Patent Publication Number: US-2009228049-A1

Title: Connecting Cannulated Bone Screws

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This method of bone fracture fixation is of relevance to the field of orthopaedic surgery. 
     2. Description of the Prior Art 
     There are several ways to achieve bone fracture fixation in orthopaedic surgery. The simplest is the use of a lag screw. It consists of a head and a shaft which is partially threaded only distally while the proximal portion of the shaft immediately adjacent to the head is smooth. It works by engaging the threaded portion of the shaft into the bone at the far side of the fracture and compressing this against the bone at the near side of the fracture using the head of the screw. However, there are some limitations to this technique. One limitation is that the lag screw cannot cross more than one fracture line and thus is ineffective against fractures with fracture lines that run roughly parallel. And another limitation is that the lag screw is dependent on the quality of screw thread purchase on the bone at the far side of the fracture. Poor bone quality means poor fixation. Thus in these two instances, a more invasive and time-consuming option such as plate fixation is used. 
     Through the current invention, the above problems with the lag screws are circumvented. The current invention would allow fixation through multiple fracture lines that are roughly parallel and is not dependent on the quality of screw purchase into the bone. Rather, the screw thread purchase involving the internal threads of a larger-diameter cannulated screw and the external threads of a smaller-diameter cannulated screw, which are in effect screwed into each other, will provide a stronger construct of bone fracture fixation than the lag screw and the ability to traverse multiple fracture lines. 
     BRIEF SUMMARY OF THE INVENTION 
     This method of bone fracture fixation seeks to provide a rigid construct which is effective and offers a means to circumvent the limitations of the commonly used lag screw by utilizing engagement of internal threads on an implantable object associated with the bone at one side of the fracture with the external threads on the shaft of another implantable object associated with the bone at the opposite side of the fracture. The preferred embodiment comprises two cannulated screws of different diameters which associate with the bones at the opposite sides of the fracture via their screw heads contacting the bone surfaces, and the screws are screwed into the bones manually or drilled towards each other and engage by means of internal threads of the larger-diameter cannulated screw and the external threads of the smaller-diameter cannulated screw. The engagement of the two cannulated screws is aided by a guiding mechanism involving a guide pin which guides the trajectory of both screws into the bones and toward each other so that their threads may engage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross sectional view of the larger-diameter cannulated screw 
         FIG. 2  is a cross sectional view of the smaller-diameter cannulated screw 
         FIG. 3  is a view of the guide pin 
         FIG. 4  is a cross sectional view of the engagement of the cannulated screws across the fracture site 
         FIG. 5  is an exterior view of the engagement of the cannulated screws 
         FIG. 6  is a cross sectional view the larger-diameter cannulated screw and the smaller-diameter cannulated screw using an alternative embodiment 
         FIG. 7  is a view of the guiding mechanism to be used with an alternative embodiment 
         FIG. 8  is a cross sectional view of the engagement of the cannulated screws across the fracture site using an alternative embodiment 
         FIG. 9  is a view of disrupted ankle syndesmosis 
         FIG. 10  is a view of disrupted ankle syndesmosis fixed with current invention 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross-sectional view of the larger-diameter cannulated screw with internal threads  4  demonstrating head  1 , centrally located proximal bore  2 , and the larger distal bore  3  which is in continuity with the proximal bore  2 . The larger-diameter cannulated screw may possess external threads  5  along the entire length of the screw shaft as demonstrated in  FIG. 1  or partially along the length of the screw shaft. 
       FIG. 2  is a cross-sectional view of the smaller-diameter cannulated screw with external threads  7  demonstrating head  6  and a centrally located bore  8 . The smaller-diameter cannulated screw is sized so that the external threads  7  thread onto and engage the internal internal threads  4  of the larger-diameter cannulated screw. The diameter of bore  8  of the smaller-diameter cannulated screw is equal to the diameter of proximal bore  2  of the larger cannulated screw. The external threads  7  of the smaller-diameter cannulated screw may be present along the entire length of the shaft as demonstrated in  FIG. 2  or partially along the length of the shaft. 
