Abstract:
A ball joint for use with an external fixator has a sleeve in engagement with a rod of a strut for the external fixator. A ring insert and a blocking element are provided, having a combined circumferential surface being able to be introduced into a hole of a ring of an external fixator. The ring insert and blocking element have complementary wedge surfaces allowing a relative radial displacement therebetween to enlarge the diameter of the combined circumferential surface for blocking the ball joint inside the hole. Additionally the sleeve comprises a spherical hollow ball portion with concentrical inner and outer spherical portions, wherein the outer spherical portion engages a complementary spherical socket of the blocking element and wherein the inner spherical portion is engaged by a ball nut. A connection element extends through the sleeve, blocking element and ring insert, wherein a radial displacement and axial displacement of the ring insert and blocking element are activated by the connection element.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a ball joint for a rod, especially a telescopic rod for an external fixator, especially for use with an external ring fixator. It is further related to a strut comprising a rod and two ball or universal joints at the free ends of the rod. 
     A plurality of compression-distraction apparatus have been designed and improved by Ilizarov and his group using two external rings to be placed around the limb to be fixed. There are usually at least two such rings, one proximal and one distal ring, which are connected with a plurality of struts or rods. Preferably, these struts are linked to the rings in a way that the attachment points can be pivoted and the length of the strut can be varied to enable adjustment of the external fixation rings. 
     Ilizarov has also provided some improvements for said systems. EP 0 377 744 shows a telescopic rod for such an external fixator. U.S. Pat. No. 4,615,338 shows a further device to control the length of such telescopic rods. The rods are extending through the rings and are attached to said rings via nuts. The use of said devices is inconvenient since the two rings, between which the struts are to be arranged, need to be in nearly perfect orientation to fit the straight rods through the ring holes of said rings. 
     A different external ring fixator having telescopic rods is shown in U.S. Pat. No. 5,702,389. There the connection between the rods and the two related rings use a different structure. That patent discloses spheres incorporated into the ring system, being half-spheres attaching two rods to such a hole. Further, in order to achieve adjustability, one part of the connector is introduced into a hole of the fixator ring and consists of a flexible material which then attaches to the rod part of the strut. This allows for pivotal connection of said rod/strut to one hole of the ring of the fixator. 
     SUMMARY OF THE INVENTION 
     However, there is still a need for improved universal joints for that use, i.e. joints being able to allow moving attached struts in (almost) every direction, which is necessary, when the telescopic struts are shortened or lengthened or—being connected to such a manipulation—if the two rings are positioned in a different direction and orientation in space. It is one aspect of the invention to improve the ease of using such rods with a ring fixator, especially to fit the struts into the holes if they are at least in a certain area, not necessarily at the ideal place, and that these rings can stay angulated to each other. 
     It is therefore one aspect of the invention to provide a joint for a strut of an external fixator, especially a ball joint for a telescopic rod for a ring fixator, which can be readily and quickly changed in its orientation as well as securely fixed in a determined orientation. 
     It is a further object of the invention to provide a strut comprising such ball joints for allowing an easy connection of two rings of an external fixator. 
     The invention provides inter alia a ball joint for use with an external fixator comprising a sleeve being in engagement with a rod of a strut for said external fixator. A ring insert and a blocking element are provided, having a combined circumferential surface being able to be introduced into a hole of an external fixator element, especially into a hole of a ring of an external fixator. Ring insert and blocking element have complementary wedge surfaces allowing a relative radial displacement of ring insert and blocking element one against the other to enlarge the diameter of the combined circumferential surface for blocking the ball joint inside such a hole. Additionally sleeve comprises a spherical hollow ball portion with concentrical inner and outer spherical portions, wherein the outer spherical portion engages a complementary spherical socket of blocking element and wherein the inner spherical portion is engaged by a ball nut. The ball nut is in an axial displaceable relationship with a connection element extending in and through sleeve, blocking element and ring insert, wherein the radial displacement and axial displacement are activated by the connection element. 
     The ball joint according to the invention combines two functions. A first function is to lock and unlock the rotation of the ball joint. A second function of the joint relates to the fixation of the joint and with the joint the rod and thus one end of the connecting strut within a cylindrical hole of a ring fixator. 
