Patent Publication Number: US-10758375-B2

Title: Compact mechanical joint balancer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/621,852, filed Jan. 25, 2018, the contents of which are incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     In the normal intact knee, the relative position between the femur and the tibia is controlled by the shape of the bearing surfaces and by the 4 major ligaments crossing the knee; the anterior and posterior cruciates and the medial and lateral collaterals. As the knee is flexed and extended, the lengths of the ligaments and the shape of the bearing surfaces work in harmony to maintain stability, without excessive looseness or tightness. When the components of a total knee are inserted at surgery, it is important that the components are inserted accurately, and that the geometry of the components is sufficiently close to anatomic, so that the ligaments can still maintain the correct stability between the femur and the tibia. 
     In a typical surgical procedure, the bone cuts are made, then the trial components are inserted. At this stage, the knee is flexed and extended to determine whether the knee comes into extension without hyperextending, or having an extension lag. The thickness of the tibial component is adjusted if necessary. Then an assessment is made if the balancing is correct throughout flexion. A simple evaluation method is to remove the trials, introduce a spacer block between the cut surfaces of the femur and tibia with the knee in extension, and move the tibia into varus and valgus to assess the relative stiffnesses. This is repeated at 90 degrees flexion. If either the medial or lateral side of the knee is too tight, the tight ligaments are released. 
     Methods other than the spacer blocks can be used. One method is to use a distractor tool which inserts between the joint surfaces and equal expansion forces are applied separately to the lateral and medial condyles, equal distraction gaps being an indicator of balancing. More recently, an instrumented tibial trial has been introduced, which takes the place of the tibial trial component, which measures the lateral and medial contact forces throughout flexion and displays the forces on a computer screen. 
     Thus, there is a need in the art for improved mechanical devices and methods for balancing joints, including knee joints. The present invention meets this need. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a joint balancer device, comprising: a handle having a gauge; a lower plate attached to the handle; and an upper plate aligned in parallel with the lower plate by a gap space, wherein the upper plate is displaceable relative to the lower plate; wherein the upper plate is connected by a mechanical link to the gauge, and wherein the mechanical link magnifies displacement of the upper plate such that the gauge displays the magnified displacement. 
     In one embodiment, the device further comprises: an axle casing extending posteriorly (towards a joint) from the handle and connected at a posterior end to the lower plate, the axle casing further having a lumen; an axle rotatably positioned within the lumen of the axle casing, the axle connected at an anterior end (away from a joint) to the mechanical link and at a posterior end to the upper plate. 
     In one embodiment, the mechanical link comprises: a vertical first beam positioned within a hollow interior of the handle and rotatable about a pin secured to the interior of the handle, wherein the first beam is attached at a superior end to a pointer of the gauge; and a vertical second beam positioned substantially in parallel with the first beam, the second beam being joined at a superior end to the anterior end of the axle and attached at an inferior end to an inferior end of the first beam. In one embodiment, a clockwise or a counterclockwise rotation of the axle moves the inferior end of the first and second beam in a left or a right direction, respectively, such that the first beam is rotated about the pin in a clockwise or a counterclockwise direction, respectively, to shift the pointer of the gauge in a right or left direction, respectively. 
     In one embodiment, the mechanical link comprises: a vertical first beam positioned within a hollow interior of the handle and having a flexible inferior section pinched between opposing studs attached to the interior of the handle, the studs acting as a pivot point, wherein the first beam is attached at a superior end to a pointer of the gauge; and a vertical second beam positioned substantially in parallel with the first beam, the second beam being joined at a superior end to the anterior end of the axle and attached at an inferior end to an inferior end of the first beam. In one embodiment, a clockwise or a counterclockwise rotation of the axle moves the inferior end of the first and second beam in a left or a right direction, respectively, such that the flexible inferior section of the first beam bends about the opposing studs to shift the pointer of the gauge in a right or left direction, respectively. 
     In one embodiment, the mechanical link comprises: a horizontal beam positioned within a hollow interior of the handle and rotatable about a pin secured to the interior of the handle, wherein the beam comprises a notch at a posterior end and is attached at an anterior end to a pointer of the gauge; and a tab extending in an inferior direction from the anterior end of the axle, the tab being engaged to the notch of the beam. In one embodiment, a clockwise or a counterclockwise rotation of the axle moves the tab and the posterior end of the beam in a left or a right direction, respectively, such that the beam is rotated about the pin to shift the pointer of the gauge in a right or left direction, respectively. 
     In one embodiment, the mechanical link comprises: a pointer of the gauge rotatable about a pointer axis and a superiorly positioned notch; and a tab extending in an inferior direction from the anterior end of the axle, the tab being engaged to the notch of the pointer. In one embodiment, a clockwise or a counterclockwise rotation of the axle moves the tab in a left or a right direction, respectively, such that the pointer of the gauge is rotated about the pointer axis in a counterclockwise or clockwise direction, respectively. 
     In one embodiment, the mechanical link comprises: a first and a second elongate beam adjacently positioned within a hollow interior of the handle, each elongate beam resting on a fulcrum on the interior of the handle and having an anterior end as a pointer of the gauge; a first short beam extending from a posterior end of the first elongate beam in a lateral direction underneath a left side of the upper plate; and a second short beam extending from a posterior end of the second elongate beam in a lateral direction underneath a right side of the upper plate. In one embodiment, an inferior displacement of a side of the upper plate inferiorly displaces the respective short beam underneath the side of the upper plate, such that the respective connected long beam is actuated about the fulcrum to shift the pointer of the gauge in a superior direction. In one embodiment, the upper plate comprises flexible membranes above the first and second short beams. 
