GB2598580A - Knee replacement bone preparation guide - Google Patents

Abstract

A combined differential tensioner and tibial cutting guide block is provided, for the purpose of referencing the tibial cut from the cylindrical axis 3 of the femoral component, during knee replacement surgery. The differential tensioner comprises medial 12 and lateral (13, fig 4) tensioning blocks of thickness, which can preferably be independently selected, such that the knee ligaments can be appropriately tensioned, prior to guiding a tibial cut which is parallel to the femoral component cylindrical axis. The tensioning blocks have curved keels 15, concentric with the cylindrical axis, enabling tibial cut posterior slope to be varied. The tensioning blocks preferably have one or more apertures (14, fig 4) which, when drilled, act to locate the positioning of the tibial component on the planar surface of the tibial cut 11. The invention allows knee movement and stability to be trialled, before the definitive tibial cut is made.

Description

1 KNEE REPLACEMENT BONE PREPARATION GUIDE 2

3 Background

4 Preparation of the bone to accept the prosthetic components of a total knee replacement comprises the following steps: distal femoral cut; femoral finishing cuts; excision of 6 osteophytes; tibial cut; final preparation of the tibial plateau to accept the tibial tray keel; 7 and patella resurfacing cut. (The patella preparation will not be considered further).

8 These cuts may be made using a saw, or powered burr, stabilised by cutting blocks or by a 9 robot arm.

11 Presently, the tibial cut is controversial, both in the planning and in the execution. There 12 are alternative strategies to choose between, when planning and executing a knee 13 replacement surgery, that have developed over the years. Here are three that are 14 commonly used: 16 First strategy. Both the distal femur and the proximal tibia can be prepared using a 17 standard alignment technique, aiming for a neutral mechanical axis of the limb, and a 18 new prosthetic joint line that is perpendicular to the mechanical axis. Ligament releases 19 are usually then needed to balance the joint. This is known as Mechanical Alignment technique. In this technique, the order of bone cuts is not particularly important: either 21 the femur or the tibia can be prepared first.

23 Second strategy. The proximal tibia is prepared first. Classically, this tibial cut is made 24 perpendicular to the mechanical axis of the tibia, though in modern times this cut is now often sloped into varus (Anatomical Alignment technique) in order to follow the native 26 slope of the tibial plateau more closely. Then, the knee is flexed, and the tibial cut is used 27 to guide the rotation of the femoral component. The femoral cuts are then finished.

29 Third strategy. The femur is prepared first, aiming to re-create the cylindrical axis of the native femoral condyles in the prosthetic joint. This is known as Kinematic Alignment 31 technique. It entails closely following the native articular surfaces of the femoral 1 condyles, accounting for cartilage loss, such that the implanted prosthesis is a close 2 surface replacement of the original, un-diseased joint. The surgeon then has to align the 3 tibial cut in the most appropriate way such that the joint is balanced (physiological degree 4 of tension in the soft tissue structures) as the tibia moves around the femoral condyles, from full extension to full flexion. This tibial cut is difficult to carry out, and very often a 6 're-cut is necessary, to alter either the varus-valgus alignment, or the posterior slope, or 7 the depth of the cut, or any combination of these three variables. Clinical results of this 8 Kinematic Alignment strategy to date are promising, and there is much interest in 9 improving the user-friendliness of the technique -and particularly in getting the tibial cut right first time.

12 The actual methods used to implement these alignment strategies include: 13 Measured resection instruments (traditional method); 14 Patient-specific cutting blocks, designed and 3D-printed from an MRI or CT scan ahead of the operation date; 16 Computer navigation 17 Robot-assisted surgery 18 These methods used for execution of the surgical plan should not to be confused with the 19 alignment strategy followed when designing the plan.

