Abstract:
A medical instrument includes a base; an upper member; and a linkage mechanism coupled to the base and the upper member and being operable to maintain the upper member in a variable fixed position that is substantially parallel to the base and spaced a predetermined, variable distance therefrom, wherein the linkage mechanism includes at least three linkage elements that are arranged at angles relative to one another such that when one link element opens, the other link elements open with at an equal angle resulting in the upper member being constrained to remain parallel to the base.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. patent application Ser. No. 60/784,343, filed Mar. 20, 2006, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of joint replacement or joint resurfacing surgery, and more particularly, to a distractor (distraction device) that is used in a prosthesis implantation operation, such as a knee prosthesis implantation. 
       BACKGROUND  
       [0003]    The success of joint replacement surgery is primarily dependent on two factors, namely, (1) the position of the implant components with respect to the boney anatomy; and (2) the postoperative state of the surrounding soft tissues. Joints are surrounded by ligamenteous and capsular tissue. The state of these soft tissues determines the laxity and stability of the joint. The state of the soft tissues is effected by the following two factors: (1) the position of the components; and (2) the amount of soft tissue releases performed by the surgeon, as discusses in U.S. patent application publication No. 2005/0267485, which is hereby incorporated by reference in its entirety. In addition, a number of devices and techniques have been described that attempt to facilitate ligament balancing during a TKA procedure and in particular, some of these techniques involve trial prosthesis components which are used after femoral and tibial bone cuts are made to assess ligament tissue. See, U.S. Pat. No. 5,733,292, which is hereby incorporated by reference in its entirety. 
         [0004]    Other devices are used to measure a gap between the distal femur and proximal tibia in extension and to help a surgeon recreate that same gap when the distal femur and proximal tibia in extension to help a surgeon recreate that same gap when the knee is in flexion. See, U.S. patent application publication No. 2003/0187452 and U.S. Pat. No. 6,575,980, both of which describe “gap checking” devices, each of which is hereby incorporated by reference in its entirety. Other devices have been developed to help measure an amount of ligament tension or to apply a desired amount of tension to the ligaments and in addition, paddle-like devices have been suggested for applying or measuring tension across a knee joint. See, U.S. Pat. Nos. 4,501,266; 5,597,379; 5,540,696; 5,800,438; 5,860,980; 5,911,723; and 6,022,377, each of which is hereby incorporated by reference in its entirety. 
         [0005]    The device described in the above mentioned &#39;485 publication is designed to determine the rotational alignment of the femoral component such that the knee is in optional tension; however, there are a number of disadvantages and limitations associated with this device. For example, the following are disadvantages associated with this device: (1) the distal femoral cut must be made first before the device is inserted and therefore, one can not change the planning in varus/valgus; proximal/distal, and flexion/extension; (2) the device must be fixed to the femur and therefore, requires bone screws on the medial and lateral sides which add invasiveness to the bone, as well as the soft tissues since access is required to the lateral side of the joint; (3) a vast number of components sizes for the tibia and the femur are still required; and (4) the system has constraints due to it not being able to account for different prosthetic designs using the same components, for example, different degrees of constraint or concavities of the tibial or femoral components. 
       SUMMARY 
       [0006]    A medical instrument includes a base; an upper member; and a linkage mechanism coupled to the base and the upper member and being operable to maintain the upper member in a variable fixed position that is substantially parallel to the base and spaced a predetermined, variable distance therefrom, wherein the linkage mechanism includes at least three linkage elements that are arranged at angles relative to one another such that when one link element opens, the other link elements open with at an equal angle resulting in the upper member being constrained to remain parallel to the base. 
         [0007]    In one embodiment, the instrument is a distraction device and it is intended for use in implant surgery, such as knee joint replacement, and there are two upper parts constructed to support condyles of the femur. 
