Abstract:
A method and device for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient, comprising inserting a distractor into the intervertebral space. The vertebral endplate depth is measured by moving a posterior rod on the distractor longitudinally to a position at the posterior margin of the vertebral endplate and moving an anterior rod on the distractor to a position at the anterior margin of the vertebral endplate. The force applied during distraction is measured using at least one transducer mounted on the distractor and the intervertebral space is distracted by rotating the posterior and anterior rods on the distractor until the measured force reaches a predetermined value. The posterior height of the intervertebral space is measured based on the amount that the posterior rod is rotated and the anterior height of the intervertebral space is measured based on the amount that the anterior rod is rotated. The intervertebral angle is measured based on the angular orientation of two pivotally connected handles on the distractor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/080,030, filed Jul. 11, 2008, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This application relates generally to surgical measuring devices, and more particularly, to an intervertebral measuring device for selecting an optimal size implant. 
       BACKGROUND 
       [0003]    Chronic lower back pain caused by degenerative disc disease is one of the leading causes of disability in adults. Intervertebral disc degeneration can occur as part of the normal aging process in which the nucleus of the disc dehydrates, reducing the shock absorbing capability of the disc. Patients who fail to obtain adequate pain relief from non-surgical treatment (e.g., rest, pain medication, physical therapy, exercise, epidural steroid injections, chiropractic manipulation, ultrasound, massage, orthotics, etc.) may require spinal surgery to alleviate discogenic pain and disability. 
         [0004]    One method of treating degenerative disc disease is spinal fusion or arthrodesis surgery in which the affected vertebrae are fused together using a bone graft. During spinal fusion, a perforated titanium cage may be surgically implanted within the space between two adjacent vertebrae after the pain-generating intervertebral disc is removed. The implanted spinal fusion cage must be appropriately sized to restore the normal disc height at the affected vertebral segment. The fusion cage is packed with bone graft, which grows through the perforated walls of the cage and eventually forms a solid bond or fusion with the adjacent vertebrae to prevent motion in the affected vertebral segment and reduces chronic discogenic pain. 
         [0005]    Another approach for treating degenerative disc disease is total disc replacement (“TDR”), which seeks to alleviate discogenic pain, while maintaining or minimizing the loss of motion in the affected vertebral segment. During TDR, the pain-generating intervertebral disc is removed and a metallic artificial disc implant that allows motion, such as the ProDisc™-L manufactured by Synthes Spine of West Chester, Pa. and Charité® manufactured by DePuy Spine, Inc. of Raynham, Mass., is inserted into the space between the adjacent vertebrae. The TDR implant must be appropriately sized to restore the normal disc height at the affected vertebral segment, thereby reducing chronic discogenic pain, while maintaining or minimizing loss of range of motion in the affected vertebral segment. 
         [0006]    While spinal fusion and TDR have been successfully used to treat degenerative disc disease, it is estimated that between 3-8% of these surgeries must be surgically revised. Some of these revisions are believed to be due to the improper selection, sizing and placement of the artificial disc or fusion cage, which is based predominantly on the judgment of the surgeon at the time of the procedure. 
         [0007]    For example, surgeons today must rely on their experience and “feel” when using intervertebral spreaders or distractors to spread the affected intervertebral segment during TDR or fusion surgery. Both the artificial disc and fusion cage must fit in a “snug” intervertebral space that has appropriate ligament tension provided by the anterior longitudinal ligament, posterior longitudinal ligament (if it has not been removed), ligamentum flavum, facet capsular ligament (in the case of TDR where the facet is not removed), and the inter-transverse and inter-spinous ligaments. Too much distraction by the surgeon performing a TDR procedure will result in the placement of an implant that is too large for the intervertebral segment, reducing the ideal range of motion that the implant can provide. In contrast, insufficient ligament tension will not produce enough force on the endplates of the arthroplasty, resulting in loosening and ultimately subsidence (i.e., migration of the implant from its optimal position). Similarly, since fusion cages or devices should be placed in a stretched intervertebral segment for better fusion, insufficient ligament tension may result in an unsatisfactory fusion and/or undesired range of motion of the affected vertebral segment. 
