WO2013113909A1

WO2013113909A1 - Method for manufacturing a mechanism for interconnecting two bones

Info

Publication number: WO2013113909A1
Application number: PCT/EP2013/052099
Authority: WO
Inventors: Floris Paulus Jacobus Gerardus LAFEBER; Petrus Maria VAN ROERMUND; Thijmen STRUIK
Original assignee: Umc Utrecht Holding B.V.
Priority date: 2012-02-03
Filing date: 2013-02-01
Publication date: 2013-08-08

Abstract

Method for manufacturing an articulation mechanism for connecting a first fixator on a first bone to a second fixator on a second bone, the articulation mechanism providing a limitation of the freedom of movement between the first bone and second bone while allowing articulation of an anatomical joint between the bones, comprising the following steps of: − defining a plane substantially parallel to the flexion extension plane; - connecting a first connector to the first fixator and aligning the first connector to the plane; - connecting a second connector to the second fixator and aligning the second connector to the plane; - determining the relative movement between the first and second connectors during articulation of the anatomical joint, and; - fabricating a patient specific interconnecting element based on the determined relative movement arranged for interconnecting the first and second connectors for forming the articulation mechanism dictating the relative movement to the first and second fixator.

Description

METHOD FOR MANUFACTURING A MECHANISM FOR INTERCONNECTING TWO BONES

The invention relates to a mechanism for connecting two fixators on two separate bones while allowing articulation of the anatomical joint between the bones. The invention further relates to a system for dictating an articulating movement of an anatomical joint and the parts of said system.

The so called articulated fixator is known for treatment of anatomical joints with degenerative joint damage e.g.

osteoarthritis (OA) . The aim of the mechanism in the

articulated fixator is to:

1. give the necessary stability to the anatomical joint (e.g. unload the anatomical joint of forces resulting from external loading)

2. meanwhile allow articulation of the anatomical joint (e.g. for more comfort and better recuperation)

At designing the mechanism in an articulated fixator both aims have to be taken into account. It is known that

anatomical joints mostly articulate in a complex motion with not a fixed center of rotation. A particular example is the knee joint, which has a rotation and translation component during articulation making the rotation axis moving along a trajectory during articulation.

For practical reasons the motion that the mechanism in articulated fixators can allow is always an approximation of the natural anatomical movement. The articulated fixator described in US2010/0145336 is designed to control the distance between the first and second bone. The motion is fixed in the sagittal plane by the pivots and on the proximodistal axis the distance is set by a fixed length of the link assembly. Degrees of freedom are the rotation and slight transverse translation in the sagittal plane. The advantage of this mechanism is that the complex natural movement of the knee in the sagittal plane is possible, but the disadvantage is that there is no stability in the sagittal plane.

The mechanism described in US2010/0222727 does define an exact movement in the sagittal plane, giving a larger stability to the joint. The mechanism provides a replicate of the range of motion of the natural joint motion to the greatest extent possible. The comfort is nevertheless not optimal for an individual patient, because the movement that is dictated will be just "off" his own natural movement, resulting in forces that might harm the joint, and might be perceived by the patient as unpleasant and even painful.

When providing an articulated fixator with a mechanism dictating an almost exact motion, the position of the mechanism relative to the anatomical joint is very

important.

The articulated fixator described in W02010 /029412 provides a mechanism using a four-bar linkage. This approximated movement of the anatomical joint is positioned with a guide thread. Due to complexity of the procedure and risk of collateral damage to the anatomical joint, it is not favorable to operate a guide thread (e.g. Kirschner-wire) in the anatomical joint of a patient only to position the mechanism.

It is therefore a goal of the current invention, amongst other goals, to provide an efficient method for

manufacturing an improved articulation mechanism for an anatomical joint.

This goal, amongst other goals, is met by a method according to claim 1.

