since the introduction of anterior cervical discectomy and fusion acdf in 19584 ) , the procedure has become the gold - standard operation for single or multiple level degenerative cervical spondylosis , especially for the patients with herniated intervertebral cervical discs9,12 ) . earlier , iliac bone graft was used after removal of cervical intervertebral discs . with the advent of iliac bone graft , cervical cages were introduced to avoid donor site complications and reduce patient pain . more recently and with extensive surgeon - mediated modifications combined with technological refinements , the anterior cervical plate cage technique has gained global popularity . advantages of this technique include improved sagittal alignment , prevention of interbody graft dislocation , and increased interbody fusion rate and stability8,14,16 ) . however , the application of anterior cervical plate may face with some complications ; increased incidence of dysphagia , tracheo - esophageal injuries , and adjacent level degeneration2,7,13,18 ) . to lessen potential complications while maintaining the benefits of anterior cervical plating , the zero - profile anchored spacer ( zero - p ; synthes gmbh , oberdorf , switzerland ) was invented for use in patients with degenerative cervical spondylosis ( fig . the present study analyzed clinical outcomes and radiological findings after application of the zero - p device and the conventional cage - plate ( ccp ) device for single level acdf , and sought to compare the incidence and difference of postoperative dysphagia for both devices . this retrospective and comparative study of clinical outcomes initially involved 92 patients who received single level acdf using the ccp or zero - p device . of these , 19 patients were excluded because of identified use of a different procedure such as combined acdf with circumferential fusion and cage - only acdf . another 25 patients were excluded since surgery involved two or more levels ( adjacent or distant ) . thus , 48 patients were included in this study ( 30 men , 18 women ; mean age 52.3 years , range , 36 - 78 years ) . all patients suffered from typical symptoms , such as posterior neck pain , radiating pain , and limb numbness . these patients were divided into two groups based on the device used : the ccp group comprised 27 patients and the zero - p group comprised 21 patients . most of the patients were followed - up around 6 months post - operatively ; thus , this study focused on short - term results . after routine skin preparation and drape , a skin incision was made with a left side approach . gently dissection identified the platysma , which was divided in line with the skin incision using a monopolar coagulator . the sternocleidomastoid was retracted laterally to allow palpation for deep structures of neck and extension of exposure . after confirmation of operation levels , casper distractor ( aesculap , burlingame , ca , usa ) blade was placed beneath the elevated longus colli . after trial spacers were used to confirm the suitable polyetheretherketone ( peek ) spacer , a zero - p implant filled with allograft material was inserted at the operation level . fluoroscopy at the antero - posterior and lateral view was used to establish the best position for the zero - p spacer . the first pilot hole was drilled with an aiming device and a one - step first locking screw was fixed through the first pilot hole with a torque limiter ( 1.2 nm ) . the locking screws formed a bone wedge with a 405 cranial / caudal angle and 2.5 medial / lateral angle . lengths of 12 , 14 , and 16 mm were assessed ; 16 mm screws provided the best suitable length in most patients . a plain film image of a representative successful procedure is shown in fig . a peek cage with a zephir anterior cervical plate ( medtronic spine , minneapolis , mn , usa ) was used as the ccp system . the length varied from 22.5 mm to 70 mm with a 2.5 mm differential , with the suitable length determined on a case - by - case basis . all cases were routinely examined on the admission date or pre - operative work - up period for the operation with cervical spine plain film with antero - posterior , lateral , and oblique views , with excessive flexion and extension series , computed tomography ( ct ) and magnetic resonance imaging ( mri ) used for analysis of radiologic findings . follow - up was 6 months on an outpatient basis ; plain film examination , ct scan , and mri were also performed . prevertebral soft tissue thickness was assessed during the