Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the measurement of hardness of the tissue of a limb by using a noninvasive technique. More specifically, the present invention relates to a warning device that safeguards against the development of compartment syndrome by relating [harness] hardness to intracompartmental interstitial pressure. 
     2. Description of the Background Art 
     The diagnosis of compartment syndrome is made by the direct measurement of intracompartmental interstitial pressure based on a technique developed by Dr. Thomas E. Whitesides, Jr. In this technique, a small amount of fluid is injected into a limb compartment. The pressure necessary to advance the fluid into the compartment is the measurement of the pressure of the compartment. If the intracompartmental interstitial pressure should increase to within 30 mmHg of the diastolic pressure, this could result in irreversible damage of the tissue within the compartment. Treatment for such a condition is emergency surgical release of the fascia overlying the muscle, which is constricting the compartment. Delay in the diagnosis of compartment syndrome and subsequently delay in performing the fasciotomy can result in the needless loss of function, contracture and possible amputation of the limb. 
     The decision to perform a fasciotomy for a suspected compartment syndrome is frequently difficult. In the classic article by Dr. Thomas E. Whitesides, Jr., “Tissue Pressure Measurements as a Determinant of the Need for Fasciotomy”, Clin. Orthop., 113:43, 1975, even if physicians are well versed in the signs and symptoms of compartment syndrome, the clinical analysis sometimes is indefinite and confusing, resulting in delay in performing the fasciotomy. 
     According to Dr. Whitesides Jr., the one factor that must be present in a compartment syndrome is increased intracompartmental interstitial pressure. Therefore, the effectiveness of the fasciotomy is based on relieving this pressure and re-establishing tissue perfusion. In order to effectively diagnose compartment syndrome, a technique for measuring tissue pressure has been established. For details of the technique of direct intracompartmental interstitial pressure measurement, refer to the article cited above by Dr. Thomas E. Whitesides, Jr. 
     Compartment syndrome occurs in skeletal muscles enclosed by osseofascial boundaries. The condition develops when accumulating fluid creates high interstitial pressure within a closed osseofascial space, reducing perfusion of surrounding tissues below a level necessary for viability. As the interstitial pressure within the compartment increases, the expansion of the compartment is limited by the compliance of the osseofascial envelope. Like a balloon about to burst, the envelope becomes less and less compliant as the interstitial pressure increases. The change in compliance can be detected by palpation. 
     Dr. Bruce Steinberg is the inventor of the device described in U.S. Pat. No. 5,564,435. That device quantitatively measures palpation, linear regression of force applied to volume displaced, and has shown a correlation between quantitative modulus of hardness and the interstitial pressure within a compartment. Dr. Steinberg et al. in an article “Evaluation of Limb Compartments with Suspected Increased Interstitial Pressure”, Clin. Ortho. No. 300, p 248-253, 1994, demonstrates how such a device can be used to assess compartment pressure with quantitative hardness measurements. Dr. Steinberg, however, has found that this particular device is cumbersome because of its difficulty in application. The device must be applied to a limb with a continuous stable force while a piston mounted within the platform moves to compress the limb. Measurements become inaccurate if there is any movement of the limb or the device. In the setting of a painful limb in trauma, this measurement becomes very difficult because the patient has difficulty maintaining the limb still. The device described by Dr. Steinberg in U.S. Pat. No. 5,564,435 requires that two separate forces be applied simultaneously, the continuous stable force for the force plate and a second force to increase the pressure within the piston. The measurements that derive the hardness result from the piston. As the pressure increases and as the piston compresses the limb compartment, measurements of pressure and displacement are simultaneously recorded while the device is held stable against the limb at a known force plate pressure. 
     The present invention overcomes this complexity by applying only one force to obtain the measurement of both pressure and displacement. This is done by mounting a stable pressure measuring probe where the piston was previously located. In addition, instead of having the platform as stable and nonmovable, the platform is now spring loaded and moves as pressure is applied to the limb. In effect, the probe pushes against the limb and the platform or force plate moves as the probe forces itself into the limb. The displacement of the probe is now measured by the distance between the probe tip and the movable platform. When removed from the limb, the spring loaded platform realigns to an even level with the probe tip (the spring force is slightly greater than the weight of the platform). In this way, the measurement of pressure within the probe is obtained electronically and the distance between the tip of the probe and the platform is measured as well electronically. A quantitative hardness can be obtained by the relationship between probe pressure and platform displacement. This quantitative measurement of palpation can then be used to assess the interstitial pressure within a compartment. This is a significant improvement over the prior art in that one can now apply a device to the limb with one hand and not worry about the difficulty of maintaining a constant force against the limb with one hand, while then pressurizing the piston mounted within the platform with the other hand. Dr. Steinberg has found with the new device application is faster 
     SUMMARY OF THE INVENTION 
     It is therefore one of the objectives of this invention to provide a system for noninvasively evaluating a limb suspected of compartment syndrome. 
