Patent Publication Number: US-2005124989-A1

Title: Mechanical bone tamping device for repair of osteoporotic bone fractures

Description:
This application is a continuation of Ser. No. 10/230,256, filed Aug. 29, 2002. 
    
    
     BACKGROUND OF THE INVENTION  
      Pathologic fracture of the spinal vertebral body is very common. Bones weakened by osteoporosis or by malignant processes account for a large proportion of vertebral fractures. Most such fractures occur as a result of trivial trauma and are due to the weakened architecture of the bone through loss of bone calcium and associated alteration of bony trabecular support or through frank replacement of bony tissue by malignant cells.  
      The injection of bone cement into the vertebral body to strengthen or stabilize it is a well recognized process that provides immediate stability to the weakened or compressed vertebral body that has been altered by disease. Present systems designed to inject bone cement into the vertebra weakened by disease (malignant or benign) generally utilize two types of processes. The first process involves simply injecting liquid bone cement into the interstices of the bone under pressure. The problem with this process is that it requires the bone cement to be in a relatively liquid state to allow it to fill the interstices of the bone. Because venous channels within the bone communicate with epidural veins in the spinal canal and with veins in the general vasculature, numerous complications have arisen from this injection process whereby bone cement has inadvertently entered the spinal canal causing paralysis from compressing the spinal cord or, alternately, cement has entered the general venous system, causing death by pulmonary embolism. Obviously, these consequences of injecting bone cement under pressure into the interstices or trabeculae of vertebral bodies are unacceptable.  
      A second and safer method has been developed to strengthen osteoporotic or malignant vertebral fractures. This involves placing a balloon into the intervertebral body and inflating it so that a cavity is formed in the weakened bone. This cavity can then be filled with a more viscous form of bone cement, thereby reducing the risk of embolism to the spinal canal or lungs as is seen with high pressure less viscous injection. The problem with this technique is that the balloons used to create the cavity within the bone frequently break when spicules of bone puncture them, or, because they expand along the path of least resistance, an aberrant or asymmetrical cavity is formed which inhibits or compromises the ideal placement of the cement support for stabilization of the weakened vertebrae. A more desirable system is required to allow placement of bone cement in the exact position required by the treating surgeon and in a manner that acceptably lessens the risk of bone cement migration or embolization.  
     SUMMARY OF THE INVENTION  
      The system described herein is a simple mechanical mechanism whereby a cavity can be created in any desirable location within the vertebral body to allow the instillation of bone cement in a viscous configuration thereby minimizing the risk of malplacement of the bone cement or embolization of bone cement through the trabecular channels as may happen when less viscous bone cement is administered to strengthen pathologic cancellous bone.  
      To achieve this greater safety and efficacy, a mechanical device for creating a cavity within the soft cancellous bone is used. This form of cavity creation is much more controllable than with balloon inflation insofar as it does not depend on the elastic properties of a balloon wall expanding along the path of least resistance to create a cavity, whereas the dimensions of a balloon-created cavity are largely beyond the surgeon&#39;s control and more or less dependent upon the extent of disruption of the architecture of the pathologic bone.  
      According to this invention, a cavity is formed by compressing cancellous trabeculae outward, much as one might form a cavity in moist snow by inserting a hand, fingers extended, and then closing it to form a fist. To produce the cavity by purely mechanical action, a screw jack or other expanding mechanism is employed to compress or tamp the surrounding weakened cancellous bone. The mechanism, when operated, forces the arms apart, thereby directly compressing or tamping the cancellous bone.  
      By employing a screw jack mechanism to form the cavity, the exact dimensions of the cavity as well as the placement of the cavity can be controlled by the treating surgeon. Passive placement of liquid bone cement by injection under pressure is not required and the highly inaccurate and uncontrollable cavity formation afforded by balloon insufflation is avoided. The screw jack mechanism affords a more direct, exquisitely controllable and safer means by which cavities can be formed for bone cement stabilization of vertebrae weakened or fractured by benign or malignant disease states. Although a screw jack mechanism is envisioned in the preferred embodiment, it is recognized that other mechanisms such as levers could be substituted to achieve the same result, i.e., mechanical compression of cancellous bone to formulate a cavity within the confines of the vertebral body.  