More specifically, this goal, amongst other goals, is met by a method for manufacturing an articulation mechanism for connecting a first fixator on a first bone to a second fixator on a second bone, the articulation mechanism

providing a limitation of the freedom of movement between the first bone and second bone while allowing articulation of an anatomical joint between the bones, comprising the following steps of:

- defining a plane substantially parallel to the flexion extension plane;

connecting a first connector to the first fixator and aligning the first connector to the plane;

connecting a second connector to the second fixator and aligning the second connector to the plane;

determining the relative movement between the first and second connectors during articulation of the anatomical joint, and;

fabricating a patient specific interconnecting element based on the determined relative movement arranged for interconnecting the first and second connectors for forming the articulation mechanism dictating the relative movement to the first and second fixator. This method provides a simple way to create a patient specific and fixator specific mechanism. Only a small portion of the mechanism has to be custom made and bringing the mechanism into position before fabricating the

interconnecting element is relatively easy, because only the alignment with the flexion extension plane has to be done and not the exact alignment of the mechanism to the center of rotation of the anatomical joint. Moreover, the design of the interconnecting element, responsible for the hinging movement between the fixators, is based on the relative movement of the already connected fixators according to the natural anatomical joint flexion-extension. The prior placement of the fixators is therefore less critical.

It should be noted that with the term dictating or

prescribing a relative movement is meant that there is only one degree of freedom of movement between the bones . The position of the first bone with respect to the second bone, that is in X, Y and Z-direction is therefore prescribed or dictated along at least a part of the articulation movement between said bones.

The fixators may for instance comprise bone pins.

Preferably, two bone pins are inserted on either side of the joint. More preferably, the fixators are exclusively formed by bone pins, wherein the connectors directly and rigidly connect to said pins. The advantage for the patient is that he will have high comfort while carrying an articulated fixator with a

mechanism fabricated as described above. When the patient articulates the anatomical joint, the rotating axis of the mechanism will be aligned, or at least correspond to a high extend, with the rotating axis of the anatomical joint in any position of the anatomical joint. On the other hand this articulated fixator can enforce a higher stability on the anatomical joint in all the other possible degrees of freedom. Because the mechanism is according to a preferred embodiment designed to follow the exact natural flexion-extension of the anatomical joint during articulation, no compensation (e.g. backlash) for bad alignment of the rotating axis of the articulated fixator with the rotating axis of the anatomical joint is required.

According to a preferred embodiment, the step of determining the relative movement consists of determining the relative movement between the connectors in said plane, wherein the step of fabricating the interconnecting element comprises fabricating an articulating interconnecting element arranged for limiting relative movement between said connectors for at least a part of the relative movement between said connectors along a movement path, wherein said movement path is based on the determined relative movement between the connectors in said plane. The relative movement which the interconnecting element allows between the two connectors, which are connected to said element, is therefore also limited to this plane. As described, the interconnecting element can be arranged to only dictate the relative movement during a part of the relative movement of the joint. In case of for instance a knee, the interconnecting element can be arranged to only prescribe the relative movement between an angle of 10° and 30° between the bones. It is further not necessary that the prescribed relative movement is exactly the same as the measured, preferably natural, relative movement in the earlier step. It is for instance allowed to deviate from this relative movement. However, according to a further preferred embodiment, at least a part of the movement path for prescribing the relative movement is substantially, or even exactly, equal to the determined relative movement. In other words, at least a part of the prescribed movement path is hereby substantially the same as the measured movement path.

It is however also possible that the prescribed movement path is offset in a direction to allow distraction. The measured relative movement is measured as a movement path and said path is for instance translated in a direction for the step of fabricating the interconnecting element to increase the relative distance between the bones. For instance for a knee, the direction is preferably

substantially vertical. According to a further preferred embodiment, the

interconnecting element is designed as a cam system

comprising at least two, preferably three cam followers; an interconnecting element with a guiding surface fabricated based on the determined relative movement, wherein the guiding surface is uniquely shaped based on the determined relative movement to guide all cam followers, wherein the cam system dictates the determined relative movement to the articulated fixator. The step of determining the relative movement between the first connectors, or more preferably the first and second fixators, preferably comprises using at least one measuring device fixedly positioned to the first or second connector, a counterpart fixedly positioned to the second connector; and recording the relative movement between the measuring device and the counterpart. This counterpart may be part of the measuring device, and/or may even be formed integrally with the connectors, as will be discussed in more detail below.

