Patent Publication Number: US-2005143735-A1

Title: Double compression unloadable screw system

Description:
PRIORITY  
      This application claims priority to Provisional Application No. 60/466,498 filed Apr. 29, 2003. 
    
    
     FIELD OF THE INVENTION  
      The invention generally relates to an apparatus and method of bone surgery. In particular, this invention is for uniting bone fragments, such as in a fractured hip.  
     BACKGROUND OF THE INVENTION  
      Hip fractures commonly occur through the neck of the femur and less frequently through the femoral head. Surgical repair of such fractures involves insertion of a screw from the lateral proximal femur, through the neck of the femur and into the femoral head. This results in uniting of the bone fragments under a certain amount of compression and allows for bone healing. These screws commonly have threads at the proximal and distal ends and have a central portion with no threads. The result is that the distal threads engage the most internal bone to be joined and the proximal threads engage the most proximal portion of bone to be joined, pulling together the bone fragments across the fracture line. This results in a certain degree of pressure across the fracture which promotes healing of the bone. However, it is desirable to increase the amount of pressure across the fractured bone fragments to stabilize the bones and improve bone healing. Furthermore, after the patient returns to weight bearing activities, these hip screws are placed under stress. The forces acting upon the hip screws can cause them to fail in a predictable region along their lengths. There is therefore a need to improve hip screws to avoid these predictable failures.  
      One known method of providing increased compression to bone fragments involves the use of a bone screw with threads at the proximal and distal ends and a separate head. After insertion of the screw, the head is threaded onto the proximal end of the screw to provide additional compression to the bone fragments.  
     SUMMARY OF THE INVENTION  
      The present invention provides an apparatus and method for both improving the compression of the bone fragments as well as strengthening of the bone screw. It includes a screw shaft with threads on the proximal and distal ends to unite the bone fragments, a head which threads over the proximal end of the screw shaft and provides increased compression on the bone fragments and a sleeve which slides or threads around the bone screw and provides reinforcing strength. The compression head and the strengthening sleeve may be a single unit or may be two distinct components. Therefore, the invention has the advantage that the combined head and sleeve, or the head and sleeve individually, can be removed in the future after initial insertion of the assembly.  
      The compression head and the sleeve serve to increase the compression pressure on the bone fragments and to decrease the likelihood of screw failure. After the screw shaft is inserted through the bone fragments to unite them under pressure, the reinforcing sleeve (or head and sleeve combination) slides or threads around the proximal end of the inserted screw shaft. It is of a length sufficient to extend across the region of the screw shaft that is under stress and prone to failure. The sleeve thus provides reinforcing strength to this area of the bone screw to decrease the likelihood of future bone screw breakage. The compression head (if not part of a head and sleeve combination) is then threaded onto the proximal threaded end of the bone screw. In this way, the head (or the head portion of the combined head and sleeve) creates further compression across the fracture line such that there is a double compression of the bone fragments, which is desirable for bone healing.  
      The removable nature of the compression head and reinforcing sleeve can serve various purposes. For example, the sleeve may be made of a material which reveals signs of stress when imaged, such as by an x-ray. When a stressed sleeve is thus detected, it can then be removed and replaced by a new sleeve. The new sleeve then continues to provide reinforcement to the bone screw and further decreases the chance of bone screw failure. Such a sleeve therefore provides protection against bone screw failure in multiple ways. The sleeve provides physical reinforcement to the screw when initially installed. It also prevents bone screw failures by revealing, through imaging or other manner, the need for replacement of the stressed sleeve with a new sleeve.  
      The system can also be used to provide bone dynamization to promote bone healing. After insertion of the bone screw assembly and a period of bone regrowth under static conditions, either the compression head, the sleeve or both may be moved or removed. Removal of these elements decreases the compression on the bone fragments and allows some increased mobility of the bone fragments. This change in conditions may be used to stimulate continued bone re-growth and strengthening after initial re-growth has occurred. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view showing a bone screw assembly with a compression unit and a screw shaft.  
       FIG. 2  is an exploded sectional view showing a bone screw assembly with a compression unit and a screw shaft.  
       FIG. 3  is an exploded perspective view of a bone screw assembly with a compression unit and a screw shaft.  
       FIG. 4  is an exploded sectional view showing a bone screw assembly with a compression head, a sleeve, and a screw shaft.  
       FIG. 5  is an exploded sectional view showing a bone screw assembly with a compression head, a sleeve, and a screw shaft.  
       FIG. 6  is a sectional view showing a fractured femoral neck with the bone screw of  FIG. 1  joining the bone fragments.  
       FIG. 7  is a sectional view showing a fractured femoral neck with the bone screw and compression unit of  FIG. 1  joining the bone fragments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The bone screw assembly of the present invention is shown in  FIGS. 1 through 7 . As shown in  FIGS. 1 through 3 , the bone screw assembly  10  includes a screw shaft  14  having an elongate wall structure and external threads  18 ,  22  at the proximal and distal ends, respectively. The distal end of the screw shaft is shaped to allow it to penetrate the bone. The screw shaft  14  may be partially or fully canulated or may be solid. The proximal screw threads  18  are shaped to engage the internal threads  26  of the compression unit  30 . The distal screw threads  22  are shaped to engage the distal bone fragments and may comprise any of a plurality of functional patterns/shapes.  
