DE20304922U1 - Device for determining an articulation point of two bones - Google Patents

Description

A 57 235 u AESCULAP AG & Co. KG

25 March 2003 At Aesculap-Platz

u-248 78532 Tuttlingen

DEVICE FOR DETERMINING A JOINT POINT OF TWO BONES

The invention relates to a device for determining a joint point of two bones connected by a joint, which are pivotably connected relative to one another about at least two non-parallel axes of rotation, in which determination the two bones are moved relative to one another, measured values describing the movement of a point of a bone in a reference system of the other bone are recorded, and from these measured values an axis of rotation describing the movement as rotation is determined for several parts of the path or for one of several paths recorded one after the other, with a navigation system, at least one marking element of the navigation system which can be fixed to one bone, and with a data processing system which determines an axis of rotation describing the movement as rotation from the measured values recorded for one part of the path or for one of several paths recorded one after the other.

The position of anatomical joint points can be determined under certain conditions by moving the bones that are connected to each other at these joint points against each other and by analyzing the movement, for example using a navigation system, to calculate the position of the joint point from the recorded trajectories. This is easily possible, for example, with joints that

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as a ball joint, as is the case with the hip joint. Such a procedure is described in EP 0 969 780 Bl.

However, it is more difficult to determine the exact articulation point for joints that are not designed as pure ball joints, but are essentially hinge joints or hinge-like joints. If a joint is a pure hinge joint, there is a fixed axis of rotation, but no central articulation point. For such joints, it therefore makes no sense to determine such a articulation point.

However, most hinge joints or hinge-like joints in the body of mammals are designed in such a way that they can not only be pivoted about a single axis of rotation, but that - at least to a limited extent - pivoting about a second axis of rotation is also possible. This applies, for example, to the knee joint. The tibia can be pivoted to a small extent about the longitudinal axis in addition to the normal pivoting movement about the transverse axis, so there are essentially two axes of rotation that are perpendicular to one another. These axes of rotation together determine a joint point for the knee, around which every movement takes place, even if it is formed from the rotations about the two axes of rotation described as a superimposed movement. In these cases, a joint point can be determined, but it is necessary to actually pivot the two bones against each other about these two axes of rotation in order to be able to determine this joint point. A pivoting movement alone about one of the two axes of rotation would not be sufficient, since the determination

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an axis of rotation alone does not determine a hinge point along this axis of rotation.

It is an object of the invention to design a generic device in such a way that when two bones move, a joint point can be determined with the desired accuracy.

This object is achieved according to the invention in a device of the type described at the outset in that the data processing system calculates the dispersion in orientation for the differently oriented axes of rotation determined in this way and calculates a hinge point from the differently oriented axes of rotation only if the dispersion in the orientation of the axes of rotation exceeds a certain value.

The invention is therefore based on the knowledge that all actually determined axes of rotation are formed by superimposing the joint's own axes of rotation, for example in the knee by superimposing the flexion axis of the knee on the one hand and the tibial longitudinal axis as the second axis of rotation on the other. The scatter of this orientation is a measure of how strongly the parts of these two main axes of rotation are represented in the differently oriented axes of rotation. If the joint is only moved in such a way that all axes of rotation are essentially parallel, then it is impossible to determine a joint point. The more the differently oriented axes of rotation deviate from one another in their orientation, the more precisely they define the joint point being sought through their intersection.

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By determining the scatter, measurements can be eliminated in which the axes of rotation differ too little from each other in their orientation, so that an exact determination of the position of the hinge point is not possible. Conversely, if the different orientations of the axes of rotation differ sufficiently from each other and thus show a large scatter, reliable values for the hinge point are calculated.

When recording the movement, it is therefore not necessary to ensure that the movement of the two bones occurs precisely around the two main axes; it is sufficient if the dispersion of the orientation of the recorded axes of rotation is large enough to ensure that sufficient parts of both main axes of rotation have been taken into account. For example, the tibia can be moved in any way relative to the femur, making use of the degrees of freedom provided by the knee, and as soon as sufficient dispersion is achieved, one can be sure that a reliable determination of the joint point can be carried out.

In principle, a variety of methods can be used to determine differently oriented axes of rotation from the trajectories of the two bones moving against each other.

In particular, it can be provided that the data processing system during a movement of the two bones relative to each other in different

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Relative positions determine the instantaneous orientation of one bone in the reference system of the other bone and calculate for this instantaneous orientation an instantaneous axis of rotation which corresponds to a rotation of the bone from an assumed reference orientation into this instantaneous orientation, and that the data processing system determines the scatter in the orientation by the scatter of the instantaneous axes of rotation.

