Radiographic Analysis of the Sagittal Alignment and Balance of the Spine in Asymptomatic Subjects

Vialle, Raphaël MD; Levassor, Nicolas MD; Rillardon, Ludovic MD; Templier, Alexandre MD; Skalli, Wafa MD; Guigui, Pierre MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.D.02043
Scientific Articles
Abstract

Background: There is an increasing recognition of the clinical importance of the sagittal plane alignment of the spine. A prospective study of several radiographic parameters of the sagittal profile of the spine was conducted to determine the physiological values of these parameters, to calculate the variations of these parameters according to epidemiological and morphological data, and to study the relationships among all of these parameters.

Methods: Sagittal radiographs of the head, spine, and pelvis of 300 asymptomatic volunteers, made with the subject standing, were evaluated. The following parameters were measured: lumbar lordosis, thoracic kyphosis, T9 sagittal offset, sacral slope, pelvic incidence, pelvic tilt, intervertebral angulation, and vertebral wedging angle from T9 to S1. The radiographs were digitized, and all measurements were performed with use of a software program. Two different analyses, a descriptive analysis characterizing these parameters and a multivariate analysis, were performed in order to study the relationships among all of them.

Results: The mean values (and standard deviations) were 60° ± 10° for maximum lumbar lordosis, 41° ± 8.4° for sacral slope, 13° ± 6° for pelvic tilt, 55° ± 10.6° for pelvic incidence, and 10.3° ± 3.1° for T9 sagittal offset. A strong correlation was found between the sacral slope and the pelvic incidence (r = 0.8); between maximum lumbar lordosis and sacral slope (r = 0.86); between pelvic incidence and pelvic tilt (r = 0.66); between maximum lumbar lordosis and pelvic incidence, pelvic tilt, and maximum thoracic kyphosis (r = 0.9); and, finally, between pelvic incidence and T9 sagittal offset, sacral slope, pelvic tilt, maximum lumbar lordosis, and thoracic kyphosis (r = 0.98). The T9 sagittal offset, reflecting the sagittal balance of the spine, was dependent on three separate factors: a linear combination of the pelvic incidence, maximum lumbar lordosis, and sacral slope; the pelvic tilt; and the thoracic kyphosis.

Conclusions and Clinical Relevance: This description of the physiological spinal sagittal balance should serve as a baseline in the evaluation of pathological conditions associated with abnormal angular parameter values. Before a patient with spinal sagittal imbalance is treated, the reciprocal balance between various spinal angular parameters needs to be taken into account. The correlations between angular parameters may also be useful in calculating the corrections to be obtained during treatment.

Author Information

1 Department of Orthopaedic Surgery, Hôpital Beaujon, 100 Boulevard de Général Leclerc, F-92110 Clichy, France. E-mail address for R. Vialle: ravialle@noos.fr

2 Department of Biomechanics, ENSAM-PARIS, 151 Boulevard de L'Hôpital, F-75013 Paris, France

Article Outline

There has been an increasing recognition of the importance of the sagittal plane contour in the normal function of the spine and in its various disease states1-15. The reciprocal curves of cervical lordosis, thoracic kyphosis, and lumbar lordosis allow efficient absorption of the loads applied to the spinal column and increase the efficiency of the spinal musculature. These curves, especially lumbar lordosis, play a role in the maintenance of an efficient upright posture. Many authors have reported the negative effects of a reduced lumbar lordosis with fixed sagittal imbalance after spinal instrumentation, also known as a flat-back deformity1,2,9,10,16,17. Therefore, in the treatment of complex spinal deformities such as scoliosis or kyphosis, it is important to restore both frontal and sagittal balance. The most important radiographic parameters of the sagittal balance of the spine in upright posture are well defined1,10,15,18,19, but very few reports present the normal physiological values. Moreover, it is well established that the physiological upright standing posture can be reached in a different way for each person with a unique and individual pattern of spinopelvic balance and sagittal alignment1. These patterns can be affected by numerous variables, such as the age, gender, weight, and, especially, the pelvic morphology and orientation of the patient. Thus, it is necessary to study the normative values and relationships among all of the parameters of spinopelvic balance in asymptomatic patients.

The aim of this study was to determine the radiographic parameters of the sagittal profile of the spine in a volunteer cohort. We determined the variations of these parameters according to epidemiological and morphological criteria and studied the relationships among all of these parameters.

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Materials and Methods

Three hundred subjects agreed to participate in the study. All were volunteers and met the following inclusion criteria: an age between twenty and seventy years, no history of a spinal disorder or spinal surgery, and no radiographic abnormality detected prior to or during the study. Hip, knee, and ankle abnormalities were ruled out by clinical examination. All volunteers provided informed consent. The study population consisted of 300 volunteers (110 women and 190 men), with a mean age of thirty-five years (range, twenty to seventy years). The epidemiological and morphological characteristics of this cohort were obtained from the following data: age, gender, weight, and height. The body mass index was calculated as the weight in kilograms divided by the square of the height in meters.

