Patent Document

TECHNICAL FIELD 
       [0001]    This invention relates generally to the medical implant field, and more specifically to a new and useful device and method for force management within a joint in the field of artificial joints for medical implantation. 
       BACKGROUND 
       [0002]    In the field of skeletal joint rehabilitation, artificial joints have been used to replace damaged joints in the human body in order to alleviate pain and allow the patient to regain normal mobility that may have been lost due to joint damage. There has been much iteration in artificial joint design for various parts of the body, for example, the shoulder, the knee, the hip, etc. Despite the progress that has been seen in artificial joint design in recent history, several challenges remain that prevent artificial joints from lasting and functioning as well as a healthy natural joint, necessitating repair and/or replacement of the artificial joint. 
         [0003]    A major challenge that currently exists in artificial joint design is wear. Artificial joints generally consist of two major components that are made of materials that aim to minimize the coefficient of friction between the two major components and mimic that of cartilage and fluid in a natural joint. The minimization of friction between the two major components of an artificial joint accomplishes two major functions: to extend the life of the joint and to minimize wear at the interface of the two major components. With wear comes the creation of wear particles. Once the wear particles become numerous, the immune system within the body functions to send macrophages to the site of the artificial join and attack the wear particles and consequently also attack healthy bone and tissue, resulting in resorption of the healthy bone and tissue and causing further bone loss and damage to the joint site. This problem perpetuates because the more the interface of the two major components is worn down, the more wear particles are generated because the surfaces between the two major components are no longer the smooth surfaces of a new artificial joint. Because artificial joints are anchored to healthy bone, as additional bone becomes reabsorbed around the artificial joint, the artificial joint starts to loosen from the implant site (potentially causing further wear). Eventually, the joint will need to be replaced. The problem of wear also exists in joints used in other applications such as machines, linkages, mechanical bearings, and braces. 
         [0004]    Investigations into minimizing the friction, and thus the wear, between the two major components of an artificial joint have lead to innovations in new materials for the interface of the two major components. However, even with extremely low coefficients of frictions between the materials at the interface of the joint, wear particles are still produced. In addition, the new materials may have other detrimental properties such as low fracture resistance, brittleness, unknown long-term biocompatibility with the body, etc. 
         [0005]    Thus, there is a need in the medical implant field to create a new and useful device and method for force management within a joint to minimize wear within an artificial joint while remaining biocompatible, robust, and durable. This invention provides such a new and useful device and method. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]      FIG. 1  is a schematic representation of the preferred embodiments of the invention; 
           [0007]      FIG. 2  is a schematic representation of the force interactions within the preferred embodiments of the invention; 
           [0008]      FIG. 3  is a schematic representation of the first preferred embodiment of the invention; 
           [0009]      FIG. 4  is a schematic representation of the second preferred embodiment of the invention; and 
           [0010]      FIG. 5  is a schematic representation of the third preferred embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use this invention. 
         [0012]    As shown in  FIG. 1 , the artificial joint of the preferred embodiments includes a first component  10  that includes a first component joint interface  12  and a first component magnet arrangement  14  that creates a first magnetic field  15 ; a second component  20  that includes a second component joint interface  22  and a second component magnet arrangement  24  that creates a second magnetic field  25 ; and a compressible volume  26  located between the first and second magnet arrangements  14  and  24  and adapted to control the overlap of the first and second magnetic fields  15  and  25 . The first component joint interface  12 , the compressible volume  26 , and the second component joint interface  22  cooperate to transfer compression loads. Under a relatively low compression force, the compressible volume  26  reaches a low compression state, and the first magnetic field  15  and the second magnetic field  25  create a low repulsion force that acts upon the relatively low compression force. Under a relatively high compression force, however, the compressible volume  26  reaches a high compression state that is more compressed than the low compression state, and the first magnetic field  15  and the second magnetic field  25  create a high repulsion force that is greater than the low repulsion force and that acts upon the relatively high compression force. The high repulsive force decreases the friction force between the first and second components, thus decreasing wear in the artificial joint. 