21 Whatever strategy is followed, and whatever methods are used, management of the soft- 22 tissue envelope of the knee is critical. Ideally, the bone cuts should be made in such a way 23 as to align the prosthetic components such that the knee is 'balanced'. In this paper we 24 will consider a knee to be balanced if the overall alignment (Hip-Knee-Ankle) is within acceptable limits, and the capsule and ligaments have a physiologically normal degree of 26 tension as the knee moves through its range of motion from full extension to full flexion.

28 A key principle of knee replacement surgery is that the balance of the knee (in other 29 words, the degree of tension within the soft-tissue envelope) is altered during the process, and particularly by the excision of osteophytes. The degree to which this occurs 31 can vary widely, which introduces a degree of uncertainty into pre-operative planning.

32 Consider Figure 1. The femoral trial component 1 is in place, but at this point (and at the 1 start of the procedure of course) the tibial osteophytes 7 are present. Here, the medial 2 collateral ligament (MCL) 5 is tented around a large osteophyte 7, a situation very 3 commonly encountered when varus deformity is present pre-operatively. It is difficult to 4 predict exactly how the MCL 5 has been affected until the offending osteophytes 7 are removed. Similar difficulties are encountered with large posterior osteophytes with are 6 often present with a fixed-flexion deformity, and with lateral compartment osteophytes 7 which are associated with valgus deformity. In large varus deformity cases with tibial 8 plateau bone loss, although the MCL 5 is passing over osteophytes, it may even be slack.

When considering the balancing of a knee, it is useful to consider the philosophy that the 11 knee is comprised of 3 separate but connected joints: the medial and lateral 12 compartments, and the patello-femoral compartment. The Kinematic Alignment strategy 13 is fundamentally based on the concept of the 'cylindrical axis 3': this is the axis which is 14 concentric with the 'best-fit' circles fitted into the medial and lateral posterior femoral condyles (see Figure 2). This cylindrical axis 3 is then considered to be the axis about 16 which the knee flexes, for the first 100 degrees or so of movement from a starting 17 position of full extension. The surgical plan when considering the femur, when using 18 kinematic alignment strategy, is to re-surface the femur so that the reconstructed joint 19 has the same cylindrical axis 3 as the un-diseased joint initially had (see Figures 1 & 2).

Osteophytes 7 are removed from the tibia and femur.

21 Now, the proximal tibial cut 11 must be planned and executed such that this 22 physiologically normal degree of tension is produced in the soft tissues, particularly the 23 medial and lateral collateral ligaments, and in the posterior capsule and posterior 24 cruciate ligament.

26 There exists a subset of knee replacement design where the medial femoral condyle 27 surface is partially that of a sphere, its geometric centre 4 located on the cylindrical axis 3 28 (see Figure 1). If the medial tibial insert articular surface is congruent with this sphere, 29 then this imparts antero-posterior (A/P) and medio-lateral (M/L) stability to the reconstructed joint. This is known as a 'medially-stable' or 'medial-pivot' knee. By 31 contrast, the lateral compartment is much less congruent, and may even include a fully 32 flat tibial insert surface, allowing A/P movement in the lateral compartment.

2 As the knee moves into deep flexion (approximately greater than 100 degrees flexion) the 3 lateral femoral condyle slides back as well as rolling, which assists flexion of the knee. The 4 posterior slope of the tibial insert surface in a standard knee replacement (the posterior slope of the lateral compartment tibial insert surface in a medial-pivot design), and by 6 inference therefore of the tibial cut, has a crucial influence on knee balance in flexion: 7 specifically, if the knee is too tight in flexion during trialling, an increase in the posterior 8 slope is often used as a technique to improve soft-tissue balance (see Figure 2).

