proximal junctional kyphosis ( pjk ) is traditionally defined as a proximal junctional sagittal cobb angle change of 10 versus the preoperative value . although no consensus has been reached , the majority of authors have used kyphosis of 10 between the lower endplate of the uppermost instrumented vertebra ( uiv ) and the upper endplate of the two supra - adjacent vertebrae as a definition5 ) . pjk is a post - surgical radiographic status observed in the sagittal plane following multi - level fusion surgery for spinal deformity . in a broader sense , pjk is a type of adjacent segment disease associated with spinal fusion and often occurs after deformity correction . furthermore , pjk results in poor surgical results due to pain , deformity , instability , motion difficulties , and potential neurologic deficits . to minimize pjk , , several such factors have been described in the literature , namely , older age , inadequate restoration of global sagittal balance , combined anterior - posterior surgery , fusion to the sacrum , posterior only spinal fusion rather than anterior instrumentation , and low bone mineral density ( bmd)9,10,12,15,19 ) . skeletal muscles , including flexor and extensors , produce movements of the trunk and maintain body balance , and degeneration and atrophy of thoracolumbar extensor muscles lead to fatty replacement and back kyphosis as manifested by lumbar degenerative kyphosis or flat back syndrome . furthermore , sarcopenia may affect skeletal stability or surgical outcome , but the impact of thoracolumbar muscle volume on pjk has not been previously evaluated . the aim of this study was to identify the factors responsible for pjk after spinal instrumented fusion in adult spinal deformity with a minimum follow - up of 2 years . this study was conducted by retrospectively reviewing the medical charts and radiologic data of 49 patients with spinal deformity , including lumbar degenerative kyphosis , diffuse lumbar scoliosis , and post - operative flat - back syndrome , that underwent posterior fusion with instrumentation from january 2004 to december 2011 at two university hospitals . the minimum follow - up was 2 years and mean follow - up was 3.6 years . all 49 patients underwent multi - level fusion surgery at more than 4 levels from l5 or the sacral level . radiographic pjk was defined as angle change of > 10 on dynamic plain radiographs at the proximal end of a construct14 ) . preoperative evaluations included plain radiographic examinations and magnetic resonance imaging ( mri ) , a postoperative radiograph examination was conducted before discharge ( early postop , ep ) , and then annually . postoperative radiographs included standing antero - posterior , lateral whole spine , and dynamic flexion / extension views , and were used to determine the presence or not of pjk at proximal adjacent level from instrumentation . the 49 patients were divided in terms of two groups ( pjk and non - pjk groups ) based on the presence of pjk at final follow - up . these two groups were compared with respect to demographic factors , bmd , sagittal plane angle parameter , pelvic parameter , instrumentation factor , surgical approach , and the back muscle volume of thoracolumbar junction . group bmd values were compared using t scores determined by dual energy x - ray absorptiometry . c7pl was defined distance between c7 plumb line which is the vertical line originating the center of the c7 vertebral body and the posterior superior corner of s1 , and its length was measured preoperatively and at final follow - up . distance from the center of the upper instrumented vertebra ( uiv ) to c7pl was also measured at early postoperatively and final follow - up ( fig . pelvic incidence ( pi ) is defined as the angle between the perpendicular to the sacral plate at its midpoint and the line connecting this point to the middle axis of the femoral head . sacral slope ( ss ) is the angle between the superior endplate of s1 and the horizontal line . pelvic tilt ( pt ) is the angle between the line connecting the midpoint of the sacral plate to center of the axis of femoral heads , and the vertical line . data related to surgical procedures , including surgical approach ( concomitant anterior - posterior or not ) , cross - link usage , whether screws and rods were fractured or not , and uiv level , were also analyzed . contiguous t2-weighted mri scans were acquired in 5-mm slices using a 1.5-tesla ge ( general electric , milwaukee , wi , usa ) twin - speed scanner from t10 to l2 . patients were positioned supine on the imaging table and numbers of scan slices were adjusted to include the anatomic