Patent Publication Number: US-2003236572-A1

Title: Total joint replacements using magnetism to control instability

Description:
REFERENCE TO RELATED APPLICATION  
     [0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10/087,052, filed Oct. 18, 2001, which claims priority from U.S. Provisional Patent Application Serial No. 60/241,401, filed Oct. 18, 2000, the entire content of both applications being incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates generally to prosthetic components and, in particular, to total joint replacements using magnetism to control instability.  
       BACKGROUND OF THE INVENTION  
       [0003] Total joint replacement has become a common procedure in the United States and elsewhere in the world. Arthroplasty of the hip, knee, shoulder, ankle, and elbow are the most frequent applications. Smaller joints are replaced as well.  
       [0004] Unfortunately, instability continues to be a problem. The most common reasons for instability of joint replacements is muscular weakness, which makes the joint less stable and more prone to dislocation or uncoupling. Other reasons include decreases in mental acuity, malposition of components, and alcohol.  
       [0005] It is estimated that $75 million is now spent in the United States annually in conjunction with total hip replacement instability. This includes costs associated with repeat surgeries and hospitalizations to correct instability problems. Surgical methods for controlling instability are not entirely effective. The current solution is bracing, repeat surgery to correct any malposition of components, and muscle advancement or retensioning.  
       [0006] A certain percentage of patients have unsolvable problems, necessitating drastic measures to address their situations. This usually involves performing a Girdlestone procedure, which involves removal of the prosthesis altogether, leaving nothing in the joint. Frequently this results in a “flail” limb, with significant functional deficits. An inability to solve these problems, not infrequently, leads to litigation because of the frustration felt by the patient. These, in turn, lead to additional costs, exascerbating the problem.  
       [0007] Although certain inventions have been disclosed and patented wherein magnetism is used in joint-replacement surgery, none so far have been specifically directed to solving the problems associated with instability. U.S. Pat. No. 5,879,386, entitled “Magnetic Prosthetic System” uses magnetism to hold the bones apart during articulation to reduce friction. U.S. Pat. No. 5,571,195 to Johnson, entitled “Prosthesis For An Artificial Joint Having A Wear Particle Collection Capability” utilizes magnetism to collect metal wear particles. U.S. Pat. No. 5,092,320 to Maurer uses magnets (70) to secure a knee brace to the leg of a wearer. U.S. Pat. No. 4,216,548 to Kraus utilizes magnets and electromagnetism to stimulate bone growth/ingrowth. U.S. Pat. No. 3,140,712, entitled “Articulated Joint,” for example, artificially duplicates the vacuum or suction [of a joint] by means of a magnetizable metal cup.  
       [0008] U.S. Pat. Nos. 4,743,264 and 4,781,720 to Sherva-Parker use magnetic traction to retain external prosthetic devices, i.e., amputation apparatus. U.S. Pat. No. 5,062,855 to Rincoe teaches the use of magnetism to control an artificial limb. U.S. Pat. No. 5,507,835 to Jore discloses a first embodiment wherein magnetic fixtures are used to hold an external prosthesis in place, and a second embodiment wherein repelling magnetic forces are used to hold bones apart.  
       [0009] European patents EP0578969A3 and EP0578969B1 disclose magnet arrangements for a prosthesis in particular, for a dental prosthesis. In addition, European patents EP0533384A1 and EP0533384B1 disclose a prosthesis for use with an ossicular chain to allow a magnetic to be coupled to the ossicular chain. European patents EP0638293A1 and EP0638293B1 show a device for positioning a magnet in a prosthesis, apparently a dental prosthesis as well.  
       SUMMARY OF THE INVENTION  
       [0010] The instant invention solves problems evident in the current art by employing magnetic force fields to control the instability of joint-replacement operations. According to the preferred embodiment, prosthetic components according to the invention are fabricated with opposite-polarity magnets on either side of the joint surface, so that an inherent stability is conferred to the joint. The magnets are of sufficient strength so that dislocation or uncoupling of the components would be very difficult, but not impossible. The forces would, however, allow motion between the bearing surfaces, without increasing friction between the joint surfaces. The approach is applicable to various artificial joint situations, including the hip, shoulder, ankle, elbow, knee, and smaller joints. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is an exploded view of a prior-art total hip system, showing how the components are assembled;  
     [0012]FIG. 2 is a drawing which shows how the head of a prior-art femoral component fits on the trunion of the prosthesis;  
     [0013]FIG. 3 is a drawing which shows a prior-art acetabular component with an apical hole;  
     [0014]FIG. 4 is a drawing which illustrates the components of a priora-art shoulder replacement;  
     [0015]FIG. 5A depicts the initial separation associated with the so-called “pistoning effect,” wherein, during the swing phase of gait, hip joint components briefly separate;  
     [0016]FIG. 5B depicts the heel-strike rim contact stage associated with the pistoning effect;  
     [0017]FIG. 5C depicts static relocation associated with the so-called pistoning effect;  
     [0018]FIG. 6 is a cross-sectional view of a hip system constructed in accordance with a preferred embodiment of the invention; and  
     [0019]FIG. 7 is a cross-sectional view of a hip system constructed in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] The invention will now be described in detail with reference to the accompanying figures. As discussed above, although the embodiments will be described in conjunction with a total hip replacement, it will be apparent to those of skill that the approach is applicable to alternate joint situations, including the shoulder, elbow, knee, and smaller joints.  
