Apparatus and methods for inter-operative verification of appropriate spinal prosthesis size and placement

A distractor and measuring device includes a first handle operatively connected to a first paddle and a second handle operatively connected to a second paddle. The first and second handles are pivotally connected to one another such that movement of the handles relative to one another causes the paddles to move relative to each other. At least one transducer is positioned on at least one of the first and second handles for measuring an amount of force applied during distraction. A measurement rod is pivotally connected to the device and is movable in a longitudinal direction relative to the first and second paddles. A potentiometer is operatively connected to the measurement rod to measure the angular orientation of the measurement rod.

BACKGROUND

This disclosure relates generally to apparatus and methods for use with spinal surgery and, more particularly, to apparatus and methods for precise spinal surgery disc placement.

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.

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. Another approach for treating degenerative disc disease is total disc replacement (“TDR”) in which the pain-generating intervertebral disc is removed and a metallic artificial disc implant that allows motion is inserted into the intervertebral space between the adjacent vertebrae. The implanted spinal fusion cage or TDR implant (collectively referred to as “intervertebral 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.

It is estimated that approximately 3% of fusions and 3-8% of TDR procedures performed each year require revision. Some of these revisions are believed to be due to the misplacement and incorrect size selection of the intervertebral implant (e.g., artificial disc or fusion cage), which is based predominantly on the judgment of the surgeon at the time of the procedure.

Currently, there exist a variety of devices and methods for use in spinal surgery that are related to spinal disc space distractors. For instance,FIG. 1shows a spinal disc space distractor that is depicted in U.S. Pat. No. 6,261,296 (“the '296 patent”). Devices such as those disclosed in the '296 patent provide grip and tightening mechanisms that are useful due to their mechanical advantages. Although the distractor device disclosed in the '296 patent is capable of opening up the disc (intervertebral) space, such conventional distractor devices do not provide the surgeon with the ability to accurately measure endplate length and disc height for the optimal selection, sizing and placement of the artificial disc, fusion cage or other intervertebral implant. Instead, the selection, sizing and placement of the artificial disc or fusion cage are based predominantly on the judgment of the surgeon at the time of the procedure.

Because conventional distractors lack measuring capabilities, discs may be distracted too much or too little, or the intervertebral implant may be placed in the wrong position. Incorrect selection, sizing and placement of the artificial disc or fusion cage may lead to many serious post-operative complications.

For instance, if the implant selected is too large for the intervertebral space, the implant could over-stuff the intervertebral space, which can reduce the patient's range of motion. Furthermore, if the surgery is a total disc replacement and the implant selected is too large for the space, the implant could create a fusion, instead of preserving the patient's range of motion. If the implant selected is too small, the implant could slip out when the disc annulus is relaxed and nick an artery or the spinal cord, resulting in paralysis or death.

Improper placement of the implant can also lead to improper stress on the surrounding intervertebral bodies, which often necessitates more surgery. Placement of the implant too far anterior may cause the spine to have reduced flexion and enhanced extension. Placement of the implant too far posterior may cause the spine to have enhanced flexion and reduced extension of the spine. Both of the aforementioned scenarios are abnormal for the function of the spine and could also lead to improper loading and stressing of the entire spine.

Conventional spinal distractor devices do not provide the surgeon with the ability to accurately measure the intervertebral disc space to facilitate the proper selection, sizing and placement of the intervertebral implant (e.g., artificial disc or fusion cage).

BRIEF SUMMARY

In one aspect of this disclosure, a distractor and measuring device is disclosed that comprises a first handle operatively connected to a first paddle and a second handle operatively connected to a second paddle. The first and second handles are pivotally connected to one another such that movement of the handles relative to one another causes the paddles to move relative to each other. At least one transducer is positioned on at least one of the first and second handles for measuring an amount of force applied during distraction. A measurement rod is pivotally connected to the device and is movable in a longitudinal direction relative to the first and second paddles. A potentiometer is operatively connected to the measurement rod to measure the angular orientation of the measurement rod.

