Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-08-31
2001-04-17
Mancene, Gene (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06217616
ABSTRACT:
BACKGROUND OF THE INVENTION
Fractures of the radial head constitute one of the two most common fractures of the elbow, and treatment of radial head fractures has been a controversial subject. Although excision of the head of the radius can sometimes give good long-term results, such may often cause persistent elbow or wrist pain associated with cubitus valgus or with proximal radial migration. As a result, prosthetic replacements have been frequently recommended to obviate these problems and to improve stability of the elbow joint.
Radial heads have sometimes been replaced with flexible silicone rubber prostheses, particularly in cases of severe rheumatoid arthritis and degenerative arthritis, and such replacements have been suggested for comminuted fractures of the radial head. However, because a silicone rubber radial head is quite flexible, compared to bone, the ability of such a flexible implant to transfer dynamic forces across the radial-capitellar joint without excessive deformation is questioned. Clinical and biomechanical studies have now indicated that replacement of a radial head with a prosthesis made of silicone rubber, which is much less stiff than bone and deforms under load, does not reestablish the true mechanical function of the natural radial head because such a silicone rubber prosthesis may be unable to transmit physiological forces from the proximal radius to the capitellum; this indicates that a less flexible radial head prosthesis should result in more normal physiologic stress transfers and provide improved clinical results. Moreover, fractures of silicone rubber devices have also been reported.
Both metal and acrylic radial head replacements have now been clinically implanted. Although there is only limited experience with prostheses made of metal, the elastic moduli of certain metals, such as titanium, can be as much as
10
times greater than the elastic modulus of bone, and this very substantial difference often results in a biomechanical incompatibility between the bone of the capitulum and the radial head replacement prosthesis. Moreover, the difference has also been such as to cause undesirable changes in cartilage in situations where there is cartilage at the wear surface.
Shown in U.S. Pat. No. 5,782,923 is one endoprosthesis for replacing the entire elbow joint wherein a radius component is mounted in the bone by a shaft portion and has a spherical head that is received in a recess of a member that slides in a socket formed in the undersurface of a flange portion of the ulnar component. U.S. Pat. No. 5,030,237 shows an elbow prosthesis which includes a humeral insert made of a metal alloy, such as Vitallium, and an ulna prosthesis, wherein the end of the radius may be optionally replaced.
Although the results of certain metal and polymeric implants have been promising in some aspects, none of these replacements have been totally satisfactory. Accordingly, the search has continued for more satisfactory radial head replacements that better mimic the properties of the natural radius head.
SUMMARY OF THE PRESENT INVENTION
In accordance with one aspect of the present invention, an improved prosthesis for the replacement of the end portion or head of the radius is provided which includes a body having a head, a collar and a stem with the stem being proportioned to be received within the medullary cavity of the radius. The head is shaped so as to smoothly interface with the capitulum of the humerus, having a peripheral surface which extends completely about the head, an undersurface and a concave proximal end surface for contact with the capitulum. This concave surface is surrounded by a rim, and in one aspect, the peripheral surface varies in height with the medial region extending further in a distal direction than the lateral region so that the medial region of the head is substantially thicker. In another aspect, the prosthesis is designed to be axisymmetric so that height of the peripheral surface is uniform. The collar is located between the undersurface of the head and the stem, and in one aspect, it is aligned at an oblique angle to the axis of rotation of the radius in pronation-supination movement.
In these various embodiments, the collar has a lesser cross-sectional dimension than the head; thus, the undersurface of the head circumscribes the collar and provides a peripheral shoulder against which the annular ligament of the elbow, in which the collar portion of the head resides, may articulate. The undersurface is preferably substantially planar and, in one aspect, is aligned at an angle of between about 95° and about 105° to the axis of rotation of the radius in pronationsupination movement.
In one particular aspect of the invention, the concave surface in the end of the prosthesis is not centered; instead, it is offset from the center of the head in a direction toward the lateral region. Moreover, viewing the prosthesis looking toward the proximal end surface, a cross-sectional shape is seen, which is generally ovoid, with the medial region of the head being wider in the interior-posterior plane than is the lateral region, preferably with the medial region of the peripheral surface having a radius of curvature at least about 15% greater than that of the lateral region. In such construction, the centerline of the stem that is received within the medullary cavity is preferably aligned at an angle of between about 8° and about 22° to the axis of rotation of the radius in pronationsupination movement.
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Cook, et al., “Wear Characteristics of the Canine Acetabulum Against Different Femoral Prostheses”,1989 British Editorial Society of Bone and Joint Surgery, vol. 71-B, No. 2, Mar. 1998, pp. 189-197.
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Ascension Orthopedics, Inc.
Fitch Even Tabin & Flannery
Mancene Gene
Robert Eduardo C.
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