WO2010125474A2

WO2010125474A2 - Device and method of determination of the knee flexion axis in computer assisted surgery

Info

Publication number: WO2010125474A2
Application number: PCT/IB2010/001285
Authority: WO
Inventors: James B. Stiehl; Stéphane Lavallee; Anthony Boyer
Original assignee: Blue Ortho
Priority date: 2009-05-01
Filing date: 2010-04-29
Publication date: 2010-11-04
Also published as: WO2010125474A3

Abstract

Device and method of determination of the knee flexion axis in Computer Assisted Surgery The present invention relates to a computer assisted surgical navigation method for determining the called flexion axis of a kneewherein a tensor (22) has previously been inserted, the pressure of which having been controlled to tighten the ligaments, comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia (8) with respect to the femur(2)along a flexion/extension movement, computing said relative positions to determine the flexion axis of the knee. The invention also relates to a surgical device for determining the flexion axis of a knee.

Description

Device and method of determination of the knee flexion axis in Computer Assisted Surgery

Purpose of the Invention [01] The present invention relates to a method to define the flexion axis of the knee during a surgical navigation procedure. The present invention is directed to knee arthroplasty such as total knee arthroplasty, uni knee arthroplasty, and knee revision procedures.

Background of the invention

[02] Arthroplasty is surgery to relieve pain and restore range of motion by realigning or reconstructing a joint. Typical arthroplastic options include joint resection, interpositional reconstruction, and total joint replacement. Joint resection involves removing a portion of a bone from a joint to create a gap between the bone and the corresponding socket, thereby improving the range of motion. Scar tissue eventually fills the gap. Pain may be relieved and motion restored, but the joint is typically less stable. Interpositional reconstruction reshapes the joint and adds a prosthetic disk between the bones forming the joint. The prosthesis can be made of plastic and metal or from body tissue such as fascia and skin. If the joint does not respond to the more conservative treatments (which may include medication, weight loss, activity restriction, and/or use of walking aids such as a cane), joint replacement is often considered appropriate. Joint replacement (i.e., total joint arthroplasty) is the surgical replacement of a joint with a prosthesis. Many joint replacements are needed because arthritis has caused the joint to stiffen and become painful to the point where normal daily activities are no longer possible. Arthroplasty, especially joint replacement, is becoming an increasingly prevalent treatment. For example, it has been reported that more than 200,000 knee replacements are performed in the United States each year. A knee prosthesis has three main components, a femoral implant, a tibial implant, and an disk-like insert or cushion. The femoral and tibial implants cap the ends of the distal femur and the proximal tibia, respectively. They are typically made of metal and include posts for driving them into the femur and tibia, respectively. The cushion is typically made of a strong, smooth, low-wearing plastic. [03] In a typical knee replacement operation, the surgeon makes an anterior incision spanning over the distal femur, the knee, and the proximal tibia, and then separates the femur and the tibia from the surrounding tissues. Next, the surgeon secures a resection guide to the proximal end of the tibia. A resection guide is a jig or template configured to provide a desired cutting angle for a saw plate or other resection tool. Conventional resection guides are used somewhat similarly to the manner in which a carpenter uses a miter box to achieve a desired angle for cutting wood. The surgeon uses the resection guide to position a saw plate or other suitable resection tool and cuts off the tibial plateau (i.e., the upper end of the tibia which forms the lower part of the knee joint). This prepares the tibia to receive the tibial implant (which will form an artificial tibial plateau). To determine the longitudinal axis of the femur, the surgeon inserts an intramedullary ("IM") rod through a hole near the center of the joint surface of the lower end of the femur and into the medullary (i.e., bone marrow) canal that runs longitudinally in the center of the femur. Then, the surgeon aligns one or more additional resection guides for cutting the distal femur as required for receiving the femoral implant (which will form the upper part of the artificial knee). Typically, the surgeon aligns these resection guides relative to the angle of the artificial tibial plateau, the longitudinal axis of the femur and anatomical features such as the transepicondylar axis of the distal femur. Finally, the surgeon drives the posts of the implants longitudinally into the distal femur and proximal tibia, respectively, cements them in place, secures the cushion to the top of the tibial implant, and closes the incision. [04] In general, artificial knees are designed to mimic the operation of natural knees. A healthy, natural knee is not merely a simple hinged joint that bends backward (flexion). It also has a rotary motion that locks the femoral condyles