Abstract:
A method of determining the accuracy of a pressure sensor in a surgical cassette is disclosed. The method involves displacing a diaphragm of the sensor a pre-defined amount of displacement, and measuring the force exerted on the diaphragm by the displacing step. The accuracy of the pressure sensor is determined by comparing the force measured in the measuring step to a pre-defined force for the pre-defined amount of displacement.

Description:
[0001]    This application claims the priority of U.S. Provisional Application No. 60/419,062 filed Oct. 16, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to pressure sensors used on surgical cassettes and more particularly to a method of testing the accuracy of such sensors prior to surgery.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    Surgical cassettes utilized in phacoemsulsification, vitreoretinal, or other ophthalmic surgical procedures typically have an aspiration manifold within the cassette. When the cassette is inserted into an ophthalmic surgical console, the aspiration manifold is operatively coupled to a source of vacuum. The cassette is also fluidly coupled to the aspiration port of an ophthalmic surgical handpiece, typically via flexible plastic tubing. Ophthalmic tissue is aspirated by the handpiece into a collection bag that is also fluidly coupled to the aspiration manifold of the cassette. Such cassettes typically employ a variety of pressure sensors to measure the vacuum level within the aspiration manifold of the cassette and thus the eye. For example, such cassettes have utilized both conventional vacuum transducers and non-invasive pressure sensors to measure such vacuum. Exemplary non-invasive pressure sensors are disclosed in U.S. Pat. Nos. 5,910,110 to Bastable and 5,470,312 to Zanger et al., both of which are incorporated herein in their entirety by reference.  
           [0004]    Communicating an accurate reading of the vacuum level within the aspiration manifold of such surgical cassettes to the surgeon is critical to the success of the surgical procedure and the safety of the patient. For example, during a phacoemulsification procedure, the tip of the phacoemulsification handpiece may become occluded with ophthalmic tissue. When the tip occludes, the peristaltic pump vacuum source of the surgical system continues to pump, increasing the vacuum within the aspiration line of the handpiece. When the blockage on the tip is removed, the patient&#39;s eye may be exposed to a dangerous surge of vacuum. However, if the vacuum level within the aspiration manifold of the cassette is measured and provided to the surgeon, the surgeon can use the user interface of the surgical console to slow down or stop the peristaltic pump to bring the vacuum to the desired level before the blockage breaks free. To insure that an accurate aspiration manifold vacuum reading is provided to the surgeon, certain ophthalmic surgical systems utilize two pressure sensors to measure vacuum in the aspiration manifold of the cassette. With this design, the surgeon still receives an accurate measurement of the vacuum level within the aspiration manifold of the cassette even if one of the sensors fails or is not working properly. However, such dual redundancy increases the cost and complexity of the surgical system and cassette. Therefore, a need exists for an improved apparatus and method of insuring the accuracy of such pressure sensors.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to a method of determining the accuracy of a pressure sensor in a surgical cassette. A surgical cassette having a pressure sensor is provided. The pressure sensor has a diaphragm. A surgical console with a cassette receiving area is also provided. The cassette is disposed in the cassette receiving area. The diaphragm is displaced a pre-defined amount of displacement, and the force exerted on the diaphragm by the displacing step is measured. The accuracy of the pressure sensor is determined by comparing the force measured in the measuring step to a pre-defined force for the pre-defined amount of displacement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    For a more complete understanding of the present invention, and for further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings in which:  
         [0007]    [0007]FIG. 1 is a top, partially sectional view schematically illustrating the relevant portions of a surgical system and cassette according to a preferred embodiment of the present invention;  
         [0008]    [0008]FIG. 2 is a front view of the non-invasive pressure sensor of the surgical cassette of FIG. 1 according to a preferred embodiment of the present invention;  
         [0009]    [0009]FIG. 3 is a side, sectional view of the sensor of FIG. 2 along line  3 - 3 ;  