extent of muscles of interest . cross - section areas were calculated using software installed in the mri unit in the following way . after choosing a region of interest in the axial plane , the range of interest ( roi ) was enlarged on a monitor screen as much as necessary and subjected to planimetry . all measurements were obtained using a picture archiving and communication system and its software ( pacs , infinitt , seoul , korea ) ( fig . data were analyzed using spss for windows ( version 14.0 ; spss inc . , chicago , il , usa ) , and statistical significance was accepted for p values<0.05 . c7pl was defined distance between c7 plumb line which is the vertical line originating the center of the c7 vertebral body and the posterior superior corner of s1 , and its length was measured preoperatively and at final follow - up . distance from the center of the upper instrumented vertebra ( uiv ) to c7pl was also measured at early postoperatively and final follow - up ( fig . pelvic incidence ( pi ) is defined as the angle between the perpendicular to the sacral plate at its midpoint and the line connecting this point to the middle axis of the femoral head . sacral slope ( ss ) is the angle between the superior endplate of s1 and the horizontal line . pelvic tilt ( pt ) is the angle between the line connecting the midpoint of the sacral plate to center of the axis of femoral heads , and the vertical line . data related to surgical procedures , including surgical approach ( concomitant anterior - posterior or not ) , cross - link usage , whether screws and rods were fractured or not , and uiv level , were also analyzed . contiguous t2-weighted mri scans were acquired in 5-mm slices using a 1.5-tesla ge ( general electric , milwaukee , wi , usa ) twin - speed scanner from t10 to l2 . patients were positioned supine on the imaging table and numbers of scan slices were adjusted to include the anatomic extent of muscles of interest . cross - section areas were calculated using software installed in the mri unit in the following way . after choosing a region of interest in the axial plane , the range of interest ( roi ) was enlarged on a monitor screen as much as necessary and subjected to planimetry . all measurements were obtained using a picture archiving and communication system and its software ( pacs , infinitt , seoul , korea ) ( fig . data were analyzed using spss for windows ( version 14.0 ; spss inc . , chicago , il , usa ) , and statistical significance was accepted for p values<0.05 . the pjk group and non - pjk group contained 16 and 33 patients , respectively . women dominated in both groups  2 men and 14 women in pjk group and 4 men and 29 women in non - pjk group . mean overall age was 62.5 in pjk group ( range of 5669 ) and 61.9 in non - pjk group . sexual distributions and ages were non - significantly different ( table 1 ) . mean t score was 2.300.85 in pjk group and 1.010.67 in non - pjk group . bmd was significantly lower in the pjk group than in the non - pjk group ( p=0.027 ) ( table 1 ) . mean c7pl before surgery was 89.9551.60 mm in pjk group and 58.9766.98 mm in non - pjk group , and mean c7pl at final follow - up was 71.9658.04 mm in pjk group and 55.8337.00 mm in non - pjk group . mean distance from uiv to c7pl before surgery was 1.96.0 mm in pjk group and 0.53.0 mm in non - pjk group , and mean distance from uiv to c7pl at final follow - up was 5.55.4 mm in pjk group and 2.53.4 mm in non - pjk group . no intergroup difference was observed for mean c7pl at pre - op or at final follow - up and for mean uiv distance to c7pl distance at early post - op . however , mean uiv distance to c7pl distance at final follow - up was significantly greater in pjk group ( p=0.019 ) ( table 1 ) . mean preoperative pi was 57.9412.70 in pjk group and 58.458.37 in non - pjk group , and mean preoperative ss was 15.385.88 in pjk group and 16.916.59 in non - pjk group . mean preoperative pt was 42.0612.00 in pjk group and 41.219.16 in non - pjk group . pre - op mean pi , ss , and pt were not significantly different in the two groups ( table 1 ) . concomitant anterior - posterior surgery was performed in 6 of 16 ( 37.5% ) in pjk group and in 16 of 33 ( 48.4% ) in non - pjk group . cross - linkages were used in 13 of 16 ( 81.2% ) in pjk group and in 26 of 33 ( 78.8% ) in non - pjk group . instrument fractures occurred in 5 of 16 ( 31.2% ) in pjk group and in 10 of 33 ( 30.0% ) in non - pjk group . in pjk group , number of uiv levels were 2 of 16 ( t10 , 12.5% ) , 3 of 16 ( t11 , 18.8% ) , 5 of 16 ( t12 , 31.2% ) , 4 of 16 ( l1 , 25.0% ) , and 2 of 16 ( l2 , 12.5% ) , and in non - pjk group were 5 