Currently, the Kinematic Alignment recommended method to achieve this balance is to 11 make an initial tibial cut that aims to mirror the native tibial plateau slope in both the 12 corona! plane (for the so-called 'varus-valgus' slope) and in the sagittal plane (for the so- 13 called 'posterior slope 9'), and a measured depth of resection of the cut. Technically, this 14 can be difficult to achieve first time, as these multiple variables have to be considered concurrently. The balance is then assessed using spacer blocks. If the medial or lateral 16 compartment is too tight in knee extension, then the tibial cut can be re-cut in the 17 coronal plane, such as for example using a 2-degree re-cut block. If the knee is too tight in 18 flexion, the posterior slope can be increased, by again carrying out a tibial re-cut with 19 increased posterior slope. The balance, range of movement, alignment, stability, and patella tracking are then assessed using trial prostheses, prior to implanting the definitive 21 prostheses.

22 Alternatively, a patient-specific 3D printed cutting block that can produced from MRI/CT 23 can be used to guide the tibial cut. One flaw inherent in this technique is that soft-tissue 24 tension cannot be reliably estimated from scans: the ligaments 5,6 are frequently tented around osteophytes 7 (see Figure 1), and there is cartilage-loss, possibly bone-loss, and 26 deformity present at the time of the scan.

28 There is an additional, related factor when balancing the knee: the cut tibial surface 11 is 29 prepared to accept the tibial component keel. This in turn sets the M/L and A/P positioning of the tibial component, which has important effect on the knee balance.

31 Currently, this positioning is made by referencing the anatomy of the tibia: the exact 32 technique varies between surgeons and implants, but essentially it involves selecting a 1 'best-fit' position of the tibial tray that matches the perimeter of the cut tibial surface 11, 2 and also paying attention to the position of the tibial tubercle when considering the 3 rotational positioning of the tibial tray around the long axis of the tibia 2.

Clearly, it would be desirable to be able to trial the soft-tissue balance that a particular 6 plane of tibial cut would lead to should it be selected (with the appropriate insert 7 thickness), before the definitive cut is actually made. This is an area of current research 8 and development: there is prior art in this field, and significantly the paper by Yaron Bar- 9 Ziv and others which can be viewed open-access at: Pdx.doi.org/11).210371aoi,2019.08.03. This academic paper describes the use of a 11 'linked soft-tissue guided technique'. The technique initially resurfaces the femur, using 12 Kinematic Alignment. The soft-tissues of the knee ligaments and capsule are then 13 tensioned using spacers (of a range of thickness from 1-6mm) introduced into the medial 14 and lateral compartments, initially with the knee extended (in a similar fashion to that shown in Figure 3 of this present paper), and then with the knee flexed. If the tibial 16 plateau surface is in contact with the femoral trial, preventing the insertion of the spacer 17 blocks then a preliminary tibial cut is made, just sufficient to insert spacers. The tibial 18 cutting block is initially attached to the femoral trial with the knee flexed, with an 19 assembly that aligns the planned tibial cut parallel to the cylindrical axis, then the cutting block is pinned to the tibia, allowing the tibial cut to be made.

21 The authors noted some issues which they encountered when using their technique: 22 Firstly they noted that the posterior slope of the tibial cut is set from the femoral trial: 23 this must be positioned correctly, otherwise any flexed/extended positioning of the 24 femoral trial will be translated into a similar error in the posterior slope of the tibial cut.

Secondly, the posterior slope of the tibial cut will change if the knee is not held at exactly 26 90 degrees of flexion, again because this posterior slope is set from the femur when using 27 this technique. Thirdly, they found that although use of the independent spacers for the 28 medial and lateral compartments (which they name 'special shims') do allow the knee to 29 be balanced, they are unstable in their positioning and may fall out, either medially or laterally.

1 There is no prior art that describes a method of trialling the position of the tibial 2 component (with a standard, un-modified femoral trial or definitive component in situ) in 3 a way that assesses all of the important positioning factors before the definitive tibial 4 cut is made. These factors being: varus-valgus slope; posterior slope; depth of cut; medial-lateral positioning of the component on the cut surface; and antero-posterior 6 positioning. This would require a method and device that allows adjustment of the tibial 7 slope in both planes, combined with a method of differentially distracting the medial and 8 lateral compartments in order to physiologically tension the medial and lateral collateral 9 ligaments, while controlling the A/P and M/L positioning of this provisional tibial articular surface, therefore allowing the surgeon to trial the knee balance throughout the range of 11 knee movement, before executing the tibial cut.