      The compression unit  30  is shaped to advance along and around a portion of the screw shaft  14 . The compression unit  30  includes a compression unit head  34  and a compression unit sleeve  38 . The proximal first portion of the compression unit  30  is the compression unit head  34 . The compression unit head  34  includes internal threads  26  for connecting with the proximal threads  18  of the screw shaft  14  and external threads  42  for providing the double compression bone-engagement function. The distal second portion of the compression unit  30  is the compression unit sleeve  38 . The compression unit sleeve  38  may slide along a portion of the screw shaft  14  such that it is in contact with the screw shaft  14  or there may be a gap between the screw shaft  14  and the compression unit sleeve  38 . The central portion of the screw shaft  14  may be without external threads, may have external threads running its entire length or may have one or more segments of external threads.  
      Alternatively, as shown in  FIGS. 4 and 5 , the compression head  46  and sleeve  50  may be separate components of the bone screw assembly, as shown in  FIGS. 4 and 5 , rather than combined as a single unit. In this alternative, the compression head  46  contains internal threads  26  and external threads  42 . The sleeve  50  contains internal threads  54  and may or may not contain external threads  58 . When the sleeve has external threads  58 , as shown in  FIG. 4 , these threads are shaped to connect with the internal threads  26  of the compression head  42 , which advances around the sleeve  50 . The internal threads  58  of the sleeve  50  are shaped to connect with the proximal threads  18  of the screw shaft  14 . When the sleeve  50  is without external threads, as shown in  FIG. 5 , the sleeve  50  advances along the screw shaft  14  with its internal threads  54  engaging the proximal threads  18  of the screw shaft  14 . The compression head  46  then follows, after the sleeve  50 , advancing around the proximal end of the screw shaft  14  in a location proximal to the sleeve  50  such that the internal threads  26  of the compression head  46  engage with the proximal threads  18  of the screw shaft  14 .  
      As shown in  FIG. 6 , the screw shaft  14  is inserted through the neck  62  of the femur and into the head  66  of the femur. After insertion of the screw shaft  14 , the compression unit  30 , as shown in  FIG. 7 , or the separate sleeve  50  and compression head  46 , are advanced along and around the screw shaft  14  to provide double compression of the bone fragments and strengthening of the screw shaft  14 . Such strengthening may be quite important in view of the stresses on the screw shaft generally at between about 20%-35% of the length of the screw shaft as measured from the proximal end  73 .  
      The proximal ends of both the screw shaft  14  and the compression unit  30  may be shaped to allow selective engagement with one or more insertion devices or drivers, as shown in  FIGS. 2 and 3 . For example, these components may have insets  70 , such as hexagonal insets, or other features to allow an insertion device with a hexagonally shaped tip or other type of insertion device to drive the elements into the bone.  
      In order to prevent the compression unit  30 , the sleeve  50  or the compression head  46  from advancing too far distally on the screw shaft, the invention may include structure for stopping advancement of these elements at a desired point. For example, the proximal end of the compression unit  30  may include a rim which would abut against the proximal end of the screw shaft after the compression unit is completely advanced onto the screw shaft. Alternatively, the proximal threads  18  of the screw shaft  14  could include a closed end to the threading such that the advancing internal threads  26  of the compression unit would be stopped at that point during the threading process. Another alternative would be an external projection on the screw shaft such as a nub or a ring around the screw shaft which would block the distal advancement of the compression unit  30  or the sleeve  50  at a predetermined point.  
      The invention may also include the use of a coating or coatings between the screw shaft and the compression unit sleeve. This coating could be supplied on the screw shaft  14  or in the lumen of the sleeve  50  or of the compression unit sleeve  38 . The coating could serve to facilitate sliding of the sleeve  50 , to prevent bony in-growth between the sleeve  50  and the screw shaft  14 , to do both, or to serve any other biomedical device-related function, according to need. In addition, the screw shaft  14  could include apertures through which material could be delivered to the surrounding tissue or which could be used to allow tissue in-growth.  
      The compression unit  30 , the compression head  46  and the sleeve  50  have the advantageous option of being removable and replaceable. For example, they could be removably connected to the screw shaft at the time of insertion of the bone screw assembly. Later, they might be removed from the screw shaft in order to facilitate dynamization. In addition, the sleeve  50  or the compression unit sleeve  38  could be made of a material that indicates stress points when imaged radiologically or with other medical imaging techniques, including, for example, X-ray, ultrasound, or others. When imaging reveals stress in the reinforcing sleeve, the compression unit  30  or the sleeve  50  could be adjusted, removed or replaced. However, when it is not desirable for the compression unit  30  or the sleeve  50  and head  46  to be removable and replaceable, they could be permanently connected to the screw shaft  14  after installation. It is further recognized that the pitch, height and other features of the threads referred to herein may be modified to achieve single compression ratios for low values, double compression ratios for high value, or may comprise more standard or universal ratios. The sleeves  50  are a unique combination when used for strengthening, imaging for stress, or for implementing the guiding force to prevent retrograde and lateral motion of the screw shaft or attached components after implant. For example, in one embodiment, initial compression is achieved by placing at least one screw shaft into a patient across a fracture site. This achieves initial compression which is then further assisted by use of the compression unit and sleeve. The bones then stick together and commence the healing process. However, it is often essential to subsequently release the compression forces of the compression unit and allow the natural muscle force of the patient to further accelerate healing at the site. During this process the sleeve maintains natural compression and guiding alignment of the screw shaft to prevent undesired migration. This is particularly advantageous when more than one screw is inserted, which is common, to prevent interference or disturbance by one screw against another. The invention enables single then double compression, followed by aligned natural fracture impaction, while serving as an anchor or backstop to prevent retrograde motion of the screw shaft. The invention may utilize or enable various combinations of the above features and results, and may include components having bio-resorbable characteristics.