upper cervical spine instrumentation is always challenging for spine surgeons due to the highly variable pedicle anatomy and vertebral artery anomalies .
particularly , c1 - 2 fixation techniques have been developed in order to overcome those problems , and c1 lateral mass - c2 pedicle screw fixation has been increasingly used since it was introduced in 1994 by goel and laheri3 ) and modified in 2001 by harms and melcher5 ) .
many published reports describe the technique 's superiority to transarticular c1 - 2 fixation and several posterior fusion methods in biomechanics and feasibility when vascular and bony anomalies exist , although the procedure is still challenging and its accuracy does not provide any certainty to surgeons . because it is a free - hand technique , abumi 's method and the fluoroscopy - guided technique have their own limitations in terms of reliability9 ) , so navigation systems have been recently introduced to aid in these procedures . among them , the computed tomographic ( ct)-based navigation system was recently revealed to have excellent results : over 89% accuracy reported , even in the thoracolumbar spine2 ) and an 85% to 97% accuracy rate in lower cervical spine instrumentation , as reported in previous articles12,13 ) .
however , the accuracy rate for the ct - based navigation system at the upper cervical spine level , especially in c1 and c2 instrumentation , has not been reported yet .
we have used the ct - based navigation system in several cervical instrumentation surgeries , and report its safety and efficiency through the comparison of previous reports .
this retrospective study included 25 patients who underwent upper cervical instrumentation , including c1 lateral mass - c2 pedicle screw fixation , between may 2005 and march 2014 .
the patients underwent the usual posterior approach and instrumentation using ct - based navigation systems ( steal - station ; sofamor - danek , memphis , tn , usa in cases 1 to 4 ; and navigation cart ii ; stryker , kalamazoo , mi , usa in the remaining cases ) by one surgeon . among them , 18 patients who has postoperative ct scans participated in this study and the accuracy of the screw positions at c1 and c2 only was analyzed by a third observer .
the usual posterior approaches were used and ultimately , two occipito - cervical fusions , 13 c1 lateral mass - c2 pedicle screw fixations , and three c2 - 3 - 4 posterior instrumentation were performed ( table 1 ) .
the laminectomies were performed in neurologically compromised patients , and a ct - based navigation system and fluoroscopy were used during the pedicle screw instrumentation procedure ( vertex ; sofamor - danek , memphis , tn , usa in cases 1 to 11 and synapse system ; synthes , west chester , pa , usa in the remaining cases ) .
the precise position of the screw was evaluated by a postoperative computed tomographic scan and categorized into one of three groups : absolute in - pedicle , non - critical breach ( pedicle violation < 2 mm ) , and critical breach ( pedicle violation > 2 mm ) . in the prone position ,
a usual mid - line incision was performed and the reference device was secured at the c2 spinous process after exposure .
navigation procedures included acquiring a preoperative ct image , activating a patient tracker , adjusting the camera angle , registration , tool calibration , and simulation of screw placement . during the registration process
, both a point - to - point registration and surface - matching process were used to reduce the mean deviation , and simulation of the screw trajectories was acquired on the navigation monitor ( fig .
the c1 trajectory was changed from a lateral mass under the c1 posterior arch ( cases 1 to 11 ) to modification of the pedicle screw with a partial removal of the lower c1 arch ( remaining cases ) ( fig .
in the prone position , a usual mid - line incision was performed and the reference device was secured at the c2 spinous process after exposure .
navigation procedures included acquiring a preoperative ct image , activating a patient tracker , adjusting the camera angle , registration , tool calibration , and simulation of screw placement . during the registration process
, both a point - to - point registration and surface - matching process were used to reduce the mean deviation , and simulation of the screw trajectories was acquired on the navigation monitor ( fig .
the c1 trajectory was changed from a lateral mass under the c1 posterior arch ( cases 1 to 11 ) to modification of the pedicle screw with a partial removal of the lower c1 arch ( remaining cases ) ( fig .
a total of 58 screws , including 26 c1 screws and 32 c2 screws were analyzed .
postoperatively , the precise positions of the c1 lateral mass - c2 pedicle screws were 81.1% ( 47/58 ) ; 8.6% ( 5/58 ) of the screws were in a non - critical breach position and 10.3% ( 6/58 ) of the screws were critical breaches .
most ( 5/6 ) of the critical breaches and all of non - critical breaches were observed at the c2 pedicle screws , and only one case of a c1 lateral mass screw showed a critical breach ( p=0.033 , kruskal - wallis test ) .
all cases of breach occurred in traumatic instances , except for one case of degenerative spine disease , which resulted in a c2 critical breach ( p=0.091 , mann - whitney u test ) .
