it is usually due to congenital dysplasia , trauma , strain , or other causes of abnormalities in the bony connection between adjacent vertebrae , leading to partial or complete slippage of one vertebrae on adjacent vertebrae . the typical symptoms of this condition are neurological deficits , including low back pain , nerve root irritation , and neural dysfunctions . a variety of surgical fusion techniques , such as anterior interbody fusion , posterior interbody fusion , posterolateral fusion , repair of the pars interarticularis , and reduction and fusion have been applied to stabilize the spine , relieve pain , and improve the patients life quality . radiological investigation is the key component of evaluation of lumbar spondylolisthesis to determine its anatomical abnormalities , etiologies , severity , and possible pathogenic mechanisms to guide the clinical management and assess the prognosis . for this purpose , a number of x - ray , ct , and mri techniques have been employed to analyze anatomy of vertebrae , lumbar lordosis ( ll ) , and the facet joints associated with the occurrence of slippage . in contrast to the abundant radiological data on spondylolisthesis of single vertebral bodies , data for consecutive lumbar spondylolisthesis are absent , although multilevel lumbar spondylolisthesis does occur and accounts for up to 11% of spondylolisthesis . importantly , multilevel segmental involvement is of considerable significance for the occurrence of cauda equina syndrome . we identified a correlation between the forward displacement of the involved vertebrae and pelvic sagittal parameters . our findings suggest that pelvic compensatory mechanisms play a role in maintaining the overall sagittal spinal and pelvic stability . a total of 967 patients were diagnosed with and treated for spondylolisthesis at our hospital from june 2005 to march 2012 . among them , 17 consecutive spondylolisthesis cases ( 1.75% ) were identified in 5 males and 12 females with a median age of 56 years . out of these 17 patients , 7 had spondylolisthesis involving l3l4 , and 10 had spondylolisthesis involving l4l5 . the enrollment criteria for these patients were as follows : no spinal fractures , or scoliosis history ; no history of spinal surgery ; diagnosis of consecutive spondylolisthesis on a lateral lumbar spine x - ray . the lateral x - ray images for individual patients were retrieved from our picture archiving and communication systems ( pacs ) . the taillard index was defined as the relative displacement distance between the involved vertebrae divided by the horizontal length of the upper vertebral body ( fig . the pelvic incidence ( pi ) was denoted as the angle between the vertical line of s1 endplate and the line connecting midpoint of s1 endplate to midpoint of the femoral heads . the sacrum slope ( ss ) was defined as the angle between the s1 endplate and the horizontal line , while the pelvic tilt ( pt ) was the angle formed by the vertical line and the line connecting the midpoint of s1 endplate to the midpoint of the femoral heads ( fig . the displacement between the upper and lower intervertebral space was determined on flexion  extension dynamic x - ray radiographs ( fig . 3 ) . the measurement of taillard index : the forward displacement distance of upper vertebral body / the length of the upper vertebral body    100% . the measurement of the lumbar lordosis ( ll ) , pelvic incidence ( pi ) , pelvic tilt ( pt ) , and sacrum slope ( ss ) . the angular displacement of the upper intervertebral space was calculated as ( a1b1 ) ; the angular displacement of lower intervertebral space was determined by calculating ( a2b2 ) . student t test was used to analyze the relative anterior displacement of the vertebrae and the angular displacement of the intervertebral spaces . pearson correlation analysis was applied to investigate the correlation between ll and pelvic sagittal parameters and the correlation between taillard index and angular displacements . this study was approved by the ethical committee of the third hospital of hebei medical university , shijiazhuang , hebei , china . a total of 967 patients were diagnosed with and treated for spondylolisthesis at our hospital from june 2005 to march 2012 . among them , 17 consecutive spondylolisthesis cases ( 1.75% ) were identified in 5 males and 12 females with a median age of 56 years . out of these 17 patients , 7 had spondylolisthesis involving l3l4 , and 10 had spondylolisthesis involving l4l5 . the enrollment criteria for these patients were as follows : no spinal fractures , or scoliosis history ; no history of spinal surgery ; diagnosis of consecutive spondylolisthesis on a lateral lumbar spine x - ray . the lateral x - ray images for individual patients were retrieved from our picture archiving and communication systems ( pacs ) . the taillard index was defined as the relative displacement distance between the involved vertebrae divided by the horizontal length of the upper vertebral body ( fig . 1 ) . the ll was the angle between the l1 endplate and s1 endplate . the pelvic incidence ( pi ) was denoted as the angle between the vertical line of s1 endplate and the line connecting midpoint of s1 endplate to midpoint of the femoral heads . the sacrum slope ( ss ) was defined as the angle between the s1 endplate and the horizontal line , while the pelvic tilt ( pt ) was the angle formed by the vertical line and the line connecting the midpoint of s1 endplate to the midpoint of the femoral heads ( fig . the displacement between the upper and lower intervertebral space was determined on flexion  extension dynamic x - ray radiographs ( fig . 