     It is also an object of the present invention to evaluate a limb by measuring and recording simultaneous pressure and distance values. 
     It is a further object of the present invention to make a medical diagnosis on the basis of recorded pressure and distance values. 
     These and others objects of the present invention are achieved via a noninvasive technique which monitors the condition of limb tissue. More particularly, a noninvasive technique is disclosed for diagnosing and monitoring compartment syndrome. In the preferred embodiment of the invention, a pressure measuring probe is mounted within a spring loaded platform, where the platform is movable and distance is measured relative to the probe. Using this device, one may obtain measurements to assess the hardness of a limb compartment. More particularly, the preferred embodiment of the invention includes an apparatus and method for evaluating the condition of tissue within a limb. The method comprises of the following steps. First applying the apparatus to a limb with a force of application. Second, as this force is increased the change in the pressure of the mounted probe is recorded while the distance that the probe moves into the limb is recorded by the movement of a platform also applied against the limb. The method also includes the step of determining the relationship of multiple points of pressure [within the probe to the distance the probe to the compression measured, formulating a quantitative harness curve] applied to the probe and travel distance of the probe such that a quantitative hardness curve can be formulated. Additionally, this invention also includes a linear regression analysis of the multiple points of the curve to determine a quantitative harness modulus. 
     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a schematic overview of the system of the present invention. 
     FIG. 2 is an exploded view of the force applicator employed in the system in the present invention. 
     FIG. 3 is an assembled view of the force applicator of the present invention 
     FIG. 4 is an electronic schematic of the force applicator of the present invention. 
     FIG. 5 is an electronic schematic of the breakout box of the system of present invention. 
     FIG. 6 is chart of displacement vs. pressure as determined by the applicator instrument. 
    
    
     Similar reference characters refer to similar parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to a system and method for noninvasively evaluating a limb for compartment syndrome. The system utilizes a force applicator instrument which is applied to a limb suspected of having the syndrome. The instrument includes a spring biased force plate which is mounted about a probe. This plate is displaced as the instrument is employed in applying an increasing force to the limb. The pressure applied to the probe is detected and recorded, as is the displacement of the force plate relative to the force probe. Linear regression techniques are applied to the pressure and distance data to compute hardness of the limb. A compartment syndrome diagnosis is then made in accordance with the hardness computation. Details of the system and method are elaborated upon more fully hereinafter. 
     System of the Present Invention 
     With reference now to FIG. 1, the system  10  of the present invention is depicted. The system  10  employs an applicator instrument  20 , a break out box  22  and a computer  24 . Applicator instrument  20  is preferably a hand held device employed by the doctor in applying pressure to a limb  26  of a patient. With reference to FIG. 2, the various components of instrument  20  are depicted in an exploded view. These include: a base portion  28 , a housing portion  32 , a force plate  34 , and a force probe  36 . Force probe  36  is preferably positioned through a central aperture within force plate  34 . A spring  38  serves to interconnect force probe  36  and force plate  34 . More specifically, a helical spring is wound about the end of probe  36  positioned within force plate  34 . This connection results in probe  36  and plate  34  being biased with respect to one another. That is, force plate  34  is permitted to slide relative to the probe  36  against the bias of the spring tension. Ideally, the spring tension is selected such that the distal end of probe  36  is flush with the outer face of the force plate  34  with spring  38  in an unbiased state. Thus, prior to the instrument  20  being applied to the limb  26  of a patient, the distal end  42  of instrument  20  presents a relatively flat surface. Thereafter, as the instrument  20  is driven into a limb  26  the force plate  34  is caused to slide rearwardly to expose force probe  36 . 
     With continuing reference to FIG. 2, the stabilizing columns  44  and  45  of force plate  34  are depicted. More specifically, two shorter columns  44  and one elongated column  45  are included. These columns  44  and  45  move linearly within corresponding apertures formed within the housing and base portions ( 48  and  52 , respectively). These columns  44  and  45  are mounted to the interior of force plate  34  and function in guiding the plate  34  as it passes rearwardly over house and base portions ( 32  and  28 , respectively). 