into the tibial plateau on straightening (extension) of the leg. On extension of the knee, the ligaments become tight and convert the knee into a rigid locked structure. The knee unlocks on flexion, allowing an increased range of motion as the lower leg swings backward. In operation of a conventional artificial knee, the lower surface of the femoral implant glides on the upper surface of the cushion (which stays sandwiched between the femoral implant and the tibial implant). [05] But complications may result if the distal femur is not resected properly (i.e., if the surgeon does not cut the distal femur at proper angles relative to the artificial tibial plateau and the longitudinal axis of the femur). Such complications can include increased wear of the plastic surfaces of the prosthesis; bending, cracking or fracture of the bones; dislocation, excessive rotation or loss of motion of the prosthesis; and/or angular deformity of the joint. Naturally, proper resection requires proper alignment of the resection guide(s). [06] Specifically, there are a variety of methods by which the surgeon may order the bone cuts utilized in performing the arthroplasty procedure. In one method, the surgeon uses known anatomical relationships to cut the bones in a prescribed and measured manner. However, for the prosthetic implant to successfully replicate normal function, the ligaments that hold the two knees together must be carefully tensioned or adjusted such that they hold the knee in tight articulation throughout the range of motion. This balancing procedure is referred to as "soft tissue balancing". Balancing may involve releasing the medial or lateral collateral ligaments to correct the varus or valgus deformity that may result from the arthritic disease. A proper ligament tension requires that only small gaps of a few millimeters may occur throughout the normal range of motion of the knee. Proper ligament tension provides a naturally acting joint, and minimizes pain and discomfort. Further, properly balanced ligaments reduce stress and wear on the prosthesis that extends its in vivo life. [07] Recent anatomical studies of Eckhoff et al [1] have shown that the normal articulation of the distal femur with the tibia can be defined by fixed axis or "Flexion/Extension Axis". This is because the normal articulation of the posterior femoral condyles with the tibia after the first few degrees of motion can be described by symmetrical cylinders that maintain a fixed axis throughout the range of motion. They showed that this axis is close to but not exactly matched to the transepicondylar axis of the distal femur. The transepicondylar axis is an anatomical feature, as noted above, that many surgeons use as guide for making the measured distal femoral resection. It has been described a method of making the normal distal femoral cuts based upon the relationship of the perpendicular axis of the tibial shaft to the transepicondylar axis, calling that method the Tibial Shaft Axis Method for femoral rotational resection. They pointed out that for the normal position to be achieved, tension provided by simple distractors would be needed to replicate the normal anatomical ligament tension distorted by loss of the articular surface, scarring, and arthritic osteophytes. Coughlin et al [2] performed anatomical studies that proved the relationship of the Flexion/Extension Axis as the correct axis which is perpendicular to the normal anatomical axis of the tibial shaft which extends from the center of the proximal tibia to the center of the distal ankle joint which is the midpoint of the proximal talus.

[08] The Flexion/Extension Axis of the knee joint then localizes in the distal femur and is co-linear with the centers of the radii of curvature of the posterior femoral condyles. Additionally, this axis represents the constant loci of the medial and lateral collateral ligaments as the knee articulates through the normal range of motion. In other words, this axis defines the normal distance between the two bones, femur and tibia, such that the ligament tension is symmetrical and correctly maintained in all positions of motion. The utility of determining the Flexion/Extension Axis is that the correct placement of prosthetic implants can be made directly from this axis, and may represent an improvement over other indirect or inferential methods.

[09] The Flexion Axis is defined as the femoral axis around which the tibia turns all along the flexion-extension movement of the knee. Therefore, the most relevant way to measure the flexion axis is to record the position of the femur and the tibia along the flexion-extension movement of the knee. However, a knee that requires an arthroplasty presents some defects, mostly on cartilage. For a given flexion angle, especially in extension where the defect is the most important, if both medial and lateral femoral condyles are in contact with respectively medial and lateral tibial plateaus, the position of the tibia with respect to the femur is not representative of a natural knee. As a consequence, for a pathological knee, the measurement of the flexion axis must not be done with the tibial plateau in contact with the femoral condyle, but must be done with the ligaments that are tightened, using a tensor that allows the operator to quantify and control the pressure that tightens the ligaments.

