Abstract:
An improved socket for a prosthesis uses a liner material providing fluid flow through a porous matrix whose local pressure is adjusted by a control system communicating with multiple valves and pressure sensors. Control of pressure in a viscoelastic material provides an improved trade-off between comfort and stability.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. provisional application 60/870,492 filed Dec. 18, 2006 hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to prosthetic devices, and in particular to a prosthetic device that may sense points of high pressure contact with a patient&#39;s residual limb and locally reduce those pressures on a dynamic basis. 
   Lower limb amputees may be fitted with a prosthetic leg that attaches to their residual limb by means of a socket surrounding the residual limb. In such cases, it is important that the socket fit closely to the residual limb so that the amputee has a sense that the prosthetic limb is secure and stable and the forces of walking are distributed evenly over the entire residual limb. It is also important that the fit not be so tight as to restrict blood flow or to be uncomfortable. 
   Unfortunately the volume of the residual limb changes significantly over time and even within the course of a day making a good fit between the residual limb and socket difficult to maintain. This change in the volume of the residual limb can be managed to some extent by the use of one or more socks which may be placed over the residual limb or removed from the residual limb at different times to adjust the fit of the socket. 
   Volume changes in the residual limb may also be accommodated by means of a socket incorporating one or more bladders that may be inflated manually by the patient, or automatically, to provide a desired pressure between the socket and the residual limb. For example, U.S. Pat. No. 6,585,774 describes a socket in a “pumping bladder” between the socket and the residual limb that makes use of forces developed during walking to pump additional fluids into other bladders within the socket to maintain a desired predetermined pressure between the socket and the residual limb. In this way, a good fit between the socket and the residual limb may be obtained despite variations in the volume of the residual limb. 
   A difficult problem for patients, even with padded sockets that properly conform to the surface of the residual limb, is the persistence of localized points of high pressure in the contact between the residual limb. Such persistent high pressure points can result from difficult to correct socket interface pressure that occurs during human activities such as running, jumping, leaning and lifting which cause large variations in the loading on the residual limb, and the need to maintain a relatively stiff interface between the residual limb and the socket to prevent undue “bouncing” movement of the prosthetic leg during use. Such persistent high pressure points may cause discomfort to the patient which can deter the use of the prosthetic leg and, in extreme cases, can cause tissue damage. 
   SUMMARY OF THE INVENTION 
   The present invention provides a cushioned socket for a prosthetic leg using a dynamically controlled system of pressure sensors and valves that selectively control the local pressure in a fluid filled and fluid permeable liner. The liner provides viscoelastic properties that moderate the pressure equalization process to reduce high pressure points while still providing a secure fit between the residual limb and the socket and facilitating multiple points of pressure control. 
   Specifically, the present invention provides a socket for the attachment of a prosthesis to a residual limb, the socket providing a compliant fluid charged material creating an interface between a rigid shell attached to the prosthesis and the residual limb. A plurality of sensors sense local pressure in the compliant fluid charged material and a set of actuators provide local control of the pressure between the compliant material and the residual limb. A control system communicates with the sensors and the actuators to anticipate points of high pressure between the residual limb and the compliant material to adjust the local pressure to reduce points of high pressure. 
   It is thus one object of one embodiment of the invention to provide for multiple zones of control pressure that anticipate the creation of high pressure points during use of the prosthesis. 
   The compliant fluid charged material may provide a substantially incompressible fluid with a predetermined viscosity moving through at least one porous matrix to provide viscous flow of the fluid under pressure. 
   It is thus another object of one embodiment of the invention to provide a material permitting the blending of multiple points of pressure control through viscoelastic flow. 
   The compliant fluid charged material may consist of at least two layers allowing fluid flow therebetween. 
   It is thus another object of one embodiment of the invention to provide for multiple dimensions of fluid flow permitting greater flexibility in the tailoring of the property at a given zone. 
   The porous matrix may be an elastomeric foam. 
   It is thus another object of one embodiment of the invention to provide a material that can have a controlled porosity allowing a variety of different materials to be readily implemented. 
   The porous matrix may be a gel material. 
   It is thus another object of one embodiment of the invention to make use of gel materials in controlling viscosity. 
