WO2013110269A1 - Artificial joint - Google Patents

Description

 Artificial joint

The invention relates to an artificial prosthesis part intended for the construction of an endoprosthesis for the human knee joint and which functions as a kinematic function, illustrated in the front view (FIGS. 1 a, 1 b) in the view from the side (Fig. 2) and in a three-dimensional view (Fig. 3a, 3b). This prosthesis part is a composite joint and consists of two rigidly connected rod ends (3 and 6) and two rigidly interconnected joint cups (4 and 7). The articular surface of the first condyle (3) is like the articular surface of the second condyle (6) in the front view (Fig. 1 a, Fig. 3a, 3b): section line with the plane M _T MM _F M or section line with the plane M _F LM _T M) and also in the view from the side always convexly curved (Fig. 2: contact lines C _F M and c _FL ). The joint surface of the first joint socket (4) and the joint surface of the second joint socket (7) are always concave curved in the front view (Fig. 1 a, Fig. 3a, 3b: section line with the plane M _T MM _FM or section line with the plane M _FL -M _T M) - In the view from the side but the articular surface of the first socket (4) is still concavely curved, while the second socket (7) always convex curved (Fig. 2: contact lines C _T M and c _T i _) - Accordingly, the joint has a first, by a first condyle (3) and a first socket (4) formed Gelenkkompartiment (2) and a second Gelenkkompar-- (5) formed by a second condyle (6 ) and a second socket (7). In the case of compressive adhesion, the joint function is established and the first joint head (3) and the first joint socket (4) contact K _m in the first joint compartment (2) and the second joint socket (6) and the second joint socket in the second joint compartment (5) (7) at the contact point K |. During the main function, the flexion / extension of the composite joint (Fig. 2), the contact point K _m moves in the functional plane FE _{Km of} the first joint compartment (2) on the first condyle (3) along the plane contact line c _FM and simultaneously in the first socket (4) along the plane contact line C _T M and the contact point K | in the functional plane FE _K i parallel to the functional plane FE _Km on the second condyle (6) along the plane contact line c _FL and at the same time on the second joint socket (7) along the flat contact line c _T i_- Under adhesion created by the Hertzian surface pressure a small contact surface C _m in the contact point K _m and a small contact surface Ci in the contact point K | (Fig. 1 a).

In the knee endoprosthesis according to EP 1 382 314 A2, the articular surface of the first condyle (3) is rotationally symmetrical with the axis of rotation M _F M which is perpendicular to the functional plane FE _Km and thus passes through the center of curvature of the contact line C _F M. The joint surface of the second condyle (6) is also rotationally symmetrical and accordingly has the axis of rotation M _FI _, which is perpendicular to the functional plane FE _K i and thus passes through the center of curvature of the contact line c _F i_. The axes of rotation M _FM and M _FI _ are parallel to each other and have an offset f, which is designed so that the rotational axis M _FL with respect to the rotation axis M _{FM is} always moved to the rear (med.: To posterior) and in special versions additionally upwards (Fig. 2, Fig. 1 b) or downwards (med. after cranial or cadaver).

The joint surface of the first joint socket (4) is rotationally symmetrical with the rotation axis MTM, which is perpendicular to the functional plane FE _Km and thus extends through the center of curvature of the contact line C _T M. The joint surface of the second joint socket (7) is also rotationally symmetrical and accordingly has the rotation axis MTL, which is perpendicular to the functional plane FE _K i and thus passes through the center of curvature of the convex contact line c _T i_. The rotation axes M _T M and MTL are parallel to each other and have the distance t from each other.

