Abstract:
A weight sensor includes a pad of electrically insulative material having a through aperture and electrode plates secured to its top and bottom surfaces at least partially covering the aperture. An electrical contactor in the aperture responds to a compressive force applied across the pad thickness to electrically interconnect the electrodes and close a circuit that permits an alarm to be actuated. The sensor is disposed in footwear worn by a patient being rehabilitated from a lower limb surgery or injury. The stiffness/compressibility of the pad determines the applied patient weight or compressive force required to permit the contactor to close the circuit between the electrodes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/264,946 entitled “Patient Weight Bearing Monitor” filed Nov. 30, 2009. The disclosure of this provisional patent application is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a weight bearing monitor system for recuperating orthopedic patients and, more particularly, to a lower limb load monitoring device for measuring or detecting the amount of force applied to, or weight borne by, a lower limb or joint of the body (either natural or prosthetic) and providing a signal to the user when a predetermined weight threshold level is exceeded. 
     BACKGROUND 
     Numerous situations exist where it is important to limit the load or force applied to or borne by a lower natural or prosthetic limb or joint of the body during standing, walking, stepping, running or jumping activities or during rehabilitation therapy. Situations also exist where it is important that the lower limb or joint be exposed to a certain load or force, particularly during rehabilitation therapy. In both situations it is important to monitor such load or force and to provide a signal to the user when such force is exceeded or met. Examples include post-surgery or injury rehabilitation of hips, knees, ankles or any other portion of the body which is affected by force applied to or borne by at least one of the user&#39;s legs, or any other situation in which monitoring of the weight on a lower limb during standing, walking, jumping or other activities is desired. 
     Prior art systems for monitoring the load or force applied to joints in a patient&#39;s leg are found in U.S. Patent Application Pub. No. 2006/0282018 and in the following U.S. Pat. Nos. 3,702,999 (Gradisar); 3,791,375 (Pfeiffer); 3,974,491 (Sipe); 4,745,930 (Confer); 5,107,854 (Knotts et al); 5,253,654 (Thomas et al); 5,511,561 (Wanderman et al); 5,619,186 (Schmidt et al); 6,174,294 (Crabb et al); and 6,273,863 (Avni et al). 
     Prior art monitors are typically complex mechanically and/or electrically with the result that they are expensive and add significantly to a patient&#39;s cost of recuperation. 
     SUMMARY OF THE INVENTION 
     The present invention provides an inexpensive alternative to methods and apparatus utilized heretofore to monitor the load applied to a lower limb or joint by a patient&#39;s weight and to provide an alarm when a predetermined and selectable weight is applied or exceeded. 
     A sensor according to the present invention includes a pad of electrically insulative material, having top and bottom surfaces defining its thickness dimension, and provided with a through aperture communicating between those surfaces. First and second thin electrically conductive plates or sheets serve as electrodes and are secured to the top and bottom surfaces of the pad, covering or partially covering the ends of the aperture. A resilient electrical contactor secured to the underside of the top plate depends through a portion of the aperture toward the bottom plate. The resulting structure serves as an electrical switch that is normally open but closes when the compressive force applied across the pad thickness is sufficient to permit the contactor to contact the bottom plate through the pad aperture. Electrical lead wires secured to respective electrodes are connected to a circuit that permits an audible and/or visible alarm to be actuated when the switch is closed. 
     The pad is typically disposed between the sole of a patient&#39;s foot and the ground, preferably in a patient&#39;s sock, shoe or other footwear or foot-worn orthopedic structure. The stiffness or compressibility of the pad determines the applied weight or compressive force required to permit the contactor to close the circuit between the electrodes, resulting in the closure of the switch. By selecting pad material having appropriate compressibility characteristics, one can design the switch to close in response to different applied forces. Patients can use sensors of increasing stiffness as rehabilitation progresses and greater weight loads are permitted on the affected limb. 
     The above and still further features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view in perspective of the sensor portion of a weight monitor assembly according to the present invention. 
         FIG. 2  is an exploded view in perspective of a sensor unit employed in the assembly of  FIG. 1 . 
         FIG. 3  is a view in perspective of one form of an indicator portion of the weight monitor assembly of the present invention. 
         FIG. 4  is a view in perspective of a compressible pad employed in the sensor unit of  FIG. 2 . 
         FIG. 5  is a view in perspective of one form of an electrode and contactor employed in the sensor unit of  FIG. 2 . 
         FIG. 6  is a view an elevation view in section of the sensor of  FIG. 2 . 
         FIG. 7  is an electrical schematic diagram of the weight monitor of the present invention. 
         FIG. 8  is a broken view in elevation showing the weight monitor assembly of the present invention deployed on a patient with the sensor located in the patient&#39;s shoe. 
         FIG. 9  is a view in elevation showing the sensor of the weight monitor assembly located in an orthopedic appliance worn by the patient during rehabilitation therapy. 