More specifically, according to a further preferred

embodiment, the step of determining the relative movement comprises connecting a measuring device between said

connectors, wherein a first section of the measuring device is connected to the first connector and a second section is connected to the second connector, wherein the first and second sections are provided with marking means for marking the relative movement between the first and second sections during articulation of the anatomical joint, and;

wherein the patient specific interconnecting element is fabricated based on the marked relative movement. The measuring device is provided with two sections which are allowed to move with respect to each other during

articulation of the joint. The sections may for instance be plate shaped and extend in or parallel to the flexion plane. Preferably, the marking means comprise at least two markers, for instance in the form of scratching means, provided on one of the sections of the measuring device which are arranged for marking the relative movement on the other section, wherein the step of fabricating the patient

specific interconnecting element comprises providing cam followers on a first section of the interconnecting element at locations corresponding to the locations of the markers and providing a guiding surface on a second section of the interconnecting element, the guiding surface having a shape based on the marked relative movement for guiding the cam followers along the path of the marked relative movement. The position of the markers is hereby used as the position for the cam followers. It is for instance possible that the first section of the measuring device is reused as a section of the interconnecting element. This first section can be a standard part, wherein the second section is custom made for the patient. It is in particular advantageous if this standard section of the measuring device and interconnecting element is formed integrally with a connector.

According to a further preferred embodiment, also the second section comprises cam followers engaging a guiding surface on the first section. Both the first and the second sections are hereby provided with cam followers engaging guide surfaces provided on the other sections. The steps of measuring and fabricating the second set of cooperating cam followers and guiding surface preferably correspond to the steps of manufacturing the first set, as discussed above.

In a preferred embodiment the measuring device comprises sharp pens positioned on the center of the cam followers on the first connector. The relative movement is recorded by the scratching of the sharp pens into the counterpart leaving a groove. Preferably the groove is scanned and the scanned picture is vectorized to use as input for computer aided manufacturing of the interconnecting element, in particular the guiding surface thereof.

The method can then comprise the step of providing a first section of the measuring device provided with marking means and determining the relative movement during articulation by marking said movement on the second section, wherein the step of fabricating the interconnecting element comprises replacing the marking means with cam followers. The first section does not need to be removed from the connector and can for instance be formed integrally therewith.

It is also possible that the second section of the measuring device, containing the markings of the determined relative movement, is processed to fabricate a part of the

interconnecting element. The for instance plate shaped section can be drilled or sawed along the markings for producing the guiding surface.

The method of fabricating the interconnecting element then preferably comprises machining the second section based on the markings thereon, for instance by following the contours of the markings, for creating said guiding surface, wherein the method further preferably comprises the step of

contacting the cam followers of the first section and the guiding surface of the second section to assemble the interconnecting element. The second section of the measuring device can for instance be removed from the connector for easy processing of said section. At the same time, the cam followers can be placed efficiently on the first section. After machining of the second section, the second section can be reconnected to the connector and placed in operative contact with the first section.

In another preferred embodiment the measurement device is a digital camera, the counterpart has a surface facing the digital camera with a checkered pattern. The digital camera records the relative movement of the checkered pattern and the recorded information is used to define the design for fabricating the interconnecting element.

Outside the operating room the relative movement can be easily determined. Also outside the operating room the interconnecting element can be easily fabricated.

Furthermore outside the operating room the interconnecting element can be integrated with the mechanism and the

mechanism will dictate the relative movement to the first and second fixator resulting in a beneficial articulated fixator .

A further preferred embodiment comprises manufacturing a first articulation mechanism aligned on the first plane, and a second articulation mechanism aligned on a second plane, wherein the first and second articulation mechanisms extend between the first and second fixators, wherein the second plane is preferably on the opposite side (contra-lateral) of the anatomical joint to the first plane.

The benefit of this second mechanism in combination with the first mechanism is that both mechanisms provide stability to the anatomical joint. The rotating axis follows a unique movement pattern at a mechanism. The movement pattern for both mechanisms is specific, resulting in an even better approximation of the natural anatomical movement of the anatomical joint, resulting in even more comfort and higher stability. A unilateral mechanism needs to resist high torsion forces. An articulated fixator with bilateral mechanisms is able to resist higher forces needed for e.g. distraction . As discussed earlier, to limit the number of parts of the system, the measuring device and/or the interconnecting element, or a section thereof, can be an integral part of the first or second connector.

Another benefit of the articulated fixator according to the invention, having a dictated relative movement which is a relatively precise approximation of the natural anatomical movement of the articulation of the anatomical joint, is that it can be used for a distraction treatment of an anatomical joint. For this treatment the bones of an

anatomical joint are pulled away from each other providing a joint space width (JSW) . The dictated relative movement ensures a JSW that is relatively constant in different positions of articulation. As discussed above, it is possible that the dictated relative movement is offset from the measured natural relative movement.