         [0008]    In one aspect, the base includes a first base plate and a second base plate that is pivotably coupled to the first base plate, wherein at least a portion of the first and second base plates lie in the same plane and is intended to be mounted to a planar surface, such as a tibial cut. The instrument further includes a controller operably coupled to the linkage mechanism to cause the controlled movement thereof resulting in movement of the upper part relative to the base. The controller is operably connected to an adjustable spacer element that has a body disposed around the linkage mechanism, with the body being fluidly connected to a fluid source such that controlled delivery of fluid to the body results in expansion of the spacer element and a distance between the upper part and base increasing. In one embodiment, the body of the spacer element is in the form of an expandable pouch that has a hollow interior in which the linkage mechanism is disposed, with the pouch being secured to the upper part and the base by means of fastening elements. 
         [0009]    Other features and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 
         [0010]    The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings figures of illustrative embodiments of the invention in which: 
           [0011]      FIG. 1  is a side view of a knee joint defined by a femur bone and a tibia bone; 
           [0012]      FIG. 2  is a perspective view a distraction device according to one embodiment relative to the femur bone; 
           [0013]      FIG. 3  is a side perspective view of the distraction device of  FIG. 2 ; and 
           [0014]      FIG. 4  is a perspective view of one exemplary spacer means for changing the position of the distraction device of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]      FIGS. 1-4  illustrate a distraction device (distractor)  100  according to one exemplary embodiment of the present invention. The distraction device  100  is compatible with minimally invasive procedures, where the patella is not reflected (or reduced) and the knee joint is not opened completely. 
         [0016]    The distraction device  100  has a flat/planar base  110  which is configured and intended to rest or sit on a plateau cut that is made in a bone as part of the implant surgery. For purposes of illustration only, the distraction device  100  will be described as being used in a knee implant operation and thus  FIG. 1  shows a femur bone  10  and tibia bone  20 ; however, the potential applications of the distraction device  100  extend and go beyond the knee implant surgery and thus, the following description of the application of the distraction device  100  in knee implant surgery is merely exemplary and not limiting of the present invention. In the case where the distraction device  100  is used in knee implant surgery, the base  110  thereof rests on a tibial plateau cut that is made near the end of the tibia  20 . 
         [0017]    The base  110  is configured so that it is adjustable to accommodate a range of knee sizes. More particularly, the base  110  is in the form of a plate and more specifically, the base  110  is informed of two base plates, namely, a first plate  120  (internal plate) and a second plate  130  (external plate). The first and second plates  120 ,  130  are adjustable relative to one another and in particular, the first and second plates  120 ,  130  are pivotably connected to one another by a pivot joint  140 . In order for the first and second plates  120 ,  130  to lie in the same plane and be pivotally connected, the first plate  120  has a main portion  122  and a raised portion  124  that is connected to the main portion  122  by means of a ramp  126 . As illustrated in  FIG. 2 , when the lower surface of the main portion  122  rests on the ground, the portion  124  is elevated relative to the ground such that a space  125  is formed under the lower surface of the raised portion  124 . Similarly, the second plate  130  has a main portion  132  and a raised portion  134  that is connected to the main portion  132  by means of a ramp  136 . As illustrated in  FIG. 2 , when the lower surface of the main portion  132  rests on the ground, the portion  134  is elevated relative to the ground such that a space  135  is formed under the lower surface of the raised portion  124 . 
         [0018]    The first and second plates  120 ,  130  are pivotally connected at the raised portions  124 ,  134  and as shown in  FIG. 2 , one raised portion (e.g., portion  124 ) overlies the other raised portion (e.g., portion  134 ). The pivot joint  140  extends through both of the raised portions  124 ,  134  and permits the two plates  120 ,  130  to pivot at the raised portions  124 ,  134  thereof. The raised portion  124  of the first plate  120  has an opening  128  formed therethrough proximate the pivot joint  140 . Similarly, the raised portion  134  includes an opening or slot  138  proximate the pivot joint  140 . A U-shaped piece (not shown) can be inserted into opening  128  and slot  138 . By turning a nut (not shown) or the like that is part of a threaded post of the U-shaped piece that traverses the slot  138 , the size and arrangement of the base  110 , and in particular, the relative positions of the first and second plates  120 ,  130  can be locked into a fixed position. Since the portions  124 ,  134  are raised relative to the main portions  122 ,  132 , respectively, receiving an object (e.g., the U-shaped piece) through the opening  128  and slot  138  does not interfere with the main portions  122 ,  132  resting on the planar cut since it can be received in the space  125 ,  135 . 
