Abstract:
Braces for applying dynamic forces and methods for applying dynamic forces on limbs are disclosed herein. In one embodiment, a brace has an upper frame, a lower frame, a hinge coupling the upper frame to the lower frame, a hydraulic pump, and a bladder. The upper frame is moveable relative to the lower frame. The hydraulic pump is coupled to the hinge, the upper frame or the lower frame. The hydraulic pump generates a fluid flow that flows into and expands the bladder. The bladder is positioned to apply a force to the limb as it expands.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of provisional U.S. Patent Application Ser. No. 60/484,050, filed Jun. 30, 2003, which is hereby incorporated by reference in its entirety. 

   TECHNICAL FIELD 
   The present invention relates to knee braces with dynamic counterforces. 
   BACKGROUND 
   Investigations have shown that the anterior cruciate ligament (ACL) is rarely lax and experiences the greatest tension at full-extension. If an ACL is torn, or if for some other reason the knee encounters an anterior instability, the knee requires support at full-extension. Conventional knee braces provide support to the tibial condyles by applying a force with a static strap or a rigid frame. 
   Application of a force with a static strap or a rigid frame has several disadvantages. First, the static strap and rigid frame spread the force over a relatively large amount of generally soft tissue. Second, if the static strap or rigid frame were to produce a sustained, concentrated force on the tibial tuberosity, it would be extremely uncomfortable for the wearer over time. Consequently, the wearer would likely loosen the straps and thereby eliminate the necessary supportive force. 
   Moreover, studies have shown that the posterior cruciate ligament (PCL) prevents posterior displacement of the tibia on the femur and prevents hyperextension at the knee joint. These studies also indicate that the PCL is taut at full-extension, becomes progressively more lax until 30 degrees of flexion, and thereafter becomes increasingly more tense until it reaches a maximum tension at 130 degrees of flexion. When the PCL is torn, a posterior instability can result that produces a greater susceptibility to further weaken or tear the PCL due to hyperextension. 
   Typical knee braces attempt to prevent hyperextension through range of motion stops. It is difficult, however, to prevent hyperextension using range of motion stops because of the soft tissue in and around the knee. The range of motion stops stop the rigid frame from rotating, but the soft tissue in the knee can give, allowing hyperextension. 
   SUMMARY 
   The present invention is directed toward braces for applying dynamic forces and methods for applying dynamic forces on limbs. In one embodiment of the invention, a brace has an upper frame, a lower frame, a hinge coupling the upper frame to the lower frame, a hydraulic pump, and a bladder. The upper frame is moveable relative to the lower frame. The hydraulic pump is coupled to the hinge, the upper frame, or the lower frame. The hydraulic pump also generates a fluid flow that flows into and expands the bladder. The bladder is positioned to apply a force to the limb as it expands. 
   An embodiment for applying the dynamic force on the limb includes placing the bladder proximate to the limb and filling the bladder at least partially with the fluid to expand the bladder. The method further includes exerting the force on the limb as the bladder expands and draining the bladder at least partially to at least partially release the force. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a knee brace with a hinge in accordance with one embodiment of the invention. 
       FIG. 2  is an exploded view of a plate, a first hinge member, and a second hinge member of the hinge of  FIG. 1 . 
       FIG. 3A  is a top plan view of an assembly including a resilient member with the plate, the first hinge member, and the second hinge member of  FIG. 2 . 
       FIG. 3B  is a top plan view of an assembly including a first torsion spring attached to a first hinge member and a second torsion spring attached to a second hinge member in accordance with another embodiment of the invention. 
       FIG. 4  is a top plan view of first and second adjustable range restrictors. 
       FIG. 5A  is a top plan view of an adjustable range restrictor system in accordance with one embodiment of the invention. 
       FIG. 5B  is an isometric view of the adjustable range restrictor system of  FIG. 5A  with the first and second adjustable range restrictors removed from a cover plate. 
       FIG. 6  is an isometric exploded view of a hinge and range restrictor in accordance with an embodiment of the invention. 
       FIGS. 7A–7C  are top plan views illustrating a hinge with a rocker in accordance with another embodiment of the invention. 
       FIG. 8  is an isometric exploded view of a power pack in accordance with one embodiment of the invention. 
       FIG. 9  is an isometric exploded view of a hinge having a power pack, a first hinge member, and a second hinge member in accordance with one embodiment of the invention. 
       FIGS. 10A–10C  are top plan views of a power pack attached to a first hinge member and a second hinge member in accordance with another embodiment of the invention. 
       FIG. 11  is an isometric exploded view of a power pack having valves to control the fluid flow in accordance with another embodiment of the invention. 
       FIG. 12  is a front view of a knee brace having a bladder to exert a force on the tibial tuberosity in accordance with one embodiment of the invention. 
       FIG. 13  is a partial cross-sectional view taken substantially along line  13 — 13  of  FIG. 12 . 
       FIGS. 14A and 14B  are partial cross-sectional views illustrating the bladder empty and filled with fluid. 
       FIG. 15  is a rear isometric view of the knee brace with the bladder positioned to exert a posterior force on the femur in accordance with one embodiment of the invention. 
       FIG. 16  is a partial cross-sectional view taken substantially along line  16 — 16  of  FIG. 15 . 
       FIG. 17  is a side view of a knee brace with a lower frame having a front portion and a rear portion connected by a strap in accordance with another embodiment of the invention. 
       FIGS. 18A and 18B  are partial side cross-sectional views illustrating the strap in the elongated and contracted positions. 
   

   DETAILED DESCRIPTION 
   The following disclosure describes several embodiments of braces with dynamic counterforces and methods for applying counterforces to limbs. Many specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1–18  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the invention may have additional embodiments or that the invention may be practiced without several of the details described in the following description. For example, even though many embodiments of the braces with dynamic counterforces are described with reference to a knee brace, they can also be used in elbow braces or other braces. 
