to stand upright , the human body must maintain a balance between spinal curvature and pelvic morphology1 ) . as the spine and the pelvis are connected in a biomechanical system420 ) , the morphology of pelvic curve will change to compensate for changes in spinal curvature . when lumbar spondylolisthesis occurs , the sagittal shape of the pelvis will be inevitably affected . recently , a few studies have reported the pelvic parameters in lumbar spondylolisthesis11151619 ) . however , the pelvic parameters only record local changes in pelvic morphology and do not take into account how the spine compensates for pelvic changes . jackson et al.56 ) notes that the pelvis and spine should be considered a whole unit when considering biomechanics , and for this , he proposed the pelvic radius ( pr ) technique , which reportedly has similar reliability to pelvic parameters7 ) . sergides notes that the pr technique can help to prevent poor visualization of c7 , which is typical when using sagittal vertical axis from c7 plumb line ( sva ) measurements18 ) . to the best of our knowledge , there are few studies assessing sagittal alignment in spondylolisthesis using the pr technique6 ) . the aim of this study was to investigate the sagittal pelvic morphology in lumbar isthmic spondylolisthesis using the pr technique , compared to controls . we retrospectively reviewed 74 chinese patients who had a confirmed diagnosis of lumbar isthmic spondylolisthesis on x - ray and a three - dimensional ct from january 2011 to april 2014 . the average age of the subjects was 49.29.5 years ( range : 3066 years ) . there were 17 males and 57 females , which included 30 grade i slippages and 44 grade ii slippages according to the system of meyerding . they all had low - grade spondylolisthesis ; 34 subjects had spondylolisthesis at l4 and 40 subjects at l5 . the exclusion criteria were as follows : 1 ) previous spinal fracture or surgery , 2 ) spinal infection or tumor , 3 ) hip disease , and 4 ) lower limb deformities . healthy volunteers consisted of 23 females and 24 males with an average age of 23.3 years ( range : 2127 years ) . they were informed of the risks of participating in this study and signed an informed consent form . for each subject , the lateral view of spinal radiograph , including the base of the skull and the upper section of the femur , was obtained . subjects were asked to stand upright with their knees extended , with their elbows bent to accommodate shoulder flexion to 3013 ) . 1 ) : 1 ) pelvic radius ( pr ) , the distance from the hip axis to the posterior - superior corner of the s1 endplate ; the hip axis was located in the middle between the two femoral bead mid - points . 2 ) the pelvic angle ( pa ) , the angle between the pr line and the plumb line . 3 ) pelvic morphology ( pr - s1 ) , the angle between the pr line and the s1 endplate line . 4 ) total lumbopelvic lordosis ( pr - t12 ) and thoracic kyphosis ( tk ) , both t12-pr and t412 , measured using the cobb technique . statistical analysis was performed using spss software ( spss for windows version 13.0 , spss inc . , we compared the demographic factors and spinal parameters of the patients and the healthy volunteers using an independent - samples t - test , with subgroups for gender , l4 and l5 spondylolisthesis , and i and ii spondylolisthesis . we retrospectively reviewed 74 chinese patients who had a confirmed diagnosis of lumbar isthmic spondylolisthesis on x - ray and a three - dimensional ct from january 2011 to april 2014 . the average age of the subjects was 49.29.5 years ( range : 3066 years ) . there were 17 males and 57 females , which included 30 grade i slippages and 44 grade ii slippages according to the system of meyerding . they all had low - grade spondylolisthesis ; 34 subjects had spondylolisthesis at l4 and 40 subjects at l5 . the exclusion criteria were as follows : 1 ) previous spinal fracture or surgery , 2 ) spinal infection or tumor , 3 ) hip disease , and 4 ) lower limb deformities . healthy volunteers consisted of 23 females and 24 males with an average age of 23.3 years ( range : 2127 years ) . they were informed of the risks of participating in this study and signed an informed consent form . for each subject , the lateral view of spinal radiograph , including the base of the skull and the upper section of the femur , was obtained . subjects were asked to stand upright with their knees extended , with their elbows bent to accommodate shoulder flexion to 3013 ) . 