[10] It is known that tensors are used during knee arthroplasty procedures in order to control the ligament tensions of the knee.

[H] It is known that some navigation systems are tracking Instrument position during their position adjustment. The tracking technology of trackers and navigation system is independent of the invention, provided that the trackers are tracked in real-time by the navigation system. It includes, but is not limited to optical active technology with active infrared Light Emitting Diodes (LEDs) on trackers, optical passive technology with passive retro-reflective markers on trackers, or magnetic technology, radio-frequency technology, ultrasonic technology, gyroscopic devices and accelerometers, mechanical arms with encoders. Those tracking technologies are known as prior art. Tracking the tibia with respect to the femur during a flexion-extension motion is a well known characteristic of navigation systems. [12] During knee prosthesis implantation, the Flexion Axis must be determined as precise as possible because it is used to orientate the femoral component of the prosthesis, especially the axial rotation. Because Flexion Axis cannot be acquired with a reproducible and accurate method in conventional procedures with or without navigation, other references are usually used to orientate the femoral component, such as transepicondyle axis, posterior condyle axis, or Whiteside line. However, those axes are approximation to the Flexion Axis as the femoral axis around which the tibia turns all along the flexion-extension movement of the knee. [13] The purpose of the invention is therefore to provide a device and a method to determine more accurately the Flexion Axis of the knee.

Brief Description of the invention

[14] A first object of the invention is a computer assisted surgical navigation method for determining an axis of motion, called flexion axis, of a knee wherein a tensor has previously been inserted, the pressure of which having been controlled to tighten the ligaments, comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia with respect to the femur along a flexion/extension movement, computing said relative positions to determine the flexion axis of the knee.

[15] This method involves a navigation system that tracks in real-time the positions of the tibia and the femur.

[16] The purpose of those acquisitions is to define a relationship of the femur with respect to the tibia, e.g. in the form of a matrix of the affine transformation (rotation and translation) between the tibial coordinate system and the femoral coordinate system.

[17] This flexion axis will then become a reference for the placement of arthroplasty resection jigs used for completing the bone resections needed for placement of the prosthesis (especially the femoral component). The surgeon uses these computer-originated visual cues to precisely place the arthroplasty resection jigs.

[18] The use of a tensor that tightens the ligaments of the knee during the acquisitions of the tibia with respect to the femur at different flexion angles of the knee, ensures, on the one hand, that the acquisitions are reproducible, and, on the other hand, that they are representative of the natural which is non- arthritic.

[19] According to a first embodiment of the invention, two positions of the tibia with respect to the femur are acquired, wherein the flexion angle of the knee differs more than 15° between said two positions.

[20] The flexion axis can be defined as the unique rotation axis that is extracted from the transformation matrix between the two positions of the tibia.

[21] Otherwise, the flexion axis is defined as the axis orthogonal to the plane that contains the femoral center and the ankle centers in the two positions of the tibia.

[22] According to a second embodiment of the invention, more than two positions of the tibia with respect to the femur are acquired with a flexion angle of the knee ranging from 0° to 90°.

[23] The flexion axis can thus be defined as the axis orthogonal to the plane that fits the ankle centers that are recorded for the different flexion angles of the knee.

[24] Otherwise, the flexion axis is defined as the axis orthogonal to the circle that fits the ankle centers that are recorded for the different flexion angles of the knee, and goes through the center of this circle. [25] According to another possibility, the flexion axis is the average of instantaneous axis of rotation computed for the different flexion angles of the knee.

