smith - petersen osteotomy utilizes the posterior margin of the body as the pivot , with the posterior column closed and the anterior column opened . with sufficient elasticity in the intervertebral disc , a certain degree of lengthening can be achieved during surgery . however , when all discs are fully ossified , as in as patients , elasticity is absent . in such cases , anterior opening may occur when disruption of the disc happens or osteoclasis of the end plate develops . when the angle of opening is ~30 , there are substantial defects of the anterior column , thus calling into question the necessity for anterior gap reconstruction . earlier studies rarely mention such themes while some authors have emphasized the need for combined anterior and posterior procedures [ 15 ] . with 15  years of experience , comprising more than 300 cases of patients with as , we previously reported the need for reconstruction of the anterior column in cases of spo on segments with pseudarthrosis . however , questions arose regarding the usage of spo on fully ossified segments . with small gaps , bone union is easily accomplished ( as is characteristic of as ) , with good healing results . however with gaps larger than a certain degree , the indications for the need for anterior reconstruction seemed questionable . we also felt the obliged to take into account the differing healing patterns which depend on the disruption sites . the aim of this study is to determine whether anterior column support is required in spo procedure with correction angles of more than 10 , while examining the subsequent healing patterns in relation to the disrupted area . corrective osteotomy for kyphotic deformity was performed in 327 as patients from april 1995 to august 2008 . the indications for corrective surgery were as follows : patients with ankylosis of the cervical vertebrae that hindered forward gazing , patients complaining of cosmetic problems and lastly patients who were easily fatigued due to kyphotic deformity . spo was performed at the level that required a correction angle of 1020. in the case of pseudarthrosis , spo was performed on the lesion segment , and anterior reconstruction was also carried out ( 19 segments in 12 patients ) . of 54 patients who underwent spo , patients with a correction of less than 10 and those who received anterior reconstruction because of pseudarthrosis were excluded from this study . an analysis was done on 26 segments of 19 patients who showed a correction angle of more than 10 in the anterior opening of the surgery segment . eleven of these 19 patients received spo alone , while eight patients received spo in combination with pedicle subtraction osteotomy in the lumbar region , due to insufficient correction after pso . usually , surgical plan was decided preoperatively and we performed pso prior to spo in most of cases . study subjects included 17 male and two female patients with a mean age of 40  years ( 2456  years ) ( table  1 ) . the mean follow - up period was 6.5  years ( 29.1  years).table  1chosen osteotomies and osteotomy levelpatient no.sex/agespo levelpso level1m/49t12-l12m/29l1 - 23m/31t11 - 12 , l2 - 34m/26t10 - 11 , l1 - 25m/38t11 - 12 , l1 - 26m/46t8 - 9l27m/37t12-l1l38m/33t12-l1l39m/46t11 - 12 , l1 - 210m/45t11 - 12 , l1 - 2l311m/56t12-l1 , l2 - 312m/53t12-l1l313m/36t12-l1l314f/31t12-l1l315m/37t12-l116f/26t11 - 12l217m/43t12-l1 , l2 - 318m/48t10 - 1119m/24l1 - 2spo smith - petersen osteotomy ; pso pedicle subtraction osteotomy chosen osteotomies and osteotomy level spo smith - petersen osteotomy ; pso pedicle subtraction osteotomy visual analog scale ( vas ) score and modified srs 24 instrument were clinically assessed prior to surgery and at the last follow - up . complications were also analyzed . in the radiological evaluation , the sagittal angle of the disc ( dsa ) , the segmental angle ( sa ) , and the c7 plumb line were analyzed prior to surgery , after surgery and at the final follow - up ( fig .  1 ) . at the every follow - up visits , the fusion state was analyzed in order to evaluate the bony fusion . the criteria of bony fusion were determined as with a bony bridge in the anterior gap , < 5 of motion in flexion and extension in the simple lateral view without radiolucent lesion around the screw.fig . a sagittal angle of disc ( dsa ) and b segmental angle radiographic measurement methods . a sagittal angle of disc ( dsa ) and b segmental angle the patients were classified according to the site of the anterior opening , as the disc level , the lower end - plate of the upper body ( upper body ) , or the upper end - plate of the lower body ( lower body ) ( fig .  2 ) . the healing patterns of anterior opening were classified as no change and ossification , and the radiological correction angles and loss of correction angles were evaluated relative to the opening site . radiological analysis was surveyed by three separate spine specialists in the department of spinal orthopedics and neurosurgery.fig .  