Abstract:
Apparatus and method for measuring spinal instability through the use of a distractor arm assembly having segments pivotal with respect to each other and movable through the movement of a pivotal collar assembly along a centrally positioned jackscrew.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to an apparatus and method for measuring instability of a motion segment unit of a spine in which at least two force applying members are attached in spaced apart locations to the motion segment unit and a force applied thereto to assist a surgeon in selecting a suitable course of treatment to correct or improve the instability of the motion segment unit. 
     BACKGROUND OF THE INVENTION 
     It is well known that back pain is one of the most frequently occurring and expensive disabling ailments, especially for patients in the 30–60 year age bracket. Although back pain syndrome is a very common occurrence, its diagnosis to this day is very difficult. 
     The vertebral column (spine) is a biomechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebrae discs. The biomechanical functions of the spine include (1) support of the body (trunk and appendages), which involves the transfer of the weight and the bending moments of the head, trunk and arms to the pelvis and legs, (2) complex physiologic motion between these body parts, and (3) protection of the spinal cord and nerve roots. 
     The major regions of the spine are the cervical, thoracic, lumbar and sacral. The vertebrae increase in size and mass from the cervical to the lumbar regions. The increase in size of the vertebrae is directly related to an increased capacity for supporting larger loads. The lumbar region is therefore the major load bearer of the spine. However, this increase in load bearing capacity is paralleled by a decrease in flexibility. Because the lumbar regions bears heavier loads than other regions of the spine, the lumbar trunk (low back structure) is more, but not exclusively, susceptible to strain and hence back pain. 
     The spine is comprised of different levels known as motion segment units. The lumbar spine, for example, is comprised of five motion segment units. The motion segment unit is the smallest component of the spine that exhibits kinematic behavior similar to that of the whole spine. The motion segment unit is capable of flexion, extension, lateral bending, torsion and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc and the connecting ligamentous tissue. 
     Many causes of back pain and related neurological pain, are attributed to the instability of the motion segment unit. Segmental instability is defined as “the loss of ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots, and, in addition, there is no development of incapacitating deformity or pain due to structural changes”. Instability is therefore an abnormal response to applied loads characterized by motion in the motion segment unit beyond normal constraints. Excess motion can be abnormal in quality (i.e., abnormal coupling patterns) or in quantity (abnormal increased motion) or both. Excess motion may well result in damage to the nerve roots, the spinal cord, and other spinal structures. 
     The underlying causes of the structural changes in the motion segment unit leading to instability are trauma, degeneration, aging, disease (tumor, infection, etc.), surgery, or a combination thereof. It is known that a mechanically unstable motion segment unit can originate due to loss of biomechanical function of the spine joint ligaments and degeneration of the intervertebral disc and nucleus pulposus. A degenerate nucleus polposus causes disc space narrowing, loss of viscoelastic properties and the subsequent transfer of compressive loads to the annulus fibrosus. The altered anatomic dimensions and subsequent abnormal response to loading can cause loss of pretension in the ligamenum flavum, and longitudinal ligaments, degeneration of the facet capsules (and possible subluxation) with a consequence of secondary degenerative osteoarthritis of the joints. 
     Spinal disorders requiring neural decompressive surgery can leave motion segment units unstable due to the removal of supporting structures of the joint. A severely unstable motion segment unit is most likely to be fused to insure postsurgical stability. The need to fuse the vertebrae of a motion segment unit is dependent on the pre-operative symptoms and clinical (radiographic) findings and on the outcome of the surgical procedure. 
     One effort at mechanically determining spinal instability is disclosed in “A Technique for Mechanical Assessment of the Intervertebral Joint”, Mark Lubin et al., Biomech. Sym. ADM vol. 43 (1981). A Cloward lamina spreader is fitted with a strain gauge and a loading and unloading of force is provided manually. The device disclosed in the aforementioned publication is disadvantageous because there is no recognition of the need to control the rate of displacement nor a means for doing so which enables precise measurements of relative stiffness of the motion segment unit. The motion segment unit is a viscoelastic structure and therefore its resistance to deformation is dependent on the loading rate. Objective criteria for determining the degree of instability of the motion segment unit is therefore important in assessing whether spinal fusion surgery is necessary to relieve back pain in the patient. 
