Abstract:
A teletibial implant is provided for measuring forces between a femur having first and second condylar surfaces and a tibia when a joint is articulated. The implant includes a medial tibial insert engaging the first condylar surface and a lateral tibial insert engaging the second condylar surface. A transducer includes a medial plate coupled to the medial tibial insert, a lateral plate coupled to the lateral tibial insert, and a bottom plate supporting the medial and lateral plates. The medial and lateral plates receive forces from the medial and lateral inserts, respectively. The bottom plate also has a plurality of spaced apart force sensors for measuring forces exerted on the medial and lateral plates.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains generally to a joint prosthesis and, more particularly, to a system that measures forces on a joint prosthesis to determine proper implantation of the prosthesis on a patient.  
           [0002]    The human knee is the single largest joint of the human body, but due to its structure, is arguably the most vulnerable to damage. The leg consists principally of a lower bone called a tibia and an upper bone known as a femur. The tibia and femur are hinged together at the knee joint. The knee joint includes femoral condyles supported in an engagement with crescentic fibrocartilages that are positioned on the upper end of the tibia and receive the femur. The joint is held together by numerous ligaments, muscles and tendons. The patella is a similarly supported bone positioned in front of the knee joint and acts as a shield for it.  
           [0003]    When the knee joint has been severely damaged from accident, wear, or disease, partial or total knee replacement may be the only viable solution. One type of knee replacement is shown in U.S. Pat. No. 4,340,978 issued to Buechel et al. In this patent, the tibia is resected to form a flat, horizontal platform known as tibial plateau. The amount of bone structure removed corresponding to the severity of damage to the joint and the necessary allowance needed for the prosthesis. A tibial platform is secured to the tibial plateau with posts or anchors fixed normal or perpendicular to the tibia plateau. The anchors provide additional support to the tibial platform when the joint is subjected to shear, tipping and torque forces present under normal knee articulation.  
           [0004]    A femoral component, comprising a curved convex semi-spherical shell, covers the femoral condyles and slidably engages a concave tibial bearing insert. On a side opposite the femoral component, the tibial insert is substantially flat and slidably engages the tibial platform. Interaction of opposing surfaces of these three elements, the femoral component, the tibial insert and the tibial platform allows the prosthesis to function in a manner equivalent to a natural knee joint.  
           [0005]    Another tibial platform and a surgical procedure for implantation is described in U.S. Pat. No. 4,822,362 issued to Walker et al.  
           [0006]    Crucial to either the complete joint of Buechel et al. or the tibial platform of Walker et al. is proper alignment of the tibial platform on the tibial plateau. Without proper alignment, neither will function correctly whereby uneven forces on the prosthesis may result in excessive contact stresses leading to deformation and/or early wear and thus undesirable short prosthetic life.  
           [0007]    Template assemblies have been used in implantation surgical procedures to resect the tibia and align the tibial platform. One such assembly is disclosed in U.S. Pat. No. 4,211,228 issued to Cloutier. This assembly comprises a Y-shaped handle having two flat prongs that are used to check the planes of the resected tibia for overall flatness and to hold temporarily the tibia inserts. An alignment rod, fixed to the flat handle, is aligned visually along the long axis of the tibia, as viewed laterally and anteriorally, to ensure correct positioning of the tibial platform onto the patient&#39;s tibia. Since tibial platform alignment does not include movement of the prosthetic components in order to access force loads on the joint, alignment of the tibial platform may not be optimum, realizing pressure differences across the surface of the platform which under normal articulation of the joint may cause fatigue in the prosthesis.  
           [0008]    Developments have been made for a system to dynamically measure and analyze forces present on components of a knee joint prosthesis and all other types of prostheses. One such system is described in U.S. Pat. No. 5,197,488. The system measures forces throughout the normal range of motion of the joint using a first member attached to an outer surface of a first bone and a second member attached to an outer surface of a second bone. A transducer is located between the first and second member to measure forces thereon. However, this system does not provide isolated quantitative indications of forces present on the medial and lateral portions of the tibia. Thus, a system is needed to provide indications of forces in specific sections of the prostheses, including the medial and lateral portions of the tibia.  
         SUMMARY OF THE INVENTION  
         [0009]    A teletibial implant is provided for measuring forces between a femur having first and second condylar surfaces and a tibia when a joint is articulated. The implant has a medial tibial insert engaging the first condylar surface and a lateral tibial insert engaging the second condylar surface. A transducer includes a medial plate coupled to the medial tibial insert, a lateral plate coupled to the lateral tibial insert, and a bottom plate supporting the medial and lateral plates. The medial and lateral plates receive forces from the medial and tibial insert, respectively. The bottom plate has a plurality of spaced apart force sensors for measuring forces exerted on the medial and lateral plates.  
