Ligament-tensioning device for non-spheroid joints The invention concerns a device for the tensioning of ligaments on non-spherical joints in human or animal bodies according to the generic part of patent claim 1. 5 V Accordingly, the device according to the invention concerns a surgical instrument that allows the implantation of joint prostheses with a certain pretensioning of the ligaments and capsule structures controlling the joint. 10 A prerequisite for the good functioning of the joint provided with a prosthesis is a conflict-free interaction of the kinematics of the artificial joint and the soft parts surrounding the joint. In the case of non-spherical joints, e.g. knee joint or elbow joint, the movement of the joints is controlled by the surrounding capsule-ligament structures. In the case of a natural joint the geometry of the articulation surfaces 15 and of the capsule-ligament apparatus are so harmonised that the physiological movement is possible. When replacing the diseased natural joint by an artificial joint, the harmonising of the kinematics of the artificial joint with the capsule ligament apparatus is decisive as far as the mobility and stability of the joint provided are concerned. The natural structures of the capsules and the ligaments 20 contain receptors to generate neurological signals to control the muscles of the extremities. These receptors convert mechanical stimuli, produced by the stretching of the surrounding tissue, into neurological signals. Depending on the fitted situation of the artificial joint, this information system gets damaged to a lesser or greater degree, and consequently the patient will suffer a loss of 25 proprioceptivity of the extremity. Those patients in whom during the implantation a good harmonisation of the soft part tensioning is achieved over the extent of the movement of the artificial joint, have a b tter proprioceptivity than those in whom the harmonisation was not as 30 good (The Journal of Bone and Joint Surgery, Vol. 73-B, Jan. 1991 and Vol. 78 B, July 1996). This information was established based on the replacement of knee joints and these modern surgical techniques are described, for example, in EP 0 322 363 WEHRLI and WO 96/17552 TODD. These surgical techniques are supported by an instrument, that allows to restore the statics of the leg to be operated with high accuracy and to securely and permanently anchor it on the skeleton. In addition to the restoring of a painless mobility, the alignment of the extremity with regard 5 to the axis (varus/valgus) and the rotational positioning (internal/external rotation) is an important aim of the operation. Today's stand of instruments allows, on a large scale, the reproduction of correct relationship of the axes. The soft part situation, as a rule, is surgically adapted to suit the artificial joint, but the tensioning of the soft part structure, passively controlling the joint, is not 10 quantified. A leading parameter of the surgical techniques described is the statically correct positioning of the artificial joint. After the statics has been restored, the soft part situation is corrected by so called capsule and ligament releases, while this correction is carried out in various bent positions of the joint. 15 Various authors have suggested instruments, the aim of which is a controlled dealing with the capsule-ligament structures during the implantation of an artificial joint. All these instruments have a spreading function using various technical solutions. 20 A spreading instrument for the tensioning of ligament structures in the knee during the implantation of a knee prosthesis is disclosed in US 5,649,929 CALLAWAY. The instrument is based on a scissors-like concept and allows the spreading of the knee joint when bent. On this occasion one arm of the instrument is resting on the proximal tibia, and the second arm is introduced 25 centrally into the bent knee. The spreading movement can be locked by a spindle provided between the handles. The instrument is intended to be used exclusively for bent knees. In EP 0 322 363 WEHRLI a ligament tensioning instrument is disclosed, that 30 achieves a spreading effect also by means of two scissors-like levers, that can be locked with a spindle and nut provided between the handles. The instrument rests on the tibia on the bone screw introduced below the joint surface and is 7> introducedd in the centre of the bent knee and then spread out. In the same patent instrument is described for the tensioning of the ligaments with extended knee, that drives two spreaders guided in a longitudinal guide by means of two threaded spindles. The two spreaders, which can be separately operated, make a separate spreading of the two compartments of the knee joint possible. In contrast to the above described one, this double-spreader produces a parallel 5 spreading movement. This produces a spreading movement that does not result in the angular change of the spreader at various spreading paths, as this is the case for the scissors-type one. In WO 96/17552 TODD a ligament tensioner for the implantation of knee 10 prostheses is disclosed, that has two separate spreaders, which are also guided in a longitudinal guide. The operation is carried out by means of two hand levers, while the spread position can be locked by a ratchet mechanism. When compared with spreaders operating with threaded spindles, hand lever 15 operated spreaders have the advantage that the surgeon has