       FIG. 3  shows the guide pin  9  which guides the trajectory of the larger- and smaller-diameter cannulated screws into the bones and aids in their engagement. The guide pin  9  allows for appropriate fitting into both the proximal bore  2  of the larger-diameter cannulated screw and the bore  8  of the smaller-diameter cannulated screw. The appropriate fit is defined as the guide pin having a diameter slightly less than the diameter of the bores such that the cannulated screws are free to move along the length of the guide pin with minimal toggling around the guide pin. 
       FIG. 4  shows the engagement of the screws aided by the guide pin  9 . 
     An alternative embodiment of bone fracture fixation is demonstrated in  FIGS. 6 ,  7 , and  8 . In this alternative embodiment, the larger-diameter cannulated screw has one central bore  10  and internal threads  4 . The guiding mechanism is demonstrated in  FIG. 7 . The cannulated guide pin  14  has a central bore  15  to allow for a guide pin  16  to fit appropriately so that the guide pin  16  can slide along the length of the cannulated guide pin  14  with minimal toggle.  FIG. 8  demonstrates the engagement of the cannulated screws across the fracture site using the alternative embodiment. 
     EXAMPLE OF USE 
     Please reference  FIG. 4  for this example. The guide pin  9  is drilled into the bone across the fracture site and comes out at the opposite side of the bone. The larger cannulated screw is inserted onto the guide pin  9  and is screwed manually or drilled into the bone until the head  1  contacts the surface of the bone. The appropriate fit of the guide pin  9  within the proximal bore  2  of the larger cannulated screw guides the trajectory of the screw into the bone along the path of the guide pin. Subsequently, the smaller cannulated screw of appropriate length is chosen. Its length must be less than the difference between the width of bone traversed by the guide pin and the length of proximal bore  2  of the larger cannulated screw, but greater than the difference between the width of bone traversed by the guide pin and the length of the larger cannulated screw. The smaller cannulated screw is inserted over the guide pin end which comes out the opposite side of the bone and is screwed manually or drilled into the bone. Its external threads  7  will engage the internal threads  4  of the larger cannulated screw (i.e., the smaller cannulated screw is screwed into the shaft of the larger cannulated screw), and the smaller cannulated screw is inserted until the head  6  contacts the surface of the bone and is tightened. The guide pin  9  is then removed from the bone. 
     In the alternative embodiment, please reference  FIG. 8 . The alternative embodiment facilitates choosing the appropriate length of the smaller-diameter cannulated screw. The cannulated guide pin  14  is drilled into the bone. The larger-diameter cannulated screw is inserted onto the cannulated guide pin and is manually screwed or drilled into the bone following the trajectory of the cannulated guide pin into the bone. The guide pin  16  is inserted into the bore  15  of the cannulated guide pin  16  and is drilled into the bone across the fracture and comes out at the opposite side of the fracture. The smaller-diameter cannulated screw need only be of length greater than the difference between the width of the bone and the length of the larger-diameter cannulated screw. It is inserted onto the guide pin  16  which comes out the other side of the bone and is screwed manually or drilled toward the larger-diameter cannulated screw until the internal threads  4  and external threads  7  start to engage, at which point, the cannulated guide pin  14  can be backed out and removed from the bone. After the head  6  of the smaller-diameter cannulated screw contacts the bone and is tightened, the guide pin  16  is removed. 
     In another alternative embodiment, please reference  FIG. 9  which shows a syndesmotic injury of the ankle in which the distal tibio-fibular joint is disrupted. There is no fracture of a bone per se, but the separation of these two bones can be brought together by the current invention as shown in  FIG. 10 . 
     Thus a method for bone fracture fixation has been shown and described above. It will be apparent that many changes, modifications, variations, and other uses and applications are possible and contemplated, and all such changes, modifications, variations, and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention as is described in the Claims section.