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Variable geometry rim surface acetabular shell linerRelated Patent Categories: Prosthesis (i.e., Artificial Body Members), Parts Thereof, Or Aids And Accessories Therefor, Implantable Prosthesis, Bone, Joint Bone, Hip Joint Bone, Acetabular Cup, And An Inner Insert Liner CupVariable geometry rim surface acetabular shell liner description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060241780, Variable geometry rim surface acetabular shell liner. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation of U.S. patent application Ser. No. 09/808,228 filed on Mar. 14, 2001, which claims priority to U.S. Provisional Patent Application No. 60/189,182 filed on Mar. 14, 2000. The disclosure of this prior application is incorporated by reference in its entirety. BACKGROUND OF THE INVENTION [0002] 1. Field of The Invention [0003] This invention relates generally to acetabular prosthetic devices and more particularly to an improved acetabular shell liner wherein the liner has a variable geometry rim surface. [0004] 2. Related Art [0005] Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. Hip joint prostheses are common. The human hip joint acts mechanically as a ball and socket joint, wherein the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. In a total hip joint replacement, both the femoral head and the surface of the acetabulum are replaced with prosthetic devices. [0006] A first general class of hip prosthetic devices included an acetabular component in which the head of a prosthetic femoral component was intended to articulate relative to the acetabular component. Initial designs included an acetabular component with a thin bearing surface, or liner, which interfaced with a large femoral component head. This design allowed for good range of motion and a low incidence of dislocation or subluxation of the femoral component head, but the thin liners proved to wear poorly, requiring replacement. [0007] Acetabular components generally comprise an assembly of a shell and a liner, but may comprise the liner alone. Generally, a metal shell and a polymeric liner are used. However, the liner may be made of a variety of materials, including but not limited to, polyethylene, ultra high molecular weight polyethylene, and ceramic materials. The shell is usually of generally hemispherical shape and features an outer, convex surface and an inner, concave surface that is adapted to receive a polymeric shell liner. The shell liner fits inside the shell and has a convex and concave surface. The shell liner is the bearing element in the acetabular component assembly. The convex surface of the liner corresponds to the inner concave surface of the shell or acetabulum, and the liner concave surface receives the head of a femoral component. [0008] The liner concave surface, or internal concave surface, is characterized by features relative to an axis through the center of the concave surface. This axis may or may not be aligned with the central axis of the shell. In a typical liner the concave surface has a hemispherical geometry and is also referred to as the internal diameter. In such liners, the geometry is characterized by features that are concentric to an axis that runs through the center of the internal diameter. [0009] The acetabular component is configured to be received and fixed within the acetabulum of a pelvis. Typically, the acetabular component comprises an assembly of a shell and a liner. If only a liner is used, it is most often fixed within the acetabulum with bone cement. [0010] The femoral component generally comprises a spherical or near-spherical head attached to an elongate stem with a neck connecting the head and stem. In use, the elongate stem is located in the intramedullary canal of the femur and the spherical or near-spherical head articulates in the liner internal diameter. [0011] Currently, a hip joint prosthesis may comprise an acetabular component having a thicker liner and a femoral component having a smaller sized head than the initial designs. Acetabular designs that include thicker liners provide more bearing support and less surface area for wear but presents problems with dislocation and subluxation, as well as reduced range of motion, due to the smaller head size. Thus, one of the critical concerns in designing total hip joint replacement components is how to design the components to minimize contact of the neck of the femoral component with the rim of the liner during articulation, thus reducing rim contact-induced subluxation, dislocation, and wear, while allowing a maximum desired range of motion. There are a variety of acetabular liners available for use in hip replacement procedures that seek to address the issues of limited range of motion, rim-contact wear, and dislocation or subluxation. [0012] For example, the standard, non-anteverted liner, also called a flat or zero degree liner, has a wide rim, or impingement, surface. Typically, the center of rotation of the femoral head on a standard liner is concentric with the acetabular shell. This type of standard liner is used to provide a broad range of motion. Use of this liner requires optimal positioning of the acetabular component in the acetabulum in order to provide the required range of movement for a patient. While standard liners allow a broad range of motion, if malpositioned, they present an increased possibility of dislocation. To address this problem, a high wall liner may be used. [0013] In contrast to standard liners, high wall liners, also known as shouldered or lipped liners, employ an extended, elevated portion over a segment of the periphery of the liner internal diameter in order to increase coverage of the femoral head and thus reduce the likelihood of dislocation and aid in reduction of the head should subluxation occur. The use of high wall liners may be beneficial in cases of tenuous stability in order to avoid dislocation. See e.g. T. Cobb, et al., The Elevated-Rim Acetabular Liner in Total Hip Arthroplasty: Relationship to Postoperative Dislocation, Journal of Bone and Joint Surgery, Vol. 78-A, No. 1, January 1996, pp. 80-86. However, high wall liners of all designs have