to achieve a satisfactory implant , this invention utilizes , as shown in fig1 a base plate 27 having a pair of rods 28 and 29 received in sockets 30 and 31 , respectively , mounted on the base plate . base plate 27 is preferably a contoured , slightly malleable implant made from stainless steel ( as are all other components utilized in the implant ). the center portion 33 of the base plate has an upwardly extending ridge 34 thereat with sockets 30 and 31 being positioned parallel to and along the opposite sides of the ridge . sockets 30 and 31 are tubular sections which are fastened to the top side of the base plate in a conventional manner , such as , for example , by welding . projections , or wings , 36 and 37 extend outwardly from center portion 33 and each projection has three apertures thereon ( designated as apertures 39 , 40 and 41 in projection 36 and apertures 43 , 44 and 45 in projection 37 ) formed near the outer edges , which edges are shown curved at the front portion ( at apertures 39 and 43 ). it is to be realized , however , that the curvature of the base plate , depends upon the particular sacrum and / or particular fixation to be utilized , as brought out more fully hereinafter . rods 28 and 29 are also preferably formed of stainless steel , and are contoured as needed for each particular application . the system is a total system , and may utilize one or more types of securing elements for securing vertebrae of the spinal column along the rods . as shown in fig1 by way of example , each rod 28 and 29 may have a hook element 47 thereon , which hook element , as shown in fig1 and 2 , includes a body portion 48 having a bore 49 therethrough through which the associated rod extends with the bore being slightly larger than the diameter of the rod to enable free axial and rotational movement of the rod in the bore . hook portion 50 extends below the body portion , and , as shown in fig3 a tapered wedge 51 with head 52 thereon , is utilized to engage the rod extending through the bore to lock the hook element against movement ( including precluding rotation and axial movement ) independently of the rod . as indicated in fig2 and 3 , bore 49 and wedge 51 are lengthwise serrated , to that upon final placement , sufficient pressure can be applied to cold weld the components in position . rods 28 and 29 are maintained in spaced relationship with respect to one another by use of bridging element 54 , as shown in fig1 and 4 . as shown , bridging element 54 includes outwardly facing arcuate sections 55 and 56 ( the inner surfaces 57 of which are lengthwise serrated ) having a v - shaped strut 58 therebetween , with the inner surfaces 57 of arcuate sections 55 and 56 engaging the inner portions of rods 28 and 29 when the bridging element is positioned between the rods , as indicated in fig1 , 20 and 21 of the drawings . as shown in fig1 and 5 , a yoke and foot element 59 may also be placed between rods 28 and 29 . element 59 includes a pair of outwardly facing arcuate sections 60 and 61 ( the inner surfaces 62 of which are lengthwise serrated ) at opposite sides of block 63 with the inner surfaces 62 of the arcuate sections 60 and 61 being engagable with the inner portions of rods 28 and 29 when positioned between the rods , as shown in fig1 , 18 and 19 of the drawings . block 63 has a bore therethrough for receiving a threaded shank 64 , which shank has a foot , or hook , 65 at one end . as indicated , foot 65 is tapered toward the front , or freely extending edge , of the foot to facilitate insertion adjacent to a vertebra or the like . shank 64 has nut 66 thereon adjacent to block 63 at the side opposite foot 65 so that upon rotation of nut 66 , foot 65 can be drawn toward or moved away from block 63 ( or , alternately , the bore of block 63 can be threaded so that the foot is drawn toward or moved away from the block by rotation of the shank , and nut 66 can be used to lock the shank in place with respect to the bore ). as shown best in fig6 rod 28 ( rod 29 is identical and therefore not separately discussed ) has a body portion 67 that terminates at shoulder 68 with a reduced diameter end portion 69 extending therefrom , which end portion is lengthwise serrated and is received in socket 30 ( as shown in fig1 and 13 ), the inner surface of which is also lengthwise serrated ( as is socket 31 ). a wedge 71 ( likewise lenghtwise serrated ) having head 72 thereon , as shown in fig1 and 7 , is used to lock the rod in the shaft so that , when so locked , the rod cannot rotate and the rod is therefore constrained to movement with the base plate . as shown in fig8 and 9 , rod 28 has threads 74 formed thereon , with the threads being flat topped , as indicated in fig9 to lessen the cut into the rod so as to maintain maximum strength , and each thread has lengthwise serrations 75 therein . the threads are 20 per inch which allows small changes in the locking points for the securing elements along the rods ( of about 1 mm ). typical use of the implant of this invention is shown in fig1 and 11 . it is meant to be emphasized that while only one bridging element , yoke and foot element , and hook element has been shown in