pedicle screw fixation is a necessary part of various spinal surgeries . despite many advantages of using pedicle screws , their usage in the spine is replete with danger and has the potential for permanent neurologic deficit , especially when placing screws near the spinal cord at the concave apex of a scoliotic spine5151620 ) . there are several methods of pedicle screw insertion that have been developed to enhance the safety of its usage . some surgeons first identify anatomical landmarks by intraoperative c - arm image intensifier or k - wire placement into the pedicles1221 ) . others utilize stereotactic image guided systems that are based on preoperative computerized tomographic ( ct ) scan or fluoroscopy1919 ) . the incidence of pedicle screw misplacement ranges from 2% to 44.2% , with screw - related neurologic complications in the 0 - 0.9% range , using the aformentioned methods25678111314151820212223 ) . previous studies on the morphometry of thoracic pedicles indicate that there is a high inter - individual variability in the pedicle dimensions17 ) . the pedicles exhibited significant variability in their shape and orientation , not only between different vertebrae within the thoracic spine , but also in the same vertebra and even in the same pedicle among individuals . therefore , the starting point for each thoracic screw is slightly variable and is based on the anatomy of the posterior elements that are visualized intraoperatively . it is time consuming to take c - arm images after each pedicle screw placement and to rearrange the drapes each time an image is taken . the stereotactic image guided systems requires preoperative ct scan with exposure to high levels of radiation , higher cost , the need for expensive equipment , and prolonged operation time . exposure to radiation is considered to be a serious health hazard1919 ) . to improve pedicle screw placement accuracy with minimal radiation and maintain low cost 1 ) . we can identify the desired starting and aiming points using this guide pin . the purpose of this study was to describe a novel pedicle screw placement technique using a special guide - pin and present the results of our first 183 pedicle screw placement analyzed by ct scan . in the first phase , 2 thoracic spine cadaver specimens were assigned to test the accuracy of the guide - pin for transpedicular screw fixation . a specially designed guide - pin with a marker was inserted into the pedicle to identify the correct starting ( 2 - 3 mm lateral to the center of the pedicle ) and aiming points ( center of the pedicle isthmus ) in posteroanterior ( pa ) and lateral x - rays ( fig . 2 ) . after confirming the safety and accuracy of transpedicular screws using ct scan and naked eye examination in cadavers , eighteen patients underwent posterior stabilization utilizing transpedicular screws by two surgeons from two institutions using the same methods described above ( phase 2 ) . all pedicle screws were inserted using the free hand technique after confirmation of the guide - pin on pa and lateral portable / c - arm images1011 ) . to objectively evaluate the position of the screws inserted , postoperative computed axial tomography ( cat ) scans were performed . the relative position of the screw inside the pedicle was graded as follows : 1 ) contained : axis of the pedicle screw between the medial and lateral pedicle walls completely ; 2 ) medial violation : axis of the pedicle screw is out of medial wall of the pedicle ; and 3 ) lateral violation : axis of the pedicle screw is out of the lateral pedicle wall . the positions of the scanned pedicle screws were independently determined by two senior spine surgeons . the surgical technique of free hand transpedicular screw placement using guide - pin can be broken down into specific steps that are repeated at each level . the first component of successful thoracic screw placement is meticulous exposure of the posterior elements . the spine was exposed to the tips of the transverse processes bilaterally , staying strictly subperiosteal to reduce bleeding . we placed the guide - pins into the pedicles according to the recommended starting point following 215 mm hole by cortical burr10 ) . we checked pa and lateral portable x - rays to adjust the starting and aiming points after all the guide - pins were placed ( fig . 