METHOD FOR DESIGNING A JOINT PROSTHESIS

Abstract

A method for designing a two-part joint prosthesis (830) comprises: providing kinematic data of a subject's joint under load; and designing the joint prosthesis using the kinematic data, wherein the working surfaces of the two-part prosthesis comprise, consist essentially of or consist of cellular material. Advantageously, the method may not require any intra-operative adjustments to replace one or more of the components (831, 832), e.g. with a component of a different size. In particular, if components are made of biological tissues, such as a patient's own cells, it is advantageous to design and produce an implant that requires no adjustments intra-operatively as each implant may be manufactured specifically for each patient, and the time and costs of producing a range of sizes, most of which would not be required, would otherwise be prohibitive.

Claims

1. A method for designing a two-part joint prosthesis, the method comprising: providing kinematic data of a subject's joint under load; and designing the joint prosthesis using the kinematic data, wherein the working surfaces of the two-part prosthesis comprise, consist essentially of or consist of cellular material.

2. A method according to claim 1, comprising constructing the two-part joint prosthesis.

3. A method according to claim 1, wherein the joint prosthesis is a knee prosthesis.

4. A method according to claim 3, wherein the knee prosthesis comprises a femoral component and a tibial component, or a femoral component and a patellar component.

5. A method according to claim 1, wherein the prosthesis is devoid of an insert.

6. A method according to claim 1, wherein at least a portion of the two-part joint prosthesis comprises 3D bioprinted cells.

7. A method according to claim 6, wherein a cartilage portion or the cartilage portions or a subchondral bone portion or a bone portion of the two-part joint prosthesis comprises 3D bioprinted cells.

8. (canceled)

9. A method according to claim 6, comprising determining the thickness of a/the femoral component and/or of a/the tibial component.

10. A method according to claim 9, comprising determining the thickness of a/the femoral component and/or of a/the tibial component with respect to one or more parameters selected from the list consisting of cartilage thickness, thickness of sub-chondral structure, cancellous bone, and knee kinematics.

11.-15. (canceled)

16. A method according to claim 1, comprising adjusting the thickness of a/the femoral component and/or of a/the tibial component of the prosthesis design, based on dynamic kinematic data of the subject's joint.

17. A method according to claim 1, comprising obtaining kinematic data of the subject's joint under load in the coronal plane.

18. (canceled)

19. A method according to claim 17, comprising measuring alignment of the subject's joint in the coronal plane, without application of a load and with application of a load.

20. A method according to claim 19, comprising measuring alignment of the subject's joint in the coronal plane, without application of a load at one or more degrees of flexion of the joint between about 0° and about 100° of joint flexion.

21. A method according to claim 19, comprising measuring alignment of the subject's joint in the coronal plane, under application of an external force on the subject's joint, wherein the external force applied is selected so as to reduce or correct a joint deformity to a predetermined value or limiting value defined by a soft tissue envelope.

22.-24. (canceled)

25. A method according to claim 20, comprising calculating the difference between the alignment of the subject's joint without load and under application of an external force.

26.-29. (canceled)

30. A method according to claim 1, comprising manufacturing the joint prosthesis using 3D bioprinting.

31. (canceled)

32. A model of a two-part joint prosthesis obtained or obtainable by the method according to claim 1.

33. A two-part joint prosthesis obtained or obtainable by the method according to claim 1.

34. A two-part joint prosthesis according to claim 33, wherein at least a portion of the two-part joint prosthesis is 3D-bioprinted.

