FR2846875A1 - Rotating tibial base for knee prosthesis comprises insert on plate able to move along several rotational axes due to asymmetric stud engaged in cavity in plate - Google Patents

Abstract

The base for a knee prosthesis consists of an insert (4) on a plate (2) able to move along several rotational axes in the plate internal half. It follows an asymmetric rotational movement associated with a lateral translation which makes the prosthesis movement close to the movement of a normal knee. The rotation is linked to the movement of an asymmetric stud (3) with a cylindrical pin (5) which is engaged in a cavity (1) located in the plate.

Description

-1 The present invention relates to a tibial prosthesis base

  total knee, formed of a metal plate fixed in the tibial bone and a movable insert provided with a controlled asymmetric rotation. This base can be adapted to any prosthetic femoral part, by adapting the upper face of the movable insert, and thus completes a preexisting range. A total knee prosthesis comprises a femoral implant, as a rule made of metal, a tibial plate also metallic, and between these two parts slides an insert usually made of polyethylene, which acts as a wear joint, and which is more or less closely linked to the tibial plateau. An essential problem to be solved in knee prostheses resides in the solution provided simultaneously in the mobility of the two femoral and tibial implants between them and in their state of congruence, that is to say their quality to fit perfectly together. on the other. The desired mobility is found in two orthogonal planes of the anatomical space, the sagittal plane (flexion - extension) and the transverse plane

(internal and external rotations).

  In a total knee prosthesis, a high mobility of the femoral implant relative to the tibial implant can be obtained by producing inconsistent contact surfaces between the femoral condyles and the insert.

  tibial (i.e. areas with limited contact areas).

  Unfortunately, this weak congruence is offset by pressures

  high exerted on the insert, which has the consequence of causing a high wear of the polyethylene which constitutes it.

  To reduce as much as possible the wear of the polyethylene of the tibial plateau, it is therefore advisable to produce the prostheses so that the femoral implant and the tibial implant have excellent congruence. However, this in turn offers a drawback since it considerably reduces the rotational mobility of the parts in relation to one another, thus creating constraints

  harmful in particular at the level of the fixation of the tibial plate in the bone.

  It is for this reason that we have seen knee prostheses with strong congruence whose tibial insert is capable of rotation in a transverse plane. - 2 Almost all prostheses with movable insert offer a perfectly centered axis of rotation. This symmetry in rotation does not correspond to the mechanics of the knee as described for example by Professor Bousquet (Saint-tienne). It appears that the outer half of the knee. 5 is much more mobile than the inner half, regardless of the state of the cruciate ligaments, preserved or not during the implantation of a total knee prosthesis. Michael J.; Pappas has shown that the movement of the external meniscus of the knee is three times greater than that of the internal meniscus during rotations. This inadequacy of the symmetrical rotation of the prostheses 10 results in the well-known risk of rotary luxation of the polyethylene insert. Indeed, during forced rotation-flexion movements, the significant (and normal) displacement of the external compartment of the insert causes the opposite movement, just as important but not physiological, of the internal half of the same insert, which will then decoaptation of the femoral condyle and dislocation. For this reason, some tibial trays have safety stops, but cases of rupture of the stop and / or of polyethylene have

  been reported, which betrays the importance of duress.

  The prostheses described above have a fixed, punctual axis of rotation.

  Here again the study of the kinetics of the knee (Bousquet) shows that there are 20 instantaneous centers of rotation, which describe an area of more than one square centimeter, located essentially in the inner half of the spinal surface (place of 'insertion of the crusaders) of the upper end of the tibia. It is therefore not recommended to retain the path of a constrained axis of rotation, by using either studs fitted in a cavity, or rails, which allows the polyethylene insert only one and the same movement. , whatever

knee positions.

  The adaptation of the prosthesis to the kinetics of the knee, to the different tension of the peripheral ligament structures, guaranteeing a reduction in mechanical stresses in the implant, a greater longevity of the latter, and a optimal functional result therefore depends on the choice made

  in the invention, which avoids the pitfalls analyzed above.

