WO2022189892A1

WO2022189892A1 - Instrument

Info

Publication number: WO2022189892A1
Application number: PCT/IB2022/051770
Authority: WO
Inventors: Philip WINNOCK DE GRAVE
Original assignee: Dr. Philip Winnock De Grave - Orthopedie
Priority date: 2021-03-12
Filing date: 2022-03-01
Publication date: 2022-09-15
Also published as: BE1028689B1

Abstract

The instrument according to the invention comprises a longitudinal guide with a longitudinal axis and an orientation element fixably connected to the longitudinal guide, wherein the orientation element is arranged with respect to the longitudinal guide so as to be rotatable about a first axis, and wherein the orientation element is slidable with respect to the longitudinal guide along the longitudinal axis of the longitudinal guide. The instrument comprises a first and second measuring pin, in each case with a measuring point, wherein the measuring point can be positioned in the deepest point of the condylus lateralis and the condylus medialis, wherein each of the first and second measuring pins is fixably connected to the orientation element so as to be releasable.

Description

INSTRUMENT

Technical field

[01] The invention relates to instruments, that is medical instruments for orthopaedic purposes, more particularly to instruments for determining the saw cut direction when shortening the proximal end of a tibia.

Prior art

[02] In recent years, fitting a knee prosthesis, also referred to as artificial knee, has become increasingly widespread in the medical world and is regarded as a routine operation nowadays. A knee prosthesis is given to patients whose knee joint is damaged to a significant degree, and in which the chance of regaining the knee function without an operation is very slim. In order to fit the knee prosthesis on the femur and the tibia, sawn-off surfaces have to be provided on the tibia and the femur in order to be able to fit and fix the prosthesis to these surfaces.

[03] One of the underlying problems is aligning the saw cut for the tibia according to the best orientation and shortening the tibia. When fitting an artificial knee, a bottom prosthesis element is fitted to the saw cut of the tibia.

[04] Various schools of thought exist in orthopaedics with regard to this shortening of the tibia. A first school of thought ‘mechanical alignment’ will always align the saw cut on the tibia at right angles to the longitudinal axis of the tibia. A second school of thought bases alignment of the sawn surface on the ‘kinematic axis’ of the femur. This action is referred to as ‘kinematic alignment’. A very recent different school of thought bases the alignment of the saw cut, and consequently the artificial knee elements, on the so-called ‘inverse kinematic alignment’, which starts from the plane of the tibial joint surface.

[05] In the publication ‘Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment’, one of the founders of inverse kinematic alignment, Dr. Ph. Winnock de Grave, explains which advantages this method of alignment offers compared to the ‘mechanical alignment’ and ‘kinematic alignment’ which are currently in use in the medical world.

[06] In order to be able to perform inverse kinematic alignment properly, the upper part of the tibia has to be sawn off below the two deepest points of the condylus medialis and the condylus lateralis of the tibial plateau. According to recent findings, it is best if the saw cut runs parallel to the connecting line between these two deepest points of the condylus medialis and the condylus lateralis.

[07] In this case, the problem is the correct alignment of the saw cut in the medial-lateral direction with respect to these two points.

[08] US4952213 describes an instrument which is suitable for aligning the saw cut with the proximal end of the tibia, but is not suitable for aligning the saw cut with respect to the two deepest points of the condylus medialis and the condylus lateralis.

Summary of the invention

[09] It is an object of the invention to provide an instrument, being a medical instrument, which is able to bring about correct and parallel alignment of the saw cut with respect to the two deepest points of the condylus medialis and the condylus lateralis. [10] According to a first aspect of the invention, an instrument is provided for determining the saw cut direction during shortening of the proximal end of a tibia. The instrument comprises:

- a longitudinal guide with a longitudinal axis, wherein the longitudinal guide is designed such that its longitudinal axis is positioned parallel to the central axis of the tibia outside the tibia;

- an orientation element which is fixably connected to the longitudinal guide, wherein the orientation element is arranged so as to be rotatable with respect to the longitudinal guide about a first axis which is preferably at right angles to the longitudinal axis of the longitudinal guide, and wherein the orientation element is slidable with respect to the longitudinal guide along the longitudinal axis of the longitudinal guide; and

- a first and second measuring pin, in each case with a measuring point on the end of the measuring pin, wherein the measuring point of the first measuring pin can be positioned in the deepest point of one of the condylus lateralis and the condylus medialis, wherein the measuring point of the second measuring pin can be positioned in the deepest point of the other of the condylus lateralis and the condylus medialis, wherein each of the first and second measuring pins is fixably connected to the orientation element so as to be releasable, each at the location of a fixing point on the top side of the orientation element.