35. A computer program comprising computer executable instructions that, when executed by a processor, cause the processor to control an additive manufacturing apparatus to manufacture the prosthesis of claim 33.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0089] Embodiments of the invention are described with reference to the accompanying drawings, in which:

[0090] FIG. 1 shows a radiograph of a subject's legs showing alignments of lower limbs, femurs and tibias in the coronal plane;

[0091] FIG. 2 shows and anterior-posterior (AP) view of a knee without valgus stress correction;

[0092] FIG. 3 shows an AP view of the knee of FIG. 2 with valgus stress correction using standardise force correction showing gap in medial compartment;

[0093] FIG. 4 shows a graph illustrating data processing of the alignment of a subject's joint according to an embodiment of the invention;

[0094] FIG. 5 shows an apparatus used for measuring the alignment angle in the coronal plane for various degrees of flexion of a subject's knee;

[0095] FIG. 6 shows an apparatus used for measuring the alignment angle in the coronal plane for various degrees of flexion of a subject's knee with application of any external force using a Force Measurement Device;

[0096] FIGS. 7-10 show schematic representations illustrating a method of designing a joint prosthesis according to an embodiment;

[0097] FIG. 11 illustrates a two-component UKA initial design for femoral component according to an embodiment;

[0098] FIG. 12 illustrates a two-component UKA adjusted thickness design for femoral component according to an embodiment;

[0099] FIG. 13 illustrates a two-part knee prosthesis for a UKA, according to an embodiment of the present invention;

[0100] FIG. 14 illustrates a two-part knee prosthesis for a PFJR, according to an embodiment of the present invention;

[0101] FIG. 15 illustrates a two-part knee prosthesis for a TKA, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF DRAWINGS

[0102] FIG. 1 shows a radiograph 100 of a subject's legs showing alignments of legs, femurs and tibias in the coronal plane. On the right leg 2, line 12 represents the mechanical axis of the femur which refers to a line drawn from the centre of the femoral head to the centre of the knee. Line 14 represents the anatomical axis of the femur and refers to a line drawn along the centre of the intramedullary canal (broadly following the main axis of the diaphysis). The anatomical and mechanical axes of the tibia are both represented by line 16, and in FIG. 1 both coincide.

[0103] The global mechanical axis, also referred to as Maquet's line, labelled as line 18 on the left leg 4, extends from the femoral head to the centre of the talus. If this line 18 passes through the centre of the knee, it is considered that the knee is “balanced” or has no deformity in the coronal plane. In this example, line 18 passes through the medial (inner) side of the knee, which indicates a varus deformity.

[0104] In a conventional approach, in order to reduce the degree of varus alignment in the knee, a surgeon would use a conventional 3-part implant and adjust intra-operatively the dimensions, e.g. thickness, of the polyethylene implant in order to correct alignment. Alternatively, or additionally, intra-operative cut adjustments may be performed to the tibial and/or femoral bone(s) to which the implant is intended to be fixed.

[0105] In contrast, the method of the present invention comprises designing a two-part prosthesis, in this embodiment a knee implant. The method comprises determining and/or adjusting the dimensions, e.g. thickness, of at least one of the components of the implant design, in this embodiment of the tibial component or the femoral component, based on dynamic kinematic data of the subject's knee.

[0106] For simplicity, the embodiments described herein exemplify a procedure for designing a prosthesis for a unicompartmental knee arthroplasty (UKA), with the adjustment to correct a subject's knee deformity being applied to the tibial component or the femoral compartment. However, it will be understood that the teachings described herein may equally apply to a TKA procedure.

[0107] Unicompartmental knee arthroplasty (UKA) surgery may typically be carried out on patients with medial compartment osteoarthritis (OA). In these patients the medial compartment of the knee has worn away, leaving them with a varus deformity in their coronal alignment. The aim of the surgery is to replace the worn surfaces and to correct the coronal deformity to being close to neutral.

[0108] When a varus coronal deformity is corrected during surgery this typically causes a gap to appear in the medial compartment of the knee between the two bearing surfaces. This is illustrated in FIGS. 2 and 3.

[0109] FIG. 2 shows and anterior-posterior (AP) view of knee 200 without valgus stress correction, and FIG. 3 shows an AP view of the knee with valgus stress correction using standardise force correction showing gap in medial compartment.

[0110] The size of the gap created depends of the angle of the deformity correction and can be calculated via trigonometry. This gap needs to be filled by the UKA implant. This is conventionally done by using different sizes of polyethylene inserts that sit between the tibial and femoral components, and is currently assessed and adjusted intra-operatively.

