after decompression of spinal cord or nerve roots , interbody fusion should be performed for spinal stabilization .
there are several fusion materials which have been used for several decades with effectiveness and safety10 ) .
most classical fusion grafts are autologous iliac bone grafts and tricortical iliac crest bone block .
autologous bone has achieved favorable fusion , but it results in an additional wound of the harvest site with risk of morbidity2 ) .
another fusion material is polyetheretherketone ( peek ) cage and has been used with or without anterior cervical plate augmentation . without the anterior plate ,
higher subsidence rate has been reported12 ) . besides the peek cage , allograft or autograft acdf
many studies regarding the peek cage and autograft have been reported but there have been fewer studies about allograft in cervical spine3,14 ) .
allografting has been used for decades and the safety and effectiveness of allografts in acdf was reported11,20 ) .
in addition , we also studied the anterior screw insertion angle with two types of screws .
between november 2010 and june 2012 , a total of 55 patients who were diagnosed with degenerative cervical disc disease underwent acdf using allograft bone substitute and combined reinforcement with rigid plate system were selected .
exclusion criteria was less than 10 months follow up period , posterior instrumentation after acdf , and inability to measure radiologic parameters .
the patients were divided into 2 groups according to used screws because the mean screw insertion angle was significantly different between the two screws - fixed and variable types .
group a consisted of 13 patients who underwent anterior plate fixation with fixed type screws .
insertion angle was limited at 12 degrees ( atlantis ) and 8 degrees ( vectra ) in each direction from cephalad to caudal .
the mean age was 55.310.32 years ( range , 44 to 63 years ) in group a , and 52.610.32 years ( range , 44 to 63 years ) in group b. there were 9 males and 5 females in group a , and 9 males and 7 females in group b. the mean follow - up period was 14.576.09 months ( range , 12 to 34 months ) . there were 3 one - level fusions and 8 two - level fusions and 4 three - level fusions .
there were 2 segments of c3 - 4 fusion ( zero in group b ) , 7 segments of c4 - 5 fusion ( 6 in group b ) , 11 segments of c5 - 6 fusion ( 15 in group b ) , 7 segments of c6 - 7 fusion ( 7 in group b ) ( table 1 ) .
a single surgeon performed all operations with a standard smith - robinson anteromedial approach using a surgical microscope5,16 ) .
after discectomy and decompression of the neural component , the graft bone was inserted into the disc space during gentle distraction of vertebral bodies . the allograft was commercially used freeze - dried bone from a cadaver donor .
vertebral plating system was selected according to the fusion level , from single to maximally 3 levels .
regular follow up was provided immediately after surgery , at 1 month , 6 months , and 1 year after surgery , and the last follow - up .
flexion - extension lateral views were also obtained at 6 months and 1 year after surgery .
two independent neurosurgeons measured the following parameters with pacs digital software system(piviewstar , infinitt co. , ltd , seoul , korea ) .
segmental height , segmental lordotic angle and segmental interscrew angle in each time and change of segmental interspinous distance between flexion and extension in the last follow up dynamic radiography .
three dimensional computed tomography ( 3d ct ) was obtained at 1 year follow - up and compared with plain radiographs of bony fusion .
segmental height was measured on the radiographs , which was the mean value of anterior and posterior height of fusion segment ( fig .
the subsidence was defined as  a 2 mm reduction in the segmental height at 1 level fusion ,  a 4 mm reduction in the segmental height at 2 level fusion and  a 6 mm reduction in the segmental height at 3 level fusion between immediately at the last follow - up .
segmental lordosis was measured using cobb 's method to assess the sagittal alignment . also , the angle between the most upper and lower screws was also measured serially .
the segmental screw angle was defined as the measured value by cobb 's methods between the upper and lower screws .
nonunion was defined as the appearance of segmental instability with 2 mm widening of the interspinous distance on the flexion - extension lateral views at the last follow - up .
we used the bridwell 's fusion grading system in ct scan4 ) : grade i : complete fusion , grade ii : partial fusion , grade iii : unipolar pseudoarthrosis , grade iv : bipolar pseudoarthrosis .
between november 2010 and june 2012 , a total of 55 patients who were diagnosed with degenerative cervical disc disease underwent acdf using allograft bone substitute and combined reinforcement with rigid plate system were selected .
exclusion criteria was less than 10 months follow up period , posterior instrumentation after acdf , and inability to measure radiologic parameters .
the patients were divided into 2 groups according to used screws because the mean screw insertion angle was significantly different between the two screws - fixed and variable types .
group a consisted of 13 patients who underwent anterior plate fixation with fixed type screws .
insertion angle was limited at 12 degrees ( atlantis ) and 8 degrees ( vectra ) in each direction from cephalad to caudal .