For each patient, one lateral radiograph of the spine was made with use of a vertical 30 × 90-cm film with a constant distance between the subject and the radiographic source. The subject stood in a comfortable position, with the knees fully extended and the arms flexed forward to 45° and resting on supports. The radiograph was centered on the twelfth thoracic vertebra and was made during inhalation.

The radiographs were digitized, and all measurements were performed by means of SpineView software (SurgiView, Paris, France), which was validated in a previous study20. The end-plate lines used for measurements were identified by three of the authors (N.L., L.R., and P.G.).

On each lateral radiograph, three pelvic parameters were measured (Fig. 1). The sacral slope is the angle between the horizontal line and the cranial sacral end-plate tangent. The pelvic tilt is the angle between the vertical line and the line joining the middle of the sacral plate and the center of the bicoxofemoral axis (the line between the geometric center of both femoral heads). According to Legaye et al.10, the pelvic incidence is the key anatomical parameter for determining the spinal balance. The pelvic incidence is the angle between the line perpendicular to the middle of the cranial sacral end plate and the line joining the middle of the cranial sacral end plate to the center of the bicoxofemoral axis.

On each lateral radiograph, three spinal parameters were measured (Fig. 2). The lumbar lordosis is the angle between the cranial end plate of L1 and the caudal end plate of L5. The thoracic kyphosis is the angle between the cranial end plate of T4 and the caudal end plate of T12. The T9 sagittal offset is the angle between the vertical plumb line and the line between the center of the vertebral body of T9 and the center of the bicoxofemoral axis. In addition, the T1 sagittal offset is the angle between the vertical plumb line and the line between the center of the vertebral body of T1 and the center of the bicoxofemoral axis. Vertebral wedging and the intervertebral angles were also measured for all vertebral levels between T9 and L5. By summing the intervertebral angles and the vertebral wedging, we obtained the values of the segmental angulations from T9 to S1. Furthermore, we noted the so-called transitional vertebra located at the junction of the lumbar lordosis and the thoracic kyphosis, in order to measure the maximum lordosis (the angle between the cranial end plate of the transitional vertebra and the cranial end plate of S1) and the maximum kyphosis (the angle between the cranial end plate of T4 and the caudal end plate of the transitional vertebra).

The data were analyzed with use of SPSS software (SPSS, Chicago, Illinois). Our analysis was conducted in four steps. First, we performed a descriptive analysis of the epidemiological and morphological parameters of the cohort. Second, we determined the mean value, standard deviation, standard error, and range of the angular parameters previously defined. Third, we investigated the relationship between the epidemiological and morphological parameters using simple and multivariate linear regression analysis and the unpaired t test. Finally, we studied the variations of all of the parameters with respect to each other. In order to perform this final analysis, our study was based on three hypotheses formulated by During et al.1, Legaye et al.10, and Duval-Beaupère et al.21,22. First, the pelvic incidence is a morphological parameter for which the value is fixed and invariable for each subject. Second, sagittal balance is obtained because of the adaptation of other parameters to this fixed parameter. Third, the T9 sagittal offset is an indicator of the position of the center of gravity of the part of the body above the femoral heads and therefore of the sagittal balance of the spine. Using simple and multivariate linear regression and principal components analysis, we attempted to describe the relationships between different parameters and the pelvic incidence and to determine how the T9 sagittal offset varied with regard to all of these parameters.

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Results

The main characteristics of the population and the values of the angular parameters are reported in Table I. The distribution of each angular parameter was normal. The transitional vertebra, which is the junction between the thoracic kyphosis and the lumbar lordosis, was T11 in 12.6% of the spines, T12 in 23.7% of the spines, L1 in 37.7% of the spines, and L2 in 26% of the spines. By summing up the intervertebral angles and the vertebral wedging, we obtained the values of the segmental angulations from T9 to S1 (Table II).

The distributions of the lumbar lordosis, the sacral slope, and the pelvic incidence with regard to gender were not normal. The mean values for women were higher than those obtained for men. No significant difference was detected, on the basis of the numbers available, for the T9 sagittal offset or for the pelvic tilt with regard to gender (Table III). No significant correlation was found, on the basis of the numbers, between the body mass index and any of the angular parameters. No significant correlation was found, on the basis of the numbers, between the subject age and the T9 sagittal offset, the sacral slope, and the pelvic tilt. We found small correlations between age and the following parameters: thoracic kyphosis (r = 0.2), pelvic incidence (r = 0.14), and lumbar lordosis (r = 0.14). When gender was considered in the multivariate analysis, age no longer influenced the thoracic kyphosis, the pelvic incidence, or the lumbar lordosis.