fig1 and 11 , any number of each could be utilized as might be needed for a particular application . in addition , the use of wire could be expanded , and pedicle screws ( not shown ) could also be utilized . as shown in fig1 and 11 , base plate 27 is mounted on sacrum 78 of the spinal column 79 of the body and contoured rods 28 and 29 , when positioned in sockets 30 and 31 ( as shown in fig1 through 14 ), extends along the vertebrae 80 of spinal column 79 so that such vertebrae can be secured to rods 28 and 29 to cause alignment of the spinal column and thereafter support the aligned spinal column in the manner generally dictated by the contour of the rods . the sacrum offers several possibilities for fixation , which , when combined , appear to provide a secure foundation for a dorsally placed transacrally fixed implant . the dorsal sacral landmarks , and particularly the first and second dorsal foramina , are readily identifiable and relate in a consistent manner both qualitatively and quantitatively to the major mass of the sacral bone , which is located ventrally to the vital structures of the sacrum . ideal lumbosacral fixation should secure directly to the sacrum rather than indirectly by way of the iliae , and be modular for ease of insertion , versatile to allow use in a wide variety of diseases and deformities , and integratable with existing and evolving cephalad spine implants . the sacral foundation of this invention provides a dorsal sacral implant that is secured ventrally by multiple transacral anchors . to determine the best configuration for such a sacral implant , implant configuration and transacral fixation site possibilities were first studied on a variety of anatomical models and four embalmed cadaver specimens . to study contour , plaster of paris negative and laminated paper positive molds were prepared and from these 0 . 5 mm ( 0 . 02 inch ) thick stainless steel prototypes were prepared . to study transacral anchor possibilities , drill holes were placed in six different sites . the placements were selected to safeguard neurovascular structures , avoid sacroiliac articulations , and provide maximum bone fixation . the three dimensional placement of the drill sites was confirmed by faxitron radiography . the relative strength of fixation site combinations was also estimated in the embalmed human sacrae and in fresh calf sacrae using hand held spring scales . to study the dimensions of the dorsum of the first two sacral vertebral segments , and the depth of the transacral fixation sites , 18 cadaver sacrae of known sex ( 9 males and 9 females ) and 31 sacrae of unknown sex were measured with calipers . in addition , anthropometric sacral dimensional measurements were determined in the 18 sacrae of known sex . student t - test and linear correlation coefficient analyses , where appropriate , were also performed . to study the effect of transitional vertebrae on sacral configuration , 8 sacrae so involved were studied . the transacral fixation depths and dorsal sacral measurements of the 18 sacrae of known sex are tabulated in tables 1 and 2 as follows : table 1______________________________________transacral fixation ( tsf ) depth distance ( mm ) males ( 9 ) females ( 9 ) p______________________________________tsf 1 49 . 7 (± 3 . 7 ) 46 . 9 (± 3 . 3 ) nstsf 2 38 . 8 (± 2 . 7 ) 37 . 2 (± 2 . 5 ) nstsf 3 30 . 7 (± 3 . 1 ) 28 . 8 (± 2 . 3 ) ns______________________________________ table 2______________________________________human sacrum : dorsal measurements distance ( mm ) males ( 9 ) females ( 9 ) p______________________________________s1df to s1df 48 . 8 (± 5 . 0 ) 47 . 4 (± 3 . 8 ) nss2df to s2df 43 . 1 (± 4 . 2 ) 44 . 0 (± 3 . 0 ) nss1df to s2df 26 . 2 (± 2 . 4 ) 21 . 4 (± 2 . 5 ) & lt ;. 001sn to s1df 21 . 7 (± 2 . 3 ) 22 . 3 (± 1 . 6 ) ns______________________________________ legend : s1df = first sacral dorsal foramen s2df = second sacral dorsal foramen sn = superior sacral notch of the sacral ala the longest , and apparently most secure , transacral fixation site was previously described by harrington and dixon ( harrington , p . r . and dixon , j . h . &# 34 ; spinal instrumentation in the treatment of severe progressive spondyloisthesis &# 34 ;, clin . orthop . 117 : 157 - 163 , 1976 ) and passes through the first sacral pedicle to the sacral prominatory ( tsf 1 ). the dorsal entry point is caudad and lateral to the superior facet , 10 mm lateral and five mm cephalad from the first sacral dorsal foramen . the direction with reference to a staring line perpendicular to the flat dorsal surface of the first and second sacral laminae was 15 degrees cephalad in the frontal plane , 20 degrees cephalad in the sagittal plane , and 35 degrees medially in the horizontal plane . access to the entry site necessitated a dowel cut window 81 in the iliac crest 82 ( as indicated in fig1 ). the tsf 1 depth was 49 . 7 mm (± 3 . 7 ) in the nine males and 46 . 9 mm (± 3 . 3 ) in the nine females . it was not significantly different than the distance between the outer edges of the first sacral dorsal foraminae ( s1 -- s1 ) which was 48 . 8 mm (± 5 . 0 ) in males and 47 . 4 mm (± 3 . 8 ) in females . the distance from the superior sacral notch of the sacral ala to the cephalad edge of the first dorsal sacral foramen was similar for males , 21 . 7 mm (± 2 . 3 ) and females , 22 . 3 mm (± 1 . 6 ). the distance between the cephalad edges of the first and second dorsal sacral foramina was significantly shorter in females , 21 . 4 mm (± 2 . 5 ), than in males , 26 . 2 mm (± 2 . 4 ), ( p & lt ;. 001 ). a common measurement used to separate male and female sacrae , a ratio of the transverse diameter of the first sacral body to the breadth of the sacrum , was not significantly different , males 41 . 9 (± 3 . 2 ), and females 39 . 8 (± 3 . 3 ), as shown in table 3 as follows : table 3______________________________________human sacrum : cephalad edge measurements distance males ( 9 ) females ( 9 ) ______________________________________transverse 43 . 7 mm (± 3 . 9 ) 42 . 4 mm (± 4 . 4 ) nsdiameters1 body ( tb ) anterior 104 . 0 mm (± 6 . 9 ) 106 . 4 mm (± 4 . 7 ) nsbreadth ats1 ( brd ) tb / brd × 100 41 . 9 (± 3 . 2 ) 39 . 8 (± 3 . 3 ) ns______________________________________ the distance between the outer edges of the second dorsal sacral foramina was similar , males 43 . 1 mm (± 4 . 2 ) and females 44 . 0 mm (± 3 . 0 ). of the five additional transacral fixation sites studies , two appear superior . one ( tsf 2 ) transversed from the first sacral transverse tubercle to the lateral mass of the sacrum . the direction with reference to a staring line perpendicular to the flat dorsal surface of the first and second sacral laminae was 25 degrees cephalad in the frontal plane , 25 degrees cephalad in the sagittal plane , and 25 degrees lateral in the horizontal plane . this distance was similar for males , 38 . 8 mm (± 2 . 7 ), and females , 37 . 2 mm (± 2 . 5 ). the other site ( tsf 3 ) passed from a point midway between the intermediate and lateral sacral crest and midway between the first and second dorsal foramina to the lateral mass . the tsf 3 direction with reference to a starting line perpendicular to the flat dorsal surface of the first and second sacral laminae was 20 degrees caudad in the frontal plane , five degrees caudad in the sagittal plane , and 50 degrees lateral in the horizontal plane . this distance was similar for males , 30 . 7 mm (± 3 . 1 ) and females , 28 . 2 mm (± 2 . 3 ). for the 49 normal sacrae studied , without regard to sex , there was a positive correlation between the first sacral dorsal interforaminal distance and tsf 1 depth ( p & lt ; 0 . 001 ), between tsf 1 depth and tsf 2 depth ( p & lt ; 0 . 005 ), and between tsf 1 depth and tsf 3 depth ( p & lt ; 0 . 001 ), as shown in table 4 as follows : table 4______________________________________dorsal and transacral fixation sitemeasurement correlations : without regard to sex ( n = 49 ) p______________________________________s1df -- s1df vs . tsf 1 . 5297 & lt ;. 001tsf 1 vs . tsf 2 . 4137 & lt ;. 005tsf 1 vs . tsf 3 . 5294 & lt ;. 001______________________________________ legend : s1df = first sacral foramen tsf = transacral fixation there were eight sacrae with transitional vertebrae , five had complete sacralization of l5 with the lateral mass appearing to coincide with the &# 34 ; new &# 34 ; s1 vertebrae , and three had imcomplete sacralization of l5 and the lateral mass coincided with the true s1 vertebrae . in these , as well as in the one sacrum with complete lumbarization of s1 , the tsf 1 distance was relatively short . although there are significant sex and race related differences in the anthropometry of the human sacrum , these differences are small , approximately 5 to 6 mm . in the study , for the implant of this invention , the major sex difference was not in the s1 body / sacral breadth ratio , but in the spacial relations of the first and second dorsal sacral foramina , the distance being shorter in females . however , the relatively large s1 pedicle approach to the prminatory appears to offer adequate compensation for this difference in an implant with fixed distance fixation entry sites . while interracial differences were not considered , they appear to be less significant than the intersex differences . thus , design of sacral implants based on sex and race differences does not appear to be necessary . as a result of these studies , the dorsal sacral implant of this invention has been designed to meet the implant needs and requirements for sacral fixation . to achieve sacral fixation , base plate 27 is secured on sacrum 78 by means of screws 84 ( are more specifically designated as screws 84a - 84f as shown best in fig1 , 15 , 16 and 17 ). the positioning of screws 84a and 84d are best shown in fig1 and 15 , while the positioning of screws 84b and 84c are best shown in fig1 and 16 , and the positioning of screws 84c and 84f are best shown in fig1 and 17 . as can be appreciated from fig1 and 15 through 17 , the screws are placed in a three - dimensional configuration in the sacrum with the location and angles being predetermined , as brought out hereinabove , to provide maximum retention of the base plate on the sacrum even though severe forces might be later applied thereto . after the base plate is fixed to the sacrum , and after the