KNEE SURFACE REPLACEMENT PROSTHESIS

Abstract: The invention relates to knee surface replacement prosthesis comprising a femoral component (1) provided with lock pins and a plastic tibial component (2) placed in a metal tray (3) fixed to the tibia by means of a pin or a screw. The knee surface replacement prosthesis is characterized in that the external surface of its femoral component (1) having a movement determined by their conical surfaces is formed in accordance with its logarithmic spiral generatrix (4) while its internal surface corresponds to the conical spiral generatrix (5).

Description: Knee surface replacement prosthesis Technical Field The present invention relates to a knee surface replacement prosthesis comprising a femoral component provided with lock pins and a plastic tibial component placed in a metal tray fixed to the tibia by means of a pin or a screw Background Art The injured knee joint can be replaced by an artificial knee joint In cases where high degrees of shape deformations are to be corrected, the necessary movements can only be ensured by a tight mechanical connection of the two surfaces The various hinged resp axial total prostheses are used for such purposes The disadvantage of the well known prostheses is that after the removal of the bone fragments the large quantity of metal is covered by only a thin layer of human tissue i e only by the human skin Another disadvantage is that the bearing providing the connection can not be positioned physiologically, which may lead to the prosthesis becoming loose If the ligament systems providing the function of a rope system and limiting the sliding of the two surfaces on each other are intact, it is sufficient to replace only the surface participating in the movement In these cases, so-called surface replacement prostheses are used In the case of these so-called surface replacement prostheses only a smaller part of the bone end must be removed They have no mechanical connection Prior Art The femoral component of the known surface replacement prostheses consists of two slides positioned on the condyle opposite to which a plastic block combined with a metal tray is placed on the tibia The femoral component consists essentially of two approximately parallel rolls as described by Fick at the beginning of the century when this has become known in the technical literature as the so-called Fick principle The shape of these rolls is approximately ellipsoidal and the center of the generatrix curves is located on a spiral so-called evolute This ensures a tight connection of the two bones in accordance with the components of the knee joint during flexion and extension of the knee The femoral component of most of the known knee prostheses is a mirrored symmetrical duplicate of prostheses corresponding to a prosthesis designed to one condyle, which, however, does not correspond to the normal three- dimensional movement of the knee A considerable part of the known knee prostheses has a flat plane opponent element which results in a sliding of the components on each other, which damages the stability of the knee-joint Meanwhile there is a single point connection between the curved femoral component and the plane surface causing an excessive abrasion on a plastic tibial component In known solutions where the opponent element is a plastic part with a shape of a trough, but due to its geometry does not allows the end rotation, therefore a rotation stress develops between the prosthesis and the bone, which may lead to a dislocation of the prosthesis component To overcome this, rotation is ensured by a rotatable plastic insert positioned in a metal tray like ratable base In this case the prosthesis manufactured by Waldemar Link may inhibit rotation because of the connective tissue of the human body, which causes pain The stops limiting the movement and transmitting the forces, which may cause dislocation at the bone boundary in their end position Dislocations can be eliminated e.g by application of anchoring procedures disclosed in the CH 665 55, UK 2 150 441 , UK 2 219 942, US 4 659 331 , US 4 714 473 and US 4 731 086 Patent Specifications In the most known prostheses the depth of the trough in the sagittal plane of the opposite element is essentially constant Such a solution is disclosed in the DE 2 550774 Patent Specification The disadvantage of this embodiment is that the knee may be sprained backwards when the leg is extended. The common disadvantage of the known knee prostheses discussed above is that the movements necessary during the normal human life can not be ensured without any difficulty Meanwhile the movements or the abrasion of the components may cause a wear taking place very quickly or a mechanical loosing of the prosthesis. Summary of the invention The purpose of the present invention is the establishment of a prosthesis for replacing the surface of the human knee joint comprising components for realizing the normal movements necessary for perfect function of the knee-joint by sliding and rolling on each other. During the creation of the knee prosthesis according to the invention it was started from the assumption that joints in humans and animals have evolved according to the needs of their movement and that their final shape ensures perfect functioning. Accordingly, the