[11] According to some embodiments, the orientation element may be arranged so as to be rotatable with respect to the longitudinal guide about a first axis which is at right angles to the longitudinal axis of the longitudinal guide.

[12] For ‘inverse kinematic alignment’ when fitting an artificial knee, the sawn surface at the proximal end of the tibia in the medial-lateral direction has to run parallel to the connecting line between the deepest points of the condylus lateralis and the condylus medialis. By means of the instrument according to the invention, the two measuring points can transfer the position of these two deepest points to the orientation element, so that the orientation becomes visible, tangible and usable. By positioning the two measuring points in the deepest points of the condylus lateralis and the condylus medialis, one measuring point in each of these deepest points, and due to the known mutual position of the measuring points and the respective fixing point on the top side of the orientation element, the orientation of the connecting line between these two deepest points on the outer side of the tibia is known.

[13] According to preferred embodiments, the height difference between a measuring point and a corresponding fixing point in the direction of the axis of the longitudinal guide may be identical for both measuring points. The direction of the connecting line between the two deepest points of the condylus lateralis and the condylus medialis is then transferred one-on-one to the direction of the connecting line between the two fixing points on the orientation element.

[14] According to some embodiments, the height difference between the fixing point of the first measuring pin and the measuring point of the first measuring pin may be adjustable.

[15] According to some embodiments, the height difference between the fixing point of the second measuring pin and the measuring point of the second measuring pin may be adjustable.

[16] This adjustability may be required because the upper surface of the tibia may be worn locally in many patients. Thus, the deepest points of the condylus lateralis and/or the condylus medialis may have worn down or worn away to such a degree that bone tissue has been damaged or worn away. In order to compensate for this damage and to reconstruct the position of the original deepest points of the condylus lateralis and/or the condylus medialis, this wear has to be compensated for. If no bone wear is visible or measurable in the deepest points of the condylus lateralis and the condylus medialis, the height difference with their fixing point is identical for both measuring points. When one of the deepest points has been worn down, the height difference between the corresponding measuring point and its corresponding fixing point has to be reduced to such a degree that this adjustment compensates for the wear.

[17] According to some embodiments, each of the measuring pins may be rotatable about an axis of rotation which passes through the fixing point and which is at right angles to the connecting line between the fixing points.

[18] According to some embodiments, each of the measuring points is able to perform translations in a plane at right angles to the axis of rotation of the respective measuring pin.

[19] The measuring pins in each case comprise an arm to which the measuring point is fastened at one end of the arm. The measuring pin is connected to the axis of rotation on the other side of the arm by means of a pin which fits into an opening at the location of the fixing point of the orientation element. The arm comprises a slot in which a guide is able to slide. The guide is installed on the fixing point via the pin and is parallel to the axis of rotation of the measuring pin. By sliding the arm with respect to the guide, the measuring point performs a translational movement in the plane at right angles to the axis of rotation at the first end of the measuring pin, which plane passes through the measuring point. By rotating the measuring pin, the measuring point is able to perform a circular motion in the plane at right angles to the axis of rotation, which plane passes through the measuring point.

[20] By rotating the measuring pin and by performing a translation in the plane, one measuring point can be positioned in the deepest point of one of the condylus lateralis and the condylus medialis. By rotating the other measuring pin and by performing a translation in the plane, the other measuring point can be positioned in the deepest point of the other of the condylus lateralis and the condylus medialis. Once the correct position for the measuring points has been found, the position with respect to the fixing point may be secured or fixed for each of the measuring pins, for example by means of a clamping system.

