Patent Abstract:
A device and method for treating a bone includes a bone plate including first and second portions joined to one another via a connecting portion, a rigidity of the connecting portion being less than rigidities of each of the first and second portions in combination with a first sensor mounted on the first portion measuring strain on the first portion and a second sensor mounted on the second portion measuring strain on the second portion.

Full Description:
BACKGROUND 
       [0001]    Strain gages can be placed on orthopedic implants to track the progress of bone healing. Upon initial implantation, the implants are expected to experience higher levels of strain which decrease during healing as the bone begins to share more of the load with the implant. Currently, however, implant strain values need to be assessed with a known load applied to the bone in order to evaluate bone healing. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention relates to a device and method for treating a bone includes a bone plate including first and second portions joined to one another via a connecting portion, a rigidity of the connecting portion being less than rigidities of each of the first and second portions along with a first sensor mounted on the first portion measuring strain on the first portion and a second sensor mounted on the second portion measuring strain on the second portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  shows a perspective view of a system according to a first exemplary embodiment of the present invention; 
           [0004]      FIG. 2  shows a perspective view of a system according to a second exemplary embodiment of the present invention; 
           [0005]      FIG. 3  shows a perspective view of a system according to a third exemplary embodiment of the present invention; 
           [0006]      FIG. 4  shows a side view of a bone fixation element of the system of  FIG. 3 ; 
           [0007]      FIG. 5  shows a perspective view of a system according to a fourth exemplary embodiment of the present invention; 
           [0008]      FIG. 6  shows a top plan view of a system according to a fifth exemplary embodiment of the present invention; and 
           [0009]      FIG. 7  shows a top plan view of a system according to an alternate embodiment of the present invention, 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present invention may be further understood with reference to the following description and the appended drawings, wherein like elements are referred to with the same reference numerals. The exemplary embodiment of the present invention relate to a system and method for tracking the progress of bone healing. In particular, the exemplary embodiments describe systems and methods that calculate a ratio of strain at multiple locations along an implant and/or a bone. An exemplary embodiment of the system may include a first sensor on a surface of the implant adapted to be positioned at a location proximate a weakened portion of the bone. Strain on the implant at this location will be affected by the strength or stiffness of the weakened bone and the load placed on the bone by the patient. A second sensor may be placed on the implant at a location in which strain measured by the second sensor is affected only by the load placed on the bone such that the measured strain is substantially unchanged by the bone healing process. Thus, a ratio between the strains measured by the first and second sensors provides information corresponding to bone healing, regardless of the load on the bone. It will be understood by those of skill in the art that although the exemplary embodiment specifically describe tracking the healing progress of a leg bone, the present invention may be used to track the progress of healing of any load bearing bone. It will also be understood by those of skill in the art that although the exemplary embodiments specifically show and describe two sensors, the present invention may include additional sensors along different areas of the bone to determine ratios corresponding to the bone healing progress of the different areas. In addition, although exemplary embodiments show a bone plate, the present invention may be used with any other fixation element such as, for example, screws, intramedullary devices, external fixators, spine fixation implants and prosthetics. 
         [0011]    As shown in  FIG. 1 , a system  100  according to a first exemplary embodiment of the invention comprises an implant  102  (e.g., a bone plate) and first and second sensors  104 ,  106 , respectively. The implant  102  is configured for fixation over a target portion of a bone  108  to, for example, fix a fracture  110  or to support a weakened portion of the bone  108 . The first and second sensors  104 ,  106  are mounted along a surface  114  of the implant  102  such that the first and second sensors  104 ,  106  may be mechanically coupled to the bone  108 . Although the surface  114  is shown as facing away from the bone  108  when the implant  102  is fixed to the bone  108  in a desired location, it will be understood by those of skill in the art that the sensors  104 ,  106  may be mounted along any surface of the implant  102 . For example, the sensors  104 ,  106  may also be mounted on a surface of the implant  102  facing the bone  108  or a surface on a side of the implant  102 . The first and second sensors  104 ,  106 , respectively, are positioned on the implant  102  so that, when the implant is in a desired position on the bone  108 , the first sensor  104  is located over a site of the fracture  110  while the second sensor  106  is separated from the fracture  110  over a healthy (i.e., solid) portion  112  of the bone  108  to Measure levels of strain and/or load on the implant  102 , at these positions along the implant  102 . The second sensor  106  should be isolated between two screws locked in a healthy portion  112  of the bone  108  to measure a load on the bone  108 . 