  According to the invention, the polyethylene insert is therefore mobile on the metal tibial plateau, by sliding on a polished surface. Means for controlling this rotary sliding are provided on the metal plate and the insert, which one has a centered cavity, and of a symmetrical design - 3 (having axes of symmetry but devoid of center of symmetry) , the other a protrusion or outgrowth, of an asymmetrical design and which comes to fit freely in the cavity. The nesting depth is at least

minus 6 mm.

  There is therefore, according to the invention, a right knee system and a system

for left knee.

  According to the invention, the respective shapes of the cavity and of the protrusion are thus produced that the insert can perform, by sliding on the plate which serves as a reference, a rotational movement whose total amplitude is preferably close to 10 40 (20 of internal rotation and 200 of external rotation, which are figures close to the maxima encountered in recent physiological studies), this rotation taking place for the benefit of the external half of the insert. A single center of rotation should not be defined, but at least five different centers, at least 1 mm apart, all compatible with the desired displacement, and all located in an area of approximately one square centimeter . This area is located in the inner half of the plateau, along the midline, and halfway between the anterior edge and the posterior edge of the same plateau. This type of rotation excludes the use of a

  any additional stop, located at the edge of the plate.

  The large amplitude of the rotation of the implants also excludes the abutment contact of the insert in the tray, except to make forced movements, for example during a fall, in which case the invention plays a role.

  security, when the 200 rotation is reached.

  According to the invention, the congruence of the insert and the plate allows a moderate lateral translation, intended to improve the placement of the femur on

  the tibia, with regard to ligament tensions.

  In total, thanks to the instantaneous centers of rotation and to the lateral translation, several positions of this insert can be defined for a given angle of rotation of the insert, which results in a better adaptation of the prosthesis to the anatomy of each patient. The rotation is done freely according to several random axes, depending on the subject's anatomy

  operated and not a constraining guide means.

  It should be added that the manufacture of knee prostheses supposes the existence of a range of sizes (generally 6) adapted to the morphology of the operated patients. As the invention essentially applies to -4 allowing a given angle of rotation, it will be noted that this angle is independent of the size of the prosthesis and therefore the cavity-pad assembly can keep the same dimensions, which simplifies the manufacturing and authorizing the cross-use of a congruent insert on its upper face with a given size of femur (for example n0 4) and on its lower face with the neighboring size of the tibial plateau (n03 or n05). This cross-use is not

rare in knee surgery.

  The accompanying drawings illustrate the invention. FIG. 1 represents an exploded perspective front view of an embodiment, according to the invention, of the polyethylene insert (4) with its stud (3) and its rod. (5) and the metal plate (2) with its cavity (1) and its tube

hollow (6).

  Figure 2 shows the underside of the insert with its stud

  asymmetrical (3) according to an embodiment according to the invention.

  Figure 3 shows the upper face of the metal tray with its cavity

  symmetrical (1) according to an embodiment according to the invention.

  FIG. 4 represents the plate and the insert in neutral position, one with respect to

  the other, that is to say without rotation.

  FIG. 5 represents the plate and the insert, the latter with an internal rotation effect of 200.

  Figure 6 shows the tray and the insert, the latter with rotation effect

external 200.

  FIG. 7 represents the location area of the instantaneous centers of rotation. Figure 8 shows a variant shape of the cavity (in diabolo) such

  that it can be made in the metal tray according to the invention.

  FIG. 9 represents another variant of cavity (in V).

  With reference to these drawings, the invention, in one of its variants, comprises a symmetrical cavity (1) hollowed out in the tibial metallic plate (2) 30 of a knee prosthesis, as well as an asymmetrical stud (3) formed on the

  underside of the afferent tibial insert (4). The stud (3) is provided with a cylindrical appendage (5), the whole coming to engage in the cavity (1) and the underlying hollow tube (6).

  The asymmetrical shape of the stud (3) allows a rotary sliding of the insert (4) 35 on the plate (2) in polished metal at an angle of 200 in internal rotation and 20 in external rotation in front and behind the line (8) reference neutral -5

(rotation = 0).