     It is an advantage of the present invention to allow the tightening step of both functions to be executed with one movement. For a complete loosening of the device usually two operations are necessary due to the self locking property of the hole fixation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is now described with reference to the enclosed drawings, showing preferred embodiments of the joint in connection with a telescopic rod: 
         FIG. 1  is a perspective view of a strut comprising two ball joints, according to the invention; 
         FIG. 2  is a perspective exploded view of a joint according to the invention as used in  FIG. 1 ; 
         FIG. 3  is a perspective view of the joint according to  FIG. 2  in an assembled state; 
         FIG. 4  is a cross section of the joint according to  FIG. 2  in its longitudinal direction; 
         FIG. 5  is an enlarged side view of the sleeve of the joint according to  FIG. 2 ; and 
         FIG. 6  is a perspective view of three struts according to  FIG. 1  attached to two external fixator rings. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a perspective view of a strut  10  comprising a rod  11  and two ball joints  12 , according to an embodiment of the invention. The strut  10  comprises usually a plurality of single elements, as can be seen for the ball joint in  FIG. 2 . Preferably, the strut  10  comprises three units, rod  11  and two joints  12 , which are usually connected either with an element of the rod  11  being also part of a corresponding joint  12  or wherein the joint is connected with a screw or bayonet connection with the rod  11 .  FIG. 4  shows an inner rod connection thread  27 . 
       FIG. 2  shows a perspective view of a ball joint  12  according to an embodiment of the invention. As mentioned above the ball joint  12  comprises a sleeve  20  being connected to a rod  11  of the strut  10  in a predetermined way. This can be done in many ways known to the person skilled in the art. Sleeve  20  of  FIG. 2  therefore comprises a cylindrical portion  21  which can be part of the rod  11  or is connected thereto at its free end, where ball nut  30  is shown in  FIG. 2 . Ball nut  30  comprises a polygonal rod or cylinder  32 , i.e. a mathematical so called generalized cylinder having a polygon as basic top and bottom surface, followed by a spherical portion  33 . The basic polygon of the generalized cylinder can be an octagon. Here the basic circle of the cross-section has received four flat surfaces, wherein neighbouring surfaces are in a 90 degree angle one to another, i.e. like a tetragon, with intervening spherical portions. The engagement of ball nut  30  inside sleeve blocking element  40  will be explained in connection with  FIG. 3 . 
     Sleeve  20  comprises at the opposite side a ball like element, which is called ball  22  in the subsequent description. 
     Ball  22 , thus being indirectly fixedly connected to rod  11 , is rotatable within a socket  41  of blocking element  40 . Blocking element  40  comprises a first inclined surface  42  being complementary to a second inclined surface  52  being part of a ring insert  50 . All three elements, sleeve  20 , blocking element  40  as well as ring insert  50  have a central bore, indicated through reference numerals  23 ,  43  and  53 , respectively. Opposite to ball nut  30  a fixation screw  60  is provided, extending through all of the above-mentioned bores  23 ,  43  and  53  and threaded with its outer thread  61  into the inner thread  31  of ball nut  30 , not shown in  FIG. 2  but visible in  FIG. 4 . Furthermore a spring  70  is shown in  FIG. 2  being arranged between two abutment shoulders  64  and  54  of fixation screw  60  and ring insert  50 , respectively. Abutment shoulder  64  is provided by flange  63  which is near the screw head, using a square-head end  62 . 
     Fixation screw  60  can be turned into the ball-nut  30 . This operation tightens the screw  60  and reduces the relative distance of sleeve  20 , blocking element  40  as well as ring insert  50  one to another. This movement presses ball  22  into socket  41  and the ball joint/rotation movement of the joint is locked. In other words: tightening screw  60  blocks the determined orientation of the axis of ring insert  50  in relation to the axis of sleeve  20 . The orientation is shown in  FIG. 2 to 4  to be identical, being the same axis.  FIG. 1  shows different orientation in the context of the use of a strut  10  comprising two of these ball joints  12 . 
     The clamping action is generated due to friction and an additional shape fitting because the ball  22  has hardened ribs  24  that dig into the softer opposite shape of the socket  41 . Ribs  24  describe concentrical rings around the axis of sleeve  20 , wherein the envelope of the ribs in longitudinal direction of the sleeve  20  comprises a portion of a sphere with intervening grooves  25 . Different materials are used for elements  20  and  40 ; especially sleeve  20  is harder than blocking element  40 . This ensures that the hard ribs are digging into the softer socket  41  of the blocking element  40 , providing for a positive fit. It would be possible to separately harden the ball portion  25 . 
     Inside the sleeve  20  is arranged ball nut  30 . The central bore  23  through sleeve  20  has two different portions. The first one is simply big enough to allow introduction of ball nut  30 . The second portion is a spherical portion forming a hollow ball portion  26  which is concentrical to the sphere of the ribs  24 . 
     One portion of the bore inside blocking element  40  is complementary to the polygon cylinder  32  of ball nut  30  to allow accommodating ball nut  30  in a rotation-free manner inside of blocking element  40  in the area of the cylindrical portion  45 , i.e. behind the socket  41 . 
