atlantoaxial instability ( aai ) may be caused by trauma , arthritis , tumor , infection , or congenital malformation316 ) .
atlantoaxial stabilization resulted from bony fusion is needed to correct deformities and prevent neurological impairment .
the surgical method of c1 - 2 transarticular screw fixation was introduced by margerl and has been the main technique for wire fusion of c1 and c2 vertebrae358 ) .
however , fusion rates may be altered by the depth of the vertebral artery ( va ) groove of the lateral mass and the width of the c2 pedicle1 ) .
additionally , screw fixation might be challenging due to the high risk of injury to the va , which can lead to massive bleeding and cerebral infarction1620 ) .
therefore , goel and laheri introduced c1 lateral mass and c2 pedicle screw fixation ( c1lm - c2p ) , which was popularized by harms and melcher , to reduce the risk of injury to the va56101116 ) .
furthermore , in the inevitable situation of injury to the va or in the case of unfavorable bone structures , c1lm - c2p can not be performed using the bilateral method .
therefore , the aim of this study is to compare the fusion rates of the unilateral and bilateral methods using the c1lm - c2p technique .
between march 2009 and december 2013 , 25 consecutive patients in our institution underwent c1lm - c2p to address aai due to rheumatoid arthritis . among those patients ,
preoperative radiologic imaging studies were performed to obtain accurate information on the anatomy of the atlas and axis .
cervical x - rays , including hyperextension and hyperflexion views , were obtained to confirm the instability of the c1 - 2 complex . the atlanto - dental interval ( adi )
was examined to determine the instability status at the preoperative ; immediate postoperative ; and 1 , 3 , 6 , and 12-month postoperative periods . moreover
, computed tomography ( ct ) scans with 1-mm slices were performed to examine the potential insertion pathway of the c1 lateral mass screw and c2 pedicle screw , and ct angiography in the area of c1 - 2 was used to visualize the pathway of the va for examining potential anomalies and reducing the possible complication of va injury . magnetic resonance ( mr )
posterior c1lm - c2p screw fixation , the surgical technique of harms and melcher6 ) , was performed in all patients in this study group .
after discharge , patients reported to an outpatient clinic at postoperative months 1 , 3 , 6 , and 12 to measure adis as the index of bony fusion .
bony fusion was defined as a bony bridge which was seen at the c1 - 2 inter - spinous space on lateral radiograph images , no difference of adi between the immediate postoperative period and the 12 month time point , and no instrument failure during the follow - up period21 ) .
after wide exposure of the c1 - 2 posterior structures , the starting point in the c1 lateral mass was formed with a high speed drill at the connection point of the posterior arch and the lateral mass . after tapping the drilled pathway , screws with a diameter between 3.5 mm and 4.0 mm
were inserted into the lateral mass . before marking the entry point of the c2 screw ,
the medial border of the c2 pedicle was identified to prevent screw malposition in the spinal canal .
after identification of the transitional corner , which is the more cephalad portion of the lamina and c2 isthmus , an entry point was made 4 mm lateral and 4 mm caudal to the transitional corner using a drill .
the direction was 20 to 30 medial and cephalad , with palpation of the medial border of the c2 pars interarticularis .
after drilling and tapering , a polyaxial screw with the proper length and diameter was inserted .
an interlaminar spacer and wiring between c1 and c2 were tightly fastened , and the rod was applied to the screw head .
preoperative and postoperative adis were measured on lateral plain radiographs using a picture archiving and communication system ( pacs ) and then compared side - by - side to evaluate the time of bony fusion and differences between unilateral and bilateral screw fixation .
statistical analyses were performed with spss version 18.0 ( spss , chicago , il , usa ) .
non - parametric analysis was performed in the both unilateral and bilateral screw fixation groups .
the mann - whitney u test was used to compare the bony fusion rate between the unilateral and bilateral screw fixation methods .
after wide exposure of the c1 - 2 posterior structures , the starting point in the c1 lateral mass was formed with a high speed drill at the connection point of the posterior arch and the lateral mass . after tapping the drilled pathway , screws with a diameter between 3.5 mm and 4.0 mm
were inserted into the lateral mass . before marking the entry point of the c2 screw
, the medial border of the c2 pedicle was identified to prevent screw malposition in the spinal canal .
after identification of the transitional corner , which is the more cephalad portion of the lamina and c2 isthmus , an entry point was made 4 mm lateral and 4 mm caudal to the transitional corner using a drill .
the direction was 20 to 30 medial and cephalad , with palpation of the medial border of the c2 pars interarticularis .
after drilling and tapering , a polyaxial screw with the proper length and diameter was inserted .
an interlaminar spacer and wiring between c1 and c2 were tightly fastened , and the rod was applied to the screw head .
preoperative and postoperative adis were measured on lateral plain radiographs using a picture archiving and communication system ( pacs ) and then compared side - by - side to evaluate the time of bony fusion and differences between unilateral and bilateral screw fixation .
statistical analyses were performed with spss version 18.0 ( spss , chicago , il , usa ) .
non - parametric analysis was performed in the both unilateral and bilateral screw fixation groups .
the mann - whitney u test was used to compare the bony fusion rate between the unilateral and bilateral screw fixation methods .
unilateral c1 lateral mass and c2 pedicle screw fixation was performed in 11 patients ( 44% ) in this study ( fig .