         [0013]    In conventional artificial joints, the coupling between the first and second joint interfaces during use create perpendicular (or “normal”) forces on the first and second components. As the first and second joint interfaces move relative to each other when there are applied normal forces, a friction force is created between the first and second joint interfaces, resulting in wear on the first and second joint interfaces. In the preferred embodiments, as shown in  FIG. 2 , normal force is generated on the first and second components  10  and  20  in Step S 201 , which result in compressive forces felt by the compressible volume  26 , which cause the compressible volume  26  to compress in Step S 203 . The compression of the compressible volume  26  brings the first and second magnetic fields  15  and  25  closer together. The first and second magnetic fields  15  and  25  are preferably arranged such that contact and overlap of the two magnetic fields  15  and  25  creates a repulsive force in a direction opposite of the normal force created by the coupling between the first and second components  10  and  20  in Step S 205 , resulting in reduced overall normal force, Step S 207 . As a result, the overall friction force is decreased due to the decreased overall normal force, reducing wear on the first and second joint interfaces  12  and  22 . Because the overall normal force is reduced, the repulsive forces may be used to extend the life and usability for materials that succumb to high normal forces. Alternatively, the first and second magnetic fields  15  and  25  may be arranged to create an attractive force upon contact and overlap in situations where the first and second joint interfaces  12  and  22  benefit from an increased normal force. For example, the first joint interface  12  may be designed to have geometry where a certain portion of the interface  12  may be better suited for higher normal forces than another. The magnetic fields  15  and  25  can then be used to direct normal forces towards this portion of the interface  12 . The first and second magnetic fields  15  and  25  may also be a combination of attractive and repulsive forces along the range of the magnetic fields. For example, within the range of motion of the joint, the first and second magnetic fields  15  and  25  may be attractive within certain portions of the range and may be repulsive in other portions of the range, resulting in force management within the joint that is specific to motion type, motion range, and location of contact point between the first and second components  10  and  20 . However, any other type of magnetic field  15  and  25  suitable for the application may be used. 
         [0014]    In the preferred embodiments, the first component  10  preferably moves relative to the second component  20 , which remains relatively stationary to the body. The first component  10  may rotate, roll, or translate with respect to the second component  20 . For example, in the first preferred embodiment  30  as shown in  FIG. 3 , the invention is applied to replace a shoulder joint and the first component  10  is the humeral component of the shoulder joint and the second component  20  is the glenoid component of the shoulder joint. In the second preferred embodiment  40 , as shown in  FIG. 4 , the invention is applied to replace a hip joint and the first component  10  is the femoral head of the hip joint and the second component  20  is attached to the hip. However, based upon the joint geometry, joint location, force distribution, and other factors, any other combination of movement in the first and second components  10  and  20  may be used. For example, in the third preferred embodiment  50 , as shown in  FIG. 5 , the invention may be applied to replace a knee joint and the first component  10  is attached to the femur and the second component  20  is attached to the tibia. 
         [0015]    In the preferred embodiments, the first component joint interface  12  and the second component joint interface  22  are preferably of materials that, when in contact with each other, yield a very low coefficient of friction. The first component joint interface  12  is preferably of cobalt chrome. The first component  10  is also preferably composed entirely of the same material, for example, in a shoulder joint, the stem of the humeral head is preferably also of cobalt chrome. The second component joint interface  22  is preferably of Ultra High Molecular Weight Polyethylene (UHMWPE), which has a very low coefficient of friction when in contact with cobalt chrome. However, any other combination of materials that provides a suitably low coefficient of friction when in contact may be used. 
         [0016]    In the preferred embodiments, the first component magnet arrangement  14  preferably creates a first magnetic field  15  that is uniform relative to the second magnetic field  25  throughout the range of motion of the joint. For example, for the range of motion of the joint and without compression of the compressible volume  26 , the distance and strength of the first magnetic field  15  as seen by the second magnetic field  25  is approximately unchanged. This is preferably achieved using a plurality of magnets within the first component magnet arrangement  14  placed along or close to the surface of the first joint interface  12 , as shown in  FIGS. 1 ,  3 , and  5 . The magnets of the first component magnet arrangement  14  are preferably embedded into the material of the first joint interface  12  to prevent movement of the magnets upon contact between the first and second magnetic fields  15  and  25  and to prevent direct contact between the potentially non-biocompatible materials of the magnets with the body. Alternatively, as shown in  FIG. 4 , depending on the type of movement and the expected force distribution, the first component magnet arrangement  14  may consist of a single magnet. The magnets of the first component arrangement  14  are preferably neodymium magnets. However, any other magnet arrangement or magnet type within the first component magnet arrangement  14  suitable to the application may be used. 
         [0017]    The second component magnetic field  24  preferably creates a second magnetic field  25  that is strong and localized and provides a strong repulsive force once in contact and overlap with the first magnetic field  15 . This is preferably achieved using a single strong magnet placed behind the compressible volume  26 , as shown in  FIGS. 1 and 3 . Alternatively, as shown in  FIGS. 4 and 5 , depending on the expected movement and force distribution, the second component magnet arrangement  24  may consist of a plurality of magnets placed behind the compressible volume  26 . The magnets of the second component arrangement  24  are preferably neodymium magnets. However, any other magnet arrangement or magnet type within the second magnet arrangement  24  suitable to the application may be used. 
