AT391264B - Hip joint prosthetic thigh part - Google Patents

Description

No. 391 264

The invention relates to a hip joint endoprosthesis thigh part for cement-free anchoring with a leaf-like shaft that widens on all sides from the distal end of the fuselage, which is provided with step-shaped approaches at least in the region of the fuselage of its blade side directed towards and from the rear and on its outward-facing narrow side in the region of the fuselage Shaft has a trochanter wing, from which a shoulder leads to a prosthesis neck, the envelope of the shaft in the area close to the fuselage having a lens-like disk shape comprising the trochanter wing and the transition into the prosthesis neck.

A thigh part of the type mentioned above is shown and described in in1 0 112 435 published after the priority date of the present application. In contrast to the present thigh part, this earlier construction is intended for anchoring in a cement bed. Its step-like structure serves to ensure mechanical anchoring after the bone cement has shrunk.

In the case of cementless anchoring of thigh parts of hip prostheses, the aim is to fix the shaft in the bone as close as possible to the trunk. There are two main types of stress: a bending load due to the moment exerted by the weight acting on the prosthesis head and a torsional load that is essentially caused by the forces that occur when the joint is moved. In the case of loading and unloading, the bending moment causes, among other things, forces at the interface between the bone and shaft, which in the proximal area are essentially medial in the case of loads and are relieved due to a " relaxation " of the elastic deformations of the shaft occurring under loads - are directed laterally.

The torsional forces act on the shaft essentially as a pair of forces with respect to a " axis of rotation " lying from the central plane running to the rear.

As is known, local stress peaks must be avoided in the flow of force between the prosthesis and bone, since they can lead to local bone loss.

Therefore, for a cement-free anchoring - for example in a shaft known from DE-OS 32 16 539, which has a solid core and a rib-like surface structure - the surgically created recess which receives the shaft is matched to the core cross-section; when the shaft is driven in, the surface structure then penetrates into the cancellous tissue which delimits the recess and displaces and compresses it; this results in a firm " seat " the structure in this tissue.

The cancellous tissue, however, is unevenly thick across the cross-section of the femur. At the edges of the cross-section, i.e. H. In the areas of the trochanter wing and the inward-facing arch, the cancellous bone is considerably thinner than in the `` middle '', for example in the area of a `` middle plane '' running from the back.

There is therefore the task of adapting the surface structure of the prosthesis socket to the different cancellous bone thicknesses. Because on the one hand, the " seat " the structure is stronger, the larger the area available for the tissue to grow; on the other hand, new tissue can only grow into or regenerate into a surface structure to a certain depth.

With the invention, this object is achieved in that the stepped lugs form longitudinal ribs which protrude from a lens-like core cross-section, and in that the radius of the disc shape for the envelope is smaller than that for the disc shape of the core cross-section. The radius of the " wrapping " is advantageously at most 60% of the core radius.

The straight longitudinal ribs running in the direction of the longitudinal axis of the shaft enable the shaft to be driven in easily and ensure its " clean " Penetration of the cancellous bone. In addition, they promote and improve a large-area distribution of the forces which occur, the double lens film of the shaft having a relatively slim " double wedge "results; whose symmetry with respect to the " median plane " enables a relatively evenly distributed outflow of force over its relatively wide flanks for both loading and unloading as a result of the bending moment that occurs; in the same way, the torsional loads are distributed over a considerable width.

Due to the different radii of " wrapping " and " core " it is possible to adapt the height of the ribs to the different possibilities for densifying the cancellous tissue.

Furthermore, due to the stepless transition of the cross-sectional shape of the shaft, both on the prosthesis neck and on the trochanter wing, discontinuities in which stress peaks can occur are largely avoided in the circumferential direction of the shaft.

As mentioned at the beginning, the shaft is fixed in the area close to the fuselage, the fuselage-free part then only serves for lateral support to the outside, without wedging or a clamping effect occurring below the area near the fuselage. Because of the relatively long lever arm of the " high " Above the hull-free shaft part, it is favorable if the hull-free shaft part has a high degree of flexural strength. Such can be achieved with simple means - while at the same time ensuring that the trunk-free shaft part does not become wedged in the bone - if the longitudinal ribs of the leaf sides from the inward-facing transition into the prosthesis neck in an arcuate shoulder which is about 1/3 of the - Measured from the fuselage-free end - The shaft height runs downwards parallel to the longitudinal axis of the shaft, merging into a sheet of reduced thickness. It has proven to be particularly effective if the

Claims (4)