     FIG. 1  is a side view of a knee brace  60  including an upper frame  30 , a lower frame  32 , and hinges  10  connecting the upper frame  30  to the lower frame  32 . The upper frame  30  can include at least one strap  20  to wrap around the quadriceps or hamstring, and the lower frame  32  can also include one or more straps. In other embodiments, the upper and lower frames  30  and  32  can have different configurations and include different configurations of straps. For example, the knee brace  60  can also include a flexible, elastic sleeve  62  coupled either directly or indirectly to the upper and lower frames  30  and  32 . 
     FIG. 2  is an exploded view and  FIG. 3A  is a top plan view of one embodiment of the hinge  10 . In this embodiment, the hinge  10  includes a back plate  200 , a first hinge member  260 , and a second hinge member  261 . The first hinge member  260  rotatably mounts to the back plate  200  and is configured to attach to the upper frame  30  ( FIG. 1 ) to permit the upper frame  30  to pivot relative to the back plate  200 . The second hinge member  261  also rotatably mounts to the back plate  200  and is configured to attach to the lower frame  32  ( FIG. 1 ) to permit the lower frame  32  to pivot relative to the back plate  200  independently of the upper frame  30 . Accordingly, the upper and lower frames  30  and  32  pivot independently about two different axes of rotation. 
   Referring to  FIG. 2 , the first hinge member  260  is a generally flat plate with a front surface  266  and a back surface (not shown) opposite the front surface  266 . Between the front surface  266  and the back surface are a top edge  276 , a bottom edge  274  and a side edge  272  configured for attachment to a portion of the upper frame  30 . For example, the first hinge member  260  can include two apertures  262  and  264  proximate to the side edge  272  for receiving fasteners (not shown) to connect the upper frame  30  to the first hinge member  260 . The second hinge member  261 , similarly, has a front surface  267  and a back surface (not shown) opposite the front surface  267 . Between the front surface  267  and the back surface are a top edge  277 , a bottom edge  275  and a side edge  273  configured for attachment to a portion of the lower frame  32 . The second hinge member  261  can also include two apertures  263  and  265  proximate to the side edge  273  for receiving fasteners (not shown) to connect the lower frame  32  to the second hinge member  261 . In additional embodiments, the first hinge member  260  can be an integral portion of the upper frame  30  and the second hinge member  261  can be an integral portion of the lower frame  32 . The first and second hinge members  260  and  261  can have different configurations in other embodiments. 
   Referring to  FIGS. 2 and 3A  together, the first hinge member  260  is pivotally connected to the back plate  200  by a fastener  320 . The first hinge member  260  rotates relative to the back plate  200  about a first axis of rotation A 1  ( FIG. 3A ). The first hinge member  260  has a pin  252  that projects from the front surface  266  and the back surface. In additional embodiments, the pin  252  can have a different configuration or shape. For example, the pin  252  can extend or project from either the front surface  266  or the back surface. The portion of the pin  252  projecting from the back surface is received within an annular slot  220  in the back plate  200 . The annular slot  220  is accordingly centered about the first axis of rotation A 1  with a centerline at a radius R 1  corresponding to the distance from the first axis of rotation A 1  to the pin  252 . Accordingly, as the first hinge member  260  rotates relative to the back plate  200  about the first axis of rotation A 1 , the pin  252  slides in the annular slot  220 . A first endpoint  224  and a second endpoint  226  of the slot  220  define the maximum range of motion for the first hinge member  260 . Accordingly, the length of the slot  220  determines the pivoting range of the first hinge member  260  relative to the back plate  200 . In additional embodiments, the slot  220  can have different lengths to change the pivoting range of the first hinge member  260 . In other embodiments, the position of the slot  220  and the pin  252  can be different, such as the slot  220  can be in the first hinge member  260  and the pin  252  can be attached to the back plate  200 . 
   The second hinge member  261  is pivotally connected to the back plate  200  by a fastener  322 . The second hinge member  261  rotates relative to the back plate  200  about a second axis of rotation A 2  ( FIG. 3A ). The second hinge member  261  has a pin  253  that projects from the front surface  267  and the back surface. In additional embodiments, the pin  253  can have a different configuration or shape. For example, the pin  253  can extend or project from either the front surface  267  or the back surface, or there can be two separate pins with one extending from each surface. The portion of the pin  253  projecting from the back surface is received within an annular slot  222  in the back plate  200 . The annular slot  222  is accordingly centered about the second axis of rotation A 2  with a centerline at a radius R 2  corresponding to the distance from the second axis of rotation A 2  to the pin  253 . As the second hinge member  261  rotates relative to the back plate  200  about the second axis of rotation A 2 , the pin  253  slides in the annular slot  222 . A first endpoint  225  and a second endpoint  227  of the slot  222  define the maximum range of motion for the second hinge member  261 . The length of the slot  222  determines the pivoting range of the second hinge member  261  relative to the back plate  200 . In additional embodiments, the slot  222  can have a different length to change the pivoting range of the second hinge member  261 . In other embodiments, the position of the slot  222  and the pin  253  can be different, such as the slot  222  can be in the second hinge member  261  and the pin  253  can be attached to the back plate  200 . 
   Referring to  FIG. 3A , the curved edge  270  on the first hinge member  260  is spaced away from the curved edge  271  on the second hinge member by a gap G. Accordingly, the first hinge member  260  and the second hinge member  261  pivot independently about the two different axes of rotation A 1  and A 2 . Because the hinge has two different and independent axes of rotation, it better simulates the natural motion of the knee joint. This is expected to mitigate the sliding of the knee brace down the leg and reduce the exertion of unnatural forces against the knee joint. 
   In the illustrated embodiment, the back plate  200  has a cutout portion  250 . 
   The cutout portion  250  allows the first and second hinge members  260  and  261  to rotate through the full pivoting range without the upper and lower frames  30  and  32  ( FIG. 1 ) striking the back plate  200 . 