a direct control of the spreading force via the manual force. Depending on the lever ratios the conversion of the manual force into spreading force can be freely chosen, similarly to surgical pliers and scissors. The knowledge and training of a surgeon guarantee the reliable handling of such instruments. The manual intuitive control 20 of the spreading force via a threaded spindle is clearly less accurate and may lead to an overstraining of the soft parts. It is a disadvantage of the aforementioned spreading instruments that in addition to the intuitive control no quantification of the force, with which the soft parts are 25 tensioned, is possible by the instrument. The invention wants to provide a remedy for this. The object of the invention is to tension the capsule-ligament structures of a joint to be provided with a prosthesis with a parallel spreading movement, while making a manual control and 30 additionally a simultaneous quantitative control of the spreading force possible. The instrument should be able to be used both in bent and extended joints. For this purpose two separate spreaders are used, that are operated by means of two pairs of handles and the two four-bar mechanisms are spread parallel. To quantify the force per spreader a leaf spring element is used that indicates the spreading force in Newtons via an indicator and a graduated scale. The invention achieves this objective with a device to tension the ligaments on 5 non-spherical joints that has the features of claim 1. Further advantageous developments of the invention are characterised in the dependent claims. 10 The device according to the invention comprises a prism-shaped, cylindrical or plate-shaped base body and two blades connected with the base body and symmetrical to an anterior-posterior extending plane, which blades can be brought to rest parallel on the flat osteotomised surface on the side of the joint of a first bone adjacent to a non-spherical joint, a left and a right handle, a left and a 15 right operating lever which simultaneously with the holding of the device with one hand each can be actuated by the same hand on the corresponding handle, and a left and a right tensioning lever which with their contact surfaces can be brought to rest on the surface on the side of the joint of a second bone adjacent to the joint. The movement of the tensioning lever relative to the blades is carried out by 20 a left and a right parallel displacing device, that can be operated by means of the corresponding operating lever and are so connected with a tensioning lever each, that during the movement of the operating lever the contact surfaces of the tensioning levers can be moved independently from one another parallel to the contact surfaces on the blades. The parallel displacing devices are constructed 25 as four-bar lever mechanisms. The magnitude of the spreading force between the contact surfaces on the claws and the contact surfaces on the tensioning levers can be read off on a graduated force meter, while during the tensioning of the ligaments by actuating the device a separate reading off of the force exerted on the left and on the right operating levers is possible. The position of the 30 tensioning lever relative the blades can be detachably locked by means of locking elements. In a preferred embodiment of the device according to the invention the force indicator has a moving indicator lever, the edges of which move relative to the scale. This indicator lever is moved by the longitudinal bending of an operating lever part that can be bent relative to the other fork-like provided operating lever part by a manually applied tensioning force that is not subjected to this tensioning force. If by means of the tensioning force both operating lever parts are moved 5 relative one another, the indicator lever moves relative to the graduated scale. The parallel displacing devices, constructed as four-bar lever mechanisms, preferably comprise four levers, while an upper lever and a lower lever are provided parallel, the upper lever is connected with the corresponding tensioning 10 lever, the lower lever is connected with the base body and the connecting levers form the scissors bars, so that the upper lever and the lower lever can move parallel towards or away from one another. The lengths of the levers are so chosen, that with regard to the force transfer 15 between the tensioning force exerted with the respective hand between the corresponding operating lever and handles and the distractive force exerted on the bones adjacent to the joint by the corresponding tensioning lever and the corresponding blade within a tensioning width between the contact surfaces of the blades and the contact surfaces of the tensioning levers between 5 mm and 20 35 mm, preferably between 7 mm and 25 mm, the parallel displacing devices ensure a transmission ratio of 1:1. The advantages achieved by the invention are essentially that by virtue of using the ligament tensioner according to the invention the tensioning forces exerted on 25 the ligament during the tensioning of the ligaments can be intuitively controlled by the forces applied by hand to the operating elements and the transmission ratio of 1:1 on the one hand, and on the other by simultaneously indicating the force a quantitative control can be carried out. This quantitative indication of the force ensures that the medial and the lateral ligaments, which are separately tensioned 30 by the tensioning force applied by the left or the right hand, can be acted upon with the same pretensioning force or with deliberately different pretensioning forces.