           [0010]    In another aspect of the present invention, a system is provided for measuring forces applied to a joint prosthesis and adapted to be located between a first bone and a second bone that form an articulation joint. The system includes a first member attached to an outer surface of the first bone and a second member attached to an outer surface of the second bone. A transducer is positioned between the first member and the second member. The transducer has a first plate receiving forces exerted between the first bone and the second bone, a second plate receiving forces exerted between the first bone and the second bone, and a bottom plate supporting the first and second plates. The bottom plate further has a plurality of spaced apart force sensors for measuring forces exerted on the first and second plates. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 illustrates a front view of a knee prostheses according to the present invention.  
         [0012]    [0012]FIG. 2 illustrates a rear bottom perspective view of a tibial component.  
         [0013]    [0013]FIG. 3 illustrates a rear bottom perspective view of the tibial component of FIG. 2 with some elements illustrated in dashed lines.  
         [0014]    [0014]FIG. 4 is a top plan view of a transducer according to the present invention.  
         [0015]    [0015]FIG. 5 is a bottom perspective view of the transducer in FIG. 2.  
         [0016]    [0016]FIG. 6 is a bottom plan view of the transducer in FIG. 2.  
         [0017]    [0017]FIG. 7 is a sectional view of the transducer taken along the line  7 - 7  in FIG. 6.  
         [0018]    [0018]FIG. 8 is a bottom plan view of the transducer in FIG. 2 with sensing elements.  
         [0019]    [0019]FIG. 9 is a front elevational view of a lower portion of the knee prosthesis shown in FIG. 1.  
         [0020]    [0020]FIG. 10 is a top perspective view of the lower portion shown in FIG. 9.  
         [0021]    [0021]FIG. 11 is a side view of the lower portion illustrated in FIG. 9.  
         [0022]    [0022]FIG. 12 is an exemplary environment for transmitting force signals. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    An exemplary prosthetic according to the present invention will now be described. Generally, a prosthetic includes a component mounted to the femur  2  and another component mounted to the tibia  4 . Both femur  2  and tibia  4  are shown in dotted lines in FIG. 1. Measuring forces between the components aid in aligning the components properly and analyzing forces exerted on the components.  
         [0024]    [0024]FIG. 1 further illustrates assembly  10  in accordance with an exemplary embodiment of the present invention. Assembly  10  includes femoral component  12  mounted to the femur  2  and tibial component  14  mounted to the tibia  4 . Femoral component  12  includes flange  18  formed integrally with two condyles  20 . Femoral component  12  includes fixing posts or anchors  22  integrally formed on femoral component  12 . Posts  22  are used to fix the femoral component  12  to femur  2 .  
         [0025]    An outside surface  26  of flange  18  provides most of the bearing surface for a patella, not shown, which cooperates with femur  2  to protect the joint. Condyles  20  are provided for replacing the condylar surfaces of femur  2  and include medial condylar surface  27  and lateral condylar surface  28 .  
         [0026]    Tibial component  14  includes tibial inserts  30  and  32 , transducer  34  and lower portion  35 . Lower portion  35  is secured to tibia  4  and may be a solid or a hollow construction. Medial tibial insert  30  is adapted to engage medial condylar surface  27 . Lateral tibial insert  32  engages lateral condylar surface  28 . The medial and lateral condylar surfaces  27  and  28  exert force on medial tibial insert  30  and lateral tibial insert  32 , respectively. Medial and lateral inserts  30  and  32  can be made from polyethylene or any other suitable material. In turn, inserts  30  and  32  exert forces on transducer  34 . Transducer  34  includes medial plate  36 , lateral plate  38  and lower plate  40 . Support posts  42  support the medial plate  36  and lateral plate  38 . Strain gauges (described below) are mounted directly below support post  42  and sense strain therein. When installed as a replacement assembly for a natural human knee joint, assembly  10  provides quantitative feedback in force load balance across the tibial-femoral joint.  
         [0027]    FIGS.  2 - 3  illustrated rear, bottom perspective views of tibial component  14 . Lower portion  35  is mounted to transducer  34  with cover plate  43 . Cover plate  43  protects transducer  34  from entry of unwanted elements. Cylindrical portion  45  forms a pocket (described later) for storage of components connected to transducer  34 . Ribs  46  support tibial component  14  and attach cover plate  43  to stem  48 . Stem  48  is inserted into the tibia  4  (FIG. 1). FIG. 3 illustrates certain elements with dashed lines to illustrate the construction of transducer  34 , which is described further with reference to FIG. 5.  
