Abstract:
A method for determination of mechanical integrity of bone includes embedding a threaded rod into the bone, extracting the threaded rod out of the bone, and measuring the force required to extract the threaded rod from the bone, wherein the force required to extract the threaded rod is indicative of the mechanical integrity of the bone. A device for determining the mechanical integrity of bone includes a housing, a rod holder mounted to the housing and configured to hold a threaded rod which is embedded in the bone, a pulling force applicator which applies a pulling force to the rod holder to extract the threaded rod from the bone, and a pulling force measuring instrument which measures the pulling force applied to the rod holder by the pulling force applicator as the threaded rod is extracted from the bone.

Description:
BACKGROUND 
       [0001]    Fragility fractures represent a major cause of morbidity in the elderly in the United States and other industrialized countries. These fractures are characterized by their spontaneous or low-energy injury mechanisms, such as falls from standing heights, with mechanical forces that would not ordinarily cause fractures in healthy, young adults. Fragility fractures most commonly occur near the ends of the bone, such as the metaphysis, where there is less cortical bone and more trabecular bone. 
         [0002]    Bone&#39;s remarkable strength and toughness is due to the nanoscale interaction of the inorganic, mineral portion of the bone material with the organic, collagen portion. Numerous diseases can disturb the mineral portion of the bone, or the collagen portion, or both, all resulting in a bone prone to fracture. In current clinical practice, bone strength and fracture risk associated with disease of the bone is estimated based on evaluation of bone mineral density, while also taking into consideration other factors, such as the patient&#39;s age, gender, race, and history of prior fracture. The evaluation of bone mineral density is typically performed by noninvasive means such as dual-energy x-ray absorptiometry (DXA) scans. When appropriate, treatment is available in the form of bisphosphonates, selective estrogen receptor modulators (SERMs), or recombinant parathyroid hormone (PTH) injection. 
         [0003]    Unfortunately, the predictive value of an abnormal DXA scan in estimating bone strength and fracture risk is relatively low, particularly with respect to proximal femur fractures, the most morbid of the common fragility fractures. Moreover, when using DXA scans, pathological states involving the organic portion of bone can be underestimated, as only the mineral component of the bone is evaluated. 
         [0004]    Additionally, complications of bisphosphonate drug therapy have been highlighted in case series describing an unusual or “atypical” or fatigue fracture thought to be caused by prolonged bisphosphonate treatment. These unique fractures are thought to occur due to a change in a mechanical property of the bone, making the bone less tough and more prone to mechanical fatigue. These “atypical” fatigue fractures occur despite increases in the inorganic or mineral density of bone and again illustrate that measurement of bone mineral density via DXA scan does not provide all the necessary information regarding the material properties of the bone. As the various factors affecting risk of fragility fracture have proven difficult to quantify, a need exists for a more accurate method of predicting this risk. 
       SUMMARY 
       [0005]    During treatment of patients with fragility fractures, a guide wire can be extracted by reverse threading the surgical guide wire, or by pulling on the guide wire by hand, or with vice grips or pliers. For example, in the treatment of osteoporotic hip fractures, a threaded guide-wire is inserted into and later removed from the center portion of the femoral head. 
         [0006]    The force required to extract the guide wire can give the physician a qualitative measure of the bone strength of the patient. This qualitative assessment can vary from doctor to doctor, and from day to day, and does not always correlate with the patient&#39;s bone mineral density as determined by, for example, DXA scans. Possible reasons for this include (1) poor sensitivity of the surgeon&#39;s judgment, (2) a pathological disturbance in the structure of the mineral component of the bone, such as a lack of connectivity of the trabecula, or (3) a defect in the non-mineral, structural component of the bone, that is, the type-one collagen portion of the bone. However, if the force required to remove the threaded guide wire is known, biomechanical information, which can be used, for example, to validate noninvasive measurement of bone disease and/or guide future patient care, can be obtained without any additional trauma to the patient. Such information can also be used in research studies involving determining the local material properties of bone, including monitoring the effects of pharmacologic treatments of bone. 