     In one embodiment, the upper plate has an upper surface contoured to fit a femur&#39;s condyles or a trial femoral component of a total knee replacement, and the lower plate has a lower surface contoured to fit a tibia&#39;s condyles or a resected proximal surface of a tibia in a total knee replacement. 
     In one embodiment, the upper plate and the lower plate are separated by a substantially parallel distance between about 1 and 5 mm. In one embodiment, the distance is maintained in a neutral state by one or more springs positioned between the upper plate and the lower plate, the one or more springs selected from the group consisting of: coil springs, conical springs, wave springs, and leaf springs. In one embodiment, the gauge includes a scale having at least one of force units, distance units, angle degrees, or unitless markings. In one embodiment, the device further comprises one or more pressure sensors positioned between the upper plate and the lower plate. 
     In one embodiment, the device is configured to measure a relative difference in force between a lateral side and a medial side of a joint. 
     In another aspect, the present invention relates to a method of balancing a joint, comprising the steps of: providing the balancer device of the present invention; inserting the balancer device into a joint in need of balancing such that the upper and lower plates rest against opposing surfaces of the joint; flexing the joint through at least part of its full range of motion; recording the magnitudes of displacement between left and right sides of the joint indicated by the gauge throughout the flexing of the joint; removing the balancer device from the joint; modifying the balance of the joint to reduce or eliminate the magnitudes of displacement between left and right sides of the joint. 
     In one embodiment, the joint is a knee joint or an elbow joint. In one embodiment, the opposing joint surfaces are opposing bone surfaces, opposing implant surfaces, or combinations thereof. In one embodiment, the balance of the joint is modified by trimming the opposing surfaces, raising the opposing surfaces, adjusting a spacer component between the opposing surfaces, or combinations thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of exemplary embodiments of the invention will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings. 
         FIG. 1  depicts a perspective view of an exemplary balancer device. 
         FIG. 2A  and  FIG. 2B  depict perspective views of the handle component of an exemplary balancer device.  FIG. 2A  depicts the handle alone.  FIG. 2B  depicts a partial cutaway view of the handle. 
         FIG. 3A  and  FIG. 3B  depict perspective views of the balancer component of an exemplary balancer device comprising a condyle and a mechanical linkage.  FIG. 3A  depicts the balancer component alone.  FIG. 3B  depicts a partial view of the bottom surface of the balancer component. 
         FIG. 4A  through  FIG. 4C  depict various views of the balancer component of an exemplary balancer device as it receives a load.  FIG. 4A  depicts a frontal view of the balancer plate and handle plate at rest.  FIG. 4B  depicts a frontal view of the balancer plate and handle plate receiving a load on the right condyle.  FIG. 4C  depicts a perspective view of the balancer component receiving a load on the right condyle, causing the pointer or gauge to provide a reading to the right. 
         FIG. 5A  through  FIG. 5C  depict various views of another exemplary balancer device.  FIG. 5A  depicts the balancer device near a knee joint.  FIG. 5B  depicts a partially cutaway view of the underside of the balancer device.  FIG. 5C  depicts the balancer component and pointer or gauge in isolation. 
         FIG. 6  depicts perspective views of another exemplary balancer device. The balancer device (left) includes a handle (middle) and a balancer component (right). 
         FIG. 7  depicts certain views of the balancer device in  FIG. 6 , including the underside of the plate of a condylar component (left), a partial cutaway view of the device under no load (middle), and a partial cutaway view of the device under load (right). 
         FIG. 8  depicts perspective views of the balancer device in  FIG. 6  as separable components that can be snap fit together. The separable components include a balancer component (far left) and a handle component (middle left) that can be combined to form the balancer device (middle right), and optionally a cap (far right). 
         FIG. 9  depicts a partial cutaway view (top) and the internal components (bottom) of another exemplary balancer device. 
         FIG. 10  depicts a perspective view (top), an underside view (middle), and a partial cutaway view (bottom) of another exemplary balancer device. 
         FIG. 11  depicts a perspective view (top) and a proximal view (bottom) of another exemplary balancer device. 
         FIG. 12  is a flowchart depicting an exemplary method of balancing a joint using a balancer device. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to devices and methods for balancing joints, including knee joints. The devices convert unequal forces at a joint to a rotation or displacement of a pointer or gauge. The devices are able to display the relative difference in force between the lateral and medial sides of a joint from flexion to extension. In certain aspects, the devices are useful for balancing the knee during total knee surgery. The devices can be inserted between the trial femoral and tibial components of the knee and measures whether one condyle experiences more force than the other. 
     Definitions 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements typically found in the art. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art. 
     Unless defined elsewhere, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, exemplary methods and materials are described. 
     As used herein, each of the following terms has the meaning associated with it in this section. 
     The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element. 
     “About” as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20%, ±10%, ±5%, ±1%, and ±0.1% from the specified value, as such variations are appropriate. 