We observed a very strong correlation between the maximum thoracic kyphosis and the T9 sagittal offset, between the maximum lumbar lordosis and the sacral slope (Fig. 3), between the maximum lumbar lordosis and the pelvic incidence, between the sacral slope and the pelvic incidence, and between the pelvic tilt and the pelvic incidence. The correlation matrix among all of the angular parameters is shown in Table IV. Several simple or multiple regression models were then established (Table V). Principal components analysis indicated that 90% of the variation of all of the parameters considered could be explained by three uncorrelated new variables. The first variable was the pelvic incidence, the maximum lumbar lordosis, and the sacral slope; the second one was the thoracic kyphosis; and the third one was the pelvic tilt. A multivariate regression on principal components showed a very good correlation (r = 0.8, R2 = 0.7) between the T9 sagittal offset and the set of the three new variables.

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Discussion

The current study yields a physiological standard for several angular pelvic and spinal parameters that describe spinal balance, measured in a cohort of 300 asymptomatic adult volunteers. Many authors have conducted studies with similar objectives1,3-8,10,15,17,23-27. The measurements of various parameters taken into account in these previous studies were facilitated by the use of data-processing software. In the current study, we used the SpineView software package, which enables a rapid and precise measurement of all angular parameters on digitized radiographs. In a previous study20, we showed that the intraobserver and interobserver reliability is very high and that the results obtained by the numerical process are similar to those obtained by manual measurement.

We think that it is important to make radiographs in a standardized fashion. According to Vedantam et al.28, positioning the arms at 90° rather than 30° resulted in a negative shift of the sagittal vertical axis. According to Marks et al.29, shoulder flexion of 45° is the best position to use when a lateral radiograph is made, in order to repeatedly measure the sagittal vertical axis. We believe that it is possible to use these positions in daily practice for the evaluation and treatment of spinal disorders affecting sagittal balance.

Because of the large number of patients in this study, we were able to provide physiological values for the segmental angulations from T9 to S1. We consider these values important in the management of spinal trauma. In a study on the long-term follow-up after treatment of lumbar or thoracolumbar fractures30, the authors stressed the importance of obtaining anatomic realignment in the sagittal plane from a functional point of view. According to that study, the parameter called “traumatic regional angulation” is dependent on the regional kyphosis and on the physiological angulation of the considered segment. The values obtained in the current study should be useful in establishing goals and planning an optimal therapeutic course.

The mean values for lumbar lordosis and sacral slope were different between women and men. Gelb et al.24, Legaye et al.10, and Korovessis et al.7,8 came to the same conclusion. On the contrary, lumbar lordosis as well as thoracic kyphosis were independent of gender in studies on fifty adult volunteers by Jackson et al.3-6. The variations in lumbar lordosis and sacral slope observed in those studies may be explained by a pelvic incidence that was slightly higher in women than in men.

Providing correct sagittal balance by surgical correction of a spinal deformity is of paramount importance. In the short term, it ensures a good position of the fused segment with regard to the gravity plumb line, allowing the best conditions for fusion. In the long term, good sagittal balance facilitates preservation of the adjacent levels16. During et al.1 showed that there is a relationship between the pelvic inclination angle and the sacral slope, between the sacral slope and the lumbar lordosis, and between the pelvisacral angle and the pelvic inclination angle. Legaye et al.10 defined the spatial position of the pelvis by means of the pelvic incidence and the pelvic tilt. In a cohort of thirty-eight volunteers, they demonstrated an interdependence between a complete set of parameters describing the spinal sagittal balance. This interdependence was directly linked to the pelvic incidence, which influences, by means of the sacral slope, the “ideal” lumbar lordosis allowing the maintenance of an efficient standing position. Statistical analysis (principal components analysis) of the data in the present study also showed that a well-balanced position is obtained by means of three uncorrelated factors: the pelvic incidence, the sacral slope, and the lumbar lordosis. This finding confirms the conclusions of During et al.1. Within this group of parameters, the pelvic incidence has a predominant role, since its value is not supposed to vary in an individual. Furthermore, the sacral slope also influences the lumbar lordosis.

The example in Figure 4 illustrates the importance of sagittal balance with respect to the surgical treatment of spinal disorders. The postoperative lateral radiograph of a patient with fixed sagittal imbalance after a posterolateral arthrodesis for the treatment of low-back pain shows that the lumbar lordosis was not adequately reestablished. The pelvic incidence was 55°, and the sacral slope was 20° instead of its theoretical value, which we determined was 41°. The lumbar lordosis was 30° instead of its theoretical value, which should have been 60°. The lateral radiograph (Fig. 4, A), made five years after surgery, shows an L3-L4 pseudarthrosis associated with degenerative spondylolisthesis. We believe that this complication was secondary to the fixed sagittal imbalance created by the first surgical procedure. The main objective of the subsequent operation was to restore the lumbar lordosis and the sacral slope in order to adapt them to the pelvic incidence. This goal was partially fulfilled by an L4 posterior wedge osteotomy (Fig. 4, B). The postoperative lumbar lordosis was 54°. In the case of this patient, we believe that obtaining a better sagittal balance improved the likelihood of ultimately achieving a solid fusion. ▪

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Department of Orthopaedic Surgery, Hôpital Beaujon, Clichy, and the Department of Biomechanics, ENSAM-PARIS, Paris, France

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