pre - operative work - up period , the day of surgery , and 2 weeks and 6 months post - operatively . using the lateral images of cervical plain film , prevertebral soft tissue thickness was calculated as the mean value of the distance between the anterior surface of each vertebral body or plate and the air shadow of the airway ( fig . these data were subtracted from the pre - operative data to identify the difference between the ccp and zero - p groups . all radiological image analyses were performed three separate times by an independent person to ensure accuracy . clinical outcomes were evaluated pre - operatively on the date of admission and post - operatively 6 months after surgery with the modified japanese orthopaedic score ( mjoa ) proposed by benzel3 ) . . recovery rate ( rr , % ) was also calculated as [ ( postoperative mjoa - preoperative mjoa)/(18-preoperative mjoa)]1006 ) . the operation time and estimated blood loss ( ebl ) chi - square test and the fisher 's exact test were performed for qualitative analyses . this retrospective and comparative study of clinical outcomes initially involved 92 patients who received single level acdf using the ccp or zero - p device . of these , 19 patients were excluded because of identified use of a different procedure such as combined acdf with circumferential fusion and cage - only acdf . another 25 patients were excluded since surgery involved two or more levels ( adjacent or distant ) . thus , 48 patients were included in this study ( 30 men , 18 women ; mean age 52.3 years , range , 36 - 78 years ) . all patients suffered from typical symptoms , such as posterior neck pain , radiating pain , and limb numbness . these patients were divided into two groups based on the device used : the ccp group comprised 27 patients and the zero - p group comprised 21 patients . most of the patients were followed - up around 6 months post - operatively ; thus , this study focused on short - term results . after routine skin preparation and drape , a skin incision was made with a left side approach . gently dissection identified the platysma , which was divided in line with the skin incision using a monopolar coagulator . the sternocleidomastoid was retracted laterally to allow palpation for deep structures of neck and extension of exposure . after confirmation of operation levels , casper distractor ( aesculap , burlingame , ca , usa ) blade was placed beneath the elevated longus colli . after trial spacers were used to confirm the suitable polyetheretherketone ( peek ) spacer , a zero - p implant filled with allograft material was inserted at the operation level . fluoroscopy at the antero - posterior and lateral view was used to establish the best position for the zero - p spacer . the first pilot hole was drilled with an aiming device and a one - step first locking screw was fixed through the first pilot hole with a torque limiter ( 1.2 nm ) . the locking screws formed a bone wedge with a 405 cranial / caudal angle and 2.5 medial / lateral angle . lengths of 12 , 14 , and 16 mm were assessed ; 16 mm screws provided the best suitable length in most patients . a plain film image of a representative successful procedure is shown in fig . a peek cage with a zephir anterior cervical plate ( medtronic spine , minneapolis , mn , usa ) was used as the ccp system . the length varied from 22.5 mm to 70 mm with a 2.5 mm differential , with the suitable length determined on a case - by - case basis . all cases were routinely examined on the admission date or pre - operative work - up period for the operation with cervical spine plain film with antero - posterior , lateral , and oblique views , with excessive flexion and extension series , computed tomography ( ct ) and magnetic resonance imaging ( mri ) used for analysis of radiologic findings . follow - up was 6 months on an outpatient basis ; plain film examination , ct scan , and mri were also performed . prevertebral soft tissue thickness was assessed during the pre - operative work - up period , the day of surgery , and 2 weeks and 6 months post - operatively . using the lateral images of cervical plain film , prevertebral soft tissue thickness was calculated as the mean value of the distance between the anterior surface of each vertebral body or plate and the air shadow of the airway ( fig . these data were subtracted from the pre - operative data to identify the difference between the ccp and zero - p groups . all radiological image analyses were performed three separate times by an independent person to ensure