Abstract:
An interspinous implant kit includes a receiver body and multiple modules. The receiver body is adapted to be implanted between a spinous processes and includes corresponding saddles with an intermediate section therebetween. The intermediate section includes a module-receiving bay. The modules are configured to be interchangeably received in the bay. The first module may have a sensor adapted to generate measurement data indicative of at least one of forces acting on said receiver body and strain when mated to said receiver body. The second and third modules have mechanical material properties that are the of the same type, but different. One of the modules is advantageously mated to the receiver body to form a long-term implant.

Description:
This application is a divisional of U.S. application Ser. No. 11/340,974, filed Jan. 27, 2006, now U.S. Pat. No. 7,691,130, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present application is directed generally to spinal implants, such as interspinous implants, and more particularly to spinal implants having a sensor associated therewith and methods of using the sensor. 
     The spine includes a number of vertebral members that are typically vertically spaced apart by intervertebral discs. This arrangement permits the spine to undergo slight flexion, extension, lateral flexion, and rotation. In addition, the vertebrae typically include bony protrusions, called spinous processes, that extend posteriorly from the spinal axis. 
     Various medical procedures include spacing apart the vertebral members that extend along a section of the spine. These procedures may be required due to damage to one or more of the vertebral members and/or intervertebral discs caused by a specific traumatic event, a degenerative condition, a tumor, infection, or the like. These procedures typically involve decompressing the relevant vertebral members and installing some form of spinal implant. For example, a bone plate may be secured to adjacent vertebral members to fix their relative positions. However, many procedures call for the vertebral members to remain moveable relative to each other, at least to some extent. For example, a spacer may be inserted between adjacent spinous processes, or other vertebral elements, to provide elastic resistance to relative compression between the elements. The mechanical properties of the spacer (e.g., its size and stiffness) help determine how and to what extent the spacer stabilizes the spinal column. As such, it may be desirable to determine target mechanical properties for the spacer, which is not always possible to predict a priori with the desired level of accuracy. 
     SUMMARY 
     In one illustrative embodiment, the present application provides a method of spacing spinal elements comprising: installing a first spinal implant having a sensor associated therewith; selecting a second spinal implant based on measurement data provided by the sensor; and replacing the first spinal implant with the second spinal implant. The first and second implants may be installed in separate surgical procedures, or during the same surgical procedure, and the implants may be positioned between a superior spinous process and an inferior spinous process and advantageously directly engage the same. The selection of the second implant may be based on the data provided by the sensor and a material property of the second spinal implant, such as its stiffness. The measurement data may correspond to strain or force data. The sensor may be, but is not required to be, embedded in the first spinal implant, such as in a removable module. A corresponding apparatus is described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a spinal implant in accordance with one embodiment. 
         FIG. 2  shows a portion of a spine with an installed sensor-equipped test spinal implant. 
         FIG. 3  shows a portion of a spine with an installed long-term spinal implant replacing the sensor-equipped test spinal implant. 
         FIG. 4  shows alternative embodiments of the spinal implants using a common receiver body and multiple modular inserts. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment, the present application relates to an interspinous implant  20  that has an associated sensor  30 . See  FIG. 1 . Such an implant  20  may be used to space a superior spinous process  12  from an inferior spinous process  14  in adjacent vertebrae  10 . See  FIG. 2 . The implant  20  may be put in place temporarily in order to obtain measurement data, and may therefore be sometimes referred to herein as a test implant  20 . 
     The implant  20 , sometimes referred to as a spacer, typically includes a superior saddle  22 , an inferior saddle  24 , and a midsection  26  therebetween. The superior and inferior saddles  22 , 24  are formed by respective lugs  22   a , 24   a  extending from the midsection  26 , and are configured to receive the superior and inferior spinous processes  12 , 14 , respectively. The saddles  22 , 24  may be symmetric or asymmetric, as is desired. The midsection  26  is intended to substantially fill the space between the spinous processes  12 , 14 , and is advantageously flexible and elastic. For additional information, attention is directed to U.S. Pat. No. 6,626,944 to Taylor, which is incorporated herein by reference. 
     A sensor  30  is associated with the implant  20 . The sensor  30 , in one embodiment, may take the form of a conventional strain gage. Such a strain gage  30  should be oriented to measure vertical strain on the implant  20  between the spinous processes  12 , 14 ; however, the sensor  30  may alternatively or additionally measure strains in other directions. In various alternative embodiments, the sensor  30  measures load, pressure, stress, strain, vibration frequency, and the like, either at a moment in time, or over time. The sensor  30  may advantageously be disposed in the midsection  26  of the implant  20 , such as embedded therein, although this is not required. 