A further preferred embodiment of the method further therefore comprises the step of providing distraction means between the first fixator and the second fixator for distracting the anatomical joint.

In a preferred embodiment the distraction means is a filling element that is mounted between the first connector and the fixator, after the mechanism has been fabricated as

described. The distance between the bones will be increased with the thickness of the filling element. An even further aspect of the invention is the addition of a resilient element between the first fixator and the second fixator as distraction means. This provides for intra^¬ articular intermitted fluid pressure changes. The combination of articulation and intra-articular intermitted fluid pressure changes is assumed to provide the best treatment conditions for the recuperation of degenerative joint diseases e.g. OA. In a further preferred embodiment the resilient element is combined with the filling element.

The invention further relates to a system for dictating an articulating movement of an anatomical joint, in particular a knee joint, between a first bone and a second bone, wherein the system comprises:

- a first connector connected to a first fixator, such as a bone pin, inserted in the first bone;

- a second connector connected to a second fixator, such as a bone pin, inserted in the second bone, wherein the fixators and/or the connectors are provided with adjusting means which are moveable between an adjusting position wherein the connectors can be aligned in a plane substantially parallel to a flexion extension plane of the anatomical joint and a locked position wherein the position of the connectors is fixed with respect to the bones;

- a measuring device which is connectable between the

first and second connectors for measuring the relative movement between the bones during articulation of the anatomical, and;

- a patient specific interconnecting element between the first and second connectors for allowing an articulated movement there between, wherein the interconnecting element comprises limiting means for limiting the relative movement between the two connectors, wherein the limiting means are designed and fabricated based on the determined relative movement by the measuring device . Preferably the connectors are provided with the adjusting means, for instance in the form of lockable ball joint. A first end of the connectors can be arranged to rigidly connect to the fixators, in particular bone pins, wherein the adjusting means are provided between this first end and a second end which second end is arranged for connecting with the measuring device and the interconnecting element.

According to a preferred embodiment, the system comprises at least two cam followers, wherein the interconnecting element comprises a guiding surface as limiting means for receiving and guiding cam followers connected to one of the

connectors. The cam followers, for instance in the form of bearing mounted wheels or sliding contacts, hereby engage the guiding surface. The guiding surface and the cam

follower are arranged such that upon relative movement between connectors, the movement of the connectors, and thereby the bones, is dictated by the shape of the guiding surface .

According to a further preferred embodiment, the measuring device, or at least a part thereof, is removably connectable between the connectors. It is then possible to attach the measuring device for determining the relative movement and to subsequently replace at least a part of the measuring device with the interconnecting element. It is therefore preferred that also the interconnecting element is at least partly connectable between the connectors. The measuring device and/or the interconnecting element, or sections thereof as will be explained below, can thereto be provided with suitable connection means for removably connecting with corresponding connection means provided on the connectors. It is however also possible that the measuring device stays connected between the connectors, wherein the measuring device is adapted to fabricate the interconnecting element as discussed above.

According to a further preferred embodiment, a first section of the measuring device is connected to the first connector and a second section is connected to the second connector, wherein the first and second sections are provided with marking means for marking the relative movement between the first and second sections during articulation of the

anatomical joint. This allows efficient measuring of the moving path during articulation. It is preferred if the marking means comprise at least two markers, for instance in the form of scratching means, provided on one of the sections of the measuring device which are arranged for marking the relative movement on the other section, wherein the locations of the cam followers on a first section of the interconnecting element correspond to the locations of the markers and wherein the guiding surface on a second section of the interconnecting element has a shape based on the marked movement for guiding the cam followers along a path based on the marked movement.

According to a further preferred embodiment, the

interconnecting element comprises a plate shaped element extending in or parallel to the flexion extension plane as a first section, wherein one end of the plate shaped element comprises the guiding surface, wherein the plate shaped element is received in a correspondingly shaped slot of a second section, wherein the cam followers are provided in the slot and are in contact with the guiding surface. The slot shaped section hereby limits the movement in the flexion extension plane while at the same time accommodating the cam followers or slide contacts. As described earlier, at least one of sections of the interconnecting element or the measuring device can be formed integrally with one of the connectors to reduce the number of parts of the system. To distraction of the joint using the system, a further preferred embodiment further comprises distraction means between the first fixator and the second fixator for

distracting the anatomical joint.