1a ) . the mean age of the unilateral screw fixation group was 53.1 years ( range 30 - 74 ) with female predominance ( 90.9% ) .
an anomalous course of the va was the main reason for unilateral screw fixation , which was required in eight patients .
meanwhile , in the bilateral screw fixation group , the mean age was 55.9 years ( range 40 - 72 ) with a female predominance ( 85.7% ) ( table 1 , fig .
one showed va stenosis compressed by the pedicle screw on digital subtraction angiography , but no other complications , such as neurologic impairment , massive bleeding , or infection , were found on further evaluation . in
the 11 patients in the unilateral screw fixation group , the preoperative mean adi was 8.14 mm , and the mean immediate postoperative adi was reduced to 1.82 mm .
moreover , measurements of mean adi at 1 , 3 , 6 , and 12 months postoperatively were 1.99 mm , 1.99 mm , 1.92 mm , and 1.91 mm , respectively . on the other hand , in the 14 patients in the bilateral screw fixation group , the mean preoperative adi was 8.48 mm , and that at postoperative 1 , 3 , 6 , and 12 months was 1.99 mm , 1.96 mm , 1.91 mm , and 1.90 mm , respectively ( table 1 ) .
the postoperative adi values at the immediate versus 1 month postoperative time points were not statistically different ( unilateral group , p=0.241 and bilateral group , p=0.520 ) .
similar results were found in the immediate versus 6 month time points unilateral group , p=0.859 and bilateral group , ( p=0.052 ) .
moreover , when comparing the whole follow - up period of 12 months for both techniques , no meaningful postoperative changes in adis were found , and atlantoaxial fusion was seen in all patients included in the study ( 100% ) . according to the mann - whitney test , there was no statistically significant difference between the unilateral and bilateral screw fixation groups ( p=0.893 ) ( table 2 ) . when evaluating bony fusion at 12 months , instrument - related failure was not noted in any case .
bony bridge between the interlaminar space of c1 - 2 was demonstrated on ct scans in all patients .
differences in adi between the immediate postoperative and 12 month time points were not statistically significant .
chronic nuchal pain with or without neurological deficits may be encountered , and improperly treated upper cervical cord injury may trigger severe neurologic impairment .
the surgical method of c1 - 2 complex fixation has been introduced by many surgeons , and bilateral transarticular screw fixation using the wiring technique of gallie or brooks - jenkins has achieved the highest stability and stiffness2712 ) . however , bilateral transarticular screw fixation ( taf ) is not always possible because of va anomalies or fracture of the c1 - 2 complex1314 ) .
therefore , stability achieved through bilateral taf has been compared with that of unilateral taf in a previous study .
hue et al.7 ) mentioned that there were no differences in fusion rate as measured by postoperative adi change between unilateral and bilateral taf . to overcome the major complication of vascular and neural injuries ,
c1lm - c2p was introduced by goel and laheri in 1994 and modified by harms and melcher in 200156 ) .
the superiority of c1lm - c2p fixation compared to taf is a lower risk of va injury due to the use of individual screw systems .
the pathway of the screws in the taf method , which crosses c2 through c1 , is relatively complex and narrow compared with that of c1lm - c2p , which uses individual screw systems .
however , some problems have been encountered in bilateral screw placement including severe anomalous va pathways and restricted pathway for the 3.5 mm screw .
in other words , if the risk of injuring the va and spinal cord can be predicted preoperatively , abandoning screw placement in an ipsilateral side with anatomical variation may be a wise decision .
in the 25 patients enrolled in the present study , no massive bleeding or neurologic damage was detected .
vertebral canal violation without other complications was noted in one case of va stenosis caused by a c2 pedicle screw .
the risk of va injury can not be completely eliminated even with c1lm - c2p .
if preoperative ct angiography suggests a high risk of va injury , the screw may be inserted on the unilateral side only .
therefore , our study was designed to identify differences in bony fusion between unilateral and bilateral fixation .
when we compared the preoperative and postoperative adi measures , there was no statistically significant difference between unilateral and bilateral fixation ( p=0.893 ) .
this suggests that unilateral and bilateral c1lm - c2p produce the same bony fusion and reduction of aai .
in addition , the strength of fusion in the unilateral screw fixation groups was as strong as that in the bilateral screw fixation group .