     Another effort at measuring the relative instability of the motion segment unit of the spine is disclosed in Mark D. Brown and David C. Holmes (U.S. Pat. No. 4,899,761). The apparatus disclosed in this reference provides a vertebrae distractor including a device for applying a constant rate of increasing force against adjacent vertebrae of a motion segment unit to thereby distract or separate the vertebrae. Means for detecting and recording the changes in the resistance to distraction are also provided. The device disclosed in the &#39;761 Patent, while providing useful objective criteria regarding the relative stiffness of a motion segment unit of the spine, nonetheless, requires the removal of spinal tissue in order to place the distractor legs in a suitable position for operating the device as shown in  FIG. 2  of the reference. In particular, it is often necessary to remove the interspinous ligaments from adjacent vertebrae in order to provide placement of the distractor device in an operable position to measure spinal stiffness. The removal of spinal tissue with this procedure may contribute to the instability of the motion segment unit. Thus, the surgeon must first further destabilize the motion segment unit before a measurement can be taken and this may have a bearing on the type of implantable spinal assist device that may be used to correct the instability and the degree to which the patient may recover from the spinal surgery. 
     It would therefore be a distinct advantage in the art for measuring and treating instability of a motion segment unit of the spine if a device used to determine the relative stiffness of a motion segment unit did not result in significant damage and/or removal of spinal tissue in order to make the appropriate measurements of spinal stiffness. 
     It would be a further advantage in the art to provide a device for measuring spinal instability which can be readily attached to preselected positions of the motion segment unit during operation without significant tissue damage. 
     It would be a still further advantage in the art to provide a device for measuring spinal instability which can be employed in a comprehensive system in which spinal stiffness or other characteristics of the motion segment unit can be matched with a suitable spinal assist device such as a spinal implant device for reducing or eliminating instability of the motion segment unit. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an apparatus and method for measuring instability of a motion segment unit of the spine in which significant damage and/or removal of spinal tissue is reduced or eliminated during the process of making measurements of the relative stiffness of the spine. The apparatus and method may be effectively employed in a comprehensive system of associating a measured characteristic of the motion segment unit (e.g. stiffness) which an appropriate spinal assist device (e.g. a spinal implant device) to reduce or eliminate the instability of the affected motion segment unit. 
     In one aspect of the present invention, there is provided an apparatus and method for measuring instability of a motion segment unit of a spine comprising:
         a) motor means for applying a controllable force to a distractor arm assembly;   b) a distractor arm assembly operatively engaged to the motor means comprising:
           1) a collar assembly fixedly secured to the motor means,   2) screw means operatively engaged to the motor means through said collar assembly, and rotatable when the motor means is operational, and   3) a pair of arms each having at least two arm segments pivotal with respect to each other, a first of said arm segments being attached to the collar assembly and a second arm segment having a remote end for engaging a portion of the motion segment unit of the spine, and   4) a pivot collar assembly for engaging the second arm segments enabling the second arm segments to be movable with respect to each other,
 
wherein rotation of the screw means causes the pivot collar assembly to move causing the first and second arm segments to move relative to each other whereby the remote ends of the arms move away from each other to provide a controllable force on adjacent portions of a motion segment unit and toward each other to release the controllable force against said adjacent portions of the motion segment unit.
   
               

     The controllable force or load generated by the motor means and applied through the distractor arm assembly may be a force sufficient to provide a constant rate of distraction on the motion segment unit or may be a constant rate of force resulting in a particular displacement profile of the motion segment unit. In either application the resulting force or displacement readings can be associated with a characteristic of the motion segment unit (e.g. stiffness) facilitating the adaptation by the surgeon of a suitable course of treatment. 