4 ) . we started gearshift from the dorsal lamina 2 mm lateral to the center of the pedicle . gearshift was advanced from the desired starting point toward the center of the pedicle isthmus , which was the center of pedicle on a pa x - ray and isthmus on the lateral radiograph . after inserting the gearshift tip approximately 20 mm ( past the pedicle isthmus ) , we confirmed the integrity of 5 bony walls with a flexible , ball - tipped probe . the pedicle finder was then advanced further with an ultimate depth averaging 45 - 55 mm for the lumbar spine and 25 - 45 mm in the thoracic region . a flexible ball - tipped pedicle sounding or palpating device was utilized to palpate five distinct bony borders : a floor and four walls ( medial , lateral , superior , and inferior ) . at this point , if a soft tissue breach was palpated , this provided an opportunity to redirect the screw into an appropriate position into the pedicle . the pedicle tract was undertapped with a 1 mm less diameter tap than the intended screw . following this , the pedicle tract was palpated again to make sure that the five osseous borders were intact . this second inspection often allowed palpation of distinct bony ridges confirming intraosseous position and the tract length was remeasured with a hemostat . the coronal plane radiograph is evaluated for the harmonious position of all screws especially when any rotational scoliosis deformity exists . in cases of shorter or longer screws compared to adjacent screws , the screw length as viewed on the lateral x - ray is mandatory to check for harmonious position . on the lateral x - ray , the screws should be parallel to the superior endplates , and not extending past the anterior border of the vertebral body . these two plane radiographs should confirm the harmonious position of the screws as noted by the surgeon in vivo ( fig . in addition , various osteotomy procedures were used depending on the etiology of the deformity . the surgical technique of free hand transpedicular screw placement using guide - pin can be broken down into specific steps that are repeated at each level . the first component of successful thoracic screw placement is meticulous exposure of the posterior elements . the spine was exposed to the tips of the transverse processes bilaterally , staying strictly subperiosteal to reduce bleeding . we placed the guide - pins into the pedicles according to the recommended starting point following 215 mm hole by cortical burr10 ) . we checked pa and lateral portable x - rays to adjust the starting and aiming points after all the guide - pins were placed ( fig . 4 ) . we started gearshift from the dorsal lamina 2 mm lateral to the center of the pedicle . gearshift was advanced from the desired starting point toward the center of the pedicle isthmus , which was the center of pedicle on a pa x - ray and isthmus on the lateral radiograph . after inserting the gearshift tip approximately 20 mm ( past the pedicle isthmus ) , we confirmed the integrity of 5 bony walls with a flexible , ball - tipped probe . the pedicle finder was then advanced further with an ultimate depth averaging 45 - 55 mm for the lumbar spine and 25 - 45 mm in the thoracic region . once the pedicle seeker was removed , the tract was visualized . a flexible ball - tipped pedicle sounding or palpating device was utilized to palpate five distinct bony borders : a floor and four walls ( medial , lateral , superior , and inferior ) . at this point , if a soft tissue breach was palpated , this provided an opportunity to redirect the screw into an appropriate position into the pedicle . the pedicle tract was undertapped with a 1 mm less diameter tap than the intended screw . following this , the pedicle tract was palpated again to make sure that the five osseous borders were intact . this second inspection often allowed palpation of distinct bony ridges confirming intraosseous position and the tract length was remeasured with a hemostat . the coronal plane radiograph is evaluated for the harmonious position of all screws especially when any rotational scoliosis deformity exists . in cases of shorter or longer screws compared to adjacent screws , the screw length as viewed on the lateral x - ray is mandatory to check for harmonious position . on the lateral x - ray , the screws should be parallel to the superior endplates , and not extending past the anterior border of the vertebral body . these two plane radiographs should confirm the harmonious position of the screws as noted by the surgeon in vivo ( fig . in addition , various osteotomy procedures were used depending on the etiology of the deformity . the first component of successful thoracic screw placement is meticulous exposure of the posterior elements . the spine was exposed to the tips of