Patent Grant
7160332
The invention is directed generally to methods, systems and devices related to prosthetic implants, including devices for preventing prosthetic components from separating from each other, and more specifically to assemblies for adapting a prosthetic component to receive a device for retaining or preventing an implant stem head from dislocating from a prosthetic component.
Artificial implants, including hip joints, shoulder joints and knee joints, are widely used in orthopedic surgery. Artificial hip joints are generally ball and/or socket joints, designed to match as closely as possible the function of the natural joint. The ball and socket joint of the human hip connects the femur with the pelvis, wherein the ball-shaped head of the femur is positioned within a socket-shaped acetabulum of the pelvis. The head of the femur or ball fits into the acetabulum, forming a joint which allows the leg to move forward, backward and sideways in a wide range of motion. The acetabulum is lined with cartilage, which cushions the bones and allows the joint to rotate smoothly and with minimal friction. An envelope of ligaments connect the pelvis and femur, covering the joint and stabilizing it. The cartilage also renders the joint sufficiently strong to support the weight of the upper body, and sufficiently resilient to absorb the impact of exercise and activity. A healthy hip allows the leg to move freely within its range of motion, while supporting the upper body and absorbing the impact that accompanies certain activities. Various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials. Prosthetic components are generally made from any or combinations of metal, ceramic, and/or plastic material.
Total hip arthroplasty and hemi-arthroplasty are two procedures well known within the medical industry for replacing all or part of a patient's hip. A total hip arthroplasty replaces both the femoral component and the acetabular surface of the joint, so that both a femoral prosthesis and an acetabular prosthesis are required. To replicate the natural kinematics of a hip joint, a total hip prosthesis has three parts: the stem, which fits into the femur and provides stability; the ball, which replaces the spherical head of the femur; and an acetabular prosthesis, which replaces the hip socket. Each part is available in various sizes in order to accommodate a range of body sizes and types. In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization in fit.
A conventional acetabular prosthesis may include a cup, a cup and a liner, or in some cases only a liner, all of which may be formed in various shapes and sizes. Generally, a metal cup and a polymeric liner are used. The liner may be made of a variety of materials, including polyethylene, ultra high molecular weight polyethylene, metal, and ceramic materials. The cup is usually of generally hemispherical or partially hemispherical in shape and features an outer surface and an inner surface that is adapted to receive a cup liner. The liner fits inside the cup and has inner and outer surfaces. The cup liner is the bearing element in this type of acetabular component assembly. The outer surface of the liner corresponds to the inner surface of the cup or acetabulum, and the liner inner surface receives the head of a femoral component. An acetabular cup may include a highly polished inner surface in order to decrease wear.
The liner inner surface can be characterized by features relative to an axis, such as an axis of rotation through the center of the inner surface. This axis may or may not be aligned with the central axis or axis of rotation of the cup. In a typical liner the inner surface has a hemispherical or partially hemispherical geometry and is also referred to as the internal diameter. In such liners, the geometry of the internal diameter can be characterized as concentric to an axis that runs through the center of the internal diameter such as the axis of rotation of the cup, outer diameter of the liner or otherwise.
An acetabular prosthesis may be fixed in the reamed acetabulum of a patient. Such a prosthesis may include a cup (or a cup and liner assembly) that is fixed by placing screws or other retaining devices through apertures in the cup, by securing the cup with cement, or by using bone ingrowth material on the outer surface of the cup. In other cases, spikes, pegs, or fins around the rim of the cup are used to help hold the implant in place until new bone forms. In some cases, only a liner is cemented in a patient due to poor bone stock. Any combination of these structures or techniques may be used.
A femoral prosthesis used in total hip arthroplasty generally includes 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 moves in a manner corresponding to relative motion between the pelvis and femur, (“articulates”) relative to the acetabular component. Femoral prostheses used in total hip arthroplasty procedures may or may not differ from a prosthesis used in a hemi-arthroplasty, described below. However, the femoral head of each type prosthesis is generally a standard size and shape. Various cups, liners, shells, stems and other components may be provided in each type arthroplasty to form modular prostheses to restore function of the hip joint.