[21] After positioning the measuring points, one of the two measuring points of one of the two measuring pins may be arranged in the deepest point of one of the condylus lateralis and the condylus medialis by sliding the orientation element downwards along the longitudinal guide. Assuming that the height difference between the fixing point of the first measuring pin and the measuring point of the first measuring pin is equal to the height difference between the fixing point of the second measuring pin and the measuring point of the second measuring pin, the orientation element may be rotated with respect to the longitudinal guide until both measuring points are situated in both deepest points of the condylus lateralis and the condylus medialis in order to orient the orientation element parallel to the line which connects the deepest point of the condylus lateralis and the deepest point of the condylus medialis.

[22] If the tibia at the location of the condylus lateralis or the condylus medialis is worn away, for example if the cartilage has partly or completely worn away or if even bone tissue in one of the two points has worn away, then the height of the measuring pin which will be placed in the worn-out deepest point will be adjusted with respect to the associated fixing point, compared to the height difference between the other measuring point and the associated fixing point. In this way, the wearing away of cartilage or bone may be compensated for when the measuring point touches the excessively worn surface.

[23] According to some embodiments, each of the measuring pins may comprise an arm for coupling the measuring point to the fixing point, wherein the arm of the first measuring pin is positioned at a different level along the longitudinal axis of the longitudinal guide than the arm of the second measuring pin. [24] This height difference ensures that the two arms can rotate freely with respect to each other and the one arm can, if necessary, rotate over the other arm.

[25] According to some embodiments, the instrument may comprise an angular indicator which indicates the inclination between the orientation element and the longitudinal guide.

[26] According to some embodiments, the inclination between the orientation element and the longitudinal guide may be limited and/or adjustable.

[27] The inclination between the orientation element and the longitudinal guide may be limited to a inclination angle of +/- 6°, wherein the inclination angle is the angle a between the connecting line between the fixing points and the perpendicular to the longitudinal axis of the longitudinal guide, and wherein a positive angle indicates an inclination of the connecting line above the perpendicular to the longitudinal axis of the longitudinal guide on the lateral side of the tibia, thus from the laterally downwards to medial side of the tibia. Preferably, the angle is limited to between -2° and +6°.

[28] According to some embodiments, the inclination angle may be fixable.

[29] This angular indicator, and the adjustability and fixability thereof, may be important to the surgeon who uses the instrument. He or she can limit the angle to an adjustable angle if the measured angle is outside the desired range, or if the angle is not suitable for the artificial knee which is to be implanted.

[30] This adjusting and fixing of the inclination may, for example, be effected by providing an element which is fitted at right angles to the orientation element and rotates before the longitudinal guide upon rotation of the orientation element. The longitudinal guide and the element at right angles to the orientation element may be provided with fitting holes in which a fixing element, such as a tab, a pin or a nut, may be positioned in two corresponding holes if the inclination angle has a well-defined value.

[31] According to some embodiments, the orientation element may be fixably connectable on both the left side and the right side of the longitudinal guide.

[32] In order to perform the orientation for plotting out the direction of the desired saw cut for both the left and right tibia, it has to be possible for the position of the orientation element to be either to the left or to the right of the longitudinal guide. Optionally, the orientation element is rotatable about 180° about the first axis around which the orientation element can rotate.

[33] According to some embodiments, the instrument may furthermore comprise a fixing pin which is fittable in the cavity of the tibia.

[34] When a fixing pin is used in this way, this fixing pin will always indicate the correct orientation of the central tibia axis. This so-called endomedullar orienting of the fixing pin, and consequently the longitudinal guide, has the advantage that few, if any, errors can be made when looking for the correct orientation of the tibia axis.

[35] According to some embodiments, the longitudinal guide may be fitted parallel to and rotatable about the fixing pin.

[36] This may be achieved by providing a coupling rod which is fitted at a given angle, preferably at right angles, to the fixing pin, and to which the longitudinal guide is fitted at a given angle, preferably a perpendicular angle. The coupling rod is preferably fitted to the fixing pin so as to be rotatable, but fixable. According to preferred embodiments, the longitudinal guide may be displaced away from and towards the fixing pin along this coupling rod. Preferably, the position of the longitudinal guide on the coupling rod may be fixed, for example by means of a clamping device.