     In a further aspect of the present invention, there is provided an apparatus and method for measuring instability of a motion segment unit of a spine comprising:
         c) motor means for applying a controllable force to a distractor arm assembly;   d) a distractor arm assembly operatively engaged to the motor means comprising:
           1) a collar assembly fixedly secured to the motor means,   2) screw means operatively engaged to the motor means through said collar assembly and rotatable when the motor means is operational,   3) a pair of arms each having at least two arm segments pivotal with respect to each other, a first of said arm segments being attached to the collar assembly and a second arm segment having a remote end in the form of a dual leg assembly for engaging of the motion segment unit of the spine, and   4) a pivot collar assembly for engaging the second arm segments enabling the second arm segments to be movable with respect to each other.   
               

     wherein rotation of the screw means causes the pivot collar assembly to move causing the first and second arm segments to move relative to each other whereby the dual leg assembly exerts a controllable force on adjacent portions of a motion segment unit. 
     In a further aspect of the invention, the apparatus is adapted to engage a spine imbedded attachment device within a preselected portion of the motion segment unit and through the attachment device the constant rate of force is exerted against respective portions of the motion segment unit. 
     In a further aspect of the invention there is provided a detection means connected to the distractor arm assembly for measuring the resistance of the pair of arms to said distraction which is related to the resistance of the adjacent vertebrae of the motion segment unit to said distraction, at a plurality of force exerting positions, said detection means generating an output signal corresponding to said resistance. There may be further provided translation means adapted to receive the output signal for the detection means and for translating the output signal into interpretable data such as a stiffness value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings in which like reference characters indicate like parts are illustrative of embodiments of the present invention and are not intended to limit the invention as encompassed by the application including the claims appended hereto. 
         FIG. 1  is a side view of a first embodiment of the apparatus of the present invention including a distractor arm assembly including two distractor arms for placement between adjacent vertebrae of a motion segment unit and a motor assembly for operating the distractor arm assembly contained with a housing; 
         FIG. 2  is a perspective view of a further embodiment of a distractor arm assembly with two distractor arms in accordance with the present invention; 
         FIG. 3  is a perspective view of a portion of a distractor arm employing reusable or disposable pins for contacting the motion segment unit; 
         FIG. 4  is a cross-sectional view of the embodiment of the apparatus of the present invention shown in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of a lumbar vertebrae showing a pedicle screw inserted therein wherein the pedicle screw is engageable by the distractor arm assembly of the present invention; 
         FIG. 6  is a schematic view of a portion of an arm of a distractor assembly adapted to engage a pedicle screw of the type shown in  FIG. 5 ; 
         FIG. 7  is a schematic view of a system for evaluating instability of the motion segment unit using the apparatus of the present invention; 
         FIG. 8  is a side elevational view of a further embodiment of the apparatus of the present invention employing a distractor arm assembly including a dual leg assembly; 
         FIG. 9  is a perspective view of a further embodiment of a distractor arm assembly having a dual leg assembly in accordance with the present invention; 
         FIG. 10  is a perspective view of a further embodiment of the distractor arm assembly having a dual leg assembly in accordance with the present invention; and 
         FIG. 11  is a perspective view of a still further embodiment of the distractor arm assembly having a dual leg assembly of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring the drawings and first to the embodiment of the invention shown in  FIGS. 1–6 , there is shown an apparatus  2  of the present invention which includes a stepper motor assembly  4  (See  FIG. 4 ) which is capable of applying force preferably at a constant rate to a distractor arm assembly  8  as hereinafter described. On example of a stepper motor assembly  4  is described in U.S. Pat. No. 4,899,761 incorporated herein by reference. The stepper motor assembly  4  is contained within a housing  6 . The apparatus also includes a distractor arm assembly  8  which, as described in detail hereinafter, is able to apply a constant force to spaced apart locations of a motion segment unit to separate and/or distract, and/or torque the same and thereby to enable the determination of reliable data on a characteristic (e.g. relative stiffness) of the motion segment unit. 