         [0007]    In this regard, an exemplary method for determination of mechanical integrity of bone includes embedding a threaded rod into the bone, extracting the threaded rod out of the bone, and measuring the force required to extract the threaded rod from the bone, wherein the force required to extract the threaded rod is indicative of the mechanical integrity of the bone. Additionally, an exemplary device for determining the mechanical integrity of bone includes a housing, a rod holder mounted to the housing and configured to hold a threaded rod which is embedded in the bone, a pulling force applicator which applies a pulling force to the rod holder to extract the threaded rod from the bone, and a pulling force measuring instrument which measures the pulling force applied to the rod holder by the pulling force applicator as the threaded rod is extracted from the bone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0008]    Other objects and advantages will be apparent to those skilled in the art from reading the following detailed description of exemplary embodiments in conduction with the drawings, wherein like elements are represented by like reference numerals, and wherein: 
           [0009]      FIG. 1  illustrates a front side view of a diagnostic device according to a first point engaged to a threaded rod; 
           [0010]      FIG. 2  illustrates an exploded view of the diagnostic device according to the first embodiment; 
           [0011]      FIG. 3  illustrates a cutaway view of the diagnostic device according to the first embodiment engaged to a threaded rod and with the cannulated tube removed; 
           [0012]      FIG. 4  illustrates a rear side view of the diagnostic device according to the first point engaged to a threaded rod and with the cannulated tube removed; 
           [0013]      FIGS. 5A through 5D  illustrate a front view, side view of a front portion, rear view, and side view of a rear portion, respectively of an exemplary threaded rod for use with an exemplary diagnostic device; 
           [0014]      FIG. 6  illustrates a front side view of the diagnostic device according to the first embodiment engaged with a threaded rod embedded in bone; 
           [0015]      FIGS. 7A through 7C  illustrate a cutaway view of a method employing the diagnostic device according to the first embodiment; 
           [0016]      FIG. 8  illustrates an exploded view of a diagnostic device according to a second embodiment; 
           [0017]      FIG. 9  illustrates a front side view of the diagnostic device according to the second embodiment; 
           [0018]      FIG. 10  illustrates a cutaway view of the diagnostic device according to the second embodiment; 
           [0019]      FIG. 11  illustrates a front side view of the diagnostic device according to the second embodiment engaged with a threaded rod embedded in bone; and 
           [0020]      FIGS. 12A through 12C  illustrate a cutaway view of a method employing the diagnostic device according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A diagnostic device  10  according to a first embodiment is illustrated in  FIGS. 1-4 . The device  10  includes a housing  20 , a rod holder  30  movably mounted in the housing  20 , and, as discussed in detail below, mechanisms for pulling the rod holder  30  relative to the housing  20  and measuring the pulling force applied to the rod holder  30 . 
         [0022]    The housing  20  can be formed of a hollow cylinder, and has a handle  40  mounted on a bearing  70  at the proximal end  22  of the housing  20 . A tip holder  50  is removably fixed to its distal end  24 . In the illustrated embodiment, the outer surface of the distal end  24  is inclined to help provide a friction fit for the tip holder  50 . However, the tip holder  50  can be removably fixed to the distal end  24  by other means, such as by a threaded engagement, a pinned arrangement, or a bayonet connection. The housing  20  is also provided with a guide slot  26  on its back side and an access window  28  on its front side. 
         [0023]    The rod holder  30  is configured to releasably hold the threaded rod  100  that is to be pulled out from the patient&#39;s bone. For example, the rod holder  30  can include a main bore  32  for receiving the threaded rod  100  and one or more set screws  34  threadably received in respective threaded bores perpendicular to the main bore  32 . The set screws  34  can be tightened against the threaded rod  100  to hold the threaded rod  100  in the main bore  32 . However, the rod holder  30  can be configured to hold the threaded rod  100  by other means, such as by a chuck arrangement. 
         [0024]    The handle  40  includes a main bore  42  engaged with the bearing  70  so that the handle  40  is rotatably retained on the housing  20 . The handle  40  also includes a threaded bore  44  in threaded engagement with a main screw  60 . The main screw  60  extends through the main bore  42 , is threaded to the threaded bore  44 , and is operatively connected to one or more measuring devices in the housing, as discussed in detail below. 