     The terms “proximal,” “distal,” “anterior,” “posterior,” “medial,” “lateral,” “superior,” and “inferior” are defined by their standard usage indicating a directional term of reference. For example, “proximal” refers to an upper location from a point of reference, while “distal” refers to a lower location from a point of reference. In another example, “anterior” refers to the front of a body or structure, while “posterior” refers to the rear of a body or structure. In another example, “medial” refers to the direction towards the midline of a body or structure, and “lateral” refers to the direction away from the midline of a body or structure. In some examples, “lateral” or “laterally” may refer to any sideways direction. In another example, “superior” refers to the top of a body or structure, while “inferior” refers to the bottom of a body or structure. It should be understood, however, that the directional term of reference may be interpreted within the context of a specific body or structure, such that a directional term referring to a location in the context of the reference body or structure may remain consistent as the orientation of the body or structure changes. 
     Throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, 6, and any whole and partial increments there between. This applies regardless of the breadth of the range. 
     Balancer Device 
     Referring now to  FIG. 1 , an exemplary balancer device  10  is depicted. Balancer device  10  has a superior end  12 , an inferior end  14 , and comprises a balancer component  40  partially encased within a handle component  20 . 
     Referring now to  FIG. 2A  and  FIG. 2B , handle  20  of an exemplary balancer device  10  is depicted in isolation. Handle  20  comprises housing  22 , axle casing  30 , and plate  32 . Housing  22  comprises a substantially hollow construction having a superior end  12  and an inferior end  14 . Housing  22  comprises a window  24  and a scale  26  positioned near its superior end  12 . Scale  26  provides a graded measure of any suitable unit adjacent to window  24 . For example, scale  26  can include at least one of force units, distance units, angle degrees, or unitless markings. Housing  22  further comprises a first tab slot  28   a  and a second tab slot  28   b  positioned on the top of its superior end  12 . Pin  38 , aligned along axis  39 , is positioned within the hollow interior of housing  22  and projects from the anterior surface of housing  22  in an anterior to posterior direction. 
     Axle casing  30  extends posteriorly from housing  22  and connects at its posterior end to plate  32 . Axle casing  30  comprises a lumen extending from the hollow interior of housing  22  (at axle opening  36 ) to an open end adjacent to plate  32 . Plate  32  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. Plate  32  has an upper surface that interfaces with balancer  40 . In some embodiments, plate  32  has a flat upper surface. In other embodiments, plate  32  has a curved upper surface. In some embodiments, plate  32  has a flat lower surface. In other embodiments, plate  32  has a contoured periphery or lower surface having the shape of a joint space, such as the resected proximal surface of a tibia in a total knee replacement. Plate  32  includes axle slot  34  positioned opposite from the open end of axle casing  30  and aligned with the lumen of axle casing  30 . 
     Referring now to  FIG. 3A  and  FIG. 3B , balancer  40  of an exemplary balancer device  10  is depicted in isolation viewed from an anterior direction, alternately called the underside. Balancer  40  comprises first beam  42 , second beam  50 , axle  56 , and plate  58 . First beam  42  and second beam  50  each comprises an elongate shape having a superior end  12  and an inferior end  14 . First beam  42  comprises pin bore  44  positioned between its superior end  12  and inferior end  14 . Pin bore  44  is sized to fit around pin  38  such that first beam  42  is at least partially rotatable about pin  38 . First beam  42  further comprises pointer or gauge  46  positioned near its superior end  12  and first tab  48   a  positioned on the top end of its superior end  12 . Pointer or gauge  46  is visible through window  24  adjacent to scale  26 , such that the position of pointer or gauge  46  indicates the instant numerical measure as measured by balancer device  10 . First tab  48   a  fits within first tab slot  28   a  and limits the movement of first beam  42  to the boundaries of first tab slot  28   a.    
     First beam  42  is attached to second beam  50  at their inferior ends  14  by extension  54 . In some embodiments the inferior half of first beam  42  below pin bore  44  is at least partially flexible. A flexible inferior half of first beam  42  below pin bore  44  compensates for the different centers of rotation of second beam  50  and first beam  42 . The flexible portion also acts as a spring to center pointer or gauge  46  when no load is present in device  10 . In some embodiments, the attachment between the inferior end of first beam  42  and second beam  50  is a rigid attachment, such as by an adhesive, a welding, or by forming first beam  42  and second beam  50  as a single continuous piece. In other embodiments, the attachment between the inferior end of first beam  42  and second beam  50  is a separable attachment, such as by slotting the inferior end  14  of first beam  42  into an opening in extension  54 . Second beam  50  is thereby positioned parallel and adjacent to first beam  42 , such that both can fit within the hollow interior of housing  22 . Second beam  50  comprises indent  52  between its superior end  12  and inferior end  14 . Indent  52  provides freedom for second beam  50  to travel around pin bore  44 . Second beam  50  comprises second tab  48   b  positioned on the top end of its superior end  12 . Second tab  48   b  fits within second tab slot  28   b  and limits the movement of second beam  50  to the boundaries of second tab slot  28   b.    
     Axle  56  extends posteriorly from the superior end  12  of second beam  50  along axis  31  and connects at its posterior end to plate  58 . Plate  58  comprises axle tip  60  positioned opposite from axle  56  and aligned with axis  31 . Plate  58  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. In some embodiments, plate  58  has a flat upper surface. In other embodiments, plate  58  has a contoured upper surface having a shape that fits in a joint space, such as a shape that fits against the condyles of a trial femoral component of a total knee replacement. As described elsewhere herein, the bottom surface of plate  58  is contoured to fit the upper surface of plate  32 . For example, a plate  58  having a flat lower surface fits with a plate  32  having a flat upper surface, and a plate  58  having a curved lower surface fits with a plate  32  having a curved upper surface. 