the mean age was 55.310.32 years ( range , 44 to 63 years ) in group a , and 52.610.32 years ( range , 44 to 63 years ) in group b. there were 9 males and 5 females in group a , and 9 males and 7 females in group b. the mean follow - up period was 14.576.09 months ( range , 12 to 34 months ) . there were 3 one - level fusions and 8 two - level fusions and 4 three - level fusions .
there were 2 segments of c3 - 4 fusion ( zero in group b ) , 7 segments of c4 - 5 fusion ( 6 in group b ) , 11 segments of c5 - 6 fusion ( 15 in group b ) , 7 segments of c6 - 7 fusion ( 7 in group b ) ( table 1 ) .
a single surgeon performed all operations with a standard smith - robinson anteromedial approach using a surgical microscope5,16 ) .
after discectomy and decompression of the neural component , the graft bone was inserted into the disc space during gentle distraction of vertebral bodies . the allograft was commercially used freeze - dried bone from a cadaver donor .
vertebral plating system was selected according to the fusion level , from single to maximally 3 levels .
regular follow up was provided immediately after surgery , at 1 month , 6 months , and 1 year after surgery , and the last follow - up .
flexion - extension lateral views were also obtained at 6 months and 1 year after surgery .
two independent neurosurgeons measured the following parameters with pacs digital software system(piviewstar , infinitt co. , ltd , seoul , korea ) .
segmental height , segmental lordotic angle and segmental interscrew angle in each time and change of segmental interspinous distance between flexion and extension in the last follow up dynamic radiography .
three dimensional computed tomography ( 3d ct ) was obtained at 1 year follow - up and compared with plain radiographs of bony fusion .
segmental height was measured on the radiographs , which was the mean value of anterior and posterior height of fusion segment ( fig .
the subsidence was defined as  a 2 mm reduction in the segmental height at 1 level fusion ,  a 4 mm reduction in the segmental height at 2 level fusion and  a 6 mm reduction in the segmental height at 3 level fusion between immediately at the last follow - up .
segmental lordosis was measured using cobb 's method to assess the sagittal alignment . also , the angle between the most upper and lower screws was also measured serially .
the segmental screw angle was defined as the measured value by cobb 's methods between the upper and lower screws .
nonunion was defined as the appearance of segmental instability with 2 mm widening of the interspinous distance on the flexion - extension lateral views at the last follow - up .
we used the bridwell 's fusion grading system in ct scan4 ) : grade i : complete fusion , grade ii : partial fusion , grade iii : unipolar pseudoarthrosis , grade iv : bipolar pseudoarthrosis .
there was no statistically significant difference between groups a and b in demographic data ( table 1 ) .
fusion rates were 100% in group a and 75% in group b in our previous suggested definition , the interspinous distance of fusion segment less than 2 mm per level ( table 2 ) .
there were 41 fusion segments assessed by 3 dimensional ct scan ( table 3 ) .
complete fusion was achieved in 5 segments ( 29.4% ) of group a and 13 ( 54.1% ) of group b. grade ii fusion was seen in 2 segments ( 11.8% ) of group a and 4 segments ( 16.7% ) of group b. grade iii fusion , 4 segments ( 23.5% ) of group a and 3 segments ( 12.5% ) of group b. grade iv fusion , 6 segments ( 35.3% ) of group a and 4 segments ( 16.7% ) of group b. grade i fusion was more frequent in group b , but fisher 's exact test did not show statistically significance .
subsidence was found 2 cases ( 15.4% ) of group a and 3 cases ( 18.8% ) of group b and did not showed significant difference .
significant difference between the two groups was seen in the pre - operative segmental angle and immediate postoperative interscrew angle . between the preoperative and each follow - up period
, many parameters showed significant differences ( table 5 ) . in group a the segmental angle was increased after fusion surgery and slightly decreased at the 1 year follow up period , but was not significant in later statistical analysis .
segmental height was not changed after surgery , but decreased at 1 year follow up .
b , there was no difference in segmental angles , but the segmental height was increased after surgery then decreased at 1 year follow up ( fig .
mean value of interscrew angle was decreased more in group b , but there was no significant difference between two groups .
dense radiolucent line was identified around the allograft block , however intra graft bony bridge formation was seen .
also , the interspinous distance change did not exceed 2 mm during flexion and extension .
no graft malposition , migration , or mechanical failure of instruments was observed , and there was no revision surgery .
no graft malposition , migration , or mechanical failure of instruments was observed , and there was no revision surgery .
acdf is the most favorable and familiar method for treatment of cervical degenerative diseases , and also for trauma .
myelopathy or radiculopathy is treated with decompression of neural elemetns , and osseous fusion is established to stabilize the cervical spine .
there have been many studies about several fusion materials and plate augmentation , but they are still controversial .