     Referring specifically to  FIGS. 1 ,  2  and  5 , the housing  6  contains a stepper motor assembly  4  which includes a stepper motor  10  which provides rotational movement through a drive gear  12  and an idler gear  14 . 
     Rotational movement is provided through an assembly including a coupler  18  which secures a jackscrew  20  therein. A series of ball bearings  22  secures the remote end of the jackscrew in the motor housing  6 . There is also provided a load cell or strain gauge  16  for measuring resistance to the force applied by the distractor arm assembly  8 . 
     A collar assembly  24  is operably connected to a lower portion of the housing  6 . The collar assembly  24  is secured to the housing  6  through a port  26 . An opening  28  is provided within the collar assembly  24  to allow the jackscrew  20  to pass therethrough into the housing  6 . 
     Attached to the collar assembly  24  is the distractor arm assembly  8  which in accordance with the present invention includes two segmented distractor arms  32   a  and  32   b . Each distractor arm  32   a ,  32   b  is provided with first arm segments  34   a  and  34   b , and second arm segments  36   a  and  36   b , respectively. The distractor arm assembly  8  may be released from the collar assembly  24  by operation of a release assembly  60  which can be a projection  62  reversibly securable within a slot  64  or by any other suitable means. 
     The first segments  34   a  and  34   b  are operably and rotatably connected to the collar assembly  24  through a connecting device  38   a  and  38   b  such as a screw, bolt or the like. The first segments  34  are operatively and rotatably connected to the second segments  36  through a similar type of connecting device  40   a  and  40   b.    
     As shown in  FIGS. 2 and 4 , the second segments  36   a  and  36   b  are linked to the jackscrew  20  through a pivot collar assembly  37 . As a result, the .second segments  36   a  and  36   b  are able to rotate with respect to each other thereby enabling the remote ends of the second segments to move toward and away from each other as described hereinafter. 
     At the remote end of the second segments  36   a  and  36   b , there is provided motion segment unit engaging devices which engage a portion of the motion segment unit of the spine. As shown in  FIGS. 2–4 , there is provided a pair of motion segment unit engaging devices in the form of pins  42  which are adapted to engage opposed portions of motion segment units as the second segments  36   a  and  36   b  move away from each other to thereby force apart the adjacent portions of the motion segment units as hereinafter described. The pins  42  which may be reusable or disposable may be any shape so long as the pin ends can engage the motion segment unit. It is desirable for the pins  42  to have a relatively small contact surface that engages the motion segment unit. It is also preferred that the pins are adjacent each other when placed in proximity to the motion segment unit to minimize damage to adjacent tissue. In addition to pins, pedicle screws and bone drill bits and similar devices may be used. 
     In an alternative embodiment as shown in  FIGS. 5 and 6 , the motion segment unit of a spine shown generally by the numeral  44  in  FIG. 5  may be provided with a pin, pedicle screw, bone drill bit, or the like shown generally by the numeral  46  which is preinserted into the motion segment unit and has a head portion  48  which is adapted to be engaged by the remote end of the second segment  36  of the distractor arm  32 . In particular and referring to  FIG. 6 , the remote end of the second segment  36  may be provided with a cavity  50  having a shape complimentary to the head portion  48  of the pin  46  so that the head portion  48  may be inserted into the cavity  50  to provide reversible locking engagement with the second segment  36  of the distractor arm  32 . 