[26] Another object of the invention is a surgical device for determining an axis of motion, called flexion axis, of a knee, comprising: a tensor adapted to be inserted inside the knee to tighten the ligaments, wherein the tensor is connected to a control unit that is able to control the pressure of the tensor, a navigation system adapted to track the respective positions of the femur and the tibia, an acquisition system for acquiring intraoperatively at least two relative positions of the tibia with respect to the femur, computing means for computing said relative positions to determine the flexion axis of the knee. [27] A third object of the invention concerns the use of a tensor inserted inside the knee to tighten the ligaments, in a computer assisted surgical navigation method for determining an axis of motion, called flexion axis, of a knee, said method comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia with respect to the femur along a flexion/extension movement, computing said relative positions to determine the flexion axis of the knee.

[28] A fourth object of the invention is a computer assisted surgical navigation method for optimizing the position of knee implants with respect to the kinematics of the knee, in a knee wherein a tensor has previously been inserted, the pressure of which having been controlled to tighten the ligaments, comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia with respect to the femur along a flexion/extension movement, computing the gaps between virtual tibial and femoral implants for each position of said flexion/extension movement, adjusting the implants parameters in order to obtain a desired pattern of the gaps.

Brief Description of the drawings

[29] Fig. 1 is a sequential view showing the device according to the invention.

[30] Fig. 2 shows a frontal view of the femur and tibia with the four tensor plates inserted between the femoral distal condyles and the tibial cut. [31] Fig. 3 shows an axial view of the femur and tibia with the four tensor plates inserted between the femoral posterior condyles and the tibial cut. [32] Fig. 4 shows a sagittal view of the femur and tibia. [33] Figs. 5A to 5C show an illustration of femoral component prosthesis for a total knee arthroplasty as well as an illustration of the Flexion Axis of the femoral component respectively in sagittal view, axial view and frontal view.

Detailed Description of the invention Tensor [34] The Tensor is any surgical instrument that is inserted between the tibia and the femur to generate pressure and/or spacing. It is usually positioned on the tibial cut flat surface and contains plates in contact with the femur and the tibia. It has the following characteristics:

• The Tensor is a device that is used intraoperatively to control lateral ligaments tension. In most surgical procedures, the ligaments are the lateral collateral ligament (LCL) and the medial collateral ligament (MCL). In some cases, the Posterior Cruciate Ligament (PCL) and the Anterior Cruciate Ligament (ACL) are also preserved.

• The Tensor delivers a pressure that can be controlled. The pressure applied on the medial condyle is independent of the one applied on the lateral condyle.

• The technology that applies pressure is independent of the invention. The Tensor mechanism could be pneumatic, mechanical, hydraulic, or manual. It can use one pressure on both sides of the knee or two independent pressures. The mechanical design uses plates between the femur and the tibia that are joined together externally or internally. An external fixation of the plates is common in most tensors. However, if the fixation between the plates is done internally using a small displacement system like a balloon or a jack, it will have many advantages since it will have no deviation from the contacts with soft tissues and it will have no lever effect that necessitates stiff mechanisms to compensate for it. If a tensor cannot be used, it is also possible to replace it by an adjustable spacer mechanism that just creates spacing between the internal and external side of the knee, but in that case it is necessary for each flexion angle to adjust the spacing height to put the ligament in tension and feel manually that the ligaments are tight enough. A tensor can be also constituted by a variable spacer system that is used in conjunction with a flat pressure sensor mounted on the plates that are in contact with the femur or the tibia; the height of such variable spacer can be adjusted manually with a screw, or with a compact motorized system, or it can be simply made of series of wedges with different thickness that are inserted manually. • The pressure is given as an input to the Tensor. Two possibilities: i. The Tensor is connected to the navigation system. The communication protocol can be wired such as USB, or wireless, such as radiofrequency, Bluetooth or Wifi. The pressure is given as an input to the Tensor by the navigation system, ii. The Tensor is not connected to the navigation system. The pressure is given as an input by an operator such as a nurse to the Tensor. [35] For the rest of this document, and for the explanation of the invention, the Tensor is described as a pneumatic device, activated by air pressure and the navigation system is described with an optical tracking system [36] Fig. 1 is a sequential view showing the Tensor 22 which is inserted between the tibia 8 and the femur 2, and that is connected to the navigation system 15 through a control unit 12.

[37] The navigation system comprises a camera 9 and a computer with a display 10. These features are well-known from the one skilled in the art and will not be described in detail.