         [0018]    The second component  20  may also include a backing  28  that encases the second component magnet arrangement  24  to prevent movement of the magnets relative to the second component  20  and prevents non-biocompatible materials form the magnet to come into direct contact with the body. When the first and second magnetic fields  15  and  25  are brought together, because of the strong nature of the fields, the created forces are very strong and the magnets of the first and second component magnet arrangements  14  and  24  will experience a strong tendency to move, either to bring like poles together or to pull towards each other. Thus, it is preferred that the magnets of both the first and second component magnet arrangements  14  and  24  are securely held to prevent any undesired motion of individual magnets. The backing  28  is preferably of a material with a high modulus of elasticity to prevent elastic deformation while under the high stresses that may be experienced during joint use. The material also preferably has a high Young&#39;s Modulus to prevent plastic deformation due to the high stresses that may be experienced during joint use. The backing  28  is preferably of a titanium material. Titanium is also a highly biocompatible material, is relatively light, and is used often in existing artificial joints. The backing  28  may also function to help anchor the second component  20  to healthy bone for implantation of the joint. 
         [0019]    The compressible volume  26  functions to control the distance between the first and second magnetic fields  15  and  25  based upon the application of a compressive force. As shown in  FIGS. 1-5 , the compressible volume  26  is coupled to the second component  20  and is placed in between the first component magnet arrangement  14  and the second component magnet arrangement  24  and behind the second joint interface  22 . As a result, the compressible volume  26  functions like a switch to control the separation between the first and second magnetic fields  15  and  25 . Because it is placed behind the second joint interface  22 , interaction properties of the compressible volume  26  are relatively unimportant and the material for the compressible volume  26  can be selected from a wide range of materials for the appropriate compressive properties. The compressible volume  26  preferably compresses to an amount that allows enough contact and overlap between the first and second magnetic fields  15  and  25  to create a repulsive force adequate to significantly reduce the overall normal force between the first and second components  10  and  20  and thus significantly decrease the amount of wear on the first and second joint interfaces  12  and  22 . Depending on the application, the adequate force to accomplish the desired wear reduction may vary. The compressible volume  26  is preferably composed of an elastomer with a modulus of elasticity that allows adequate elastic compression of the compressible volume  26  when compressed with expected forces during joint use to provide the desired repulsive force for the application. When the forces are no longer applied, the elastomer preferably expands to the original volume. Alternatively, the compressible volume  26  may be of any other material type. 
         [0020]    The following descriptions of the preferred embodiments include all of the features and functions as described above. Further embodiments may include use of the joint in mechanical bearings, linkages, braces, machinery, and any other suitable application where it may be beneficial to decrease or regulate the overall normal force within a joint. 
       1. First Preferred Embodiment 
       [0021]    As shown in  FIG. 4 , the first preferred embodiment  30  of the invention is applied to a shoulder joint. In the first preferred embodiment, the first component  10  includes a humeral head element  36  as the first joint interface  12  and a stem element  38 ; both preferably made of cobalt chrome, but may alternatively be made of any other suitable material. The second component  20  includes a socket  39  as the second joint interface  22  and is preferably made of polyethylene, but may alternatively be made of any other suitable material. The humeral head  36  rotates, rolls, and translates relative to the socket  39 . Because of this, first component magnet arrangement  12  preferably consists of a plurality of magnets  32  located beneath the convex surface of the humeral head  36  that provide a uniform first magnetic field  15  relative to the second magnetic field  25 . The humeral head  38  preferably includes crevices, each to hold one magnet  32 , preventing movement of the magnet relative to the first component  10  and to prevent contact of the magnet  32  with the body. The second component magnet arrangement  22  preferably consists of a single strong magnet  34  that provides a strong localized magnetic field  25  that creates a strong repulsive force when in significant overlap with the first magnetic field  15 . The magnets  32  are preferably each cylindrical neodymium magnets of diameter ⅛ of an inch and thickness 1/16 of an inch that provide 0.92 lb of repulsive force when arranged with like magnetic poles in close proximity, such as K&amp;J Magnets Inc. N42 D21 magnets. The strong magnet  34  is preferably a cylindrical neodymium magnet of diameter 1 inch and thickness ⅛ of an inch that provide 81.5 lbs of repulsive force when arranged with like magnetic poles in close proximity, such as K&amp;J Magnets Inc N50 DX02 magnets. However, any other suitable arrangement, type, force attributes, and size of the magnets within the first and second component magnet arrangements  12  and  22  may be used. 