     The lower surface of plate  58  has at least two springs  62  positioned opposite from each other to the left and to the right of axle  56 . Springs  62  can be any suitable spring, such as a coil spring, a conical spring, a wave spring, a leaf spring, and the like. The lower surface of plate  58  is in parallel alignment with the upper surface of plate  32  at rest, with springs  62  maintaining a constant height difference between both plates at rest, as well as restoring parallel alignment to both plates after displacement of plate  58 , thereby functioning as a centering mechanism to zero pointer or gauge  46  of device  10 . Plate  58  and plate  32  can be separated by any suitable height, such as a height between about 1 and 5 mm. In some embodiments, the height between plate  58  and plate  32  is 2.5 mm. 
     Balancer device  10  uses balancer  40  and handle  20  to provide a reading of the relative difference in force between the lateral and medial sides of a joint by translating displacement of plate  58  into a proportional movement of pointer or gauge  46 . The displacement can be an angular displacement, a rotation, or a vertical displacement. Referring now to  FIG. 4A  through  FIG. 4C , the operational relationship between balancer  40  and handle  20  is depicted. It should be understood that while joints attached to a body as a point of reference can have medial and lateral sides, for the sake of example a hypothetical joint being measured by balancer device  10  has a left and a right side as depicted in  FIG. 4B  and  FIG. 4C . In  FIG. 4A , balancer device  10  is shown at rest. Plate  58  of balancer  40  and plate  32  of handle  20  are held in parallel alignment by springs  62 . In  FIG. 4B , an imbalance in a hypothetical joint leads to a relative difference in force that is greater on the right side, which depresses right spring  62  on plate  58 . In  FIG. 4C , the greater force on the right side of plate  58  causes axle  56  to rotate about axis  31  in a clockwise direction, such as up to 5 degrees or up to 2.5 degrees. Second beam  50 , being connected at its superior end to axle  56 , pivots in a clockwise direction about its connection with axle  56 , causing its inferior end to move laterally to the left. The leftward movement of the inferior end of second beam  50  also shifts the inferior end of first beam  42  in a leftward direction due to the connection between second beam  50  and first beam  42  at extension  54 . First beam  42 , being rotatably connected to pin  38 , thereby pivots in a clockwise direction about axis  39 , causing the superior end of first beam  42 , along with pointer or gauge  46  to move laterally to the right. In various embodiments, the position of axis  39  can be raised or lowered to adjust the magnification of movement between the inferior end  14  of second beam  50  and pointer or gauge  46 . 
     In summary, a relative difference in force between the left and right side of a joint exerts a greater load on a respective side of plate  58 . The greater load displaces plate  58 , such as an angular displacement, a rotation, or a vertical displacement, which rotates axle  56 . The rotation of axle  56  and the mechanically linked movement of second beam  50  and first beam  42  is translated to a shift in the position of pointer or gauge  46  to the left or to the right, whereupon pointer or gauge  46  points to a unit on scale  26  of housing  22  to indicate the relative difference in force in the joint on the side with the greater load. Movement in pointer or gauge  46  is thereby representative of a magnified magnitude of the movement of plate  58 . 
     Referring now to  FIG. 5A  through  FIG. 5C , an exemplary balancer device  70  is depicted. Balancer device  70  comprises a housing  72  with a superior end  12  and an inferior end  14 . Housing  72  comprises a window  74  near its superior end  12  sized to fit a pointer or gauge  90 . Housing  72  may further comprise a scale positioned near its superior end  12  that provides a graded measure of any suitable unit adjacent to window  74  (not pictured). For example, the scale can include at least one of force units, distance units, angle degrees, or unitless markings. Axle casing  76  extends posteriorly from the superior end  12  of housing  72  and connects at its posterior end to plate  78 . Axle casing  76  comprises a lumen extending from housing  72  to an open end adjacent to plate  78 . Plate  78  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. Plate  78  has an upper surface that interfaces with the lower surface of plate  82 , such that a flat upper surface of plate  78  fits with a flat lower surface of plate  82 , and a curved upper surface of plate  78  fits with a curved lower surface of plate  82 . In some embodiments, plate  78  has a flat lower surface. In other embodiments, plate  78  has a contoured lower surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a tibia in a knee joint. Plate  78  includes axle slot  80  positioned opposite from the open end of axle casing  76  and aligned with the lumen of axle casing  76 . 
     Plate  82  rests on the upper surface of plate  78  and is connected to the posterior end of axle  84 . Axle  84 , aligned along axis  86 , extends anteriorly through the lumen of axle casing  76  into housing  72 . Plate  82  comprises axle tip  83  positioned opposite from axle  84  and aligned with axis  86 . Plate  82  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. In some embodiments, plate  82  has a flat upper surface. In other embodiments, plate  82  has a contoured upper surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a femur in a knee joint. The lower surface of plate  82  has at least two springs  88  positioned opposite from each other to the left and to the right of axle  84 . Springs  88  can be any suitable spring, such as a coil spring, a conical spring, a wave spring, a leaf spring, and the like. The lower surface of plate  82  is in parallel alignment with the upper surface of plate  78  at rest, with springs  88  maintaining a constant height difference between both plates at rest. Plate  82  and plate  78  can be separated by any suitable height, such as a height between about 1 and 5 mm. In some embodiments, the height between plate  82  and plate  78  is 2.5 mm. 