[38] A femoral tracker 1 is rigidly fixed on the patient's femur 2, so that the navigation system 15 can track in real-time the position of the femur.

[39] A tibial tracker 7 is rigidly fixed on the patient's tibia 8, so that the navigation system 15 can track in real-time the position of the tibia. [40] The control unit 12 of the tensor is connected to the air pressure 13 that is available in every standard operating room, powered by the power cable 14, and connected to the navigation system through a USB cable 11.

[41] When the air pressure controlled by the control unit 12 is injected into the cable 6, the jacks 5 spread the lower plate 4 from the upper plate 3. [42] Fig. 2 shows a frontal view of the femur 2, the patella 16, and the tibia 8 with the four tensor plates 3, 4, 19, 20 inserted between the femoral condyles and the tibial cut. On the medial side, one plate 20 is in contact with the tibial cut, and one plate 9 is in contact with the femoral medial condyle. On the lateral side, one plate 4 is in contact with the tibial cut, and one plate 3 is in contact with the femoral lateral condyle. The lateral collateral ligament (LCL) and the medial collateral ligament (MCL) are respectively illustrated as reference numerals 17 and 18. When the pressure increases (resp. decreases) on the medial side, the two plates 19 and 20 are moving apart (resp. closer). When the pressure increases (resp. decreases) on the lateral side side, the two plates 3 and 4 are moving apart (resp. closer). Numerals 26 and 27 refer respectively to the transepicondylar axis of the knee and to the flexion axis of the knee. [43] Fig. 3 shows an axial view of the bones and the tensor of Fig. 2. Flexion angle

[44] Referring to Fig.4, the flexion angle σ is defined as the angle between the femoral mechanical axis and the tibial mechanical axis in a sagittal view. The femoral mechanical axis is usually determined as the axis that links the hip center 21 and the femoral center 24. The mechanical tibial axis is usually determined as the axis that links the ankle center 23a to the tibia center 25a. (here aligned with the femoral mechanical axis). Segments [25b, 23b] and [25c, 23c] represent other angular positions of the mechanical tibial axis. The detailed definitions of the tibial and femoral mechanical axes are independent of the invention.

Measurement with the Tensor

[45] Referring to Fig. 4, the navigation system tracks the femoral tracker 1 and the tibial tracker 7 on which a coordinate system is attached. For a given flexion angle σ, the navigation system records the matrix Ma of the affine transformation (rotation and translation) between the tibial coordinate system and the femoral coordinate system. For a better understanding the tensor is not represented on Fig. 4. For all the following embodiments, the measures are done with the tensor inserted between the tibial cut and the femoral condyles, and the air pressure value is selected so that the ligaments are tightened.

[46] In the different methods described below to compute the Flexion Axis of the knee, some methods compute the Flexion Axis as an Affine Axis, which is mathematically defined by a point and a vector; other methods compute the Flexion Axis as a Vectorial Axis only, which is mathematically defined by a vector that represents the direction of the Affine Axis. Therefore, every Affine Axis contains a Vectorial Axis, but the opposite is not true. The femoral component axis is an affine axis known by design of the prosthesis. [47] If the Flexion Axis is determined as a Vectorial Axis, the navigation system offers the surgeon to set the rotations of the femoral component with respect to the femur. In that case, the surgeon can for instance adjust the varus/valgus of the femoral component axis 29 with respect to the Flexion Axis 27 of the knee in a frontal view of the femur (as shown on Fig. 2), or control the axial rotation of the femoral component axis 29 with respect to the Flexion Axis 27 of the knee in an axial view of the femur (as illustrated on Fig. 3). Depending of the surgical strategy, the surgeon can decide to align the femoral component axis with the Flexion Axis or set several degrees between them. [48] If the Flexion Axis is determined as an Affine Axis, the navigation system offers the surgeon to set the translations of the femoral component with respect to the femur, in addition of the rotations as it is described above. In that case, the surgeon can for instance make coincident the Flexion Axis with the femoral component axis, instead of just adjusting rotations. [49] Acquisition Method A: In one preferred embodiment, the surgeon records the position Ma of the tibia with respect to the femur for two different value of σ, different enough to guaranty the accuracy of the following computations. The minimum between the two positions is 15°. A convenient proposal for the surgeon is to ask him to place the knee in full extension, where σ is around 0° and in flexion, where σ is around 90°.