   In the illustrated embodiment, the first hinge member  260  and the second hinge member  261  are operatively coupled by a resilient member  300 . The resilient member  300  has a first end  302  attached to the first hinge member  260  and a second end  304  attached to the second hinge member  261 . The first end  302  is received within an aperture  282  in the first hinge member  260 . A channel  284  connects the aperture  282  to an edge  268  and is sized to receive a portion of the resilient member  300 . Similarly, the second end  304  of the resilient member  300  is received within an aperture  283  of the second hinge member  261 . A channel  285  connects the aperture  283  to the edge  277  and is sized to receive a portion of the resilient member  300 . The first end  302  and the second end  304  of the resilient member  300  are enlarged so that they are not pulled through the smaller channels  284  and  285 . In one embodiment, the first end  302  and the second end  304  of the resilient member  300  have a donut shape with a pin in the center. In other embodiments, the first end  302  and second end  304  of the resilient member  300  can be clamped or bonded. 
   The resilient member  300  is elastic and provides resistance to the hinge members  260  and  261  during flexion. In one embodiment, urethane can be used; in other embodiments other materials may be used. The resilient member  300  stretches as the first hinge member  260  rotates in a direction D 1  and/or the second hinge member  261  rotates in a direction D 2 . The resilient member  300  urges the first hinge member  260  to rotate in a direction D 3  and the second hinge member  261  to rotate in a direction D 4 . Accordingly, when no external force is placed on the first and second hinge members  260  and  261 , the pins  252  and  253  are drawn toward the first endpoints  224  and  225  of the slots  220  and  222 . When an external force is applied to the first hinge member  260  causing rotation in the direction D 1 , the resilient member  300  stretches elastically and rides along a curved edge  270  of the first hinge member  260 . In the illustrated embodiment, the curved edge  270  has a radius R 3 . In one embodiment, the curved edge  270  may not have a constant radius. Similarly, when an external force is applied to the second hinge member  261  causing rotation in the direction D 2 , the resilient member  300  stretches elastically and rides along a curved edge  271  of the second hinge member  261 . In the illustrated embodiment, the curved edge  271  has a radius R 4  that is greater than the radius R 3 . In additional embodiments, the radius R 3  can be equal to or greater than the radius R 4 . 
   The resilient member  300  and the radii of the hinge members  260  and  261  operate together to control the rotation of the hinge members  260  and  261 . For example, when R 3  is less than R 4 , the first hinge member  260  rotates in direction D 1  for an arc length before the second hinge member  261  rotates in direction D 2  for an arc length. This is because a greater external force must be applied to rotate a member with a greater radius in light of the counter force applied by the resilient member  300 . Accordingly, in the illustrated embodiment, when an external force is applied to the hinge  310 , the first hinge member  260  rotates first because its radius R 3  is less than the radius R 4  of the second hinge member  261 . The second hinge member  261  will begin to rotate after the pin  252  of the first hinge member  260  has rotated through at least a portion of its range of motion. The rotation of one hinge member before the rotation of the other hinge member simulates the natural anatomical motion of the knee joint during extension and flexion. A better simulation of the natural motion of the knee joint reduces the movement of the knee brace down the leg of the user and the tendency of the knee brace to force the knee into unnatural positions. 
     FIG. 3B  is a top plan view of an assembly including a first torsion spring  398  attached to a first hinge member  360  and a second torsion spring  399  attached to a second hinge member  361  in accordance with another embodiment of the invention. Each torsion spring  398  and  399  is also attached to the back plate  200 . The first torsion spring  398  urges the first hinge member  360  to rotate in the direction D 3  and the second torsion spring  399  urges the second hinge member to rotate in the direction D 4 . Accordingly, when no external force is placed on the first and second hinge members  360  and  361 , the pins  252  and  253  are drawn toward the first endpoints  224  and  225  of the slots  220  and  222 . In one embodiment, the torsion springs can have different spring coefficients causing one hinge member to rotate before the other. 
     FIG. 4  is a top plan view of the hinge  310  of  FIG. 3A  with first and second adjustable range restrictors  402  and  404 .  FIG. 5A  is a top plan view of an adjustable range restrictor system  406  in accordance with one embodiment of the invention.  FIG. 5B  is an isometric view of the adjustable range restrictor system  406  of  FIG. 5A  with the first and second adjustable range restrictors  402  and  404  removed from a housing  540 . As explained in more detail below, the adjustable range restrictor system  406  allows a user to adjust the pivoting range of the first hinge member  260  and/or the second hinge member  261 . 
   Referring to the illustrated embodiment in  FIG. 4 , the fastener  320  is received in an aperture  432  of the first adjustable range restrictor  402  so that the first adjustable range restrictor  402  is positionable about the first axis of rotation A 1 . The first adjustable range restrictor  402  has an annular slot  422  extending about the first axis of rotation A 1  with a centerline at the radius R 1 . The slot  422  is positioned and sized to receive the pin  252  of the first hinge member  260 . Accordingly, when the first hinge member  260  pivots, the pin  252  moves within the slot  422 . Similarly, the fastener  322  is received in an aperture  430  of the second adjustable range restrictor  404  so that the second adjustable range restrictor  404  is positionable about the second axis of rotation A 2 . The second adjustable range restrictor  404  has an annular slot  420  extending about the second axis of rotation A 2  with a centerline at the radius R 2 . The slot  420  is positioned and sized to receive the pin  253  of the second hinge member  261 . Accordingly, when the second hinge member  261  pivots, the pin  253  moves within the slot  420 . In the illustrated embodiment, the length of the slot  420  is approximately equal to the length of the slot  222 , and the length of the slot  422  is approximately equal to the length of the slot  220 . In other embodiments, the slots  420  and  422  can have different lengths. 
   The first and second adjustable range restrictors  402  and  404  can be rotated so that their slots  422  and  420  limit the rotation of the first and second hinge members  260  and  261 . For example, referring to the embodiment in  FIG. 4 , the first adjustable range restrictor  402  is positioned so that the slot  422  is offset from the slot  220  of the first hinge member  260 . Consequently, a first endpoint  424  of the slot  422  and the second endpoint  226  of the slot  220  define stops for the pin  252  to limit the rotation of the first hinge member  260  about the first axis of rotation A 1 . The first adjustable range restrictor  402  can be rotated further in the direction D 1  to further limit the rotation of the first hinge member  260 . Conversely, the first adjustable range restrictor  402  can be rotated in the direction D 3  to increase the range of rotation. The second adjustable range restrictor  404  can similarly be positioned about the second axis of rotation A 2  so that the slot  420  is offset from the slot  222  to define stops for the pin  253  that limit the rotation of the second hinge member  261  about the second axis of rotation A 2 . 