         [0008]    In addition, anterior and posterior pre- and post-distraction disc heights are important parameters for a surgeon to consider when placing an intervertebral disc arthroplasty. It is believed that there is an optimal window of anterior and posterior disc height that allows the optimal range of motion by the arthroplasty. Currently, surgeons must either “eyeball” anterior and posterior disc heights before and after distraction on a fluoroscope, and/or use so called “Yo-Yo” or trial devices to measure the size of the intervertebral space by forcing different Yo-Yo&#39;s into the space between adjacent vertebras until the correct sized Yo-Yo is placed. However, these Yo-Yo devices do not allow for ligament tension measurement and/or feel by the surgeon. Conventional intervertebral spreaders do not measure anterior and posterior disc heights. 
         [0009]    Yet another parameter for a surgeon to consider when placing an intervertebral disc arthroplasty is the depth of the patient&#39;s vertebral endplate. New arthroplasties should cover the entire vertebral endplate outer-rim because this is where the strongest bone lies. However, neither conventional distractors/spreaders nor fluoroscopes provide a surgeon with the precise depth of the patient&#39;s vertebral endplate to select the correct size arthroplasty or fusion device that will cover the maximum amount of outer rim. 
         [0010]    Intervertebral angle is also an important parameter that surgeons must estimate based on their experience using fluoroscopes and x-rays. Estimating the correct “lordotic” or “kyphotic” angle is important because it allows for the correct spinal segment alignment in the case of fusion devices, or for the maximum range of motion in the case of total disc arthroplasties. Conventional spreaders (distractors) or Yo-Yo devices do not provide for a precise determination of the intervertebral angle. 
         [0011]    It is, therefore, desirable to reduce post-operative complications of spinal fusion and TDR arising from improper selection, sizing and placement of the artificial disc or fusion cage. 
       SUMMARY 
       [0012]    In accordance with one aspect of this disclosure, a device and method are disclosed for intraoperatively and accurately measuring: (1) ligament tension while the surgeon is distracting (spreading) the affected intervertebral segment; (2) the anterior and posterior disc heights before and after distraction; (3) the vertebral endplate depth or distance; and (4) intervertebral angle during surgery. 
         [0013]    In accordance with one aspect of this disclosure, a method is disclosed for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. The method comprises inserting a distractor into the intervertebral space and measuring a vertebral endplate depth. The amount of force applied during distraction is measured with at least one transducer mounted on the distractor. The intervertebral space is distracted. The intervertebral angle and the posterior and anterior heights of the intervertebral space are measured. The optimal size of the intervertebral implant is selected corresponding to the measured vertebral endplate depth, posterior height, anterior height, and intervertebral angle. 
         [0014]    In accordance with another aspect of this disclosure, a method is disclosed for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. The method comprises inserting a distractor into the intervertebral space. The vertebral endplate depth is measured by moving a posterior rod on the distractor longitudinally to a position at the posterior margin of the vertebral endplate and moving an anterior rod on the distractor to a position at the anterior margin of the vertebral endplate. The force applied during distraction is measured using at least one transducer mounted on the distractor and the intervertebral space is distracted by rotating the posterior and anterior rods on the distractor until the measured force reaches a predetermined value. The posterior height of the intervertebral space is measured based on amount that the posterior rod is rotated and the anterior height of the intervertebral space is measured based on amount that the anterior rod is rotated. The intervertebral angle is measured based on the angular orientation of two pivotally connected handles on the distractor. The optimal size of the intervertebral implant is selected corresponding to the measured vertebral endplate depth, posterior height, anterior height, and intervertebral angle. 
         [0015]    In accordance with another aspect of this disclosure, a device is disclosed for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. The device comprises a first plate that is insertable within the intervertebral space for engagement with a first vertebral endplate and a second plate that is insertable within the intervertebral space for engagement with a second vertebral endplate. The first plate is operatively connected to a first handle and movable in response to movement of the first handle. The second plate is operatively connected to a second handle and movable in response to movement of the second handle. At least one transducer measures the force applied during distraction of the intervertebral space. 
         [0016]    In accordance with another aspect of this disclosure, a device is provided for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. The device comprises a movable posterior rod operatively connected to at least one posterior distracting member, the posterior distracting member being movable in a radial direction in response to rotation of the posterior rod. A movable anterior rod is operatively connected to at least one anterior distracting member, the anterior distracting member being movable in a radial direction in response to rotation of the anterior rod. At least one transducer is used to measure the force applied during distraction of the intervertebral space. 