Since all four axes, M _FM and M _FL and M _T M and M _T i_, are parallel to each other, the joint with respect to the main function, flexion / extension, is a positive four-bar linkage in which f, t, r _m and η den Distances of the four axes are. Each plane section, perpendicular to the four axes, represents a functional plane. In this section, the distance f defines the position of the rigidly interconnected rod ends (3 and 6), hereinafter referred to as the femur, and the distance t fixes the position of the rod connected joint sockets (4 and 7), called tibia further, firmly. In the reference system of the tibia (t), the femur (f), guided by the wings r _m and η, inflected, the position of the femur is clearly given by the angle of flexion Δφ, by which the femur (f) from the initial angular position φ _{0 has} turned (Fig. 2). During flexion, the instantaneous axis of rotation about which the femur (f) rotates moves. The position of the instantaneous axis of rotation (MDA) is uniquely defined by the angle of rotation Δφ of the femur (f) because the instantaneous axis of rotation (MDA) passes through the intersection point (P) of the two wings r _m and η (Fig. 2 and Fig. 3a ). As illustrated in Fig. 3a, it is possible, by suitable dimensions of the distances f, t, r _m and η of the four morphologically defined axes, the instantaneous axis of rotation MDA at small angles of inflection in or near the contacts K _m and K | as is also the case in the knee endoprosthesis according to EP 1 382 314 A2, because it means kinematically that then the articular surfaces of the rod ends predominantly roll on the articular surfaces of the socket joints. Since the human knee experiences the highest loads when walking and running, as long as the foot in question must press on the ground, in which case the flexed knee is in a flexion angle range of approximately Δφ = 0 ° to Δφ = 30 °, it is extremely advantageous tribologically in that the articulation surfaces in the knee in this flexion region of the so-called "stance phase" predominantly roll on each other during walking and running, because then the sliding friction in the joint is replaced by the orders of magnitude smaller rolling friction, which in turn is expected to result in a lower wear of the joint surfaces tribological optimization is realized in the knee endoprosthesis known from EP 1 382 314 A2 and has proven itself in practice, but the disadvantage is that the four-bar joint of this knee prosthesis allows only a maximum knee bend which is smaller than that possessed by the natural knee. The reason for this is that in this prosthesis the entire Funktionsbere I the flexion / extension is based on the four-joint described above with constant distances f, t, r _m and η.

In order to further approximate the natural knee function in flexion / extension, it is advantageous to make the distances f, t, r _m and η dependent on the angle of flexion Δφ. It is particularly advantageous to design essentially only the distance f from the angle of flexion Δφ. The distance f should be chosen so that in the flexion 0 ° <Δφ <30 ° optimal rolling of the articular surfaces prevails, then the distance f as a function of the rotation angle Δφ decreases monotonically and completely disappears at large flexion angles, so then the two femoral axes M _F M and M _F i_ coincide. This "fluid" construction implies that the contact lines c _FM , C _T M _> C _F L and c _T i_ continue to be plane curves, but that they are no longer circular in their total extent Axes of the "fluent" successive four joints for each flexion angle Δφ through the radii of curvature of the contact lines c _FM , C _T M _> c _FL and C _T L in the positions of the contacts K _m and K | certainly. The successive positive quadrilaterals with decreasing distance f finally degenerate into a positive, single hinged joint in which the contacts K _m and K | then remain stationary on the contact lines C _T M and c _T i_. This automatically adjusts at large angles of flexion spatial constancy of the contacts K _m and K | on the associated sockets (4 and 7) has the additional advantage that the contacts K _m and K | can not leave the geometrically relatively small articular surfaces of the pans (4 and 7) even at the largest flexion angles, although in the initial roles the contacts K _m and K | hike backwards on the pans. It should also be pointed out that the small contact surfaces C _m and C 1 that result under the Hertz surface pressure of the frictional connection do not significantly influence the gear parameters f, t, r _m and η.

Overall, as an endoprosthesis intended for the human knee joint, artificial joint represents the parallel connection of two planar cam gears, which are characterized by the plane contact line pairs C _F M; C _T M and c _F i_; C _T L are given, which causes the forced run in flexion / extension in the artificial joint.

The four parallel axes have a common point of intersection at infinity and therefore produce even movements of the joint section. Without loss of generality, the above and other descriptions apply, even in cases where the four axes have a common point of intersection. The prosthesis then inevitably performs spherical movements. The "function levels" become concentric spherical surfaces around the common point of intersection of the axes.

In the first joint compartment (2), at the point of contact K _m, the joint force _{m is} mated, whose line of action coincides with the common normal n _m at the contact point K _{m of} the two joint surfaces (contact surface C _m ), and in the second joint compartment (5) the joint force F _l , whose line of action with the common normal ni at the contact point K | of the two articular surfaces (contact surface C |) coincides (Fig. 3a, 3b). The normals n _m and ni generally do not intersect. They have a shortest distance d, which is both perpendicular to the normal n _m and perpendicular to the

Normal ni stands. Consequently, each the lines of action of the two joint forces F _m F _l un6 skewed run. The two helical forces F _m and F ₁ result uniquely in a force screw, consisting of the resultant force F _R = F _m + F ₁ whose line of action intersects the distance d at the point Γ plus a torque T _R whose direction which matches the resultant force F _R. The position of the point Γ on the distance d is uniquely determined by the amounts of the two joint forces, eg, lies for in the middle of d, as drawn in Fig. 3b.

