a spacer block may be inserted in the flexion gap and the balance of the joint may be assessed and any soft tissue release carried out to improve the balance of the joint.

The rest of the surgical procedure is then largely conventional. The remaining femoral cuts are made at926to complete preparation of the femur. Trial implants may be attached during a trialing stage928and a trial reduction of the joint carried out. As will be appreciated trialing can give rise to iterative changes to the cuts and/or soft tissues until the surgeon is happy. Eventually, the prosthetic tibial and femoral components are implanted at930and then the knee is closed932.

As noted above the invention may include various operations from each of the general patient data acquisition102, planning104and surgical method106steps ofFIG.1. Also, the planning method may be implemented in a variety of ways ranging from software to printed media providing instructions to guide a user through the planning method and/or including fields for entering information or data and/or carrying out calculations to determine the planned tibial and femoral cut angles.

Generally, some embodiments of some of the aspects of the invention, for example some embodiments the planning and/or surgical method, may employ various processes involving data stored in or transferred through one or more computer systems. Embodiments of the present invention also relate to an apparatus for performing these operations. This apparatus may be specially constructed for the required purposes, or it may be a general-purpose computer selectively activated or reconfigured by a computer program and/or data structure stored in the computer. The processes presented herein are not inherently related to any particular computer or other apparatus. In particular, various general-purpose machines may be used with programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required method steps. A particular structure for a variety of these machines will appear from the description given below.

In addition, embodiments of the present invention relate to computer readable media or computer program products that include program instructions and/or data (including data structures) for performing various computer-implemented operations. Examples of computer-readable media include, but are not limited to, magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media; semiconductor memory devices, and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM). Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter.

FIG.11illustrates a typical computer system that, when appropriately configured or designed, can serve as a planning computer or CAS computer or part of a CAS system according to the invention. The computer system970includes any number of processors972(also referred to as central processing units, or CPUs) that are coupled to storage devices including primary storage976(typically a random access memory, or RAM), primary storage974(typically a read only memory, or ROM). CPU972may be of various types including microcontrollers and microprocessors such as programmable devices (e.g., CPLDs and FPGAs) and unprogrammable devices such as gate array ASICs or general purpose microprocessors. As is well known in the art, primary storage974acts to transfer data and instructions uni-directionally to the CPU and primary storage976is used typically to transfer data and instructions in a bi-directional manner. Both of these primary storage devices may include any suitable computer-readable media such as those described above. A mass storage device978is also coupled bi-directionally to CPU972and provides additional data storage capacity and may include any of the computer-readable media described above. Mass storage device978may be used to store programs, data and the like and is typically a secondary storage medium such as a hard disk. It will be appreciated that the information retained within the mass storage device978, may, in appropriate cases, be incorporated in standard fashion as part of primary storage497as virtual memory. A specific mass storage device such as a CD-ROM974may also pass data uni-directionally to the CPU.

CPU972is also coupled to an interface980that connects to one or more input/output devices such as such as video monitors, track balls, mice, keyboards, microphones, touch-sensitive displays, transducer card readers, magnetic or paper tape readers, tablets, styluses, voice or handwriting recognizers, or other well-known input devices such as, of course, other computers. Finally, CPU972optionally may be coupled to an external device such as a tracking system, a database or a computer or telecommunications network using an external connection as shown generally at982. With such a connection, it is contemplated that the CPU might receive information from the tracking system, network, or might output information to the tracking system, network or other device in the course of performing the method steps described herein.

Although the above has generally described the present invention according to specific planning methods and surgical procedures, the present invention has a much broader range of applicability. One of ordinary skill in the art would recognize other variants, modifications and alternatives in light of the foregoing discussion.