         [0017]    In accordance with another aspect of this disclosure, a device is disclosed for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. The device comprises a movable posterior rod operatively connected to first and second posterior distracting members. The posterior distracting members are movable in opposing radial directions in response to rotation of the posterior rod, which includes markings corresponding to the distance that the posterior distracting members have moved for measuring posterior disc height. A movable anterior rod is operatively connected to first and second anterior distracting members. The anterior distracting members are movable in opposing radial directions in response to rotation of the anterior rod, which includes markings corresponding to the distance that the anterior distracting members have moved for measuring anterior disc height. The posterior and anterior rods are movable in a longitudinal direction, the anterior rod including a linear scale and the posterior rod including a position indicator for measuring the distance between the distal end of the posterior rod and the distal end of the anterior rod. The first posterior and anterior distracting members each having a terminal end that engages a first load sensor plate, the first load sensor plate being movable in response to movement of either the first posterior distracting member or the first anterior distracting member. The second posterior and anterior distracting members each having a terminal end that engages a second load sensor plate, the second load sensor plate being movable in response to movement of either the second posterior distracting member or the second anterior distracting member. A first handle is pivotally connected to a proximal end of the first load sensing plate and a second handle is pivotally connected to a proximal end of the second load sensing plate. The first and second handles are pivotally connected to one another and a protractor measures the angular orientation of the first and second handles. At least one transducer measures the force applied during distraction of the intervertebral space. 
         [0018]    These and other advantages of the present disclosure will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a side elevation view of a spinal measuring device and distractor in accordance with this disclosure; and 
           [0020]      FIG. 2  is a flow chart illustrating a preferred sequence of steps for determining an optimal size of an intervertebral implant to be inserted within an intervertebral space between two vertebrae in a patient. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIG. 1 , a measuring device and distractor  10  is shown in  FIG. 1  inserted within the space between adjacent vertebrae V 1 , V 2  after a pain-generating or diseased intervertebral disc is removed. Although the measuring device and distractor  10  is described herein as an intervertebral measuring device for selecting an optimal size arthroplasty for a patient undergoing a TDR or spinal fusion procedure, it is understood that the device  10  is not limited to use with this type of procedure and may be used in connection with a variety of other medical procedures, including, but not limited to, hip, knee and shoulder arthroplasty procedures. 
         [0022]    The measuring device and distractor  10  preferably includes a housing  12 . The housing  12  is preferably hollow and has a proximal end  12   a  and a distal end  12   b.  The housing  12  is preferably made from titanium, stainless steel or any other biologically suitable material that is safe for use within the body of a patient. 
         [0023]    Two elongated rods—a posterior rod  20  and an anterior rod  30 -are mounted within the housing  12  so that each rod may be rotated relative to and slide longitudinally within the housing. Each rod  20 ,  30  may, for example, be supported by one or more bearings or sleeves (not shown) mounted on the housing  12 . One end of each rod  20 ,  30  preferably extends outwardly from the proximal end  12   a  of the housing  12  and terminates in a handle or knob  22 ,  32 , which the surgeon may use to turn the rod or slide the rod longitudinally. The rods  20 ,  30  are preferably made from titanium, stainless steel or any other biologically suitable material that is safe for use within the body of a patient. 
         [0024]    The opposing or distal end of the posterior rod  20  is coupled to and engages a pair of opposing posterior distracting members or pulls  28   a,    28   b.  The posterior distracting members  28   a,    28   b  project outwardly from the housing  12 , preferably in a direction that is generally perpendicular to the longitudinal axis of the rod  20 . The first posterior distracting member  28   a  preferably extends upwardly through the top  12   c  of the housing  12  in a direction toward the superior vertebrae V 1  as illustrated in  FIG. 1 . The second posterior distracting member  28   b  preferably extends downwardly through the bottom  12   d  of the housing  12  in a direction toward the inferior vertebrae V 2  as illustrated in  FIG. 1 . The top  12   c  and bottom  12   d  of the housing  12  are at least partially relieved to permit the distracting members  28   a,    28   b  to extend outwardly therethrough and slide longitudinally with the posterior rod  20  as the rod is moved longitudinally relative to the housing  12 . 
         [0025]    The distracting members  28   a,    28   b  are coupled to the distal end of the posterior rod  20  in a conventional manner that permits rod  20  to rotate relative to the distracting members  28   a,    28   b,  while also permitting the distracting members  28   a,    28   b  to move longitudinally with the posterior rod  20  as the rod is moved longitudinally relative to the housing  12 . 
         [0026]    The posterior rod  20  preferably includes a plurality of teeth about its circumference to form a gear (e.g., a circular pinion) at least in proximity to the location where the rod engages the distracting members  28   a,    28   b.  The distracting members  28   a,    28   b  also include a plurality of teeth on at least one surface to form a gear (e.g., a rack) for meshing engagement with the teeth (circular pinion) on the posterior rod  20  to convert rotational movement of the rod  20  into linear movement of the distracting members  28   a,    28   b  toward or away from the adjacent vertebrae. 