rods are contoured to the desired contour needed for the particular subject , the rods are placed in sockets 30 and 31 and when so placed , wedges 71 are driven into the space between the rods and sockets as shown in fig1 and 14 , with the serrated surfaces maintaining the wedges in place ( preferably the wedges are wired in place to give even more protection against dislodging by later patient movements ). the body , or main , portions of rods 28 and 29 are utilized to secure spinal column 79 in the desired position and alignment dependent upon the curvature of the rods , with the vertebrae of the spinal column being secured to the rods as generally shown in fig1 and 11 , and as more specifically shown in fig1 to 23 . as shown , rods 28 and 29 , when finally positioned , are adjacent to the lamina 86 of vertebrae 80 of spinal column 79 with the rods having the spinous process 87 of the vertebrae between the rods . as shown , hook elements 47 on rods 28 and 29 are utilized to engage vertebrae 80 of spinal column 79 with hooks 50 engaging the underside of a vertebra ( above spinal cord 88 ) to maintain the vertebrae adjacent to the rods . bridging elements 54 maintain rods 28 and 29 spaced , and vertebrae can be held in place adjacent to the spacing elements by wire 90 , as needed . as shown , wire 90 may also be wrapped around rods 28 and 29 and bridging elements 54 as needed for maintaining the positioning of the bridging elements . yoke and foot elements 592 are utilized with foot 65 hooked under a vertebra 80 so that the vertebrae can be lifted or otherwise moved toward the rods by lengthening or shortening the shank relative to the spacer block . all removable elements ( including wedges ) are preferably wired in place upon final implant to assure against loosening and / or loss of any such element or component from its intended position . in performing the surgical implant , the sacral anatomy is preferably first defined , after which staged surgery ( anterior disectomy and fusion if necessary to restore alignment and / or provide stabilization ) may be carried out prior to sacral fixation . to prepare the sacral fixation , the sacrum is first exposed , measurements are then taken for determining the best positioning of the base plate on the sacrum , the site is thereafter prepared and sculptured , and the tsf 3 holes in each side of the base plate are located and drilled ( 25 mm ). the iliac crest entry portals are next located and drilled as necessary to provide the portals , and the tsf 1 holes are then located and drilled ( 45 mm ). the base plate is next placed into position on the sacrum , the tsf 2 holes are confirmed over the s1 transverse tubercle , the tsf 1 are then drilled , and screws are securely screwed into all of the drilled holes . after fixation of the base plate , lumbar and thoracolumbar fixation is carried out by selecting the correction mode to be utilized ( distraction , compression , extension , flexion , rotation , or combination ), preparing the proximal fixation sites , contouring the rods and placing the rods in the sockets of the base plate , wedging the rods in the desired position , and then placing the necessary securing elements on the rods to enable the spinal column to be secured in the desired shape by means of the hooks and / or wire utilized ( with various instruments being utilized , as necessary , to facilitate the fixation ). as an operation proceeds , the various securing elements used as hooks can be adjusted by removing the tapered wedges , where necessary , by utilizing the heads on the wedges to easily withdraw the wedges , and then moving the elements , after which the elements are again screwed in place . the hooks can , of course , be placed for compression or distraction . it is meant to be realized , that various elements and / or method steps as described herein , could be modified as would be obvious to one skilled in the art without departing from the intended scope of this invention . as can be appreciated from the foregoing , this invention provides an improved surgical implant and method that utilizes direct sacral fixation .
a dorsally placed transacrally fixed surgical implant is described for correcting and supporting the spinal column with the implant being particularly useful for aligning and stabilizing the lumbosacral junction . direct sacral fixation is achieved utilizing a contoured metallic base plate that is firmly fixed to the sacrum by means of a plurality of screws , which screws are angularly disposed with respect to one another to maximize retention of the base plate on the sacrum . a pair of spine engagable rods , contoured to the desired spinal column configuration , have one end inserted into sockets on the base plate with the rods being locked against rotation or other movement relative to the base plate . a total fixation system is provided that has flexibility with respect to rod utilizable components , with the rods having bridges , hooks and / or wires spaced along the rods for engagement with vertebrae of the spinal column to hold the spinal column in the desired position , once achieved , as determined by the contour of the rods .