   The adjustable range restrictors  402  and  404  are held in place by the housing  540 . Referring to  FIGS. 5A and 5B , at least a portion of the outer edge  442  of the first adjustable range restrictor  402  has teeth  412 , and the outer edge  440  of the second adjustable range restrictor  404  also has teeth  414 . The housing  540  has a recess  570  with teeth  550  that engage the teeth  412  and  414  of the first and second adjustable range restrictors  402  and  404 . When the housing  540  is attached to a front plate  400  ( FIG. 4 ), the teeth  550  preclude the first and second adjustable range restrictors  402  and  404  from rotating about the first and second axes of rotation A 1  and A 2 . The housing  540 , for example, can have a lip  560  that snap-fits onto the front plate  400  to lock the first and second range restrictors  402  and  404  in the desired positions for limiting the range of motion. The first and second adjustable range restrictors  402  and  404  are rotatably adjusted by removing the housing  540 , rotating the first and second adjustable range restrictors  402  and  404 , and replacing the housing  540 . The configuration of the teeth  412 ,  414  and  550  in the illustrated embodiment permits the first and second adjustable range restrictors  402  and  404  to be adjusted in 10-degree increments. In additional embodiments, the teeth  412 ,  414  and  550  can be sized and spaced differently. 
   One advantage of the embodiment of the range restrictor system  406  shown in  FIGS. 4–5B  is the ease with which a user can adjust the pivoting range of the first and second hinge members  260  and  261 . It will be appreciated that the range restrictor system  406  can have other configurations. For example, in additional embodiments, other types of devices can be used to restrict the first and second adjustable range restrictors  402  and  404  from rotating about the first and second axes of rotation A 1  and A 2 . In one such embodiment, the front plate  400  could have a projection with teeth that engage the teeth of one or both of the adjustable range restrictors  402  and  404 , thus eliminating the need for the housing  540 . In the illustrated embodiment, the front plate  400  is similar to the back plate  200 , but is positioned on the other side of the hinge members  260  and  261 . In still other embodiments, the front plate  400  can have a different configuration, or the hinge may not have the front plate  400 . In further embodiments, the first and second adjustable range restrictors  402  and  404  can be placed proximate to the first and second hinge members  260  and  261 , or the adjustable range restrictor system  406  can be placed adjacent to the back surface of the back plate  200 . In additional embodiments, the hinge may not have the adjustable range restrictor system  406 . 
     FIG. 6  is an exploded view of the hinge  10  of  FIG. 1 . In the illustrated embodiment, the first and second hinge members.  260  and  261  are held between the back plate  200  and the front plate  400  by the fasteners  320  and  322 . The hinge  10  can have spacers  600 ,  620 ,  630  and  632  to assist the first and second hinge members  260  and  261  to rotate more easily between the plates  400  and  200 . The spacers  600  and  630  have an aperture  604  through which the fastener  320  is placed, and an aperture  602  through which the first pin  252  is placed. Similarly, the spacers  620  and  632  have an aperture  624  through which the fastener  322  is placed, and an aperture  622  through which the second pin  253  is placed. In additional embodiments, the spacers  600 ,  620 ,  630  and  632  can have different configurations, or the hinge  10  may not have one or more of the spacers  600 ,  620 ,  630  and  632 . The range restrictor system  406  attaches to the front plate  400  as explained above. 
     FIG. 6  also illustrates the compactness of the hinge  10  and the range restrictor system  406 . The hinge  10  and the range restrictor system  406  together can have a thickness of between 0.125 inch and 1 inch. In one embodiment, the hinge  10  and the range restrictor system  406  together have a thickness of approximately 0.31 inch. The compact size of the hinge  10  and the range restrictor system  406  makes it easier to wear clothes over the knee brace and reduces the risk of the hinge interfering with the other knee joint during activities. 
     FIGS. 7A–7C  are top plan views illustrating a hinge  710  in accordance with another embodiment of the invention. The hinge  710  is similar to the hinge  10  described above, and like reference numbers refer to like components in  FIGS. 1–7C . In the illustrated embodiment, the hinge  710  includes a first hinge member  660  with a first recess  662  and a second hinge member  661  with a second recess  663 . The first and second hinge members  660  and  661  are pivotally coupled to the back plate  200 . Referring to  FIG. 7A , the pin  252  of the first hinge member  660  is positioned at the first endpoint  224  of the slot  220  in the back plate  200 , and the pin  253  of the second hinge member  661  is positioned at the first endpoint  225  of the slot  222  in the back plate  200 . The hinge  710  also includes a rocker  650  attached to the back plate  200 . The rocker  650  has a flexible arm  698  and a head  697  positioned between the first hinge member  660  and the second hinge member  661 . 
   When the hinge  710  is in the full-extension position shown in  FIG. 7A , the head  697  is proximate to a curved edge  670  of the first hinge member  660  and at least partially within the second recess  663  of the second hinge member  661 . Because the head  697  of the rocker  650  is at least partially within the second recess  663  of the second hinge member  661 , the second hinge member  661  is effectively jammed and restricted from movement. Accordingly, a force applied to either hinge member  660  or  661  will cause the first hinge member  660  to pivot in a direction S 1  about the first axis of rotation A 1 . 
   Referring to  FIG. 7B , the first hinge member  660  has pivoted about the first axis of rotation A 1  to a position where the pin  252  is at the second endpoint  226  of the slot  220  in the back plate  200 . The first hinge member  660  accordingly cannot pivot further about the first axis of rotation A 1  in the direction S 1 . In this position, the head  697  of the rocker  650  is received at least partially within the first recess  662  of the first hinge member  660 , releasing the bending force on the arm  698 . In this position the head  697  is free to move between the two recesses  662  and  663 . As the second hinge member  261  begins to rotate about the second axis of rotation A 2 , the cam shape of the surface  671  forces the head  697  of the rocker  650  into the first recess  662  of the first hinge member  660 , effectively jamming and precluding rotation of the first hinge member  660  about the first axis of rotation A 1 . 