[0111] To create a two-component implant (femoral and tibial components), e.g. with a view to create a 3D bioprinted UKA, it is necessary to design a correctly sized 2-part implant for the patient pre-operatively. It is not possible to adjust the fit of the implant intra-operatively as per current practice. Therefore the size of the implant required to correct the coronal alignment deformity needs to be calculated pre-operatively.

[0112] When carrying out a UKA (unlike a TKA) the existing soft tissue envelope (ligaments around the knee) is maintained. It is therefore this soft tissue envelope that defines the correctability of the coronal alignment i.e. how close to neutral alignment the knee can go before the soft tissues constrain any further movement. This soft tissue envelope varies as the knee goes from extension to full flexion. Therefore the gaps created vary as the knee goes from extension to flexion. These gaps need to be calculated and this information used in the implant designs to ensure that the components are the correct thickness.

[0113] In an embodiment, the method comprises determining and/or adjusting the dimensions, advantageously the thickness, of the tibial component, based on dynamic kinematic data of the subject's knee.

[0114] In the present embodiment, as illustrated in FIG. 4, we consider a varus knee with medial knee osteoarthritis, with 10° varus in full weight bearing.

[0115] In the present embodiment, the predetermined value to which the varus alignment should be corrected was selected as 2°.

[0116] Alignment of the subject's knee in the coronal plane was measured in 10-degree increments between flexion angles of 0° and 90°. These measurements were carried out using a non-invasive measurement system 300, in this embodiment PhysioPilot®, as described in more detail below, and as shown in FIG. 5. The degree of varus alignment in the coronal place over the range of flexion is shown as “30” in FIG. 4.

[0117] Alignment of the subject's knee in the coronal plane under full weight-bearing conditions was then measured under application of an external force on the subject's joint. The force was applied so as to reduce the varus alignment to the predetermined value of about 2°. These measurements were also carried out using a non-invasive measurement system 400, in this embodiment PhysioPilot®, as described in more detail below, and as shown in FIG. 6. The degree of varus alignment in the coronal place over the range of flexion is shown as “32” in FIG. 4. As shown in FIG. 6, a force measurement device 41 is held in the clinician's left hand and placed over the medial malleolus. The trackers 42 are attached to the lower limb with straps 43 and also another tracker is attached to the force measurement device 41. The clinician's right hand (unseen) is placed on the lateral epicondyle of the knee.

[0118] The difference between the alignment of the subject's knee in unloaded condition and under application of an external force, was then calculated, as shown as “34” in FIG. 4.

[0119] As mentioned above, in this embodiment, these measurements were also carried out using a non-invasive measurement system, in this embodiment PhysioPilot®.

[0120] In this system, as shown in FIG. 5 the subject is typically positioned supine with active infrared (IR) trackers 42 non-invasively secured to the distal thigh and proximal calf using straps and mounting plates 43. Movement is captured by a camera 45 connected to a computer 46. The subject is instructed to relax their leg muscles. Anatomical landmarks (femoral epicondyles, centre of the knee, ankle malleoli, anterior ankle centre) are palpated and kinematic hip and knee joint centres are located in three dimensions through a tracked sequence of clinical manoeuvres. These points are used to “register” the lower limb in order to determine coronal and sagittal mechanical femoro-tibial (MFT) angles. The coronal MFT angle (alignment) with the lower limb in maximum passive extension can then be recorded by supporting the limb only under the heel.

[0121] The passive range of motion of the knee, from full extension (0°) to full flexion (in this embodiment 90°), is then assessed. The knee is passively flexed with the clinician supporting the limb under the thigh and at the heel.

[0122] The anterior-posterior (AP) movement of the knee can also be measured using PhysioPilot® to confirm that the ACL is intact and that the patient is suitable for UKA. The AP laxity is measured using the Lachman test. The knee is held at 15°-30° of flexion as measured by the PhysioPilot®. The clinician holds the patient's thigh with one hand and the calf with the other with their thumb on the tibial tuberosity. The tibia (shank) is pulled forwarded and the amount of relative motion in mm to the femur (thigh) is recorded by PhysioPilot®. This measurement can be compared to known limits to determine whether the ACL is intact and so the patient is suitable for a UKA.

