Patent Publication Number: US-2016220150-A1

Title: Surgical instrument with magnetic sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application filed under 35 U.S.C. §371(a) of International Patent Application No. PCT/US2014/050825, filed Aug. 13, 2014, which claims benefit of, and priority to, U.S. Provisional Patent Application 61/882,323, filed on Sep. 25, 2013. The entire contents of each of the above applications is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a surgical instrument, and more particularly, to a surgical instrument including a magnetic field sensor assembly for determining tissue thickness. 
     2. Background of Related Art 
     Various surgical procedures are performed in a minimally invasive manner. This includes forming a small opening through a body wall of a patient, e.g., in the abdomen, and inserting surgical instruments therethrough to perform surgical procedures. Due to the relatively small interior dimensions of the access devices used in endoscopic procedures, only elongated, small-diametered instrumentation may be used to access the internal body cavities and organs. Typically, such instruments are limited in their ability to sense and/or control conditions and/or parameters during an operation, such as, for example, the thickness of tissue positioned between tissue contacting surfaces of an end effector of the surgical instrument. 
     Accordingly, a need exists for surgical instruments that can sense the amount of tissue positioned between tissue contacting surfaces of an end effector of the surgical instrument and provide this information to the user prior to operation of the surgical instrument. 
     SUMMARY 
     In accordance with an embodiment of the present disclosure, there is provided a surgical instrument including an end effector, a magnetic field sensor assembly, and a processor. The end effector includes first and second tissue contacting surfaces configured to receive tissue therebetween. The first tissue contacting surface is movable relative to the second tissue contacting surface between a spaced apart position and an approximated position. The magnetic field sensor assembly includes a first magnetic field sensor disposed on the first tissue contacting surface and a first magnet disposed on the second tissue contacting surface. Alternatively, the first magnetic field sensor may be disposed on the second tissue contacting surface and the first magnet may be disposed on the first tissue contacting surface. The processor is connected to the first magnetic field sensor. The processor determines a distance between the first and second tissue contacting surfaces based on a detectable signal received from the first magnetic field sensor. 
     In an embodiment, the surgical instrument may further include a contact sensor disposed on the first tissue contacting surface. The contact sensor may monitor contact between tissue and the first tissue contacting surface during approximation of the first tissue contacting surface toward the second tissue contacting surface. 
     In another embodiment, the first tissue contacting surface may be pivotably coupled with the second tissue contacting surface about a pivot. In particular, the first magnetic field sensor may be disposed adjacent the pivot. The magnetic field sensor assembly may further include a second magnetic field sensor disposed distal of the first magnetic field sensor and a second magnet disposed distal of the first magnet, such that during approximation of the first tissue contacting surface toward the second tissue contacting surface, the first magnetic field sensor contacts tissue while the second magnetic field sensor is spaced apart from tissue. 
     In an embodiment, the first magnet and the first magnetic field sensor may be in a superposed relation in the approximated position. The first magnetic field sensor may be a Hall effect sensor. Alternatively, the first magnetic field sensor may include a magnetoresistive film. 
     In accordance with another aspect of the present disclosure, there is provided a method of determining tissue thickness. The method includes placing tissue between a first tissue contacting surface and a second tissue contacting surface of an end effector of a surgical instrument; approximating the first and second tissue contacting surfaces; generating a detectable signal; and calculating a distance between the first and second tissue contacting surfaces based on the detectable signal. The detectable signal is generated by a magnetic field sensor on the first tissue contacting surface in response to a magnetic field of a magnet on the second tissue contacting surface. 
     In an embodiment, the method may further include determining an initial contact between tissue and the first tissue contacting surface. Furthermore, generating a detectable signal may include generating the detectable signal at the time of initial contact between tissue and the first tissue contacting surface. 
