Sagittal plane analysis of the spine and pelvis in degenerative lumbar scoliosis

Abstract

Background:

Previous studies have reported the normative values of pelvic sagittal parameters, but no study has analyzed the sagittal spino-pelvic alignment in degenerative lumbar scoliosis (DLS) and its role in the pathogenesis.

Methods:

Retrospective analysis was applied to 104 patients with DLS, together with 100 cases of asymptomatic young adults as a control group and another control group consisting of 145 cases with cervical spondylosis. The coronal and sagittal parameters were measured on the anteroposterior and lateral radiograph of the whole spine in the DLS group as well as in the two control groups.

Results:

Statistical analysis showed that the DLS group had a higher pelvic incidence (PI) value (50.5° ± 10.2°), than the normal control group (with PI 47.2° ± 8.8°) and the cervical spondylosis group (46.9° ± 9.1°). In DLS group, there were 38 cases (36.5%) complicated with degenerative lumbar spondylolisthesis, who had higher PI values than patients without it. Besides, the lumbar lordosis (LL) and sacral slope (SS) of DLS group were lower; the scoliosis Cobb’s angle was correlated with pelvic tilt (PT); thoracic kyphosis was correlated with LL, SS, and PT; and LL was correlated with other sagittal parameters.

Conclusion:

Patients with DLS may have a higher PI, which may impact the pathogenesis of DLS. A high PI value is probably associated with the high prevalence of degenerative lumbar spondylolisthesis among DLS patients. In DLS patients, the lumbar spine maintains the ability of regulating the sagittal balance, and the regulation depends more on thoracic curve.

Keywords

lumbar pelvis scoliosis spine

Background

Degenerative lumbar scoliosis (DLS) presents not only coronal deformity, but also changes in sagittal alignment. Previous studies have reported lower lumbar lordosis (LL)^1,2 and sacral slope (SS)³ in DLS patients, which may participate in the pathogenesis of DLS,⁴ and have a close correlation with the clinical outcomes.^5,6 Moreover, studies on the sagittal alignment of the spine and pelvis in DLS are helpful for making classification systems and surgical decisions, so the analysis of sagittal spino-pelvic alignment in DLS has vital clinical significance. However, previous studies usually chose adult spinal deformity (ASD) patients as the subjects investigated, and few of them focused on DLS, especially for PI and other pelvic parameters. So this study measured the sagittal parameters of a group of DLS patients, compared them with asymptomatic young adults and patients with cervical spondylosis, and tried to figure out the characteristics of sagittal alignment in DLS.

Methods

Patients’ inclusion

This was a retrospective study approved by the Regional Ethics Committee of our hospital (IRB00006761-2012066). Three groups were selected for the study. The DLS group included 104 subjects, age greater than 40 years, whose Cobb’s angle were more than 10°, with diagnosis of DLS, including cases with spinal stenosis and degenerative spondylolisthesis (a slippage of >3 mm). Subjects were excluded if they met the following criteria: cases of adolescent idiopathic scoliosis, especially Lenke type V patients; complicated with spinal tumor, isthmic spondylolisthesis, pelvic fracture; history of operations on spine or lower extremities; and scoliosis caused by a forced posture. Orthopedic patients in our hospital during January 2012–January 2014 who met the criteria above were included in this study, and all the studies were carried out before surgery.

Considering the lack of data of elder population, and according to the previous literature, pelvic incidence (PI) presented no correlation with age^7
–9 and gender^10,11 as a morphological parameter after reaching maturity, we set up two control groups. A cohort of 100 asymptomatic young adult volunteers (age 18–35 years, mean 26.1 ± 3.8 years) were included as the normal control group.¹² Meanwhile, 145 cases of patients with cervical spondylosis were set to the cervical spondylosis group, in which the lumbar degeneration was considered similar to normal elder adults. The inclusion criteria were diagnosis of cervical spondylosis, with detailed image data; strength level of lower extremity above IV, able to keep upright independently and cooperate on taking the whole spine radiography. The exclusive criteria of DLS group was applied to cervical spondylosis group.

Radiographic measurements

For all the subjects, anteroposterior and lateral radiographs of the entire spine and pelvis (standing position, full extension of hip and knee, with elbow flexion and hands on the handrail, including bilateral femoral heads) were taken. As the measurement is partially dependent on where you assign the point which represents the endplate and the center point between the two femoral heads, there may be differences in measurement between different observers, so all the measurement was done by a single researcher, and the average value of two measurements was taken. In some DLS cases with severe bone hyperplasia, the osteophyma was not added into calculation when assigning the point in measurement. The following parameters were measured using the PACS system (Picture Archiving and Communication System, GE healthcare, Mount prospect, IL, USA).