   The compliant fluid charged material may include pockets of a compressible fluid held within the porous matrix. 
   It is thus another object of one embodiment of the invention to allow flexible tailoring of the elasticity of the liner independently from its viscous flow properties. 
   The liquid may be an oil of predetermined viscosity. 
   It is thus an object of one embodiment of the invention to provide an alternative method of tailoring the viscous properties of the material through the use of different weights of oil. 
   These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view in partial cross-section of a prosthetic leg suitable for use with the present invention having an upwardly open socket for receiving the residual limb of the patient and an attached control computer; 
       FIG. 2  is a fragmentary cross-section of a portion of the socket of  FIG. 1  showing the interface between the residual limb and a set of porous cushioning materials having valves and pressure sensors positioned for local peak pressure control; 
       FIG. 3  is a simplified version of  FIG. 2  showing fluid flow through the cushioning materials with pressure against the socket by the residual limb in an area where peak pressure is being moderated; 
       FIG. 4  is a pressure diagram of a region around the valves of  FIG. 2  and  FIG. 3  showing local boundaries of pressure control possible with the present invention; 
       FIG. 5  is a set of graphs showing the use of an external stride cycle signal and a set of corresponding pressure sensing signals used to provide the predictive control system of the present invention; and 
       FIG. 6  is a flow chart showing the steps of the program executed on the computer of  FIGS. 1 and 3  in effecting the control of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a prosthetic leg  10  may include a leg shaft  12  terminating at its lower end with a foot portion  14  and at its upper end by a socket  16 . The socket  16  defines a volume  18  opening upwardly to receive the residual limb  28  of a patient (not shown in  FIG. 1 ). A battery powered micro computer  20  may be attached to the prosthetic leg  10  to receive signals over signal lines  22  from valves and sensors in the socket  16  (to be described below) and a stride sensor  24 , such as a spring-loaded switch, accelerometer, or strain gauge, sensing force or acceleration on the leg shaft  12  indicating a stride portion of the patient&#39;s gait. 
   Referring now also to  FIG. 2 , the socket  16  may include an outer rigid shell  26  conforming generally to the outer surface the residual limb  28 . The shell  26  may be attached directly to the shaft  12  to communicate forces there between. 
   At various locations on the inner surface of the shell  26 , pressure sensors  29  are placed, near corresponding ports  30  in the shell  26  to be described. The pressure sensors  29  may preferably provided not only normal pressures but also shear pressures and may optionally provide rotational pressures directly or by aggregating shear pressures of multiple adjacent pressure sensors  29 . Multi-axis pressure sensors of this type are known in the field of tactile sensing and may use technologies, for example, described in U.S. Pat. Nos. 4,521,685 and 4,745,812. The locations of the pressure sensors  29  will be in areas that are most prone to high contact pressures, for example the medial and lateral tibial supracondyles, medial and lateral tibia, medial and lateral gastrocnemius, patellar tendon, popliteal depression, distal tibia and fibula head. 
   Also attached to the inner surface of the shell  26  may be air bladders  32  such as will be used to tune the springiness at the interface between the residual limb  28  and the shell  26 . 
   The pressure sensors  29 , ports  30 , and air bladders  32  are covered with a first permeable foam layer  36  allowing resistive flow of a saturating fluid such as water or oil. A permeable interface layer  38  (or a different permeable foam layer) covers the foam layer  36  separating it from a second permeable foam layer  40  having similar or different resistive flow properties to that of layer  36 . The interface layer  38  controls a flow of fluid between the layers  36  and  40  to provide for a general long-term pressure equalization among these layers  36  and  40  moderated by their intrinsic resistance to flow. A flexible impermeable membrane  42  covers layer  40  separating it from the residual limb  28  which may in turn be covered with a sock  44 . 
   The precise viscoelastic properties of the liner formed by layers  36 ,  38 , and  40  may be tailored by changing the viscosity of a fluid within the layers  36  and  40  or the porosity of the matrix through which the fluid flows and the number of air bladders  32 . The matrix may be a porous medium such as an elastomeric foam or may be a gel material. The fluid may simply be an oil of known viscosity providing an essentially incompressible fluid. 