In another aspect of this disclosure, a method is disclosed for measuring an intervertebral space between two vertebrae in a patient. The method comprises inserting a distractor into the intervertebral space and measuring a force applied during distraction with at least one transducer positioned on the distractor. The intervertebral space is distracted until the measured force reaches a value corresponding to a predetermined distance an intervertebral disc annulus stretches between the vertebrae. A first longitudinal displacement of a measurement rod on the distractor is measured when the rod is moved to a position at an anterior side of an intervertebral endplate, and a second longitudinal displacement of the measurement rod on the distractor is measured when the rod is moved to a position at a posterior side of the intervertebral endplate. A length of the intervertebral endplate is calculated by taking the difference between the first and second longitudinal displacements. A first value is generated using a potentiometer on the distractor corresponding to a first angle when the measurement rod is pivoted to contact the anterior side of the endplate. A second value is generated using the potentiometer on the distractor corresponding to a second angle when the measurement rod is pivoted to contact the posterior side of the endplate. An anterior height of the intervertebral space is calculated based on the first longitudinal displacement and the first angle, and a posterior height of the intervertebral space is calculated based on the second longitudinal displacement and the second angle.

The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of this disclosure in order that the following detailed description may be better understood. Additional features and advantages of this disclosure will be described hereinafter, which may form the subject of the claims of this application.

DETAILED DESCRIPTION

A preferred spinal distractor and measuring device10is disclosed herein that may be used during the anterior approach to lumbar fusion or total disc replacement (“TDR”). The preferred spinal distractor and measuring device10utilizes sensors and a measurement rod to provide intra-operative feedback to a surgeon during spinal surgery by measuring the length of the vertebral endplate, the disc height between the anterior/posterior upper and lower vertebrae, and the magnitude of hand force applied to the device as a function of annulus distraction. This feedback allows the surgeon to quantitatively assess the amount of force applied to the distractor versus the distraction of the intervertebral disc annulus, the length of the intervertebral endplate that serves as the footprint for the implant, and the distraction of the intervertebral bodies at both anterior and posterior locations. This information will enable the surgeon to make a highly informed decision regarding the optimal size of the intervertebral implant (e.g., disc implant, fusion cage, intervertebral spacer) and the final placement of the implant for a patient undergoing a spinal fusion or TDR procedure.

FIGS. 2-5illustrate the preferred spinal distractor and measuring device10in accordance with an illustrative embodiment. The spinal distractor10preferably includes split handles12. The split handles12preferably include a pair of upper handles12a,12band a pair of lower handles12c,12d. The upper handles12a,12bare preferably separated from one another to form a gap13aand the lower handles12c,12dare preferably separated from one another to form a gap13b. The gaps13a,13ballow for a measurement rod14to be operated on the device10without the handles12a,12b,12c,12dlimiting the travel distance or movement of the measurement rod14. This feature is especially useful when the surgeon intends to adjust the angle of the measurement rod14while the handles12a,12cand12b,12dare squeezed together during distraction (seeFIG. 5).

The upper and lower handles12a,12b,12c,12dpreferably include a hand-graspable portion15a,15b,15c,15d. The hand-graspable portions15a,15b,15c,15dare adapted to be gripped by a surgeon or user so that the upper handles12a,12band lower handles12c,12dmay be squeezed toward one another during distraction. The upper handles12a,12bare preferably pivotally connected to the lower handles12c,12dby a yoke mechanism18(seeFIGS. 3-5).

As will be described further below, one or more sensors or transducers11, such as (but not limited to) pressure transducer(s), force transducer(s), load cell(s), strain gauge(s), piezoelectric transducer(s) or the like, are preferably located on the hand-graspable portion15a,15b,15c,15dto measure the amount of force/pressure being applied by the surgeon or user to the upper and lower handles12a,12b,12c,12dduring distraction. The sensor(s)11is preferably electrically connected to an amplifier circuit to produce a voltage value corresponding to the hand force/pressure applied by the surgeon during distraction, which will ultimately correspond to a displacement value for the amount of distance that the annulus ligament between the vertebrae stretches during distraction. Knowing this value will provide the surgeon with a greater understanding as to how far to distract and when to stop distraction of the vertebrae, which will avoid over distraction and under distraction, and reduce surgical complications that may require revision surgery.