13 Field of invention

14 Knee replacement surgery, or arthroplasty. The alignment of the tibial component(s).

16 Description

17 A combined ligament differential tensioner and tibial cutting guideblock is provided, to 18 align the tibial cut 11 during knee replacement. This comprises a medial compartment 19 tensioning block 12 with curved keel 15 that is available in a range of thickness, a lateral compartment tensioning block 13 with curved keel 15 that is of a range of thickness that 21 is selected independently from the medial block thickness; a tibial cutting guideblock 17 22 with integral saw slot; and a handle 18. This entire device is formed in one piece, so that 23 the tensioning blocks 12,13 are attached to each other, forming a single block that can be 24 inserted into the joint space with the femoral trial 1 in situ, so that the tibial cut 11 is now directly referenced from the cylindrical axis 3 in the coronal plane, while posterior slope 9 26 is selected by rotating the entire device about the cylindrical axis 3, this rotation being 27 enabled by the curved keels 15 which are concentric with the cylindrical axis 3. Means of 28 temporary fixation 16 to the tibia 2 are provided to enable range of knee movement and 29 knee stability to be assessed by the surgeon before the definitive tibial cut 11 is made.

1 It should be noted that the inventive device can be used in conjunction with standard, 2 unmodified femoral trial and definitive components, without the need for tracks or other 3 attachment points on the femoral component 1.

Providing a range of this device allows the medial 12 and lateral tensioning block 13 6 respective thickness to be independently selected, the range typically extending from 7 1mm to 10mm thicknesses, in 1mm increments. It is envisaged that this range of devices 8 with varying medial and lateral tensioning block thickness can be 3D-printed in a stiff, 9 high-density plastic, although metal construction may also be used. The undersurface of each tensioning block 12,13 is in the form of a curved keel 15, this curve being concentric 11 with the cylindrical axis 3 of the femoral trial component 1, allowing the whole device to 12 be positioned with more or less degree of posterior slope 9, on an uneven tibial surface 13 that may be in its original diseased state, or may have been preliminarily cut 14 conservatively 10, in order to remove any bony contact between this tibial surface and the femoral trial component 1.

17 In addition to guiding the plane of the definitive tibial cut 11 with reference to the 18 femoral condylar cylindrical axis 3, the inventive device acts to guide the A/P and M/L 19 positioning of the tibial component on this planar cut surface 11. This is achieved by incorporating an aperture (or apertures) in the device which can be step drilled in order 21 to mark a reference (or references) on the prepared tibial cut surface 11. This reference 22 can then be used in turn when preparing the cut surface to accept the tibial component 23 keel, the step which determines the M/L positioning, A/P positioning and axial rotation of 24 the implanted tibial component. In the specific example described, a medially-stable knee is used. A single central aperture 14 in the device is provided at the geometric centre of 26 the medial tensioning block 12, in order to guide a step-drill such that the resulting 27 reference drill-hole is directly in line with the sphere centre 4 of the medial condyle 28 sphere.

Variants 1 The device can be used together with 'conventional condylar' primary knee replacement 2 type, where the articular surface geometry is not congruent, but a cylindrical axis 3 can be 3 approximately determined in the femoral component 1, and the tibial component rotates 4 around the femoral condyles in a track that is approximately concentric with the cylindrical axis 3.

7 The device can be used together with the 'medially-stable' or 'medial-pivot' primary knee 8 replacement type, where the articular surface geometry of the medial compartment is 9 congruent and includes a segment of a sphere (thereby imparting A/P and M/L stability to the reconstructed joint), and the articular surface of the lateral compartment is less 11 congruent. Again, a cylindrical axis 3 is present in the femoral component, and the device 12 rotates around the femoral condyles in a curved track with an axis that is concentric with 13 the cylindrical axis 3.

