1. Field Of The Invention
This invention relates to artificial joints and in particular to artificial joints of the ball and socket type.
2. Description Of The Prior Art
As 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 xe2x80x9csemi-constrainedxe2x80x9d 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 without contact of the arm with the edge of the socket. The surgeon, when installing such a semi-constrained joint, aligns the ball and socket 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 joint. As a general proposition, such precise alignment is easiest the first time an artificial joint is placed in a patient. 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.
In order to increase the inherent stability against dislocation of such semi-constrained constructions, it has become conventional to add a cylindrical portion to the hemispherical socket to make it deeper. Although the ball is not physically constrained by the socket by this adjustment, the ball does not have further to travel than if just a hemisphere had been used and thus some reduction in the propensity towards dislocation is achieved. Ball and socket joints of this type generally provide an arc or range of motion of approximately 115xc2x0 when a 28 mm diameter sphere is used and the socket is made a few millimeters deeper than a hemisphere. Larger ranges of motion can be obtained by keeping the size of the arm attached to the ball constant and increasing the diameter of the ball. In this way, the angular extent of the arm relative to the ball becomes smaller. In the limit, if the ball could be made progressively larger and larger, a range of motion of 180xc2x0 could be achieved. In practice, however, the largest sphere in common use in artificial joints, and in particular artificial hip joints, has a diameter of 32 mm and provides a range of motion of approximately 120xc2x0. It should be noted however, that such larger sphere sizes are not universally favored because frictional torque increases with diameter.
A recent study by the Mayo clinic, which appeared in December, 1982 edition of The Journal of Bone and Joint Surgery, reported a dislocation frequency of 3.2% for 10,500 hip joint implant procedures using the semi-constrained construction. 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. Accordingly, even higher dislocation frequencies are encountered for second and subsequent implantations.
An alternative to the semi-constrained construction is the construction wherein the ball is physically constrained within the socket. In this construction, a spherically-shaped bearing surrounds the ball and serves as the socket. 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 and thus constrains the ball and its attached arm from dislocation.
The bearing is typically made from plastic, such as ultra-high molecular weight polyethylene (UHMWPE), or metal. 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 Noiles, U.S. Pat. No. 3,996,625. As can be seen in FIG. 1 of this patent, a plastic bearing 17 fitted with a metal reinforcing band (un-numbered) extends beyond the diameter of ball 24 so as to physically constrain the ball within the bearing. The bearing itself is attached to fixation element 12. The metal reinforcing band is assembled over the lip of the opening of bearing 17 after that bearing has been forced over sphere 24. The reinforcing band increases the force required to dislocate the joint. In practice, the design shown in FIG. 1 of U.S. Pat. No. 3,966,625 3,996,625 has been found to provide a range of motion of approximately 85xc2x0 when a sphere diameter of 28 mm is used and to resist direct dislocating forces of several hundred pounds.
For constrained constructions such as that shown in U.S. Pat. No. 3,996,625, 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 bearing. Because of the leverage associated with the arm and the long bone of the patient to which it is attached, e.g., the patient""s femur, the dislocating force produced when the neck contacts the rim of the bearing can be considerable. For example, a force on the order of 25 pounds applied to a patient""s leg can produce a dislocating force of over several hundred pounds because of the leverages involved. This type of dislocation force can be avoided by geometrically aligning the artificial joint with the patient""s anatomy so that the neck does not come in contact with the rim of the bearing during normal motion of the patient""s limb. That is, the leverage based dislocation forces can be avoided in the same way as dislocations are avoided in the seim-constrained semi-constrained construction, i.e., through precise alignment of the artificial joint with the natural anatomy of the patient. 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. In practice, artificial hips having the construction shown in U.S. Pat. No. 3,996,625 have been found to suffer dislocation due to the leverage effect in fewer than 0.5% of the implantations performed. This is significantly better than the 3.5% dislocation frequency reported in the Mayo clinic study discussed above, but an even lower dislocation frequency is obviously desirable.