essence of this invention is to form the shape of the surfaces of the prosthesis participating in the movements, which differs significantly from those of currently known and patented knee surface replacement prostheses. The invention is based on the recognition - which has already been supported by three-dimensional analysis - that ..spherical movement of a rigid body" known from mechanics must be realized by means of the new prosthesis. The purpose of the invention is achieved by a knee surface replacement prosthesis consisting of a femoral component provided with lock pins and a plastic tibial component placed in a metal tray fixed to the tibia by means of a pin or a screw wherein the external surface of its femoral component having a movement determined by conical surfaces, is formed by the logarithmic spiral generatrix playing role in the movement while its internal surface corresponds to the conical spiral generathx. In a preferred embodiment of the knee surface replacement prosthesis according to the invention the circular arcs - circle segments - running along the generatrices of the femoral component are shaped perpendicularly to the logarithmic spiral generatrix in case of the external surface while in case of the internal surface they are formed rotatable along the conical spiral generatrix in accordance with the segment curves of the human knee-joint. In another preferred embodiment of the knee surface replacement prosthesis according to the invention, the surface of the tibial component is provided with a trough-shaped notch, the surface of the exterior arch of this trough-shaped notch is formed in accordance with the rolling down of the logarithmic spiral generatrix of the femoral component while the surface of its internal surface corresponds to the rolling down of the conical spiral generatrix of the femoral component In a further preferred embodiment of the knee surface replacement prosthesis according to the invention the trough-shaped camber of the tibial component is higher than that of the femoral component Brief description of the drawings The knee surface replacement prosthesis according to the invention will now be described by way of example with reference to the appended drawings, wherein Fig 1 shows an axonometnc view of the knee surface replacement prosthesis according to the invention, Fig 2 shows an axonometnc view of the femoral component of the knee surface replacement prosthesis according to Fig 1 illustrating the generatrices of the external and internal surface, Fig 3 shows a rear view of the femoral component according to Fig 2, illustrating the circle segments, Fig 4 shows an axonometnc drawing of tibial component of the knee surface replacement prosthesis according to Fig 1 , Fig 5 shows a top view of another, single-sided embodiment applied on the internal condyle of the femur according to Fig 2, and Fig 6 shows an axonometnc drawing of another single-sided embodiment of the knee surface replacement prosthesis according to Fig 4 applied on the internal tibial plateau Detailed description of the drawings The knee surface replacement prosthesis shown in Fig 1 consists essentially of a femoral component 1 and a tibial component 2 as well as a metal tray 3 for fixing the tibial component 2 which itself is already known Fig 1 shows a double-sided embodiment Fig 2 is an axonometnc drawing of the femoral component 1 shown in Fig 1 illustrating the external surface of the logarithmic spiral generatrix 4 and the internal surface of the conical spiral generatrix 5 which play role in the movement Fig 3 is a rear view of the femoral component shown in Fig 1 illustrating the circle segment 6 perpendicular to the logarithmic spiral generatrix 4 resp the circle segments 7 rotated spatially in an appropriate degree along the conical spiral generatrix 5 of the femoral component Fig 4 is an axonometnc drawing of the trough-shaped surfaces 8 and 9 implemented in accordance with the rolling down of the logarithmic spiral generatrix 4 resp the conical spiral generatrix 5 of the femoral component 1 It has also a significant sagittal plane camber, the protrusion of the frontal part prevents namely the knee from spraining backwards The camber 10 of the tibial component 2 in frontal plane is higher than that of the femoral component 1 , so that the freedom of movement is allowed and simultaneously all external mechanical influences acting on a body are compensated For realizing the spherical movement of a rigid body the surfaces of the femoral component 1 playing role in the movement are shaped on a so-called polhodic cone The generatrix of the external surface is a logarithmic spiral generatrix 4 while that of the internal surface is a conical spiral generatrix 5 The counterpart, i e the tibial component 2 which is positioned in a conventional metal tray 3 is formed so that it is trough-shaped The surface 8 of the trough-shaped external arc of the tibial component 2 is formed in accordance with the rolling down of the logarithmic spiral generatrix 4 of the femoral component 1 while the surface 9 of its internal arc corresponds to the rolling down of the conical spiral generatrix 5 of the femoral component 1 p = ce kφ (π/ 6< Φ < 2π /3) where „c" is a constant characteristic of the curvatures of the condyle, in accordance