         [0019]    The lower surfaces of the first and second base plates  120 ,  130  can be rough or can have protrusions, such as spikes, so as to prevent the distraction device  100  from sliding around on the tibial plateau cut. In addition, openings can also be included so that the surgeon can fix the distraction device  100  to the tibial bone (at tibial cut) by means of pins or screws that are received through openings formed through the first and second plates  120 ,  130 . 
         [0020]    The distraction device  100  includes two upper femoral plateaus, namely, a first upper femoral plateau member  150  (internal) and a second upper femoral plateau member  160  (external). The first upper femoral plateau member  150  is configured and intended to support the internal (medial) condyle  12  of the femur  10 , while the second upper femoral plateau member  160  is configured and intended to support the external (lateral) condyle  14  of the femur  10 . 
         [0021]    As described above in more detail and based on the pivoting action between the plates  120 ,  130 , the distance of separation between each plateau members  150 ,  160  is adjustable. More specifically, the optimal distance of separation between the plateau members  150 ,  160  can be automatically computed from the femoral bone model, by for example, calculating the distance between the most posterior or most distal points on the femoral condyles  12 ,  14 . An average of these two distances can be selected so that the distraction device  100  fits the femur  10  when the knee is in both flexion and in extension. Markings can be incorporated onto the distraction device  100 , for example, on the base  110  (plates  120 ,  130 ) to indicate the separation distance so that the surgeon can adjust the tibial base distance to the appropriate value as determined by various techniques. Alternatively, a caliper system or similar tool can be used to measure the distance between the plateau members  150 ,  160 . Alternatively, pair of holes can be made in the base plates  120 ,  130  of the distraction device  100  corresponding to predefined discrete distance that correspond to various sizes of a knee implant. The surgeon can then easily insert a peg or the like into the proper holes in order to replicate a particular size of the implant that corresponds to the planned implant size. 
         [0022]    An upper surface of each of the first and second femoral plateau members  150 ,  160  is constructed to support and complement the respective condyle and can be convex in form in both the sagittal and frontal planes to better fit with the femoral condyles  12 ,  14 , respectively. Thus, they can be spherical or they can have different curvatures in the different planes to simulate different levels of constraints. 
         [0023]    The first and second upper femoral plateau members  150 ,  160  are coupled to the first and second base plates  120 ,  130 , respectively, by means of a linkage mechanism  200  that ensures that each of the plateau members  150 ,  160  remains parallel to the respective lower base plate  120 ,  130  throughout the course of the distraction motion (i.e., the range of motion of a distraction operation). 
         [0024]    The linkage mechanism  200  is formed of a plurality of link pairs  210 ,  220  connected to each other and coupled to one of the femoral plateau members  150 ,  160  and the respective base plate  120 ,  130  by pins or the like  230 . As shown, the link  210  is connected at one end to one of the femoral plateau members  150 ,  160  and is connected at its other end to one end of the other link  220 . The pins  230  permit pivoting of the links  210 ,  220  with respect to each other and with respect to the femoral plateau plates  150 ,  160  and the base plates  120 ,  130 . 
         [0025]    The links  210 ,  220  are arranged at angles to each other such that when one pair of links  210 ,  220  hinges or pivots open, all other link pairs  210 ,  220  open at an equal angle, thereby constraining the first and second upper femoral plateau members  150 ,  160  to remain parallel to the first and second lower base plates  120 ,  130 . 
         [0026]    In one exemplary embodiment, at least three linkage mechanisms  200  for each of the first and second femoral plateau members  150 ,  160  and the respective base plate  120 ,  130  are chosen to optimize the stability, strength and size of the linkage mechanism  200 . However, it will be appreciated that each mechanism  200  can have more or less than three pairs of links  210 ,  220 . Thus, two or four pairs of links  210 ,  220  can be used. 