Hemi-arthroplasty refers to replacing part of a hip joint, such as replacing a femoral component so that a femoral prosthesis similar to those used in a total hip replacement articulates against natural body tissue in the patient's acetabulum. In most cases, the acetabulum is left intact and the head of the femur is replaced, using a component similar to those employed in a total hip replacement. In other cases, a hemi-surface prosthesis fits over the head of the femur so that the bone of the femoral head is spared. This hemi-surface prosthesis is then fixed to the femur with cement around the femoral head and has a short stem that passes into the femoral neck. Generally, a femoral prosthesis implanted during a hemi-arthroplasty is referred to as an endoprosthesis and includes a stem and a head, and may include additional components such as shells and liners. Current designs include monoblock, and two, three and five component designs. A monoblock endoprosthesis is a one-piece structure including a femoral stem and head. Polarity refers to the number of articulating surfaces a prosthesis contains. A monoblock endoprosthesis has one articulation surface between the head and the patient's natural acetabulum, and is therefore referred to as monopolar. Thus, a prosthesis may be described both with respect to the number of components and with respect to the number of articulating surfaces as installed in a patient. Some current designs may also include a mechanical device, such as a snap-ring, for constraining the femoral head, further described below.
Prostheses used in hip replacement surgery may also be described as constrained and non-constrained prostheses. Non-constrained prostheses rely on the downward force of the body through the joint and the tension created by the soft tissue, including the muscles, ligaments and tendons, to retain the femoral head relative to the acetabular prosthesis in its implanted position. Non-constrained prostheses generally allow the greatest range of motion. Other prostheses include mechanisms for preventing dislocation of the stem head from the acetabular component. Typically, these prostheses have restraint mechanisms that result in a smaller range of motion of the hip joint, and are generally referred to as “constrained” components. Dislocation may be the result of trauma to the hip, abnormal anatomy, soft tissue laxity, or impingement.
One example of a restraint mechanism is a shell or liner having greater than hemispherical coverage around the head such that the head is constrained within the internal diameter, thus preventing subluxation and dislocation. In contrast to standard liners, constrained liners may 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. While use of a constrained components is generally not desirable due to resulting decreased range of motion, the use of constrained components 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, which is incorporated by reference herein. However, constrained components reduce the range of motion in part because of the elevated lip segment; there is thus a substantial loss of overall range of motion compared to a standard liner. An implant stem head constrained by a shell or liner may dislocate if the femoral component rotates beyond the range of motion permitted by the assembly. Dislocation may occur because the edge or lip of the liner or shell that retains the implant stem head acts as a fulcrum about which the femoral component pivots, thereby causing the implant stem head to dislocate from its position within the liner or shell of the prosthesis. Dislocation of a hip prosthesis is painful and often requires medical intervention. Finally, a liner utilized in a constrained component must have a strong lock mechanism for retention in the cup due to the forces exerted on the liner by the other components of a constrained prosthesis. Lever out force is the moment required to dislocate the head from the liner. The ability of a prosthesis, such as an implant stem head, to withstand forces exerted on the liner is referred to as lever-out, or shuck-out. Pull out force is generally a tensile force applied in the direction of the rotational axis of the cup so as to separate the head from the cup.
During a total hip replacement procedure, the surgeon generally obtains measurements to ensure proper prosthesis selection, limb length and hip rotation. After making the incision, the surgeon works between the hip muscles to gain access to the joint. The femur is pushed out of the acetabulum and removed so that the exposed joint cavity may be cleaned and enlarged with reamers of gradually increasing size. The cup of the acetabular prosthesis is then placed in the prepared hemispherical socket. A liner may then be inserted into the cup and fixed into place. The femur is then prepared to receive the stem by reaming the center of the bone and planing and smoothing the top end of the femur. If the ball is a separate piece, the proper size is selected and attached. The assembled femoral component is then placed within the acetabular component and the joint is properly aligned. If complications such as dislocation require a surgeon to perform a revision procedure and utilize a constrained component, the surgeon may be forced to remove the acetabular component in its entirety, causing damage and bone loss.
Current constrained liner designs have many disadvantages. One design currently used does not adequately constrict movement of the components, resulting in component wear. Also, some current constrained liners are not easily removable after installation in a patient because the mechanism retaining the components is not easily accessible or reverse-operable. Other locking mechanisms are located on the exterior surface of the assembly, allowing deformation and dislocation of the locking mechanism and subsequent failure of the component. Finally, currently available designs do not offer both adequate range of motion and sufficient lever-out.
Thus, there is a need for a constrained liner assembly that offers both adequate range of motion and sufficient lever-out.
There is also a need for a constrained liner assembly that is easily disassembled after installation in a patient.
There is also a need for a constrained liner assembly that allows an existing, implanted acetabular shell to be converted to a constrained prosthesis without requiring removal of the shell and thus damage to the patient's bone and other structures.
Finally, there is a need for a liner assembly that is capable of reducing the movement of components, in order to decrease wear of the components.
These are some needs which exist in conventional designs, one, some or all of which needs are fulfilled by some or all structures of various embodiments of the invention.