3 ) . the measurement of taillard index : the forward displacement distance of upper vertebral body / the length of the upper vertebral body    100% . the measurement of the lumbar lordosis ( ll ) , pelvic incidence ( pi ) , pelvic tilt ( pt ) , and sacrum slope ( ss ) . the angular displacement of the upper intervertebral space was calculated as ( a1b1 ) ; the angular displacement of lower intervertebral space was determined by calculating ( a2b2 ) . the data were analyzed with spss13.0 . student t test was used to analyze the relative anterior displacement of the vertebrae and the angular displacement of the intervertebral spaces . pearson correlation analysis was applied to investigate the correlation between ll and pelvic sagittal parameters and the correlation between taillard index and angular displacements . this study was approved by the ethical committee of the third hospital of hebei medical university , shijiazhuang , hebei , china . twenty isthmic ( upper and lower vertebrae ) and 14 ( upper and lower vertebrae ) degenerative consecutive spondylolisthesis were identified in 34 vertebral bodies in 17 patients ( table 1 ) . among the above mentioned 7 patients with l3l4 consecutive spondylolisthesis . out of the 10 patients with consecutive spondylolisthesis , the average taillard index of the upper vertebrae was 17.6    4.1% , whereas its value for the lower vertebrae was 22.4    4.1% ( t  =  7.672 , p the average angular displacement of the upper vertebrae was 10.8    2.6 , whereas that of the lower vertebrae was 18.6    5.5 ( t  =  5.251 , p  <  0.001 ) . in the 17 patients , the average ll was 58.1    4.3 , the average pi 68.7    4.8 , the average ss 37.2    3.8 , and the average pt 31.6    4.0. a correlation between ll and pi and pt was identified , while pi was correlated with pt and ss ( table 2 ) . in contrast , no correlation was present between taillard index and the angular displacement ( p  >  0.05 ) . for example , isthmus crack spondylolisthesis can be due to a congenital isthmus defect , an acute lumbar trauma leading to an isthmus fracture , stress - related fractures caused by chronic fatigue on the basis of congenital isthmus dysplasia . this condition is commonly detected in the 5th lumbar vertebral body in 30- to 40-year - old adults , as males and females are affected approximately equally . on the other hand , degenerative spondylolisthesis is usually secondary to intervertebral disc degeneration and commonly affects l4l5 in 50- to 60-year - old women . although the diverse causative factors usually lead to single - level spondylolisthesis , we found that consecutive spondylolisthesis is not a rare condition , especially in individuals performing long - term heavy physical labor , such as the subjects enrolled in the present study . we also discovered that pelvic compensation mechanisms play a key role in maintaining the overall sagittal spinal and pelvic stability in this condition . to the best of our knowledge , due to its distinct anatomical nature , isthmic spondylolisthesis is considered to be inherently more instable than degenerative spondylolisthesis , a fact reflected by the difference in the angular displacement existing between these 2 types . in the present study , identical types of spondylolisthesis in the 2 levels of vertebrae were present in each individual . therefore , we believe that the effects exerted by the type of spondylolisthesis on angle displacement were minimal in the studied patients . interbody gravity force is transmitted and divided into a compression force perpendicular to the shift force that is parallel to the vertebral endplates . in general , lower vertebral bodies bear greater shearing forces with respect to the upper vertebrae , which can accelerate intervertebral degeneration in the disc and facet joints . consequently , from an anatomical standpoint , the range of movement in the discs that are closer to the lumbosacral region is greater than that in those located away from the lumbosacral region . in line with this mechanism , we found that the angular displacement in the lower vertebral bodies was much more prominent than that in the upper vertebrae in the clinical setting of consecutive spondylolisthesis . it is worth noting that the antishearing force mechanisms are compromised in lumbar spondylolisthesis . to compensate for this defect , a series of compensatory changes these adaptive changes are usually reflected by the alteration of spine  pelvis sagittal parameters , such as pi . a number of studies have shown that the extent of increase of pi values in patients with single - level lumbar spondylolisthesis is positively correlated with the severity of displacement of the involved vertebrae . barrey et al has further suggested that pi value is to some extent predictive for displacement of affected vertebrae in 1-level spondylolisthesis . we calculated the pi values in 2-level spondylolisthesis and found that the average pi value in consecutive spondylolisthesis was higher than the reported ones in single - level segment spondylolisthesis ( 68.7 vs 66.3 ) , and not surprisingly either , higher than that in healthy individuals . we postulated that the high pi values in consecutive spondylolisthesis might be a risk factor for displacement of the affected vertebrae . nevertheless , to evaluate the biomechanical relevance of an increased pi value in spondylolisthesis requires further prospective studies . any change in orientation within an anatomical spine segment will cause adaptive changes in adjacent segments in order to maintain the stability of the body . for example , vialle et al have reported that ss values in single - vertebral lumbar spondylolisthesis gradually increase along with the degree of displacement in spondylolisthesis grade i although our results clearly demonstrated that consecutive spondylolisthesis resulted in a modification of pelvic parameters , our data indicated that the values of ss ( 37.2 ) and pt ( 31.6 ) in patients with consecutive spondylolisthesis were higher than those observed in the healthy population ( pt  =  25.1 and ss  =  30.7 ) in the same age group . in the present study , all the taillard indexes were below 50% , classifying the grade of spondylolisthesis as i or ii . we hypothesize , that the compensatory mechanisms in the spine  pelvis sequence in 2-segment spondylolisthesis might be different from that in 1-level spondylolisthesis . another set of data supporting this notion was the failure of identification of the existence of a significant correlation between pt and ss , which was compatible with the reported finding indicating that pi  =  ss  +  pt , and there are opposite trend shifts of pt and ss in spondylolisthesis . first , consecutive spondylolisthesis may cause a distinct pelvic  spine compensation which leads to a discordant change in pt and ss . second , the small sample size and selection bias in the present study might have led to the occurrence of this discrepancy . as a limitation of the current investigation , we have to point out that the data collected in the present study are derived from x - ray images only . since whole - spine imaging was not employed , we were unable to conduct an analysis of the whole - spine sequence . in summary , our study showed that in the spondylolisthesis of 2 adjacent lumbar segments both the degree of the vertebral slip and the angular displacement of the lower vertebrae were greater , than those of the upper vertebrae , indicating that the compensatory mechanism of the pelvis plays an important role in maintaining the sagittal balance .