     Movement of the distractor arm assembly  8  is provided in the following manner. The stepper motor assembly  4  provides rotational movement to the jackscrew  20  through the drive gear  12  and idler gear  14 . The jackscrew  20  is secured to the stepper motor assembly  4  through the employment of the coupler  18  and ball bearings  22 . Rotational movement of the stepper motor causes the jackscrew  20  to rotate and thereby enable the pivot collar assembly  37  to move upwardly along the jackscrew  20  towards the stepper motor assembly  4 . As the pivot collar assembly  37  moves upwardly, the first segments  34   a  and  34   b  move away from each other thereby causing a similar movement in the second segments  36   a  and  36   b  thereby causing the remote end of the second segments  36  to move away from each other and thereby move the respective portions of the motion segment unit away from each other. As a result, a measurable and preferable constant rate of distraction (displacement control) is applied, or a constant rate of force (force control), is applied against the motion segment unit and the resulting force, or displacement, can be associated with a rating of a characteristic of the motion segment (e.g. stiffness) which can assist the surgeon in deciding on an appropriate course of treatment including the implantation of spinal assist devices. Depending on the direction of rotation of the jackscrew  20 , the segments may move toward each other thereby measuring the compressive stiffness of the motion segment unit. 
     For example, a method of measuring the relative stiffness of a motion segment unit is disclosed in U.S. patent application Ser. No. 10/683,505 filed on Oct. 10, 2003 (Attorney docket No. 508.1.014), incorporated herein by reference. The method includes applying a force against at least one pair of “targeted” adjacent vertebrae of the patient. The application of force can be applied by the apparatus of the present application. 
     Thereafter, a measurement is taken of at least one characteristic of the targeted motion segment unit as a function of the applied force (e.g. stiffness, displacement at a predetermined force and/or hysteresis). An output signal corresponding to the characteristic of the motion segment unit is then generated. 
     The output signal is then compared to a data bank of values of the same characteristic obtained from sample pairs of targeted adjacent vertebrae tested in the same manner as the targeted adjacent vertebrae. The values of the characteristic of the sample targeted vertebrae are matched with implantable spinal assist devices capable of reducing or eliminating instability of the targeted adjacent vertebrae. 
     The surgeon then selects the suitable spinal implant device, if any, and installs the same in a manner which improves stability of an otherwise unstable targeted adjacent vertebrae. 
     Returning to  FIGS. 1–6 , by reversing movement of the stepper motor  10 , the pivot collar assembly  37  is forced to move downwardly, thereby causing the first segments  34   a  and  34   b  to move towards each other and thus cause the second segments  36   a  and  36   b  to likewise move toward each other and thereby relieve the force applied to the adjacent portions of the motion segment unit. 
     Translation of the movement of the distractor arms into a signal for determining the relative stiffness of the spine can be made in accordance with U.S. Pat. No. 4,899,761 incorporated herein by reference. 
     As is apparent from the description of the embodiment of the present invention provided herein, the apparatus of the present invention is designed to minimize invasion of spinal tissues and may be used by both posterior and anterior surgical procedures, including posterior lateral and anterior lateral, and lateral procedures. The apparatus provides unimpaired line of sight and provides for ready separation of the distractor arm assembly from the stepper motor assembly. The apparatus further provides an efficient, less invasive means of applying a force against targeted motion segment units to enable a surgeon to perform the diagnostic and implantation procedures described in U.S. patent application Ser. No. 10/683,505 filed on Oct. 10, 2003. 
     The apparatus in  FIGS. 1–6  may be used as part of a system for evaluating instability of a motion segment unit as shown for example in U.S. Pat. No. 4,899,761 incorporated herein by reference. By way of example and referring to  FIG. 7 , the apparatus  2  of the present invention includes a distractor arm assembly  108  including a stepper motor assembly  104  which is capable of applying a force to the pair of distractor arms  32   a  and  32   b . The stepper motor assembly is variable in both speed and torque. The torque and rotational speed produced by the motor is dependent upon the power available to the motor. Thus, the rotational speed of the motor is variable, depending on the rate at which a computer  122  sends voltage impulses via an input/output port  120  through a motor stepping circuit  122  to the stepper motor assembly  4 . Each voltage pulse can be set to a constant rate of motor revolution (e.g., 1.8 degrees) so that, for instance, 200 pulses are required for each revolution of the stepper motor assembly  4 . 