there were three complications ( two vertebral artery occlusions and a deep wound infection ) , but no postoperative instrument - related neurological deteriorations were seen , even in the critical breach cases . in case 1 and case 15 ,
the right c2 pedicle screw protruded over 50% into the vertebral foramen , even though correct navigation procedures were performed .
the postoperative ct angiograms showed that retrograde flow from the opposite vertebral artery covered the distal area ( fig .
one patient ( case 14 ) had a deep wound infection postoperatively and wound revision with massive irrigation was performed in response .
according to the 2000 cadaveric study of ludwig et al.10 ) , the accuracy rates of c3 - 7 pedicle screws using the ct - based navigation system are 76% ( in pedicle ) and 13.4% ( non - critical breach ) .
a 2005 clinical study by rampersaud et al.12 ) reported an 85% accuracy rate in pedicles ; and a 13.1% rate of non - critical breaches and 2.2% occurrence of critical breaches in the thoracolumbar spine . in 2012 , gelalis et al.2 ) published a systemic review comparing the free - hand technique , and found that the percentage of the screws fully contained in the pedicle ranged from 69 to 94% overall ; those that used fluoroscopy ranged from 28 to 85% , while those using ct navigation ranged from 89 to 100% and those using fluoroscopy - based navigation ranged from 81 to 92% .
the authors believe that the present study is the first article regarding c1 - 2 fixation using a ct - based navigation system . in the present study , an 81.1% accurate placement in the pedicle
was seen , in addition to an 8.6% rate of non - critical breaches and a 10.3% rate of critical breaches .
those results were not highly satisfactory to the authors , but the c1 - 2 junction has high mobility , which makes precise stabilization inherently problematic as compared with the lower cervical or thoracolumbar spines .
additionally , seven cases were excluded because their screw positions were correct on the postoperative x - ray so ct scans were not taken .
the accuracy rate would be higher than reported here if they were involved in the study .
another clinical study by richter et al.13 ) reported a 97% accuracy rate using a navigation system versus a 91.4% accurate rate using the conventional method for c3 - 6 pedicle screw fixation . a recent article regarding c1 lateral mass - c2 pedicle screw fixation reported a 95.8% correct screw placement14 ) , but the researchers used isocentric c - arm 3-dimensional navigation , which is a more upgraded system than ours .
the possible reasons of the relatively higher rate of breach at c2 than c1 in the present study are described below .
one reason is that most of the cases were c2 fractures , so relative instability occurred , and it could cause error and change between the preoperative ct scan and intraoperative position of the patient .
another reason may be that the reference device , which was secured to c2 spinous process , could have been touched and therefore moved , unintentionally during the procedure . finally , the authors recommend that more precision be used , both during the point - to - point registration and the surface - merging during the navigation procedure .
a modified c1 trajectory could be helpful to reduce intraoperative bleeding , postoperative occipital neuralgia , and operation time .
although a ct - based navigation system can allow surgeons to perform more precise procedures at the lower cervical and thoracolumbar spine , it still has some problems in the upper cervical spine , due to the region 's highly variable anatomy . even though there were no catastrophic complications , more experiences
 objectiveto identify the accuracy and efficiency of the computed tomographic ( ct)-based navigation system on upper cervical instrumentation , particularly c1 lateral mass and c2 pedicle screw fixation compared to previous reports.methodsbetween may 2005 and march 2014 , 25 patients underwent upper cervical instrumentation via a ct - based navigation system . 
 seven patients were excluded , while 18 patients were involved . 
 there were 13 males and five females ; resulting in four degenerative cervical diseases and 14 trauma cases . 
 a ct - based navigation system and lateral fluoroscopy were used during the screw instrumentation procedure . among the 58 screws inserted as c1 - 2 screws 
 fixation , their precise positions were evaluated by postoperative ct scans and classified into three categories : in - pedicle , non - critical breach , and critical breach.resultspostoperatively , the precise positions of the c1 - 2 screws fixation were 81.1% ( 47/58 ) , and 8.6% ( 5/58 ) were of non - critical breach , while 10.3% ( 6/58 ) were of critical breach . 
 most ( 5/6 , 83.3% ) of the critical breaches and all of non - critical breaches were observed in the c2 pedicle screws and there was only one case of a critical breach among the c1 lateral mass screws . 
 there were three complications ( two vertebral artery occlusions and a deep wound infection ) , but no postoperative instrument - related neurological deteriorations were seen , even in the critical breach cases.conclusionalthough ct - based navigation systems can result in a more precise procedure , there are still some problems at the upper cervical spine levels , where the anatomy is highly variable . even though there were no catastrophic complications , more experience 
 are needed for safer procedure . 