No. 391 264 area the width of the sheet of reduced thickness is more than 50% of the total cross-sectional width at the respective shaft height. All materials customary in implant technology are suitable as materials for the new shaft; primarily, however, the shaft is made of metal (e.g. titanium) or one of the metal alloys common in implant technology. The invention is explained in more detail below on the basis of exemplary embodiments; Fig. 1 shows a straight shaft in a plan view of one of its leaf sides; Fig. 2 is the shaft of Figure 1 seen from the left. Fig. 3 shows the section (ΙΠ-ΙΙΙ) of Fig. 1 again; FIG. 4 is the section (IV-IV) of FIG. 1. The construction according to FIGS. 1-4 is a so-called straight shaft, which extends from its distal end (9) in the direction of its longitudinal axis (1) flared, its outer narrow side (2) merging into a trochanter wing (6) at approximately half of the vertical shaft height (H) measured between the highest point of the trunk and the lowest point of the trunk, without a prosthesis neck, initially curved outwards, which runs at the end of the shaft over an at least almost horizontal shoulder (3) to the transition into the prosthesis neck (4). The widening cone of the inward-facing narrow side merges into an arc that ends in the inward-facing transition (7) to the prosthesis neck (4); this in turn carries a conical pin (13) for receiving the joint head, not shown. The blade sides of the shaft connecting the narrow sides (2) and (5) with the surface normals facing with or facing the rear when the prosthesis is inserted are covered with longitudinal ribs (8) in the shaft region near the trunk. As can be seen from FIG. 3, the shaft according to FIG. 1 has the shape of a biconvex lens in cross section perpendicular to its longitudinal axis (1), the lens shape both for the shaft core (11) and for the ridges of the ribs ( 8) tangent envelope (12) is present. At the level of the inward-facing transition (7) to the prosthesis neck (4), the major axis (14) of the " lens " for the envelope (12) about 2.5 times the length of its small axis (15). In practice, this ratio has proven to be particularly expedient and in many cases has been adapted to the bone relationships between the cortex and cancellous bone in the associated area of the femur. The radius producing the lens curvature for the envelope (12) is at most 0.6 times the corresponding radius for the " Kem lens ". By this measure, the shape of the core (11), which is inserted into a surgically created recess in the bone, and the rib height of the ribs (8), which are driven into the cancellous bone - without a recess having previously been created for them - adapted to the existing cancellous bone thickness and the existing possibilities of its compression. The ribs (8) end in the inward-pointing shaft area in a stepped section (16) which arises in the arch of the inward-facing narrow side (5) - approximately 3/4 of the shaft height - and with a constant curvature in a parallel to the longitudinal axis (1) - approximately at 1/3 of the shaft height measured from the hull-free end - opens out. In this way, a sheet (17) of reduced thickness with a smooth surface is formed in the area of the sheet sides located inward from this shoulder. As a result, the shaft has a T-shaped cross section in this area, by means of which a relatively high resistance to deformation under bending load is achieved. The cross-sectional shape of the ribs (8) can be any, i. H. The ribs (8) do not necessarily have the rectangular shape shown in Fig. 3 with rounded ends, but can also be triangular, oval or sawtooth-shaped in its basic shape, in the latter case the steep flank of the sawtooth can point inwards or outwards . PATENT CLAIMS 1. Hip joint endoprosthesis part of the thigh for cement-free anchoring with a leaf-like shaft that widens conically on all sides from the fuselage-free end, which is provided with step-shaped approaches at least in the area near the body of its blade side facing backwards and to the rear, and on its outward-facing narrow side in the area of the body near the body has a trochanter wing, from which a shoulder leads to a prosthesis neck, the envelope of the shaft in the area near the fuselage having a lens-like disk shape comprising the trochanter wing and the transition into the prosthesis neck, characterized in that the step-shaped approaches are No. 391 264 Form longitudinal ribs (8) which protrude from a lens-like core cross-section (11) and that the radius of the disc shape for the envelope (12) is smaller than that for the disc shape of the core cross-section (11). 2. Thigh part according to claim 1, characterized in that the radius of the disc shape of the envelope (12) is at most 0.6 times the radius of the disc shape of the core cross-section (11) 3. Thigh part according to claim 1, characterized in that the longitudinal ribs (8) of the leaf sides from the inward-facing transition (7) into the prosthesis neck (4) in an arcuate shoulder (16), 10 of about 1/3 of the - Measured from the fuselage-free end - shaft height runs downwards parallel to the longitudinal axis (1) of the shaft, transition into a sheet (17) of reduced thickness. 4. Thigh part according to claim 3, characterized in that in the torso-free area the width of the sheet (17) reduced thickness is more than SO% of the total cross-sectional width at the respective shaft height 15 to 2 sheet drawings