         FIG. 10  is a view in elevation showing the sensor of the weight monitor assembly attached by a hook and loop fastener to a sock worn by the patient during rehabilitation therapy. 
         FIG. 11  is a view in elevation showing the sensor of the weight monitor assembly inserted in a pocket of a sock worn by the patient during rehabilitation therapy. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed explanations of the preferred embodiments reveal the methods and apparatus of the present invention. 
     Referring to  FIGS. 1 through 7 , a weight sensor  17  comprises an electrically non-conductive pad  10 , preferably made of silicone or like material and typically approximately 0.25″ thick, is shown in a rectangular configuration with top and bottom surfaces and a centrally located aperture  13  defined through its thickness. Neither the pad  10  nor the aperture  13  is required to be rectangular and can be any shape, such as round, polygonal, irregular, etc., as may be consistent with the functional features described herein. Top and bottom electrically conductive electrode sheets or plates  11  and  12 , respectively, are disposed on respective top and bottom surfaces of pad  10  to partially or fully cover opposite ends of aperture  13 . In the illustrated embodiment sheets  11  and  12  have the same peripheral configuration, in this case rectangular, as pad  10  and overlie the entireties of respective top and bottom surfaces of the pad. A resiliently movable electrical contactor  15 , preferably made of resiliently bendable spring steel, is secured by soldering or other electrical connection to the underside of top plate  11 . Contactor  15  includes a short section extending along the underside of plate  11  and a bent section extending at an angle (e.g., 45°) into aperture  13 . It is to be understood that an angular configuration of contactor  15 , and the particular angle of 45°, are design choices, and that any electrically conductive contactor configuration that permits the contactor to move toward the opposing electrode plate under weight loading may be employed. 
     In the illustrated preferred embodiment each electrode sheet  11 ,  12  is made of steel and is approximately 0.020″ thick, although other conductive materials (e.g., copper, aluminum) may be employed. Electrically conductive wire leads A and B are connected, by soldering or the like, to respective electrode sheets  11  and  12 . As illustrated in  FIG. 2 , the opposite ends of wire leads A and B are connected to a plug  25  which is adapted to connect to respective terminals in a control box receptacle or jack  21 . The control box  20  contains a buzzer  22 , or other audible or visual alarm, and a voltage source such as a battery  28  (or batteries) connected in series between the terminals of jack  21 . Thus, when there is contact between electrode plates  11  and  12  by means of contactor  15  being forced further through aperture  13 , a circuit is closed across the batteries and alarm  22  as best illustrated in the schematic diagram of  FIG. 7 . Such contact is made when the force applied across weight sensor unit  17  is sufficient to compress pad  10  to cause contactor  15  to contact bottom plate  12  and resiliently bend so that the circuit between plates  11  and  12  is closed. That force is the weight of a patient applied across the unit by the sole of a patient&#39;s foot urging the unit toward a floor or the ground through a sock, shoe or other footwear in which the sensor is located. It will be appreciated that contactor  15  may be secured instead to the bottom plate  12  and positioned to be forced into contact with top plate  11  when the pressure applied across the unit is sufficient to compress pad  10  to cause contactor  15  to contact top plate  12  and resiliently bend so that the circuit between plates is closed. In either case the resilience of the contactor permits the predetermined force to be exceeded without damaging the contactor. 
     As best illustrated in  FIG. 4 , pad  10  may be a two inch square of molded silicone with a density selected to permit compression of the pad to effect contact between the electrode sheets in response to application of a predetermined force across the pad. Pads of different densities can be provided to permit different applied weights to actuate the alarm as a patient&#39;s rehabilitation progresses; that is, differently calibrated sensors can be provided to sound an alarm at different applied weights. 
     As illustrated in  FIG. 2 , the electrode plates may be substantially identical except for the provision of contactor  15  on the underside of plate  11 . The wire leads A and B are preferably 22 gauge wire. As noted elsewhere herein, the electrodes need not be identical, and only one electrode is required to flex to permit contactor movement as the pad  10  is compressed under a weight load. 
     Referring to  FIG. 6 , the sensor unit may be encased in an electrically insulative sleeve  18  that can be placed in footwear such as a sock, shoe or orthopedic appliance when used by a patient. Sleeve  18  may be made of cotton, polyester or any such deformable material that serves as a cover for the unit and does not interfere with its operation. Alternatively, sleeve  18  may be encapsulated about the sensor unit by dipping the sensor in molten encapsulation material (e.g., a rubber like compound) or brushing on the compound. That compound may be color coded to indicate the compression weight at which electrical contact is made between the electrodes of the unit. As assembled, the unit  17  typically weighs two pounds or less. 
     The weight sensor of the present invention may be used in several ways. One such way is illustrated in  FIG. 8  wherein the sensor unit  17  is shown removably secured, by means of hook and loop fastener, or the like, to the interior sole of a patent&#39;s shoe  30 . The sole of the patient&#39;s foot applies the patient&#39;s weight to the top plate  11  ( FIG. 1 ) of the sensor, thereby compressing the sensor pad  10  and gradually moving the contactor  15  toward the bottom plate  12 . When the maximum permitted weight is applied across the sensor, contactor  15  contacts the bottom plate, closing the circuit and sounding or flashing an alarm at the control box  20  ( FIG. 7 ). The control box may be connected by means of a clip  40 , or the like, to the top of a sock or a belt, etc., worn by the patient. 