         [0010]    [0010]FIG. 4 is a top, partially sectional view similar to FIG. 1 showing the plunger of the surgical system loading the diaphragm of the sensor of FIGS.  2 - 3 ;  
         [0011]    [0011]FIG. 5. is the preferred force versus displacement curve for the diaphragm of the sensor of FIGS.  2 - 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS.  1 - 5  of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
         [0013]    Referring to FIGS.  1 - 3 , a surgical system  10  generally includes a surgical console  12  and a surgical cassette  14 . Console  12  and cassette  14  are preferably for use in ophthalmic surgery, although the present invention is applicable to other surgical systems that provide aspiration to a surgical handpiece. Surgical console  12  includes a cassette receiving area  16  for removably receiving cassette  14 , a linear actuator  18 , a load cell or force gage  20 , and a computer or microprocessor  22 . Linear actuator  18  includes a lead screw  24  having a plunger  26  on one end. Linear actuator  18  is preferably a conventional linear stepper motor having a shaft  24 . A preferred linear stepper motor  18  is the Model ZB17GBKR-13 available from Eastern Air Devices (EAD) of Dover, N.H. The rotation of linear stepper motor  18  one step preferably results in a 0.0003125 inch linear displacement of shaft  24  and plunger  26 . However, linear actuator  18  may also be a DC motor with position feedback, a pneumatically actuated piston, or other conventional means of moving a plunger with a known displacement. A preferred load cell for load cell  20  is the Model 31 available from Sensotec of Columbus, Ohio. Linear stepper motor  18  and load cell  20  are electronically coupled to computer  22  in a conventional manner, as schematically illustrated by lines  28  and  30 , respectively. Cassette receiving area  16  has a front plate  32  for interfacing with cassette  14  including an aperture  34  for plunger  26  and apertures  36  and  38  for other plungers of console  12  used to interface with various portions of cassette  14 .  
         [0014]    Surgical cassette  14  generally includes a body  50  having a pressure sensor receiving area  52 , a non-invasive pressure sensor  54  disposed in receiving area  52 , and an aspiration manifold  56  fluidly coupled to sensor  54 . Body  50  is preferably a rigid thermoplastic and may be made from any suitable method, such as machining or injection molding. Although not shown if the Figures, cassette  14  may also include additional fluid channels, manifolds, or ports that provide control of aspiration or irrigation fluid. A preferred ophthalmic surgical cassette for cassette  14  is disclosed in U.S. Pat. No. 6,293,926, which is incorporated herein in its entirety by this reference.  
         [0015]    Pressure sensor  54  has a body  58  having a cavity  60 , a port  62  for fluidly coupling with aspiration manifold  56 , and a diaphragm or membrane  64 . Body  58  is preferably a rigid thermoplastic, and diaphragm  64  is preferably made of stainless steel. Diaphragm  64  has a rim  66  that mates with a recess  68  in body  58  to retain diaphragm  64  within body  58 . Diaphragm  64  preferably has a diameter of about 0.996 inches (not including rim  66 ). Diaphragm  64  preferably has a thickness of about 0.0027 inches to about 0.0033 inches, and most preferably about 0.003 inches. Diaphragm  64  is preferably made of 17-7 stainless steel.  
         [0016]    When cassette  14  is inserted into cassette receiving area  16  of console  12 , computer  22  rotates stepper motor  18 , causing shaft  24  and plunger  26  to be moved linearly through aperture  34  toward diaphragm  64  of sensor  54 . Stepper motor  18  moves plunger  26  until it contacts and displaces diaphragm  64 , as shown in FIG. 4. Plunger  26  preferably displaces diaphragm  64  until a known pre-load force (“F preload ”) is placed on diaphragm  64  as measured by load cell  20 . F preload  must be greater than the largest vacuum exerted on diaphragm  64  via aspiration manifold  56  of cassette  14  and cavity  60  of sensor  54 . F preload  for diaphragm  64  is preferably about 4.0 lb f .  
         [0017]    When console  12  provides vacuum to aspiration manifold  56  of cassette  14  and thus cavity  60  of sensor  54 , the absolute value of the force exerted on diaphragm  64  by plunger  26  varies in an inversely proportional manner with the absolute value of the vacuum level. In other words, larger absolute values of vacuum yield smaller absolute values of force exerted by plunger  64 , and smaller absolute values of vacuum yield larger absolute values of force exerted by plunger  64 . This relationship may be calibrated so that when load cell  20  provides a force measurement to computer  22 , computer  22  can calculate the vacuum level within cavity  60 , aspiration manifold  56 , and the eye.  