     In accordance with another embodiment of the present disclosure, there is provided a method of determining tissue thickness. The method includes placing a magnet on a first side of tissue; placing a magnetic field sensor mounted on a surgical instrument on a second side of tissue; generating a detectable signal; and calculating a distance between the magnet and the magnetic field sensor based on the detectable signal. The second side is opposite of the first side. The detectable signal is generated by the magnetic field sensor in response to a magnetic field of the magnet. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present disclosure are described hereinbelow with reference to the drawings, wherein: 
         FIG. 1  is a perspective view of a surgical instrument in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a side, cross-sectional view of a main body of the surgical instrument of  FIG. 1 , shown in a first, unapproximated condition; 
         FIG. 3  is an enlarged, side cross-sectional view of a tool assembly of the surgical instrument of  FIGS. 1 and 2 , shown in the first, unapproximated condition; 
         FIG. 4  is an enlarged, side cross-sectional view of the tool assembly of the surgical instrument of  FIGS. 1 and 2 , shown in a second, approximated condition; 
         FIG. 5  in an enlarged, side cross-sectional view of the tool assembly of the surgical instrument of  FIGS. 1 and 2 , shown after completion of a firing stroke; 
         FIG. 6  is a perspective view of a surgical instrument in accordance with another embodiment of the present disclosure; 
         FIG. 7  is a top perspective view of a handle assembly of the surgical instrument of  FIG. 6  with a portion of a handle section removed therefrom; 
         FIG. 8  is a side cross-sectional view of the distal end of the surgical instrument of  FIGS. 6 and 7 , shown in a first condition; 
         FIG. 9  is a side cross-sectional view of the distal end of the surgical instrument of  FIGS. 6 and 7 , shown in a second condition; 
         FIG. 10  is a perspective view of a surgical instrument in accordance with another embodiment of the present disclosure; 
         FIG. 11  is a side, cross-sectional view of the surgical instrument of  FIG. 10 ; and 
         FIG. 12  is a partial perspective view of a magnet assembly for use with the surgical instrument of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described in detail with reference to the drawings, in which like reference numerals designate identical or corresponding elements in each of the several views. As used herein, the term “distal,” as is conventional, will refer to that portion of the instrument, apparatus, device or component thereof which is farther from the user while, the term “proximal,” will refer to that portion of the instrument, apparatus, device or component thereof which is closer to the user. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
     With reference now to  FIGS. 1 and 2 , there is illustrated a surgical instrument  300  including a magnetic field sensor assembly  3000  ( FIG. 2 ) in accordance with an embodiment of the present disclosure. Surgical instrument  300  includes a handle assembly  312  and an elongated body  314 . Handle assembly  312  includes a stationary handle member  326 , a movable handle or trigger  328  and a barrel portion  330 . A disposable loading unit or DLU  316  is releasably secured to a distal end of elongated body  314 . DLU  316  includes a proximal body portion  318 , which forms an extension of elongated body  314 , and a distal tool assembly or end effector  320  including a cartridge assembly  322  and an anvil assembly  324 . Tool assembly  320  is pivotably connected to body portion  318  about an axis substantially perpendicular to the longitudinal axis of elongated body  314 . Reference may be made to U.S. Pat. No. 8,281,937, the entire contents of which are incorporated herein by reference, for a more detailed discussion of the structure and operation of surgical instrument  300 . 
     With particular reference now to  FIGS. 2-4 , surgical instrument  300  includes a magnetic field sensor assembly  3000  disposed in tool assembly  320 . Magnetic field sensor assembly  3000  includes a plurality of magnets  362   a,    362   b,    362   c,    362   d  disposed on tissue contacting surface  322   a  ( FIG. 3 ) of cartridge assembly  322  and a plurality of magnetic field sensors  360   a,    360   b,    360   c,    360   d  disposed on tissue contacting surface  324   a  ( FIG. 3 ) of anvil assembly  324 . Magnets  362   a,    362   b,    362   c,    362   d  may be permanent magnets or electromagnets. 
     Magnetic field sensors  360   a,    360   b,    360   c,    360   d  may be any type of sensor capable of generating a detectable signal in response to the presence of a magnetic field. In embodiments, the magnitude of the detectable signal generated by the sensor varies with the strength of the magnetic field detected. Suitable magnetic field sensors include, e.g., Hall effect sensors. As those skilled in the art will appreciate, a Hall effect sensor is a transducer that varies its output voltage (the detectable signal) in response to a magnetic field. Magnetoresistive films may be used in making the magnetic field sensor. For example, a magnetic field sensor made from thin film giant magnetoresistive (GMR) materials may be placed adjacent a source for producing a magnetic field. In embodiments, the GMR material and the source for producing the magnetic field may be placed on respective tissue contacting surfaces  322   a,    324   a  of surgical instrument  300 . Accordingly, the distance from the GMR material to the source for producing the magnetic field would vary with changes in the thickness of tissue. The distance from the GMR material to the source for producing the magnetic field may be calculated based on the magnitude of the detectable signal generated by the GMR material based on the strength of the magnetic field at any given time. 