Methods, systems and devices according to this invention seek to prevent prosthetic components from separating from each other. They also seek to provide improved orthopedic prosthetic components and more specifically, improved constrained components that may be utilized with existing implanted prosthetic components. Liner assemblies and systems according to various embodiments of this invention include an adapter component designed to link current prosthetic cup geometry to a constrained liner, so that implanted prosthetic cups may be retrofit with a constrained liner, while providing optimal range of motion and sufficient lever out. Liner assemblies and systems according to various embodiments of this invention provide a sufficient range of motion without sacrificing lever out.
Certain embodiments of an adapter component according to this invention include first and second adapter elements and a locking member. The first adapter element is adapted to engage a prosthetic cup, and is adapted to receive the locking member. The first and second adapter elements include opposing tapers, so that when assembled, a taper lock is formed by the two elements. As assembled, the locking member of the adapter component couples the adapter component and the liner, retaining the liner in position.
Certain embodiments of this invention also include a constrained liner assembly having structure designed to couple an implant stem head to a cup. Components of a liner assembly according to this invention may include an adapter component, a liner, including a retaining member, and a capture member. An adapter component is coupled to a prosthetic cup, and a liner is inserted into the adapter component and cup. An implant stem head is inserted into the liner, deforming a retaining member housed in a channel in the liner. A capture member is inserted into slots on the liner, deforming the retaining member and retaining the implant stem head in position in the liner.
Various methods, systems and devices according to this invention seek to provide improved orthopedic prosthetic components and more specifically, improved constrained components that may be utilized with existing implanted prosthetic components. Liner assemblies and systems according to certain embodiments of this invention may include an adapter component designed to link current prosthetic cup geometry to a constrained liner, while providing optimal range of motion and sufficient lever out. Generally, a liner assembly according to certain embodiments of this invention includes an adapter component designed to resist disassembly as force is applied to the assembly.
Liner assemblies according to certain embodiments of this invention includes structure designed to couple an implant stem head to a cup. The components of the liner assembly generally include an adapter component, a liner, including a retaining member, and a capture member. For example, in one embodiment, an adapter component includes first and second adapter elements having opposing tapers, so that when assembled, a taper lock is formed by the two elements. In one embodiment, the first adapter element and the cup include opposing tapers, so that when the adapter element is assembled and inserted into the cup, a taper lock is also formed between the adapter component and the cup. In one embodiment, the first adapter element includes geometry permitting the first adapter element to mate with the cup. In one embodiment, a locking member of the adapter component couples the first adapter element and the liner, retaining the liner in position.
Consider one example of systems and devices according to this invention. As shown in
The implant stem head 44 is generally spherical in shape and includes cavity 50, which is adapted to receive a femoral stem 52, the implant stem head 44 and femoral stem 52 forming a femoral prosthetic component 54 when assembled. The cavity 50 of the implant stem head 44 may be generally cylindrical, conical, cubical, or any other suitable shape. In other alternative embodiments, the cavity 50 may include threads, barbs, rings or any other suitable mechanical connectors to couple the head 44 to the stem 52. In yet another embodiment, adhesive or cement may be used to couple the head 44 to the femoral stem 52. The components of the various prostheses described may be made from metal, such as stainless steel and titanium, ceramic, plastic, such as polyethylene, or any other suitable material. In one embodiment, the cup and implant stem head are either metal or ceramic, while the liner is polyethylene. In other embodiments, the liner is metal or ceramic. In yet another embodiment, the liner is plastic and includes a metal outer surface.
A liner assembly 56, shown in
As shown in
In one embodiment, the inner surface 80 of the second adapter element 62 includes geometry that mates with geometry on the liner 40, providing alignment and rotation resistance of the liner. In one embodiment, the inner surface 80 of the second adapter element 62 includes splines 82 that mate with splines 84 on the outer surface of the liner 40, as shown in
In one embodiment, a positioning element 86 is used to position and align the first and second elements 60, 62 of the adapter component 58 in order to accurately position the elements for engagement and locking. In one embodiment, the positioning element 86 is a plastic member, shown in
In one embodiment, shown in
As shown in
As shown in
The face 124 of the liner 40 includes slots 126 adapted to receive the tabs 128 of a capture member 130, as shown in
In an alternative embodiment, a metal or ceramic liner is coupled to an implant stem head and an acetabular cup. In this embodiment, a single adapter element is coupled to a cup, and a liner is press fit into a taper lock engagement with the cup and adapter element.
In another alternative embodiment, the adapter component includes a first adapter element and a locking member. In this embodiment, a second adapter element is included in the liner, so that insertion of the liner locks the adapter elements and retains the liner in the cup.