[0123] Knee laxity in the coronal plane can be quantified using varus and valgus stress manoeuvres applying manual force directly with one hand over the medial ankle malleolus and with the supporting hand placed over the lateral femoral epicondyle for a valgus stress or with one hand over the lateral ankle malleolus and with the supporting hand placed over the medial femoral epicondyle for varus stress. The application of the force is directed in the coronal plane and perpendicular to the mechanical axis of the tibia, as best illustrated in FIG. 6. During laxity assessment, the moment arm is determined as the perpendicular distance from the knee centre to the line of action of the applied force; this distance is determined by PhysioPilot® using the tracked force measuring device. When carrying out these varus and valgus stress manoeuvres the tracked force measuring device is used to measure the magnitude, point of application and direction of the force applied. This allows the actual moment being applied to the knee in the coronal plane to be calculated and so the laxity assessment can be standardised. During these stress manoeuvres, the knee is typically held at between 0° and 5° of flexion as indicated by the PhysioPilot® measurement of the sagittal MFT angle. If the knee cannot extend to 0° the stress measurements are performed within a 5° window of flexion from the maximum extension angle. The maximum possible angular correction of alignment in the coronal plane for a varus knee can be measured by applying a valgus stress to the knee as given above. This determines the corrected deformity that will be achieved intra-operatively. This measurement does not aim to reach a predetermined value but measures the limiting value of the soft tissue envelope i.e how much the knee malalignment can be corrected without altering the existing soft tissue envelope. This measurement of maximum possible correction can then be repeated at various levels of knee flexion through the range of motion with the PhysioPilot® measuring the knee flexion as well as the knee laxity. These measurements of corrected deformity will be specific to the individual being measured.

First Embodiment of Calculations: Single-Radius and Dual-Radius Designs

[0124] Having calculated the difference between the alignment of the subject's knee in full weight-bearing condition and under application of an external force, as shown in FIG. 4, an average of these values over the range of flexion was calculated. Advantageously, in this embodiment, the median average of the difference values was calculated. In the embodiment of FIG. 4, the median average adjustment was 4.4°.

[0125] Calculating the average, e.g., median average, allows a user to apply the calculated average as a target correction pre-operatively in the design of the knee prosthesis.

[0126] In this embodiment, two designs of the femur were used: single-radius and dual-radius.

[0127] In the single radius design, the centre of rotation of the knee is known and the method involved calculating the size of the tibial component using model (1):

[0128] Wherein θ is the adjustment angle, [0129] x is the distance from the joint axis to the distal part of the joint in mm, and [0130] y is the adjustment gap in mm.

[0131] The method then involved calculating the adjustment gap using equation (1):

y=x tan(θ)  Equation (1)

[0132] Wherein θ is the adjustment angle, [0133] x is the distance from the joint axis to the distal part of the joint in mm, and [0134] y is the adjustment gap in mm.

[0135] Thus, knowing x, and having calculated θ, y can be calculated using equation (1).

[0136] Using the single-radius design, the adjustment gap will be a fixed number, and this design can be used to draw different varus/valgus curves over degrees of flexion for different y gap values.

[0137] In the dual radius design, a geometric construction of the femur in the sagittal view can be represented by Model (2):

[0138] The model is composed of two arcs from circles with different radii, with an internal tangential at a point during flexion. θ is the degree of flexion in the knee relative to the axis of rotation around point A from 0 to 90 degrees.

[0139] To calculate the size of the tibial component in this case, the change in gap distance during the rotation through the dual-radius model is adjusted. This can be represented by Model (3):

[0140] During rotation, the tibia reaches a point when the arc of the first circle and second circle are tangential (at point D). As the knee continues to rotate about point A (the centre of the knee), an adjustment in the calculate gap distance is needed as line CE brings the tibia closer to the centre of the knee than if the knee was modelled exclusively with an arc from the circle with the large radii. Thus, the method involves calculating CE is as a function of θ.sub.1 in order to calculate the loss in gap space over the range of motion (as the tibial will move up by distance CE once it reaches the tangent at point D.

