Artificial hip and shoulder ball joints conventionally employ ball and socket articulation components. In a hip prosthesis, the acetabular portion is embedded in the bony structure of the acetabulum and the femoral portion is embedded in the femur. The femoral portion normally includes the ball while the acetabular portion normally includes the socket or cup. The ball is attached to an arm composed of a neck which in turn is attached to a stem or shaft.
It has been found in use that a dislocating force is created when the neck of the arm attached to the ball impinges on the rim of the acetabular component. Because of the leverage associated with the patient's femur, the dislocating force produced when the neck contacts the rim of the bearing can be considerable. For example, a force applied to a patient's leg can produce a dislocating force of several fold because of the leverages involved. 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.
A number of methods are known for retaining the ball in the cup. In the most common method, the patient's own anatomy, i.e., his or her muscles, tendons and ligaments, are used to retain the ball within the socket. A hemispherical cup typically is used which allows the ball and its attached neck the maximum amount of movement without contact of the neck with the edge of the cup. The surgeon when installing such a prosthesis aligns the ball and cup as closely as possible with the patient's natural anatomy so that the patient's movements do not tend to dislocate the ball from the cup. Such precise alignment is easiest the first time the prosthesis is implanted in a patient. Subsequent reconstructions are much more difficult to align because of deterioration of the anatomy as a result of the first operation, the healing process after the operation, the incompetency of soft tissue, and changes in the anatomy caused by the presence of the prosthesis itself.
Notwithstanding the various retaining systems attempted in the prior art, a significant number of prostheses dislocate. Such dislocations immobilize the patient, can be painful, and can necessitate the discomfort and expense of a second operation. As discussed above, the critical alignment is even more difficult to achieve and maintain when a second implantation is performed. Accordingly, even higher dislocation frequencies are encountered for second and subsequent implantations.
An alternative to the semi-constrained construction is a construction wherein the cup is physically constrained. In this construction, a spherically-shaped bearing surrounds the ball and serves as the cup. The bearing is attached to a fixation element which is embedded in, for example, the patient's pelvic bone. The bearing encompasses more than one-half of the ball surface and thus constrains the ball and its attached arm from dislocation. For plastic bearings, the ball and bearing are usually assembled by forcing the bearing over the ball. The more of the ball which is encompassed by the bearing, the greater the required assembly force, and the greater the constraining force to prevent postoperative dislocation of the joint. In addition, the more that the bearing encompasses the ball, the smaller the range of motion for the ball prior to contact of the bearing with the arm attached to the ball. An example of a constrained artificial joint employing a plastic bearing is shown in U.S. Pat. No. 3,996,625 by Noiles.
A constrained construction using a metal socket bearing is shown in U.S. Reissue Pat. No. 28,895 by Noiles. In a practical sense, the metal bearing of Noiles can be said to be non-dislocatable, since the force required to extract the metal sphere from the enclosing metal socket bearing is at least several thousand pounds. Accordingly, in use, rather than the metal ball dislocating from the metal socket bearing, a high dislocating force will cause the fixation element to be disrupted from the bone in which it has been embedded. Metal balls in metal socket bearings are used in only a minority of joint reconstructions.
Another type of artificial ball and socket joint, referred to as an endoprosthesis, eliminates the fixation element associated with the socket and simply uses a ball surrounded by a plastic socket bearing in a spherical metal head, which head is placed in the patient's natural socket but not secured to bone. For this construction, the ball can rotate within the bearing up to the rim of the bearing (the bearing is greater than a hemisphere so as to be retained on the ball), and then the bearing and its attached head rotate in the patient's socket. As with certain other constructions, anatomical alignment is used to avoid dislocations, in this case between the metal head and the natural socket.
A device which is adaptable to employ several different constraining systems is shown in U.S. Pat. No. 4,960,427 by Noiles. Examples of other prostheses are disclosed in U.S. Pat. Nos. 5,314,489 by Hoffman, U.S. Pat. No. 5,201,767 by Caldanse, U.S. Pat. No. 4,778,473 by Mathews, U.S. Pat. No. 5,108,445 by Ashby, U.S. Pat. No. 5,370,704 by DeCarlo and U.S. Pat. No. 5,413,610 by Amino.