         [0022]    The compressible volume  26  of the first preferred embodiment  20  is preferably an elastomer. The expected force between the first and second components  10  and  20  during joint use is approximately of the range 10N-400N. Thus, at the maximum expected force of 400N, the elastomer preferably compresses enough to allow for enough overlap of the first and second magnetic fields  15  and  25  to create the maximum repulsive force. With an assumption of a cylindrical compressible volume  26  with diameter 0.0254 meters with a thickness of 0.01 meters (uncompressed) and a desired compression distance of 0.008 meters (based upon the reach of the first and second magnetic fields  15  and  25 ), the desired modulus of elasticity is approximately 0.986 MPa. The compressible volume  26  of the first preferred embodiment is preferably an a Dynaflex® Polymer with a modulus of elasticity of 0.965 MPa. However, any other suitable material and arrangement for the compressible volume  26  may be used. 
       2. Second Preferred Embodiment 
       [0023]    As shown in  FIG. 5 , the second preferred embodiment  40  of the invention is applied to a hip joint. In the second preferred embodiment, the first component  10  includes a femoral head  46  as the first joint interface  12  and a femoral stem  48 , both preferably made of cobalt chrome, but may alternatively be made of any other suitable material. The second component  20  includes a socket  49  as the second joint interface  22 , preferably made of polyethylene but may alternatively be made of any other suitable material, and an acetabular shell as the backing  28 . The femoral head  46  mostly rotates relative to the socket  49 . Because of this, the first component magnet arrangement  12  preferably consists of a single high gradient magnet  42  that is positioned within and concentric with femoral head  46 . The high gradient magnet  42  functions to provide a significant increase in repulsive force with a small increase in overlap of the first and second magnetic fields  15  and  25 . The high gradient magnet  42  may be produced by including a high concentration of neodymium at the center of the magnet  42  and a lower concentration of neodymium closer to the surface of the magnet  42 , but may alternatively be produced using any other suitable method. The first component magnet arrangement  12  may alternatively be a matrix of a plurality of magnets arranged under the surface of the femoral head  46 . The second component magnet arrangement  22  preferably consists of a plurality of magnets  44  placed behind the compressible volume  26  and concentric to the high gradient magnet  42  to provide a uniform relative second magnetic field  25 . The plurality of magnets  44  are preferably embedded into the acetabular shell and held stationary relative to the second component  20 , preventing direct contact with the body. However, any other suitable arrangement, type, force attributes, and size of the magnets within the first and second component magnet arrangements  12  and  22  may be used. 
         [0024]    The compressible volume  26  of the second preferred embodiment  40  is preferably an elastomer placed concentric with the magnets  44  and the high gradient magnet  42 , the plurality of magnets  44 , and the femoral head  46  to provide equal compressive properties throughout the range of motion of the first component  10 . However, any suitable material or arrangement of the compressible volume  26  may be used. 
       2. Third Preferred Embodiment 
       [0025]    As shown in  FIG. 6 , the third preferred embodiment  50  of the invention is applied to a knee joint. In the third preferred embodiment, the first component  10  includes a femur cap  56 , preferably made of cobalt chrome, as the first joint interface  12  and the second component  20  includes a tibia cap  58 , preferably made of polyethylene, that includes a stem as the second joint interface  22 . The second component  20  also includes a titanium disk with a stem that serves as the backing  28 . The stem of the tibia cap  58  is inserted into the stem of the backing  28  and secured to the backing  28 . The femur cap  56  rotates, rolls, and translates with respect to the tibia cap  58 . Because of this the first component magnet arrangement  12  preferably consists of a plurality of magnets  52  located beneath the contact surfaces of the femur cap  56  that provide a uniform first magnetic field  15  relative to the second magnetic field  25 . The femur cap  56  preferably includes crevices, each to hold one magnet  52 , preventing movement of the magnet relative to the first component  10  and to prevent contact of the magnet  52  with the body. The second component magnet arrangement  22  preferably consists of a ring of magnets  54  embedded along the outer circumference of the titanium disk underneath the contact surface areas of the femur cap  56 , providing an uniform magnetic field relative to the first magnetic field  15 . However, any other suitable arrangement, type, force attributes, and size of the magnets within the first and second component magnet arrangements  12  and  22  may be used. 
         [0026]    The compressible volume  26  of the third preferred embodiment  50  is preferably an elastomer. The elastomer is preferably a ring that surrounds the stem of the tibia cap  56 , is placed in between the contact surface areas of the first and second components  10  and  20 , and is supported by the stem of the titanium disk of the backing  28  to prevent shifting of the elastomer. Due to the geometry of the knee joint, stems and anchors are preferably used to anchor the components of the joint to each other. However, any other suitable material or arrangement of the compressible volume  26  may be used. 
         [0027]    As a person skilled in the art of will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.

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