   Referring to  FIG. 7C , the second hinge member  661  has pivoted about the second axis of rotation A 2  to a position where the pin  253  is at the second endpoint  227  of the slot  222  in the back plate  200 . The second hinge member  661  accordingly cannot pivot further about the second axis of rotation A 2  in the direction S 2 . Throughout the rotation of the second hinge member  661  from the position in  FIG. 7B  to the position in  FIG. 7C , the head  697  of the rocker  650  remains in the first recess  662  of the first hinge member  660  precluding the first hinge member  660  from pivoting about the first axis of rotation A 1 . Because the head  697  of the rocker  650  is at least partially within the first recess  662  of the first hinge member  660 , the first hinge member  660  requires a greater force to rotate in a direction S 3  than the force required for the second hinge member  661  to rotate in a direction S 4 . Accordingly, the rocker  650  encourages the second hinge member  661  to pivot in the direction S 4  about the second axis of rotation A 2  before the first hinge member  660  pivots in the direction S 3  about the first axis of rotation A 1 . In additional embodiments, the hinge  710  can have a rocker with a different configuration, or the hinge may not have a rocker. Furthermore,  FIGS. 7A–7C  illustrate the full range of extension ( FIGS. 7A–B ) and flexion ( FIGS. 7B–C ) of the illustrated embodiment. Other embodiments can also have this range of extension and flexion without the rocker  650  or other components. 
     FIG. 8  is an isometric exploded view of a power pack  800  in accordance with one embodiment of the invention that can be used with embodiments of the hinges  10  and  710  described above, and also with other types of single axis or bicentric hinges. In the illustrated embodiment, the power pack  800  includes a first piston  810 , a second piston  830 , and a housing  802  having a front portion  850  and a rear portion  860 . The first piston  810  in the embodiment shown in  FIG. 8  is a rotary piston that is received within an upper cavity  870  in a front side  866  of the rear portion  860  of the housing  802 , and a similar cavity (not shown) in the backside (not shown) of the front portion  850  of the housing  802 . Similarly, the second piston  830  in the embodiment shown in  FIG. 8  is a rotary piston that is received within a lower cavity  880  in the front side  866  of the rear portion  860  of the housing  802  and a similar cavity (not shown) in the backside (not shown) of the front portion  850  of the housing  802 . In other embodiments, the pistons can be linear pistons, and a portion of the pistons can extend outside the housing  802 . 
   The first piston  810  of the illustrated embodiment includes a hub  812 , an arm  814  attached to the hub  812 , and a head  816  attached to a distal portion  815  of the arm  814 . A portion of the hub  812  projects beyond a back surface (not shown) of the arm  814  and is received within an aperture  862  in the rear portion  860  of the housing  802 . Another portion of the hub  812  projects beyond a front surface  822  of the arm  814  and is received within an aperture  852  in the front portion  850  of the housing  802 . The apertures  852  and  862  and the hub  812  are aligned with the first axis of rotation A 1  about which the first piston  810  rotates. The arm  814  of the first piston  810  is received within a channel  872  in the upper cavity  870  of the housing  802 . The channel  872  is sized and configured to permit the arm  814  to pivot about the first axis of rotation A 1 . The head  816  of the piston  810  is received within an annular chamber  874  in the upper cavity  870  of the housing  802  in this embodiment. The annular chamber  874  is sized and configured to permit the head  816  to pivot about the first axis of rotation A 1 . As the first piston  810  rotates about the first axis of rotation A 1 , the head  816  moves through the annular chamber  874  from a position in which a surface  817  on the head  816  contacts a first wall  873  in the chamber  874  to a position in which a top surface  824  on the head  816  contacts a second wall  875  in the chamber  874 . Thus, the first wall  873  and the second wall  875  of the chamber  874  define the stops for the first piston  810 . 
   The second piston  830  of the illustrated embodiment includes a hub  832 , an arm  834  attached to the hub  832 , and a head  836  attached to a distal portion  835  of the arm  834 . A portion of the hub  832  projects beyond a back surface (not shown) of the arm  834  and is received within an aperture  864  in the rear portion  860  of the housing  802 . Another portion of the hub  832  projects beyond a front surface  842  of the arm  834  and is received within an aperture  854  in the front portion  850  of the housing  802 . The apertures  854  and  864  and the hub  832  are aligned with the second axis of rotation A 2  about which the second piston  830  rotates. The arm  834  of the second piston  830  is received within a channel  882  in the lower cavity  880  of the housing  802 . The channel  882  is sized and configured to permit the arm  834  to pivot about the second axis of rotation A 2 . The head  836  of the second piston  830  is received within an annular chamber  884  in the lower cavity  880  of the housing  802  in this embodiment. The annular chamber  884  is sized and configured to permit the head  836  to pivot about the second axis of rotation A 2 . As the second piston  830  rotates about the second axis of rotation A 2 , the head  836  moves through the annular chamber  884  from a position in which a surface  837  on the head  836  contacts a first wall  883  in the chamber  884  to a position in which a top surface  844  on the head  836  contacts a second wall  885  in the chamber  884 . Thus, the first wall  883  and the second wall  885  of the chamber  884  define the stops for the second piston  830 . 
   In the illustrated embodiment, the first and second pistons  810  and  830  are the same size and shape. In additional embodiments, the pistons  810  and  830  can be shaped or configured differently. For example, one piston can have an annular arm with a greater radius than the arm of the other piston, or one piston can have a head with a different size or shape than the head of the other piston. In still other embodiments, the power pack can have only one piston. In the illustrated embodiment, the annular chamber  884  in the upper cavity  870  has a longer arc length than the annular chamber  874  in the lower cavity  880 , and the upper annular channel  882  is bigger than the lower annular channel  872 . These differences in size allow the second piston  830  to pivot further about the second axis of rotation A 2  than the first piston can pivot about the first axis of rotation A 1 . In additional embodiments, the range of pivot and the size of the channels and chambers can be the same. Or alternatively, the first piston  810  can have a greater range of pivot than the second piston  830 . In additional embodiments, the housing may not have a channel, or the channel and chamber can be shaped or configured differently. For example, the chamber can be linear rather than annular. 