The invention further relates to an articulated fixator, comprising a first connector connected to the first fixator, a second connector connected to the second fixator and an interconnecting element as described above interconnecting the first and second connectors.

To be able to use the fixator for distracting a joint as described above, the fixator preferably comprises

distraction means to distract the anatomical joint,

preferably in the form of a resilient element between the first fixator and the second fixator for distracting the anatomical joint.

The invention further relates to an interconnecting element or measuring device as described. The invention furthermore relates to a kit of parts comprising a measuring device and an interconnecting element as described above. The kit may further comprise two connectors. The present invention is further illustrated by the

following Figures, which show preferred embodiments of the device according to the invention, and are not intended to limit the scope of the invention in any way, wherein: figure la: shows a schematic view of an articulated fixator with unilateral a mechanism, according to a preferred embodiment of the present invention;

figures lb shows the articulated fixator of figure la in perspective;

figure 2: shows two planes substantially parallel to the flexion extension plane on both sides of the knee joint, figure 3: shows a picture of an articulated fixator with bilateral custom fabricated mechanisms according to the present invention.

figure 4 : shows a first connecting element describing a first movement pattern in a first position,

figure 5: shows a first connecting element describing a first movement pattern in a second position.

- figure 6: shows a first connecting element of a cam

system in several positions.

figure 7 : shows a second connecting element describing a second movement pattern in a first position,

figure 8 : shows a second connecting element describing a second movement pattern in a second position.

figure 9: shows another preferred embodiment of a mechanism.

figure 10: shows another preferred embodiment of a mechanism.

- figure 11: shows a preferred embodiment of a device to determine the relative movement, using sharp pens, leaving a groove on a counterpart. figure 12: shows a picture of a preferred embodiment of a device to determine the relative movement, using sharp pens, leaving a groove on a counterpart,

figure 13: shows a preferred embodiment of a filling element with different thicknesses mounted between the first connector and the fixator, after the mechanism has been fabricated.

figures 14a-c: show a preferred embodiment in cross- section of a mechanism with a resilient element shown at different displacements.

figures 15a-c: show the embodiments of figures 14a-c in perspective ;

figures 16 - 19 show further embodiments of the

mechanism.

With reference to for instance figure la, 2 and 3, a general aspect of the present invention is to provide a method for manufacturing a mechanism 2 for connecting a first fixator 3 on a first bone 4 to a second fixator 5 on a second bone 6, the mechanism providing a limitation of the freedom of movement between the first bone and second bone while allowing articulation of an anatomical joint 7 between the bones, comprising the following steps of:

defining a first plane 8 substantially parallel to the flexion extension plane;

connecting a first connector 9 to the first fixator and aligning the first connector to the first plane;

connecting a second connector 10 to the second fixator and aligning the second connector to the first plane;

- determining the relative movement between the first and second connectors during articulation of the anatomical joint, and; fabricating a patient specific interconnecting element 11 based on the determined relative movement arranged for interconnecting the first and second connector for forming the mechanism dictating the relative movement to the first and second fixator.

Figures lb shows the system in greater detail. The

connectors 9 and 10 are provided with clamp shaped ends 9a and 10a which are arranged to rigidly connect to two bone pins 3, 5 inserted in the bones 4 and 6 on either side of the joint. The other ends 9c and 10c are arranged to connect to the interconnecting element 11 or the measuring device. Between the first ends 9a, 10a and the second ends 9c, 10c joints 9b, 10b are provided. Joint 9b, 10b allow the

alignment of the second ends 9c, 10c in the plane 8. After alignment, the joints 9b, 10b can be locked. Lockable joints as such are known in the field. Alignment of the ends 9c and 10c of the connectors ensure that the interconnecting element 11, and also the measuring device, extends in a plane parallel to the plan 8 such that the articulation between the bones 4 and 6 can be measured and prescribed efficiently. The placement of the pins 3 and 5 is hereby less critical. With reference to figures 2 and 3, it is further possible that the method comprises manufacturing a first mechanism 2 aligned on the first plane 8, and a second mechanism 102 aligned on a second plane 12, wherein the first and second mechanism extend between the first and second fixators. The second plane is preferably on the opposite side (contra^¬ lateral) of the anatomical joint to the first plane. The benefit of this second mechanism in combination with the first mechanism is that both mechanisms provide stability to the anatomical joint. The rotating axis 22, see for instance figures 4, 5, 7 and 8, follows a unique movement pattern 13 at a mechanism. The movement pattern for both mechanisms is specific, resulting in an even better approximation of the natural anatomical movement of the anatomical joint, resulting in even more comfort and higher stability. A unilateral mechanism needs to resist high torsion forces. An articulated fixator with bilateral mechanisms is able to resist higher forces needed for e.g. distraction.