of 33 ( t10 , 15.1% ) , 4 of 33 ( t11 , 12.2% ) , 12 of 33 ( t12 , 36.3% ) , 6 of 33 ( l1 , 18.2% ) , and 6 of 33 ( l2 , 18.2% ) . surgical approaches , cross - link - usage , instrument fracture , and uiv level were non - significantly different in the two groups ( table 1 ) . mean thoracolumbar junction back muscle volumes in pjk and non - pjk group were 1042149 and 1315143 mm , respectively , at t10 , 1113101 and 1445176 mm at t11 , 1227111 and 1603230 mm at t12 , 133399 and 1783326 mm at l1 , and 1443175 and 1905303 mm , respectively , at l2 . thoracolumbar junction back muscle volume was significant lower in the pjk group than in the non - pjk group ( p<0.001 ) ( table 1 , fig . the correlation of thoracolumbar junction back muscle volume and preoperative sagittal imbalance in overall group was also evaluated . the mean muscle volume of t10 to l2 showed negative correlation with mean c7pl before surgery in the overall group . but the correlation was not strong with pearson correlation 0.313 ( p=0.029 ) ( fig . the pjk group and non - pjk group contained 16 and 33 patients , respectively . women dominated in both groups  2 men and 14 women in pjk group and 4 men and 29 women in non - pjk group . mean overall age was 62.5 in pjk group ( range of 5669 ) and 61.9 in non - pjk group . mean t score was 2.300.85 in pjk group and 1.010.67 in non - pjk group . bmd was significantly lower in the pjk group than in the non - pjk group ( p=0.027 ) ( table 1 ) . mean c7pl before surgery was 89.9551.60 mm in pjk group and 58.9766.98 mm in non - pjk group , and mean c7pl at final follow - up was 71.9658.04 mm in pjk group and 55.8337.00 mm in non - pjk group . mean distance from uiv to c7pl before surgery was 1.96.0 mm in pjk group and 0.53.0 mm in non - pjk group , and mean distance from uiv to c7pl at final follow - up was 5.55.4 mm in pjk group and 2.53.4 mm in non - pjk group . no intergroup difference was observed for mean c7pl at pre - op or at final follow - up and for mean uiv distance to c7pl distance at early post - op . however , mean uiv distance to c7pl distance at final follow - up was significantly greater in pjk group ( p=0.019 ) ( table 1 ) . mean preoperative pi was 57.9412.70 in pjk group and 58.458.37 in non - pjk group , and mean preoperative ss was 15.385.88 in pjk group and 16.916.59 in non - pjk group . mean preoperative pt was 42.0612.00 in pjk group and 41.219.16 in non - pjk group . op mean pi , ss , and pt were not significantly different in the two groups ( table 1 ) . concomitant anterior - posterior surgery was performed in 6 of 16 ( 37.5% ) in pjk group and in 16 of 33 ( 48.4% ) in non - pjk group . cross - linkages were used in 13 of 16 ( 81.2% ) in pjk group and in 26 of 33 ( 78.8% ) in non - pjk group . instrument fractures occurred in 5 of 16 ( 31.2% ) in pjk group and in 10 of 33 ( 30.0% ) in non - pjk group . in pjk group , number of uiv levels were 2 of 16 ( t10 , 12.5% ) , 3 of 16 ( t11 , 18.8% ) , 5 of 16 ( t12 , 31.2% ) , 4 of 16 ( l1 , 25.0% ) , and 2 of 16 ( l2 , 12.5% ) , and in non - pjk group were 5 of 33 ( t10 , 15.1% ) , 4 of 33 ( t11 , 12.2% ) , 12 of 33 ( t12 , 36.3% ) , 6 of 33 ( l1 , 18.2% ) , and 6 of 33 ( l2 , 18.2% ) . surgical approaches , cross - link - usage , instrument fracture , and uiv level were non - significantly different in the two groups ( table 1 ) . mean thoracolumbar junction back muscle volumes in pjk and non - pjk group were 1042149 and 1315143 mm , respectively , at t10 , 1113101 and 1445176 mm at t11 , 1227111 and 1603230 mm at t12 , 133399 and 1783326 mm at l1 , and 1443175 and 1905303 mm , respectively , at l2 . thoracolumbar junction back muscle volume was significant lower in the pjk group than in the non - pjk group ( p<0.001 ) ( table 1 , fig . the correlation of thoracolumbar junction back muscle volume and preoperative sagittal imbalance in overall group was also evaluated . the mean muscle volume of t10 to l2 showed negative correlation with mean c7pl before surgery in the overall group . but the correlation was not strong with pearson correlation 0.313 ( p=0.029 ) ( fig . the present study confirms preoperative bmd and thoracolumbar back muscle volume are related to the presence of pjk after segmental spinal instrumented fusion in adults with spinal deformity , and that although sagittal balance is achieved immediately after surgery , newly developed sagittal imbalance after deformity surgery is associated with increased pjk at final follow - up . many studies have identified risk factors of the postoperative development of pjk3,5,6,9,10,12,15,19 ) , such as , patient age , bone quality , and inadequate restoration of global sagittal balance requiring substantial correction to restore global alignment . furthermore , uiv level may play a role because the uiv in many deformity cases ends at the thoracolumbar junction , a transition zone between the mobile lumbar spine and the less mobile thoracic spine and rib cage . it is also possible that body mass index contributes to the development of pjk as the uiv may be biomechanically stressed in larger patients . takemitsu et al . measured the cross - sectional areas of paraspinal muscles in patients with lumbar degenerative kyphosis , which lumbar extensor muscles overwork to maintain secure balance by ct , and weakness of spinal extensors is caused by definite atrophy , which may produce round back kyphosis and more fatty replacement as compared with normal individuals8,18 ) . harrison et al . showed that anterior trunk translation in standing subjects increases extensor muscle activity and loads and stresses acting on intervertebral discs in the lower thoracic and lumbar regions7 ) . it has been established altered biomechanical stresses on an intervertebral disc may cause adjacent segment degeneration . in the present study , thoracolumbar back muscle volume was smaller in the pjk group than in the non - pjk group . we could know relation between preoperative thoracolumbar back muscle volume and pjk , even if this result could not answer what is the preceding between two factors . osteoporosis is a characterized by reduced bone strength , increased skeletal fragility , and fracture susceptibility . furthermore , a weak bone / screw interface is a known risk factor of screw pull - out after a spinal instrumentation procedure , and numerous studies have concluded low bone mineral density is a risk factor for adjacent segment disease after spinal fusion . terracciano et al.2 ) showed muscle atrophy is prominent in osteoporosis and preferentially affects type ii muscle fibers with little or no impact on type i fibers , and that this atrophy is associated with bmd , which suggests disease severity plays a central role in the pathogenesis of osteoporosis - related muscle atrophy . in the present study , patients allocated to two groups based on the presence of absence of pjk . osteoporosis and sarcopenia are aging related , but in the present study age was not found to be related with pjk . decreased skeletal muscle volume increases axial loading during the maintenance of skeletal stability , and osteoporotic bone can not sufficiently support skeletal loading , which means back muscles are exposed and a vicious cycle is initiated . furthermore , reduced screw pull - out strength imposes a heavy burden on back muscles and causes micro - motion of proximal instrumentation . resultantly , lower bmd in combination reduced thoracolumbar back muscle volume might in combination induce skeletal instability and facilitate proximal junctional kyphosis . line of gravity and center of gravity have been studied by several authors11,16 ) . in a stabilized standing position , the c7 plumb line is located behind the gravity line in the sagittal plane . however , in most pathological situations , the center of gravity is too far forward with a mechanical axis located in front of the femoral heads , and this imbalance will induce body reactions . the present study shows uiv distance from the c7 plumb line was greater in the pjk group . furthermore , center of gravity is located more frontally for greater uiv distances and stress loading on proximal instrumentation might be dependent on uiv distance , and thus , when compensatory mechanisms , such as , thoracic kyphosis , hip or knee bending can not tolerate a critical point , pjk might occur . second , fat volume could not be excluded from muscle mass measurements because of technical limitations . accordingly , we suggest a well designed prospective study be performed to identify more precisely factors that contribute to pjk , and that a biomechanical study be undertaken to confirm our results . nevertheless , the study provides surgeons with information useful for deciding on instrumentation level and surgical approach . the incidence of pjk following multi - level fusion surgery was higher related to preoperative lower bmd , thoracolumbar back muscle volume , and insufficient postoperative sagittal balance . surgeons should make treatment plan carefully to prevent pjk after multi - level fusion surgery , especially in cases of osteoporosis and sarcopenia . we believe the result of this study will be useful for preventing pjk after surgery , but more studies with long - term follow - up and more subjects are also needed in the future .