     The input/output port  120  of the computer  122  sends a signal in the form of a voltage pulse (e.g., 12 volts) to the motor stepping circuit  122  to control the rate at which the stepper motor assembly  4  rotates. Each pulse is sufficient to cause the stepper motor assembly  4  to rotate at a constant rate (e.g. 1.8 degrees). A desired pulse rate has been found to be between about 30 to about 60 pulses (e.g. 40 pulses per second). If the rate of rotation is too slow, the motion segment unit will tend to “creep” or undergo additional distraction which leads to a false reading of stiffness, which is measured by dividing the resulting force by the distance of distraction. 
     The load cell or strain gauge  116  operates as a load transducer and detects the resistance of the adjacent vertebrae to the force being applied by the pair of distractor arms  32   a  and  32   b , and translates the same into a voltage (in milivolts). Accordingly, the voltage produced by strain gauge  116  is a function of the resistance to the force applied, and is translated into a voltage, which is typically in the range from 0 (no load) to about 12 millivolts (maximum load). A maximum voltage of about 12 millivolts is equivalent to about 200 newtons of stress, since the voltage varies directly with the stress, since the voltage varies directly with the stress. 
     The load cell or strain gauge  116  transmits a signal corresponding to the change in voltage to a signal conditioning circuit  112  which has a two-fold function. First, the signal conditioning circuit  112  filters out extraneous voltage interference such as minute voltage signals emanating from fluorescent lights, etc. and, second, it amplifies the voltage signal from the strain gauges  116  from mV to V units so that the change in voltage may be read by an analog to digital converter  114 . The signal conditioning circuit  112  translates the millivolt signal from the stress gauge  116  into a voltage readout of from 0–10 volts, or other suitable range as desired. 
     The analog to digital converter  114  converts the amplified signal from the signal condition circuit  112  into force, units, (e.g. newtons) which can be read by computer  122 . For example, the converter  114  converts the voltage from the signal conditioning voltage (e.g. 0–10 volts) to a digital readout of, for example, 0 to 255 units. The computer  122  is equipped with stored data which can interpret and convert the maximum load (e.g. 200 newton load) applied to the vertebrae. The results may be viewed on a monitor  118  and compared to previously acquired data such as data acquired from motion segment units of normal subject having similar physiologic backgrounds as described in U.S. Pat. No. 10/683,505 filed on Oct. 10, 2003. As shown in  FIG. 7 , the apparatus of the present invention is placed into operation by placing the distractor arm assembly  108  into position between adjacent vertebrae and then activating the stepper motor assembly  104  by moving switch  110  to the “on” position. The system is deactivated by moving the switch  110  to the “off” position and removing the distractor arm assembly  108  from its position between the adjacent vertebrae. In accordance with the present invention, the system may be used in the operating room to enable the surgeon to quantitatively determine whether fusion of a motion segment unit is necessary to insure stability at the level of the spine in question, or reconstruction of a motion segment unit, or intervertebral disc replacement, or disc nucleus replacement, or ligament replacement, is necessary to insure stability at the level of the spine in question. 
     A method of measuring the relative stiffness of a motion segment unit as disclosed in U.S. patent application Ser. No. 10/683,505 filed on Oct. 10, 2003, incorporated herein by reference may be used with the apparatus of the present invention. The method includes applying a force against at least one pair of “targeted” adjacent vertebrae of the patient. The application of force can be applied by the apparatus of the present application. 
     Thereafter, a measurement is taken of at least one characteristic of the targeted motion segment unit as a function of the applied force (e.g. stiffness, displacement at a predetermined force and/or hysteresis). An output signal corresponding to the characteristic of the motion segment unit is then generated. 
     The output signal is then compared to a data bank of values of the same characteristic obtained from sample pairs of targeted adjacent vertebrae tested in the same manner as the targeted adjacent vertebrae. The values of the characteristic of the sample targeted vertebrae are matched with implantable spinal assist devices capable of reducing or eliminating instability of the targeted adjacent vertebrae. 