Referring toFIG. 3, the upper handles12a,12band lower handles12c,12dpreferably include a semi-circular or arcuate protrusion19a,19b,19c,19dadjacent to a recessed portion21a,21b,21c,21dconfigured to receive and rotatably support a corresponding protrusion from an opposing handle. When assembled, protrusion19aof upper handle12ais located within and rotatably supported by recess portion21cof lower handle12c, and protrusion19cis located within and rotatably supported by recess portion21aof upper handle12a. Similarly, protrusion19bof upper handle12bis located within and rotatably supported by recess portion21dof lower handle12d, and protrusion19dis located within and rotatably supported by recess portion21bof upper handle12b. The yoke mechanism18preferably extends through an opening within the protrusions19a,19b,19c,19dto pivotally connect the upper handle12ato the lower handle12cand the upper handle12bto the lower handle12d.

A potentiometer mounting bracket16is preferably mounted on top of the portion of the distractor10that contains the yoke mechanism18(seeFIG. 3). The bracket16preferably includes a fixation point for the stationary portion of a potentiometer16a, which is preferably concentric with the yoke mechanism18and which allows for a rotating arm of the potentiometer to fit into the yoke mechanism18. The purpose of this assembly is for determining the angle of the measurement rod14, which will permit measurement of the disc height to permit a more accurate implant size selection and avoid post-operative complications.

The distractor10preferably includes a handle resistance mechanism20. The handle resistance mechanism20includes, for example, one or more springs22coiled around handle adjustment bars24. A first handle adjustment bar24preferably extends through an opening formed near the proximal end of the upper handle12aand lower handle12c. A second handle adjustment bar24preferably extends through an opening formed near the proximal end of the upper handle12band lower handle12d. The springs22provide the resistance needed for smooth operation of the distractor10and provides sufficient resistance to prevent the surgeon from unintentionally over-distracting the annulus. A handle locking screw26is preferably threaded onto the ends of each handle adjustment bar24to allow for the surgeon to accurately pause the distraction and lock the distractor10into place. The ability to accurately pause the distraction permits the annulus between the vertebrae being distracted to adjust to its new position without overstretching the annulus. It is understood that other biasing means or springs may be utilized, such as, for example, leaf springs or the like.

Referring toFIGS. 2 and 3, the upper and lower handles12a,12b,12c,12dpreferably terminate in a distal portion17a,17b,17c,17d. The upper handles12a,12band lower handles12c,12dmay be pinned together at their respective distal portions17a,17band17c,17d. The ends of the distal portions17a,17,b,17c,17dare relieved so that, when assembled, a gap27extends between distal ends17a,17band between distal ends17c,17d.

The distractor10includes an upper paddle or jaw32athat is pivotally connected to the distal ends17a,17bof the upper handles12a,12b, and a lower paddle or jaw32bthat is pivotally connected to the distal ends17c,17dof the lower handles12c,12d. An upper connecting member34apreferably projects from the upper paddle32aand a lower connecting member34bprojects from the upper paddle32b. The end of the upper connecting member34ais located within the gap27between the distal ends17a,17bof the upper handles12a,12b. A pin35apreferably pivotally connects the end of the upper connecting member34ato the distal ends17a,17bof the upper handles12a,12b. Similarly, the end of the upper connecting member34bis located within the gap27between the distal ends17c,17dof the lower handles12c,12d. A pin35bpreferably pivotally connects the end of the lower connecting member34bto the distal ends17c,17dof the lower handles12c,12d.

The upper and lower connecting members34a,34bpreferably include an opening or slot36extending therethrough. At least one cross link40is preferably connected to the connecting members34a,34bto ensure that the connecting members (and their respective paddles32a,32b) remain parallel to one another during operation of the device10. It is preferred that two cross links40be utilized, one on each side of the connecting members34a,34b.