we have come into conclusion with the unilateral c1lm - c2p showed as much strength as the bilateral c1lm - c2p .
likewise , the unilateral taf showed as much strength as the bilateral taf , which was mentioned by hue et al.7 ) comparison between the unilateral taf and unilateral c1lm - c2p should be made in the future study of aai .
however , intraoperative reduction and smaller surgical site exposure are the advantages of c1lm - c2p which is more selective in choosing surgical technique in aai9 ) .
several studies have been performed to reduce the complication rate when stabilizing the c1 - 2 complex .
anatomical variations of the va and c1 lateral mass deformation due to systemic diseases like rheumatoid arthritis may increase the risk of va injury .
also , as the surgical procedure is performed , the risk of injuring the va increases with every 2% when the screw is inserted3141620 ) . therefore , comprehensive understanding of the atlantoaxial and va anatomy by ct angiography can assist with successful surgical planning and reduce the complication rate related to the va .
hypoglossal nerve injury may occur because of the anatomical proximity to the anterior aspect of the lateral mass315 ) .
c2 dermatome anesthesia or pain may result in patients with c2 ganglion injury due to insufficient exposure of the c1 lateral mass and c1 - 2 joint34 ) .
such improvements in navigation systems and monitoring devices may allow for improved accuracy when determining screw trajectory pathways1316 ) .
however , these navigation systems have limitations with regard to calibration of three - dimensional ct scans with fluoroscopy - based images , inaccuracies caused by manipulating surgical instruments in the operation field , and motion of the operating field or craniocervical junction even when fixed with a head clamp16171819 ) . therefore , navigation systems require increased accuracy when operating on the c1 - 2 complex .
intraoperative fluoroscopy with abundant surgical experience can reduce complications in c1 - 2 surgical techniques16 ) .
if there is a risk of va damage , the choice to abandon predictable screw insertion will increase the safety and completeness of the surgical technique of c1lm - c2p screw fixation .
our study illustrated that unilateral screw fixation shows similar strength , bony fusion , and reduction rate as bilateral screw fixation .
even though we were able to demonstrate that unilateral screw fixation is similar to bilateral fixation in terms of strength , bony fusion , and reduction rate , several limitations of our study must be considered .
the existing limitations are a lack of retrospective study , small number of enrolled cases , and deficiency of the biomechanical study . due to our small study cases ,
further prospective study using a multi - center design may help increasing the reliability of our unilateral surgical technique .
furthermore , future biomechanical studies using animal or human cadavers might provide numeric evidence supporting the present study conclusion .
c1 lateral mass and c2 pedicle screw fixation can performed in patients with aai due to rheumatoid arthritis .
although bilateral fixation is the main treatment method for such cases , unilateral screw fixation can be performed if a neurovascular contraindication is predicted . in our study , by means of adi changes , unilateral fixation showed similar strength to the bilateral fixation with bone fusion .
radiologic images , such as ct angiography with cervical three dimensional ct and mr images , should be performed to reduce the complications of c1lm - c2p prior to surgery .
 objectivebilateral c1 lateral mass and c2 pedicle screw fixation ( c1lm - c2p ) is an ideal technique for correcting atlantoaxial instability ( aai ) . 
 however , the inevitable situation of vertebral artery injury or unfavorable bone structure may necessitate the use of unilateral c1lm - c2p . 
 this study compares the fusion rates of the c1 lateral mass and c2 pedicle screw in the unilateral and bilateral methods.methodsover five years , c1lm - c2p was performed in 25 patients with aai in our institute . 
 preoperative studies including cervical x - ray , three - dimensional computed tomography ( ct ) , ct angiogram , and magnetic resonance imaging were performed . to evaluate bony fusion , measurements of the atlanto - dental interval ( adi ) and 
 ct scans were performed in the preoperative period , immediate postoperative period , and postoperatively at 1 , 3 , 6 , and 12 months.resultsunilateral c1lm - c2p was performed in 11 patients ( 44% ) . the need to perform unilateral 
 c1lm - c2p was due to anomalous course of the vertebral artery in eight patients ( 73% ) and severe degenerative arthritis in three patients ( 27% ) . 
 the mean adi in the bilateral group was 2.09 mm in the immediate postoperative period and 1.75 mm in 12-months postoperatively . 
 the mean adi in the unilateral group was 1.82 mm in the immediate postoperative period and 1.91 mm in 12-months postoperatively . 
 comparison of adi measurements showed no significant differences in either group ( p=0.893 ) , and the fusion rate was 100% in both groups.conclusionalthough bilateral c1lm - c2p is effective for aai from a biomechanical perspective , unilateral screw fixation is a useful alternative in patients with anatomical variations . 
INTRODUCTION
MATERIALS AND METHODS
Operative technique
Statistical analysis
RESULTS
DISCUSSION
CONCLUSION