   The ports  30  may each communicate with an electrically actuated valve  50  that may connect the port  30  to a sealed reservoir  52  when the valve  50  is actuated, or disconnect the port  30  from the sealed reservoir  52  when the valve  50  is deactivated. Signal lines  22  from the pressure sensors  29  and a valve  50  may pass through guides on the outer surface of the shell  26  to the computer  20  (not shown). 
   Referring now to  FIGS. 3 and 4 , a local area of low pressure  54  may be dynamically generated about any of the ports  30  by opening the valve  50  so that with pressure  56  by the residual limb  28  against the membrane  42 , fluid  58  flows into the port  30  through the valve  50  and into the reservoir  52 . Migration of the fluid through the layer  38  and through the layers  36  and  40  controls the size and duration of the low pressure area  54 , with higher flow resistance materials localizing the low pressure area and preserving it longer, and lower flow resistance materials expanding the low pressure area  54  but decreasing its duration. The character of the low-pressure area may also be controlled by the size of the port  30 , the size of the reservoir  52 , and the control of the valve  50  as will be described. 
   Absent movement of the residual limb  28 , the low-pressure area  54  gradually decays and equalizes as liquid flows through these layers  36 ,  38 , and  40 . 
   Referring now to  FIGS. 3 ,  5 , and  6 , computer  20  executing a stored program  57  and communicating with the pressure sensors  29  and valves  50  may generate these local, low pressure areas  54 , dynamically, and on an anticipatory basis. At process block  60  of the program  57 , the program reads a stride signal  67  from the stride sensor  24  during a walking by the patient to benchmark pressure readings from each of the pressure sensors  29 . This benchmarking measures pressures at each pressure sensor  29  during a normal stride cycle  62  having a stride portion  64  (when the prosthetic leg  10  is bearing weight) and a swing portion  66  (when the prosthetic leg is not bearing weight). 
   During the normal stride cycle  62 , pressure readings  68  from each of the pressure sensors  29  may be collected and those pressure readings having peak pressures exceeding a predetermined threshold  70  may be identified. 
   After benchmarking has been obtained per process block  60 , the program may proceed to decision block  71  to detect the beginning of the next stride portion  64  (indicated by the rising edge of stride signal  67 ). At this time as indicated by process block  72 , selected valves  50  associated with benchmarked signals from pressure sensors  29  having pressure values exceeding threshold  70  may be opened to anticipate over pressure at the locations of the particular pressure sensors  29 . This creates graduated low-pressure areas  54  at those points while maintaining higher pressures outside of these points, the net effect being to counteract the high pressure that would otherwise occur. 
   The actual pressures reached during the stride portion  64  are then monitored as indicated by process block  74  and then these pressures may be used to create new benchmarks which may be used in the next stride portion  64 . In this way, each stride portion  64  takes the data obtained from the previous stride portion  64  and uses that to anticipate conditions during the next stride portion  64 . If the pressure associated with the valves  50  opening during the stride portion  64  drops below a predetermined value, the valves  50  may no longer be opened or may be opened for a lesser period of time or less time may be allowed to vent the reservoirs  52  as will be described. Generally, the process attempts to maintain a high-pressure contact between the socket  16  and the residual limb  28  within the bounds of limit  70 . 
   At process block  76  when a swing cycle is detected as indicated by signal  67  the selected valves  50  that were opened at process block  72  may be reopened to allow the reservoir  52  to vent back into the layers  36 ,  38  and  40  in preparation for the next stride portion  64  at process block  78 . 
   Generally, the duration of the opening of the valves  50  may be determined according to models developed for the particular materials of the layers  36 ,  38 , and  40  to precisely control a pressure profile over the interface between the residual limb  28  and the socket  16 . 
   The present invention provides localized pressure control independent of the general or average pressure asserted against the residual limb  28  and thus can be used in conjunction with other systems intended to control that average pressure between the residual limb  28  and the socket  16 , for example those that use a pump or the like to increased the pressure of liquid within the layers  40  and  36  to accommodate volume changes in the residual limb  28  such as are taught in the prior art. 
   Different limits  70  may be applied to different regions of the socket  16  and may be manually adjusted by the patient. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.