[37] When using an alternative way for orienting the longitudinal guide along the tibia axis, the so-called extramedullary orientation, a set of elements is fitted on the outer side of the tibia between knee and ankle, which set of elements indicates this central axis of the tibia. The set of elements has at least one direction-indicating element, which indicates the direction of the tibial axis and extends upwards in the direction of the proximal tibia. This direction- indicating element is coupled in parallel to the longitudinal guide, as a result of which the axis of this longitudinal guide adopts the direction of the axis of the tibia.

[38] According to some embodiments, the instrument may thus furthermore comprise a set of elements which indicate the direction of the axis of the tibia extramedullary.

[39] According to some embodiments, this set of elements may comprise a direction-indicating element, which direction-indicating element runs parallel to the direction of the tibial axis and to which the longitudinal guide is coupled in parallel.

[40] When using endomedullary orientation, this direction-indicating element may be fitted and used for monitoring. During correct introduction of the fixing pin, and the additional fitting of the monitoring element to the longitudinal guide, the treating physician is able to see if the tibia axis does indeed pass through the central point in the ankle of the patient, as indicated by the direction-indicating element. This may be inferred by comparing the position of the direction-indicating element with the position of the central point of the ankle of the patient. If this direction-indicating element deviates from the position of the central point of the ankle, the fixing pin has possibly not been fitted correctly. [41] According to some embodiments, the orientation element may furthermore comprise one or more fixing holes for securing the orientation element to the tibia.

[42] This fixing may be achieved by means of bolts or screws which are fixed in the bone via the fixing hole. This ensures that the orientation of the orientation element and tibia does not change when removing the measuring pins and fitting any saw guide elements.

[43] According to some embodiments, the instrument may at least comprise one saw guide element, which saw guide element comprises a guide body which can be fitted to the orientation element, and which guide body comprises a guide opening for guiding a saw blade.

[44] Via a saw guide element which may be fitted to the orientation element, the orientation of this orientation element may be transferred to the direction of the saw cut which is effectively to be used, being the guide opening for the guiding of a saw blade.

[45] According to some embodiments, the guide body may be fitted to the orientation element by means of pins which fit into the orientation element at the location of the fixing points.

[46] According to some embodiments, the guide opening may define a guiding surface parallel to the connecting line of the fixing points.

[47] The use of the same fixing points ensures the correct transfer of the orientation.

[48] According to some embodiments, the guiding surface may make an angle with the plane defined by the longitudinal axis of the longitudinal guide and the connecting line between the fixing points, which angle differs from 90°. This angle is then the anterior-posterior inclination, also referred to as slope. [49] According to some embodiments, the instrument may comprise a plurality of saw guide elements, wherein the anterior-posterior inclination angles or slope produced by the respective guiding surfaces and the plane defined by the longitudinal axis of the longitudinal guide and the connecting line between the fixing points are different with respect to each other.

[50] Using the measuring pins and the orientation element, the instrument transfers the direction of the connecting line between the deepest points of the condylus lateralis and the condylus medialis to the orientation element and to the saw guide elements. This largely corresponds to the angle of the saw cut which is readable in the coronal plane. The angle between the guiding surface and the plane defined by the longitudinal axis of the longitudinal guide and the connecting line between the fixing points corresponds to the angle of the cut surface or slope readable in the sagittal plane. By adjusting these two angles, it is possible to achieve an optimum orientation of the saw cut in all directions. Preferably, two trios of saw guide elements are provided, one trio for shortening the left tibia, one trio for shortening the right tibia, and wherein each trio has a saw guide element with a 3° slope, a saw guide element with a 5° slope, and a saw guide element with a 7° slope. A positive angle is understood to mean an angle which runs downwards from the front side to the rear side of the knee.

[51] According to some embodiments, the at least one saw guide element may comprise one or more fixing holes for securing the saw guide element to the tibia.

[52] This fixing is achieved by means of bolts or screws. This ensures that the orientation of the saw guide element on the tibia does not change when removing the other parts of the instrument before shortening of the tibia takes place.