         [0027]    It will also be appreciated that instead of having link pairs defined by parts  210 ,  220  that are coupled to and between the first and second femoral plateau members  150 ,  160  and the respective base plate  120 ,  130 , there can be more than two links in each set. In other words, link triplets defined by three link members pivotally attached to one another and to the first and second femoral plateau members  150 ,  160  and the respective base plate  120 ,  130  can be provided or link quadruplet defined by four link members can be employed instead of the illustrated link pairs  210 ,  220 . The illustrated linkage mechanism  200  has been designed such that it has a low profile height on the order of about  5 mm when fully retracted as illustrated in  FIG. 2 , and a considerably higher height of about 15 mm or 20 mm when fully extended. If additional heights are required beyond the maximum height range, spacer blocks can be fastened onto the first and second upper femoral plateau members to augment the maximum achievable height. 
         [0028]    The fastening mechanism that is incorporated into the distraction device  100  can be any number of different types, including but not limited to, a quick-clip or snap type mechanism, or a peg and hole type mechanism, or a sliding dove tail joint arrangement, etc. In addition, in the case where the above mentioned spacer blocks are used, these blocks can have similar surfaces to those of the first and second femoral plateau members  150 ,  160  and are constructed to mate in a complementary manner with the condyles  12 ,  14  of the femur  10 . Alternatively, the spacer blocks can have different shaped surfaces, such as flat planes so that they can fit the femur  10  after the distal femoral and posterior femoral cuts are made. By measuring the gap spaces between the femur  10  and tibia  20 , the physician can determine if the required distraction height is greater than the maximum height achievable by the distraction device  100 . The system can also advise the surgeon as to which height of spacer block to use in order to sufficiently augment the distraction height, while keeping the distraction device&#39;s dynamic range of motion or workspace in a suitable location. 
         [0029]    The height of each of the first and second upper femoral plateau members  150 ,  160  is preferably independently controlled by a controller or some other type of mechanism. There are any number of different techniques that can be used to control the movement of the first and second upper femoral plateau members  150 ,  160  relative to the first and second base plates  120 ,  130 . For example, the height can be controlled by a hydraulic system. Since the height of the distraction device  100  can be readily changed, the portion of the device  100  that is inserted into the joint can remain as small as possible, and require only a minimum opening of the joint. 
         [0030]      FIG. 4  illustrates one exemplary means  300  for controlling the height of the first and second upper femoral plateau members  150 ,  160  relative to the first and second base plates  120 ,  130 . The illustrated means  300  is a fluid based system and includes a first fluid holding member that is expandable (first pouch)  310  that is intended to be associated with one of the linkage mechanisms  200  and a second fluid holding member that is expandable (second pouch)  320  that is intended to be associated with another linkage mechanism  200 . More specifically, the first pouch  310  is constructed to surround one linkage mechanism  200  and receive and hold a fluid (e.g., water) and the second pouch  320  is constructed to surround another linkage mechanism  200 . The first pouch  310  is thus a flexible member that has a hollow interior  310  that is constructed to accommodate the linkage mechanisms  200  which in the illustrated embodiments is defined by three pairs of links pairs. 
         [0031]    Each of the first and second pouches  310 ,  320  has an upper part  312  and an opposing lower part  314 , with the upper part  312  being coupled to a first intermediate plate  330 , while the lower part  314  is coupled to a second intermediate plate  340 . The intermediate plates  330 ,  340  can have any number of different sizes and shapes so long as they are complementary to the other parts and perform the function of providing a mounting surface or substrate that permits the linkage mechanism  200  to be mounted between the base  110  and the upper femoral plateau members  150 ,  160 . In the illustrated embodiment, the intermediate plates  330 ,  340  are in the form of disks or the like. 