Coronal parameters on anteroposterior radiograph, including L3 tilt, the angle between the upper endplate of L3 and the horizontal line; Cobb’s angle, the angle between the upper endplate of the superior end vertebra and the lower endplate of the inferior end vertebra; and coronal balance, distance from C7 plumb line to the sacral midpoint.

Sagittal parameters on lateral radiograph, including LL, the angle between the upper endplate of L1 and the lower endplate of S1; sagittal balance (sagittal vertical axis, SVA), the perpendicular distance between the C7 sagittal plumb line and the posterior superior corner of the sacrum, which is negative when C7PL lies posterior to the posterior superior corner of the sacrum; thoracic kyphosis (TK), the angle between the upper endplate of the T4 and the lower endplate of the T12; PI, the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the femoral head axis (midway between the centers of the femoral heads); pelvic tilt (PT), the angle between the vertical line and the line joining the middle of the sacral plate and the hip axis, which is positive when the hip axis lies in front of the middle of the sacral plate; and SS, the angle between the sacral endplate and the horizontal line.

Statistical analysis

An independent-sample Student’s t-test was used to compare the parameters, especially PI, between DLS group and the two control groups. The relationship between the parameters was analyzed using Pearson’s coefficients. All the data were analyzed using SPSS 20.0 statistical software (SPSS Inc., Chicago, Illinois, USA). The statistical level of significance was set at 0.05.

Results

In DLS group, a total of 104 subjects (36 males and 68 females) were enrolled, with a mean age of 62.7 ± 7.2 years (43–78 years), complicated diagnosis including lumbar spinal stenosis, lumbar disc herniation, lumbar degenerative spondylolisthesis, and so on. Results of the measurement were shown in Table 1. Measurements of the two control groups were listed in Table 2.

Table 1.

Radiographic measurements of DLS group.

	Minimum	Maximum	Mean	Standard deviation
Age	43	78	62.71	7.219
L3 tilt	0.0	20.5	6.504	4.6988
Cobb’s angle	10.2	46.8	16.809	7.5987
Coronal balance	−62.75	60.89	15.1387	17.03025
LL	−7.5	65.6	34.710	15.7859
PI	16.9	72.5	50.536	10.2044
SS	−5.4	51.2	28.081	10.3853
PT	0.8	48.5	22.446	9.8956
TK	−7.1	61.1	26.316	13.1342
SVA	−52.64	210.60	39.8083	50.48249

LL: lumbar lordosis; PI: pelvic incidence; SS: sacral slope; PT: pelvic tilt; TK: thoracic kyphosis; SVA: sagittal vertical axis.

Table 2.

Mean values of sagittal parameters in the two control groups.

	LL	PI	SS	PT	TK	SVA
Young adults group	50.8 ± 10.2	47.2 ± 8.8	37.5 ± 7.1	9.6 ± 5.9	28.4 ± 8.2	−18.4 ± 32.8
Cervical spondylosis group	50.8 ± 10.2	47.2 ± 8.8	37.5 ± 7.1	9.6 ± 5.9	28.4 ± 8.2	−18.4 ± 32.8
Cervical spondylosis group	52.9 ± 9.1	46.9 ± 9.1	36.8 ± 6.8	10.5 ± 7.0	36.9 ± 9.6	−18.4 ± 32.9

LL: lumbar lordosis; PI: pelvic incidence; SS: sacral slope; PT: pelvic tilt; TK: thoracic kyphosis; SVA: sagittal vertical axis.

The mean PI of DLS group was 50.5° ± 10.2°, which was significantly higher than the asymptomatic young adults (18- to 35-years old, average age 26.1 ± 3.8 years, mean PI 47.2° ± 8.8°, p = 0.014) according to an independent-sample Student’s t-test. Among them, there were 38 patients complicated with degenerative lumbar spondylolisthesis, accounting for 36.5%, and the average PI was 53.4° ± 8.9°, which had a significant difference than the other 66 cases without degenerative spondylolisthesis (mean PI 48.9° ± 10.6°, p < 0.05) and the asymptomatic young adults (p < 0.01). Between the cases with and without spondylolisthesis, there were no difference in LL, SS, PT, TK, and SVA (p < 0.05).

