Artificial acetabulum

#The invention relates to an artificial acetabulum made of an outer shell which houses an inner shell that cooperates with a spherical head of the joint.

#Artificial acetabula are often made as double shells in which the outer shell permits attachment in osseous tissue, and the inner shell has a spherical surface to receive the spherical head of a prosthesis shaft. The outer and inner shells may be of different materials, to use, on the one hand, an outer shell which is tissue-tolerated as regards growing-in, and, on the other hand, to use for the inner shell materials, which have good properties as regards wear and anti-seizure behavior.

#European patent publication EP 0 313 762 A1 is known an acetabulum in which the inner shell of plastics is fixed in the outer shell with a snap connection. The outer shell has a high rigidity which causes a high shape stability. The inner shell of plastics is elastically deformable, depending on the influence of forces acting through the spherical head, and is supported in this by the rigid outer shell. However, the rigid outer shell has the disadvantage that, in the case when the bone elastically yields under high compressional forces, tensile and shearing forces act on the side relieved of load between the outer shell and the bone.

#FIG. 1a is a view from below of an outer shell;

#FIG. 1b is a section through the outer shell along line A--A in FIG. 1a;

#FIG. 1c is a plan view of a further outer shell;

#FIG. 1 shows a view from below into the opening of a metallic outer shell 1, which is formed substantially by an attachment zone 1a and two outer shell segments 1b and 1c, which are connected by resilient connection elements 1d to the attachment zone la. The attachment zone 1a may be primarily attached in osseous tissue by attachment means, such as spines 6 or bone screws which extend through bores 8a, 8b. The position of the pole P2 can be seen in FIGS. 1a and 1b. The pole P2, which forms part of the attachment zone 1a, is situated, with respect to the whole outer shell 1, at its pole. In the region of the pole P2 is a centering or guiding bore 3 which extends along an assembly axis 18 (shown in FIG. 3); the bore 3 has a smooth inner surface or is provided with a thread 4. The centering bore 3 extends into a conical, outwardly open recess 5. In the region of the equator of the outer shell 1 is at each side of the attachment zone 1a provided a spring element 1d, which extends in the peripheral direction, has a width d and each connects one outer shell segment 1b, 1c to the attachment zone 1a. The outer shell segments 1b, 1c can also be primarily attached in osseous tissue by attachment means, such as spines 6 or bone screws which extend through the bores 8c, 8d. In the present example an S-shaped recess 2b, 2c is formed between the outer shell segments 1b, 1c and the attachment zone 1a. Between the outer shell segments 1b, 1c is formed a radial recess 2 extending in the direction of a meridian circle 9. The extension of the recesses 2, 2b, 2c and also their width may be carried out in a number of embodiments. The resilient properties between an outer shell segment 1b, 1c and the attachment zone 1a may be varied within wide limits by the arrangement and design of the spring element 1d. The illustrated spring element 1d has in the radial direction a relatively small wall thickness d, and in its extension parallel to the assembly axis 18 a wall thickness d2, which is greater than the wall thickness d. As is best seen in FIG. 1a spring element 1d is least rigid during compression or expansion of the outer shell 1 in the peripheral direction. The resilient properties between the attachment zone 1a and the outer shell segments 1b, 1c may be varied, within wide limits, in dependence on the arrangement and the design of spring element 1d.

#FIG. 1c shows a plan view of a further embodiment of an outer shell 1. The attachment zone 1a is of the same design as that in FIG. 1a and has therefore a bore 8b, a pole P2 with a centering bore 3 and a spring element 1d extending in peripheral direction. After the spring element 1d follow, in the peripheral direction, several outer shell segments 1e, 1b, which are interconnected in each case by at least one spring element 1d. Between the outer shell segments are recesses 2b, 2d, which are bridged only by spring elements 1d. The outer shell segments 1e, 1b may have bores 8c for the attachment of the outer shell segment to the bone with a bone screw.

#FIG. 1d shows a side elevation of spring element 1d, which has a height d2 and bridges a recess width e between the attachment zone 1a and the outer shell segment 1e. The extension of the recess between the attachment zone 1a and the outer shell segment 1e may be designed in a number of ways, for instance as shown, such that the recess width narrows after the spring element 1d. A recess 2a may, naturally, also widen along its way, and may also have parallel side walls or side walls making an angle with each other. For every outer shell segment 1e, 1b is needed at least one spring element 1d to obtain a continuous connection to the attachment zone 1a. In the illustrated embodiment the spring elements are arranged along the peripheral direction of the outer shell 1. Depending on the desired spring action, the spring elements 1d can also be situated at other places to obtain direct connection between the outer shell segment 1e, 1b and the attachment zone 1a, or to connect together outer shell segments 1e, 1b. Outer shells 1 are normally symmetrical, so that the plane 19 shown in FIG. 1c is a plane of symmetry, and a recess 2 is therefore between the outer shell segments 1b, 1c.