The interconnecting element is preferably designed as a cam system comprising at least two, preferably three cam

followers 14, see again figures 4, 5, 7 and 8, but also figures 6a-d;

an interconnecting element with a guiding surface 15

fabricated based on the determined relative movement;

the guiding surface uniquely shaped to guide all cam

followers;

the cam system dictating the determined relative movement to the articulated fixator.

Determining the relative movement between the first and second connectors, or more preferably the first and second fixators, comprises using at least one measuring device 16, see figures lla-e and 12, fixedly positioned to the first or second connector, a counterpart 17 fixedly positioned to the second connector; and record the relative movement between the measuring device and the counterpart.

The measuring device in this example comprises sharp pens 18 positioned on the center of the cam followers 14 on the first connector 9. The relative movement is recorded by the scratching of the sharp pens into the counterpart leaving a groove 19. Preferably the groove is scanned and the scanned picture is vectorized to use as input for computer aided manufacturing of the interconnecting element.

After the connection of the connectors 9 and 10 to the bone pins 3 and 5 and the connectors are aligned to the plane 8 as described above, a first plate shaped section 16 is connected to the first connector 9. A second plate shape section 17 is connected to the second connector 10 such that the plate shaped sections extend parallel and at a small distance from each other. Upon articulation of the bones, the sections 17 and 16 will also rotate with respect to each other, see figure 11c. Provided on the first section 17 are pens 18 the ends of which are in contact with the surface of the second section 17 such that upon rotation, the movement of the pens 18 with respect to this second section is marked, as indicated with the grooves 19 in figure lie, see also figure 12.

Based on the grooves 19, the guiding surfaces 15 as for instance shown in figures 4 - 8 can be constructed. By tracing the path of the grooves 19 with a circular shape identical to the shape and size of the cam followers to be used, the shape of the guiding surface 15 is obtained in this example. This shape is shown in figures lib and lid. The location of the pens 18 in the first section 17 hereby correspond to the center of rotation of the cam followers to be used.

It is even possible that the same section 17 of the

measuring device is used in the construction of the articulating system. By replacing the pens by the cam followers, or by providing cam followers on the same

location as the pens, for instance on the opposite side of the plate shaped section 17, this section can be used as part of the interconnecting element. The same applies to the second section 16, which can be converted to a part of the interconnecting element by machining the plate shaped section provided with the markings. Figures 6a-d show the interconnecting element in greater detail. A first section, indicated with 9 as this part is formed integrally with the first connector 9, is provided with a slot wherein the plate shaped section 11 of the interconnecting element can be received. Plate shaped element 11 is connected to the second connector 10. The slot is covered at both sides, see for instance figure lb, such that the plate shaped section 11 is confined in the slot and the movement is therefore limited to a plane parallel to the plane 8. Provided in the slot are the cam followers 14 which engage the guiding surface 15 of the plate shaped element 11. The cam followers 14 and the guiding surface 15 are arranged such the relative movement of the plate shaped element 11 and the connector 9 is prescribed, see figures 6b-d .

Figures 13a-c, 14 and 15 show the distraction means

according to the invention. The distraction means can be in the form of filling elements 22, 122, 222 that can be mounted between the first connector 9 and the fixator 5 after the mechanism has been fabricated as described. The distance between the bones will be increased with the thickness of the filling element, see figures 13a-c. It is also possible that distraction means are provided between the connector 9, see figure lb and the interconnecting element 11.

It is further possible to add a resilient element 21 between the first fixator and the second fixator. This provides for intra-articular intermitted fluid pressure changes. The combination of articulation and intra-articular intermitted fluid pressure changes is assumed to provide the best treatment conditions for the recuperation of degenerative joint diseases e.g. OA.