     Base portion  28  includes an encoder  46  that is employed in measuring the travel distance of elongated stabilizing column  45 . Specifically, column  45  is received within apertures formed within housing portion  32 , base portion  28 , and within an elongated cylindrical aperture formed in L-shaped bracket  47 . A portion of bracket  47  is translucent. Encoder  46  includes Light Emitting Diodes (LEDS) that both transmit and detect light. Consequently, light transmitted through the translucent portion of bracket  47  can detect the presence or absence of column  45 . In this manner, the position of the end of column  45  within bracket  47  can be detected by encoder  46 . This distance measurement corresponds to the travel of force plate  34  relative to force probe  36 . This distance measurement is recorded and used in future calculations, as described more fully hereinafter. 
     A centrally located load cell  56  is interconnected to force probe  36  for use in measuring the pressure applied to force probe  36 . This load cell  56  is interconnected to the opposite end of force probe  36  and is positioned intermediate housing portion  32  and base position  28 . Specifically, probe  36  extends through the central aperture of housing portion  32  and contacts load cell  56 . A lead  58  is included for passing signals from cell  56  to breakout box  22 . As such, pressure applied to probe  36  is transmitted to the load cell  56  where it is measured and recorded. Additionally, force probe  36  and load cell  56  are fixed with respect to the remainder of instrument  20 . Consequently, the force applied to instrument  20  by the operator is transferred to both force probe  36  and the limb region  26 . 
     The system thus described is employed in sensing and measuring both pressure and distance values. The pressure values reflect the pressure encountered by force probe  36  as instrument  20  is pressed into a limb  26 . The distance measurement reflects the distance between force plate  34  and probe  36  which occurs as instrument  20  is pressed into a limb  26 . The components of the system employed in utilizing and analyzing this data are described next. 
     With reference to FIG. 1 the breakout box  22  of the system  10  is depicted. Box  22  is electrically coupled to applicator instrument  20  by suitable cabling  62 . The breakout box  22  functions in receiving pressure measurements from the load cell  56 . These values are then compared against preset set minimum and maximum values. When the minimum pressure is met an indicator means  64  within box  22  signals the start of the data sampling period. That is, simultaneous pressure and distance values are recorded for a predetermined length of time only after a threshold pressure value is met. In the preferred embodiment, the threshold pressure value is 25 grams. Reaching this value starts the data acquisition cycle within computer  24 , the user is also alerted to the initiation of the cycle by indicator means  64 . In the preferred embodiment the sampling time is 3 seconds. Indicator  64  provides audible beeps during the acquisition cycle, preferably one beep per second. Likewise, if the doctor applies too much pressure with instrument  20  the indicator means provides a warning signal. In the preferred embodiment, the maximum pressure is between 7.5-10 lbs. The indicator means  64  can take the form of a audible beep or can be carried out by way of a visual monitor. 
     The computer  24  is also electrically coupled to the breakout box by suitable cabling  66 . This computer  24  preferably takes the form of a laptop or desktop computer. However, the computer can also take the form of a specialized data processor specifically adapted for carrying out the present invention. Whatever the form, computer  24  is used in providing electrical power to instrument  20  as well as breakout box  22 . Furthermore, computer  24  is employed in collecting, storing, and analyzing the pressure and distance measurements collected by the applicator instrument  20 . Once stored within computer  24 , the data is analyzed and employed in making diagnostic assessments. 
     Method of the Present Invention 
     The inventive method carried out by the system of the present invention is next described. In accordance with the method, the applicator is used by a doctor to apply increasing pressure to a limb region suspected of compartment syndrome. This increasing pressure is applied over a predetermined time period by the distal end of the applicator instrument. 
     In the next step of the method, the pressure applied to the force probe is repeatedly sensed and measured. These values are then stored over the predetermined time period. 
     In a similar fashion, the distance between the force plate and force probe at the distal end of the instrument is measured and stored. Again, this measurement is repeatedly taken over the course of a predetermined time period. 
     The distance and pressure values are then plotted as a curve. An analysis of the curve is then carried out by linear regression techniques to determine a limb hardness. In a final step of the method, a medial diagnosis is made on the basis of the computed hardness. 