2three anterior opening level . a disc level , b lower endplate of upper body and c upper endplate of lower body three anterior opening level . a disc level , b lower endplate of upper body and c upper endplate of lower body an anova test ( variance test ) was performed to determine whether the correction angle and healing differed according to the opening level . the distances from the apex to the disc level , the lower end - plate of the upper body ( upper body ) , and the upper end - plate of the lower body ( lower body ) were measured and the differences in their correction angles and healing were analyzed , using anova with duncan s post - hoc test . for statistical analysis , sas version 9.1 was used , and a p value of < 5% was considered to be significant . to assess possible injuries to the spinal cord or spinal nerves during surgery , electrophysiological monitoring of somatosensory - evoked potentials and motor - evoked potentials was carried out under general anesthesia in all patients . a bronchoscope - assisted intubation was performed when an endotracheal intubation was impossible because of complete loss of neck motion . we used an operating table that allowed for adjustment of flexion and extension . to assure that the apex of kyphotic deformity was located centrally , the chest and iliac crests were fixed firmly in a prone position using a silicone bolster and sponge pads . using a posterior approach , the segments to be corrected were exposed both superiorly and inferiorly , and the pedicle screws were then inserted . in most of cases , pedicle screws were inserted above 23 levels and below 2 levels at the osteotomy site . however , number of fixation points cephalad and caudad to the osteotomy site was different according to individual patient s situation . structures such as spinous processes , lamina and facet joints were removed in an oblique manner from the area in which the osteotomy would be performed . with firm grasping of the cranial and caudal spinous processes with towel clamps , the operating table was extended gradually , at the same time closing the osteotomy site carefully . in cases in which satisfactory deformity correction could not be obtained by a single spo , additional spo was carried out ( table  1 ) . upon satisfactory deformity correction , posterior fusion using local bone obtained from the osteotomy site that involved lamina and fused facet joint were applied over the entire levels of the corrected segments after decortication with osteotome . if there was substantial defect , we covered this area with flat block bone . after three to seven days of postoperative bed rest , patients were ambulated in tlso braces . the braces were used for at least three months , at which time antero - posterior and lateral radiographs of the lumbar or thoracolumbar spine and standing lateral radiographs of the entire spine were obtained . eleven patients underwent only spo , and eight additional patients underwent spo and subsequent pso . spo was performed on a total of 26 segments , with 12 patients having undergone spo on one segment only , whereas seven patients underwent spo on two segments each . the most common site of spo was t12-l1 , occurring in nine segments , followed by l1 - 2 for six segments , t11 - 12 for five segments , l2 - 3 for three segments , t10 - 11 for two segments , and t8 - 9 for one segment ( table  1 ; fig . a local kyphosis caused by ankylosing spondylitis was 44. b lateral view after smith - petersen osteotomy at t11 - 12 and l1 - 2 , sagittal angle of disc at t11 - 12 and l1 - 2 was 18 and 19 , respectively . c whole spine lateral view at 3  years and 4  months after surgery showing improved sagittal imbalance and no loss of correction . d lateral radiograph showing complete fusion at t11 - 12 and l1 - 2 ( arrows ) from the growth of bridge bone a 46-year - old man with kyphotic deformity . a local kyphosis caused by ankylosing spondylitis was 44. b lateral view after smith - petersen osteotomy at t11 - 12 and l1 - 2 , sagittal angle of disc at t11 - 12 and l1 - 2 was 18 and 19 , respectively . c whole spine lateral view at 3  years and 4  months after surgery showing improved sagittal imbalance and no loss of correction . d lateral radiograph showing complete fusion at t11 - 12 and l1 - 2 ( arrows ) from the growth of bridge bone radiological evaluation revealed that the average sagittal angle of the disc prior to surgery was 0.1 ( 7 to 8 ) and was 20.1 ( 1031 ) after surgery . this angle was 18.7 ( 11 to 30 ) at the final follow - up . after surgery it was 8.8 ( 10 to 22 ) , and at the final follow - up , it was 7.6 ( 12 to 21 ) . the average c7 plumb line was 149.7  mm prior to surgery , 42.4  mm after surgery , and 45.2  mm at the final follow - up ( table  2).table  2preoperative and postoperative radiographic datapreopimpoim corrfinal follow - upfinal corrlocdsa0.1 ( 7 to 8)20.1 ( 10 to 31)20.218.7 ( 11 to 30)18.81.4sa12.8 ( 34 to 1)8.8 ( 10 to 22)21.67.6 ( 12 to 21)20.41.2c7 pl ( mm)149.7  42.4  107.345.2  104.52.8values in parentheses are rangesdsa sagittal angle of disc ; sa segmental angle ; c7 pl c7 plumb line ; preop preoperative ; impo immediate postoperative ; i m corr immediate correction ; final corr final correction ; loc loss of correction preoperative and postoperative radiographic data values in parentheses are ranges dsa sagittal angle of disc ; sa segmental angle ; c7 pl c7 plumb line ; preop preoperative ; impo immediate postoperative ; i m corr immediate correction ; final corr final correction ; loc loss of correction the site of the anterior opening was divided into the disc level , the lower end - plate of the upper body ( upper body ) , and the upper end - plate of the lower body ( lower body ) . of the 26 total segments , 11 segments had an opening in the upper body while 8 segments had an opening in the disc level and 7 segments in the lower body . in all cases , bony fusion was confirmed at a mean period of 5.6  months ( 36.7  months ) after surgery and the anterior opening gap was healed in 18 segments ( 69.2% ) . for patients that developed an opening in the upper body , the gaps in the lower body opening group were healed in 85.7% of the cases , and for the opening at the disc level , the gaps were