     An alternative manner of using the weight sensor is illustrated in  FIG. 9  wherein an orthopedic appliance  50  or similar structure is shown being worn on a patient&#39;s foot. The sensor is removably secured to the interior sole of the appliance. 
     In another embodiment the sensor is removably secured to the bottom of a patient&#39;s sock  60 , as illustrated in  FIG. 10 . Alternatively, as shown in  FIG. 11 , the sock  60  may be provided with a pocket  61  on the bottom side of the sock sole to receive the sensor. The lead wires connect to the control box which may be attached to the pants or other garment worn by the patient. 
     The foregoing describes only a few of the many ways in which the sensor  17  may be deployed for use. 
     The dimensions described herein for the preferred embodiments are presented simply as examples and can vary as desired in order to provide a suitably functional and comfortable unit. For example, the pad  10  need not be square; instead it can be round, oval, rectangular, contoured to a portion of the patient&#39;s foot, or irregular in shape. The pad length and/or width can be adjusted as desired, although it is believed that 1.5″ and 3.0″ are practical limits on these dimensions. The thickness of the pad  10  can range from 0.100″ to 0.400″. 
     The material for the pad  10  is chosen to provide a desired compression versus applied weight characteristic and can be solid/dense and sponge-like material including neoprene, silicone, natural rubber, latex, buna N, buna S, hypalon, EPDM, or polyurethane. The pad can be cut from sheet material, using a steel ruler die, shearing or cutting by hand. The material can also be molded to the needed shape. 
     Aperture  13  can be square, rectangular, round or any convenient regular or irregular shape and can have any length and width dimensions appropriate to the described function. Examples would be in the range of 0.250″ to 1.000″. 
     The top and bottom plates  11 ,  12  can be conductive steel or aluminum, or they can be plastic with attached metal strips to provide the desired electrical conductivity. The thickness of the plates may, for example, range from 0.015″ to 0.125″. Importantly, in the illustrated embodiment the plates must be sufficiently thin to resiliently flex and follow the pad surfaces to which they are attached under a weight load. It will be appreciated that only one of the plates is required to flex as the pad compresses and, therefore, the plates need not be identical in structure or function. 
     In order for electrical contact to be made between electrodes  11  and  12  by contactor  15 , pad  10  is typically required to be compressed by approximately 50%, depending on the configuration, dimensions and positioning of the contactor. This compression is effected at different applied weights, typically between 5 and 40 lbs., depending on the compression characteristics of the pad which are predetermined by blending various compounds and then testing for the compression needed. 
     It will be appreciated that the important feature of unit is that, as pad  10  is compressed under the load of a patient&#39;s weight applied across the pad thickness dimension, contact will ultimately be made between electrodes  11  and  12  when a predetermined load force is reached. The use of contactor  15  attached to one of the electrodes and functioning through an aperture  10  is only one way of establishing this contact. For example, an electrically conductive member may be partially embedded in pad  10  in spaced relation to one or both electrodes and positioned to make contact between the electrodes through a recess or other opening in a pad surface in response to a predetermined pad compression. 
     Contactor  15  can be made of a strip of spring steel, or a small spring. As an alternative to the single contactor, the top and bottom plates can be provided with a lip, each facing the other, so that lips can be used to make the electrical contact upon compression of the pad. 
     The lead wires can be as short as a few inches and as long as necessary to reach the control box from the sensor, depending on where the control box is to be worn. The control box can be made of metal or plastic and can have any suitable size and shape to house the indicated components. 
     The battery voltage may be between 1.5 volts to 9 volts. A buzzer or other alarm that is operative with the chosen voltage supplies a sound or flashing light to alert the user that that he has reached the critical weight set by the physician or therapist. That is, the unit is used to indicate to the patient that the weight being applied to the limb is at the maximum weight that the person conducting the therapy has indicated to be appropriate. Sensor units can be made to respond to a large number of different weights needed by the medical profession. 
     The sensor of the present invention prevents patients who are undergoing therapy for an injured limb or replaced joint from suffering damage caused by placing more weight on the limb or joint than can be safely applied at different stages of recovery. A primary advantage of the sensor of the present invention is that can be manufactured and sold for a price that is far less than other sensors currently being used for the same purpose, thereby allowing patients to purchase the unit. 
     As described and illustrated, the sensor can be used attached to a stocking, placed in a shoe or sandal, or attached to a form fitting appliance. 
     Having described preferred embodiments of new and improved weight bearing monitor system, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the teachings set forth herein. It is therefore to be understood that all such variations, modifications and changes are believed to fall within the scope of the present invention as defined by the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.