A constrained construction using a metal socket bearing is shown in Noiles, U.S. Pat. Re. 28,895 (reissue of U.S. Pat. No. 3,848,272). This construction provides approximately a 90xc2x0 range of motion where the sphere diameter is 28 mm. In a practical sense, the metal bearing can be said to be non-dislocatable. The force required to extract the metal sphere from the enclosing metal socket bearing is more than several thousand pounds. 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.
A third 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 rotates in the patient""s socket. As with the semi-constrained construction, anatomical alignment is used to avoid dislocations, in this case between the metal head and the natural socket.
In view of the foregoing, it is apparent that in semi-constrained and endoprosthesis hip joints, reconstructive geometry of the prosthesic components is critical in ensuring the stability of the prosthesis against dislocation. Moreover, in ball and socket constructions which constrain the elements against dislocation, the range of motion inherent in the prosthesis is reduced and thus because of the possibility of leverage type dislocations, similar demands are placed on the surgeon to establish the geometry of the reconstruction within rather narrow limits.
Accordingly, an object of this invention is to provide a ball and socket joint which provides the surgeon with increased latitude in geometric positioning of the prosthetic components over those ball and socket joints presently available.
A further object of the invention is to provide a ball and socket joint the materials and configuration of at least a portion of which can be selected and/or changed in situ, that is, during or after implantation of the joint in the patient.
An additional object of the invention is to provide a ball and socket joint including a bearing member which can be readily replaced in situ with either a bearing member of the same or of a different type depending on the patient""s post-operative history.
Another object of the invention is to provide a ball and socket joint wherein (1) the socket portion of the joint has more than one orientation with respect to the ball portion of the joint, the preferred orientation being a function of the patient""s anatomy, and (2) the orientation of the socket portion with respect to the ball portion can be selected and/or changed in situ, that is, during or after implantation of the joint in the patient.
A further object of the invention is to provide a prosthetic ball and socket joint of increased inherent range of motion which is readily assembled and disassembled at the surgical site.
An additional object of the invention is to provide a ball and socket bearing for an artificial joint which constrains the joint from dislocating and at the same time provides a range of motion which is greater than that available in the construction of the constrained type described above.
To achieve these and other objects, the invention, in accordance with one of its aspects, provides a ball and socket joint for implantation in a patient""s body comprising a ball portion and a socket portion,
the ball portion including:
a ball; and
first fixation means for implantation in a first bony structure, said fixation means being connected to said ball; and
the socket portion including:
a bearing for receiving the ball; said bearing defining an orientation between itself and the patient""s body;
second fixation means for implantation in a second bony structure; and
connecting means associated with the bearing and the second fixation means for connecting the bearing to the second fixation means in more than one orientation.
In accordance with a further one of its aspects, the invention provides a ball and socket joint for implantation in a patient""s body comprising a ball portion and a socket portion,
the ball portion including:
a ball; and
first fixation means for implantation in a first bony structure, said fixation means being connected to said ball; and
the socket portion including:
a bearing for receiving the ball, said bearing being one member of a family of interchangeable bearings, the family including at least one member which is made from a different material or which has a different configuration or which is both made from a different material and has a different configuration from at least one other member of the family;
second fixation means for implantation in a second bony structure; and
connecting means associated with the bearing and the second fixation means for interchangeably connecting any bearing in the family to the second fixation means.
In accordance with an additional one of its aspects, the invention provides a ball and socket joint for implanting in the body which comprises:
a ball;
a cup with a spherical cavity, said cup to be affixed to bone; and
a bearing member surrounding a portion of the ball and rotatable within said spherical cavity about only one axis, said bearing member having an asymmetric opening therein, the opening having an angular extent of less than 180xc2x0 in at least one plane.
In accordance with another one of its aspects, the invention provides an artificial joint of the ball and socket type for implantation in the body which comprises:
a ball;
a bearing for forming a socket to receive the ball, the bearing having an asymmetric opening therein, the opening having an angular extent of less than 180xc2x0 in a first plane;
means for pivoting the bearing about an axis lying in a plant other than the first plane; and
means for affixing the means for pivoting to bone.