         [0018]    As mentioned above, it is critical that sensor  54  accurately measure the vacuum within aspiration manifold  56  of cassette  14 . It has been discovered that the accuracy of sensor  54  is largely dependent on the material properties and geometry of diaphragm  64 . It has been further discovered that the thickness of diaphragm  64  is particularly important to the accuracy of sensor  54 . Given the fact that this thickness is very small (e.g. on the order of 0.003 inches), such diaphragms may be somewhat challenging to manufacture to exactly the desired thickness.  
         [0019]    The following describes the preferred procedure for insuring the accuracy of sensor  54  prior to surgery. Cassette  14  is inserted into cassette receiving area  16  of console  12 . Computer  22  rotates linear stepper motor  18  so that load cell  20  just begins to provide a measurement to computer  22  of the force exerted by plunger  26  against diaphragm  64  (“F plunger ”). Computer  22  then rotates linear stepper motor  18  back 1 step. This plunger displacement is defined as “D 0 ”. The linear displacement of plunger  26  beyond D 0  is equal to the displacement of diaphragm  64  by plunger  26 , is a function of the rotation of linear stepper motor  18 , and is defined as “D”. Computer  22  then rotates linear stepper motor  18  in a step by step fashion until F plunger  equals F preload . Load cell  20  measures F plunger  for each step and provides this force to computer  22 . Computer  22  stores the value for D and the associated value of F plunger  for each step. Computer  22  also compares the measured value of F plunger  to the desired value of F plunger  for each value of D. If the measured value of F plunger  is not within a pre-defined tolerance of the desired value of F plunger , computer  22  signals the surgeon via console  12  that the pressure sensor is defective and to insert a new cassette. Computer  22  may also prevent any surgical procedure due to the defective pressure sensor. If the measured value of F plunger  is within the pre-defined tolerance of the desired value of F plunger  for all values of D, the surgical procedure may proceed.  
         [0020]    [0020]FIG. 5 shows the preferred force F plunger  vs. displacement D curves for three diaphragms  64 , the preferred diaphragm  64  made of 17-7 stainless steel, having a diameter of about 0.996 inches (not including rim  66 ), and a thickness of 0.003 inches; a diaphragm  64  having the above-described characteristics of the preferred diaphragm  64  but having a thickness of 0.0027 inches; and diaphragm  64  having the above-described characteristics of the preferred diaphragm  64  but having a thickness of 0.0033 inches. The three curves may be generated from actual operation of such diaphragms  64  in surgical console  12 , or using a conventional finite element modeling package. The “0.003 inch” curve (or its mathematical equivalent) may be utilized to define the desired value of F plunger  utilized by computer  22  when testing pressure sensor  54  as described above. Information from the “0.0033 inch” and “0.0027 inch” curves (or their mathematical equivalents) may be utilized to define the tolerances for the desired value of F plunger  utilized by computer  22  when testing pressure sensor  54  as described above. Of course, different tolerance curves may be generated for different diaphragms  64  or different applications of cassette  14 , if desired.  
         [0021]    From the above, it may be appreciated that the present invention provides a simple and reliable apparatus and method of insuring the accuracy of a non-invasive pressure sensor of a surgical cassette. The present invention is illustrated herein by example, and various modifications may be made by a person of ordinary skill in the art. For example, computer  22  may generate a force F plunger  versus displacement D curve for a given diaphragm  64  for the entire range of values of D, and then compare this curve to the “tolerance” curves in a batch mode rather than comparing each measured value of F plunger  to see if it is within the pre-defined tolerance at the time its measured, as described above. As another example, F plunger  may be measured at intervals of a pre-defined number of steps of linear stepper motor  18  instead of at each step of linear stepper motor  18  as described above.  
         [0022]    It is believed that the operation and construction of the present invention will be apparent from the foregoing description. While the apparatus and methods shown or described above have been characterized as being preferred, various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the following claims.