In another alternative embodiment, the liner does not include a retaining member. In this embodiment, the cavity of the liner is of sufficient diameter to receive an implant stem head without deformation of the liner. A capture member is inserted into slots on the face of the liner, deforming the plastic liner toward the implant stem head and thereby retaining the head in the liner.
A liner assembly according to embodiments of this invention is removable in steps, without destroying the individual components of the assembly and without requiring removal of the femoral or acetabular implants. Components of the liner assembly may be removed using a standard removal tool, or any other suitable tool. In addition, a liner assembly according to one embodiment of this invention may be used to retrofit an implanted acetabular cup as a constrained device.
One method of using one form of structure according to this invention is as follows:
For a surgical revision of an implanted hip prosthesis, the existing prosthetic liner is removed using a liner removal tool and the apex hole cover of the acetabular cup is removed using a device such as a screwdriver. The method described may also be used for a primary hip arthroplasty, wherein the acetabulum is prepared and a cup is inserted, using any screws, pegs or other devices, according to the prescribed surgical technique.
As shown in the Figures, an appropriately sized adapter component 58, including first and second adapter elements 60, 62 and a locking member 64, is chosen. After placing the locking member 64 in recess 74 of first adapter element 60 and locating second adapter element 62 adjacent first adapter element 60, a positioning element 86 is placed on adapter component 58 to ensure proper alignment of the cup 22 and the adapter component 58, and of the first and second adapter elements 60, 62. Before mating with the second adapter element 62 and insertion into cup 22, first adapter element 60 is in a first position, as shown in
The adapter component 58 is inserted into the cup 22 so that the adapter component 58 mates with the existing cup geometry, in this embodiment, splines 36, until an audible click is heard. In general, for ease of insertion, the open end of the adapter component is inserted over the removal slot 28 of the cup 22, or where best visibility of opening is possible. An alignment rod 98 is inserted and is threaded to the apex hole of the cup 22 either by hand or with a screwdriver. A hollow impactor slides over the alignment rod 98 and the legs 104 of the impactor 100 are positioned between the arms 90 of the positioning element 86. A mallet is used to strike the platform 106 on the impactor 100 until sufficient force causes the second adapter element 62 to be driven off of the positioning element 86 and to be urged toward and locked into the first adapter element 60. This engagement of the first and second adapter elements 60, 62 expands the outer diameter of the first adapter element 60 so that the first adapter element 60 engages the cup 22, as shown in
The assembled liner 40 and retaining member 110 are then inserted into the assembled cup 22 and adapter component 58. Rotation of the liner may be required to obtain optimal positioning. A liner impactor tool is used to completely seat the liner 40, forcing the locking member 64 into the groove 114 on the liner 40, holding the liner 40 in position in the cup 22.
An appropriately sized implant stem head 44 and femoral stem 52 are chosen and assembled as femoral component 54. The capture member 130 is then placed over the implant stem head 44 of the femoral component 54 so that the tabs 128 face away from the femur. The implant stem head 44 of the femoral component 54 is placed into the cavity 42 of the liner 40 and rotated to check mobility of the implant stem head 44 in the liner 40. Retaining member 110 expands to allow insertion of implant stem head 44 and contracts after implant stem head 44 passes into cavity 42 of liner 40. The tabs 128 of capture member 130 are then positioned on the slots 126 on the face 124 of the liner 40 and pressed into the slots 126 of the liner 40, using an impactor to fully seat the capture member. After insertion, the capture member 130 compresses retaining member 110, capturing and retaining implant stem head 44 in cavity 42.
A hip prosthesis incorporating a liner assembly according to embodiments of this invention offers advantages over current designs, including increases in lever out values and allowed range of motion, in part based on the fact that designs according to embodiments of this invention allow larger stem head sizes to be used for a given cup, than do conventional designs. Liner assemblies according to embodiments of this invention provide a desirable compromise between lever out values and range of motion, affording a sufficient range of motion without sacrificing lever out. The chart below summarizes various parameters for various sources for a hip prosthesis according to one embodiment of this invention and for three separate designs that are currently available.
Various embodiments of a liner assembly as described above all relate to prostheses used in hip arthroplasty. Similar embodiments of a liner assembly according to this invention may be incorporated into other orthopedic prostheses. Alternative embodiments of a liner assembly according to this invention may be utilized to capture and retain other prosthetic components. As various changes could be made in the above structures and methods without departing from the scope of the invention as defined in the claims, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.
This application is a divisional of U.S. patent application Ser. No. 10/113,095 entitled “Liner Assembly for Prosthetic Components,” filed on Apr. 1, 2002 now U.S. Pat No. 6,916,342, now allowed, the contents of which are incorporated herein by reference.
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