[53] According to some embodiments, the height between the guide opening and the connecting line between the fixing points may be adjustable. [54] By means of this adjustment, the height of the saw cut with respect to the central axis of the tibia is determined.

[55] The independent and dependent claims indicate specific and preferred features of the embodiments of the invention. Features of the dependent claims may be combined with features of the independent and dependent claims, or with features described above and/or below, in any suitable manner, as will be clear to someone skilled in the art.

[56] The abovementioned and other features, properties and advantages of the present invention will be clarified by means of the following examples of embodiments, optionally in combination with the drawings.

[57] The description of these examples of embodiments is given for clarification and is not intended to limit the scope of the invention. The reference numerals in the following description refer to the drawings. Identical reference numerals in possibly different figures refer to identical or similar elements.

Brief description of the figures

[58] With a view to providing a better illustration of the characteristic features of the invention, the following text describes a number of preferred embodiments by way of example, without any limiting nature, with reference to the appended drawings, in which:

• Fig. 1 shows a diagrammatic representation of an instrument according to the invention, shown in combination with a left tibia;

• Figs. 2 shows a detail of the instrument from Fig. 1 ;

• Fig. 3 shows a diagrammatic representation of the instrument from Fig.

1 ;

• Fig. 4 shows a diagrammatic representation of the instrument according to Fig. 1 , shown in combination with a saw guide element and in combination with the left tibia. Description of examples of embodiments

[59] The present invention will be described below by means of specific embodiments.

[60] It should be noted that the term ‘comprising’, as used, for example, in the claims, should not be interpreted in a limiting sense, limited to the elements, features and/or steps which follow it. The term ‘comprising’ does not exclude the presence of other elements, features or steps.

[61] Thus, the scope of an expression ‘an object comprising the elements A and B’, is not limited to an object which only comprises the elements A and B. The scope of an expression ‘a method comprising the steps A and B’, is not limited to a method which only comprises the steps A and B.

[62] In the light of the present invention, these expressions only mean that the relevant elements or steps, respectively, for the invention are the elements or steps A and B, respectively. [63] In the following specification, reference is made to ‘an embodiment’ or

‘the embodiment’. Such a reference means that a specific element or feature, described by way of this embodiment, is comprised in at least this one embodiment.

[64] The use of the terms ‘in an embodiment’ or ‘in the embodiment’ at various locations in this description does, however, not necessarily refer to the same embodiment, although it may indeed refer to the same embodiment.

[65] In addition, the properties or the features may be combined in one or more embodiments in any suitable way, as will be clear to a person skilled in the art. [66] Fig. 1 shows an instrument 1 according to the invention, illustrated in combination with a left tibia 2. Fig. 2 shows a detail of the measuring instrument 1 , and Fig. 3 shows this instrument 1 again, but without tibia.

[67] The instrument 1 is aligned endomedullary. A fixing pin 70 is introduced in the medullary canal of the tibia 2, and in this case follows the axis 21 of this tibia. Via a coupling rod 72, which is rotatable about the fixing pin 70 which is coupled to one end of the coupling rod 72, the rest of the instrument 1 is coupled to this fixing pin 70. The rest of the instrument 1 can rotate about the tibial axis 21 , but the position of the coupling rod and fixing pin 70 can be fixed by means of a clamping system 74.

[68] On the other end of the coupling rod 72, the longitudinal guide 10 is coupled. The longitudinal guide 10 can be arranged further away from or closer to the fixing pin 70 along the coupling rod 72. Once the correct distance has been established between the fixing pin 70 and the longitudinal guide 10, the mutual distance can be fixed by means of a clamping system 76. The longitudinal axis 11 of the longitudinal guide 10 has a direction which runs parallel to the axis 21 of the tibia. In order to verify the direction of the tibial axis 21 , a direction-indicating element 90 may be fitted to this longitudinal guide, parallel to its axis 11 . The end of the direction-indicating element 90 which is not coupled to the longitudinal guide should be arranged at the location of the centre of the ankle.