     Handpiece Electronics 
     The handpiece schematic (FIG. 4) discloses the electronics within the handheld applicator instrument. DC power is supplied by the laptop computer  5  volt bus. A regulated 4.5 VDC is created by circuitry E 1  (FIG.  4 )—a National Semiconductor chip (LM2931CM) to power the load cell. This output voltage powers the force probe load cell FIG. 4, circuitry E 3  (Entran Part # ELFM-B1-10L, Fairfield, N.J.) and FIG.  2 . When force is applied to the handheld applicator instrument, the force signal from the probe E 3  (FIG. 4) is converted to an analog signal in circuit E 2  (FIG. 4) and output to connector cable J 1  position  2 . Within circuitry E 2  (FIG. 4) the load cell is wired to J 2  and instrumentation amplifier U 7  (INA114AU—BurrBrown, Tucson, Ariz.) converts the differential input from the force probe to a calibrated analog output. An output of 10 volts corresponds to the maximum force reading of 10 pounds. This pressure signal goes through connector cable J 1  to the breakout box. 
     Breakout Box Electronics 
     The breakout box (FIG. 5) performs several functions, as described hereinafter. The box allows an interface from a  50  conductor flat computer cable to a durable small diameter handpiece cable. 
     FIG. 5 circuitry B 1  also utilizes +5 VDC laptop bus and U 3  (DCPO 105  Burr-Brown Tucson, Ariz.) to create a ±12 VDC supply for analog IC requirements; FIG. 5 circuitry B 2  comparator U 4  (LM 311  National Semiconductor) detects when ten pounds of force is present on load cell, and sounds an alarm to alert the operator (i.e. doctor) that the maximum load cell pressure is being applied. 
     FIG. 5 circuitry B 3  comparator U 2  provides a “MEASURE” signal when load cell pressure reaches 25 grams which zeroes the Data Acquisition Card (DAC) in the handpiece and triggers the laptop to begin data acquisition. A  50  conductor flex cable interfaces with the laptop through a data acquisition card (DAQ700) and DAQ software from National Instruments (Lab View). 
     The displacement of the force plate (FIG. 2) occurs as progressive force is applied to the handheld instrument pushing probe into the limb. Force plate is normally maintained flush with the tip of probe by a spring that maintains a constant force slightly greater than the weight of force plate. (This force is essentially constant regardless of the position of force plate.) This spring (partially shown in FIG. 2) is installed around probe. The force plate has three attached columns that stabilize and direct the displacement of the force plate through the housing of the instrument. As the elongated column (FIG. 2) moves through the housing, optical encoder (USDigital—Vancouver, Wash.) of FIG. 4 circuitry E 4  and FIG. 2 generates a series of pulses which are fed to U 2 , a quadrature decoder interface IC (LS7064 USDigital—Vancouver, Wash.). Signals from U 2 , (UP/DOWN* directional signal and CLK distance signal) feed into circuitry E 5 . 
     FIG. 4 circuitry E 5 , a “ripple counter” (12 bit up/down counter) comprised of U 1 , U 4  and U 5  increments or decrements based on the input signals from U 2 . This “displacement” data feeds U 3 , a 12-bit DAC (MAX507 Maxim Integrated Products Sunnyvale, Calif.). U 3  output signal “LIN_OUT” is then routed to connector cable J 1  position  3  and then to the breakout box. 
     Once initiated, simultaneous pressure and distance measurements are read by the laptop computer data acquisition card (DAC-700 National Instruments Austin, Tex.) through flex cable L 1  from the breakout box. 
     The laptop performs the following functions: Upon detection of a start signal from circuitry B 1  of the breakout box (FIG.  5 ), begins a three second sample of pressure and distance data; Provides the timing signal for the operator (i.e. doctor) for the duration of the data acquisition phase; Sounds an alarm when 7.5 pounds, and then 10 pounds, of pressure is detected at the load cell. 
     The software program (LabVIEW National Instruments—Austin, Tex.) plots and displays the points of pressure vs. distance. It also performs a mathematical analysis of the data, including a linear regression analysis based on user selected data ranges. The program is capable of saving the data and repeatedly analyzing the data for replotting and reprinting. 
     The linear regression of a specific portion of the curve of pressure vs. distance is the hardness measurement of the compartment, note FIG.  6 . The plot of the pressure to distance curve has information regarding subcutaneous fat thickness as well as muscle compartment pressure and underlying muscle tone (FIG.  6 ). These three parts of the curve include the beginning portion (A), mid portion (B) and end portion (C). The initial portion (A) corresponds to the subcutaneous fat portion overlying the compartment. The mid linear portion (B) corresponds to the pressure within the compartment. The end part of the curve (C) is the compaction of the muscle compartment which gives information regarding the tone of the muscle (FIG.  5 ). The linear regression plot (D) of mid portion (B) gives the hardness value (E) of the muscle. 
     The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. 
     Now that the invention has been described,

Technology Category: 3