The device can be used in a particular knee revision arthroplasty scenario: that where the 16 size, positioning and fixation of the femoral component is satisfactory, such that this 17 component can be retained, but where the tibial component is either loose, or is 18 unsatisfactory in positioning. In this situation, the device is used in conjunction with the 19 definitive femoral component, so that the tibial plateau preparation is referenced from the femoral cylindrical axis 3 with the soft tissues in physiological tension. In this revision 21 scenario, the tensioning blocks 12,13 may be necessarily thicker to accommodate tibial 22 plateau bone loss: it is envisaged that a range of thickness up to 25mm be provided, again 23 with selection of the medial and lateral tensioning block thickness independent from each 24 other, guided by the trial spacers.

26 A simpler variant of the device is provided to be used in medial or lateral uni- 27 compartment replacement arthroplasty. Consider Figure 8. The lateral tensioning block 28 13 can be omitted, allowing the device to be used in one compartment only, retaining 29 both cruciate ligaments and the articular surfaces of the opposite compartment. The articular surface geometry of the tensioning block 12 may be part-spherical, as shown in 31 Figures 5 8/. 8, or of a flatter, less congruent geometry. The curved keel 15 may be single, 32 and larger, or alternatively two curved keels 15 are provided within the single 1 compartment. The central aperture 14 is provided, to guide the M/L and A/P positioning 2 of the tibial component, referencing from the cylindrical axis 3 and from the medial 3 condylar sphere centre 4 if the femoral component is formed as a sphere segment, as 4 some are.

6 Essential features of the component parts 7 The features of these component parts will be described, and how these features are 8 inter-related, such that the device can be used to simultaneously control the following 9 variables: soft-tissue tension; tibial cut resection depth; varus-valgus tibial cut alignment; tibial cut posterior slope; and location of the tibial component on the plane of the 11 definitive tibial cut 11 surface, controlling translation of the tibial component in the A/P 12 and MIL directions.

14 TENSIONING BLOCKS Tensioning blocks 12,13 are provided, each with an upper articular surface and a lower, 16 keel 15 surface.

17 The articular surface geometries can be anywhere on the continuum that goes from a 18 fully congruent part-spherical surface for the medial block 12, to partially-congruent 19 concave surfaces for both medial 12 and lateral 13 blocks, to a flat surface for the lateral block 13. The essential point is that the articular surface geometry is set so that the 21 device is able to be rotated around an axis that is concentric with the cylindrical axis 3 of 22 the femoral component 1: this femoral component 1 being either a trial, or the definitive 23 implanted prosthesis.

24 Each block 12,13 is of a range of thickness that can be selected independently from the opposite block 12,13. It is envisaged that the range extend between 1-10mm in primary 26 cases, and up to 25mm in revision cases. This creates a large potential inventory of the 27 inventive device which can either be held in stock ready for use, or 3D-printed ahead of 28 surgery if the need is suspected for the case.

29 The contour of the keels 15 is such that they are formed in a segment of a circle, its axis being concentric with the cylindrical axis 3 of the femoral trial component 1.

1 In the medially-stable knee variant, the medial block 12 is provided with a central 2 aperture 14 which can be step-drilled in order to mark the position of the medial condylar 3 sphere centre 4 once the definitive tibial cut 11 planar surface has been completed, to 4 allow the A/P and M/L positioning of the tibial keel preparation to be referenced from the sphere centre 4.

6 Similarly, in the medial uni-compartment variant where the femoral component is formed 7 as a segment of a sphere, a central aperture 14 is provided.

8 In all variants, other apertures, single or multiple, in the tensioning blocks 12,13 can be 9 provided to guide M/L and A/P final positioning of the tibial component, and axial rotation of the tibial component about the long axis of the tibia 2.