healed only in 12.5% of the cases ( table  3 ) . notably , according to the opening level of each group , these gap healings ( anterior ossification ) showed a statistically significant difference ( p  <  0.01 ) . in post - hoc tests , disc level gap healing was also shown to be different from that of the upper and lower body levels.table  3radiographic data according to anterior opening levelnumber of segmentfinal correction ( sa)gap healing ( % ) upper body1123.1100lower body721.585.7disc816.212.5sa segmental angle radiographic data according to anterior opening level we examined the changes in the segmental angle in relation to the opening site . when the opening was in the upper body , a correction angle of 24.1 was obtained after surgery and that of 23.1 was observed at the final follow - up . for lower body gap openings , we obtained 23.6 of correction after surgery and 21.5 of correction at the final follow - up . for openings at disc level , however , a 16.8 angle of correction was obtained after surgery , with 16.2 of correction at the final follow - up , thus representing the least amount of correction in comparison to those of the other two sites of opening ( table  3 ) . the difference in the average angles of correction for the three groups was statistically significantly ( p  =  0.03 ) . duncan s post - hoc test showed that the average correction angle of the segmental angles of the upper body and the lower body levels was different from that at disc level . vas scores were improved from 4.5 prior to surgery to 1.4 after surgery and the modified srs instrument scale showed a substantial increase from an average of 2.9 prior to surgery to an average of 4.2 after surgery . after surgery , however , three patients experienced temporary paralytic ileus recovering spontaneously 35  days after conservative treatment . surgical treatment options for advanced ankylosing spondylitis ( as ) patients with kyphotic deformity include spo , pso , polysegmental osteotomies [ 9 , 10 ] or any combination of these [ 11 , 12 ] . of them , spo was first reported by smith - petersen et al . in 1945   and was initially performed as a single level osteotomy , mainly for the treatment of as [ 1318 ] . spo was performed in the lumbar vertebral area at a 3040 angle of correction on a single level . however , due to high mortality rates and a high possibility of neurological complications , the method has not been commonly applied [ 1 , 19 , 20 ] and several modifications of this anterior opening wedge osteotomy have been described [ 3 , 9 , 10 , 13 , 21 , 22 ] . the indication of the spo technique has gradually expanded and has been applied in various spinal diseases . polysegmental spo has been particularly common and it has been possible to achieve a certain level of correction for coronal imbalance through the performance of asymmetric osteotomy [ 9 , 10 , 21 ] . spo of a single segment has been reported to generally result in ~10 of correction . in as patients , spo could result in a correction of ~4050 , due to the brittle nature of their bones . however , the significant correction of a single segment could lead to excessive elongation of the anterior column and result in fatal complications such as aortic rupture , paraplegia , and superior mesenteric artery syndrome . some investigators have observed a loss of correction and pseudarthrosis in large anterior opening gaps . for these reasons , polysegmental anterior fusion was first mentioned by la chapelle ; according to his study published in 1946 , he performed the correction using a two - stage operation . the first surgery involved posterior osteotomy and the second surgery of the anterior column support and the strut bone graft were performed two weeks later . this two - stage system avoids fatal complications such as the rupture of the aorta and allows for safer correction of the deformity with stable bony fusion . bradford et al .   reported that serious complications such as the rupture of the aorta could nonetheless occur after spo . however , the actual incidence of these complications was not high , and the risk of complications seems to have been overestimated . in our experience mcmaster reported a study of spo performed on 14 as patients in 1985 in which the correction loss and nonunion were not reported , and an average of 33 of correction was obtained . chang et al .   reported that successful surgical outcomes could be obtained by posterior correction and fixation without anterior fusion in as patients with pseudarthrosis . they advocated that the superior fusion abilities of as patients were the reason that nonunion did not occur . however , nonunion indeed developed in 4.5% ( 3/66 ) of these patients , and revision was required . on the other hand , bridwell et al . reported that anterior grafting of the disc space may be necessary after achieving substantial correction with an spo but did not provide specific figures or data regarding this possibility . in addition , he mentioned that producing an anterior gap may make arthrodesis less reliable . all 19 patients in our study underwent a posterior approach alone with spo and complete fusion was achieved in all patients at a mean period of 5.6  months ( 36.7  months ) . these results are similar to that ( 4.2  months ) of our previous report   in which we performed spo and anterior interbody fusion for as patients with pseudarthrosis . in addition , gap healing appeared to differ according to the anterior opening sites , and we therefore examined the associations among the osteotomy site , the anterior opening area , and the distance from the osteotomy