         [0012]    The sensors  104 ,  106  in this embodiment may be passively powered MEMs sensors that are used to measure strain and include an interface for wireless connection to a data collection device as would be understood by those skilled in the art. In another embodiment, the sensors  104 ,  106  may be powered chips that are connected to a printed circuit board (PCB). This permits strain on the implant  102  to be measured and transmitted to the data collection device for further processing without physically accessing the sensors  104 ,  106 . It will be understood by those of skill in the art that the strain measurements detected by the sensors  104 ,  106  are not required to represent actual strain values, but may include any signal that changes based on changing strains of their substrates. For example, the MEMS sensors  104 ,  106  may be RF devices that deform when a strain is placed thereon, resulting in a frequency shift caused by a change in capacitance of the sensors  104 ,  106  such that the frequency shift corresponds to a change in strain. As would be understood by those skilled in the art, an external device may be employed to wirelessly provide a signal to the sensors  104 ,  106 . Changes in a returned signal may then be measured to determine a level of strain to which the sensor is subject. A ratio of the strain measured by the first sensor  104  to the strain measured by the second sensor  106  may then be determined by a physician or other professional to track healing progress. Alternatively, the ratio may be determined by a processing device that may also store the strain measurements and the determined ratios (e.g., in an internal memory or on an external storage device) so that changes in the ratio may be reviewed to more fully understand the progression of the healing over time. 
         [0013]    It will be understood by those of skill in the art that when the bone  108  is initially broken or fractured, strain on the implant  102  at the location of the fracture  110  will vary based on changing mechanical properties of the bone  108  during the healing process and the load placed on the bone  108  (e.g., the weight that the patient places on the leg) while the strain measured in the healthy portion  112  varies based only on the load placed on the bone  108 . Thus, taking a ratio of the strains measured by the two sensors  104 ,  106  normalizes the effects of the load on the sensors  104 ,  106  providing data corresponding to the stiffness of the bone  108  at the fracture site  110 . The ratio of the measurements from the first sensor  104  to the measurements from the second sensor  106  during the healing process should trend in a decreasing pattern over time, whereas a lack of healing would show no recognizable trend over time. 
         [0014]    As shown in  FIG. 2 , a system  200  according to a second exemplary embodiment of the invention is substantially similar to the system  100 , including an implant  202  and at least two sensors  204 ,  206 . However, rather than both sensors  204 ,  206  being positioned on the implant  202 , the first sensor  204  is located on a surface  214  of the implant  202  in a position corresponding to a fracture of a bone  208 , while the second sensor  206  is placed directly on a solid portion  212  of the bone  208 , outside a perimeter of the implant  202 . Thus, the first sensor  204  measures strain on the implant  202  at a position corresponding to the site of the fracture  210  while the second sensor  206  measures strain on the solid portion  212  of the bone  208 . Similarly to the system  100 , a ratio between the strains measured by the first and second sensors  204 ,  206  is determined and tracked to study the progress of healing in the bone  208 . As indicated above, the ratio of the strain measurements from the first sensor  204  to the strain measurements from the second sensor  206  trend in a decreasing pattern as the bone  208  heals, whereas a lack of healing will show no recognizable trend over time. 