         [0027]    In other words, rotation of the posterior rod  20  in one direction will cause the first posterior distracting member  28   a  to extend or move upwardly through the top  12   c  of the housing  12  and simultaneously cause the second posterior distracting member  28   b  to extend or move downwardly through the bottom  12   d  of the housing. In contrast, rotation of the posterior rod  20  in the opposite direction will cause the first posterior distracting member  28   a  to retract or move downwardly into the top  12   c  of the housing  12  and simultaneously cause the second posterior distracting member  28   b  to retract or move upwardly into the bottom  12   d  of the housing. 
         [0028]    Similarly, the opposing or distal end of the anterior rod  30  is coupled to and engages a pair of opposing anterior distracting members or pulls  38   a,    38   b.  Like the posterior distracting members  28   a,    28   b,  the anterior distracting members  38   a,    38   b  also project outwardly from the housing  12 , preferably in a direction that is generally perpendicular to the longitudinal axis of the rod  30 . 
         [0029]    Like the first posterior distracting member  28   a,  the first anterior distracting member  38   a  preferably extends upwardly through the top  12   c  of the housing  12  in a direction toward the superior vertebrae V 1  as illustrated in  FIG. 1 . Like the second posterior distracting member  28   b,  the second anterior distracting member  38   b  preferably extends downwardly through the bottom  12   d  of the housing  12  in a direction toward the inferior vertebrae V 2  as illustrated in  FIG. 1 . The top  12   c  and bottom  12   d  of the housing  12  are at least partially relieved to permit the distracting members  38   a,    38   b  to extend outwardly therethrough and slide longitudinally with the anterior rod  30  as the rod is moved longitudinally relative to the housing  12 . 
         [0030]    The anterior distracting members  38   a,    38   b  are coupled to the distal end of the anterior rod  30  in a conventional manner that permits rod  30  to rotate relative to the distracting members  38   a,    38   b,  while also permitting the distracting members  38   a,    38   b  to move longitudinally with the anterior rod  30  as the rod is moved longitudinally relative to the housing  12 . 
         [0031]    Like the posterior rod  20 , the anterior rod  30  preferably includes a plurality of teeth about its circumference to form a gear (e.g., a circular pinion) at least in proximity to the location where the rod engages the anterior distracting members  38   a,    38   b.  The distracting members  38   a,    38   b  also include a plurality of teeth on at least one surface to form a gear (e.g., a rack) for meshing engagement with the teeth (circular pinion) on the anterior rod  30  to convert rotational movement of the rod  30  into linear movement of the distracting members  38   a,    38   b  toward or away from the adjacent vertebrae. 
         [0032]    In other words, rotation of the anterior rod  30  in one direction will cause the first anterior distracting member  38   a  to extend or move upwardly through the top  12   c  of the housing  12  and simultaneously cause the second anterior distracting member  38   b  to extend or move downwardly through the bottom  12   d  of the housing. In contrast, rotation of the anterior rod  30  in the opposite direction will cause the first anterior distracting member  38   a  to retract or move downwardly into the top  12   c  of the housing  12  and simultaneously cause the second anterior distracting member  38   b  to retract or move upwardly into the bottom  12   d  of the housing. 
         [0033]    A plurality of spaced apart graduations (not shown) are preferably provided near the proximal end of each rod  20 ,  30  to provide a visual indication of and correspond to the distance that the distracting members  28   a,    28   b,    38   a,    38   b  are displaced as the rod is rotated in use. In this manner, the device  10  may be used to measure the anterior and posterior disc heights before and after intervertebral distraction to aid the surgeon in selecting the correct sized arthroplasty or fusion device for the spine of the patient undergoing the procedure. Like the rods  20 ,  30 , the distracting members  28   a,    28   b,    38   a,    38   b  are preferably made from titanium, stainless steel or any other biologically suitable material that is safe for use within the body of a patient. 
         [0034]    A ruler or linear scale  36  having a plurality of spaced apart graduations or markings is preferably mounted on or otherwise connected to the anterior rod  30  in proximity to the handle  32 . A position indicator  26  is preferably mounted on or otherwise connected to the posterior rod in proximity to the handle  22 . As the rods  20 ,  30  are moved longitudinally relative to one another, the distance between the distal end of the posterior rod  20  and the distal end of the anterior rod  30  is indicated by particular graduation on the ruler  36  that is aligned with the position of the position indicator  26 . In this manner, the device  10  may be utilized to measure the depth of the vertebral endplate to select the correct sized arthroplasty or fusion device that will cover the maximum amount of the outer rim of the vertebral endplate. 