     Magnetic field sensors  360   a,    360   b,    360   c,    360   d  are pre-calibrated for magnets  362   a,    362   b,    362   c,    362   d.  For any particular magnet  362   a,    362   b,    362   c,    362   d  and orientation of sensor  360   a,    360   b,    360   c,    360   d  with respect to that magnet, distance between sensor  360   a,    360   b ,  360   c,    360   d  and the respective magnets  362   a,    362   b,    362   c,    362   d  can be determined by means of interpolation of pre-calibrated values. A sensor reading proportional to the magnetic field is transformed to a distance measurement by means of interpolation or lookup table in which each value of the magnetic field measurement is converted to the thickness of tissue. 
     Magnetic permeability of the material is given by the equation 
       μ=μ 0 (1+χ m )   (Eq. 1)
 
     where μ 0  is permeability of free space and χ m  is a magnetic susceptibility of material. For diamagnetic and paramagnetic materials, magnetic susceptibility is extremely small χ m &lt;&lt;1) (e.g., χ m  of water is −9.035×10 −6 ). Human tissue and other nonferrous and ferrimagnetic materials do not differ substantially from that of free space in terms of magnetic field propagation. As such, the permeabilities of diamagnetic and paramagnetic materials do not differ substantially from that of free space and these materials being inserted between magnet and magnetometer substantially have no effect on distance measurements. 
     With particular reference now to  FIGS. 3 and 4 , magnets  362   a,    362   b,    362   c,    362   d  and corresponding magnetic field sensors  360   a,    360   b,    360   c,    360   d  are positioned on respective tissue contacting surfaces  322   a,    324   a,  such that a magnet  362   a,    362   b,    362   c,    362   d  and a corresponding magnetic field sensor  360   a,    360   b,    360   c,    360   d  form a pair and are in a superposed relation when anvil assembly  324  is in the approximated position ( FIG. 4 ) to clamp tissue “T” between tissue contacting surfaces  322   a,    324   a.    
     Sensors  360   a,    360   b,    360   c,    360   d  may be selectively connected to a processor or a central processing unit (CPU) ( FIG. 1 ) for monitoring, controlling, processing and/or storing information observed, measured, sensed and/or transmitted from any of the elements of components of the surgical instruments prior, during and/or after the surgical procedure. Sensors  360   a,    360   b,    360   c,    360   d  may be electrically connected via a wire  7  ( FIG. 3 ) or connected wirelessly to CPU. The data collected by sensors  360   a,    360   b,    360   c,    360   d  are sent to CPU. The data are transformed to a distance measurement by means of interpolation in which each value of magnetic field measurement is converted to a tissue thickness. The tissue thickness may be displayed on an indicator (not shown) in units of length (thickness) or, alternatively, graphically represented for potential use of the device in any particular case, e.g., whether the device is appropriately sized for the procedure. It is contemplated that the display may be the monitor to which images are shown from the camera used during laparoscopic surgery. It is also contemplated that the display may be on the instrument itself, for example, on barrel portion  330  of surgical instrument  300 , or any other portion of surgical instrument  300  that is easily viewed by the user during surgery. 
     Tool assembly  320  may further include contact sensors  77   a,    79   a  connected to CPU to detect an initial contact between tissue “T” and tissue contacting surface  324   a  of anvil assembly  324 . For example, contact sensors  77   a,    79   a  may include pressure sensors, electrical contacts and sensing circuits, force transducers, piezoelectric elements, piezoresistive elements, metal film strain gauges, semiconductor strain gauges, inductive pressure sensors, capacitive pressure sensors, and potentiometric pressure transducers. 
     Contact sensors  77   a,    79   a  may be disposed adjacent sensor  360   a  and magnet  362   a , respectively. In particular, contact sensors  77   a  detect an initial contact between tissue contacting surface  324   a  and tissue “T” during approximation of anvil assembly  324 . In this manner, magnetic field sensor  360   a  can measure tissue thickness when tissue “T” is initially brought into contact with tissue contacting surface  324   a  of anvil assembly  324 , which, in turn, enables the surgeon to measure the substantially uncompressed thickness of tissue “T”. As surgical instrument  300  is being clamped onto tissue “T”, contact sensors  77   a,    79   a  may provide the user with an indication (e.g., audio, visual, tactile, etc.) as to when tissue “T” is initially brought into contact with tissue contacting surface  324   a  of anvil assembly  324 . 