[69] An orientation element 30 is fixably connected to the longitudinal guide 10, wherein the orientation element 30 is arranged with respect to the longitudinal guide 10 so as to be rotatable about a first axis 31 which is preferably at right angles to the longitudinal axis 11 of the longitudinal guide 10. The orientation element 30 is slidable with respect to the longitudinal guide 10 along the longitudinal axis 11 of the longitudinal guide 10. A clamping system 12 is provided in order to fix the mutual orientation between the longitudinal guide 10 and the orientation element 30 with respect to each other once the correct position with respect to each other has been achieved.

[70] A first and second measuring pin 40, 50, in each case with a measuring point 41 , 51 at the end of an arm 43 or 53, respectively, are provided. These measuring pins are connected to the orientation element 30. In this embodiment, the measuring point 41 of the first measuring pin 40 can be positioned in the deepest point of the condylus medialis 101. The measuring point 51 of the second measuring pin 50 can be positioned in the deepest point of the other of the condylus lateralis 102. Each of the first and second measuring pins 40, 50 is fixably connected to the orientation element 30 so as to be releasable, each at the location of a fixing point 32, 33 on the top side 34 of the orientation element 30.

[71] The height difference H41 and H51 between the measuring points 41 and 51 and the respective fixing points 32 and 33 is adjustable. In this embodiment, the respective arms 43 and 53 are to this end adjustable in height with respect to the associated fixing points 32 and 33 by means of a control system 91 which moves the measuring pins upwards or downwards with respect to the fixing points by rotation. Each arm has its own control system. The arms are moved up and down along the corresponding axes 42 and 52.

[72] The arms 43 or 53, respectively, of the measuring pins 40 and 50 have a slot 44 or 54, respectively, along their length which allows the measuring point to perform translations in a plane at right angles to the axis of rotation 42 or 52, respectively, of the respective measuring pin 40 or 50. Each of the measuring pins 40 and 50 is rotatable about an axis of rotation 42 or 52, respectively; which passes through the fixing point 32 or 33, respectively, and which is at right angles to the connecting line 37 between the fixing points 32 and 33. In order to fix the position of the measuring pin 40 and 50, and thus the measuring point 41 and 51 , after the desired position has been reached, the arm 43 or 53, respectively, may be fixed by a clamping system 38 or 39, respectively.

[73] By means of these movement options, the measuring points 41 and 51 can be positioned at the deepest points of the condylus. The longitudinal guide 10 is fixed with respect to the fixing pin 70. The height differences between measuring pins 41 and 51 and the fixing points 32 and 33 are adjusted, if desired differently, in order to compensate for wear of cartilage or bone surface of the condylus. The measuring pins are translated and rotated about the axes 42 and 52 until their measuring points are situated above the deepest points of the respective condylus. By moving the orientation element 30 up and down along the longitudinal guide 10, a suitable height is found. The orientation element drops along the axis 11 of the longitudinal guide until the first measuring point touches the respective deepest point in the condylus. The orientation element 30 then rotates about the axis 31 until the second measuring point touches the corresponding deepest point. All elements of the instrument 1 are fixed and, if desired, the steps are repeated to adjust the position of the orientation element 30. At the end of this manipulation, the two measuring points 41 and 51 are situated in the deepest points of the condylus lateralis and medialis, and the orientation element 30 indicates the direction of the line between these two deepest points.

[74] The angle a between the connecting line between the fixing points and the perpendicular to the longitudinal axis of the longitudinal guide is the inclination angle. This is readable on the angular indicator 60. If the angle is excessively large, for example greater than -2° or +6°, the treating physician may adjust the inclination angle to an angle which, from a medical point of view, satisfies the specified demands of the operation. This selected angle may optionally be set by sliding pins into the calibration openings or fitting holes, wherein an opening 61 in the orientation element 30 and a corresponding opening in the longitudinal guide 10 make the introduction of such a pin possible. [75] The orientation of the orientation element 30 with respect to the tibia 2 can now be made permanent by fixing the orientation element 30 to the tibia by means of screws which are screwed into the bone through the fixing holes 35 and/or 36. Once the orientation has been made permanent, the measuring pins can be removed from the fixing points and space can be made for a saw guide element 80 as shown in Fig. 4. In the embodiment in Figs. 1 , 2 and 3, the instrument is shown during use on a left tibia.