         [0035]    The end of the first posterior and anterior distracting members  28   a,    38   a  that extends out of the top  12   c  of the housing  12  engages a superior side load sensor plate  40   a,  which is shown adjacent to the superior vertebrae V 1  in  FIG. 1 . The end of the distracting members may include a rounded head as is shown at the end of distracting member  28   a.  The proximal end of the superior side load sensor plate  40   a  is pivotally connected via a hinge or pin  46   a  to the distal end of a first scissor member, lever or link  42   a,  which terminates at its proximal end with a handle  48   a.    
         [0036]    Similarly, the end of the second posterior and anterior distracting members  28   b,    38   b  that extends out of the bottom  12   d  of the housing  12  engages an inferior side load sensor plate  40   b,  which is shown adjacent to the inferior vertebrae V 2  in  FIG. 1 . The end of the distracting members may include a rounded head as is shown at the end of distracting member  28   b.  The proximal end of the inferior side load sensor plate  40   b  is pivotally connected via a hinge or pin  46   b  to the distal end of a second scissor member, lever or link  42   b,  which terminates at its proximal end with a handle  48   b.  The two scissor members  42   a,    42   b  and pivot pins  44 ,  46   a,    46   b  may be made from aluminum, stainless steel, titanium or any other suitable material that is biologically safe for use in medical procedures. 
         [0037]    The two scissor members  42   a,    42   b  intersect and are pivotally connected to one another via pin or hinge  44  to form a scissor-like configuration. When the handles  48   a,    48   b  are squeezed or otherwise moved toward one another by the surgeon, the scissor members  42   a,    42   b  pivot about pin  44  causing the distal ends of the scissor members (and the pivotally connected load sensor plates  40   a,    40   b ) to spread or otherwise move away from each other. 
         [0038]    A semi-circular protractor  49  having a plurality of spaced apart graduations or markings is positioned on the housing  12  proximate to the pivot pin  44  to measure the angular orientation of the two scissor members  42   a,    42   b.  This angular orientation corresponds to the angular orientation or intervertebral angle of affected vertebral endplates when the handles  48   a,    48   b  are squeezed together to pivot the load sensor plates  40   a,    40   b  against the endplates of the superior and inferior vertebrae V 1 , V 2 . In this manner, the device  10  may be used to determine the intervertebral angle, which will provide for the correct spinal segment alignment in the case of fusion devices or for the maximum range of motion in the case of TDR. 
         [0039]    A transducer  50   a,    50   b  that converts force into a measurable electrical output, such as a conventional low-profile load sensor or load cell, is preferably mounted on each load sensor plate  40   a,    40   b  to measure ligament tension when the surgeon is distracting (spreading) the affected intervertebral segment V 1 , V 2  using the device  10 . The transducer may be constructed from stainless steel or other biologically safe material. The transducer  50   a,    50   b  may be bonded or otherwise attached to the load sensor plate  40   a,    40   b  in a conventional manner. Each transducer  50   a,    50   b  is preferably coated or covered with a thin layer of sterile plastic or other biologically safe material to protect the sensor and maintain sterility. 
         [0040]    Wiring  51   a,    51   b,  such as conventional shielded cabling, provides an electrical connection between the transducer  50   a,    50   b  and a display (not shown) that provides signal processing and a visual indication of the force (preferably in Newtons) being applied by the device  10  during distraction of the affected vertebral segment V 1 , V 2 . The wiring  51   a,    51   b  preferably extends from the sensor  50   a,    50   b  through the housing  12  and out the proximal end  12   a  of the housing, where the wiring may be connected to the display. In this manner, the device  10  may be used to measure ligament tension and aid the surgeon in determining whether the appropriate amount of tension is being applied during distraction (spreading) of the intervertebral segment. 
         [0041]    The measuring device and distractor  10  may be used by a surgeon as follows to selecting an optimal size arthroplasty for a patient undergoing a TDR or spinal fusion procedure. First, the patient may be positioned in a supine, neutral position on an operating table. Using an anterior approach, the surgeon exposes and removes the degenerative or diseased intervertebral disc, leaving a hollow intervertebral disc space between the adjacent vertebrae V 1 , V 2 . 