site to the apex of kyphosis . in cases in which the anterior opening developed at the disc level , the distance between the apex of deformity and the osteotomy site was an average of 0.5 levels ; however , when the anterior opening was in the upper body , the average was 1.73 levels , and for anterior openings at the lower body , the average was 1.57 levels . when spo was performed closer to the apex of deformity , the opening in the disc level increased , and when the osteotomy site was more distant from the apex of deformity , there was an increase in the development of the anterior opening in the upper and lower body . for osteotomies performed at the apex of deformity , we observed an opening in the disc level in five of the six cases , with only one case having an opening in the lower body . spo performed at the site close to the apex of deformity may therefore be disadvantageous for gap healing . we considered whether these results could be due to osteoclasis of the upper and lower vertebral body endplates . osteoblast reactions could be occurring not only in the cortical bone but also in the cancellous bone fracture , and anterior gap healing ( ossification ) could take place instead of marginal syndesmosis of the disc space . however , the distance from the apex did not show significant differences in relation to either correction angle ( p  =  0.69 ) or healing ( p  =  0.09 ) . in our experience , as patients show characteristic ossifications of the ligament and the joint in the vicinity of the vertebrae , suggesting that excellent bony fusion can be obtained using posterior fusion alone without any anterior column support . surgery using the anterior approach is stressful to most spinal surgeons , and its necessity must be cautiously discussed . this is especially pertinent given the recent improvements in surgical techniques , the advent of posterior fixation using pedicle screws , strong fixation , and the much improved success rates of fusion in the absence of anterior fusion . the advantages of performing posterior fusion alone without anterior fusion include reductions in the duration of operation , reduction of blood loss and the avoidance of an anterior approach that is relatively unfamiliar to spinal surgeons . this avoidance could potentially decrease morbidity and mortality and may reduce the cost as well as the burden of antero - posterior surgery on patients . we have also performed combined posterior osteotomy with anterior fusion , although an anterior approach with anterior fusion should perhaps be limited to cases in which correction could not be achieved after posterior osteotomy . the cases that require anterior reconstruction include when bony fusion is delayed for a substantial period after posterior osteotomy or those cases in which pseudarthrosis develops due to nonunion . we note that our study is limited by the relatively small number of subjects and its retrospective nature involving patients who received corrective osteotomy by posterior approach alone . in the correction of kyphotic deformity of as patients using spo , an anterior opening angle of more than 10 is a commonly encountered problem . in our study of subjects presenting with anterior opening angles from 10 to 32 , we obtained successful fusion and good clinical outcomes without the need for additional anterior interbody fusion . the superior fusion ability associated with as allowed for successful fusion regardless of complete gap healing of the anterior opening gap . when osteotomy was performed at the apex of the kyphotic segment , the anterior opening gap developed primarily in the disc space . improved gap healing and increased correction angles were obtained when the opening was present in the upper or lower body endplates compared to those at the disc space level . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
purposethe aim of this study was to determine whether anterior column support is required in smith - petersen osteotomy procedure with correction angles of more than 10 , while examining the subsequent healing patterns in relation to the disrupted area.methodsan analysis was done on 26 segments of 19 patients who showed a correction angle of more than 10 in the anterior opening after spo . 
 there were 17 male and two female patients with a mean age of 40  years ( 2456  years ) . 
 the mean follow - up period was 6.5  years ( 29.1  years ) . 
 the patients were classified according to the site of the anterior opening , as the disc level , the lower end - plate of the upper body ( upper body ) , or the upper end - plate of the lower body ( lower body ) . the healing patterns of anterior opening and the radiological correction angles were evaluated relative to the opening site.resultsin all cases , bony fusion was confirmed at a mean period of 5.6  months ( 36.7  months ) after surgery and the anterior opening gap was healed in 18 segments ( 69.2% ) . for patients that developed an opening in the upper body , 
 all of the gaps were healed . 
 the gaps in the lower body opening group were healed in 85.7% of the cases , and for the opening at the disc level , the gaps were healed only in 12.5% of the cases . 
 the least amount of correction was obtained when anterior opening occurred in disc level.conclusionsin our study of subjects presenting with anterior opening angles from 10 to 32 , we obtained successful fusion without the need for additional anterior interbody fusion . 
 improved gap healing and increased correction angles were obtained when the opening was present in the upper or lower body endplates compared to those at the disc space level .