     The surgeons then select the suitable spinal implant device, if any, and installs the same in a manner which improves stability of an otherwise unstable targeted adjacent vertebrae. 
     In a further aspect of the present invention, the apparatus is provided with dual leg assemblies pivotably connected to the remote ends of the second segments. 
     Referring to the drawings and specifically to  FIGS. 8–11 , there is shown an apparatus  2  of the present invention which includes a stepper motor assembly  4  (See  FIG. 6 ) of the type described in U.S. Pat. No. 4,899,761 incorporated herein by reference contained within a housing  6 . The apparatus also includes a distractor arm assembly  8  which, as described in detail hereinafter, is able to apply a constant rate of distraction (displacement control) and/or a constant rate of force (force control) is applied, to spaced apart locations of a motion segment unit to separate the same and/or to enable the determination of reliable data on the relative stiffness, or displacement of the motion segment unit. 
     Rotational movement is provided through an assembly including a coupler  18  which secures a jackscrew  20  therein. A series of ball bearings  22  secures the remote end of the jackscrew in the motor housing  6 . There is also provided a load cell or strain gauge  16  for measuring resistance to the force applied by the distraction arm assembly. 
     A collar assembly  24  is operably connected to a lower portion of the housing  6 . The collar assembly  24  is secured to the housing  6  through a port  26 . An opening  28  is provided within the collar assembly  24  to allow the jackscrew  20  to pass therethrough into the housing  6 . 
     Attached to the collar assembly  24  is the distractor arm assembly  8  which in accordance with the present invention includes two segmented distractor arms  32   a  and  32   b . Each distractor arm  32   a ,  32   b  is provided with first segments  34   a  and  34   b , and second segments  36   a  and  36   b , respectively. The distractor arm assembly  8  may be released from the collar assembly  24  by operation of a release means  60 . 
     The first segments  34   a  and  34   b  are operably and rotatably connected to the collar assembly  24  through a connecting device  38   a  and  38   b  such as a screw, bolt or the like. The first segments  34  are operatively and rotatably connected to the second segments  36  through a similar type of connecting device  40   a  and  40   b.    
     As shown for example in  FIG. 8 , the second segments  36   a  and  36   b  are linked to the jackscrew  20  through a pivot collar assembly  37 . As a result, the second segments  36   a  and  36   b  are able to rotate with respect to each other thereby enabling the remote ends of the second segments to move toward and away from each other as described hereinafter. 
     At the remote end of the second segments  36 , there is provided a motion segment unit engaging device for engaging a portion of a motion segment unit of the spine. There is provided a pair of dual leg assemblies  50   a  and  50   b  pivotally connected to the respective ends of the second segments  36   a  and  36   b . The pivotable connection is through a connecting device  52  which may be in the form of a screw, bolt, pin or the like. The dual leg assembly  50  is comprised of a pair of legs  54   a  and  54   b  having a forward end attached via the connecting device  52  to the second segment  36   a  and  36   b , respectively. The remote end  58  is attached to a motion segment unit engaging device  61  which may employ contact surfaces for directly engaging a portion of the motion segment unit of the spine or, as specifically shown in  FIG. 8  providing a connection means  62  for engaging a pin or other device (e.g. pedicle screw and bone drill bit) which is preinserted into a portion of the motion segment unit as described hereinafter. 
     The connection means  62  has an upper portion  64  which rotatably engages one of the legs  54   a  or  54   b  and a lower portion  66  particularly adapted to reversibly engage the head of a preinserted pin screw (e.g. pedicle screw) or bone drill bit which has been secured within the motion segment unit of the spine as shown in  FIG. 5 . 
     The upper portion  64  may comprise a collet and bushing for securing the lower portion  66  to the corresponding leg  54   a  or  54   b . The upper portion  64  includes a leg receiving slot  68  for securing the leg to the upper portion allowing at least some degree of rotational movement so that the motion segment unit engaging assembly  61  may be secured about the pin, screw or bone drill bit preinserted into the motion segment unit. 