1 ) : 1 ) pelvic radius ( pr ) , the distance from the hip axis to the posterior - superior corner of the s1 endplate ; the hip axis was located in the middle between the two femoral bead mid - points . 2 ) the pelvic angle ( pa ) , the angle between the pr line and the plumb line . 3 ) pelvic morphology ( pr - s1 ) , the angle between the pr line and the s1 endplate line . 4 ) total lumbopelvic lordosis ( pr - t12 ) and thoracic kyphosis ( tk ) , both t12-pr and t412 , measured using the cobb technique . statistical analysis was performed using spss software ( spss for windows version 13.0 , spss inc . , chicago , il , usa ) . we compared the demographic factors and spinal parameters of the patients and the healthy volunteers using an independent - samples t - test , with subgroups for gender , l4 and l5 spondylolisthesis , and i and ii spondylolisthesis . compared to the control group , the tk ( 37.29.1 ) and the pa ( 24.56.6 ) of the total patients were significantly greater ( p<0.05 ) , and the pr - s1 ( 25.411.2 ) and pr - t12 ( 83.79.8 ) were significantly lower ( p<0.05 ) , while there was no significant difference in the pr ( table 1 ) . however , the tk / pr - t12 ratio was consistently between 0.15 and 0.75 in the patient group . the pa was significantly greater for grade i patients ( 21.75.0 ) and grade ii patients ( 25.96.9 ) compared with the control group , respectively ( 13.77.8 ) ( p<0.05 ) , and the pr - s1 and pr - t12 were significantly lower . the tk was significantly greater for grade i patients ( 41.17.9 ) than that of the control group ( 31.39.3 ) ( p=0.001 ) ( table 2 ) . we compared the tk ( t412 ) , pr , pa , pr - s1 , pr - t12 , and the tk / pr - t12 ratio between l4 and l5 subgroups , and found no statistical difference between the two groups ( p>0.05 ) , as shown in table 3 . the tk and pr - s1 of grade ii were less than grade i , but the pa was greater ( table 4 ) . the pr - t12 of female patients were less than male patients , but the pa was greater ( p<0.05 ) ( table 5 ) . the physiological curvature of the spine includes cervical lordosis , thoracic kyphosis , lumbar lordosis , and sacral kyphosis . normal physiological curvature is an important factor to ensure that people can walk upright and maintain balance , as the spine and the pelvis area work together as a unit to enable normal ambulation . however , spondylolisthesis inevitably leads to changes in the sagittal pelvic morphology . studying sagittal balance helps us to understand the changes in the lumbopelvic biomechanics in lumbar spondylolisthesis and provide theories to guide clinical treatment . since the spine and pelvis are a whole , when the sagittal alignment of the lumbar spine changes , the upper spine will make compensatory changes to regain balance9 ) . therefore , the standalone use of pelvic parameters to evaluate the sagittal balance of lumbar spondylolisthesis may overlook the compensatory effect of the thoracic spine . jackson et al.6 ) described a pr technique to evaluate sagittal spinopelvic balance , which includes pr , pa , pr - t12 , and pr - s1 . they have reported that the reliability of pr - t12 , pi , and pr - s1 is similar7 ) . in addition to pr - s1 , it is not necessary to identify the whole s1 endplate when use the pr technique to measure . using pi and sva methods , clear visualization of the s1 endplate and the c7 vertebra is essential . but according to reports , the c7 vertebrae can be clearly identified on the whole spine lateral radiographs only in about half of the cases18 ) . our study also found that the c7 vertebrae could not be fully visualized in most patients . using the pr techniques , jackson et al.7 ) indicated that the pr technique was more reliable than plumbline measurement . . " congruent " alignment should meet the following three conditions : 1 ) pr - t12 is between 70 and 100 , 2 ) the pa is between 3 and 33 , and 3 ) the t412/pr - t12 ratio is between 0.15 and 0.75 . its value is in a relatively narrow range in healthy individuals , about 901018 ) . thus , the total lumbopelvic lordosis decreases when the pr - s1 increases , and vice versa . we found that the pa of spondylolisthesis was significantly increased , while the pr - s1 and pr - t12 were significantly reduced , and , as the degree of spondylolisthesis increased , the pa increased , but the pr - s1 decreased . therefore , with an increasing degree of spondylolisthesis , the pelvis rotates around the hip axis , leading to verticalization of the sacrum17 ) . meanwhile , we found that the pa of females is greater than