         [0015]    As shown in  FIGS. 3-4 , a system  300  according to a third exemplary embodiment of the invention is substantially similar to the system  200 , comprising an implant  302  and at least two sensors  304 ,  306 . Similarly to the first sensor  204 , the first sensor  304  is placed on a surface  314  of the implant  302  in a location corresponding to a position of a fracture  310  of a bone  308  (when the implant  302  is mounted on the bone  308  in a desired position) to measure strain on the implant  302  at the position of the fracture  310  while the second sensor  306  is placed directly on a solid portion  312  of the bone  308 . However, rather than being placed on an exterior surface of the bone  308 , the second sensor  306  is placed within the solid portion  312  via, for example, a bone fixation element  316  (e.g., screw). 
         [0016]    The second sensor  306  may be attached adjacent to a proximal end  318  of the bone fixation element  316  such that when the bone fixation element  316  is inserted into the solid portion  312  of the bone, the second sensor  306  contacts a cortical wall of the bone  308 . The second sensor  306  may be printed or mounted around a portion of the bone fixation element  316  to measure deformation of the bone  308  which is directly related to strain on the bone  308 . The ratio of the measurements from the first sensor  304  to those of the second sensor  306  may then be determined to track healing progress in the same manner described above. 
         [0017]    As shown in  FIG. 5 , a system  400  according to a fourth exemplary embodiment of the invention is substantially similar to the system  100 , comprising an implant  402  and first and second sensors  404 ,  406 , respectively, both of which are mounted on the implant  402 . Similarly to the first sensor  104 , the first sensor  404  is located on the implant  402  in a position which, when the implant  402  is in the desired position, corresponds to the location of a fracture  410  so that the first sensor  404  measures strain on the implant  402  at a position corresponding to the site of the fracture  410 . The second sensor  406  is positioned on a portion  420  of the implant  402  having greater flexibility than the portion of the implant  402  on which the first sensor  404  is mounted. For example, the portion  420  may be made more flexible than other portions of the implant  402  by reducing a width (i.e., an extent of the implant  402  across a bone facing surface thereof in a direction perpendicular to a longitudinal axis of the implant  402 ) and/or a thickness of the portion  420  (i.e., a distance between the bone facing surface and a surface thereof which faces away from the bone) as compared to remaining portions of the implant  402 . In a preferred embodiment, the flexible portion  420  is adjacent to an end  422  of the implant  402  so that the second sensor  406  is separated from the fracture  410  by a distance great enough to ensure that the underlying portion  412  of the bone  408  is solid. 
         [0018]    The second sensor  406  on the flexible portion  420  of the implant  402  is fixed to the solid portion  412  of the bone  408  via, for example, locking screws inserted in holes  424  on opposing sides thereof. The second sensor  406  measures strain on a portion of the implant  402  corresponding to the solid portion  412  of the bone  408  so that measurements from the second sensor  406  may be used to normalize measurements from the first sensor. Similarly to the placement of a sensor directly in or on a bone, as described in conjunction with systems  200  and  300 , placing the second sensor  406  on a more flexible portion  420  of the implant  402  between two locked screws permits a more accurate measurement of the strain on the underlying solid portion  412  of the bone  408 , as compared to the results from placing the second sensor  406  on a stiffer portion of the implant  402 . The ratio of the measurements from the first sensor  404  to the measurements from the second sensor  406  during the healing process should trend in a pattern indicating an increasing stiffness of the bone  408  over time, whereas a lack of healing should show no recognizable trend over time. 
         [0019]    As shown in  FIG. 6 , a system  500  according to another exemplary embodiment of the present invention may be substantially similar to the system  100 , comprising a bone plate  502  and first and second sensors  504 ,  506 , respectively. The first and second sensors  504 ,  506  are mounted along a surface  514  of the bone plate  502  such that the first and second sensors  504 ,  506  may be mechanically coupled to a bone via the bone plate  502 . The first and second sensors  504 ,  506  are positioned on the bone plate  502  so that when the bone plate  502  is in a desired position along the bone, the first sensor  504  is located over a site of a fracture of the bone while the second sensor  506  is separated from the fracture (preferably over a healthy (e.g., solid) portion of bone) as described above in regard to the system  100 . The first and second sensors  504 ,  506  measure a level of strain on the bone plate  502  at these positions of the bone plate  502  and a ratio of the strains measured by the first and second sensors  504 ,  506  indicates a progression of healing of the bone over time. 