Different positions and diameters for the cam followers could be chosen for an optimal stress distribution. A higher number of cam followers reduces the range of motion of the interconnecting element. In a preferred embodiment, the cam followers are configured to distribute maximal joint loads over multiple cam followers, resulting in reduced wear and more fluent motion. Figure 4 shows an example of an embodiment with three cam followers 14. The use of three cam followers provide

controlled motion of the interconnecting element 11 relative to the cam followers 14, even when the joint is not

constantly loaded. A correct positioning creates a positive locking of the interconnecting element, that only allows the measured relative movement that resembles the movement of the natural joint.

The embodiment of figure 4 shows cam followers positioned at the outside the interconnecting element 11. An advantage is that the cam followers can be dimensioned large. The

interconnecting element 11 is compact and robust. An alternative embodiment as shown in figure 9 shows smaller cam followers 314 at the inside of the interconnecting element 311. The advantage is that the outside of the interconnecting element can be used for a strong guide bearing to resist lateral forces. The cam followers 314 are guided on an inwardly faced guiding surface 315.

In the embodiment according to figure 10, three cam

followers 414 are guided in slots 415 and the cam followers 414 are surrounded within the range of motion. This

configuration provides support for various load conditions in different directions.

Figures 16a and 16b show an embodiment with two cam

followers 515. In the constant presence of joint load F, e.g. during joint distraction, the motion of the

interconnecting element relative to the cam followers can be prescribed with just two cam followers positioned at the outer surface 515 of the interconnecting element 511. An advantage is the robust shape and high stiffness of the interconnecting element. Another advantage is the easy assembly of the interconnecting element. The interconnecting element is the last part to be assembled and the cam

followers are not in the way for assembly.

In the embodiment of figure 17, one of the three cam

follower 714 moves in a slot 715 of the interconnecting element 611. The other cam followers 614 are guided on the surface 615 similar to the embodiment of figures 16a and 16b. The connection between the cam followers 614, 714 and the interconnecting element 611 is however much more

secured . In another embodiment with three cam followers 814 according to figure 18, the guiding surfaces 815a for two cam

followers 814 are chosen at the outside of the

interconnecting element 811 and one cam follower 814 is positioned at the inside of the interconnecting element 811 and guided on inner guiding surface 815b. The advantage of this embodiment is that the rotation range is about 90 degree. This is larger because only two instead of three cam followers compete for space in a 360 degree circle. Another advantage is that the part with the cam followers can be slim because the cam followers are positioned in line.

An embodiment with four cam followers is shown in figures 19a-b, wherein figure 19b is a cross-section along the line A-A in figure 19a. In this embodiment, both sections 911 and 912 of the interconnecting element are provided with guiding surfaces 915a, 915b and cam followers 914a ,914b. A first guiding surface 915a is formed on the first section 911 for guiding the cam followers 914a provided on the second section 912. Similar to the embodiment of figure 9, this guiding surface 915a is formed on the inside of the first section 911. At a different level to prevent interference, the second section 912 is provided with an outer guiding surface 915b which guides cam followers 914b provided on the first section 911.

The advantage of this embodiment is that the rotation range is almost 180 degree. To make this possible, the pair of outside cam followers 914b (like the pair of inside cam followers 914a) are positioned to the opposite of each other. The consequence is that you need two extra cam followers to balance the force in two directions instead of only one as shown in the previous embodiments with three cam followers. This embodiment has a layered setup of the interconnecting element as can be seen in the cross-section in figure 19b. This makes the interconnecting element very stiff although it has long guiding surfaces 915a, 915b for four cam followers 914a, 914b.

As an alternative, all the cam followers 914a, 914b may be provided on one section wherein the other section is provided with a body having both guiding surfaces 915a, 915b.

The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.

Claims

Method for manufacturing an articulation mechanism for connecting a first fixator on a first bone to a second fixator on a second bone, the articulation mechanism providing a limitation of the freedom of movement between the first bone and second bone while allowing articulation of an anatomical joint between the bones, comprising the following steps of:

- defining a plane substantially parallel to the

flexion extension plane;

- connecting a first connector to the first fixator and aligning the first connector to the plane;

- connecting a second connector to the second fixator and aligning the second connector to the plane;

- determining the relative movement between the first and second connectors during articulation of the anatomical joint, and;

- fabricating a patient specific interconnecting

element based on the determined relative movement arranged for interconnecting the first and second connectors for forming the articulation mechanism dictating the relative movement to the first and second fixator.