     In use, with cartridge assembly  322  and anvil assembly  324  in spaced relation to one another, target tissue “T” is placed therebetween. With the target tissue “T” positioned between cartridge assembly  322  and anvil assembly  324 , anvil assembly  324  is approximated toward cartridge assembly  322 . Contact sensor  77   a,    79   a  may detect the initial contact between tissue “T” and tissue contacting surface  324   a.  At this time, magnetic field sensor  360   a  may measure the magnetic field and send the data to CPU, which determines the substantially uncompressed thickness of tissue “T”. The tissue thickness in the uncompressed state is measured and/or recorded. Thereafter, cartridge assembly  322  and anvil assembly  324  are further approximated until all sensors  360   a,    360   b,    360   c,    360   d  are in a superposed relation with the respective magnets  362   a,    362   b,    362   c,    362   d.  Then, the tissue thickness in the compressed state is measured and/or recorded. 
     With reference to  FIGS. 1 and 5 , anvil assembly  324  is movable in relation to cartridge assembly  322  between an open position ( FIG. 3 ) spaced from cartridge assembly  322  and an approximated or clamped position ( FIG. 4 ) in juxtaposed alignment with cartridge assembly  322 . To approximate cartridge and anvil assemblies  322  and  324 , movable handle  328  is moved toward stationary handle  326 , through an actuation stroke. Subsequent movement of movable handle  328  through the actuation stroke effects advancement of an actuation shaft and a firing rod (not shown). As the actuation shaft is advanced, the firing rod is also advanced. 
     The firing rod is connected at its distal end to axial drive assembly  312   a  ( FIG. 4 ) such that advancement of the firing rod effects advancement of drive assembly  312   a.  As drive assembly  312   a  is advanced, cam roller  386  moves into engagement with cam surface  309  of anvil assembly  324  to urge anvil assembly  324  toward cartridge assembly  322 , thereby approximating cartridge and anvil assemblies  322  and  324  and clamping tissue “T” therebetween. 
     To fire surgical instrument  300 , movable handle  328  is moved through a second actuation stroke to further advance the actuation shaft and the firing rod distally. As the firing rod is advanced distally, drive assembly  312   a  ( FIG. 4 ) is advanced distally to advance actuation sled  334  through staple cartridge assembly  322  to simultaneously sever tissue “T” with knife  380  and drive pushers  348  to sequentially eject staples “S” from cartridge assembly  322 . 
     Surgical instrument  300  is adapted to receive DLU&#39;s having staple cartridges with staples in linear rows having a length of from about 30 mm to about 60 mm. For example, each actuation stroke of movable handle  328  during firing of surgical instrument  300  may advance the actuation shaft approximately 15 mm, although other lengths are envisioned. Accordingly, in embodiments to fire a cartridge assembly having a 45 mm row of staples, movable handle  328  must be moved through three actuation strokes after the approximating or clamping stroke of movable handle  328 . 
     With reference now to  FIGS. 6 through 9 , a surgical instrument including a magnetic field sensor assembly  1000  ( FIG. 8 ) in accordance with another embodiment of the present disclosure is generally designated as  100 . Surgical instrument  100  includes a proximal handle assembly  112 , an elongated central body portion  114  including a curved elongated outer tube  114   a,  and a distal head portion  116 . Alternately, in some surgical procedures, e.g., the treatment of hemorrhoids, it is desirable to have a substantially straight, preferably shortened, central body portion. The length, shape and/or the diameter of body portion  114  and head portion  116  may also be tailored to a particular surgical procedure being performed. 
     With continued reference to  FIG. 6 , handle assembly  112  includes a stationary handle  118 , a firing trigger  120 , a rotatable approximation knob  122  and an indicator  124 . Head portion  116  includes an anvil assembly  130  and a shell assembly  131 . Reference may be made to U.S. Pat. No. 7,802,712, the entire contents of which are incorporated herein by reference, for a more detailed discussion of the structure and operation of surgical instrument  100 . 
     With additional reference to  FIGS. 6, 8, and 9 , magnetic field sensor assembly  1000  includes a plurality of magnets  162  disposed on tissue contacting surface  131   a  of shell assembly  131  and a plurality of magnetic field sensors  160  disposed on tissue contacting surface  130   a  of anvil assembly  130 . Magnets  162  and magnetic field sensors  160  may be constructed as described above in connection with the embodiments of  FIGS. 1 to 5 . 