      The important point of this invention is that the expanding device is purely mechanical, as opposed to balloon-type devices which have both mechanical and pneumatic aspects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the accompanying drawings,  
       FIG. 1   a  is an axial view of vertebral body with stylet inserted via posterolateral approach;  
       FIG. 1   b  shows a cannula sleeve inserted over the stylet;  
       FIG. 1   c  illustrates the working cannula in position with the stylet removed;  
       FIG. 1   d  demonstrates the screw jack being placed into the vertebral body via the working cannula;  
       FIG. 1   e  shows the screw jack in an open configuration thereby compressing bone adjacent to the expandable arms;  
       FIG. 1   f  shows the cavity formed after the screw jack has been repeatedly expanded and contracted at the 15 degree intervals;  
       FIG. 1   g  demonstrates the cavity being filled with cement after the screw jack is removed;  
       FIG. 1   h  depicts the bone cement in situ after the working cannula is removed;  
       FIG. 2   a - 2   h  are lateral views corresponding to  FIGS. 1   a - 1   b ; and  
       FIGS. 3   a  and  3   b , and  FIGS. 4   a  and  4   b  show modifications of the tamping instrument using a lever mechanism.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      A screw jack tamp or lever arm bone compression mechanism designed to create a cavity within the bony contents of a vertebral body to allow or facilitate the stabilization of said vertebral body by instillation of bone cement or other stabilizing material (biological or inert) to repair, splint or otherwise stabilize bone structures weakened by benign or malignant processes (osteoporosis or malignant infiltration).  
      The screw jack tamp or lever arm bone compression instrument includes a shaft having a handle at one end to allow mechanical rotation of the shaft and a radially expandable structure at the other end having two, three, four or more hinged arms connected at their midpoint by a pivot and at their ends by a pair of collars separated along the length of a screw thread of the shaft. The arrangement being such that rotation of the shaft causes changes in the spacing between the collars along the threaded portion of the shaft such that the collars are approximated when the shaft is rotated in a clockwise fashion. Approximation of the collars in turn forces the pivot arms outward thereby compressing the surrounding soft cancellous bone. The arms are contracted by rotating the shaft handle counter clockwise and then re-expanded after the entire assembly is rotated such that a radially circular cavity is eventually formed.  
      Alternately, the collars could be forced together by a lever arm mechanism and achieve the same effect as a screw jack mechanism.  
      In operation, a blind hole is formed in the vertebra by inserting a stylet ( FIG. 1   a ). A cannula sleeve is then inserted over the stylet ( FIG. 1   b , and the stylet is removed ( FIG. 1   c ). Now the surgeon inserts the screw jack described above ( FIG. 1   d ), and then turns its handle (not shown) clockwise to expand the arms ( FIG. 1   e ), enlarging the cavity in the plane of the arms. The arms are then retracted, and the screw jack is turned somewhat (e.g., 15°-45°—the exact angular interval required will depend on the desired size of the cavity and the width of the arms) and then the arms are expanded again. This cycle repeated as many times necessary to cover 360° and produce a cavity which is substantially round in cross-section ( FIG. 1   f ). Bone cement in a more or less viscous state is now injected along the cannula to fill the cavity ( FIG. 1   g ). The cement is allowed to harden in the cavity to stabilize the weakened or fractured osteoporotic bone. Finally, the cannula is withdrawn and the hole closed ( FIG. 1   h ).  
       FIGS. 3   a  and  3   b  show a form of the invention in which the arms are expanded not by a screw jack, but rather by a lever-based tool, in which squeezing the handles together shortens the distance between the collars, thus expanding the arms. The effect and method of operation is the same, although the mechanical advantage may not be as great.  
      Since the invention is subject to modifications and variations, it is intended that the foregoing description and the accompanying drawings shall be interpreted as only illustrative of the invention defined by the following claims.