achievement of fusion without associated instrument complication may be the most favorable result in radiologic assessment , and this may also be correlated with clinical outcome .
non - union or mechanical failure of instrument causes pain or neurologic symptoms , and rarely dysfunction and injury of the esophagus or prevertebral tissues .
sometimes these complications may be treated by revision surgery which also has its own surgical risks . to assess the result of allograft fusion
, we studied the radiologic analysis and also insertion orientation of anterior screws which are mandatory to bone graft fusion .
allograft insertion causes distraction of the vertebral body because using small and loose grafts for fusion has a high risk of graft migration or malposition .
segmental height increased after surgery and also lordotic angulation increased because of lordotic curvature of the allograft and anterior fixation of plate and screw .
however , in the more angled screw group , postoperative segmental lordotic angle was slightly decreased and it was caused by pulling of the vertebral body by the screw insertion .
loosening of screws and retropulsing movement is not a common complication of cervical acdf with plate augmentation , but it may be associated with soft tissue injury and non - union . in our study , the angled screw group showed slightly decreased segmental angles after operation .
this may be explained by compression of vertebral bodies toward the screw and plate . according to our results ,
the screw angle or type does not have large role in fusion or subsidence . to maximize pull out strength ,
screws may be inserted with 90 degree angle to the plate and the longest size selected .
experimental ex - vivo animal or cadaver models and finite element analysis may be needed for evidence which support our results .
plain radiography is a simple exam which is cheap and with less radiation exposure than ct scans .
dynamic view of the cervical spine provides segmental stability and additional information about adjacent segment degeneration .
the most exact assessment is the 3-dimensional ct scan , but this is not always checked during follow up . in our study ,
a halo like area was seen on lateral plain radiographs , but there was a bony bridge through the graft cavity , and our 86.2% fusion rate is not significantly different from other reports13 ) .
many studies reported cage subsidence after fusion surgery and the subsidence rate varied between 43.1 - 50.5% by oh et al , and 23.4% in the study of yamagata et al.12,19 ) . the risk factors associated with subsidence have been already reported .
obesity , bone mineral density , and smoking are nonsurgical risk factors , while anteroposterior diameter of cage , and intraoperative distraction are surgical risk factors21 ) .
however subsidence does not always result in poor prognosis or aggravation of symptoms , and mostly does not cause symptoms or morbidity15,18,20 ) .
fusion may be achieved with subsidence of cage and nonunion may occur without subsidence14 ) .
medical conditions such as osteoporosis and obesity also affect nonunion or subsidence . repeated neck motion after surgery also affects cage subsidence . during the initial one or two months after surgery ,
recently , recombinant human bone morphogenic protein-2 ( rhbmp-2 ) has been used in acdf .
it still remains controversial with regard to safety in spine surgery , but additional use of rhbmp-2 may promote bone fusion after surgery7,17 ) .
another weak point of the allograft is the absence of an anchoring structure , such as metallic spikes in the peek cage .
increasing compressive force between fusion segments may be helpful for fusion and prevent migration of the allograft , but it also increases the risk of allograft breaks .
the relatively short follow up period in this study restricts evaluation of adjacent segment degeneration development .
the retrospective nature of this review is another limitation of this study . to evaluate further information about allografts
, further study may be needed with prospective , larger series and longer follow up period .
genetic study or medical status which affects bone fusion may be added in further study .
acdf with allograft bone block and plate augmentation achieves favorable radiologic results , which is not inferior to other fusion materials .
also , the type or angle of screw fixation does not affect the fusion rate but may be associated with subsidence or decrease of segmental height .
 objectiveto evaluate radiologic result of anterior cervical discectomy and fusion with allobone graft and plate augmentation , and the change of radiologic outcome between screw type and insertion angle.methodsretrospective review of clinical and radiological data of 29 patients . 
 segmental angle , height and screw angles were measured and followed . 
 the fusion rate was assessed by plain radiography and ct scans . 
 we divided the patients into two groups according to screw type and angles . 
 group a : fixed screw , group b : variable screw . 
 interscrew angle was measured between most upper and lower screws with cobb 's methods.resultsoverall fusion rate was 86.2% on plain radiography . 
 fusion was also assessed by ct scan and bridwell 's grading system . 
 there was no difference in fusion and subsidence rates between two groups . 
 subsidence was found in 5 patients ( 17.2% ) . 
 segmental lordotic angle was increased from preoperative status and maximized at the immediate postoperative period and then reduced at 1 year follow up . 
 segmental height showed similar increase and decrease values.conclusionacdf with allograft and plate showed favorable fusion rates , and the screw type and angle did not affect results of surgery . 
INTRODUCTION
MATERIALS AND METHODS
Patient Selection
Surgical Procedure
Radiographic Assessment
RESULTS
Complications
DISCUSSION
CONCLUSION