Ball and socket bearing for artificial joint

Abstract

A ball and socket joint for implanting in the body is provided wherein the socket portion of the joint can have various orientations with respect to the patient's anatomy, and the orientation used for a particular patient can be selected and / or changed in situ, that is, during or after implantation of the joint. In addition, the configuration of the joint, e.g., constrained versus semi-constrained, as well as the materials making up the socket portion of the joint, e.g., plastic versus metal, can be selected and / or changed in situ. The questions raised in reexamination request Nos. 90 / 004,732, filed Aug. 26, 1997 and 90 / 005,596, filed Jun. 22, 1999, have been considered and the results are reflected in this reissue.

Description

BACKGROUND OF THE INVENTION1. Field Of The InventionThis invention relates to artificial joints and in particular to artificial joints of the ball and socket type.2. Description Of The Prior ArtAs is well known in the art, artificial hip and shoulder joints conventionally employ ball and socket articulations. The socket is embedded in one bony structure, for example, the pelvis for a hip reconstruction. The ball is attached to an arm composed of a neck and a stem or shaft, the stem or shaft being embedded in another bony structure, for example, the femur for a hip reconstruction.A number of methods are known for retaining the ball in the socket. In the most common method, referred to herein as the "semi-constrained" construction, the patient's own anatomy, i.e., his muscles, tendons and ligaments, are used to retain the ball within the socket. For this construction, a hemispherical socket typically is used which allows the ball and its attached arm the maximum amount of movement wit...

AI Technical Summary

Benefits of technology

When the diameter of the ball is approximately the 28 mm in common use, and the socket bearing wall thickness is approximately 7 mm, the inner diameter of the cup, and thus the outer diameter of the bearing, is approximately 42 mm (28 mm+7 mm+7 mm). This outer diameter for the bearing is larger than the largest diameter sphere commonly used in semi-constrained artificial hip replacements, and thus the present constrained construction achieves a greater range of motion than the semi-constrained construction, and at the same time, restrains the ball within the socket.

In connection with both artificial hip joints and other types of ball and socket joints, it is advantageous for the surgeon to have as wide a range of joint configurations and materials to choose from as possible. It is particularly advantageous for the surgeon to be able to refine his selection of materials and configurations during the operative procedure, after he has seen the diseased joint and has a full appreciation of the patient's medical condition and anatomy. Along these same lines, it is also advantageous to be able to re-operate and change materials and / or joint configurations as a function of the patient's post-operative history without substantially disturbing the established fixation of the joint to bony structures. For example, a patient originally fitted with a semi-constrained joint may be found to be especially prone to dislocations so that a constrained construction, perhaps including a metal socket, would be more appropriate.

To achieve these types of flexibility, in accordance with certain preferred embodiments of the invention, a family of interchangeable socket bearings of different configurations and / or materials is provided to the surgeon. Each of the bearings includes means for interchangeably connecting the bearing to a fixation element for the socket portion of the joint in such a way that the bond between the fixation element and the patient's bone is not substantially disturbed by the connecting process. In view of this easy interchangeability, during the initial operation, the surgeon need not choose the specific socket bearing to be used until after completing the implantation of the fixation element, and during subsequent operations, if any, he can substitute a different bearing or replace a worn bearing without breaking the bond between the fixation element and the patient's bone.

Problems solved by technology

Subsequent reconstructions are much more difficult to align because of deterioration of anatomical landmarks as a result of the first operation, the healing process after the operation and changes in the anatomy caused by the presence of the artificial joint.

It should be noted however, that such larger sphere sizes are not universally favored because frictional torque increases with diameter.

Such dislocations essentially make the patient immobile and can necessitate a second operation.

As discussed above, the critical alignment required for the semi-constrained construction is even more difficult to achieve when a second implantation is performed.

Unfortunately, as is apparent from the geometry of the situation, the more the socket bearing encompasses the ball, the greater the restraining force on the ball, but at the same time the less the range of motion prior to the neck impinging upon the edge of the bearing to create undesired leverage.

Accordingly, is use, rather than the metal ball dislocating from the metal socket bearing, any overly severe dislocating leverage will cause the fixation element to be disrupted from the bone in which it has been embedded.

As a general proposition, metal balls in metal socket bearings are used in only a minority of joint reconstructions because the medical profession is not in agreement that a metal sphere in a metal bearing is as biologically acceptable as a metal sphere in a UHMWPE plastic bearing, even though clinical use over 15 years has failed to show the metal to metal joint to be inferior to a metal to plastic joint.

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