referring to fig1 - fig . 19 a new method for facet fixation includes the following steps . first , patient 80 is positioned prone , lying flat on an operating table in preparation for a minimally invasive surgery ( mis ) ( 100 ). next , location 114 a corresponding to a first facet joint of the l5 lumbar vertebra 84 , is radiographically identified and marked on the patient &# 39 ; s lower back . for mis procedures , a skin incision 81 is performed and a first guide wire 112 is inserted in the facet joint location 114 a ( 110 ). the placement of the guide wire 112 is verified by taking a fluoroscopic image of the patient &# 39 ; s back . the fluoroscopic image is further used to identify the location of the facet joints 114 a , 114 b of vertebra 84 and the angular relationship between them . guide wire 112 or kirschner wire ( also called k - wire ) is a thin , rigid wire that is used to stabilize bone fragments in orthopedics and other types of medical and veterinary surgery . kirschner wires were introduced in surgical procedures by martin kirschner in 1909 . they are sterilized , sharpened , smooth stainless steel pins and have different sizes . these wires can be drilled through the bone to hold bone fragments in place . they are placed percutaneously ( through the skin ), thus avoiding an operation in some cases . in other cases , the k - wires are used after an operation to hold bone fragments in place . in some cases the k - wires include threads for threading into the bone . in spine surgery k - wires are used as guide wires for the placement of spine fixation components , such as screws and pins . they are inserted either through an open surgical procedure or under fluoroscopic or x - ray observation and are removed after the insertion of the screws . in one example guide wire 112 is a threaded 140 millimeter k - wire , manufacture by spinefrontier , inc ( beverly , mass .). next , a bone needle 122 is inserted over the guide wire 112 ( 120 ) and tapped into the facet joint 114 a ( 130 ). i one example , bone needle 122 is a jamshide bone needle , manufactured by baxter - allegiance . the bone needle is then removed ( 140 ), and first arm 152 of an x - guide tool 400 is inserted over the guide wire 112 ( 150 ). second arm 154 of the x - guide tool 400 is connected to the first arm 152 at pivot point 156 ( 160 ) and the angle 60 between the two arms 152 , 154 is set so that a second guide wire 116 inserted through the second arm will meet the location of the second facet joint 114 b of vertebra 84 . angle 60 was determined from the fluoroscopic image of the patient &# 39 ; s back , as was mentioned above . second guide wire is then inserted through the second arm 154 of the x - guide tool 400 into the location of the second facet joint 114 b ( 170 ). second guide wire 116 includes two elongated members 116 a , 116 b pivotally connected at point 117 , as shown in fig2 . in one example , guide wire 116 is a threaded 50 centimeter pivoting k - wire , manufacture by spinefrontier , inc ( beverly , mass .) and the x - guide tool 400 is a two arm angular positioning guide manufactured also by spinefrontier , inc ( beverly , mass .). next , the two guide arms 154 , 152 are removed ( 180 ), ( 190 ), and the upper arm 116 a of the pivoting guide wire 116 is pivoted out of the plane of the first guide wire 112 , as shown in fig1 ( 200 ). a tissue dilator 350 is then inserted over the first guide wire 112 ( 200 ). tissue dilator 350 include an outer dilator cannula 352 and an inner dilator 354 configured to slide within the outer dilator cannula 352 , as shown in fig2 . in one example , tissue dilator 350 is a dilator manufactured by spinefrontier , inc ( beverly , mass .). the inner dilator 354 is advanced within the outer dilator cannula and the tissue around the first guide wire 112 is dilated ( 210 ). next the inner dilator 354 is removed and the outer dilator cannula 352 is advanced into the patient &# 39 ; s tissue ( 220 ), as shown in fig1 . next a hand drill 360 is inserted into the outer dilator cannula ( 230 ). referring to fig2 , hand drill 360 has a handle 364 and an adjustable stop 362 . the distance 363 between the drill stop 362 and handle 364 is adjusted to correspond to the length of the facet screw that need to be inserted into the facet joint 114 a . accordingly , in step 240 length 363 is adjusted to match the length 301 of the facet screw 21 , shown in fig2 and then the handle is advanced down until it hits the stop 362 ( 250 ). in one example , hand drill 360 is a drill manufactured by spinefrontier , inc ( beverly , mass .). next , the hand drill 360 is removed ( 260 ) and a screwdriver 370 with a removable attached screw 300 is inserted in the location 114 a via the outer dilator cannula 354 ( 270 ). the screw 300 is attached to location 114 a and the screwdriver 370 and the outer dilator cannula 354 are removed . referring to fig2 , in one example screwdriver 370 has a removable screw attaching mechanism 376 and is manufactured by spinefrontier , inc ( beverly , mass .). next , the upper arm 116 a of the second guide wire 116 is straighten and the process of dilation , drilling and screw driving is repeated for the second facet joint location 114 b resulting in two facet joint screws 300 a , 300 b being attached to locations 114 , 114 b , respectively ( 280 ). similarly , additional facet screws are driven in other facet joint locations of adjacent vertebras 82 and 86 . referring to fig2 , fig2 and fig2 , facet screw assembly 300 includes a polyaxial screw 302 and a washer 308 . screw 302 has an elongated body 302 a having a semispherical head 306 with a flat top 306 a at one end and a threaded portion 304 at the opposite end . washer 308 is non - removably fitted around head 306 . washer 308 has a semispherical through opening 308 a supporting the semispherical head 306 and is configured to pivot around the head 306 at an angle 313 , as shown in fig2 . washer 308 also has teeth 312 in its bottom surface for grabbing and holding into the bone , tissue , biologic or synthetic material , as shown in fig2 . in other embodiments , the bottom surface of washer 308 include ridges , serrations , grooves , or spikes . in one example , facet screw 300 has a length 301 of about 40 millimeters , a non - threaded portion length 303 of 20 millimeters , screw body width 311 of 4 . 