accuracy . clinical outcomes were evaluated pre - operatively on the date of admission and post - operatively 6 months after surgery with the modified japanese orthopaedic score ( mjoa ) proposed by benzel3 ) . recovery rate ( rr , % ) was also calculated as [ ( postoperative mjoa - preoperative mjoa)/(18-preoperative mjoa)]1006 ) . the operation time and estimated blood loss ( ebl ) chi - square test and the fisher 's exact test were performed for qualitative analyses . the location of surgery was c3 - 4 ( ccp , n=2 ; zero - p , n=3 ) , c4 - 5 ( ccp , n=6 ; zero - p , n=4 ) , c5 - 6 ( ccp , n=14 ; zero - p , n= 10 ) , and c6 - 7 ( ccp , n=5 ; zero - p , n=4 ) ( table 1 - 2 ) . the ccp group had a mean ebl of 146.5138 cc and an average operation time was 147.448.4 minutes . pre- and postoperative mjoa score of the ccp group was 14.51.2 and 16.70.8 , respectively . achieved mjoa score and recovery rate of mjoa score was 2.240.8% and 64.118.1% , respectively . the zero - p group had a mean ebl of 90148 cc and an average operation time was 159.552.1 minutes . pre- and postoperative mjoa score of the zero - p group was 14.71.3 and 17.00.8 , respectively . ac - hieved mjoa score and recovery rate of mjoa score was 2.270.8% and 72.119.0% , respectively . all clinical outcomes were statistically similar between the groups , except for ebl ( p=0.029 ) . concerning prevertebral soft tissue thickness , the pre - operative , immediate post - operative , 2 weeks post - operative , 6 months post - operative value was 13.51.9 , 15.62.2 , 16.82.4 and 15.21.9 mm in the ccp group , respectively , and 14.32.7 , 15.12.6 , 15.72.4 , and 14.52.6 mm in the zero - p group , respectively . the calculated subtraction was 2.11.6 mm as subtraction 1 , 3.31.4 mm as subtraction 2 , and 1.71.1 mm as subtraction 3 in the ccp group , and 0.80.7 mm as subtraction 1 , 1.40.5 as subtraction 2 , and 0.10.4 as subtraction 3 in the zero - p group . although the initial prevertebral soft tissue thicknesses of the both groups were statistically the same , their subtractions were statistically different ( table 2 ) . post - operative dysphagia was determined on the day of operation , 2 weeks post - operatively , and 6 months post - operatively date . in the ccp group , 18 patients immediately complained of dysphagia on the operation day , 13 patients had dysphagia 2 weeks post - operatively , and 6 patients still suffered from dysphagia 6 months after the operation . in the zero - p group , 13 patients immediately complained of dysphagia on the operation day , 3 patients had dysphagia 2 weeks post - operatively , and no patient had dysphagia 6 months after the operation ( table 3 ) . chronic dysphagia is a common complication of acdf ; authors have reported this complication with a wide variability from 3% up to 21%2,7,10,15 ) . post - operative soft tissue swelling , hematoma at the operative site , esophageal injury , and adhesion formation around implanted cervical plates may be possible reasons for dysphagia5 ) . dysphagia has been related to the thickness of the anterior plate at the fusion level10 ) . the zero - p device has a smaller volume than the ccp device , which allows a smaller incision . the smaller incision allows more limited resection , less exposure , and avoids mechanical stimulus to the related structures . in this study , we focused on the post - operative radiologic changes of prevertebral soft tissue swelling after acdf with both devices . we hypothesized that this swelling would be comparatively mild in the zero - p group . analyzing the radiologic finding of cervical plain films , the zero - p device definitely had an advantage of mild post - operative prevertebral soft tissue swelling ( p<0.001 ) . we also confirmed that the incidence of chronic dysphagia was low in the zero - p group ( 0% ) compared to the 22.2% in the ccp group . pre - viously described above , other studies have reported an incidence of chronic dysphagia of 20% after acdf . thus , we conclude that application of the zero - p assures mild post - operative prevertebral soft tissue swelling and less dysphagia after acdf . to date , many authors also have identified that application of the zero - p spacer can provide favorable clinical outcomes after acdf . wang et al.17 ) reported a retrospective study that both the zero - p spacer and traditional titanium plate with cage were effective for single level cervical spondylotic myelopathy , but that the zero - p spacer was associated with less dysphagia . the authors also conjectured that