   The annular chamber  874  of the upper cavity  870  is configured to receive and hold a fluid (not shown). In one embodiment, the fluid is a mineral oil; in other embodiments, water or hydraulic fluids can be used. The fluid is displaced from the chamber  874  into an upper fluid passageway  876  as the head  816  moves through the annular chamber  874  when the first piston  810  rotates about the first axis of rotation A 1 . The fluid flows from the upper fluid passageway  876  through a side fluid passageway  890  to an outlet  878  that is coupled to a reservoir  896 . Similarly, the annular chamber  884  of the lower cavity  880  is configured to receive and hold the fluid. In the illustrated embodiment, the annular chamber  884  includes a rolling bladder  804 . In other embodiments, both annular chambers can include a sleeve or a bladder, or the chambers may not include either. In the illustrated embodiment, the fluid is displaced from the chamber  884  into a lower fluid passageway  886  as the head  836  moves through the annular chamber  884  when the second piston rotates  830  about the second axis of rotation A 2 . The fluid flows from the lower fluid passageway  886  through the side fluid passageway  890  to the outlet  878 . In additional embodiments, the upper fluid passageway  876  and the lower fluid passageway  886  can remain separate, and each passageway  876  and  886  can have a separate outlet and reservoir. 
   In the illustrated embodiment, the heads  816  and  836  of the first and second pistons  810  and  830  have rectangular cross-sectional shapes to provide more surface area in the small space within the housing  802 . Furthermore, in the illustrated embodiment, the heads  816  and  836  have grooves  818  and  838  to receive seals  820  and  840 . The seals  820  and  840  prevent fluid from leaking into the channels  872  and  884 . In additional embodiment, the heads  816  and  836  can have different cross-sectional shapes such as a circular shape. In other embodiments, the heads  816  and  836  may not have seals or may have different seals. 
     FIG. 9  is an isometric exploded view of the connection between the power pack  800 , a first hinge member  910 , and a second hinge member  920  in accordance with one embodiment of the invention. The first hinge member  910  and the second hinge member  920  can be used in the hinges  10  and  710  described above, or they can be used in different bicentric hinges (including geared or non-geared hinges). In the illustrated embodiment, a first rod  912  couples the first hinge member  910  to the hub  812  of the first piston  810 . The first rod  912  is received within an aperture  914  in the first hinge member  910  and an aperture  930  in the hub  812  of the first piston  810 . Similarly, a second rod  922  couples the second hinge member  920  to the hub  832  of the second piston  830 . The second rod  922  is received within an aperture  924  in the second hinge member  920  and an aperture  932  in the hub  832  of the second piston  830 . In the illustrated embodiment, the rods  912  and  922  and the apertures  914 ,  924 ,  930  and  932  are hexagonal so that the rods  912  and  922  translate rotation of the hinge members  910  and  920  to the pistons  810  and  830 . In other embodiments, the pistons  810  and  830  can be coupled to the hinge members  910  and  920  by other methods. For example, the pistons, rods, and hinge member can be rotatably coupled with a keyway-spline connection. 
     FIGS. 10A–10C  are top plan views of the power pack  800  attached to the first hinge member  910  and the second hinge member  920 .  FIG. 10A  illustrates the power pack  800  when the first and second hinge members  910  and  920  are in the full-extension position (i.e., corresponding to full leg extension). The first piston  810  is accordingly positioned so that the top surface  824  of the head  816  contacts the second wall  875  of the chamber  874 . The second piston  830  is similarly positioned so that the top surface  844  of the head  836  contacts the second wall  885  of the chamber  884 . As a result, the fluid is displaced from the chambers  874  and  884  when the hinge members  910  and  920  are in the full-extension position. 
     FIG. 10B  illustrates the power pack  800  when the first and second hinge members  910  and  920  are in an intermediate position between full-extension and full-flexion. The rotation of the first hinge member  910  in the direction S 1  about the first axis of rotation A 1  moves the first piston  810  from the position illustrated in  FIG. 10A  to the position illustrated in  FIG. 10B . As the first piston  810  rotates, the head  816  moves through the annular chamber  874  drawing the fluid into the chamber  874  from the upper fluid passageway  876 . The first piston  810  continues to rotate until the surface  817  of the head  816  contacts the first wall  873  of the chamber  874 . The first wall  873  of the chamber  874  precludes further rotation of the first piston  810 , and consequently the first hinge member  910 , about the first axis of rotation A 1  in the direction S 1 . 
     FIG. 10C  illustrates the power pack  800  when the first and second hinges  910  and  920  are in the full-flexion position. The rotation of the second hinge member  920  in the direction S 2  about the second axis of rotation A 2  moves the second piston  830  from the position illustrated in  FIG. 10B  to the position illustrated in  FIG. 10C . As the second piston  830  rotates, the head  836  moves through the annular chamber  884  drawing the fluid into the chamber  884  from the lower fluid passageway  886 . In other embodiments, the second piston  830  can rotate before the first piston  810  rotates. Referring to  FIG. 10C , the second piston  830  is illustrated in a position with the surface  837  of the head  836  contacting the first wall  883  in the lower annular chamber  884 . The first wall  883  of the chamber  884  precludes further rotation of the second piston  830 , and consequently the second hinge member  920 , about the second axis of rotation A 2  in the direction S 2 . From the position illustrated in  FIG. 10C  the first piston  810  can displace the fluid from the chamber  874  by rotating about the first axis of rotation A 1  in the direction S 3  to the position illustrated in  FIG. 10A . Similarly, the second piston  830  and the second hinge member  920  can displace the fluid from the chamber  884  by rotating about the second axis of rotation A 2  in the direction S 4  to the position illustrated in  FIG. 10A . 