In accordance with another of its aspects, the invention provides a socket for a ball and socket joint for implantation in the body which comprises (1) a cup with a cavity, and (2) a bearing for receiving the ball of the ball and socket joint, said bearing being constrained to rotate within said cavity about a single axis.
In accordance with certain preferred embodiments of the invention, the asymmetric opening into the sockets is less in one direction than it is at 90xc2x0 to this one direction. The socket bearing is movably retained within the cup about an axis which is (1) parallel to the face of the cup and (2) in the direction of the greater opening in the socket bearing. The socket bearing is retained within the cup by two stub half round pins integral with the cup and extending part way through the wall thickness of the socket. The axes of the half round pins coincide with an axis of the spherical cup like cavity and they are also coaxial in the direction of the greater opening in the socket bearing.
When the ball and the neck of the arm of the prosthesis move in the direction of the lesser opening in the socket bearing, the total range of motion is the sum of the arc of motion which the neck can make within the bearing plus the arc of motion which the bearing can make within the cup. The cup can be a hemisphere or even less. Rotation of the ball is limited by impingement of the neck against the rim of the cup, or alternatively, and most preferably, by limiting the rotation of the socket bearing so that the neck comes just up to, but not actually into contact with, the rim of the cup. In this regard, reference is made to copending U.S. patent application Ser. No. 553,518 to Alfred Frederick DeCarlo, Jr., filed simultaneously herewith and assigned to the assignee of the present application. This application, the pertinent portions of which are incorporated herein by reference, discloses a preferred system for limiting the rotation of the socket bearing to keep the neck of the arm of the prosthesis out of contact with the rim of the cup.
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.
When the ball and the neck move in the direction of the greater opening in the socket bearing, the neck contacts the flat side of a stub half round pin, rather than the rim of the cup, or alternatively, and most preferably, a web portion of the socket bearing in the region of the stub half round pins (see, for example, element 106 in FIGS. 13, 15 and 21 below). To allow the neck and ball to move through the same arc in this direction, the flat sides of the pins can be contoured. With this feature, the total range of motion in all quadrants, using the above dimensions, is approximately 135xc2x0.
To summarize, in accordance with the above preferred embodiments of the invention, when motion is in the plane of the stub pins, the total motion is by movement of the ball within the bearing. When motion is at 90xc2x0 to the plane of the pins (the xe2x80x9c90xc2x0 planexe2x80x9d), the total motion is the sum of the motion of the ball within the bearing and the motion of the bearing within the cup. In other planes, the motion of the ball within the bearing is greater than it is in the 90xc2x0 plane and the motion of the bearing within the cup is less than it is in the 90xc2x0 plane. In this way, the invention provides a constrained ball and socket prosthesic joint with a total range of motion significantly greater than hitherto generally available.
In connection with artificial hip joints, it is advantageous to orient the cup in situ so that the axis of the stub pins is inclined according to the anatomical requirements of the patient as determined by the surgeon. For example, the axis can be inclined somewhat upward in the forward direction. In this manner almost all highly repetitive load bearing motions of the hip joint fall within the motion capability of the sphere within the socket bearing. Additional motion is furnished by movement of the bearing within the cup in such activities as crossing the legs when seated, or in significant abduction. To conveniently permit such orientation of the stub pins, in certain embodiments of the invention, the cup includes first and second portions, the first portion to be affixed to bone, the second portion having associated therewith the pin members are being moveable with respect to the first portion to provide a plurality of possible orientations for the axis of rotation of the bearing member within the spherical cavity.
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.
In the description of the preferred embodiments which appears below, constructions are shown using both plastic and metal socket bearings, as well as bearings employing a combination of metal and plastic components. Also, various assembly and disassembly constructions are illustrated. It is to be understood, of course, that both the foregoing general description and the following detailed description are explanatory only and are not restrictive of the invention.
The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention.