Abstract:
The present invention is directed to a device for fixation of a bone fracture. The device comprises at least one distal pin having a mounting end for insertion on a distal side of the fracture, at least one proximal pin having a mounting end for insertion on a proximal side of the fracture, a distal member attached to the distal pins, a proximal member attached to the proximal pins and slideably connected to the distal member for reducing or distracting the bone fracture, a distraction assembly for controlling the movement between the proximal and distal members, and a removable engagement element. The distraction assembly has two modes of operation: an active configuration and an inactive configuration. In the active configuration, incremental relative sliding movement between the proximal and distal members is allowed, and, in the inactive configuration, free relative sliding movement between the proximal and distal members is allowed. The engagement element operatively engages the distraction assembly in the inactive configuration. Removal of the engagement element places the distraction assembly in the active configuration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for fixation of bone, and in particular to an external fixation device for a long bone. 
     BACKGROUND OF THE INVENTION 
     The clinical success of external fixation of bones has been well documented in the orthopaedic literature. However, external fixation at or near a joint can be problematic due to biomechanical requirements. For example, because the wrist has degrees of freedom which allow rotation, flexion, adduction, and abduction, external fixation near the wrist should be adjustable to ensure that proper fracture alignment is achieved. Furthermore, in order to avoid damage to the tendons and nerves that surround the wrist, the pins which are used to couple the fixation device to the body are usually placed on opposite sides of the wrist in the metacarpal bone and the radius. As a result, the fixation device must be sufficiently articulated to reduce the fracture using the forces transmitted through the wrist. 
     In order to address these, as well as other complications associated with fixation at or near a joint, a number of fixation devices have been developed. Examples include those disclosed in U.S. Pat. Nos. 4,554,915, 4,611,586, 4,628,919, 4,782,842, 4,919,119, 4,922,896, Re. 34,985, 5,122,140, 5,152,280, 5,304,177, 5,320,622, 5,437,667, 5,545,162, 5,601,551, 5,683,389, and 5,743,898. One particular problem common to these, as well as other prior art devices, is the need to have both large scale distraction to facilitate implantation and precisely controlled distraction to ensure proper fracture reduction. Some of these patents attempt to overcome this dilemma by requiring assembly and/or disassembly of multiple parts. U.S. Pat. No. 5,662,649 to Huebner discloses an external fixator for repairing fractures of the distal radius and wrist which allows both rapid gross distraction and finely controlled distraction without the need for intraoperative assembly or disassembly. However, the nut and thread assembly which permits both types of movement can be cumbersome to use and unintentional switching between the movement modes is possible. 
     As the discussion above illustrates, there is a need for an improved external fixation device for bone. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a device for fixation of a bone fracture. The device comprises at least one distal pin having a mounting end for insertion into the bone on a distal side of the fracture, at least one proximal pin having a mounting end for insertion into the bone on a proximal side of the fracture, a distal member attached to the distal pins, a proximal member attached to the proximal pins and slideably connected to the distal member for reducing or distracting the bone fracture, and a distraction assembly for controlling the sliding movement between the proximal and distal members. The distraction assembly has two modes of operation: an active configuration and an inactive configuration. In the active configuration, incremental relative sliding movement between the proximal and distal members is allowed, and, in the inactive configuration, free relative sliding movement between the proximal and distal members is allowed. The free sliding movement is useful in installation of the device and the incremental relative sliding movement is useful for finely manipulating the relative positions of the proximal and distal members. 
     The device also includes a removable engagement element operatively engaging the distraction assembly in the inactive configuration. Removal of the engagement element places the distraction assembly in the active configuration. Once the engagement element is removed, it cannot easily be replaced. As a result, the distraction assembly cannot inadvertently be placed in the inactive configuration. 
     Preferably, the distraction assembly comprises a gear rack on a wall of the proximal member body cavity, an intermediate gear engageable with the gear rack to cause the incremental relative sliding movement between the proximal and distal members upon rotation of the intermediate gear, a worm gear engageable with the intermediate gear so that rotation of the worm gear causes rotation of the intermediate gear, and a resilient member which biases the worm gear into engagement with the intermediate gear. The removable engagement element may be a pin located between the intermediate gear and the worm gear to prevent engagement between the intermediate gear and the worm gear. 
     In one embodiment, the device also includes a distal mounting blocks for attaching the distal pins to the distal member and a proximal mounting block for attaching the proximal pins to the proximal member. Each pin, which may be Schanz screws or any other suitable fastener, fits into a channel in the proximal and distal mounting blocks and a locking screw secures the respective pin to the channel. In order to avoid interference with anatomical structures, the channel may be oriented at an angle of about 45° with respect to the longitudinal axis of the device. 
     In a preferred embodiment, at least one of the distal and proximal mounting blocks includes a ball joint for allowing rotational movement of the mounting blocks with respect to the rest of the device. The distal and proximal mounting blocks may include a plurality of set screws for securing the mounting blocks at a desired position with respect to the proximal and distal members. In a further preferred embodiment, two set screws oriented perpendicular to each other are used for each ball joint. 
     In a different embodiment, the distal mounting block has an arm for connection with the distal member and the distal member has a head with a track for receiving the arm. The arm is movable in the track to vary the position of the distal mounting block with respect to the distal member. The track may be curved with teeth which engage a gear on the arm upon rotation of the gear to move the distal mounting block. Preferably, a locking element is provided to prevent rotation of the gear and fix the position of the distal mounting block. 
     In order to have the sliding movement between the proximal and distal members, the proximal member may include a body and a linking section. The body has a cavity for slideably receiving the distal member. In addition, the linking section may be slideably connected to the body so that sliding motion between the linking section and the body occurs in a direction perpendicular to the direction of the sliding motion between the proximal and distal members. The sliding motion between the linking section and the body is coplanar to the sliding motion between the proximal and distal members. In a preferred embodiment, a distal end of the linking section includes a pair of slots and the proximal end of the body includes a cutout. The cutout slides in the slots to produce the sliding motion between the linking section and the body. In a further preferred embodiment, a lead screw connects the linking section to the body. Turning of the lead screw moves the linking section relative to the body. 
     In another embodiment, the proximal member has a tail pivotably connected to the linking section. The tail has a worm gear and the linking section has a curved gear rack. The worm gear engaging the gear rack to produce the pivotal movement between the tail and the linking section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
     FIG. 1 shows a perspective view of an external fixation device according to the present invention from the left side; 
     FIG. 2 shows a perspective view of the device from the right side; 
     FIG. 3 shows an exploded view of the device with scales added to provide indicia as to relative movement between two components; 
     FIG. 4 shows a top view of the device; 
     FIG. 5 is a sectional view taken along line A—A of FIG. 4 showing the ball joint of the distal mounting block; 
     FIG. 6 is a sectional view taken along line B—B of FIG. 4 showing the entire fixator; 
     FIG. 7 is a sectional view taken along line C—C of FIG. 4 showing a portion of the distraction assembly; and 
     FIG. 8 shows a side view of one embodiment of a pin for coupling the device to bone. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For convenience, the same or equivalent elements in the various embodiments of the invention illustrated in the drawings have been identified with the same reference numerals. Further, in the description that follows, any reference to either orientation or direction is intended primarily for the convenience of description and is not intended in any way to limit the scope of the present invention thereto. Finally, any reference to a particular biological application, such as fixation at or near the wrist, is simply used for convenience as one example of a possible use for the invention and is not intended to limit the scope of the present invention thereto. 
     Referring to FIGS. 1-4, an external fixation device or fixator  10  according to the present invention includes a distal member  12 , a proximal member  14  slideably connected to distal member  12 , and a distraction assembly  16  for controlling the sliding movement between proximal and distal members  12 ,  14 , and a removable engagement element  18 . Thus, as used in this application, the term distal designates the end or direction near distal member  12  of fixator  10 , and the term proximal designates the end or direction near proximal member  14  of fixator  10 . Most components of fixator  10  are made of radiolucent plastic or composite materials to minimize the radiographic interference of fixator  10 . As will be described in more detail later, distal member  12  is joined to the bone on one side of the bone fracture and proximal member  14  is joined to the bone of the other side of the bone fracture. Thus, the sliding movement between distal and proximal members  12 ,  14  results in reduction or distraction of the bone fracture. As will also be described in more detail later, distraction assembly  16  has an active configuration in which only incremental relative sliding movement between distal and proximal members  12 ,  14  is possible and an inactive configuration in which free relative sliding motion between distal and proximal members  12 ,  14  is possible. The incremental sliding motion is needed to finely control distraction and reduction and the free sliding motion is useful to facilitate installation and preoperative setup of fixator  10 . As will also be described in more detail below, engagement element  18  operatively engages distraction assembly  16  in the inactive configuration so that free sliding motion between distal and proximal members  12 ,  14  is ordinarily possible. However, upon removal of engagement element  18 , distraction assembly is placed in the active configuration to limit the sliding motion between distal and proximal members  12 ,  14  to incremental sliding motion. 
     FIG. 8 shows one embodiment of a pin  20  for coupling fixator  10  to bone. Although pin  20  is shown and described as a fastener commonly referred to as a Schanz screw, any number of known fastening devices can be used to secure fixator  10  to bone. Pin  20  has a tapered mounting end  22  with threads  24  for engaging the bone and a shaft  26 . At least one pin  20  is used to couple distal member  12  to the distal side of the bone fracture and at least one pin  20  is used to couple proximal member  14  to the proximal side of the bone fracture. Preferably, two pins  20  are used for distal member  12  and two pins  20  are used for proximal member  14 . Distal pins  20  are connected to distal member  12  by distal mounting block  28  and proximal pins  20  are connected to proximal member  14  by proximal mounting block  30 . Distal and proximal mounting blocks  28 ,  30  include a plurality of channels  32  for pins  20 . Channels  32  are preferably oriented obliquely at an angle of about 45° with respect to the longitudinal axis of fixator  10  to avoid interference with anatomical structures and provide for improved x-ray visibility in lateral views. This orientation is particularly useful if fixator  10  is used near the wrist to minimize hindrance with the thumb. A locking screw  34  locks pin  20  to channel  32 . 
     As best seen in FIGS. 4 and 5, distal and proximal mounting blocks  28 ,  30  include ball joints  36 ,  38  respectively. As ball joints  36 ,  38  provide one rotational degree of freedom and two pivotal degrees of freedom, a wide range of articulations is possible with respect to the orientation of distal and proximal mounting blocks  28  to distal and proximal members  12 ,  14 . This facilitates application of fixator  10 . Specifically, a template is typically used to insert the pins on both sides of the fracture, i.e. the proximal and distal pins and then the external fixation device is linked to the pins. In prior art devices, the placement of the pins was crucial to ensure that the external fixation device was properly aligned with the pins. With fixator  10 , the placement of pins  20  is not as critical because of the wide range of motion provided by ball joints  36 ,  38 . 
     It should be noted that ball joints are optional and, if used, can be used on either one or both of distal and proximal mounting blocks  28 ,  30 . However, for simplicity, this description will assume that both ball joints  36 ,  38  are used. Once the desired orientation of distal mounting block  28  with respect to distal member  12  has been achieved, a set screw  39  locks the two at the desired orientation. Although only one set screw  39  is needed to fix ball joint  36 , preferably two set screws  39  are present. In a further preferred embodiment, these two set screws are perpendicular to each other so that if one set screw is loosened, some movement of ball joint  36  is possible without permitting totally free movement. This is particularly useful in fine tuning the orientation of mounting blocks  28 ,  30  with respect to members  12 ,  14 . 
     FIGS. 1,  3 , and  4  show that a distal arm  40  is located medial to distal ball joint  36  for connecting distal mounting block  28  to distal member  12 . Similarly, a proximal arm  42  is located medial to proximal ball joint  38  for connecting proximal mounting block  30  to proximal member  14 . Proximal arm  42  fits into a hole  44  on proximal member  14  and a nut member  46  secures arm  42  to proximal member  14 . Distal arm  40  fits into an elongated hole or track  48  on distal member  12  and a nut member  50  secures arm  40  to distal member  12 . Because nut members  46 ,  50  can be loosened and fixator  10  has substantial lateral symmetry, distal and proximal mounting block  28 ,  30  can be placed on either side of fixator  10 . As a result, fixator  10  can be used on the left or the right side of the body and placed on the lateral or medial side of the bone as required by a particular clinical application. 
     Elongated hole  48  is located in a head portion  52  of distal member  12 . When distal nut member  50  is not tightened, distal arm can move along elongated hole  48  to vary the position of distal mounting block  28  with respect to distal member  12 . Preferably, elongated hole  48  has a curved arc shape. This is particularly useful if fixator  10  is used near the wrist so that flexion and extension movement of the hand is permitted. In order to control the flexion and extension movement, elongated hole  48  has teeth  54  that cooperate with an arm gear  56  on distal arm  40  (FIG.  6 ). By turning a first adjustment screw  58 , gear  56  turns to move along teeth  54 . Distal nut member  50  functions as a locking element by preventing rotation of gear  56  and thereby fixing the position of distal mounting block  28 . A scale  59  provides indicia as to the relative movement between distal mounting block  28  and distal member  12  (FIGS.  3  and  4 ). In order to minimize gear backlash, gear  56  can be made of metal and teeth  54  can be made of plastic. 
     Referring primarily to FIGS. 3,  4 , and  6 , proximal member  14  includes a body  60 , a linking section  62 , and a tail  64 . Body  60  has a cavity  66  for slideably receiving distal member  12 . Linking section  62  is slideably connected to body  60  in such a fashion that the sliding motion between linking section  62  and body  60  occurs in a direction perpendicular to the direction of the sliding motion between distal and proximal members  12 ,  14 . As these two sliding motions are also coplanar, if fixator  10  is used in the area near the wrist, the sliding motion between linking section  62  and body  60  is in the medial-lateral anatomical direction. A distal end of linking section  62  includes a pair of slots  68  slots and a proximal end of body  60  includes a cutout  70  which is slideable in slots  68  to thereby produce the sliding motion between body  60  and linking section  62 . A lead screw or second adjustment screw  72  connects linking section  62  to body  60  so that turning of lead screw  72  moves linking section  62  relative to body  60 . A washer  74  limits the total amount that lead screw  72  can be rotated to prevent movement of cutout  70  out of slots  68 . 
     Tail  64  is pivotably connected to linking section  62 . Thus, if fixator  10  is used near the wrist, palmar and dorsal movement of the hand is permitted. A worm gear or third adjustment screw  76  on tail  64  cooperates with a curved gear rack  77  on linking section  62  to control the movement between linking section  62  and tail  64 . By turning worm gear  76 , the position of linking section  62  relative to tail  64  can be adjusted. A scale  59  provides indicia as to the relative movement between tail  64  and linking section  62  (FIGS.  3  and  4 ). In order to minimize gear backlash, worm gear  76  can be made of metal and gear rack  77  can be made of plastic. 
     As previously discussed, distraction assembly  16  controls the sliding motion between distal and proximal members  12 ,  14 . The sliding motion results in a reduction or increase in distance between distal member  12  and proximal member  14 . Referring primarily to FIGS. 3,  6 , and  7 , distraction assembly  16  includes a gear rack  78  located on a wall of cavity  66 . An intermediate gear  79  is engageable with gear rack  78 . A scale  59  provides indicia as to the relative movement between distal and proximal members  12 ,  14  (FIGS.  3  and  4 ). In order to minimize gear backlash, intermediate gear  79  can be made of metal and gear rack  78  can be made of plastic. As intermediate gear  79  is connected to distal member  12  via distraction assembly side members  80 , movement of intermediate gear  79  along gear rack  78  causes incremental relative sliding movement between distal and proximal members  12 ,  14 . When only incremental motion between distal and proximal members  12 ,  14  is possible, distraction assembly  16  is in the active configuration. A worm gear or fourth adjustment screw  82  is engageable with intermediate gear  79  so that rotation of worm gear  82  causes rotation of intermediate gear  79 . But for the presence of engagement element  18  located between intermediate and worm gears  79 ,  82 , a resilient member  84 , such as a coil spring, would bias worm gear  82  into engagement with intermediate gear  79 . With engagement element  18  located between intermediate and worm gears  79 ,  82 , distraction assembly  16  is in the inactive configuration and distal and proximal members  12 ,  14  can freely slide relative to one another. In addition to serving as part of the coupling mechanism between distal and proximal members  12 ,  14  and keeping the various elements of distraction assembly  16  in proper relationship to each other as distraction assembly  16  moves, interference between side members  80  and the ends of window  86  on body  60  limits the total possible movement of distal member  12  with respect to proximal member  14 . 
     In order to cause engagement between intermediate and worm gears  79 ,  82  and thereby place distraction assembly  16  in the active configuration from the inactive configuration, i.e. change the sliding movement between distal and proximal members  12 ,  14  from gear-independent free sliding movement to gear-driven incremental sliding movement, engagement element  18  is simply removed to allow resilient member  84  to push the teeth of intermediate gear  79  into engagement with the teeth of gear rack  78  and worm gear  82 . Preferably, engagement element  18  is a pin with a head having knurling for ease of handling. Once engagement element  18  is removed, it cannot be easily put back into place because of interference by resilient member  84 . Thus, distraction assembly  16  cannot inadvertently be placed back into the inactive configuration once engagement element  18  is removed. In order to replace engagement element  18 , a specially designed tool is needed to counter act the biasing force of resilient member  84  and disengage intermediate gear  79  from gear rack  78  and worm gear  82 . As a result, it is extremely difficult to place distraction assembly into the inactive configuration without this tool. 
     While various descriptions of the present invention are described above, it should be understood that the various features can be used singly or in any combination thereof. Therefore, this invention is not to be limited to only the specifically preferred embodiments depicted herein. 
     Further, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains. Accordingly, all expedient modifications readily attainable by one versed in the art from the disclosure set forth herein that are within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is accordingly defined as set forth in the appended claims.