[0141] In order to calculate CE, the method uses equation (2) (it will be appreciated that derivations may be performed for the different variables):

[00001]$\begin{array}{cc}\Delta y^2=X_1^2+x_2^2-2X_1{X}_2\mathrm{Cos}\left(\frac{-1_{}-2_{}}{2}\right)& \mathrm{equation}\left(2\right)\end{array}$

[0142] Where CE=Δy; [0143] CD=X.sub.1 [0144] DE=X.sub.2

[0145] From the equation (1), the gap value ‘y’ can be adjusted using the value for Δy once the knee enters the angular range in the range of motion where the arcs are tangential such that y=x tan(θ)−Δy (equation (3)).

[0146] Using this adjustment, we can perform the same calculations as above with the single radius model where we obtain a y-value where the median corresponds to the target correction value for θ. This calculated value ‘y’ corresponds to the required thickness of the tibial component in the two-part prosthesis.

Second Embodiment of Calculations

[0147] Based on the assumptions that the bearing surfaces of the medial compartment are in contact when the knee 500 is in its pre-operative (deformed) alignment (FIG. 7) and that when the alignment is corrected that the tibia pivots around the femur about the contact point in the lateral compartment (FIG. 8), if the width of the medial tibial plateau, the distance from the centre of the knee to the contact point in the lateral compartment and the correction angle are known the maximum gap that appears (at the medial edge of the tibia) can be calculated (FIG. 9). The required anatomical measurements can be taken from medical images such as X-ray, MRI, CT etc. The correction angle can be measured by PhysioPilot®. This correction angle is the maximum correction in coronal deformity as limited by the soft tissue envelope, with the corrected deformity being the limiting value of coronal alignment. FIGS. 7-9 show a knee having a femoral end 20 and a tibial end 22, and illustrate the femoral mechanical axis 12, the tibial mechanical axis 16, and the initial tibial plateau location 52.

[0148] In FIGS. 7-9:

TABLE-US-00001 α Initial deformity (no load) β Corrected deformity (applied load) θ Correction angle σ Angle between tibial mechanical axis 16 and tibial plateau 52 (tibial mechanical angle) a Width of medial tibial plateau (distance from knee centre to medial edge of the tibial plateau) b Distance from knee centre to point of contact 56 in the lateral compartment 20b x Maximum gap width due to deformity correction

[0149] A trigonometrical calculation can be carried out at each knee flexion with the measured parameters above to calculate the maximum gap (x) throughout the range of motion. This assumes that the lateral compartment does not compress.

[0150] The correction angle is defined as

=−  (Equ 1)

[0151] The angle between the initial tibial plateau location 52 and corrected tibial plateau location 54 (FIG. 8) is)

(+)−−=−=  (Equ 2)

[0152] The maximum gap width (x) (FIG. 9) is

x=(a+b)*tan  (Equ 3)

[0153] Using Equation 3 the amount the prosthesis needs to be thickened to fill the gap (x mm) and give the correct coronal deformity correction at each degree of flexion through the range of motion can be calculated. For example: [0154] a=40 mm [0155] b=20 mm

TABLE-US-00002 Knee flexion (°) Initial deformity Corrected deformity (°) x (mm) 0 10° varus  2° varus 8 8.4 10 10° varus  2° varus 8 8.4 20 10° varus  2° varus 8 8.4 30 9° varus 1° varus 8 8.4 40 9° varus 1° varus 8 8.4 50 9° varus 1° varus 8 8.4 60 9° varus 2° varus 7 7.4 70 8° varus 2° varus 6 6.3 80 8° varus 2° varus 6 6.3 90 8° varus 3° varus 5 5.2 100 7° varus 3° varus 4 4.2

[0156] A more complicated calculation can be carried out that assumes a small constant compression (z) of the lateral compartment 20b when a valgus load is applied. This is a better approximation of the true situation of the cartilage 50 being compressed under load. The maximum gap width with compression can be given as x.sub.z.