5 millimeters , washer width 309 of about 9 millimeters , washer height of 2 millimeters and a washer pivoting angle 313 of 7 . 5 degrees . facet screw 300 is manufactured by spinefrontier , inc ( beverly , mass .) and may be made of metal such as stainless steel or titanium , plastic , bioabsorbable material , ceramic material , or other solid or porous materials . referring to fig2 , in another embodiment the facet screw assembly 160 includes a polyaxial screw 170 and a washer 180 . screw 170 has a cannulated elongated body 174 having a through opening 175 extending from its proximal end 174 a to its distal end 174 b . the elongated body 174 has a semispherical head 172 at the proximal end 174 a and a threaded portion 173 at the distal end 174 b . the semispherical head 172 is swaged locked within the opening 182 of the washer 180 , as shown in fig2 . in one example , screw 170 has a length of 35 mm , a diameter of 3 . 5 mm at the unthreaded body portion and a diameter of 4 . 5 mm at the threaded body portion . screw head 172 is polyaxially rotatable within the opening 182 of the washer 180 , as shown in fig2 , until it is secured into the bone with the screwdriver . the head 172 has a hexagonal through opening 176 for receiving a hexagonal screwdriver tip . referring to fig2 , fig2 a , and fig2 b washer 180 has a cylindrical body 184 with a top 190 a and a bottom 190 b . two concentric rows ( inner 186 a , outer 186 b ) of teeth 186 extend from the bottom 190 b of the washer body 184 . cylindrical body 184 has a semispherical through opening 182 for holding the semispherical screw head 172 . the opening 182 at the top 190 a is initially straight , as shown in fig2 , and is flared inwards ( i . e ., swaged ) after the screw head 172 is placed into the opening 182 to lock the head 172 into the opening 182 and prevent it from moving up , as shown in fig2 a and fig2 . the diameter 191 of the opening 182 at the top 190 a is initially slightly larger than the diameter of the semispherical head and becomes slightly smaller 181 after the swaging . the opening 182 at the bottom 190 b has a diameter 183 smaller than the diameter of the semispherical head 172 and larger than the diameter of the unthreaded portion of the elongated screw body 174 , thus preventing the head 172 from passing through the opening . inner row 186 a has teeth 186 with trigonal cross - section 194 and outer row 186 b has teeth with rectangular cross - section 192 . the gaps 198 , 196 between the teeth 186 in each row have alternating rectangular and trigonal shapes , respectively . this teeth geometry and arrangement allow the washer to penetrate and grab into the bone or tissue and to prevent it from rotating once it is engaged into the bone or tissue . the washer pierces the bone / tissue in order to achieve a more secure padding for compression and distributes the compression force onto a flatter wider surface . the piercing of the bone by the washer also promotes bone growth . referring to fig2 , a screwdriver assembly 370 is used to deliver and screw the facet screw assembly 160 into the bone through the cannula 202 . the screwdriver assembly 370 includes a screw retention sleeve 374 and a handle 371 attached to the proximal end of the retention sleeve 374 . handle 371 includes a detachable t - shaped component 372 that can be used to provide additional torque for driving the screw assembly 160 into the bone . the screw assembly 160 is attached to the distal end 376 of the retention sleeve 374 and can be released or retained by moving the screw release / capture component 375 up or down . detailed views of the retained and released facet screw assembly are shown in fig3 a , and fig3 b , respectively . several embodiments of the present invention have been described . nevertheless , it will be understood that various modifications may be made without departing from the spirit and scope of the invention . accordingly , other embodiments are within the scope of the following claims .
a spinal fixation element includes a screw and a washer . the screw comprises an elongated body of a first diameter and the body includes a threaded portion at a distal end and a semispherical head of a second diameter at a proximal end . the second diameter is larger than the first diameter . the washer comprises a semispherical through opening of a third diameter at the top , of a fourth diameter in the middle and of a fifth diameter at the bottom . the third diameter is slightly smaller than the second diameter , the fourth diameter is slightly larger than the second diameter and the fifth diameter is smaller than the second diameter . the washer surrounds the semispherical head , is non - removably attached to the semispherical head and is rotatable and positionable at an angle relative to the elongated body .