         [0032]    In fact, the linkage pairs defined by parts  210 ,  220  are disposed between the two intermediate plates  330 ,  340 , with the part  210  being attached to the first intermediate plate  330  and the part  220  being attached to the second intermediate plate  340 . 
         [0033]    The upper part  312  of the first pouch  310  can be coupled to the first intermediate plate  330  with fastening means  332  (such as screws or the like) and the lower part  314  can be coupled to the second intermediate plate  340  with fastening means  332 , with the plates  330 ,  340  being attached to the first upper femoral plateau member  150  and the base plate  120 . Similarly, the upper part  312  of the second pouch  320  can be coupled to the first intermediate plate  330  with fastening means  332  (such as screws or the like) and the lower part  314  can be coupled to the second intermediate plate  340  with fastening means  332 , with the plates  330 ,  340  being attached to the second upper femoral plateau member  160  and the base plate  130 . The attachment of the first and second pouches  310 ,  320  to the intermediate plates  330 ,  340  forms a tight waterproof seal. 
         [0034]    Openings  350  formed in the linkage mechanisms  200  prevent hole bosses  352  from interfering and impinging upon the links  210 ,  220  through the course of the range of distractor motion. In other words, as the distraction device  100  moves over its range of motion (up and down) the screw bosses  352  will likewise move; however, the openings  350  are formed in the links  210 ,  220  to permit reception of the screw bosses  352  and therefore, permit smooth movement of the device  100 . 
         [0035]    The pouches  310 ,  320  can be made out of a medical grade plastic or PVC or any other suitable material. Preferably, the pouches  310 ,  320  are made from a material that is the least extensible as possible so that the distraction height does not change significantly when loads are applied. The material should be bendable to accommodate changes in the shape as the plateau height is increased or decreased, but should also resist expanding or stretching like a balloon when the fluid pressure increases. In other words, if the fluid volume in the pouches  310 ,  320  is held constant, the distraction height should also remain constant even if the loads are applied since the pouches  310 ,  320  do not expand under the applied pressure. 
         [0036]    The pouches  310 ,  320  can be manufactured as two separate discs and joined together around the linkage mechanisms  200  with a seam to reduce manufacturing costs. Preferably, the seam is made using a high frequency welding machine so as to be strong and resist rupturing. 
         [0037]    In one embodiment, the pouches  310 ,  320  are fluid operated with fluid being supplied by means of conduits (tubes)  360  that can extend from the pouches  310 ,  320  to transmit the fluid. The conduits  360  can be flexible so as not to interfere with the patella and the tissues surrounding the joint as the knee joint is flexed and distracted. The fluid can be sterile water, saline solution, mineral oil, or any other appropriate fluid. A purge system can be incorporated to remove any bubbles in the system. 
         [0038]    The height of each of the first and second upper femoral plateau members  150 ,  160  is independently controlled by a controller or the like. The controller can include one or more motors or the like that are operated to control the amount of fluid in each pouch  310 ,  320  and the height of the respective first and second upper femoral plateau members  150 ,  160 . Operation of the motors results in fluid traveling through the conduits  360  into the pouches  310 ,  320  and this causes the fluid pressure to increase in the pouch  310 ,  320 . Apposing forces are applied to the intermediate plates  330 ,  340  resulting in an increase in height of the upper femoral plateau plate  150 ,  160  relative to the base plates  120 ,  130  (first and second degrees of freedom (DOF)). This in turn causes the position of the femur  10  to change relative to the tibia  20  in the knee joint. 
         [0039]    It will be appreciated that any number of different types of controllers, actuators, devices, etc., can be used to cause a controlled change in the distraction device  100 . 
         [0040]    While exemplary drawings and specific embodiments of the present invention have been described and illustrated, it is to be understood that the scope of the present invention is not to be limited to the particular embodiments discussed. Thus, the embodiments shall be regarded as illustrative rather than restrictive, and it should be understood that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as set forth in the claims that follow, and equivalents thereof. In addition, the features of the different claims set forth below may be combined in various ways in further accordance with the present invention.