a reduction in the arc of motion to contact in the direction of the elevated rim segment without a corresponding increase in motion in the opposing direction. Thus, there is a substantial loss of overall range of motion compared to a standard liner. This reduction in range of motion makes the rotational positioning or clocking of these designs in the acetabulum particularly important in order to reduce rim contact with the neck of the articulating femoral component and potential acceleration of polyethylene wear at the rim as a result of this contact. [0014] In general, anteverted liners re-orient the central axis of the internal diameter of the liner relative to the central axis of the shell. Anteverted liners shift the capture area on the head of the femoral component in order to improve hip joint stability and decrease the risk of dislocation. However, use of an anteverted liner may reduce allowed range of motion. [0015] Some liners have a constant geometry relieved rim surface around the circumference of the internal diameter of the acetabular liner. While a relieved rim surface increases range of motion, the constant geometry may not optimize the possible range of motion because it may not be correlated to the cross-section of the femoral component during a condition of femoral component neck-liner contact. At this point the femoral component is said to be in an impingement condition with the liner. [0016] Prosthesis range of motion has been evaluated in the past by creating a cone that defines the limits of motion to contact, or impingement angles, for the prosthesis, as described in Thornberry, et al., The Effects of Neck Geometry and Acetabular Design on the Motion to Impingement in Total Hip Replacement, A Scientific Exhibit at the 1998 AAOS Meeting, New Orleans, La., 1998, the entire contents of which are hereby incorporated by reference. The size of the cone depends on the design of the components. Varying the orientation of the components allows a surgeon to shift the direction of the cone. In a successful component placement, the cone is positioned so that adequate range of motion for the patient is provided. The base of the cone provides information for flexion, extension, adduction, and abduction. The direction of flexion-extension, as well as abduction-adduction, can be drawn as a line on the base of the cone. The point where the line intersects the cone is the maximum motion of prosthesis in the respective direction. Designs that provide adequate range of motion generally correlate with good clinical results. See e.g. B. McGrory, et al., Correlation of Measured Range of Hip Motion Following Total Hip Arthroplasty and Responses to a Questionnaire, Journal of Arthroplasty, Vol. 11, No. 5, 1996. [0017] Thus, there is a need for a method of forming an acetabular shell liner that provides optimization of the maximum range of motion and minimum interference with the femoral component neck. There is also a need for a liner formed by such a method. SUMMARY OF THE INVENTION [0018] Methods and structures according to this invention include a method of producing an acetabular liner in which the rim surface geometry varies, rather than being set, in order to optimize the range of motion and minimize interference with the neck of the femoral component. This variable geometry rim surface is employed around the edge of the internal concave surface of the liner, i.e. around the circumference of a generally hemispheric acetabular liner inside diameter, and allows for delayed interference, or impingement, with the neck or stem portion of the femoral component, resulting in an increased range of motion. Thus, this variable geometry rim surface delays when the neck of the femoral component contacts the rim surface of the liner during articulation, allowing an increase in the range of motion of the femoral component and optimization of the liner. [0019] Increasing the range of motion has many benefits and advantages. For example, increasing the range of motion allows a patient a greater range of movement. Second, an increase in the range of motion provides the surgeon with greater room for error in component positioning, or clocking, during surgery. Since it is not currently possible to accurately measure the precise angle required for implantation of an acetabular component in a particular patient, it is difficult to place an implant at precisely the correct angle. A surgeon generally relies on personal experience in making this assessment. While a locking mechanism, such as a spline interface between the liner and the shell, is beneficial because it allows for multiple reorientations of the liner, fine tuning the positioning of the acetabular component during the intraoperative assessment of range of motion and stability is difficult and often imprecise. Surgeons will benefit from a wider range, or larger target area for acetabular component orientation provided by the increased range of motion. [0020] Third, a broader range of motion decreases the likelihood of dislocation or subluxation, as it is less likely the femoral component will contact the rim of the liner and lever out of the internal concave surface of the acetabular component. Finally, a broader range of motion aids in preventing wear on the liner or shell. If a femoral component regularly contacts the rim surface of the liner, the liner will wear, releasing polyethylene debris. This debris may cause osteolysis when it escapes into nearby bone and tissue, which may lead to aseptic loosening of the implant. Additionally, if the liner wears thin, the neck of the femoral component may contact the metal shell, resulting in fatigue to the metal that may cause the neck or shell to break, or metal debris to be released into nearby bone and tissue. [0021] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. Continue reading about Variable geometry rim surface acetabular shell liner... Full patent description for Variable geometry rim surface acetabular shell liner Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Variable geometry rim surface acetabular shell liner patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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