     FIG. 11  is an isometric exploded view of a power pack  1000  having valves  1030  and  1032  to control the fluid flow in accordance with another embodiment of the invention. The power pack  1000  is similar to the power pack  800  described above, and like reference numbers refer to like components in  FIGS. 8–11 . The power pack  1000  of the illustrated embodiment has a first valve  1030  in the upper fluid passageway  876  and a second valve  1032  in the lower fluid passageway  886 . The valves  1030  and  1032  control the fluid flow through the respective fluid passageways  876  and  886 . When the upper valve  876  is partially closed, the fluid flow through the upper fluid passageway  876  is restricted, and consequently, the head  816  of the first piston  810  moves at a reduced speed within the chamber  874 . When the upper valve  876  is closed, no fluid can flow through the upper fluid passageway  876 , and consequently, the head  816  of the first piston  810  cannot move within the chamber  874 . 
   Accordingly, the valves  1030  and  1032  can control the ability of the pistons  810  and  830 , and therefore the hinge members  910  and  920  ( FIG. 9 ), to rotate about the first and second axes of rotation A 1  and A 2 . Furthermore, the valves  1030  and  1032  can control the speed at which the hinge members  910  and  920  rotate. In one embodiment, the valves  1030  and  1032  can be piezoelectric valves. In another embodiment, the power pack  1000  can have other valves or only one valve. 
   In the illustrated embodiment, the valves  1030  and  1032  are controlled by a controller  1002 . The controller  1002  can be a programmable chip with memory that is mounted on or in the housing  802 . The controller  1002  can be programmed using a separate hand-set with an infrared link or a hard-wired link. 
   The controller  1002  can communicate with the valves  1030  and  1032  through a wired, wireless, or infrared connection. Thus, as explained below, the controller  1002  can control the rotation of the first and second hinge members  910  and  920  by restricting or stopping the flow of fluid through the valves  1030  and  1032 . 
   In the illustrated embodiment, the power pack  1000  contains a system to automatically adjust the valves  1030  and  1032  to a particular setting corresponding to the position of the heads  816  and  836  in the chambers  874  and  834 . The system includes a first magnet  1020  disposed in the head  816  of the first piston  810  and a second magnet  1022  disposed in the head  836  of the second piston  830 . The front portion  850  of the housing  802  contains magnetic strips  1010  and  1012  positioned adjacent to the chambers  874  and  884 . The magnetic strips  1010  and  1012  sense the location of the magnets  1020  and  1022 , and consequently the position of the heads  816  and  836 . The magnetic strips  1010  and  1012  can communicate with the controller  1002  through a wired, wireless, or infrared connection. Accordingly, the valves  1030  and  1032  can be adjusted to a particular setting corresponding to the position of the heads  816  and  836 . In other embodiments, other position sensing devices can be used. 
   The ability to adjust the valves  1030  and  1032  depending on the location of the heads  816  and  836  allows the power pack  1000  to slow the pistons  810  and  830  and the first and second hinge members  910  and  920  before they reach the range of motion stops. For example, if a user is participating in a vigorous activity such as skiing, the power pack  1000  can slow the rotation of the hinge members  910  and  920 , and accordingly the movement of the knee joint, before the hinge members  910  and  920  reach the range of motion stops. In other embodiments, the power pack  1000  can slow the rotation of the hinge members  910  and  920  when a user begins rotating the hinge members  910  and  920  at a speed that could result in a knee injury. Braking or slowing the hinge members  910  and  920  before the rotation stops can reduce the high loads in the knee joint and the knee brace caused by abrupt stops. Moreover, braking can reduce the risk of hyperextension in the knee joint. Furthermore, the ability to adjust the valves  1030  and  1032  based on a corresponding position of the heads  816  and  836  also allows a user to have flexibility in setting the range of motion limitations. For example, the controller  1002  can be programmed to allow for a greater range of motion during the time of day that a user has therapy, and a limited range of motion during the time of day that the user exercises. 
   In the illustrated embodiment, a load cell  1040  is operatively coupled to the controller  1002 . The load cell  1040  can be used to trigger the controller  1002  to restrict rotation of one or both of the pistons  810  and  830  when the load cell  1040  is loaded. For example, the load cell  1040  can be placed in a shoe (not shown). In this particular embodiment, the load cell  1040  can trigger the controller  1002  to restrict rotation of the first and second pistons  810  and  830  when the shoe is subjected to a load. This embodiment could be useful, for example, for people who have lost the function of their quadriceps or have polio. In one embodiment, the load cell  1040  triggers the controller  1002  to restrict rotation of the first and second hinge members  910  and  920  at heal strike, allowing a person to put weight on the leg without concern of the knee bending. Once the toe is off the ground, the load cell  1040  triggers the controller  1002  to permit rotation of the first and second hinge members  910  and  920  so that the leg can be rotated forward. In other embodiments, the load cell  1040  can be positioned proximate to a muscle in the body. Accordingly, the tension of the muscle can trigger the load cell  1040  to allow or restrict rotation of the hinge members  910  and  920 . In additional embodiments, the load cell  1040  can be positioned in other locations. 
     FIG. 12  is a front view of a knee brace  1260  having a bladder  1200  positioned to exert a force on the tibial tuberosity in accordance with one embodiment of the invention. The knee brace  1260  is similar to the knee brace  60  described above, and like reference numbers refer to like components in  FIGS. 1–18 . The knee brace  1260  of the illustrated embodiment can be used with the embodiments of the hinges  10  and  710  described above, and also with other types of single axis or bicentric hinges. Furthermore, the knee brace  1260  of the illustrated embodiment can be used with the embodiments of the power packs  800  and  1000  described above, and also with other types of power packs. In the illustrated embodiment, the bladder  1200  is positioned proximate to an inside surface  1422  ( FIG. 14 ) of the lower frame  32 . A fluid conduit  1220  couples the bladder  1200  to the power pack  800 . The fluid conduit  1220  extends along an outside surface  1222  of the lower frame  32 . In one embodiment, the fluid conduit  1220  can be disposed within a groove in the lower frame  32 . In other embodiments, the fluid conduit  1220  can be disposed within the lower frame  32 , or proximate to the inside surface  1422  of the lower frame  32 . In the illustrated embodiment, the power pack  800  forces fluid through the fluid conduit  1220  to fill and drain the bladder  1200 . 