[76] In Fig. 4, the instrument 1 is likewise shown when used on the left tibia. The saw guide element 80 comprises a guide body 81 which is fitted to the orientation element 30 via identical supports 85 which fit into the orientation element 30 at the location of the fixing points 32 and 33. The guide opening 82 of the saw guide element 80 is now oriented parallel to the connecting line between the two deepest points in the condylus lateralis and the condylus medialis. The anterior-posterior angle made by the guiding surface with the plane defined by the longitudinal axis 11 of the longitudinal guide 10 and the connecting line 37 between the fixing points 32 and 33 may vary due to selecting a saw guide element 80 whose guiding surface is made to form this well-defined anterior-posterior angle with this plane. The selected saw guide element 80 in the embodiment illustrated in Fig. 4 forms an angle of 7° (inclined backwards) for performing the technique on a left tibia.

[77] The height of the saw cut may be adjusted by means of a measurement indication 84. This can be varied by using the control system 91 in order to change the height.

[78] Once the correct saw guide element 80 has been selected, fitted and adjusted, the saw guide element 80 may be fixed to the tibia by means of screws which are screwed into the bone via the fixing holes 83.

[79] In order to shorten a right tibia, the fixing pin 70, coupling rod 72 and longitudinal guide 10 will be fitted in and to the right tibia in an identical manner. Flowever, the orientation element 30 is rotated 180° with respect to the first axis 31. The top surface 34 of the orientation element 30 from Figs. 1 to 3 will now serve as the bottom surface of the orientation element 30 in those instances where the bottom surface of the orientation element 30 from Figs. 1 to 3 will become the top surface. All other elements of the instrument may be inserted, moved and fixed in an identical manner to that described for the left tibia. The orientation element 30 is therefore also mirror-symmetrical with respect to a plane parallel to the top and bottom surface of the orientation element 30.

[80] In the way described above, a guide is provided for shortening the tibia according to the principle of inverse kinematic alignment, both for the left and the right tibia, in which the saw cut runs parallel to the connecting line between the two deepest points of the condylus lateralis and the condylus medialis.

[81] It will be clear that, although the embodiments and/or the materials for providing embodiments according to the present invention have been discussed, various modifications or changes may be made without departing from the scope and/or the spirit of this invention. The present invention is by no means limited to the above-described embodiments, but may be brought about through different variants without departing from the scope of the present invention.

Claims

1 Instrument (1 ) for determining the saw cut direction during shortening of the proximal end of a tibia (2), the instrument comprising: - a longitudinal guide (10) with a longitudinal axis (11), wherein the longitudinal guide is designed such that its longitudinal axis (11) is positioned parallel to the central axis (21 ) of the tibia (2) outside the tibia (2);

- an orientation element (30) which is fixably connected to the longitudinal guide (10), wherein the orientation element (30) is arranged so as to be rotatable with respect to the longitudinal guide (10) about a first axis (31) which is preferably at right angles to the longitudinal axis (11) of the longitudinal guide (10), and wherein the orientation element (30) is slidable with respect to the longitudinal guide (10) along the longitudinal axis (11 ) of the longitudinal guide (10); and

- a first and second measuring pin (40, 50), in each case with a measuring point (41, 51) on the end of the measuring pin (40, 50), wherein the measuring point (41 ) of the first measuring pin (40) can be positioned in the deepest point of one of the condylus lateralis and the condylus medialis, wherein the measuring point (51 ) of the second measuring pin

(50) can be positioned in the deepest point of the other of the condylus lateralis and the condylus medialis, wherein each of the first and second measuring pins (40, 50) is fixably connected to the orientation element (30) so as to be releasable, each at the location of a fixing point (32, 33) on the top side (34) of the orientation element (30).

2.- Instrument according to Claim 1 , wherein the orientation element (30) is arranged so as to be rotatable with respect to the longitudinal guide (10) about a first axis (31) which is at right angles to the longitudinal axis (11) of the longitudinal guide (10).