the transverse processes bilaterally , staying strictly subperiosteal to reduce bleeding . we placed the guide - pins into the pedicles according to the recommended starting point following 215 mm hole by cortical burr10 ) . we checked pa and lateral portable x - rays to adjust the starting and aiming points after all the guide - pins were placed ( fig . we started gearshift from the dorsal lamina 2 mm lateral to the center of the pedicle . gearshift was advanced from the desired starting point toward the center of the pedicle isthmus , which was the center of pedicle on a pa x - ray and isthmus on the lateral radiograph . after inserting the gearshift tip approximately 20 mm ( past the pedicle isthmus ) , we confirmed the integrity of 5 bony walls with a flexible , ball - tipped probe . the pedicle finder was then advanced further with an ultimate depth averaging 45 - 55 mm for the lumbar spine and 25 - 45 mm in the thoracic region . a flexible ball - tipped pedicle sounding or palpating device was utilized to palpate five distinct bony borders : a floor and four walls ( medial , lateral , superior , and inferior ) . at this point , if a soft tissue breach was palpated , this provided an opportunity to redirect the screw into an appropriate position into the pedicle . the pedicle tract was undertapped with a 1 mm less diameter tap than the intended screw . following this , the pedicle tract was palpated again to make sure that the five osseous borders were intact . this second inspection often allowed palpation of distinct bony ridges confirming intraosseous position and the tract length was remeasured with a hemostat . the coronal plane radiograph is evaluated for the harmonious position of all screws especially when any rotational scoliosis deformity exists . in cases of shorter or longer screws compared to adjacent screws , the screw length as viewed on the lateral x - ray is mandatory to check for harmonious position . on the lateral x - ray , the screws should be parallel to the superior endplates , and not extending past the anterior border of the vertebral body . these two plane radiographs should confirm the harmonious position of the screws as noted by the surgeon in vivo ( fig . after the insertion of the pedicle screws , deformity correction was carried if necessary . in addition , various osteotomy procedures were used depending on the etiology of the deformity . there were 18 patients with a mean age of 58.2 years ( range , 34 - 68 years ) at the time of the surgical treatment . the diameter of the screws used in the thoracic spine ranged from 4.0 mm to 7.5 mm . the number of screws inserted were as follows ( total n=183 ) : t1 - 6 n=28 ; t7 - 12 n=70 ; l1 - 5 n=85 . the diagnoses were spondylolisthesis ( 7 patients ) , infection ( 3 ) , fracture ( 3 ) , spinal stenosis ( 3 ) , degenerative scoliosis ( 1 ) , and tumor ( 1 ) . all 183 thoracolumbar transpedicular screws inserted into the spine were evaluated by cat scan to assess for screw position . ninety - nine percent ( 181/183 ) of screws were contained within the pedicle . among 183 pedicle screws inserted , thoracic pedicle screws were accurately placed in 98% by the 2 surgeons ( 96 of 98 ) . only two screws ( 2% ) showed moderate cortical perforation which meant the central line of the pedicle screw was out of the outer cortex of the pedicle wall and included 1 screw ( 1% ) that violated the medial wall ( 1 lateral violation in a patient and 1 medial violation in a cadaver specimen ) . eighty - five screws inserted into the lumbar spine showed 100% accuracy without any medial or lateral pedicle wall violation . there were no screws ( out of the 135 thoracolumbar pedicle screws inserted into 18 patients ) that caused neurologic or vascular complications . there were no instances of cerebrospinal fluid ( csf ) emanating from the initial pedicle tract during the preparation of the screw holes . there were 18 patients with a mean age of 58.2 years ( range , 34 - 68 years ) at the time of the surgical treatment . the diameter of the screws used in the thoracic spine ranged from 4.0 mm to 7.5 mm . the number of screws inserted were as follows ( total n=183 ) : t1 - 6 n=28 ; t7 - 12 n=70 ; l1 - 5 n=85 . the diagnoses were spondylolisthesis ( 7 patients ) , infection ( 3 ) , fracture ( 3 ) , spinal stenosis ( 3 ) , degenerative scoliosis ( 1 ) , and tumor ( 1 ) . all 183 thoracolumbar transpedicular screws inserted into the spine were evaluated by cat scan to assess for screw position . ninety - nine percent ( 181/183 ) of screws were contained within