Each cross link40preferably includes a first link41that overlays a second link42to form a generally X-like configuration. The first and second links41,42are pivotally connected to each other with a pin43. One end of the first link41is preferably pivotally connected to the lower connecting member34bvia pin44and the opposing end of the first link41is slidingly connected to the upper connecting member34avia a pin45that slides longitudinally within the slot36in the upper connecting member. Similarly, one end of the second link42is preferably pivotally connected to the upper connecting member34avia pin46and the opposing end of the second link42is slidingly connected to the lower connecting member34bvia a pin47that slides longitudinally within the slot36in the lower connecting member.

The arrangement described above allows the upper and lower connecting members34a,34b(and their respective paddles32a,32b) to remain parallel to one another as they move away from one another when the upper handles12a,12band lower handles12c,12dare squeezed toward one another by the surgeon during distraction, as shown for example inFIG. 3.

FIGS. 3-5show a detailed view of the front portion of the distractor10. The yoke18and measurement rod14provide the measuring capabilities of the distractor10. Together, they allow for the measurement of disc height and endplate length, which will ensure a more accurate implant size selection resulting in little to no post-operative complications due to improper sizing. The measurement rod14is preferably marked with predefined graduations for taking length measurements in a similar fashion to a ruler and it extends through a slot or opening in the yoke18where the rod14may be slid back and forth in a longitudinal direction to measure endplate length. Alternatively, a displacement sensor, such as (but not limited to) a linear variable differential transformer (LVDT) or the like, may be utilized to produce a voltage value corresponding to the longitudinal displacement of the measurement rod14, which would correspond to endplate length. Additionally, the yoke18can preferably rotate freely about the distractor10, which allows the measurement rod14to be angled or pivoted up and down in the vertical direction for making disc height measurements. The disc height measurements are preferably determined by reading the potentiometer's angle and using simple geometry. Alternatively, a protractor or like device with predefined graduations may be mounted on the distractor10to measure the angle of the measurement rod14.

The distractor10also preferably includes tapered paddle ends28a,28bon the upper and lower paddles32a,32b, each of which preferably includes a split30in its center. The central split30separates the tapered paddle end28so that the paddles32a,32bhave a fork-like configuration. The tapered ends28allow for paddles32a,32bto fit easily between the discs and the split30in the center of each paddle end28allows the measurement rod14access to the discs for the measurement of disc height and endplate length. In other words the central split30allows the angle of the measurement rod14to be adjusted without interference from the paddles32a,32b(seeFIG. 5).

The upper and lower paddles32a,32balso preferably include a semicircular recess29a,29bthat is aligned with and opens into the central split30. The size or radius of the semicircular recess29a,29bis slightly larger than that of the measurement rod14so that the measurement rod may be slid longitudinally through the opening formed by the semicircular recesses29a,29bof the device10when the upper and lower paddles32a,32bare contacting one another in their normally-closed or spring-biased position.

The distractor10preferably includes features such as (but not limited to) quantitative information regarding distance and distraction force. The following is a preferred description of how the distractor10functions during an illustrative spinal procedure.

The first step is to separate the two vertebrae that border the disc space that is being worked on. To assist the surgeon during distraction, or the separation of the vertebrae without rupture of their binding ligaments, sensor(s)11, such as (but not limited to) one or more pressure transducers or strain gauges, located on the upper and/or lower handles12a,12b,12c,12dof the distractor10preferably relay the amount of pressure or force being applied by the surgeon to the handles12during distraction to a sensor amplifier that produces a voltage value. The voltage value corresponds to a displacement value for the amount of distance the annulus, or the fibrous tissue surrounding the disc, in between the vertebrae stretches. Knowing this value provides the surgeon with a greater understanding as to when to stop distracting.

After distraction, the measurement rod14is preferably extended longitudinally through the distractor10toward the spine. The displacement that occurs by the measurement rod14corresponds to the base length value of an imaginary right triangle. The rod14is then preferably pivoted or rotated to touch the upper and lower posterior and anterior parts of the vertebrae. In doing so, the potentiometer16ain the distractor10will produce a resistance value, which is preferably converted to a voltage value by means of a Wheatstone bridge, which will preferably correspond to a certain angle on the potentiometer graphs from previous calibration experiments. This angle will, in turn, correspond to a disc height. These correlations will preferably be provided on a graph for easier access to the surgeon. Alternatively, these correlations may be computed on a computer that includes a processor and memory, and displayed for viewing by the surgeon. These features will aid in the better fit and placement of disc implant or spinal fusion cages.