application of the zero - p spacer might reduce adjacent level spondylosis since these devices conveyed less stress to adjacent level . li et al.11 ) studied a prospective and randomized clinical trial compared the clinical outcomes between the zero - p and ccp devices . the zero - p spacer was superior to the ccp device in reducing the post - operative speed of degeneration . the authors also analyzed post - operative dysphagia using the dysphagia score2 ) at 1 , 3 , 6 , and 12 months after surgery . dysphagia scores were low in the zero - p group at every follow - up . however , no statistically differences were evident between the groups at 12 months follow - up , while 4 patients still suffered from mild dysphagia in the ccp group . barbagallo et al.1 ) prospectively analyzed a series of patients treated with single- or multi - level acdf with the zero - p spacer for up to 4 years after surgery . they concluded that the zero - p spacer was judged safe and efficient , even in multilevel acdf as focused on radiological outcomes , complications , and technical hints . presently , there was a complication with screw loosening in a zero - p spacer used in . 51-year - old patient who suffered from typical symptoms of cervical intervertebral disc herniation including neck pain and radiating pain . however , he complained of radiating pain on left c6 dermatome 1 month after the operation . radiologic findings revealed loosening and nonunion of the left upper screw from the vertebral body ( fig . however , the patient refused additional management and was lost from follow - up . to the best our knowledge , no other such case has been reported . more studies would be need - ed for confirming the suggested reasons of non - union . analysis of two or more operation levels would be helpful for understanding the advantages of this instrument . despite these limitations , the study demonstrates the efficacy and satisfactory clinical outcomes of the zero - p spacer . as another limtation , prevertebral structures are different between the upper cervical and lower cervical vertebrae . at the upper cervical level , only the posterior wall of the hypopharynx comprise the prevertebral structures . on the contrary , more complex structures as transverse arytenoid muscle , larynx , trachea and esophagus are components of the prevertebral structures at the lower cervical level . more detailed analysis of the relationship between the components of each operative level and soft tissue swelling is necessary for the definitive conclusions . finally , although there was a statistically difference of ebl between the two groups , we consider that its standard deviation was too large to conclude significance . acdf is a verified procedure in the treatment of cervical disc disease . in this study , both zero - p and ccp devices were effective for single level acdf , and application of the zero - p spacer resulted in minimal prevertebral soft tissue swelling and decreased post - operative dysphagia 6 months after the operation . additional long - term follow - up , larger scale , prospective , randomized controlled studies may be helpful for determining this conclusion .
objectivethis study analyzed clinical and radiological outcomes of a zero - profile anchored spacer ( zero - p ) and conventional cage - plate ( ccp ) for single level anterior cervical discectomy and fusion ( acdf ) to compare the incidence and difference of postoperative dysphagia with both devices.methodswe retrospectively reviewed our experiences of single level acdf with the ccp and zero - p . from january 2011 to december 2013 , 48 patients who had single level herniated intervertebral disc were operated on using acdf , with ccp in 27 patients and zero - p in 21 patients . 
 patients who received more than double - level acdf or combined circumferential fusion were excluded . 
 age , operation time , estimated blood loss ( ebl ) , pre - operative modified japanese orthopaedic association ( mjoa ) scores , post - operative mjoa scores , achieved mjoa scores and recovery rate of mjoa scores were assessed . 
 prevertebral soft tissue thickness and postoperative dysphagia were analyzed on the day of surgery , and 2 weeks and 6 months postoperatively.resultsthe zero - p group showed same or favorable clinical and radiological outcomes compared with the ccp group . 
 postoperative dysphagia was significantly low in the zero - p group.conclusionsapplication of zero - p may achieve favorable outcomes and reduce postoperative dysphagia in single level acdf .