         [0028]    FIGS.  4 - 8  illustrate an exemplary embodiment of a transducer according to the present invention. Transducer  34  is symmetrically u-shaped and constructed from suitable elastic material that is responsive to forces applied to medial and lateral plates  36  and  38 . Ultimately, transducer  34  is used to measure forces present on the prosthetic components. The measurements can be used to properly align the components and analyze operation of the components.  
         [0029]    [0029]FIG. 4 illustrates a top view of transducer  34 . Medial plate  36  and lateral plate  38  are spaced apart to isolate forces placed on medial and tibial inserts  30  and  32 , respectively. Both medial plate  36  and lateral plate  38  include cavities  50  and  52  to receive tibial inserts  30  and  32 , respectively, illustrated in FIG. 1. Walls  54  and  56  extend around the peripheral of plates  36  and  38  and define cavities  50  and  52 .  
         [0030]    [0030]FIG. 5 illustrates a bottom perspective view of transducer  34  and FIG. 6 illustrates a bottom view of transducer  34 . FIG. 7 illustrates a sectional of transducer  34  taken along line  7 - 7  in FIG. 6. As illustrated, lower plate  40  includes cavities  60 ,  62 ,  64 ,  66  and  68 , which define flexures  70 ,  72 ,  74 ,  76  and  78 , respectively. In the embodiment illustrated, cavities  60 ,  62 ,  64  and  66  are cylindrical with identical radii, while cavity  68  is elliptically shaped spanning across plates  36  and  38 . Forces applied to medial and lateral plates  20  and  22  are localized and directed through support posts  42  to a corresponding flexure member. Sensor  80  measures deflection of flexures  70 ,  72 ,  74 ,  76  and  78 . Sensor  80  can be resistive, capacitive, optical, etc. In the embodiment illustrated, a plurality of appropriate strain gauges (FIG. 4) are disposed in each respective cavity on a surface of each respective flexure member adjacent to support posts  42 . Sensors  80  provide a quantitative response to forces reacted between the medial and lateral plates  36 ,  38  and lower plate  40 , which correspond to forces carried by each of the condyles  20 . Flexure  78  is unique in that it is responsive to forces from both medial and lateral plates  36  and  38 . However, in order to reduce cross-talk, flexure  78  is elliptically shaped. Flexures  70 ,  72 ,  74 ,  76  and  78  allow forces to be measured across plate  40 . In this manner, changes in forces can also be measured during articulation of the knee joint. This feature thereby allows more accurate replication of forces in a normal joint. Incorrect loading on an artificial joint can cause damage to connecting tissues such as tendons and ligaments. Apertures  84  in lower plate  40  are provided for fasteners (not shown) to secure transducer  34  to stem portion  35 .  
         [0031]    [0031]FIG. 8 illustrates a bottom plan view of transducer  34 . As illustrated, channels  90  provide pathways for electrical leads from strain gauges located in cavities  60 ,  62 ,  64  and  66 . All electrical leads of the strain gauges are connected to a suitable connector or terminal strip  92  placed in cavity  94 . Additional leads can connect terminal strip  92  to other circuitry that will acquire transducer data, process the data and transmit the data outside the body.  
         [0032]    FIGS.  9 - 11  illustrate lower portion  35 . Lower portion  35  includes pocket  100  for storage of circuitry  102 . Pocket  100  opens toward transducer  34 . Circuitry  102  is used to acquire, process and transmit transducer data. Circuitry  102  is couplable to terminal strip  92  of FIG. 6. As illustrated in FIG. 12, circuitry  102  can be a telemetry device that transmits signals wirelessly to a receiver  110 . Location of circuitry  102  in pocket  100  of portion  35  provides an area for storage that is secure. More importantly though, the location below the transducer  34  and thus on the tibia does not interfere with operation or stability of the knee joint. Receiver  110  can then transmit signals received from telemetry device  102  to a computer  112  for further analysis.  
         [0033]    In summary, the present invention provides an assembly and method for implantation of knee joint prostheses. The assembly accurately measures forces present on the prosthesis in vivo without cross-talk as the knee joint is articulated through partial or complete range of movements. The resulting data is collected and transmittal wirelessly for analysis to ensure proper force load distribution across the load bearing surfaces of the knee joint prosthesis. With proper load distribution, the knee joint prosthesis is optimally aligned thereby realizing increased prosthetic life.  
         [0034]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.