fig1 and 2 show the manner in which a coupling of the general type here concerned but employing coupling units according to the prior art , is used . in fig1 and 2 , two schanz screws 4 and 5 are inserted into each of two vertebrae 2 and 3 adjacent to a damaged vertebra 1 for rigid connection of the two sound vertebrae . each schanz screw 4 of upper vertebra 2 is rigidly connected via a threaded support rod 6 to the corresponding schanz screw 5 of lower vertebra 3 . the rigidity of the connection between each schanz screw and support rod 6 is maintained by the clamps or couplings 20 . details of the prior art couplings 20 are shown in fig2 a . each coupling element has a block 7 which slides freely along support rod 6 and may be fixed at any position on the rod by means of nuts 9 , threaded on rod 6 , and retainer rings 8 . rings 8 have interior flats 25 which engage flats 26 on the rod 6 . rings 25 are free to slide along the rod 6 , and are provided with a toothed or knurled section 27 which engages a like section 28 on the face of block 7 . thus when nuts 9 are tightened , engaging retainer ring 8 with the knurled section 27 of block 7 , the block is fixed against rotation on the rod . block 7 has a threaded side extension 10 adapted to receive a ring hook 12 . the ring part 31 of ring hook 12 has a knurled or toothed portion 32 adapted to engage a like portion 33 on the block 7 . when fitted over side extension 10 and fixed by nut 11 , the ring hook is thus prevented from rotating . the hook portion 35 of ring hook 12 extends over the block 7 , forming with the block an aperture in which a schanz screw can be retained , the pressure exerted by nut 11 serving to prevent the pin from shifting or rotating . this method of clamping schanz screws has two disadvantages . first , it is difficult to gain access to the nut 11 with a small wrench . second , not much tension can be transferred through the nut to the clamp jaw . this tension is further reduced because the stress produced by the tightening of the clamp jaw causes plastic deformation . the clamp in the coupling element of the invention does not have these disadvantages . referring to fig3 - 6 , a coupling unit 36 according to the invention comprises a housing 17 , having a cylindrical sleeve 37 , which enables the housing 17 to slide and rotate on a threaded support rod 6 , identical with the support rods shown in fig1 - 2 . the housing also has an eye 13 with a conically shaped opening 13a . a tubular collet 14 is provided for receiving a schanz screw 4 identical to the schanz scraews shown in fig1 - 2 . the collet 14 has a spherical compressible head 38 , composed of a plurality of curved spring leaves 38a , at one end , and a threaded shaft 14a at the other . flats 14b are provided on each side of the shaft 14a . the collet 14 seats in the conical opening 13 ( a ) of eye 13 , the shaft of the collet extending through the eye and beyond the housing 7 . flats 41 are provided at the narrower end of opening 13 ( a ). these flats engage the flats 14b on the shaft 14a of the collet 14 and prevent the collet from rotating when it is inserted in the eye . the collet 14 is secured in the eye 13 by means of a retaining ring 16 , which slides on shaft 14a and is provided with flats 42 to match the flats 14b on shaft 14a . a nut 15 engages with the threads on shaft 14a . it is necessary for schanz screw 4 to be able to tilt during the surgical operation and also for it to be able to be fixed in a position in which it is at an angle to the axis ( shown as 13 ( b ) in fig6 ) of the conical opening 13 ( a ). the compressible head 38 of the collet , as noted above , is spherical in shape ( somewhat truncated ) enabling it to be seated in the conical opening 13 ( a ) with the ability to tilt in a vertical plane ( as shown in fig3 ), engagement of the flats 14b on the shaft of the collet with the flats 41 preventing tilting in the horizontal plane ( as shown in fig3 ). in order to maintain the desired position when tension is applied to the collet ( i . e ., when nut 15 is tightened ), retainer ring 16 has two curved recessed areas , both of which are provided with teeth 16a . these teeth 16a mesh with teeth 13c which are provided on two side arches 13d at the edges of eye 13 . in use , a schanz screw or the equivalent is inserted in collet 14 , the screw is tilted to the proper angle and the nut 15 is turned down on shaft 14a . the teeth 16a on retainer ring 16 are thus brought into engagement with teeth 13c on the sides of the opening 13a . the compressible collet head 38 is drawn down into the conical opening 13a , compressing it about the schanz screw and thus developing a firm grip on the screw . the coupling may be moved up and down and rotated about the threaded rod 6 in the manner described above in connection with fig1 and 2a . it may be set at any desired position and orientation by means of nuts 9 and retaining rings 8 , which may have the same structures as those used in the prior art ( fig1 and 2a ), teeth 17a engaging with the teeth on ring 8 . due to its novel design , the coupling unit of the invention does not have the disadvantages of the prior art coupling noted above . the collet is significantly more effective than the prior art hook clamp . furthermore , once the nut used for positioning the collet is applied using a socket wrench , its axis is in the surgeon &# 39 ; s line of sight and the nut can be turned much more easily than a nut visible only from the side .
a coupling element for holding a schanz screw and a threaded pin rigidly in the correct orientation comprising a jacket , an eye with a collet therein and a nut .