11 The medial tensioning block 12 projects forward to meet the tibial cutting guideblock 17.

12 The superior surface of this projection is planar, and is parallel to the plane of the 13 projected definitive tibial cut 11, so that acts as a visual aid to the surgeon.

CUTTING GUIDE BLOCK

16 A tibial cutting guideblock 17 with integral saw slot is provided, either formed in one 17 piece with the tensioning blocks 12,13, or securely attached. The saw slot guides the 18 projected plane of the definitive tibial cut 11. It is envisaged that this plane will usually be 19 parallel to the cylindrical axis 3 when viewed from the front, and will create a joint space that matches a particular tibial component thickness. In revision situations, where 21 unusual configurations of the tibial component may be employed, the saw slot may be 22 orientated to guide a definitive tibial cut 11 in a plane that is not parallel to, but is 23 referenced from, the cylindrical axis 3.

HANDLE

26 A handle 18 is provided, either formed in one piece with the rest of the inventive device, 27 or securely attached. The handle is orientated approximately perpendicular to the plane 28 of the projected definitive tibial cut 11, such that the handle extends over the anterior 29 aspect of the leg, towards the ankle.

Means of attaching a long alignment rod (not shown) are provided, to provide a visual aid 31 of the posterior slope 9 and of the varus/valgus (coronal plane) slope of the projected 32 definitive tibial cut 11.

2 MEANS OF ATTACHMENT 3 Means of reversibly attaching the inventive device to the tibia are provided, in the form 4 of apertures and pins 16. These apertures and pins 16 are located through the handle 18 and cutting guideblock 17. They may also be provided (not shown) orientated obliquely 6 through the projection of the medial tensioning block 12 it extends forward to meet the 7 cutting guideblock 17.

9 The device in use: the sequence of steps during a primary, medially-stable, total knee arthroplasty 11 The knee is opened, and the femoral condyles are prepared to accept a trial femoral 12 component (Figure 1) 14 Osteophytes are excised, and the joint space is then assessed. In most cases, it is anticipated that a preliminary tibial cut is then carried out freehand, removing sufficient 16 bone to ensure that there is no contact between the femoral trial and the tibial plateau 17 when the ligaments are tensioned.

19 Trial spacers 19 are inserted separately into the medial and lateral joint spaces with the knee in extension, as illustrated in Figure 3. The balance is of the knee ligaments in 21 extension is assessed, and altered by varying the thickness of the medial and lateral 22 spacers 19 independently, until the 'best fit' balance is considered to be as close to 23 physiological as the surgeon is able to determine. The spacer thicknesses are noted.

The inventive device is now selected, from a range, with the medial and lateral tensioning 26 block thickness corresponding to the previously used 'best fit' trial spacers. The device is 27 inserted into the joint space (Figure 4). The collateral ligaments are now in physiological 28 tension, although the preliminary tibial cut surface may appear to be oblique, as it is in 29 this case: it is critical to appreciate that this preliminary tibial cut is not necessarily parallel to the cylindrical axis, and if this orientation of tibial cut was accepted, an 31 unbalanced knee with deformity would result.

1 Posterior slope is assessed (see Figure 2 for an illustration of native tibial plateau 2 posterior slope: it varies from between 0 and 12 degrees, when considered relative to the 3 perpendicular to the tibial mechanical axis). The inventive device is rotated around the 4 femoral condyles until the posterior slope mirrors the original posterior slope (if using kinematic technique), or else a value is selected arbitrarily, typically 5 degrees. This 6 estimation is made technically easier by attaching a long alignment rod to the handle 18 7 of the device. The inventive device is temporarily attached to the tibia using standard 8 orthopaedic techniques, such as wires or pins 16.

The knee is now trialled throughout its range of movement, from full extension to full 11 flexion. If loose or tight in flexion, the posterior slope is altered accordingly, by removing 12 the attaching wires, rotating the device around the cylindrical axis 3 (see Figures 5 & 6) by 13 the desired angle, and re-attaching it. The knee is trialled again.