In cervical spondylosis group, a total of 145 cases were included, 92 of them were male, 53 were female, and the average age was 50.5 ± 9.6 years (range 32–83 years). The average PI value was 46.9° ± 9.1°, and the independent sample t-test showed significant difference with DLS group (p < 0.01).

In addition, the average LL in DLS group was lower than that in normal control group (37.1° ± 15.8° vs. 50.8° ± 10.2°, p < 0.01) and cervical spondylosis group (37.1° ± 15.8° vs. 52.9° ± 9.1°, p < 0.01); the average SS was lower than that in normal control group (28.1° ± 10.4° vs. 37.5° ± 7.1°, p < 0.01) and cervical spondylosis group (28.1° ± 10.4° vs. 36.8° ± 6.8°, p < 0.01); the average PT was higher than that in normal control group (22.4° ± 9.9° vs. 9.6° ± 5.9°, p < 0.01) and cervical spondylosis group (22.4° ± 9.9° vs. 10.5° ± 7.0°, p < 0.01).

Correlation analysis between coronal parameters and sagittal parameters in DLS group were carried out (Table 3). There were significant correlations between the Cobb’s angle and PT (p < 0.05). Correlation analysis among the sagittal parameters was also conducted (Table 4), significant pairwise correlations were found among LL, PI, SS, and PT (p < 0.01), PT was negatively correlated with TK (p < 0.01), and there were also significant correlations between LL and TK, LL and SVA (p < 0.01).

Table 3.

Correlation analysis between coronal parameters and sagittal parameters.

		Cobb’s angle	Coronal balance	LL	PI	SS	PT	TK	SVA
L3 tilt	Coefficient (r)	0.411^a	0.206^b	−0.098	0.034	−0.082	0.118	0.085	0.176
L3 tilt	p	0.000	0.037	0.320	0.733	0.406	0.232	0.390	0.074
Cobb’s angle	Coefficient (r)		−0.051	−0.106	0.098	−0.106	0.212^b	−0.130	0.025
Cobb’s angle	p		0.609	0.284	0.320	0.282	0.031	0.187	0.798
Coronal balance	Coefficient (r)			−0.179	−0.137	−0.159	0.019	−0.040	0.184
Coronal balance	p			0.070	0.169	0.108	0.852	0.687	0.063

LL: lumbar lordosis; PI: pelvic incidence; SS: sacral slope; PT: pelvic tilt; TK: thoracic kyphosis; SVA: sagittal vertical axis.

^aSignificant with p < 0.01 (Pearson’s test).

^bSignificant with p < 0.05 (Pearson’s test).

Table 4.

Correlations among the sagittal spino-pelvic parameters.

		LL	PI	SS	PT	TK
Age	Coefficient (r)	0.093	−0.059	0.064	−0.124	0.271^a
Age	p	0.345	0.554	0.518	0.210	0.005
LL	Coefficient (r)		0.353^a	0.785^a	−0.456^a	0.536^a
LL	p		0.000	0.000	0.000	0.000
PI	Coefficient (r)			0.537^a	0.462^a	−0.080
PI	p			0.000	0.000	0.419
SS	Coefficient (r)				−0.495^a	0.208^b
SS	p				0.000	0.034
PT	Coefficient (r)					−0.295^a
PT	p					0.002

LL: lumbar lordosis; PI: pelvic incidence; SS: sacral slope; PT: pelvic tilt; TK: thoracic kyphosis.

^aSignificant with p < 0.01 (Pearson’s test).

^bSignificant with p < 0.05 (Pearson’s test).

Discussion

Previous literature has found that adult idiopathic scoliosis patients had higher PI value than normal,^13,14 and sagittal patterns were closely related to patients’ symptoms.^2,6,15,16 Moreover, classifications for ASD and DLS were partially based on sagittal parameters.^17,18 So this study analyzes the sagittal alignment and found that patients in DLS group had higher PI than the two control groups, and there is also significant difference with the PI in 260 cases of normal Chinese adults reported by Zhu et al.¹⁹ (age 34.3 ± 12.6 years, one sample t-test, PI = 44.6° ± 11.2°, p < 0.01). This suggests that PI in DLS patients may be a little higher than that in asymptomatic young Chinese population and elder patients with cervical spondylosis.

One reason for this result may be that a high PI may participate in the pathogenesis of DLS. In a previous study,¹² we found that with the increase of PI, lower arc of the LL increased gradually and included more vertebrae, and the apex of the lordosis ascended continuously. This may lead to changes of stress distribution, and increase the shear stress around the lordosis apex in the middle part of lumbar spine, coupled with relatively larger range of flexion, rotation, lateral bending activities, and lack of connection with both sides of the iliac. In this situation, the lumbar spine is more prone to degeneration, and previous literature has also reported disc degeneration associated with high PI.²⁰ If the degeneration speed of the two sides differs, the unbalanced degeneration will cause scoliosis.