It is advantageous if the data processing system is assigned a display on which the dispersion of the orientation of the movement performed is shown. This display then immediately shows whether the dispersion of the orientation of the rotation axes is sufficiently large; if this is not the case, the dispersion can be adjusted to the desired value by changing the movement sequence.

According to a preferred embodiment, it is provided that the data processing system blocks the determination of a hinge point as long as the determined value of the dispersion of the orientation is not reached.

The navigation system can be assigned a marker element to one bone and one to the other.

The following description of preferred embodiments of the invention serves to explain in more detail in conjunction with the drawing. They show:

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Figure 1: an overall view of a device for determining the knee joint point of a patient with a navigation system and a data processing system;

Figure 2: a schematic representation of the main axes of rotation of a tibia

relative to a femur;

Figure 3: a schematic representation of the knee joint with a bundle of differently oriented knee joint pivot axes and

Figure 4: a schematic representation of the recording of different

Orientations of the bones pivoted against each other and the resulting position of an axis of rotation.

A patient 2 is lying on an operating table 1, and the articulation point of the knee joint 3 is to be determined. The drawing shows the femur 4 and the tibia 5, which are connected to each other via the knee joint 3. A marking element 6 or 7 is fixed to both the femur 4 and the tibia 5, which interacts in a known manner with a navigation system 8 set up in the operating room. The marking elements 6, 7 each carry three reference bodies 9 at a distance from each other, which either emit electromagnetic radiation themselves or emit a radiation that is noticeable to them.

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reflect electromagnetic radiation, for example infrared radiation. This radiation is received by three receivers 10 of the navigation system 8 arranged at a distance from one another, and in this way the exact position and orientation of the marking elements 6, 7 in space can be determined by the navigation system. Data sets corresponding to the respective position are fed from the navigation system 8 to a data processing system 11, which can display this data and data derived from it on a display 12.

Figure 2 shows the movements that the tibia 5 can perform relative to the femur 4. This movement is essentially a hinge-like pivoting movement about a flexure axis 13 running transversely to the longitudinal direction of the femur and tibia and, in addition, a rotation about the longitudinal axis 14 of the tibia 5. This rotation about the longitudinal axis 14 is limited and only possible over a relatively small angular range, whereas the pivoting about the flexure axis 13 can cover an angle of significantly more than 90°.

The articulation point 15 of the knee joint 3 can be determined from the intersection point of the flexion axis 13 and the longitudinal axis 14. If the two axes do not intersect exactly, but only pass very closely by each other, this point can also be defined as a point lying between the two axes and as close to them as possible.

In order to be able to determine the position of this articular point 15 solely from the movement of the tibia relative to the femur, the tibia 5 is moved relative to the femur 4

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in such a way that this movement does not only consist of a pure flexion of the knee, but also contains parts of a rotational movement around the longitudinal axis 14, i.e. the movement of the two bones against each other is as irregular as possible, whereby the practitioner does not have to pay attention to how this movement takes place exactly.

Since a marking element 6, 7 is fixed on both the femur and the tibia, the femur 4 can also move as desired; the data processing system can determine the relative movement of the tibia and femur from the movement of the marking element 6 on the femur 4 and the marking element 7 on the tibia 5 and thus the respective position of the marking element 7 and thus of the tibia 5 in a femur-specific reference system.

The axes of rotation of this movement can be determined from the movement curves of the marking element 7 in the femur's own reference system using known algorithms. Averaged axes of rotation are thus determined from the trajectories, for example by averaging different positions and orientations of the marking element 7, which will generally be different in the case of successive movements of the tibia relative to the femur, depending on how the tibia has been moved relative to the femur. However, in the case of the knee joint, these axes of rotation will normally not deviate too much from the flexion axis 13, since the movement around this flexion axis can cover a much larger angular range than the rotation around the longitudinal axis 14. The axes of rotation determined in this way then lie, for example, in a double cone 16, the longitudinal axis of which is

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the bending axis and whose tip marks the desired joint point 15.

This articulation point 15 can only be determined very imprecisely if the opening angle of the double cone 16 is very small, i.e. if the deviation of the axes of rotation determined in this way from each other is small. If, on the other hand, the angle of the double cone 16 becomes larger, a more precise determination of the exact position of the articulation point 15 is ensured; the larger angle of the double cone 16 corresponds to a larger scatter of the orientation of the axes of rotation determined in this way.

