Knee prosthesis

Abstract

A knee prosthesis for a knee joint with a medial ball-like femoral condyle and with a width “w” is characterized on the lateral compartment by:—a trace-line “Lt 1 ” of contact points “Pt 1 ” for the tibia component ( 2 ) as a predetermined curve ( 15 ) on a spherical surface “Sc 1 ”, which has its centre at the centre Mb of the medial ball 1 and which has a radius in the range “Rc 1 ”=0.65w+/−0.25w—a trace-line “Lt 2 ” of common contact points “Pt 2 ” at given flexion angles γ for a similar spherical surface “Sc 2 ” attached to the femoral component with radius “Rc 2 ”=Rc 1 , whereby at a given flexion angle γ there exists a plane “E 1 ” through the centre Mb and the common contact point Pt 1 /Pt 2 , which contains the same guiding curves “Bi”, “Be” on the tibial and on the femoral components, “Bi” towards medial and “Be” towards lateral, both of which stay in a geometrically fixed relation to the common contact point Pt 1 /Pt 2 , and—guiding curves “Bi” and “Be”, which generate an enforced gliding and rolling movement in both directions.

1 . A Knee prosthesis for a knee joint of a width “w” with a ball-like femoral condyle ( 1 ) and a corresponding tibia cavity ( 11 ) on the medial side, the ball ( 1 ) and cavity ( 11 ) having a centre “Mb”, a radius “Rb”, a spherical surface “Sb” and defining a Cartesian coordinate system X, Y, Z attached to the tibia its origin “O” at the centre “Mb”, wherein
 on the lateral compartment exists a trace-line “Lt 1 ” of contact points “Pt 1 ” for the tibia component ( 2 ) as a predetermined curve ( 15 ) on a spherical surface “Sc 1 ”, which has its centre at the origin “O” and which has a radius in a range “Rc 1 ”=0.65w+/−0.25w, 
 at a given flexion angle γ for each contact point “Pt 1 ” there exists on a trace-line “Lt 2 ” a common contact point “Pt 2 ” for a spherical surface “Sc 2 ” attached to the femur, which is identical to “Sc 1 ” and has origin “O” and radius “Rc 2 ”=Rc 1 , whereby at a given flexion angle γ there exists a plane “E 1 ” through origin “O” and the common contact point Pt 1 /Pt 2 , which contains the same guiding curves “Bi”, “Be” on the tibial and on the femoral part, “Bi” towards medial and “Be” towards lateral, both of which stay in a geometrically fixed relation to the common contact point Pt 1 /Pt 2 , 
 and at least one of the guiding curves “Bi” and “Be” progressively changes its shape by change of flexion angle γ, for generating an enforced gliding and rolling movement in flexion direction or in extension direction. 
 
     
     
         2 . The Knee prosthesis according to  claim 1 , wherein the guiding curves “Bi” and “Be” are arcs, which start from common contact points Pt 1 /Pt 2 . 
     
     
         3 . The Knee prosthesis according  claim 1 , wherein the plane E 1  is orthogonal to a tangent “T 1 ” of the trace-line Lt 1  at the common contact point Pt 1 /Pt 2  and in that the guiding curves “Bi” and “Be” at the common contact point Pt 1 /Pt 2  are tangent to a line “T 2 ” on plane E 1 , which is drawn from the common contact point to the surface “Sb” of the ball. 
     
     
         4 . The Knee prosthesis according to  claim 1 , wherein at each common contact point Pt 1 /Pt 2  the curves “Bi” and “Be” are circular arcs with radii Ri, Re. 
     
     
         5 . The Knee prosthesis according to  claim 1 , wherein the predetermined curve is generated by the interference of the spherical surface “Sc 1 ” with a surface of a cylinder, which stands orthogonal to a sagittal plane and which is constructed by a continuous curve (Lc) located on the sagittal plane. 
     
     
         6 . The Knee prosthesis according to  claim 5 , wherein the continuous curve used to construct the cylinder is lying on a sagittal plane and between two circular boundaries with radii R 1  and R 2 , which have a common centre “Ms” with the coordinates x=0.07w; y=−0.794w; z=0.5w and the radius R 1 =0.54w+0.08w respectively radius R 2 =0.54w−0.08w. 
     
     
         7 . The Knee prosthesis according to  claim 6 , wherein the radius R 1  takes R 1 =0.54w+0.03w and the radius R 2  takes R 2 =0.54w−0.03w. 
     
     
         8 . The Knee prosthesis according to  claim 1 , wherein at each flexion angle γ a tangent T 1  to the trace-line Lt 1  at the contact point Pt 1  is also the tangent for the trace line Lt 2  on the spherical surface “Sc 2 ” of the femur and in that at each flexion angle the location of the momentary rotation axis is on a plane E 1 , which passes through the centre Mb of the medial ball  1  and is perpendicular to the tangent T 1  of the three dimensional trace-line Lt 1  at contact points Pt 1  for the tibia. 
     
     
         9 . The Knee prosthesis according to  claim 1 , built as a total knee prosthesis with a femoral component, which has a groove similar to an anatomic patellar groove, and with a tibia component with an appropriate shape of a posterior cut out for maintaining one or both of the cruciate ligaments. 
     
     
         10 . The Knee prosthesis according to  claim 1 , built as lateral monocompartmental prosthesis adapted to maintain a natural patella and patella groove in combination with the congruent articulation between the natural medial femoral condyle and the convex geometry of the tibial and meniscal geometry. 
     
     
         11 . The Knee prosthesis according to  claim 1 , wherein the guiding surfaces “Bi” and “Be” for the tibial and for the femoral components are less congruent for the middle range of the flexion angle γ, than for the end positions full extension and full flexion. 
     
     
         12 . The Knee prosthesis according  claim 1 , wherein conical surfaces  17 ,  18  are added on the interior side of the trace-lines Lt 1  and Lt 2  for additional support, which have their centres at the centre Mb of the medial ball  1  and which have the trace-lines Lt 1 , Lt 2  as generator for the cones. 
     
     
         13 . The Knee prosthesis according to  claim 1 , wherein the guiding curves “Bi” and “Be” progressively change their shapes in opposite directions by change of flexion angle γ, for generating an enforced gliding and rolling movement in both flexion and extension directions.