The artificial joint described above has four kinematic degrees of freedom, one "big" and three "small" ones. The great degree of freedom (which describes the knee function of flexion / extension) is actively used by the muscles: in the reference system of the tibia (t) the femur is flexed - at small angles of flexion guided by the wings r _m and η of the quadrilateral and in large guided by the hinge axis, given by the coincident femoral axes M _F M and M _F T, where the position of the femur clearly given is by the flexion angle. The three "small" degrees of freedom are a rotation around the axis given by the surface normals n _m and b rotation about the axis given by the surface normal r i and c rotation about the axis given by the distance line d the latter represents ab-adduction in the knee joint. These three "small degrees of freedom" are not actively served by the musculature, but they are stabilized by it. The greater the amount of resultant joint force F _R , the more stable is the balance between femur and tibia. For the degree of freedom of Ab-adduction, this mechanism has already been described in EP 1 382 314 A2. There it was also suggested that the common tangent surface of the articular surfaces at the contact point K | (Contact surface Ci) and the common tangent surface of the articular surfaces in the contact point K _m (contact surface C _m ) is to be arranged inclined so that it rises in the respective contact point to the inner joint. These inclination angles λ | and A _m are uniquely defined in the sectional planes M _F LM _T L (distance η) and MFM-MTM (distance r _m ) (FIGS. 3 a, 3 b). From these inclination angles λ | and A _m hangs now a. the degree of stability of the joint when both parts of the joint have been balanced by muscular forces, and b. the amount of the maximum torque T _R.

In order to further improve the properties of the artificial joint (1 a, 1 b), it is proposed according to the invention to make these inclination angles in the joint asymmetrical in order to optimize joint stability and torque T _R.

The invention allows for various embodiments. To further clarify its basic principle, one of them is shown in the drawing and will be described below. This shows in

Fig. 1 a, an inventive joint in an extended position in one

 Front view;

Figure 1 b shows the joint shown in Figure 1 with marked axes of rotation.

Fig. 2 is a schematic representation of a joint according to the invention in one

Front view; 3a shows an artificial joint in the extended position in a three-dimensional view from the front side;

Fig. 3b shows another view of the artificial joint in the extended position in

 the spatial view from the front side.

1 a shows an artificial joint 1 in the extended position in the front view with a first joint compartment 2 comprising a first joint head 3 and a first joint socket 4. A functional plane of a first joint contact K _m is designated FE _Km . Due to the Hertzian surface pressure under adhesion, there is a small contact area C _m . Furthermore, a second joint compartment 5 with a second joint head 6 and a second joint socket 7 can be seen. A functional plane of a second joint contact K | is denoted by FE _K i. Due to the Hertzian surface pressure under frictional connection, a small contact surface C | results.

1 b, in the front view of the artificial joint 1, in an extended position to illustrate the positions, the morphologically predetermined axes of rotation (tibial: M _T M, M _T I, femoral: _FM , M _F L) are shown, which form a four-bar joint network.

For a better understanding, FIG. 2 shows an artificial joint according to the invention in an extended position in a side view, which illustrates the four-joint networking of the four morphologically given axes in the functional plane of the flexion / extension by the axial distances t, r _m , f, η. The axis M _F M is determined by the center of curvature of the femoral contact line C _F M at the contact point K _m . The axis M _FL is defined by the center of curvature of the femoral contact line c _FL at the contact point K | certainly. The axis M _TM is determined by the center of curvature of the tibial contact line C _T M at the contact point K _m . The axis M _TI _ is through the center of curvature of the tibial contact line c _T i_ in the contact point K | certainly.

For the first joint compartment 2: r _m = R _TM - R ™ with

R _T M = radius of curvature of the contact track C _T M on the medial tibial articular surface, given by the distance K _m M _TM , and

R _FM = radius of curvature of the contact track c _FM on the medial femoral articular surface, given by the distance K _m M _TM . cpo = initial position of the angle of rotation of f in relation to t.