     [0021]FIG. 1 is an exploded view of a typical prior-art total hip system  100 , showing how the components are assembled. In this configuration, an acetabular insert  110  is inserted into the pelvis  120  following appropriate reaming or other preparation. The cup  110  can be cemented or cementless, in which case some type of porous or boney ingrowth surface is generally provided. A liner  108  fits into the cup  110 , which may be of a polymer such as polyethylene, or, more modernly, a ceramic. Metal-on-metal systems are also available, any of which are applicable to the instant invention as described in further detail below.  
     [0022] On the femoral side of the system, an endoprosthesis having a stem  102  and a neck  104  is provided, allowing differently-sized ball-shape heads  106  to the neck  104 , as shown in FIG. 2.  
     [0023]FIG. 3 is a perspective view of a prior-art acetabular shell  12 . Relevant to the instant invention, such a shell may include a plurality of holes  22  extending therethrough, including an apical hole  20  for seating a bone screw.  
     [0024]FIG. 4 is a drawing of a typical prior-art shoulder system, depicted generally at  400 . Such a system includes a humeral component  402 , including a stem  406  and head  404 , the latter cooperating with a glenoid replacement surface  410  seated into the bone typically using one or more posts  412 .  
     [0025]FIGS. 5A through 5C concern the so-called “pistoning effect,” wherein, during the swing phase of gait, certain joint components briefly separate. When the components recouple or “relocate,” the effect tends to increase the wear of the surfaces. It is believed that his phenomenon accounts for the fact that higher wear rates are seen in vivo, as compared to in vitro studies. FIG. 5A depicts the initial separation associated with the so-called “pistoning effect,” wherein, during the swing phase of gait, hip joint components briefly separate. FIG. 5B depicts the heel-strike rim contact stage associated with the pistoning effect, and FIG. 5C depicts static relocation associated with the pistoning effect.  
     [0026]FIG. 6 is a drawing which shows a preferred embodiment of the invention applied to a total hip system, depicted generally at  602 . One the femoral side, a stem  608  attaches to a head  606  through a neck, and within the neck, there is disposed a magnet  604 . As with the other embodiments described herein, the magnet is based upon a very high-flux-density material, preferably those made out of the rare earth group of elements; for example, neodymium-iron-boron or samarium-cobalt systems may be used. The break at  610  is used to show that the magnet  604  may be of any appropriate length to provide more or less strength. Although not limited to a modular system, the invention preferably uses at least a modular head, enabling more straightforward construction and easier installation of the magnet  604 .  
     [0027] On the acetabular side, one or, preferably, a plurality of magnets  616  are disposed through a cup  614  in facing relationship to a liner  612 , which may be of any nonmagnetic material, including polymers such as polyethylene, ceramics, or non-magnetic metals such as stainless. In the preferred embodiment, the magnets  616  are arranged axially, and pointing generally toward the center of the head  606 , such that as the femoral component rotates, the greatest flux density is achieved between the opposing poles of the magnets on the femoral and acetabular sides. Although the magnet  604  is shown terminating with the north pole, and the magnets  616  are shown with the south poles pointing toward the head  606 , it will be appreciated that this arrangement may be reversed and have the same affect.  
     [0028] The are not necessarily shown to scale, and the cup  614  may be screwed in place, cemented or cementless, utilizing a porous or boney ingrowth surface. Although not necessary to the invention, the magnets  616  may be aligned through holes such as  20  and  22  shown in FIG. 3, thereby allowing the magnets  616  to be as close as possible to the liner  612  and may, in fact, be installed after the liner and cup are implanted by drilling through the holes through the cup  614 .  
     [0029] It will be appreciated that arrangement shown in FIG. 6 addresses various dislocation problems, as well as the pistoning affect described with reference to FIGS. 5A through 5C. In addition, as discussed above, the invention is not limited to the hip, but is applicable to other joints, including the shoulder, as shown in FIG. 7. Similar to the device of FIG. 6, the humeral component  702  includes at least one magnet  704 , and a glenoid component  708  includes one or more magnets  710 , if the polarity is reversed relative to the humeral side, thereby causing an attraction and improving stability.  
     [0030] The magnets according to the invention are preferably incorporated into the various components during the manufacturing process to prevent oxidation or other deterioration of the surfaces. Since magnets only work effectively within a certain range or “air gap,” beyond which the magnets exhibit no attraction, the magnets may form part of the total joint implants without fear of attraction from very strong magnetic fields of the type used with medical and industrial instrumentation. Compatibility with metal-on-metal replacements would effectively eliminate problems with the air-gap phenomenon.