         [0020]    In contrast to the system  100 , the bone plate  502  includes a first portion  522  and a second portion  524  connected to one another via a connecting portion  520  with the first sensor  504  mounted to the first portion  522  and the second sensor  506  is mounted to the second portion  524  on a side of the connecting portion  520  opposite the first portion  522 . Thus, when the first portion  522  is positioned such that the first sensor  504  overlies a fracture site while the second portion  524  is positioned such that the second sensor  506  extends over a healthy portion of bone, the connecting portion  520  extends between the first and second sensors  504 ,  506 . The connecting portion  520  is designed to reduce strain transmitted between the first and second portions  522 ,  524 . Specifically, the connecting portion  520  is less rigid than the first and second portions to reduce a torsion strain applied to the second sensor  524 . For example, a width of the connecting portion  520  may be reduced relative to the widths of the first and second portions  522 ,  524  to render the connecting portion  520  more flexible than the first and second portions  522 ,  524 . This reduces the rigidity of the bone plate  502  across the surface  514  in a direction substantially perpendicular to a longitudinal axis of the bone plate  502 , more substantially mechanically separating the second sensor  506  from strains associated with the fracture site (i.e., reducing torsion strains applied across the fracture site and transmitted to the second sensor  506 ) so that the second sensor  506  more accurately measures levels of strain associated only with the healthy portion of bone. It will be understood by those of skill in the art that the first sensor  504 , which is located over the fracture site, detects bending strain which are significantly greater than any torsion strains detected thereby so that these torsion strains have an insubstantial impact on the total strain measured by the first sensor  504 . 
         [0021]    In an another example, the connecting portion  520  may be formed as a frangible link designed to fail when a torsion force applied thereto exceeds a predetermined threshold level. Thus, before the frangible link is broken, the second sensor  506  is subject to torsion strains limited by the threshold level and, after the frangible link has been severed, the first and second portions  522 ,  524  of the bone plate  502  are completely separated and isolated from one another eliminating the impact on the second sensor  506  of any torsion strains arising over the fracture site. Those skilled in the art will understand that the threshold level at which the connecting portion  520  fails is preferably set so that, when the frangible link is not broken, the maximum level of torsion strain transmitted from the first portion  522  to the second sensor  506  are insignificant as compared to the strain levels expected to be seen in the healthy bone. In another alternate embodiment, as shown in  FIG. 7 , a system  500 ′ comprises two separate bone plates  522 ′,  524 ′ on which first and second sensors  504 ′,  506 ′ are mounted, respectively. The first and second bone plates  522 ′,  524 ′ are not connected to one another allowing the first plate  522 ′ to be attached over a fracture site while the second plate  524 ′ is separated therefrom and attached over a healthy portion of bone to completely eliminate the transmission of torsion from the fracture site to the second sensor  506 ′. 
         [0022]    It will be understood by those of skill in the art that other mechanisms may be employed for normalizing measurements of strain on a portion of an implant which, when mounted on a bone in a target location, corresponds to a position of a fracture or other weakened portion of that bone. For example, the patient may be provided with load sensors on which to push or stand with the affected limb such that a load measurement may be taken simultaneously with a strain measurement of the sensor on the implant. Alternatively, the patient may be provided with a sensor (e.g., placed in the sole of a shoe) to measure the load placed on the affected leg, if the affected bone is the femur or tibia. 
         [0023]    It will be apparent to those skilled in the art that various modifications and variations can be made in the structure and the methodology of the present invention, without departing from the spirit or the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.

Technology Classification (CPC): 0