[0157] If the compression, z, is assumed to be in the direction parallel to tibial mechanical axis 16 (FIG. 10) then it can be seen that this would move the whole tibial plateau z mm in that direction moving the point of contact 58. It also changes the point where lines representing the original tibial plateau location 52 and corrected tibial plateau location 55 cross as this is no longer at the original point of contact 56. It can be seen that x.sub.z (x with compression) will be

x.sub.z=x−z  (Equ 4)

[0158] This adjustment can be applied to all the maximum gap widths calculated through the range of motion. A more complex model would also take into account that as the knee flexes that the point of contact 56 in the lateral compartment 20b moves around the tibial surface so that the distance b would also change as the point of contact 56 moved. This can be measured using a series of static medical images or modelled using published information on how the point of contact tracks during flexion [Kurosawa, H., P. S. Walker, S. Abe, A. Garg, and T. Hunter. “Geometry and motion of the knee for implant and orthotic design.” Journal of biomechanics 18, no. 7 (1985): 487-499.].

[0159] An even more complex model can be created using the 3D imaging to create a solid model and then using Finite Element Analysis (FEA) techniques to model the knee. This could be generated with standard published material parameters for bone, cartilage and ligaments and then using the information gained from the kinematic assessment of the knee correction as boundary conditions and refining the material properties, particularly the ligaments, to make the model alignment for the applied load match that measured non-invasively. This model could then be used to directly measure the gaps through the range of motion. This model would include the compression of the cartilage 50 and the movement of the point of contact 56 without any specific assumptions.

[0160] The implant design can then be adjusted to ensure that these gaps are filled by the implant through the range of motion.

[0161] Using static and dynamic patient specific data, the above calculations allow a user to define the gap that needs to be filled to implement alignment correction. Adjustments of final implant position and orientation can be completed using both of these data. The thickness of both the tibia and the femur can then be implemented during manufacture of the prosthesis, e.g. of the tibial component in a UKA procedure, for example by 3D bioprinting the implant.

[0162] This can be repeated for a lateral UKA if necessary, mirroring the above methods.

[0163] An example is the implant is designed to fit a patient based on the medical imaging as per current practice. This uses the existing deformed anatomy to design a best fit implant. This would be sized to fit the current contact surfaces and the operation planned with specific bony resections to maintain the joint line. However these components would then need to be adjusted so that the deformity is corrected. This can be done by adding the calculated maximum gap widths to the thickness of the design.

[0164] Example: In an embodiment, if a medial UKA implant 600 was designed based on a single radius femoral component with a thickness of 4 mm and a flat tibial component with a 3° posterior slope and thickness of 10 mm, in the sagittal plane a cross-section of the femoral component through the points of contact in the medial compartment 60 would be represented as per FIG. 11. This would be sized to be the best fit to the medical imaging data. The femoral component 700 design could then be adjusted based on the maximum gap calculations above, as shown in FIG. 12. The radii would be adjusted to ensure a smooth transition along the contact surface.

[0165] It can easily be seen that the adjustment does not depend on the initial design of the femoral component, whether it is a single, dual or multi-radius design the thickness will be adjusted in the same way. It can also be seen that the adjustment could be made partly on the femoral component and partly on the tibial component or all on the tibial component.

[0166] It will be clear to those skilled in the art that using this method it is possible to calculate the gap required to be filled for deformity correction at any point across the tibial plateau, not just at the medial or lateral edge, and then adjust the thickness of the implant to account for this.

Total Knee Arthroplasty (TKA)

[0167] Total knee arthroplasty (TKA) surgery is usually carried out on patients with both medial and lateral compartment OA. In these patients both compartments of the knee have worn away, often with one compartment wearing more than the other leaving them with a large deformity in their coronal alignment. They also often have a deformity in the sagittal plane, called a fixed flexion deformity. The aim of the surgery is to replace the worn surfaces and to correct the coronal and sagittal deformities to being close to neutral.