Another reason may be the existence of spondylolisthesis. Previous studies have reported that degenerative lumbar spondylolisthesis is related with DLS,²¹ and Pritchett and Bortel²² also found a higher prevalence of degenerative spondylolisthesis among DLS patients (accounted for 55.5%). But previous literature did not work on the relationship between them. In this study, 38 patients in DLS group were complicated with degenerative lumbar spondylolisthesis, accounted for 36.5%. As for the natural prevalence, Iguchi et al.²³ reported that 8.7% of 3259 outpatients with low back discomfort were accompanied by degenerative spondylolisthesis, so the prevalence of spondylolisthesis in DLS was obviously higher, and the reason remains unclear. Previous studies²⁴ have confirmed the higher PI in degenerative spondylolisthesis, in this study the PI of DLS patients was also higher than normal, and the cases combined with spondylolisthesis had even higher PI, so there is a possibility that the high PI in cases with spondylolisthesis may raise the whole PI value. And another possible explanation is that the DLS and degenerative spondylolisthesis show common characteristics of higher PI, and both two diseases are easy to occur in patients with higher PI. Although the relationship among PI, DLS, and degenerative lumbar spondylolisthesis remains to be studied, this phenomenon provides a way to explore the intrinsic relevance between the two diseases, using PI to serve for etiology study and preventive work.

For other sagittal parameters, previous studies have found that LL and SS were lower in DLS patients.^1,2,3,22 In this study, we drew a similar conclusion, which suggests that the pelvis of DLS patients adjust its posture by rotating and tilting backward, so as to compensate for the sagittal imbalance. And the correlation between LL and other sagittal parameters indicates that the lumbar spine with scoliosis and severe degeneration still maintains ability to regulate the sagittal balance. This is consistent with the results in studies about normal adults and patients with other spinal diseases, such as isthmic spondylolisthesis⁸ and adult idiopathic scoliosis.² This indicated that in corrective surgery reconstruction of LL should also be emphasized.

Although measuring method of TK has not been unified,^25
–27 previous studies have not found explicit correlation between TK and pelvic parameters in normal young adults and patients with adult scoliosis.^2,28 But in this DLS group not only LL, but also SS and PT were associated with TK. One reason may be that the lumbar spine with degeneration and scoliosis present loss of lordosis and relatively stiffness, and the rotation of the pelvis is easier to transmit upward, resulting in changes in thoracic lumbar curvature. On the other hand, lumbar spine with severe degeneration in DLS patients may have poorer regulating ability of regulating sagittal balance than normal, therefore depend more on thoracic curve changes. So more attention should be paid to the sagittal balance during fixation of thoracic vertebra for patients with DLS.

This study has some limitations. Firstly, previous studies suggested that PI, as a pelvic morphologic parameter, remains stable in adulthood after maturation of bone,^7
–9 but recently some studies reported changes of PI.^29
–31 So more work should be done to distinguish whether the PI difference in this study comes from the diseases or other age-related factors (e.g. degeneration of sacroiliac joint). Secondly, although no inevitable association between cervical spondylosis and lumbar diseases has been found, cervical spondylosis patients were not perfect in spine, and some position-related parameters (such as LL etc.) were related to age, so data of elderly population remain to be collected and analyzed. Thirdly, measurement of DLS patients with severe degeneration is difficult, and the PI difference was small and easy to be influenced by the potential error for measurement. This study had a reasonable sample size, the statistical analysis showed significant difference with two independent control groups and reports from other researchers, and Vila-Casademunt et al.³² had proved the reliability on the measurement of inexperienced observers on spino-pelvic parameters after a short tutorial. So the result was meaningful, suggesting the existence of PI difference, which can be resulted from some certain causes.

Conclusions

This study is to our knowledge the first to report a higher PI in DLS. The higher PI value is probably associated with the pathogenesis of DLS and the high incidence of degenerative lumbar spondylolisthesis among DLS patients. We also found that in DLS patients the lumbar spine maintains the ability of regulating the sagittal balance, and they depend more on thoracic curve changes to regulate sagittal balance.

Footnotes

Acknowledgment

We wish to thank Beijing Municipal Science and Technology Commission.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: funded by Beijing Municipal Science and Technology Commission (Award No. Z151100004015101).

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