     FIG. 13  is a partial cross-sectional view taken substantially along line  13 — 13  of  FIG. 12 .  FIG. 13  illustrates the position of the bladder  1200  proximate to the tibial tuberosity of the leg (shown in broken lines).  FIGS. 14A and 14B  are partial cross-sectional views illustrating the bladder  1200  empty ( FIG. 14A ) and filled with fluid ( FIG. 14B ). Referring to  FIGS. 13 ,  14 A and  14 B together, the bladder  1200  is positioned against a concaved backplate  1320  in an upper portion  1300  of the lower frame  32  in the illustrated embodiment. The concaved backplate  1320  has a rigid surface that forces the bladder  1200  to expand toward the tibial tuberosity when the bladder  1200  fills with fluid. The concaved backplate  1320  has an aperture  1310  to permit the fluid conduit  1220  to connect to the bladder  1200 . The fluid conduit  1220  delivers fluid to the bladder  1200  through a fluid inlet/outlet  1400 . In additional embodiments, the fluid conduit  1220  can connect the bladder  1200  to both the power packs  800  ( FIG. 12 ) located on each side of the knee brace  1260  ( FIG. 12 ). In the illustrated embodiment, when the hinge  10  is in the full-extension position (shown in  FIG. 13 ), the bladder is expanded and filled with fluid (shown in  FIGS. 13 and 14B ) and accordingly exerts a force against the tibial tuberosity. When the hinge  710  is in the full-flexion position (shown in  FIG. 7C ), the bladder is empty (shown in  FIG. 14A ), and accordingly does not exert a force on the tibial tuberosity. One advantage of the embodiment illustrated in  FIGS. 12–14  is that the knee brace  1260  provides a concentrated, dynamic counter-anterior instability force when the ACL needs support the most—at the point of full-extension. During flexion, when the ACL is under its lowest loads, the force is removed and the knee brace  1260  follows the knee motion in a comfortable manner. 
     FIG. 15  is a rear isometric view of the knee brace  1260  with the bladder  1200  positioned to exert a posterior force on the femur in accordance with another embodiment of the invention. In the illustrated embodiment, the bladder  1200  is positioned proximate to an inside surface  1622  ( FIG. 16 ) of the upper frame  30 . The fluid conduit  1220  couples the bladder  1200  to the power pack  800 . The fluid conduit  1220  extends along an outside surface  1522  of the upper frame  30 . In one embodiment, the fluid conduit  1220  can be disposed within a groove in the upper frame  30 . In other embodiments, the fluid conduit  1220  can be disposed within the upper frame  30 , or proximate to the inside surface  1622  of the upper frame  30 . In the illustrated embodiment, the power pack  800  forces fluid through the fluid conduit  1220  to fill and drain the bladder  1200 . 
     FIG. 16  is a partial cross-sectional view taken substantially along line  16 — 16  of  FIG. 15 .  FIG. 16  illustrates the position of the bladder  1200  proximate to the femur in the leg (shown in broken lines). The bladder  1200  is positioned against a concaved backplate  1620  in a lower portion  1602  of the upper frame  30  in the illustrated embodiment. The concaved backplate  1620  has a rigid surface that forces the bladder  1200  to expand toward the femur when the bladder  1200  fills with fluid. The concaved backplate  1620  has an aperture  1610  to permit the fluid conduit  1220  to connect to the bladder  1200 . In additional embodiments, the fluid conduit can connect the bladder  1200  to both the power packs  800  ( FIG. 15 ) located on each side of the knee brace  1260  ( FIG. 15 ). In the illustrated embodiment, when the hinge  10  is in the full-extension position, the bladder is expanded and filled with fluid, and accordingly exerts a force against the femur. When the hinge  710  is in the full-flexion position (shown in  FIG. 7C ), the bladder is empty (shown in  FIG. 14A ) and accordingly does not exert a force on the femur. In additional embodiments, the knee brace can have a bladder positioned to exert a force on the femur and another bladder positioned to exert a force on the tibial tuberosity, or the knee brace can include only one of the bladders. One advantage of the embodiment illustrated in  FIGS. 15 and 16  is that the knee brace  1260  provides a concentrated, dynamic counter-hyperextension posterior force when the PCL needs support the most, at full-extension. During flexion, the point where the PCL is under its lowest loads, the force is removed and the knee brace  1260  follows the knee motion in a comfortable manner. 
     FIG. 17  is a side view of a knee brace  1760  with a lower frame  1732  having a front portion  1704  and a rear portion  1706  connected by a strap  1700  at a proximal end  1712  in accordance with another embodiment of the invention. The front portion  1704  of the lower frame  32  has a projection  1702  proximate to the proximal end  1712  of the front portion  1704 . The strap  1700  is configured to selectively contract so that the proximal end  1712  of the front portion  1704  moves in the direction F.  FIGS. 18A and 18B  are partial side cross-sectional views illustrating the strap  1700  in the elongated ( FIG. 18A ) and contracted ( FIG. 18B ) positions. In the illustrated embodiment, the bladder  1200  is positioned inside the strap  1700  and is connected to the power pack  800  ( FIG. 17 ) by the fluid conduit  1220 .  FIG. 18A  illustrates the elongated position in which the strap  1700  has a diameter of D 1  and a length of L 1 .  FIG. 18B  illustrates the strap  1700  after the bladder  1200  has filled with fluid. The expansion of the bladder  1200  causes a portion the strap  1700  proximate to the bladder  1200  to expand radially to a diameter D 2 . The radial expansion of the strap  1700  proximate the bladder  1200  causes the length of the strap  1700  to decrease to L 2 . Referring to  FIG. 17 , the contraction in the length of the strap  1700  causes the front portion  1704  to move in the direction F when the bladder  1200  is filled. The movement in the direction F causes the projection  1702  to exert a concentrated, dynamic force on the tibial tuberosity. As described above, the bladder  1200  is filled with fluid when the hinge  10  is in the full-extension position and the bladder  1200  is empty when the hinge  10  is in the full-flexion position. 
   From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.