     Axle  84  comprises tab  85  at its anterior end that seats within notch  94  of pointer or gauge  90 . Balancer device  70  thereby is able to provide a reading of the relative difference in force between the lateral and medial sides of a joint by translating displacement of plate  82  into a proportional movement of pointer or gauge  90 . The displacement can be an angular displacement, a rotation, or a vertical displacement. 
     While joints attached to a body as a point of reference can have medial and lateral sides, for the sake of describing the function of balancer device  70  in an example, a hypothetical joint being measured by balancer device  70  has a left and a right side relative to the orientation of the device in  FIG. 5A  and  FIG. 5B . An imbalance in a hypothetical joint leads to a relative difference in force that is greater on the right side, which depresses the right spring  88  of plate  85  and causes axle  84  to rotate in a clockwise direction along axis  86 , such as up to 5 degrees or up to 2.5 degrees. The rotation of axle  84  in a clockwise direction pivots tab  85  in a clockwise direction to the left, which pushes notch  94  in the left direction, causing pointer or gauge  90  to pivot along axis  92  to the left. In this embodiment, pointer or gauge  90  would point to a measure of magnitude, whereupon a user taking care to reverse the orientation may assign the greater load to the correct side of imbalance (e.g. the pointer or gauge  90  pointing to the left indicates a greater load on the right side of plate  82 ). In some embodiments, pointer or gauge  90  points towards inferior end  14 , whereupon tab  85  pushing notch  94  in the left direction causes pointer or gauge  90  to pivot along axis  92  to the right, in the same direction as the greater load on the right side of plate  82  (not pictured). In every embodiment, movement in pointer or gauge  90  is thereby representative of a magnified magnitude of the movement of plate  82 . 
     Referring now to  FIG. 6 , an exemplary balancer device  100  is depicted. Balancer device  100  has a superior end  102 , an inferior end  104 , and comprises a balancer component  124  partially encased within a handle component  106 . Handle  106  comprises housing  108 , axle casing  116 , and plate  120 . Housing  108  comprises a substantially hollow construction with a window  110  and a scale  112  positioned near its superior end  102 . Scale  112  provides a graded measure of any suitable unit adjacent to window  110 . For example, scale  112  can include at least one of force units, distance units, angle degrees, or unitless markings. Housing  108  further comprises a first tab slot  114   a  and a second tab slot  114   b  positioned on the top of its superior end  102 . Visible in  FIG. 7  (middle and right), housing  108  further comprises opposing studs  146  having a gap space in-between. 
     Axle casing  116  extends posteriorly from housing  108  and connects at its posterior end to plate  120 . Axle casing  116  comprises a lumen extending from the hollow interior of housing  108  to an open end adjacent to plate  120 . Plate  120  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. Plate  120  has an upper surface that interfaces with balancer  124 . In some embodiments, plate  120  has a flat upper surface. In other embodiments, the upper surface of plate  120  comprises recess  121 . In some embodiments, plate  120  has a flat lower surface. In other embodiments, plate  120  has a contoured periphery or lower surface having the shape of a joint space, such as the resected proximal surface of a tibia in a total knee replacement. Plate  120  includes axle slot  122  positioned opposite from the open end of axle casing  116  and aligned with the lumen of axle casing  116 . 
     Balancer  124  comprises first beam  126 , second beam  132 , axle  138 , and plate  140 . First beam  126  and second beam  132  each comprises an elongate shape. First beam  126  comprises pointer or gauge  128  and first tab  136   a  at superior end  102  and a flexible beam  130  at inferior end  104 , wherein first tab  136   a  is sized to fit within first tab slot  114   a . Flexible beam  130 , similar to the flexible inferior half of first beam  42 , magnifies the movement of pointer or gauge  128  and acts as a spring to center pointer or gauge  128  when no load is present in device  100 . Pointer or gauge  128  is visible through window  110  adjacent to scale  112 , such that the position of pointer or gauge  128  indicates the instant numerical measure as measured by balancer device  100 . Flexible beam  130  has a width that is smaller than first beam  126 , such that flexible beam  130  fits within the gap space between opposing studs  146  in the interior of housing  108  ( FIG. 7 , middle and right). In some embodiments, studs  146  are positioned from about one third of the height of handle  106 . In various embodiments, the position of studs  146  can be raised or lowered within housing  108  to adjust the magnitude of movement between the inferior end  104  of second beam  132  and pointer or gauge  128 . Second beam  132  comprises second tab  136   b  at superior end  102  and extension  134  at inferior end  104 , wherein second tab  136   b  is sized to fit within second tab slot  114   b . First beam  126  is attached by way of flexible beam  130  to second beam  132  at their inferior ends  104  by extension  134 . In some embodiments, the attachment at extension  134  is a rigid attachment, such as by an adhesive, a welding, or by forming first beam  126  and second beam  132  as a single continuous piece. In other embodiments, the attachment at extension  134  is a separable attachment, such as by slotting inferior end  104  of flexible beam  130  into an opening in extension  134 . Second beam  132  is thereby positioned parallel and adjacent to first beam  126 , such that both can fit within the hollow interior of housing  108 . 