the pedicle . among 183 pedicle screws inserted , thoracic pedicle screws were accurately placed in 98% by the 2 surgeons ( 96 of 98 ) . only two screws ( 2% ) showed moderate cortical perforation which meant the central line of the pedicle screw was out of the outer cortex of the pedicle wall and included 1 screw ( 1% ) that violated the medial wall ( 1 lateral violation in a patient and 1 medial violation in a cadaver specimen ) . eighty - five screws inserted into the lumbar spine showed 100% accuracy without any medial or lateral pedicle wall violation . there were no screws ( out of the 135 thoracolumbar pedicle screws inserted into 18 patients ) that caused neurologic or vascular complications . there were no instances of cerebrospinal fluid ( csf ) emanating from the initial pedicle tract during the preparation of the screw holes . pedicle screw fixation has the advantage of obtaining purchase of all three spinal columns without encroaching into the spinal canal4 ) . this theoretical advantage has been translated to superior clinical results in fracture fixation as well as in deformity correction111516172021 ) . however , their usage in the spine has the potential for permanent neurologic deficit , especially when placing screws near the spinal cord at the concave apex of a scoliotic spine35151823 ) . the safety margin for this technique has been improved through the use of image - guided techniques1919 ) . nevertheless , these newer techniques require additional equipment as well as the use of fluoroscopy which increases the radiation exposure . we developed specially designed guide - pin with a ball maker as it allows identification of the starting and aiming points as well as its direction without the problems mentioned earlier . the incidence of pedicle screw misplacement ranges from 1.5% to 25% using the k - wire guided methods31621 ) . although previous research showed only 1.5% misplacement , they admitted that the actual rate would be higher as their patients were primarily evaluated by the postoperative radiographs21 ) . conventional k - wire guide technique could not allow direct visualization of the starting point on the intraoperative fluoroscopy and/or radiography . in the present study , only two pedicle screws of 183 ( 1.0% ) demonstrated violations ( 1 lateral violation in a patient and 1 medial violation in a cadaver ) as confirmed by postoperative ct scan evaluation of all pedicle screws enrolled . ball tip is corresponding to " starting point " of the dorsal cortex of the pedicle . all previous guide - pin except ours failed to show starting point . some guide - pin had notch(es ) in the middle of guide - pin but they did not demonstrate starting point . tip of the guide - pin is placed in the isthmic part of the pedicle , which is 15 mm or 20 mm away from ball tip . the position of the tip should be located around isthmic part of the pedicle which is 15 mm to 20 mm away from starting point . this special guide - pin can show exact starting point in the dorsal cortex of the pedicle with ball tip . also tip of the guide pin can show the exact spot around the isthmic part of the pedicle . thus , operator can adjust the starting point and aiming spot with zero violation of the medial wall of the pedicle . kim et al.1011 ) reported a series in which 8000 screws had been placed by the free - hand technique without neurovascular complications , and only 8% had significant breaches . the free - hand technique relies on anatomy and the tactile feedback for pedicle screw insertion . we could identify the desired starting point and aiming spot using this specially designed guide - pin . thus , we feel this novel guide - pin provides additional clinically relevant information and is helpful to the spinal surgeon to undertake pedicle screw fixation using the free - hand technique even more safely . the weak point of this study was that confirmation of screw placement could only be performed by postoperative cat scan and not by direct inspection in real patients . previous studies reported accuracy rates of 68% to 87% in the ct evaluation of pedicle screw placement in cadaver scans using titanium versus cobalt chrome screws24 ) . a simple , specially designed guide - pin with portable x - rays can provide correct starting and aiming points to place the accurate pedicle screw without preoperative ct scan and intraoperative fluoroscopic assistance . the placement of thoracolumbar pedicle screws using this guide - pin is an accurate , reliable , and safe without increased radiation or surgical time .