         [0042]    Referring to  FIG. 2 , in step  100 , the measuring device and distractor  10  is then inserted intraoperatively within the body of the patient until the distal end  12   b  of the housing  12  is positioned within the intervertebral disc space at the affected vertebral segment. The distracting pulls  28   a,    28   b,    38   a,    38   b  should be sufficiently retracted to avoid interference with the vertebral endplates. 
         [0043]    In step  110 , the vertebral endplate distance or depth is then measured by sliding the posterior rod  20  longitudinally until the distracting member  28   a  is positioned at the posterior edge or margin of the vertebral endplate. The anterior rod  30  is then moved longitudinally until the distracting member  38   b  is positioned at the anterior edge or margin of the vertebral endplate. The endplate depth corresponds to the particular graduation on the ruler  36  on the anterior rod  30  that is aligned with the position indicator  26  on the posterior bar  20 . Knowing the vertebral endplate depth will allow the surgeon to select the optimal sized arthroplasty or fusion device that will cover the maximum amount of the outer rim of the vertebral endplate. The posterior and anterior rods  20 ,  30  may optionally be locked in place within the housing  12  to restrict longitudinal movement of the bars, while permitting rotational movement. 
         [0044]    Next, the surgeon will measure the pre-distraction anterior and posterior disc heights using the device  10  in step  120 . To measure the posterior disc height, the surgeon will grasp the handle  22  and turn the posterior rod  20 , causing the first and second posterior distracting members  28   a,    28   b  to move outwardly until the respective load sensor plates  40   a,    40   b  contact the posterior edge of the vertebral endplates. The posterior disc height corresponds to the particular graduation on the circumference of the posterior rod  20  indicating the amount of rotation of the rod. 
         [0045]    Similarly, to measure the anterior disc height, the surgeon will grasp the handle  32  and turn the anterior rod  30 , causing the first and second anterior distracting members  38   a,    38   b  to move outwardly until the respective load sensor plates  40   a,    40   b  contact the anterior edge of the vertebral endplates. The anterior disc height corresponds to the particular graduation on the circumference of the anterior rod  20  indicating the amount of rotation of the rod. 
         [0046]    The surgeon may also measure the intervertebral angle in step  130  by squeezing the handles  48   a,    48   b  on the proximal end of scissor members  42   a,    42   b.  This causes the scissor members  42   a,    42   b  to pivot about pivot pin  44  until the respective load sensor plates  40   a,    40   b  contact the vertebral endplates. The angular orientation of the two scissor members  42   a,    42   b  is then measured using the graduations on the protractor  49 , which corresponds to the angular orientation or intervertebral angle of affected vertebral endplates. In this manner, the device  10  may be used to determine the intervertebral angle, which will provide for the correct spinal segment alignment in the case of fusion devices or for the maximum range of motion in the case of TDR. 
         [0047]    After the pre-distraction measurements are taken, the surgeon may distract (spread) the intervertebral segment in step  150  by turning the posterior and anterior rods  20 ,  30 , which causes the distracting members  28   a,    28   b,    38   a,    38   b  to move outwardly. The transducers  50   a,    50   b  located on the load sensor plates  40   a,    40   b  measure the amount of force or tension being applied to the ligaments during distraction in step  140 , which the surgeon can monitor on a display electrically connected to the transducers via wires  51   a,    51   b.  In step  150 , the surgeon stops distracting the intervertebral segment when the desired tension is indicated on the display. This will minimize or eliminate circumstances where insufficient or excessive tension is applied to the ligaments during distraction, resulting in improper sizing of the arthroplasty or fusion device for the particular patient. 
         [0048]    When the desired ligament tension is attained and distraction discontinued, the surgeon then uses the device  10  to measure the post-distraction posterior and anterior disc heights (step  160 ) and intervertebral angle (step  170 ) in the manner indicated above. The device  10  is then removed from the patient and, in step  180 , the surgeon may use the measured endplate depth or distance, the pre- and post-distraction posterior and anterior disc heights, and the pre- and post-distraction intervertebral angle to select the optimal sized arthroplasty or fusion device for the particular patient undergoing the TDR or spinal fusion procedure. 
         [0049]    Having described and illustrated the principles of this application by reference to one or more preferred embodiments, it should be apparent that the preferred embodiment(s) may be modified in arrangement and detail without departing from the principles disclosed herein and that it is intended that the application be construed as including all such modifications and variations insofar as they come within the spirit and scope of the subject matter disclosed herein.