  According to this embodiment of the invention, the means of controlling the rotation are for a metal tibial plateau (for example 85 mm in the long axis and 52 mm in the small axis) an elliptical cavity ( 1),. 5 whose two centers are located 3 mm in front of the center of the plate, and whose dimensions are 36 mm in the major axis and 14 mm in the minor axis. The depth of the metal cavity thus produced is 6 mm. This cavity is extended by an underlying hollow rod (6), the interior of which communicates with the cavity itself. The upper side of the tray,

  the interior of the cavity and the rod are polished, in order to receive the movable insert.

  According to the same embodiment, the underside of the polyethylene insert (4), in contact with the metal plate, comprises a central stud (3) of ovoid shape (therefore asymmetrical), the most rounded side of which corresponds to the internal half of the prosthesis, and the most tapered side to the external part. Under the stud, the polyethylene forms a rod (5) descending into the metal tube, and thin enough to allow the rotations. The study of the maximum displacement of the insert on the plate shows that one obtains on each side of the neutral position, which can be found in the area (7) defined by the invention the five centers of rotation, and that the lateral translation 20 exceeds 3 mm. The external dimensions of the mobile insert, especially those of the external compartment, are smaller than those of the metal plate, in order to avoid any conflict with the soft parts. Experience with symmetrical mobile inserts, however, shows that a 5 mm overhang in maximum rotation

  (so little used) does not cause any trouble.

  According to other non-limiting embodiments - the tray cavity can take other forms, in particular characterized by a convexity inside the cavity, for example in diabolo (9), in V (10), the pad of the polyethylene insert then taking the corresponding asymmetrical shape (11). These convex shapes have the advantage of increasing the sliding capacity without frontal collision or sudden stop of the parts in contact with the approach.

of maximum rotation.

  - The hollow stem is of variable length, and its edges can merge in

  gentle curves with the main cavity.

  - The cavity can be dug in the polyethylene, which receives an asymmetrical metal pad 35 belonging to the plate.

  - the tray can have an asymmetrical general shape to adapt to 6

the anatomy of the tibia.

  To find the centers of instantaneous rotation, one of the methods is as follows:

  On the diagrams of the plate and the insert, draw the axes of symmetry.

  Overlay the diagrams. Carry out transparently five different positions of rotation of the insert on the plate, while keeping the polyethylene pad in the cavity. Five times at least more than a mm apart one of the axes of symmetry of the insert must cross with the corresponding axis of the plate

  a point located in the area defined by the invention.

Claims (4)

  1. Tibial base for total knee prosthesis, comprising: - a plate (2) made of metal with, open at the upper face, a cavity (1) centered, of symmetrical design, characteristic because it does not have a center   of symmetry, but only of axes of symmetry.   - a movable insert (4) overlying with, on the underside, a stud (3) at least 6 mm thick, and of asymmetrical shape, freely engaging in the cavity. There is an insert for right knee and an insert for left knee. The characteristic shape of the asymmetrical stud of the insert allows it to perform an asymmetrical rotary slide, for the benefit of half 1 0 external.   2. tibial base according to claim 1 o the maximum angle of rotation (internal + external) of the insert is preferably close to 400, this in order avoid the stop phenomenon.   3.Tibial base with rotary insert with asymmetrical movement, according to the preceding claims, the drawing of the stud formed under the insert.   defines not one but several centers of instantaneous rotation, preferably at least five, separated by at least one millimeter, located in an area (7) of about one square centimeter. This area is located in the inner half of the plateau, along the midline, and halfway between the anterior edge and the posterior edge of the same plateau. The rotation is done freely along several random axes, depending on the anatomy of the subject operated on and   not a binding guide means.   4. Tibial base according to one of the preceding claims, where the   wall of the cavity (1) has one or more convexities, increasing the sliding capacity without frontal collision or abrupt stop of the stud (3) on approach of maximum rotation.