     Axle  138  extends posteriorly from superior end  102  of second beam  132  along axis  118  and connects at its posterior end to plate  140 . Plate  140  comprises axle tip  142  positioned opposite from axle  138  and aligned with axis  118 . Plate  140  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. In some embodiments, plate  140  has a flat upper surface. In other embodiments, plate  140  has a contoured upper surface having a shape that fits in a joint space, such as a shape that fits against the condyles of a trial femoral component of a total knee replacement. As described elsewhere herein, the bottom surface of plate  140  is contoured to fit the upper surface of plate  120 . For example, a plate  140  having a flat lower surface fits with a plate  120  having a flat upper surface. In some embodiments the lower surface of plate  140  can include support  144  shown in  FIG. 7  (left) to increase the stiffness of plate  140  under load; support  144  can fit within a recess  121  in the upper surface of plate  120 . Support  144  comprises two downward sloping surfaces meeting at a middle edge, wherein the height of the edge combined with the depth of recess  121  define the height of parallel alignment between the lower surface of plate  140  and the upper surface of plate  120  at rest. Plate  140  and plate  120  can be separated by any suitable height, such as a height between about 1 and 5 mm. In some embodiments, the height between plate  140  and plate  120  is 2.5 mm. 
     Similar to the embodiments described elsewhere herein, balancer device  100  uses balancer  124  and handle  106  to provide a reading of the relative difference in force between the lateral and medial sides of a joint by translating displacement of plate  140  into a proportional movement of pointer or gauge  128 . The displacement can be an angular displacement, a rotation, or a vertical displacement. Referring now to  FIG. 7  (middle and right), the operational relationship between balancer  124  and handle  106  is depicted. It should be understood that while joints attached to a body as a point of reference can have medial and lateral sides, for the sake of example a hypothetical joint being measured by balancer device  100  has a left and a right side as depicted in  FIG. 7  (middle and right). In  FIG. 7  (middle), balancer device  100  is shown at rest. Plate  140  of balancer  124  and plate  120  of handle  106  are held in parallel alignment by the edge of support  144  resting at the bottom of recess  121 . In  FIG. 7  (right), an imbalance in a hypothetical joint leads to a relative difference in force that is greater on the right side, which depresses the right side of plate  140 . The greater force on the right side of plate  140  causes plate  140  to tilt and rotates axle  138  about axis  118  in a clockwise direction, such as up to 5 degrees or up to 2.5 degrees. Second beam  132 , being connected at its superior end  102  to axle  138 , pivots in a clockwise direction about its connection with axle  138 , causing its inferior end  104  to move laterally to the left. The leftward movement of the inferior end  104  of second beam  132  also shifts the inferior end of flexible beam  130  in a leftward direction due to the connection between second beam  132  and flexible beam  130  at extension  134 . The movement of flexible beam  130 , being constrained by studs  146 , thereby pivots the superior end  102  of first beam  126  in a clockwise direction, causing pointer or gauge  128  to move laterally to the right. Movement in pointer or gauge  128  is thereby representative of a magnified magnitude of the movement of plate  140 . 
     In some embodiments, handle  106  and balancer  124  can be snap fit together to assemble balancer device  100 , as depicted in  FIG. 8 . For example, extension  134  ( FIG. 8 , far left) can be sized to have a width substantially equal to flexible beam  130 , such that extension  134  and flexible beam  130  can both slide in between the gap space between opposing studs  146 . Handle  106  ( FIG. 8 , middle left) can have an axle slot  122 , an axle casing  116 , and a housing  108  having open superior ends  102 , enabling balancer  124  to be inserted into handle  106  from a superior direction ( FIG. 8 , middle right). In some embodiments, balancer device  100  can further include cap  150  ( FIG. 8 , far right) to close off the superior ends  102  of axle casing  116  and housing  108 . 
     Referring now to  FIG. 9 , an exemplary balancer device  200  is depicted. Balancer device  200  comprises a housing  206  with an anterior end  202  and a posterior end  204 . Housing  206  comprises a window  208  at its anterior end  202  sized to fit a pointer or gauge  232 . Housing  206  may further comprise a scale  210  positioned at its anterior end  202  that provides a graded measure of any suitable unit adjacent to window  208 . For example, the scale can include at least one of force units, distance units, angle degrees, or unitless markings. Axle casing  212  extends posteriorly from posterior end  204  of housing  206  and connects at its posterior end to plate  216 . Axle casing  212  comprises a lumen extending from housing  206  to an open end adjacent to plate  216 . Plate  216  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. Plate  216  has an upper surface that interfaces with the lower surface of plate  220 , such that a flat upper surface of plate  216  fits with a flat lower surface of plate  220 , and a curved upper surface of plate  216  fits with a curved lower surface of plate  220 . In some embodiments, plate  216  has a flat lower surface. In other embodiments, plate  216  has a contoured lower surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a tibia in a knee joint. Plate  216  includes axle slot  218  positioned opposite from the open end of axle casing  212  and aligned with the lumen of axle casing  212 . 
     Plate  220  rests on the upper surface of plate  216  and is connected to the posterior end of axle  224 . Axle  224 , aligned along axis  214 , extends anteriorly through the lumen of axle casing  212  into housing  206 . Plate  220  comprises axle tip  222  positioned opposite from axle  224  and aligned with axis  214 . Plate  220  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. In some embodiments, plate  220  has a flat upper surface. In other embodiments, plate  220  has a contoured upper surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a femur in a knee joint. In some embodiments, the lower surface of plate  220  has springs positioned opposite from each other to the left and to the right of axle  224 . The springs can be any suitable spring, such as a coil spring, a conical spring, a wave spring, a leaf spring, and the like. In some embodiments, the lower surface of plate  220  has a support formed by two downward sloping surfaces meeting at a middle edge resting against the top surface of plate  216 . In various embodiments, the lower surface of plate  220  and the upper surface of plate  216  are maintained in parallel alignment at rest with a gap height in between. Plate  220  and plate  216  can be separated by any suitable height, such as a height between about 1 and 5 mm. In some embodiments, the height between plate  82  and plate  78  is 2.5 mm. 