The invention to be used in a human knee joint is explained in detail in the following based on the partly schematical illustrations. They show in: 5 Fig.1 - a side view of the preferred embodiment of the device according to the invention, Fig.2 - a top view of the embodiment of the device according to the invention, 10 illustrated in Fig.1, Fig.3 - a perspective illustration of the embodiment of he device according to the invention, illustrated in Figs.1 and 2, used in a bent knee joint, and 15 Fig.4 - a perspective illustration of the embodiment of he device according to the invention, illustrated in Figs.1 and 2, used in a straight knee joint. Figs.1 and 2 illustrate the device I according to the invention with a spreading mechanism and its locking. The device comprises a plate-shaped base body 2, 20 that for the purpose of a reliable introduction of the spreading force into the tibia has a left blade 4 and a right blade 3 with contact surfaces 16 lying in one plane 5. A left handle 7 and a right handle 6 are provided on the base body 2, opposing the blades 3, 4, respectively, which handles enable the holding of the device with two hands. The device also has, corresponding to the arrangement of the blades 25 3, 4 and above the blades 3, 4, a left tensioning lever 9 and a right tensioning lever 8, which rest with their contact surfaces 15 on the opposite situated part of the joint 10. The spreading action is produced by actuating the right handle 6 together with the right operating lever 11 for the medial or lateral part of one joint. By actuating the left handle 7 together with the left operating lever 12 the 30 spreading function is produced on the other, depending on the joint, medial or lateral part of the joint. A left parallel displacing device 14 and a right parallel displacing device 13 allow a parallel displacement of both pairs of spreaders relative to the contact surfaces 16 and the contact surfaces 15. The parallel displacing devices 13, 14 are executed as four-bar mechanisms in the form of intersecting bars and each comprise four levers 18, 19, 20, 21, wherein the levers 18 on the side of the tensioning lever and the levers 21 on the side of the base body are provided parallel, whereas the two levers 19, 20 intersect. The four levers 18, 19, 20, 21 are joined with one another by means of five spindles 25, 5 26, 27, 28, 40. Two of the spindles 25, 26 are displaceably mounted in the parallel levers 18, 21 in slots 29, 30 running parallel to the contact surfaces 25,26. This construction of the parallel displacing devices 13, 14 allows that the lever 18 on the side of the tensioning lever and the lever 21 on the side of the base body can move parallel towards or away from one another. The lengths of 10 the levers 18, 19, 20, 21 are so chosen, that when the tensioning width X between the contact surfaces 16 on the blades 3, 4 and the contact surfaces 15 on the tensioning levers 8, 9 is between 5 mm and 25 mm, the transmission ratio between the manually exerted tensioning forces acting on the handles 6, 7 and the operating levers 11, 12 and the distractive forces exerted on the bones 15 adjacent to the joint 10 (Figs.3 and 4) is 1:1. Between the handles 6, 7 and the operatingjevers 11, 12 a locking element 22 each is provided, that can be locked i any position of the operating levers 11, 12 relative to the handles 6, 7. These locking elements 2,act like a sort of a ratchet 20 mechAnism that can be pushed together during the spreading and locks when the spread position is achieved. A hand lever 31 is used fqr the release of the locked position. The magnitude of the spreading force can be read off on a force meter 17 with a 25 graduated scale 24 and a displaceable indicator lever 23. This indicator lever 23 is moved by the longitudinal bending of an operating leveT part 32 that can be bent by a manually applied tensioning force relative to the other fork-like provided operating lever part 33 that is not subjected to this tensioning force. The indicator lever 23 is mounted by means of a pivot spindle 34 in the operating lever part 33 30 not subjected to the tensioning force and with a cam 35 abuts against the operating lever part 32 subjected to the tensioning force. When the two operating lever parts 32, 33 are moved relative one another by means of the tensioning force, the indicator lever 23 rotates about the pivot spindle 34 and the manually exerted tensioning force is indicated by the indicator lever 23 on the graduated scale 24. For the control of the axial relationships in the joint to be cared for, guide bars 5 (not illustrated) can be inserted into a guide 36, that is joined, angularly fixed, with the base body 2. For the use in a bent joint 10 (Fig.3) a deliberately different stability of the two parts of the joint can be produced by spreading the medial and lateral joint parts 10 to a different extent, thus achieving a rotational mobility, like the one desired, for example, for the bent knee joint. This different stability can be produced by specified different medial and lateral spreading forces. When used for a straight joint (Fig.4), the stability of the joint can be accurately 15 adjusted in an exactly specified extended petition, e.g. with slight hyperextension, as this is desirable, for example, for the knee joint.