• Computation Method A1 : In one preferred embodiment, the Flexion Axis is the unique Affine Axis of the rotation matrix that characterizes Ma.

Mathematical methods to extract the axis from an affine rotation matrix are well known in the literature.

• Computation Method A2: In another preferred embodiment, the Flexion Axis is determined as the Vectorial Axis orthogonal to the plane defined in the femoral referential by the femoral center 24, the ankle center for the first flexion angle σ (for example 23a), and the ankle center for the second flexion angle σ (for example 23c)

[50] Acquisition Method B: In another preferred embodiment, the surgeon records the position Ma of the tibia with respect to the femur for a range of n values of a, ai.. a_n, where n>2. The surgeon typically makes a flexion-extension movement of the knee and the navigation system records Ma values all along the flexion-extension movement.

• Computation Method B1 : In one preferred embodiment, the Flexion Axis is determined as the Vectorial Axis orthogonal to the plane that contains the femoral center 24, and the ankle centers for the different values of σ. As all those points are not coplanar, the plane can be defined with the least-square approximation method that fits a best plane to a cloud of points, which is well known and therefore not detailed in this document. • Computation Method B2: In another embodiment, the Flexion Axis is determined as the Affine Axis orthogonal to the circle that contains the ankle centers, and goes through the femoral center 24. As all ankle centers are not exactly on the same circle, the circle can be determined with the least-square approximation method that fits a best circle to a cloud of points, which is well known and therefore not detailed in this document.

• Computation Method B3: In another embodiment, the Flexion Axis is the average of the instantaneous axis of rotation of the tibia with respect to the femur that are computed for every flexion angle α?.. a_n, in the femoral referential. The method of computation the instantaneous axis of rotation is well known and therefore not described in this document.

• Computation Method B4: In another embodiment, one does not compute a single Flexion Axis, assuming that the Flexion Axis varies all along the flexion/extension movement and that it is relevant to consider the Flexion Axis definition as a function of the flexion angle. For instance, the surgeon may choose one axis to adjust varus/ valgus of the femoral component in the frontal view, and a different axis to adjust the axial rotation of the femoral component in the axial view.

[51] In another embodiment, the Acquisition Method B is performed with α within a range of 15° to 115° degrees of flexion, as proposed by Eckhoff et al. [1]. Indeed, this paper proposes to approximate the flexion axis by the axis of a cylinder that fits the femoral condyles in a range of flexion [15°-115°]. [52] In any of the computation method described above, it is obvious that without the tensor that tightens the ligaments, the laxity of the ligaments especially in flexion, one may record wrong positions of the tibia that will have a significant impact on the calculation of the Femoral Axis. As an example, for the computation method A1 where two positions of the tibia are recorded respectively at 0° and at 90°, without any compensation by the tensor, the laxity in flexion can induce an error up to 10°, and this error is directly reflected on the Flexion Axis on the axial view, thus leading to an error of 10° for the femoral component axial rotation adjustment.