Consider Figure 7. Now, the surgeon has selected a device that has tensioned both 16 collaterals to his/her satisfaction, throughout the range of knee movement, and the saw 17 slot in the tibial cutting guideblock 17 is parallel to the cylindrical axis 3 in the corona! 18 plane. It is not parallel to the preliminary tibial cut in this case, though on occasion it will 19 be (if both tensioning blocks are of equal thickness).

21 Definitive tibial preparation now proceeds, with the knee flexed and the tibial plateau 22 subluxed anteriorly according to usual practise. The medial tensioning block central 23 aperture 14, shown well in Figure 8, is step-drilled to a pre-determined depth that is 24 deeper than the planned definitive tibial cut 11, leaving a reference drill mark visible after the definitive tibial cut 11 is performed. The tibial cut 11 is made, using a saw guided by 26 the tibial cutting guide block 17 (Figures 7 & 8).

28 The central aperture 14-guided drill hole can now be used to guide the A/P and M/L 29 positioning of the tibial component keel preparation guide (not illustrated), so that the centre of the part-spherical depression of the medially-stable tibial component will be 31 correctly located with respect to the medial condylar sphere centre 4 (Figures 7 & 8).

1 Definitive components are implanted (together with the patella component if used), and 2 the knee is closed.

4 Figures List 6 Fig 1. Coronal (frontal) view of knee with femoral trial 1 in place, with some varus 7 deformity, large medial osteophytes 7, some erosion of medial plateau bone. The 8 cylindrical axis 3 and medial condyle sphere centre 4 are shown. The collateral ligaments 9 5,6 are illustrated 11 Fig 2. Sagittal (side) view with femoral trial 1 in place. The cylindrical axis 3, the native 12 posterior slope 9 relative to the tibial mechanical axis 8, and the MCL 5 are illustrated 14 Fig 3. Coronal view with femoral trial 1 in place, osteophytes removed from the tibia, preliminary tibial rough cut 10 sufficient to prevent any bony contact, and the 16 independent trial spacers 19 in situ, of different thickness: deformity is now corrected, 17 and the collaterals 5,6 are balanced 19 Fig 4. Coronal c/s of the inventive device in place in the joint space, showing the arrangement of the medial 12 and lateral tensioning blocks 13 with their respective keels 21 15, and the medial block central aperture 14. The collaterals 5,6 are balanced 23 Fig 5. Sagittal c/s through medial compartment, femoral trial 1 and sphere centre 4 which 24 lies on the cylindrical axis 3, the medial tensioning block 12 and central aperture 14, cutting block 17 and handle 18. The medial tensioning block curved keel 15 is shown, just 26 beyond the plane of the c/s. Part of the tibial cutting block 17 is seen curving around the 27 antero-medial aspect of the tibial plateau, beyond the plane of the c/s. The projected 28 plane of the tibial cut 11 is illustrated by a dashed line, and the posterior slope relative to 29 the tibial mechanical axis 8 is seen 1 Fig 6. Sagittal c/s through the lateral compartment and the lateral tensioning block 13.

2 The curved keel 15 is seen, just beyond the plane of the c/s 4 Fig 7. Frontal view of the inventive device pinned in place, ready for the definitive tibial cut 11 7 Fig 8. Plan view of the inventive device pinned in place on the tibial plateau 10, ready to 8 step-drill the central aperture 14. Femur and soft tissues are omitted for clarity. The 9 parallel dotted lines illustrate the inner edges of the 2 curved keels 15 which would not normally be visible on the plan view as they are located on the inferior surface 12 Figures Key 13 1 Femoral component 14 2 Tibia 3 Cylindrical axis 16 4 Medial condylar sphere centre 17 5 Medial collateral ligament (MCL) 18 6 Lateral collateral ligament (LCL) 19 7 Osteophytes 8 Mechanical axis of tibia 21 9 Posterior slope 22 10 Preliminary tibial cut 23 11 Definitive tibial cut 24 12 Medial compartment tensioning block 13 Lateral compartment tensioning block 26 14 Medial block central aperture 27 15 Tensioning block curved keel 28 16 Attaching wire and aperture 29 17 Tibial cutting guideblock 18 Handle 31 19 Trial spacers