[0168] When carrying out a TKA (unlike a UKA) the soft tissue envelope (ligaments around the knee) is more often released intra-operatively to allow correction of coronal, sagittal and transverse alignment. There are also a number of different coronal alignment paradigms that can be employed by the surgeon: aiming for mechanical alignment (0° or 180° MFT angle); aiming for kinematic alignment (usually a few degrees of varus) or anatomical alignment (reproducing the average joint line coronal alignment i.e. 3° oblique joint line with 3° varus tibia and 3° valgus femur). The surgeon does not know the releases and adjustment to end up with good MFT alignment.

[0169] To be able to design a two-part TKA implant pre-operatively requires assumptions to be made. As per the UKA the knee kinematics can be measured non-invasively. This allows the assessment of any flexion contracture or hyperextension and the assessment of any varus and valgus contracture. An example would be a knee with OA and an unloaded 20° varus coronal deformity and 15° fixed flexion contracture (FFC). The non-invasive assessment shows that the coronal alignment can be corrected to 8° varus and the sagittal alignment can be corrected to 10° FFC. If the surgeon wishes to go for kinematic alignment maintaining the soft tissue envelope that opening of the medial gap with 12° of correction can be calculated as above except that the point of rotation of the tibia around the femur will be the knee centre so b=0 in Equ 3 (due to both compartments having OA). This gap can then be added to thickness of the component(s) as described above.

[0170] However if the surgeon wishes to go for mechanical alignment there will be intra-operative releases of the soft tissues to get to around 1° varus and 0° FFC. In this case the opening of the medial gap with 19° of correction can be calculated as above except that the point of rotation of the tibia around the femur will be the knee centre so b=0 in Equ 3 (due to both compartments having OA). This gap can then be added to thickness of the component(s) as described above.

[0171] It should be clear that the methods listed here can be used with combinations of partial knee replacement implants i.e. combination of medial UKA, lateral UKA and patello-femoral joints are also possible [Heyse, Thomas Jan, Ahmed Khefacha, and Philippe Cartier. “UKA in combination with PFR at average 12-year follow-up.” Archives of Orthopaedic and Trauma Surgery 130, no. 10 (2010): 1227-1230]. Further, different knee designs are possible such as TKA preserving only the posterior cruciate ligament (PCL) or both PCL and anterior cruciate ligament (ACL).

[0172] FIGS. 13-15 show two-part joint prostheses according to embodiments of the present invention.

[0173] FIGS. 13 and 14 illustrate two-part knee prostheses for partial knee replacement. FIG. 13 shows a prosthesis 810 for a UKA (unicompartmental knee arthroplasty), with femoral component 811 and tibial component 812. FIG. 14 shows a prosthesis 820 for a PFJR (patello femoral joint replacement), with femoral component 821 and patellar component 823.

[0174] FIG. 15 shows a two-part knee prosthesis 830 for a TKA (total knee arthroplasty), with femoral component 831 and tibial component 832.

[0175] As can be seen from FIGS. 13-15, each component of prostheses 810, 820 and 830 is made of subchondral bone and bone 72, and of cartilage 74. Thus, the prostheses 810, 820 and 830 do not include an insert, and allow for implantation of the prosthesis to correct a deformity in a subject's knee without the need for any intra-operative adjustments (typically done on the insert component).

[0176] Intraoperatively, a surgeon typically opens the knee in a conventional fashion and prepares the “implant bed” by resecting the exact amount of predetermined bone on the femur and the tibia, for example using computer guided surgery.

[0177] Once the preparation is complete, the surgeon typically performs the trial using a plastic model identical to the 3D bioimplant or other fixed sizes implant. The computer assisted measurements will confirm the plan and the surgeon will replace the plastic trial with the bioimplant starting with the tibia and then the femur. The soft tissue knee approach is then closed in a normal fashion.

[0178] It will be understood that the present embodiments are provided by way of example only, and that various modifications can be made to the present embodiments without departing from the scope of the invention.