     With continued reference to  FIGS. 8 and 9 , magnets  162  and corresponding magnetic field sensors  160  are positioned on respective tissue contacting surfaces  130   a,    131   a  such that a magnet  162  and a corresponding magnetic field sensor  160  form a pair and are in a superposed relation when anvil assembly  130  is in the approximated position ( FIG. 9 ) to clamp tissue “T 1 ”, “T 2 ” between tissue contacting surfaces  130   a,    131   a.  The magnetic field reading or detectable signal from magnetic field sensors  160  is sent to a processor (CPU) ( FIG. 6 ). The data are transformed to a distance measurement by means of interpolation in which the value of the magnetic field measurement is translated to a tissue thickness. The tissue thickness can be displayed in any suitable manner, such as, for example, on indicator  124  ( FIG. 6 ) in units of length (thickness) or, alternatively, graphically represented for potential use of the device in any particular case, e.g., whether device caliber is appropriate for certain procedure. 
     Head portion  116  may further include contact sensors  177 ,  179  connected to CPU to provide indication as to when tissue interposed between anvil assembly  130  and shell assembly  131  is initially brought into contact with tissue contacting surface  130   a.  Thus, a substantially uncompressed thickness of tissue may be measured by monitoring magnetic field sensor  160  when tissue is initially brought into contact with tissue contacting surface  130   a.    
     With reference now to  FIGS. 7 and 8 , the approximation mechanism includes approximation knob  122 , a drive screw  132 , a rotatable sleeve  170 , and an anvil retainer  138  ( FIG. 8 ) for supporting an anvil assembly  130 . Rotatable sleeve  170  includes a substantially cylindrical hollow body portion and a substantially cylindrical collar  142  which together define a central bore. Collar  142  has an annular groove  144  formed thereabout, which is dimensioned to receive an inwardly extending flange formed on an inner wall of handle assembly  118 . Engagement between groove  144  and the flanges axially fixes sleeve  170  within handle assembly  118  while permitting rotation of sleeve  170  in relation to handle assembly  118 . A pair of diametrically opposed elongated ribs  148  is positioned or formed on the outer surface of the body portion. Approximation knob  122  includes a pair of internal slots (not shown) positioned to receive ribs  148  of sleeve  170  to rotatably fix sleeve  170  to knob  122 , such that rotation of knob  122  causes concomitant rotation of sleeve  170 . 
     The proximal half of screw  132  includes a helical channel  150  and is dimensioned to be slidably positioned within the central bore of rotatable sleeve  170 . Since sleeve  170  is axially fixed with respect to handle assembly  118 , rotation of sleeve  170  about screw  132  causes a pin (not shown) to move along channel  150  of screw  132  to effect axial movement of screw  132  within handle assembly  118 . 
     In use, when approximation knob  122  is manually rotated, rotatable sleeve  170  is rotated about the proximal end of screw  132  to move a pin along helical channel  150  of screw  132 . Since sleeve  170  is axially fixed to handle assembly  118 , as the pin is moved through channel  150 , screw  132  is advanced or retracted within handle assembly  118 . As a result, top and bottom screw extensions (not shown), which are fastened to the distal end of screw  132  and to anvil retainer  138 , are moved axially within elongated body portion  114 . Since anvil assembly  130  is secured to the distal end of anvil retainer  138 , rotation of approximation knob  122  will effect movement of anvil assembly  130  in relation to shell assembly  131  between spaced and approximated positions. 
     With shell assembly  131  and anvil assembly  130  in spaced relation to one another, target tissue is placed therebetween. With the target tissue positioned between shell assembly  131  and anvil assembly  130 , anvil assembly  130  is approximated towards shell assembly  131  until the target tissue makes a contact with contact sensors  177 ,  179 . At this time, magnetic field sensors  160  may measure the magnetic field and send the data to CPU, which determines the thickness of substantially uncompressed tissue. The tissue thickness in the uncompressed state is displayed and/or recorded. Thereafter, shell assembly  131  and anvil assembly  130  are further approximated until a desired gap between shell assembly  131  and anvil assembly  130  is obtained. A compressed tissue thickness may be measured by magnetic field sensors  160 , during or after approximation of shell assembly  131  and anvil assembly  130 . 
