Endoprosthesis

Method for disconnecting two components of an endoprosthesis from one another, and endoprosthesis suitable for application of this method

Endoprostheses are known (EP-B 474 015) whose components are connected to one another via a cone connection. The latter comprises a conical bore on one component and a conical plug on the other component, the conical plug matching the conical bore and being received by it. When a sufficiently small cone angle is chosen, these fit securely one within the other, as long as they have been joined together with sufficient force at the time of assembly. To safeguard it against chance damage, this cone connection is provided with a securing screw which sits in a threaded bore of one component and can be screwed forwards with its tip into a corresponding recess in the other component transverse to the direction of release. In the event of follow-up surgery, the securing screw is not readily accessible. The removal of the prosthesis which is to be replaced is thus made difficult.

The invention is therefore based on the object of making it easier to release a securing bolt of this type in the event of follow-up surgery.

The solution according to the invention lies in the method in which the connection is disconnected by the securing bolt being broken or sheared off. To do this, a load is applied which is of an amplitude not to be expected during normal use of the prosthesis. In order to make the breaking or shearing-off easier, the securing bolt is designed brittle or soft or with a predetermined break point.

For example, in the case of a brittle securing bolt, a suitable instrument is used during follow-up surgery to exert an impact on one of the two components in the direction of release, by means of which impact the securing plug is broken, and by means of which impact the connection plug in the connection bore is also generally released at the same time, so that the components can thereafter be easily separated from one another.

A screw bolt is generally used as securing bolt; however, this is not absolutely essential.

To ensure that the broken section of the securing bolt is not jammed between the two components and does not thus impede their separation, a recess should be provided in its proximity, which recess is of sufficient size to receive the broken section. For this purpose, use is expediently made of the recess into which the partial length of the bolt engages in the securing state.

The invention is explained in greater detail hereinbelow with reference to the drawing, which represents advantageous illustrative embodiments and in which:

FIG. 1 shows a cone connection between the femoral component of a knee-joint endoprosthesis and the associated medullary cavity plug, and

FIG. 2 shows a partial cross-section through the arrangement according to FIG. 1 in an alternative embodiment.

In the embodiment according to FIG. 1, the femoral component 1 of a knee-joint endoprosthesis is connected to a medullary cavity shaft whose lower portion 2 is shown partially cut away. A conical plug 3, which is fitted in a conical bore 4 of the medullary cavity shaft, is connected to the component 1. Inside the conical plug there is a threaded bore 5 which contains a securing screw 6 whose tip can be screwed forwards into a bore 7 which serves as associated securing recess. The axes 8 and 9 of the bores 5 and 7 are offset by the amount 13, so that the tip of the securing screw 6 cooperates with that edge A of the bore 7 lying on the side of the bore 7 in the direction of release. When the screw 6 is tightened, a force strengthening the cone connection 3, 4 is generated. The other edge B of the bore 7 is in normal circumstances not reached by the securing screw.

Before assembly, the securing screw 6 lies completely in the threaded bore 5, so that the cone 3 can be fitted into the bore 4. The securing screw is then accessible via the bore 12 in part 2, so that it can be screwed into the securing position. If so desired, a locking screw (not shown) can then be fitted in order to secure the securing screw 6 in the securing position.

The securing screw 6 consists of brittle material which is nevertheless sufficiently strong to guarantee the securing function during normal use of the prosthesis. In the event of follow-up surgery, a suitable instrument is used to exert an impact on the component 1 in the direction of release. The tip of the securing screw thus breaks off and drops into the bore 7. The parts can then easily be separated from one another.

Alternatively, the securing screw 6 can consist of a soft material whose strength is sufficient for the securing function, but which is nevertheless sheared off if, for example, a force separating the components 1 and 2 is exerted using a spreader instrument.

In the alternative embodiment in FIG. 2, whose constructional make-up (where not shown) can be likened to that of FIG. 1, the tip of the securing screw 6, cooperating in a securing function with the bore 7, is separated from the main part of the screw via a predetermined break point 11. This ensures that the broken section, which is formed after the screw tip is broken off or shorn off, is not too large to be received by the bore 7. If a broken section is anticipated to be of a size exceeding the receiving capacity of the bore 7, then the latter can easily be designed wider toward the side of its edge B, since the securing function depends solely on the unchanged edge A.