     Balancer device  200  further comprises an elongate beam  228  having notch  229  at its posterior end  204  and pointer or gauge  232  positioned at its anterior end  202 . Beam  228  is secured to the interior of housing  206  at pin  230  and is rotatable about pin  230  along pin axis  231 . In some embodiments, pin  230  is movable along housing  206  and fits within an elongate slot within beam  228 , enabling pin  230  to adjust the magnification of movement between the posterior end  204  of beam  228  and pointer or gauge  232 . Axle  224  comprises table  226  at its anterior end  202  that seats within notch  229  of beam  228 . In some embodiments, beam  228  further comprises spacer  234  to maintain the alignment between beam  228  and tab  226  of axle  224 . Balancer device  200  is thereby able to provide a reading of the relative difference in force between the lateral and medial sides of a joint by translating displacement of plate  220  into a proportional movement of pointer or gauge  232 . The displacement can be an angular displacement, a rotation, or a vertical displacement. 
     While joints attached to a body as a point of reference can have medial and lateral sides, for the sake of describing the function of balancer device  200  in an example, a hypothetical joint being measured by balancer device  200  has a left and a right side relative to the orientation of the device in  FIG. 9 . An imbalance in a hypothetical joint leads to a relative difference in force that is greater on the right side, which depresses the right side of plate  220  and causes axle  224  to rotate in a clockwise direction along axis  214 , such as up to 5 degrees or up to 2.5 degrees. The rotation of axle  224  in a clockwise direction pivots tab  226  in a clockwise direction to the left, which pushes notch  229  and the posterior end  204  of beam  228  laterally to the left. Beam  228  rotates about pin axis  231 , causing its anterior end  202  and pointer or gauge  232  to move laterally to the right. Movement in pointer or gauge  232  is thereby representative of a magnified magnitude of the movement of plate  220 . 
     Referring now to  FIG. 10 , an exemplary balancer device  300  is depicted. Balancer device  300  comprises a housing  306  with an anterior end  302  and a posterior end  304 . Housing  306  comprises a window  308  at its anterior end  302  wherein the anterior ends of long beam  318   a  and long beam  318   b  are visible. Housing  306  may further comprise a scale  310  positioned at its anterior end  302  that provides a graded measure of any suitable unit adjacent to window  308 . For example, the scale can include at least one of force units, distance units, angle degrees, or unitless markings. Housing  306  connects at its posterior end  304  to lower plate  312 , upper plate  314   a , and upper plate  314   b . Lower plate  312  can have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. Lower plate  312  can have a substantially flat lower surface or a contoured lower surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a tibia in a knee joint. Upper plate  314   a  and upper plate  314   b  can each have any suitable shape, such as a substantially planar shape sized to fit within a joint space between two connected bones. In some embodiments, upper plate  314   a  and upper plate  314   b  each has a flat upper surface. In other embodiments, upper plate  314   a  and upper plate  314   b  each has a contoured upper surface having a shape that fits in a joint space, such as a shape that fits with the condyles of a femur in a knee joint. Upper plate  314   a  and upper plate  314   b  are cantilevered over lower plate  312  and are maintained in parallel alignment by gap  316 . Gap  316  can have any suitable height, such as a height between about 1 and 5 mm. In some embodiments, the height between plate  82  and plate  78  is 2.5 mm. 
     Balancer device  300  further comprises long beam  318   a  adjacent to long beam  318   b , each positioned within the hollow interior of housing  306 . Long beam  318   a  and long beam  318   b  each rest on fulcrum  322  positioned on the interior of housing  306 . In some embodiments, fulcrum  322  is movable along housing  306  to adjust the magnification of movement between the posterior end  304  and the anterior end  302  of long beam  318   a  and long beam  318   b . Long beam  318   a  and long beam  318   b  are each connected to short beam  320   a  and short beam  320   b , respectively, at their posterior ends  304 . Short beam  320   a  and short beam  320   b  are each positioned underneath upper plate  314   a  and upper plate  314   b , respectively. Balancer device  300  is thereby able to provide a reading of the relative amount of force applied on the lateral and medial sides of a joint by translating displacement of upper plate  314   a  and upper plate  314   b  into a proportional movement of the anterior ends  302  of long beam  318   a  and long beam  318   b.    
     While joints attached to a body as a point of reference can have medial and lateral sides, for the sake of describing the function of balancer device  300  in an example, a hypothetical joint being measured by balancer device  300  has a left and a right side relative to the orientation of the device in  FIG. 10  (bottom). A hypothetical joint applies forces to upper plate  314   a  and upper plate  314   b  (which in this case is greater on the right side and lesser on the left side), which displaces upper plate  314   b  and short beam  320   b  a in a greater inferior direction than upper plate  314   a  and short beam  320   a . The inferior displacement of short beam  320   a  and short beam  320   b  also displaces the posterior ends  304  of long beam  318   a  and long beam  318   b  in an inferior direction, respectively, causing the anterior ends  302  of long beam  318   a  and long beam  318   b  to be displaced in a superior direction due to fulcrum  322 . The greater inferior displacement of short beam  320   b  is translated to a proportionally greater superior displacement of the anterior end  302  of long beam  318   b , and the lesser inferior displacement of short beam  320   a  is translated to a proportionally lesser superior displacement of the anterior end  302  of long beam  318   a . Movement in the anterior ends  302  of long beam  318   a  and long beam  318   b  is thereby representative of a magnified magnitude of the movement of upper plate  314   a  and upper plate  314   b , respectively. 