     Thus it is possible to turn screw  60  and block the ball portion  22  of sleeve  20  inside the complementary socket  41  of the blocking element  40 . Since all three clamping elements, the ball nut  30 , the spherical portion  26  of sleeve  20  as well as the spherical socket  41  of blocking element  40  have the same center point, the device can be oriented in every angle, especially up to nearly 20° inclined against the axis of screw  60 , i.e. the main axis of the ball joint  12  being. In a use position, parallel to a hole in the fixator. Usually a range between 0° and 15° is contemplated and can be achieved by form of the spherical portion  22 . 
       FIG. 3  shows a perspective view of the joint  12  according to  FIG. 2  in an assembled state. Similar features of the joint  12  receive the same reference numerals in all drawings.  FIG. 4  shows a cross section of the joint  12  according to  FIG. 2  in its longitudinal direction. 
     The second object of the ball joint  12  is to connect the joint, being connected to the rod  11  of a strut  10 , to a hole of a plurality of holes inside a ring of an external fixator. This contact is realized with the help of shaft or cylindrical portion  55  of the blocking ring insert element  50 . Although it is possible to provide a polygonal, elliptical or another rotation resistant exterior form of the blocking ring insert element  50 , which would have complementary polygonal, elliptical or other rotation resistant interior form of the hole of an external fixator, it is preferred to use a cylindrical blocking ring insert element  50  and a cylindrical hole in the ring of the external fixator. 
     The element allowing for blocking the shaft  55  of the blocking element  50  inside the hole of the external fixator is the wedge surface comprising the two inclined surfaces  42  and  52  of the blocking element  40  and ring insert  50 , respectively. Blocking element  40  and ring insert  50  can be arranged in a way that their exterior cylindrical surfaces  55  and  45  respectively fit inside the hole of the ring fixator. By tightening the screw  60  the blocking element  40  and the ring insert  50  are moving together in axial direction. Because of the flat wedge portions  42  and  52  the two elements  40  and  50  increase their radial distance, in other words the two diametrically opposed cylindrical portions  55  and  45  do not have the same central axis any more but are in a equal distance in the longitudinal direction of the blocking element  40  and the ring insert  50 . Therefore they compress inside the ring hole and the fix connection of the ball joint  12  or this end of strut  10  with the ring hole is realized. 
     Because the screw  50  fixes simultaneously the ball joint and the cylinder to the ring hole, there is only one tightening step necessary. 
     The wedge has a predetermined angle. This angle is usually chosen between 8° and 20° preferably between 10° and 15°, so that it is self locking within the ring hole after a first tightening operation. When the ball joint  12  is fixed within the ring hole and the screw  50  is released or untightened, then the joint stays fixed within the ring hole although the rotation of the ball joint itself, via ball nut  30  is released. It is then necessary to turn the sleeve  20  with help of a wrench engaging the tetragon or octagon loosening surface  44 , the self locking becomes loosened and the ball joint  12  can be removed from the ring hole. 
     Additionally, on ring insert  50  there is provided a nose portion  56  to push ring insert  50  down the wedge surface  42 / 52  if possible without engagement of an instrument. Furthermore said nose portion  56  provide a shoulder, guaranteeing that the cylindrical portions  45  and  55  being smaller, there is no risk that the ball joint  12  is falling through a hole of a fixator rod  11 . 
       FIG. 5  shows an enlarged side view of the sleeve  20  of the joint  15  according to  FIG. 2 , showing that the embodiment uses ten ribs  24  with nine intervening grooves  25 . The largest portion of spherical portion  22  has a smaller diameter than the cylindrical portion  21  and the inner diameter of the spherical portion  22 , i.e. the inner sphere, is unruffled and is smaller than the inner diameter of the sleeve  20  to allow introduction of ball nut  30  with its spherical portion  33  through bore  23 . 
     Finally  FIG. 6  shows a perspective view of three struts  10  according to  FIG. 1  attached to two external fixator rings  80 . These rings  80  shown in  FIG. 6  comprise a partial ring beam covering about 270 degree. Each ring  80  comprises a plurality of holes  81  having an internal diameter allowing to accommodate the cylindrical portion  55  of ring insert  50  in conjunction with the opposite cylindrical portion of blocking element  40 . Actuation of fixation screw  60  expands the diameter of the unit provided by blocking element  40  and ring insert  50  and thus fixes the neck part  50 / 60  of the ball joint  12  within one of the holes  81  of a ring  80 . It is clear from  FIG. 6  that the two rings  80  can be moved one against the other, since all three struts  10  use two ball joints  12  at each of their free ends. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.