         [0025]    The main screw  60  is constrained so that it can move only in translation in the housing  20 . For example, the main screw  60  can be fixed to a guide key  65  which is arranged to slide along the guide slot  26 . When the handle  40 , and thus the threaded bore  44  in threaded engagement with the main screw  60 , is rotated with respect to the housing  20 , the main screw  60 , because it cannot rotate with respect to the housing  20 , moves in translation with respect to the housing  20 . For a conventionally oriented threaded connection between the threaded bore  44  and the main screw  60 , clockwise rotation of the handle  40  will cause the main screw  60  to move toward the proximal end  22  of the housing  20 , while counterclockwise rotation of the handle  40  will cause the main screw  60  to move toward the distal end  24  of the housing  20 . 
         [0026]    The rod holder  30 , like the main screw  60 , can also be attached to a guide key  35  which is arranged to slide along the guide slot  26 , so as to prevent the rod holder  30  from rotating with respect to the housing  20 . The rod holder  30  is operatively connected to the same one or more measuring devices as the main screw  60 . 
         [0027]    The measuring device can be, for example, a spring scale  80 . In place of, or in addition to the spring scale  80 , a load cell capable of recording the time history of the load can be employed. The spring scale  80  is accessible through the access window  28  of the housing. 
         [0028]    In the illustrated embodiment, a slide rod  82  of the spring scale  80  is fixed to the main screw  60 , while a graduated housing  84  of the spring scale  80  is fixed to the rod holder  30 . A flange  83  is fixed at or near the distal end of the slide rod  82 . The flange  83  centers the distal end of the slide rod  82  in the graduated housing  84  and has a portion which projects into a measuring slot  86  in the graduated housing  84 . A coil spring  88  within the graduated housing  84  is interposed between the flange  83  and the proximal end of the graduated housing  84 , to thereby bias the flange  83 , and thus the slide rod  82 , toward the distal end of the graduated housing  84 . As the slide rod  82  is pulled toward the proximal end of the graduated housing  84 , for example, by the main screw  60  as the handle  40  is turned, the coil spring  88  is compressed a distance corresponding to the pulling force. Additionally, the graduated housing  84  pulls the rod holder  30  in the proximal direction as a result of the coil spring  88  being pressed against the proximal end of the graduated housing  84 . 
         [0029]    The interaction between the guide slot  26  and the guide key  35 , which constrains the rod holder  30  to move only in translation with respect to the housing  20 , helps ensure that the threaded rod  100  is not tightened or loosened by twisting while the threaded rod  100  is extracted from the bone. As only the handle  40 , but not the housing  20  itself, is rotated during extraction, the threaded rod  100  will move only in translation as it is extracted from the bone. 
         [0030]    Gradations  85  on the graduated housing  84  adjacent the measuring slot  86  are indicative of the force applied to the slide rod  82 , as the gradation  85  adjacent to the flange  83  will vary depending on the force applied to the slide rod  82  and the resultant movement of the slide rod  82  within the graduated housing  84  and the flange  83  within the measuring slot  86 . An indicator  87  can be slidably installed in the measuring slot  86 . The indicator  87  is arranged on the proximal side of the flange  83  so that it can move with and register the maximum proximal displacement of the flange  83  in an operation in which the slide rod  82  is pulled in the proximal direction. 
         [0031]    As discussed above, the proximal side of tip holder  50  is removably fixed to the distal end  24  of the housing  20 . The tip holder&#39;s removal allows for placement and removal of the load measurement cartridge, i.e., the rod holder  30 , spring scale  80 , and main screw  60 , through the opening in the distal end  24  of the housing  20 . The load-measurement cartridge can thus be switched out for one with a higher or lower load range, depending on the specific application of the device. Additionally, the load-measurement cartridge can be easily calibrated using a load cell. Specifically, a load cell can be added in series with the load-measurement cartridge, and a simple extraction test can be done to ensure that the calibration of the device is correct. 
         [0032]    The tip holder  50  can be cylindrical in shape and can include an abutment surface  52  at its distal end with a guide hole  54  to help guide the threaded rod  100 . The tip holder  50  can also include one or more access windows to provide access for tightening the set screws  34  on the threaded rod  100  when the threaded rod  100  is connected to the device  20  with the tip holder  50  in place. 