The data processing system 11 determines the deviation or scatter of the orientation values from each other from the individual orientations of the rotation axes and compares these with a specified reference value. If the scatter is below this reference value, the measured values are not used to calculate a hinge point, since the scatter corresponding to the angle of the double cone 16 is then too small and only imprecise information about the hinge point 15 can be obtained. The hinge point is only calculated in the manner described when the specified scatter value is exceeded.

The scatter value can be shown on the display 12, so that it is always possible to see whether the movement of the tibia relative to the femur has taken the two pivoting possibilities into account to a sufficient extent, i.e. whether it was sufficiently irregular. If the scatter threshold value has not yet been reached,

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The practitioner must only ensure that during subsequent movements of the tibia relative to the femur, this movement deviates as much as possible from a pure pivoting movement in a single plane.

While it is basically possible to measure the differently oriented axes of rotation through successive movements of the tibia relative to the femur, another method may provide for different components of this movement to be analyzed during a single movement and used to calculate a larger number of instantaneous axes of rotation.

For this purpose, the orientation of the marking element 7 of the tibia 5 in the femur's own reference system is determined in different positions during the movement sequence; this is symbolized in Figure 4 by a plurality of axes, each of which indicates the position of this marking element 7, namely in relation to a reference orientation R, which can be calculated, for example, from the mean value of all individual orientations.

For each position of the marking element 7 during the movement sequence, the data processing system calculates an instantaneous axis of rotation which is selected in such a way that the axis cross corresponding to a certain position is transferred to the axis cross of the reference system simply by rotating about this instantaneous axis of rotation. In other words, it is determined about which axis the marking element 7 must be rotated in order to reach the reference position corresponding to the axis cross R.

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This is carried out in this way for all positions of the marking element 7 determined along the trajectory and leads to a number of differently oriented instantaneous axes of rotation. These instantaneous axes of rotation are again examined for their scatter and if the scatter is sufficiently large, i.e. if a predetermined threshold value is exceeded, the coordinates of the articulation point 15 are calculated from these instantaneous axes of rotation, for example by determining their intersection point or by other mathematical methods.

It is also important here that the scatter of the different instantaneous orientations forms a criterion for whether such a calculation is meaningful or not based on the measurement.

Of course, it is also possible to repeat the movement of the tibia in relation to the femur with this method, but usually a few bending and stretching movements of the leg are sufficient to obtain a sufficient number of measuring points for a very precise determination of the joint point.

Claims (5)

1. Device for determining a joint point of two bones connected by a joint, which are pivotably connected relative to one another about at least two rotation axes that are not parallel to one another, in which determination the two bones are moved relative to one another, measuring values describing the movement of a point of a bone in a reference system of the other bone are recorded, and from these measuring values an axis of rotation describing the movement as a rotation is determined for several parts of the path or for one of several successively recorded paths, with a navigation system, at least one marking element of the navigation system that can be fixed to one bone and with a data processing system that determines an axis of rotation describing the movement as a rotation from the measured values recorded for a part of the path or for one of several successively recorded paths, characterized in that the data processing system ( 11 ) calculates the scatter in the orientation for the differently oriented rotation axes determined in this way and calculates a joint point ( 15 ) from the differently oriented rotation axes only if the scatter in the orientation of the rotation axes exceeds a certain value. 2. Device according to claim 1, characterized in that the data processing system ( 11 ) determines the instantaneous orientation of one bone ( 5 ) in the reference system of the other bone ( 4 ) during a movement of the two bones ( 4 , 5 ) relative to one another in different relative positions and calculates an instantaneous axis of rotation for this instantaneous orientation which corresponds to a rotation of the bone ( 5 ) from an assumed reference orientation into this instantaneous orientation, and that the scatter in the orientation is determined by the scatter of the instantaneous axes of rotation. 3. Device according to claim 1 or 2, characterized in that the data processing system ( 11 ) is associated with a display ( 12 ) on which the data processing system ( 11 ) displays the dispersion of the orientation of the movement performed. 4. Device according to one of claims 1 to 3, characterized in that the data processing system ( 11 ) blocks the determination of a hinge point ( 15 ) as long as the determined value of the dispersion of the orientation is not reached. 5. Device according to one of claims 1 to 4, characterized in that the navigation system ( 8 ) is assigned a marking element ( 6 , 7 ) on each of the one and the other bones ( 4 , 5 ).