Method according to claim 1, wherein the step of

determining the relative movement consists of

determining the relative movement between the connectors in said plane, wherein the step of fabricating the interconnecting element comprises fabricating an

articulating interconnecting element arranged for limiting relative movement between said connectors for at least a part of the relative movement between said connectors along a movement path, wherein said movement path is based on the determined relative movement between the connectors in said plane. 3. Method according to claim 2, wherein at least a part of the movement path is substantially equal to the

determined relative movement.

4. Method according to any of the preceding claims, wherein the step of determining the relative movement comprises connecting a measuring device between said connectors, wherein a first section of the measuring device is connected to the first connector and a second section is connected to the second connector, wherein the first and second sections are provided with marking means for marking the relative movement between the first and second sections during articulation of the anatomical joint, and;

wherein the patient specific interconnecting element is fabricated based on the marked relative movement.

5. Method according to claim 4, wherein the marking means comprise at least two markers, for instance in the form of scratching means, provided on one of the sections of the measuring device which are arranged for marking the movement on the other section, wherein the step of fabricating the patient specific interconnecting element comprises providing cam followers on a first section of the interconnecting element at locations corresponding to the locations of the markers and providing a guiding surface on a second section of the interconnecting element, the guiding surface having a shape based on the marked movement for guiding the cam followers along the path of the marked movement.

6. Method according to any of the preceding claims,

comprising manufacturing a first articulation mechanism aligned on the first plane, and a second articulation mechanism aligned on a second plane, wherein the first and second articulation mechanisms extend between the first and second fixators, wherein the second plane is on the opposite side of the anatomical joint to the first plane.

7. Method as described in one of the previous claims,

wherein the interconnecting element, or a section thereof, is an integral part of the first or second connector .

8. Method according to any of the preceding claims, further comprising the step of providing distraction means between the first fixator and the second fixator for distracting the anatomical joint.

9. System for dictating an articulating movement of an

anatomical joint between a first bone and a second bone, wherein the system comprises:

- a measuring device between the first and second

connectors for measuring the relative movement between the bones during articulation of the anatomical joint in connected state, and;

- a patient specific interconnecting element between the first and second connectors for allowing an articulated movement there between, wherein the interconnecting element comprises limiting means for limiting the relative movement between the two connectors, wherein the limiting means are designed and fabricated based on the determined relative movement by the measuring device.

10. System according to claim 9, comprising at least two cam followers, wherein the interconnecting element comprises a guiding surface as limiting means for receiving and guiding cam followers connected to one of the

connectors .

11. System according to claim 9 or 10, wherein a first

section of the measuring device is connected to the first connector and a second section is connected to the second connector, wherein the first and second sections are provided with marking means for marking the relative movement between the first and second sections during articulation of the anatomical joint.

12. System according to claims 10 and 11, wherein the

marking means comprise at least two markers, for

instance in the form of scratching means, provided on one of the sections of the measuring device which are arranged for marking the movement on the other section, wherein the locations of the cam followers on a first section of the interconnecting element correspond to the locations of the markers and wherein the guiding surface on a second section of the interconnecting element has a shape based on the marked movement for guiding the cam followers along a path based on the marked movement. 13. System according to any of the preceding claims 10 - 12, wherein the interconnecting element comprises a plate shaped element extending in the flexion extension plane as a first section, wherein one end of the plate shaped element comprises the guiding surface, wherein the plate shaped element is received in a correspondingly shaped slot of a second section, wherein the cam followers are provided in the slot and are in contact with the guiding surface .

System according to any of the preceding claims, wherein at least one of sections of the interconnecting element or the measuring device is formed integrally with one of the connectors .

System according to any of the preceding claims, further comprising distraction means between the first fixator and the second fixator for distracting the anatomical j oint . 16. System according to any of the preceding claims, further comprising a resilient element between the first fixator and the second fixator.

17. System according to any of the preceding claims, wherein the anatomical joint is a knee joint.

18. Articulated fixator, comprising a first connector

connected to the first fixator, a second connector connected to the second fixator and an interconnecting element according to any of the preceding claims interconnecting the first and second connectors.

19. Articulated fixator according to claim 18, further

comprising distraction means between the first fixator and the second fixator for distracting the anatomical j oint .

20. Interconnecting element or measuring device as described in any of the preceding claims.

21. Kit of parts comprising a measuring device and an

interconnecting device as described in any of the preceding claims.