Claims (1)

1. Knee replacement bone preparation guideCLAIMSClaim 1. A combined ligament differential tensioner and tibial cutting guideblock for the purpose of guiding the tibial cut 11 during primary or revision total knee arthroplasty, comprising medial 12 and lateral compartment tensioning blocks 13 with curved keels 15, and a tibial cutting guideblock 17 which guides the cut 11 with reference from the cylindrical axis 3 of the femoral trial or implanted femoral component 1 Claim 2. A combined ligament tensioner and tibial cutting block for the purpose of guiding the tibial cut 11 during primary or revision medial uni-compartment knee arthroplasty, comprising a compartment tensioning block 12 with one or more curved keels 15, and a tibial cutting guideblock 17 which guides the cut 11 with reference from the cylindrical axis 3 and/or from the medial condylar sphere centre 4 of the femoral trial or implanted femoral component 1 Claim 3. A combined ligament tensioner and tibial cutting block for the purpose of guiding the tibial cut 11 during primary or revision medial and patello-femoral bi-compartment knee arthroplasty, comprising a medial compartment tensioning block 12 with one or more curved keels 15, and a tibial cutting guideblock 17 which guides the cut 11 with reference from the cylindrical axis 3 and/or from the medial condylar sphere centre 4 of the femoral trial or implanted femoral component 1 Claim 4. As for the above Claims, where the femoral trial or implanted femoral component 1 is un-modified with tracks or other attachment points for tibial cutting guideblocks Claim 5. As for Claim land Claim 4, where the differential tensioning blocks 12,13 are available in a range of thicknesses that can be selected independently from the thickness of the other tensioning block Claim 6. As for Claims 2,3 and 4, where the tensioning block 12 is available in a range of thicknesses Claim 7. As for the above Claims, where the articular surface of the medial tensioning block 121s congruent with the part-spherical surface contour of the medial femoral condyle of the femoral component 1 Claim 8. As for Claims 1 to 4, where the articular surface of either/both of the medial 12 and lateral 13 tensioning blocks is less than fully congruent with the femoral component 1, and may be a flat surface Claim 9. As for the above Claims, where the under-surface profile of the curved keel(s) 15 is concentric with the cylindrical axis 3 of the femoral component 1 Claim 10. As for the above Claims, where a handle 18 is provided Claim 11. As for the above Claims, where means of temporary attachment to the tibia are provided by means of apertures in the handle 18, the tibial cutting guideblock 17, or the medial tensioning block 12, for the purpose of fixation wires or pins 16 Claim 12. As for the above Claims, where an aperture 14 or apertures is provided in the tensioning blocks 12,13, to guide the formation of reference mark(s) in the tibial definitive cut surface 11 Claim 13. As for Claim 12, where this aperture 14 is centrally located in the medial tensioning block 12 which has a part-spherical articular surface contour which is congruent with the part-spherical medial femoral condyle of a medially-stable knee replacement 1, such that a reference mark can be drilled or otherwise formed in the tibial plateau such that it passes through the plane of the definitive tibial cut 11, at the closest point to the medial sphere geometric centre 4 of the femoral component 1 Claim 14. A combined ligament tensioner and tibial cutting guideblock as in the preceding Claims, where the saw slot integrated in the guideblock 17 is orientated to align the definitive tibial cut 11 so that it is parallel to the cylindrical axis 3 of the femoral component 1 when viewed in the frontal plane, at a depth which correlates to a defined tibial tray and insert thickness