3.- Instrument according to one of the preceding claims, wherein the height difference (H41 ) between the fixing point (32) of the first measuring pin (40) and the measuring point (41) of the first measuring pin (40) is adjustable.

4.- Instrument according to Claim 3, wherein the height difference (H51 ) between the fixing point (33) of the second measuring pin (50) and the measuring point (51 ) of the second measuring pin (50) is adjustable.

5.- Instrument according to one of the preceding claims, wherein each of the measuring pins (40, 50) is rotatable about an axis of rotation (42, 52) which passes through the fixing point (32, 33) and which is at right angles to the connecting line (37) between the fixing points (32, 33).

6.- Instrument according to Claim 5, wherein each of the measuring points (41 , 51 ) can perform translations in a plane at right angles to the axis of rotation (42, 52) of the respective measuring pin (40, 50).

7.- Instrument according to one of the preceding claims, wherein each of the measuring pins (40, 50) comprises an arm (43, 53) for coupling the measuring point (41 , 51 ) to the fixing point (32, 33), wherein the arm (43) of the first measuring pin (40) is positioned at a different level along the longitudinal axis (11 ) of the longitudinal guide (10) than the arm (53) of the second measuring pin (50).

8.- Instrument according to one of the preceding claims, wherein the instrument (1 ) comprises an angular indicator (60) which indicates the inclination between the orientation element (30) and the longitudinal guide (10).

9.- Instrument according to one of the preceding claims, wherein the inclination between the orientation element (30) and the longitudinal guide (10) is limited and/or is adjustable.

10.- Instrument according to claim 9, wherein the inclination angle is fixable. 11.- Instrument according to one of the preceding claims, wherein the orientation element (30) is fixably connectable on both the left side and the right side of the longitudinal guide (10).

12.- Instrument according to one of the preceding claims, wherein the instrument (1 ) furthermore comprises a fixing pin (70) which is fittable in the cavity of the tibia (2).

13.- Instrument according to Claim 12, wherein the longitudinal guide (10) is fitted parallel to and rotatable about the fixing pin (70).

14.- Instrument according to one of Claims 1 to 11 , wherein the instrument (1 ) furthermore comprises a set of elements which indicate the direction of the axis of the tibia extramedullarily.

15.- Instrument according to claim 14, wherein this set of elements comprises a direction-indicating element which runs parallel to the direction of the tibial axis and to which the longitudinal guide is coupled in parallel.

16.- Instrument according to one of the preceding claims, wherein the orientation element (30) furthermore comprises one or more fixing holes (35, 36) for securing the orientation element (30) to the tibia (2).

17.- Instrument according to one of the preceding claims, wherein the instrument (1 ) comprises at least one saw guide element (80), which saw guide element (80) comprises a guide body (81 ) which can be fitted to the orientation element (30), and which guide body (81) comprises a guide opening (82) for guiding a saw blade.

18.- Instrument according to Claim 17, wherein the guide body (81 ) is fitted to the orientation element (30) by means of pins (83) which fit into the orientation element (30) at the location of the fixing points (32 and 33).

19.- Instrument according to one of Claims 17 to 18, wherein the guide opening (82) defines a guiding surface parallel to the connecting line of the fixing points (32, 33).

20.- Instrument according to Claim 19, wherein the guiding surface makes an angle with the plane defined by the longitudinal axis (11) of the longitudinal guide (10) and the connecting line (37) between the fixing points (32, 33), which angle differs from 90°.

21.- Instrument according to Claim 20, wherein the instrument (1) comprises a plurality of saw guide elements (80), wherein the angles made by the respective guiding surfaces and the plane defined by the longitudinal axis (11 ) of the longitudinal guide (10) and the connecting line (37) between the fixing points (32, 33) are different with respect to each other.

22.- Instrument according to one of Claims 17 to 21 , wherein the at least one saw guide element (80) comprises one or more fixing holes (82, 83) for securing the saw guide element to the tibia.

23.- Instrument according to one of Claims 17 to 22, wherein the height between the guide opening (82) and the connecting line (37) between the fixing points (32, 33) is adjustable.