     In operation, following purse string suturing of a first tissue “T 1 ” to anvil assembly  130  and purse string suturing of a second tissue “T 2 ” to shell assembly  131  ( FIG. 8 ), approximation knob  122  is rotated to approximate anvil assembly  130  towards shell assembly  131 . As anvil assembly  130  and shell assembly  131  are approximated toward one another, first and second tissue “T 1 , T 2 ” are extended toward one another and are tensioned. As first and second tissue “T 1 , T 2 ” are tensioned, first and second tissue “T 1 , T 2 ” tend to constrict around anvil assembly  130  and shell assembly  131 , respectively. This constriction exerts a force on each respective force measuring sensor  164 ,  166 . The force measured by each force measuring sensor  164 ,  166  may be converted, using known algorithms, to a value of tension force which is being exerted on each tissue “T 1 , T 2 ”. Surgical instrument  100  may include a gauge  140  ( FIG. 6 ) supported on stationary handle  118  of handle assembly  112 . Each sensor  160  may be operatively connected to gauge  140 . Gauge  140  functions to display, in real time, selected operational parameters, such as, for example, tissue contact, tissue compression, tissue tension, etc. 
     During a surgical anastomotic procedure, the tension on first and second tissues “T 1 , T 2 ” may be monitored to maintain the tension exerted thereon at or below a predetermined threshold level. For example, if the tension exerted on each tissue “T 1 , T 2 ”, either alone or in combination, exceeds a predetermined threshold level, elevated tension acts on the staple line and may result in undue strains exerted on the staples and/or the staple line. 
     With reference now to  FIGS. 10 and 11 , a surgical instrument including a magnetic field sensor  560  in accordance with an embodiment of the present disclosure is generally designated as  500 . Surgical instrument  500  is configured to serially deploy at least one surgical anchor  510  to attach a prosthesis in place in the repair of a defect in tissue such as an inguinal hernia. Surgical instrument  500  includes a handle assembly  520  and a delivery tube  530  extending distally from handle assembly  520 . Handle assembly  520  includes a stationary handle  521  and a firing trigger  522 . Reference may be made to U.S. Pat. No. 7,758,612, the entire contents of which are incorporated herein by reference, for a more detailed discussion of the structure and operation of surgical instrument  500  and surgical anchor  510 . 
     With particular reference now to  FIGS. 10 and 12 , magnetic field sensor  560  is disposed at a distal portion of delivery tube  530 . The placement of magnetic field sensor  560  at, e.g., a distal-most portion of delivery tube  530 , facilitates use thereof in conjunction with a magnet assembly  600 . Magnet assembly  600  includes a magnet  605  and an elongated support  607  extending from magnet  605 . Magnet  605  and magnetic field sensor  560  may be constructed as described above in connection with embodiments of  FIGS. 1-9 . 
     Magnet  605  may be positioned on one side of tissue to be measured and magnetic field sensor  560  may be placed on an opposing side of tissue. Magnetic field sensor  560  generates a detectable signal in response to a magnetic field of magnet  605 . Magnetic field sensor  560  may be connected to a processor (not shown). The processor may calculate the distance between magnet  605  and magnetic field sensor  560 , i.e., thickness of tissue, based on the detectable signal. 
     With respect to  FIG. 11 , upon determining thickness of tissue, the surgeon may then apply surgical anchor  510  to tissue by pulling trigger  522  toward stationary handle  521 . When the surgeon pulls trigger  522  toward stationary handle  521 , a lever  524  rotates counterclockwise such that a cam surface  531  of lever  524  contacts a piston  525  which drives an anchor carrier rod  526  distally. A torsion spring  527  is compressed as lever  524  is rotated counterclockwise. Anchor carrier  526  is urged distally within a queuing spring  528 , which, in turn, urges the distal-most anchor  510  past a distal end of delivery tube  530 . In this manner, anchor  510  penetrates through the prosthesis and tissue. 
     Although the illustrative embodiments of the present disclosure have been described herein with reference to the accompanying drawings, the above description, disclosure, and figures should not be construed as limiting, but merely as exemplifications of particular embodiments. For example, in the embodiments described in connection with  FIGS. 1 to 5 , it is envisioned that the plurality of magnets  362   a - d  may be disposed on tissue contacting surface  324   a  of anvil assembly  324 , and the plurality of magnetic field sensors  360   a - d  may be disposed on tissue contacting surface  322   a  of cartridge assembly  322 . Likewise, with respect to the embodiments described in connection with  FIGS. 6 to 9 , it is envisioned that the plurality of magnets  162  may be disposed on tissue contacting surface  130   a  of anvil assembly  130 , and the plurality of magnetic field sensors  160  may be disposed on tissue contacting surface  131   a  of shell assembly  131 . In addition, it is envisioned that magnet  605  may be placed on a distal portion of delivery tube  530 , and magnetic field sensor  560  may be provided as a separate element from surgical instrument  500 . It is to be understood, therefore, that the disclosure is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the disclosure.