with the population, „k" is the cotangent, characteristic of the population and also testing to avoid self- locking, of the tangent of the condyle section in the sagittal plane The equation of the conical spiral generatrix 5 of the femoral component 1 is as follows x= ae k cosΦ k= be kφ where „a" and „b" are constants depending on the femur width and the largest condyle „radιus" measured in the frontal plane, characteristic of the population The in such way formed femoral component 1 and tibial component 2 constitute a prosthesis pair, between the components of which the movement along the arc on the cone results in rotation, i e a joint movement that corresponds to the physiological movement (spherical movement) even during crouching The internal surface of the femoral component 1 is surrounded by five planes This matches the bone surfaces created during the operation by pattern generally known The tibial component 2 is a plastic block, the trough-shaped camber 10 of which is higher than that of the circle segments 6, 7 of the femoral component 1 to provide a certain degree of freedom of movement and to flexibly counteract non- physiological forces The two-parameter equation of the external surface of the femoral component 1 is as follows x = ([ce k - R] + Rcosθ) cosΦ y = ([ ce k -R] + Rcosθ) sinΦ z = ± Rsinθ where θ is the central angle of the circle arc segment with R=35 mm The two-parameter equation of the internal surface of the femoral component 1 is as follows x = ([ae kφ - R] + Rcosθ ) cosΦ y = ([ae k - R] + Rcosθ ) sinΦ z= le kφ ± Rsinθ where θ is the central angle of the circle arc segment with R=35mm Besides the double-sided prostheses it is also necessary to use smgle- sided prostheses Such single-sided prostheses form the half of a double sided knee-prostheses having external and internal curves Thus their function allows the respective physiological movements Fig 5 shows such a single-sided femoral component 1 , the generatrix of which is provided with a curve which corresponds to the conical spiral generatrix 5 of the internal surface of the femoral component 1 Fig 6 shows the tibial component 2 of a single-sided knee surface replacement prosthesis, the rolling down of the trough-like shaped surface of which corresponds to the rolling down of the femoral component 1 according to Fig 5 The single-sided femoral component 1 - Fig 5 - has a so-called anatomical internal surface It has the advantage that only a minimal part of the bone surface should be removed, further the condyle retains its anatomical shape and hence its bone structure The shell-like seating provides a stability that makes anchoring procedures mentioned in the prior art superfluous even in the case of any cementless application The advantage of the knee surface replacement prosthesis according to the invention is that provides the physiologically normal movements which are possible because of the anatomical structure of the bones forming the knee joint, which is ensured by the logarithmic spiral generatrix of the external surface of the femoral component 1 , the conical spiral generathx of its internal surface as well as the trough-shaped cambers of the tibial component 2 which corresponds to their movement. List of reference numbers 1 femoral component 2 tibial component 3 metal tray 4 logarithmic spiral generatrix 5 conical spiral generatrix 6 circle segment 7 circle segment 8 surface 9 surface 10 camber

Claims: Claims 1 Knee surface replacement prosthesis comprising a femoral component (1) provided with lock pins and a plastic tibial component (2) placed in a metal tray (3) fixed to the tibia by means of a pin or a screw, characterized in that the external surface of its femoral component (1) having a movement determined by conical surfaces is formed in accordance with its logarithmic spiral generatrix (4) playing role in the movement while its internal surface corresponds to the conical spiral generatrix (5) 2. Prosthesis according to Claim 1 , characterized in that the circular arcs - circle segments (6, 7) - running along the generatrices of the femoral component (1) are shaped perpendicularly to the logarithmic spiral generatrix (4) in case of the external surface while in case of the internal surface they are formed rotatable along the conical spiral generatrix (5) in accordance with the segment curves of the human knee-joint. 3 Prosthesis according to Claim 1 or 2, characterized in that the surface of its tibial component (2) is provided with a trough-shaped notch, the surface (8) of the exterior arch of this trough-shaped notch is formed in accordance with the rolling down of the logarithmic spiral generatrix (4) of the femoral component (1 ) while the surface (9) of its internal surface corresponds to the rolling down of the conical spiral generatrix (5) of the femoral component (1 ) 4 Prosthesis according to any of Claims 1 to 4, characterized in that the trough-shaped camber of the tibial component (2) is higher than that of the femoral component (1 )

Inventors: BARDOSI TAMASNE, JUHAROSI ZOLTAN, KRAKOVITS GABOR, BARDOSI, TAMASNE, JUHAROSI, ZOLTAN, KRAKOVITS, GABOR
Applicants: BARDOSI TAMASNE, JUHAROSI ZOLTAN, KRAKOVITS GABOR, BARDOSI, TAMASNE, JUHAROSI, ZOLTAN, KRAKOVITS, GABOR
Application year: 1997
Main classification: A61F2
IPC codes: A61F2