Δφ = change of the angle of rotation of f. For the second joint compartment 5:

RTL = radius of curvature of the contact track c _T i_ on the medial tibial articular surface, given by the distance K | M _T i_, and

RFL = radius of curvature of the contact track c _F i_ on the medial femoral articular surface given by the distance K | M _TI _.

In FIGS. 3a and 3b, an artificial joint in the extended position can be seen laterally in the three-dimensional view from the front. The current axis of rotation of the four-bar linkage is designated MDA. The sectional contours (first joint head 3, first joint socket 4) are obtained by the section of the first joint compartment 2 with the plane M _TM -M _FM . The sectional contours (second condyle 6, second acetabulum 7) result from the section of the second articulated compartment 2 with the plane M _FI _-M _TI _. This results in a pitch angle A _{m of} the common tangent of the cutting contours (first condyle 3, first socket 4) at the contact point K _m with the normal n _m at the contact point K _m and a pitch angle Ai of the common tangent of the sectional contours (second condyle 6, second socket 7) at the contact point K | with the normal ni at the contact point K |. The helical joint forces F _m and _Z are equivalent to the force screw

Claims

Applicant: AEQUOS Endoprosthetics G Am Haag 10 82166 Gräfelfing Our reference: NÄG-21-PCT 23 January 2013 PAT EN TAN SP RÜ CHE 1 . An artificial joint (1) intended in particular as an endoprosthesis for the human knee joint, comprising a first joint compartment (2) formed by a joint head (3) and a first joint socket (4), and a second joint head (6) and a second joint head Articulated socket (7) formed joint compartment (5), characterized in that in flexion / extension in the joint compartment (2) the contact line (C _F M) on the condyle (3) and the corresponding contact line (C _T M) in the socket (4 ) are plane curves that lie in the functional plane FE _Km , and in the second joint compartment (5) the contact line (c _F i_) on the condyle (6) and the corresponding contact line (c _T i_) in the socket (7) planar curves are that lie in the functional plane FE _K i, wherein the two functional levels are parallel to each other. 2. Artificial joint (1) according to claim 1, characterized in that for a given flexion angle Δφ the center of curvature (M _F i_) of the circle of curvature defined by the contact point K | on the contact line (c _F i_) of the condyle (6), which defines the axis of rotation (M _FI _), at a distance f behind, so in the medical sense posterior, the center of curvature (M _FM ) of Krümmkreises defined by the contact point K _{m is} on the contact line (c _FM ) of the condyle (3), with deviations upwards or downwards are permissible. 3. Artificial joint (1) according to claims 1 or 2, characterized in that an increasing flexion angle leads to a decreasing distance (f). 4. Artificial joint (1) according to at least one of the preceding claims, characterized in that depending on the angle of inflection Δφ, the distance f from a maximum initial value at Δφ = 0 decreases monotonically and is small at large Δφ. 5. Artificial joint (1) according to at least one of the preceding claims, characterized in that at each flexion angle Δφ the two contact surfaces C _m and C | in the contact points K _m and K | of the two Gelenkkompartimente (2, 5) are arranged inclined in such a way that they have up to the center of the joint ascending inclinations, by the pitch angle A _m and λ | in the contact points K _m and K | the cutting contours are marked. 6. Artificial joint (1) according to claim 5, characterized in that at each angle of flexion Δφ the tangents to the sectional contours in the contacts K _m and K |, whose slopes by the pitch angle A _m and λ | are not cut in space. 7. Artificial joint (1) according to at least one of the preceding claims, characterized in that at each flexion angle Δφ the normals n _m and ni of the contact surfaces C _m and C | do not intersect in the room so that the two joint forces result in a force screw {F _R , T _R ). 8. Artificial joint (1) according to at least one of the preceding claims 5 to 7, characterized in that at each flexion angle Δφ the inclination angle A _m and λ | in the angular range of 0 ° to 60 °. 9. Artificial joint (1) according to at least one of the preceding claims, characterized in that at each flexion angle Δφ the inclination angle A _m and λ | are different. 10. Artificial joint (1) according to at least one of the preceding claims, characterized in that intersect at each flexion angle Δφ all associated four axes of rotation MFM, MTM, MFL and M _T i_ in the finite.