The following is the test results of an “In-Lab” prototype of the spinal distractor and measuring device10. The spinal distractor10measures the length of the vertebral endplate, the disc height between the anterior/posterior upper and lower vertebrae, and the magnitude of hand force applied to the device as a function of annulus distraction. To validate the design of the device10, pressure transducers, a potentiometer, and strain gauges were calibrated to verify the spinal distractor measurements. The spinal distractor10was then used to evaluate intervertebral laxity following anterior lumbar discectomy to ensure the proper fit of intervertebral devices, including (but not limited to) disc implants, fusion cages, and inter-vertebral spacers.

The first set of testing performed was to calibrate the electronic devices that were used for testing the distractor10. These included a strain transducer, pressure transducers with load cells, and a potentiometer. From the strain transducer and pressure transducer tests the following equations were calculated to convert displacement from volts to millimeters and force from volts to Newtons:
Force conversionY=3.693X−1.832  (1)
Displacement conversionY=2.46X−0.006  (2)

These equations were obtained by taking the average of the trials from the strain gauge and pressure sensor calibration data. The data tables can be seen in Appendix A. The averages were graphed and a linear analysis was performed. The R2values and equations of the lines can be seen on the graphs depicted inFIG. 6.

A potentiometer was also calibrated to prepare for the second set of testing with the distractor10. The potentiometer calibration data showed was that for an increase in one degree, the potentiometer gave an output of 0.01 volts. The data for the potentiometer calibration can be seen in Appendix A.

The first experiment with the distractor10was the hand force applied versus annulus displacement tests. The team performed nine trials, four of which were using transducer number one and five of which were using transducer number two. The data from the first four trials were thrown out due to problems with the first transducer. Therefore, trials five through nine were analyzed. However, procedural errors were discovered in trials six and nine. Appendix A shows a table that is based off of three of the five trials (Trials five, seven and eight) that were conducted with the second transducer.

After each trial was run, the annulus was allowed to rest before the next trial was started. This allowed the annulus to stretch so that the next trial would go further in displacement than the last. Force values were converted to Newtons and strain values were converted to displacement in millimeters using the previously mentioned equations (1) and (2), respectively. After converting the values to new units, they were zeroed out so that all trials began at zero (see Appendix A for data tables). An exponential growth analysis was performed and the data was plotted as seen inFIG. 7.

The exponential growth analysis provided k and Y0values. The value k is the rate constant, or the rate at which strain increases relative to force, and the value Y0is the displacement value when force is zero. Using the following equation for exponential growth, the force values were plotted in increments of five starting at zero and ending at one hundred for each trial.
Exponential growth equation:Y=Y0*ekx(3)

It was found that as trials of distraction increase, the force applied in Trial five, for example, 60 Newtons, stretched he ligament 0.4 mm while applying 60 Newtons in Trial eight stretched the ligament 0.6 mm. This created a pattern where the graphs for each trial stretched further and further out as the trials were run, as depicted inFIG. 8. This is symbolic of the behavior of an annulus as it is slowly stretched during distraction.

3. Disc Height and Endplate Length Testing

To calculate disc height from the results produced during testing, the following equation was used:
2*y*[ tan (θ)]=disc height  (4)

This equation calculated disc height values based on the angle (θ) at which the measurement or toggle bar14was rotated and the distance from the pivot point to the place of measurement (y). After the distractor10was inserted, the measurement rod or toggle bar14was extended to resting position, flush with the paddles28a,28b, and advanced to both the posterior and anterior sides of the vertebrae. The displacement that occurred was recorded for each side. This stood for the “y” value for the imaginary triangle used in the calculation of disc height. Next, the measurement rod or toggle bar14was toggled or pivoted to touch the top part of the vertebrae. The movement of the potentiometer shaft produced a voltage output that was correlated to an angle value. Then, disc height was calculated using equation (4) above.

APPENDIX A

Data Tables

3. Potentiometer Calibration Data

4. Force Applied to Annulus Distraction Testing Data

5. Exponential Growth Equation Data