         [0033]    The threaded rod  100  can be a threaded Kirschner wire as illustrated, for example, in  FIG. 5 . The threads  102  of the exemplary Kirschner wire have an outer diameter approximately the same as that of its shank  104 . With this arrangement, only the threaded portion is held to the bone, regardless of the depth to which the Kirschner wire is embedded. The structure of the exemplary threaded Kirschner wire can therefore help with respect to repeatability of pullout force measurement. However, the threaded rod  100  is not limited to a Kirschner wire. For example, the threaded rod can also be, for example a screw, or a tube or needle having threads on its outer surface. 
         [0034]    A bracing tube  90  can be used to ensure that the housing  20  is not moved closer to the patient and to otherwise help with stability as the threaded rod  100  is extracted. The bracing tube  90  is configured to receive the threaded rod  100  in its cannula, and its length is selected such that its distal end will abut the bone at the site where the threaded rod  100  is embedded while its proximal end abuts the abutment surface  52  of the tip holder  50 . 
         [0035]    Various parts of the device  10 , such as the housing  20 , the rod holder  30 , the tip holder  50 , the main screw  60 , and the spring scale  80  are preferably made of stainless steel or other material able to withstand autoclave temperatures so that the device can be sterilized for use in an operating room. For embodiments which incorporate a load cell or other components which may not be able to withstand autoclave temperatures, such components would be disassembled from the device  10  prior to autoclaving. 
         [0036]    In use, after the threaded rod  100  is embedded in the bone, the bracing tube  90  is placed over the threaded rod  100  so that it abuts the bone, the device  10  is positioned such that the rod holder  30  and tip holder  50  are placed over the threaded rod  100  and the bracing tube  90  abuts the tip holder  50 , and the set screws  34  are tightened on the threaded rod  100 . The threaded rod  100  is then extracted by turning the handle  40 , and the spring scale  80  indicates the maximum force applied to the threaded rod  100  during the extraction. If a load cell is used, the load cell can provide the maximum force as well as the time history of the load.  FIG. 6  illustrates a view of the device  10  engaged with an embedded threaded rod  100 . 
         [0037]    Trabecular bone is more vulnerable to mechanical weakening by diseases that predispose patients to fragility fractures. Thus, it may be advantageous to measure the pullout force in the trabecular bone rather than the cortical bone. In the method illustrated in  FIGS. 7A through 7C , a site in which trabecular bone  200  is located under the surface of the cortical bone  300  surface, such as the center portion of the femoral head, is tested. As illustrated in  FIG. 7A , the threaded rod  100  is threaded through the cortical bone  300  and into the trabecular bone  200 . Preferably, the threaded rod  100  is embedded to a depth such that its threads  102  are engaged only with the trabecular bone  200  and not the cortical bone  300 . Known methods, such as X-ray fluoroscopy, can be used to precisely position the threaded rod  100  in the bone. As illustrated in  FIG. 7B , the bracing tube  90  is then placed over the threaded rod  100  and against the cortical bone  300  surface. Then, as illustrated in  FIG. 7C , the threaded rod  100  is extracted. The device will measure the force required to extract the threaded rod  100  in the manner discussed above. 
         [0038]    A diagnostic device  510  according to a second embodiment is illustrated in  FIGS. 8-10 . The device  510  includes a housing  520 , a rod holder  530  movably mounted in the housing between footplates  525  of the housing  520 , and, as discussed in detail below, mechanisms for pulling the rod holder  530  in translation relative to the housing  520  without rotating relative to the housing  520 , and for measuring the pulling force applied to the rod holder  530 . 
         [0039]    The housing  520  can be formed, for example, generally in the shape of a caulking gun, and has a movable handle  540  pivotably toward a fixed handle  541 . For example, the movable handle  540  and the fixed handle  541  can be squeezed together with one hand. A torsion spring  543  mounted to the fixed handle  541  biases the movable handle  540  away from the fixed handle  541 . 
         [0040]    The rod holder  530  is configured to releasably hold the threaded rod  100  that is to be pulled out from the patient&#39;s bone. For example, the rod holder  530  can include a main bore  532  for receiving the threaded rod  100  and one or more set screws  534  threadably received in respective threaded bores perpendicular to the main bore  532 . The set screws  534  can be tightened against the threaded rod  100  to hold the threaded rod  100  in the main bore  32 . However, the rod holder  530  can be configured to hold the threaded rod  100  by other means, such as by a chuck arrangement. The rod holder  530  is fixed to a pulled flange  536  which extends perpendicular to the pulling direction of the device  510 . 