     It is intended that the sensor  30  will generate measurement data about the implant  20  and/or its mechanical environment. Typically, this measurement data is indicative of the forces acting on the implant  20  and/or the resulting strain of the implant  20 . This measurement data may be transmitted from the implant using a suitable analog or digital transmitter, which may use radio frequency, thermodynamic, capacitance, or other means to convey the data signal. Alternatively, the measurement data may be supplied via suitable wires or other transmission media to a computer or other measurement data gathering station. The measurement data may be collected from the implant  20  while the implant is in the body, or after the implant  20  is removed from the body. 
     The sensor-equipped test implant  20  may be inserted between the superior and inferior spinous processes  12 , 14  of adjacent vertebrae  10  during a surgical procedure. The surgical procedure may be similar to that used for the product sold under the trade name DIAM Spinal Stabilization System, available from Medtronic Sofamor Danek, Inc. of Memphis, Tenn. Post-operatively, the implant&#39;s sensor  30  generates measurement data as discussed above. This data is reviewed and used to help select a second implant  40  which replaces the first implant  20  via a second surgical procedure. More particularly, a plurality of candidate implants, such as implants  40 , 50 , may be provided with differing material properties. Like implant  20 , the candidate implants  40 , 50  have respective superior saddles  42 , 52 , inferior saddles  44 , 54 , and midsections  46 , 56 ; but, unlike implant  20 , the candidate implants  40 , 50  advantageously do not include associated sensors. Based on the measurement data from sensor  30 , one of the candidate implants  40 , 50  is chosen so that its material properties will be appropriate for the situation. For example, based on the measurement data, a doctor may conclude that a spacer between the spinous processes  12 , 14  should have a certain overall stiffness. The candidate spacer implants  40 , 50  may have a variety of stiffness levels, indicated in the drawings as stiffness # 1  and stiffness # 2 . One of the available stiffness levels should correspond with the desired stiffness identified by the doctor, and the corresponding candidate spacer implant (e.g., candidate implant  40 ) is chosen. Then, the first implant  20  is removed and replaced with the chosen implant  40  during a second surgical procedure. Thus, the chosen implant  40  is used to create a spinal stabilizer  60  between the spinous processes  12 , 14  that helps space the two spinous processes  12 , 14  apart. It is intended that the spinal stabilizer  60  formed thereby will be in place long-term; as such, the replacement implant may be conceptually thought of as a long-term implant  40 . The long-term implant  40  may, in some embodiments, be secured in place using tethers, as disclosed in U.S. Pat. No. 6,626,944. 
     In another embodiment, various implants  20 , 40 , 50  may be formed of a common receiver body  61  in combination with various modular inserts  72 , 74   a , 74   b . See  FIG. 4 . The receiver body  61  may include a superior saddle  62 , and an inferior saddle  64 . The various inserts  72 , 74   a , 74   b  may be joined to the receiver body  61  and be disposed between the saddles  62 , 64 , such as in a corresponding bay  63  within midsection  66 . Thus, a sensor-equipped test implant  20  may be formed by inserting sensor module  72 , having sensor  30  disposed therein, into receiver body  61 . The same receiver body  61  may then be used to form candidate implant  40  by removing the sensor module  72  and inserting a different module  74   a , or form candidate implant  50  by inserting module  74   b . Advantageously, the modules  74   a , 74   b  have differing material properties, such as different stiffness levels. 
     The discussion above has assumed that the sensor-equipped test implant  20  is replaced by long-term implant  40  or  50  in a second surgical procedure. During the time period between the first and second surgical procedures, the sensor  30  may be used to collect relevant data with the patient in more “real world” circumstances. The interval between surgical procedures may be a few hours to a month or more. However, in some embodiments, the test implant  20  and the long-term implant  40  or  50  may be installed in the same surgical procedure. For example, the test implant  20  may be installed, the patient may be manipulated by the surgeon during the procedure to allow the sensor  30  to generate the relevant measurement data, long-term implant  40  selected based thereon, and test implant  20  replaced by long-term implant  40 , all within the same surgical procedure. 
     It should be noted that the term “material property” as used herein refers to elastic modulus, flexural modulus, flexural strength, stress-strain curve, Young&#39;s modulus, hardness, compression strength, dampening or viscous properties, and the like, whether of a homogenous material or of a composite, and excludes physical dimensions. 
     Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
     As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
     The present embodiments may, of course, be carried out in other specific ways than those herein set forth without departing from the essential characteristics of the application. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coining within the meaning and equivalency range of the appended claims are intended to be embraced therein.