     As specifically shown, for example, in  FIG. 8 , one embodiment of the invention provides for two pair of motion segment unit engaging assemblies  61  to enable interaction with spaced apart portions of the motion segment unit to provide a controllable force, sufficient for determining a characteristic (e.g. stiffness) of the motion segment unit. 
     Other embodiments of the dual leg assemblies are shown in  FIGS. 9–11  and include different mechanisms by which the pin, pedicle screw or bone drill bit, preinserted into the motion segment unit, may be engaged by the dual leg assembly. Specifically,  FIG. 9  shows a simple cylindrical tube  70  which has an interior profile adapted to engage and reversibly secure the pin, pedicle screw or bone drill bit that has been preinserted into the motion segment unit. 
       FIG. 10  provides for a cylindrical tube  72  which includes a knob  74  and screw down shaft  76  for reversibly securing the head of the pin, pedicle screw or bone drill bit within the dual leg assembly. 
       FIG. 11  is a further embodiment of the invention in which the dual leg assembly is comprised of a pair of parallel legs  78   a  and  78   b  which are essentially fixed with respect to each other by a connecting device  90 . Rotational movement therefore is contained within a connection means  92  which is comprised of an upper portion  94  having attached thereto a lower portion  96  similar to that describing the embodiment of  FIG. 8 . 
     As shown in  FIG. 5 , the motion segment unit of a spine shown generally by the numeral  44  may be provided with a pin, pedicle screw or the like shown generally by the numeral  46  which is preinserted into the motion segment unit and has a head portion  48  which is adapted to be engaged by the dual leg assembly attached to the second segment  36  of the distractor arm assembly  8 . The dual leg assembly and particularly the lower portion thereof is fitted in reversible locking engagement to the head portion  48  of the pin  46  so that the head portion  48  of the pin  46  may be inserted into the lower portion (e.g.  66  as shown in  FIG. 8 ) of the motion segment unit engaging assembly  61  to provide reversible locking engagement with the second segment  36  of the distractor arm assembly  8 . 
     Movement of the distractor arm assembly  8  of this embodiment is provided in a manner similar to that described for the embodiment of the present apparatus shown in  FIGS. 1–7 . As shown best in FIGS.  4  and  8 – 11 , the stepper motor assembly  4  provides rotational movement to the jackscrew  20  through the drive gear  12  and idler gear  14 . The jackscrew  20  is secured to the stepper motor assembly  4  through the employment of the coupler  18  and ball bearings  22 . Rotational movement of the stepper motor causes the jackscrew  20  to rotate and thereby enable the pivot collar assembly  37  to move upwardly along the jackscrew towards the stepper motor assembly  4 . As the pivot collar assembly  37  moves upwardly, the first segments  34   a  and  34   b  move away from each other thereby causing a similar movement in the second segments  36   a  and  36   b  thereby causing the dual leg assemblies  50   a  and  50   b  attached to the second segments  36   a  and  36   b , respectively (see  FIG. 8 ) to move away from each other thereby moving the respective portions of the motion segment unit away from each other. 
     By reversing movement of the stepper motor  10 , the pivot collar assembly  37  is forced to move downwardly, thereby causing the first segments  34   a  and  34   b  to move towards each other and thus cause the second segments  36   a  and  36   b  to likewise move toward each other thereby relieving the force applied to the adjacent portions of the motion segment unit. 
     As with the apparatus of  FIGS. 1–6 , the embodiments of  FIGS. 8–11  may be used as part of a system for evaluating instability of a motion segment unit as shown for example in U.S. Pat. No. 4,899,761 incorporated herein by reference as previously described. 
     A method of measuring a characteristic (e.g. relative stiffness) of a motion segment unit as disclosed in U.S. patent application Ser. No. 10/683,505 filed on Oct. 10, 2003 may be used with this embodiment of the present invention as previously described with respect to the embodiment of  FIGS. 1–7 .