         [0041]    The handle  540  is preferably in either direct or ratcheting engagement with a pinion gear  551 . The pinion gear  551  engages a rack gear  553 , and the rack gear  553  is fixed to a pulling flange  555  which extends perpendicular to the pulling direction of the device. Interposed between the pulled flange  536  and the pulling flange  555  is a load cell  580 . The load cell is arranged to measure the pulling force of the pulling flange  555  on the pulled flange  536 . 
         [0042]      FIG. 11  illustrates a view the device  510  engaged with an embedded threaded rod  100 . In use, after the threaded rod  100  has been embedded in the bone, the device  510  is positioned such that the rod holder  530  is placed over the threaded rod  100  and the footplates  525  abut the bone, and the set screws  534  are tightened on the threaded rod  100 . The movable handle  540  is then actuated to rotate the pinion gear  551 , which pulls the rack gear  553  to thereby pull the pulling flange  555 . The pulling flange  555  pulls the pulled flange  536  to thereby pull the rod holder  530 , which in turn extracts the threaded rod  100  from the bone. As the rod holder  530  moves back in translation only with respect to the housing  520 , the threaded rod  100  will move only in translation as it is extracted from the bone. The pulling force is measured by the load cell  580 .  FIGS. 12A-12C  illustrate the threaded rod  100  before, during, and after extraction using the device  510  of this embodiment. 
         [0043]    In an exemplary surgical technique employing a diagnostic device according to any of the various exemplary embodiments, the patient is positioned in the supine position on a fracture table with the operative limb placed in traction as is commonly performed for open reduction and internal fixation. An alternative lateral position, as is commonly used for arthroplasty treatment of osteoporotic proximal femur fractures, is also possible. The operative thigh and hindquarter is then prepped and draped in a traditional surgical fashion. 
         [0044]    Upon attaining adequate anesthesia, an approximate 1-1.5 cm longitudinal incision is performed over the lateral proximal thigh, several centimeters distal to the tip of the greater trochanter, depending on the thickness of the subcutaneous tissues. The subcutaneous tissues are incised sharply. Similarly, a 1-1.5 cm longitudinal incision is performed in the illiotibial band and underlying vastus lateralis muscle. The bone is then identified in the depth of the wound. 
         [0045]    Under fluoroscopic guidance in the anterior-posterior (AP) and lateral planes, a terminally threaded rod is drilled through the lateral femoral cortex and the tip is directed in to the central portion of the femoral head. The device is then attached to the terminally threaded rod such that the device rests upon the lateral femoral cortex. The device is then manually activated and the threaded rod is extracted from the bone in a lateral direction. The force required to remove the threaded rod is then recorded. 
         [0046]    The device is then disconnected from the threaded rod and the rod can be removed from the patients bone manually. The incision is irrigated with normal saline solution and closed primarily in a routine fashion. This procedure can also be performed with local anesthetic and IV sedation, when no other operative procedure is planned requiring a more robust anesthetic. 
         [0047]    At the completion of the operative procedure, the data obtained can be compared to known references to provide additional information regarding the biomechanical quality of the bone. This procedure can be coupled with other operative procedures and exposures. In the case of a patient undergoing operative treatment of an intertochanteric femur fracture with placement of a Dynamic Hip Screw (DHS), the placement and extraction of the threaded rod can be performed through the routine operative exposure. Similarly, the above described percutaneous technique can be performed during the treatment of an intertrochanteric femur fracture with a cephalomedulary nail. After the nail is placed, but prior to percutaneous placement of the proximal cephalomedulary screw, spiral blade, or locking bolts, the threaded rod can be placed through the nail and withdrawn as described above. 
         [0048]    The device is intended to be used in metaphyseal portion of bones. Alternative sites of use other than the proximal femur described above include, but is not limited to, the distal femur, proximal tibia, distal tibia, calcaneous, distal radius, and proximal humerus. 
         [0049]    The pulling force measured in vivo can be used to determine the mechanical integrity of living bone. In particular, the pulling force can be compared with data established empirically over time by means well-known to an ordinarily skilled artisan. For example, the diagnostic device can be used on patients with known fracture risks, and the resulting data can be used to generate standardized normative tables. 
         [0050]    It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed exemplary embodiments are therefore considered in all respects to be illustrative and not restricted.