[53] Fig. 5A shows an illustration of femoral component prosthesis 28 for a total knee arthroplasty as well as an illustration of the Flexion Axis 29 of the femoral component in sagittal view. [54] Fig. 5B shows the same prosthesis in axial view. [55] Fig. 5C shows the same prosthesis in frontal view. [56] In another embodiment of the invention, the tensor is used also for angles close to 0° in extension, in addition to angles ranging from 15° to 115°. In that situation, it is known that the kinematics that describes the motion of the tibia with respect to the femur is not a perfect rotation around a given axis. In the most general case, the flexion axis is in fact represented as a complex trajectory of two rigid bodies. This trajectory is unique and reproducible because of the placement of the tensor during the collection of data. Without the tensor, a standard manual kinematics of the leg like it has been described in many kinematic analysis systems could be very variable. It is then possible to simulate the position of tibial and femoral components on the bone, as it is commonly done in the planning phase of navigation systems. The tibial component contains an insert with a variable thickness. For a given determination of the position of the implants on the bones and the thickness of the insert, one can visualize the kinematics recorded with the tensor. At a given flexion angle, it is possible to visualize what will be the gap distances between the tibial implant surface and the femoral implant surface. This gap is virtual so it can be positive or negative. For a given position and flexion angle, the tibial and femoral implants are realigned together so that their centers coincide in an axial plane, in order to reestablish the congruence of the surfaces. This adjustment would have an impact on the kinematics if it was applied in reality. To solve this problem, the realignment of the implants is performed virtually independently from the measurements done for other flexion angles. It is represented as a tibial insert that would slide in translation and rotation on the tibial cut plane. This method acts like a virtual mobile bearing with translations and rotations and it is used even if the implant has a fixed bearing on the tibia. This realignment for each flexion angle gives the minimal gap distance between the surfaces, on the internal and external sides. The examination of these gaps is done by the surgeon or by the system. The parameters of the prosthesis position, orientation and thickness are then adjusted in order to minimize the absolute values of those gaps or in order to obtain a specific pattern, for instance a pattern that would let 2 mm gaps at 90° of flexion and 0 mm in extension. This method provides an extension of the flexion axis definition in order to optimize the implant to position in a general case.

Cited references

[1] Eckhoff, D. G. et al.: "Three-Dimensional Mechanics, Kinematics, and Morphology of the knee Viewed in Virtual Reality.", JBJS 2005;87:71 -80. [2] Coughlin KM et al.: "Tibial axis and patellar position relative to the femoral epicondylar axis during squatting." Journal of Arthroplasty 2003 Dec; 18(8):1048-55.

Claims

1. A computer assisted surgical navigation method for determining an axis of motion, called flexion axis, of a knee wherein a tensor (22) has previously been inserted, the pressure of which having been controlled to tighten the ligaments, comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia (8) with respect to the femur (2) along a flexion/extension movement, computing said relative positions to determine the flexion axis of the knee.

2. The method of claim 1 , wherein two positions of the tibia (8) with respect to the femur (2) are acquired, wherein the flexion angle of the knee differs of more than 15° between said two positions.

3. The method of claim 1 , wherein more than two positions of the tibia (8) with respect to the femur (2) are acquired with a flexion angle of the knee ranging from 0° to 90°.

4. The method of claim 2, wherein the flexion axis is the unique rotation axis that is extracted from the transformation matrix between the two positions of the tibia (8).

5. The method of claim 2, wherein the flexion axis is defined as the axis orthogonal to the plane that contains the femoral center (24) and the ankle centers (23a, 23b, 23c) in the two positions of the tibia (8).

6. The method of claim 3, wherein the flexion axis is defined as the axis orthogonal to the plane that fits the ankle centers (23a, 23b, 23c) that are recorded for the different flexion angles of the knee.

7. The method of claim 3, wherein the flexion axis is defined as the axis orthogonal to the circle that fits the ankle centers (23a, 23b, 23c) that are recorded for the different flexion angles of the knee, and goes through the center of this circle.

8. The method of claim 3, wherein the flexion axis is the average of instantaneous axis of rotation computed for the different flexion angles of the knee.

9. A surgical device for determining an axis of motion, called flexion axis, of a knee, comprising: a tensor (22) adapted to be inserted inside the knee to tighten the ligaments, wherein the tensor is connected to a control unit (12) that is able to control the pressure of the tensor (22), a navigation system (15) adapted to track the respective positions of the femur (2) and the tibia (8), an acquisition system for acquiring intraoperatively at least two relative positions of the tibia (8) with respect to the femur (2), computing means for computing said relative positions to determine the flexion axis of the knee.

10. A computer assisted surgical navigation method for optimizing the position of knee implants with respect to the kinematics of the knee, in a knee wherein a tensor (22) has previously been inserted, the pressure of which having been controlled to tighten the ligaments, comprising the following steps: acquiring intraoperatively at least two relative positions of the tibia (8) with respect to the femur (2) along a flexion/extension movement, computing the gaps between virtual tibial and femoral implants for each position of said flexion/extension movement, adjusting the implants parameters in order to obtain a desired pattern of the gaps.