that of males , indicating that the female pelvis tilts backward seriously . thus , the shear force of the female lumbosacral part is greater16 ) and the probability of the occurrence of spondylolisthesis is also greater . park et al.15 ) observed that lumbar spondylolisthesis can affect the sagittal balance of the lumbar spine . sergides et al.18 ) pointed out that although pr - t12 can quickly evaluate sagittal lumbar balance , the compensatory effect of the upper spine can not be ignored . we found that the pa , pr - s1 , and pr - t12 of low - grade spondylolisthesis differed from the normal ranges significantly , demonstrating that the local morphology of the lumbar spine and pelvis have changed . however , according to the " congruent " spinopelvic alignment criteria7 ) , for low - grade spondylolisthesis patients , the sagittal trunk is still in a state of balance . this demonstrates that when the local morphology of the lumbar spine and pelvis changes , the remainder of the spine will compensate in terms of position to maintain overall balance , such as intervertebral hyperextension and pelvic retroversion10 ) . therefore , the pr technique enables effective evaluation of spinal sagittal balance and minimizes the measurement error caused by unclear views of c7 . because the l4/l5 vertebrae are the apex of lumbar lordosis2 ) , the lower lumbar spine is a predominant region for biomechanical transmission , and thus lumbar spondylolisthesis often occurs in this region . oh et al.14 ) observed that spondylolisthesis of different segments of the lower lumbar spine incurs similar changes in pelvic morphology . we observed similar results in our study : there was no significant difference in the pr and related parameters between l4 and l5 spondylolisthesis . for severe spondylolisthesis ( slip percentage > 50% ) , the difference in the pelvic morphological changes of different segments of spondylolisthesis requires further investigation . mac - thiong et al.12 ) indicated that sagittal imbalance is more likely to cause pain and dysfunction . posterior pedicle screw fixation is the major surgical treatment for spondylolisthesis . by restoring sagittal balance , it can reduce the incidence of flat back syndrome and pathology in the adjacent segments8 ) , thus achieving a better prognosis320 ) . satisfactory clinical outcomes are dependent on preoperative measurements , which can help us to adjust the angle of pedicle screws during operations and restore sagittal spinopelvic balance . our study had several limitations , such as a small number of cases ; in addition , we did not include severe spondylolisthesis and multiple - level spondylolisthesis , which may affect the results . the gender and grade of slippage had an effect on the sagittal configuration of the pelvis , but there are no differences in terms of the impacts different segments of vertebral slip had on pelvic morphology .
objectiveto investigate the variation of pelvic radius and related parameters in low - grade isthmic lumbar spondylolisthesis.methodsseventy-four patients with isthmic lumbar spondylolisthesis and 47 controls were included in this study . 
 there were 17 males and 57 females between 30 and 66 years of age , including 30 with grade i slippages and 44 grade ii slippages ; diseased levels included 34 cases on l4 and 40 cases on l5 . 
 thoracic kyphosis ( tk ) , the pelvic radius ( pr ) , the pelvic angle ( pa ) , pelvic morphology ( pr - s1 ) , and total lumbopelvic lordosis ( pr - t12 ) were assessed from radiographs.resultsstatistically significant differences were found for the pa , pr - t12 , and pr - s1 ( 24.56.6 , 83.79.8 , and 25.411.2 , respectively ) of the patients with spondylolisthesis and the healthy volunteers ( 13.77.8 , 92.99.2 , and 40.78.9 , respectively ) . 
 the tk / pr - t12 ratios were between 0.15 and 0.75 . however , there were no differences in all the parameters between the l4 and l5 spondylolysis subgroups ( p>0.05 ) . 
 the tk and pr - s1 of grade ii were less than grade i , but the pa was greater . 
 the pr - t12 of female patients were less than male patients , but the pa was greater ( p<0.05).conclusionpelvic morphology differed in patients with low - grade isthmic lumbar spondylolisthesis compared to controls . gender and 
 the grade of slippage impacted the sagittal configuration of the pelvis , but the segment of the vertebral slip did not . 
 overall , the spine of those with spondylolisthesis remains able to maintain sagittal balance despite abnormal pelvic morphology .