     Referring now to  FIG. 11 , an exemplary balancer device  400  is depicted. Balancer device  400  is similar to balancer device  300  in many respects, including a similar housing  406  having an anterior end  402  and a posterior end  404 , a window  408 , a scale  410 , and long beam  416   a  and long beam  416   b  each resting on a fulcrum (not visible) and connected at their posterior ends  404  to short beams (not visible). Balancer  400  further employs a lower plate  412  that is covered by membrane  414 . Balancer device  400  functions similarly to balancer device  300 , wherein displacement of the left or right side of membrane  414  corresponds to an inferior displacement of a corresponding short beam and a superior displacement of the anterior end  402  of a corresponding long beam  416   a  and/or long beam  416   b.    
     While exemplary balancer devices of the present invention are described above, the balancer devices are nonetheless amenable to any suitable modification to augment their function. In various embodiments, the dimensions of the plates of the balancer devices described elsewhere herein are amenable to adjustment. For example, in certain embodiments, the plates can receive one or more additional spacer plates having substantially similar planar shapes with different heights, such that the one or more additional spacer plates can be attached to the upper surface and lower surfaces of the plates to increase the height of each plate. In other embodiments, the plates can be removable and replaceable with plates having different planar shapes or different heights. In various embodiments, the one or more additional spacer plates and the replaceable plates can be used to adjust the contours of the plates to improve the fit between the balancer devices and a joint of interest. In various embodiments, the upper surface of the upper plates are shallow, such that the femoral component of any condylar replacement artificial knee can locate at two separate points. 
     In various embodiments, the several components of the assorted balancer devices can be combined and rearranged without altering their function to accommodate different orientations and configurations. For example, exemplary balancer devices described herein have vertically oriented handles, but it should be understood that embodiments having horizontally oriented handles, handles in line with the axles and axle casings, and variously angled and dimensioned handles are contemplated. 
     The components of the balancer devices of the present invention can be made using any suitable method known in the art. The method of making may vary depending on the materials used. For example, components substantially comprising a metal may be milled from a larger block of metal or may be cast from molten metal. Likewise, components substantially comprising a plastic or polymer may be milled from a larger block, cast, or injection molded. In some embodiments, the components may be made using 3D printing or other additive manufacturing techniques commonly used in the art. 3D printing enables the entirety of the device to be printed in a single session with a brief post-processing treatment that removes sacrificial support structures between movable parts. In some embodiments, the materials can withstand commonly used sterilization techniques, enabling the devices to be reusable. In other embodiments, inexpensive methods permit the devices to be single-use and disposable. 
     Method of Use 
     The present invention also includes methods of using the balancer devices described herein to balance a joint. Referring now to  FIG. 12 , an exemplary method  500  of balancing a joint is depicted. Method  500  begins with step  502 , wherein a balancer device having a handle, a gauge, a lower plate, and an upper plate, wherein the upper plate is mechanically linked to the gauge such that the gauge displays a magnitude of displacement of the upper plate is provided. In step  504 , the balancer device is inserted into a joint in need of balancing such that the upper and lower plates rest against opposing surfaces of the joint. In step  506 , the joint is flexed through at least part of its full range of motion. In step  508 , the magnitudes of displacement between left and right sides of the joint indicated by the gauge are recorded throughout the flexing of the joint. In step  510 , the balance of the joint is modified to reduce or eliminate the magnitudes of displacement between left and right sides of the joint. 
     The joint can be any suitable joint, including but not limited to knee joints and elbow joints. The opposing surfaces of the joint can thereby be opposing bone surfaces of the joint, such as the femoral condyles and the tibial condyles of the knee. In certain embodiments, the balancer devices are useful in balancing joint replacements. In some embodiments, the balancer devices are useful in balancing total knee replacements. The opposing surfaces of the joint can thereby be opposing implant surfaces of the joint replacement, such as the trial femoral condyles and the trial tibial plateau of a replacement knee, as well as any opposing bone surfaces of the joint that contact the implant surfaces. The balance of the joint can be modified in any suitable manner. For example, the opposing surfaces may be trimmed or raised to balance any relative differences in force between the left and right sides of the joint. In certain joint replacements, the joining of the opposing surfaces can be modified by adjusting a spacer component that fits between the opposing surfaces. The gauge readings indicate whether a force is greater on the left or the right side of the joint, and the magnitudes of displacement indicate how much greater the force is. 
     In some embodiments, steps  502  through step  510  are performed in the order recited. In some embodiments, step  504  through step  508  may be repeated to verify that the modification performed in step  510  correctly balanced the joint. If the repeated step  508  indicates that the magnitudes of displacement are satisfactorily reduced or eliminated, then the joint is substantially balanced. If the repeated step  508  indicates that unsatisfactorily high magnitudes of displacement are still present, step  510  may be repeated. In some embodiments, step  504  through step  510  may be repeated until the magnitudes of displacement are satisfactorily reduced or eliminated. 
     The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims are intended to be construed to include all such embodiments and equivalent variations.