5 Sagittal Balance: The Main Parameters

10.1055/b-0039-171401

5 Sagittal Balance: The Main Parameters

João Luiz Pinheiro-Franco and Pierre Roussouly

Abstract:

While corrective surgeries that modify sagittal alignment and spinopelvic balance are highly complex and may remain the realm of specialized surgeons for the foreseeable future, a thorough understanding of these underlying concepts has become requisite for all spine surgeons to prevent iatrogenic deformities, even with very short fusions.

In humans, due to our bipedal posture and inherent pathophysiology of disk degeneration, sagittal imbalance has been determined to be a common and clinically relevant problem—termed adult degenerative spinal deformity (ADSD). A concise evaluation of spinal balance in most adults can be thus divided in the assessment of pelvic parameters, spinal parameters and global sagittal balance.

Among the most important compensation mechanisms of sagittal balance, there is the ability to position the pelvis in rotation around the femoral heads. Pelvises with smaller Pelvic Incidence (PI) have less ability to retrovert over the femoral heads. Therefore, morphology defines the functional ability of the pelvis to rotate more or less, and to provide for an adequate or inadequate sagittal imbalance compensation.

Understanding that there is a great variability of PI, and that analyses of compensatory mechanisms should not be based exclusively on absolute values of Pelvic Tilt (PT), it is paramount not to assume absolute PT values to define if the spine is balanced or not.

Historically, the human spine was divided into a lumbar lordosis, a thoracic kyphosis and a cervical lordosis. In opposition to this old anatomical segmentation of the spine, there have been different proposals for a functional segmentation, arguing that it is the orientation of the successive vertebrae that defines the curvatures: lordosis represents the area where the successive vertebrae are in extension, and kyphosis in flexion. This functional segmentation allows a true sagittal measurement to be made using the Cobb method. Several authors have tried to combine those parameters in order to define the limits between balanced and unbalanced status. Some formulae as PI-LL seemed to cover a large range of situations, but we know now that pathological evolutions are not unique but mainly depending on the various shapes linked to PI value.

5.1 Introduction

The concept of the spine having a “normal” alignment and that any deviation from this “norm” is not simply an anatomical variation but a fundamental problem, has been intrinsic to medicine since ancient times. Historic medical literature is rife with examples from Greek, Roman, and Arab sources of early attempts to manipulate or modify spinal alignment. Certain types of deformities that had been studied throughout the ages were instrumental in spurring the development of spine surgery since its inception in the early 19th and 20th centuries, particularly postinfectious (tuberculous), pediatric, and syndromic deformities. Nonetheless, a more comprehensive understanding of sagittal alignment and degenerative spinal deformity came into focus much later and may be considered the most valuable breakthrough in spinal degenerative pathology in the past 30 years. Whereas corrective surgeries that modify sagittal alignment and spinopelvic balance are highly complex and may remain the realm of specialized surgeons for the foreseeable future, a thorough understanding of these underlying concepts has become requisite for all spine surgeons to prevent iatrogenic deformities, even with very short fusions.

Ideal spinal alignment may be defined as the harmonious balance of the trunk over the pelvis, one which requires minimal energy expenditure to place the weight-bearing axis in a balanced physiological position. ¹ In humans, because of our bipedal posture and inherent pathophysiology of disk degeneration, sagittal imbalance has been determined to be a common and clinically relevant problem, termed adult degenerative spinal deformity (ADSD). A concise evaluation of spinal balance in most adults can be thus divided into the assessment of pelvic parameters, spinal parameters, and global sagittal balance.

5.2 Methods of Measurement

The study of the sagittal balance of the spine is dependent on radiographs in a standardized fashion. The Cobb method may be used: a lateral radiograph of the spine is made with vertical 30- to 90-cm film with a constant 72-in distance from the radiographic source. The knees and hips must be in a natural position (full extension is not required). Be mindful that there are controversies about the arm positions. To avoid radiological superposition of arms and spine, a forward position of arms is necessary. Two main options are possible: fist on clavicle and arms on support. In a study by Mac-Thiong et al, ² for two main French institutions, subjects stood with shoulders flexed 30° to 45° with hands resting on supports, whereas the remaining subjects recruited in North American institutions adopted the fist-on-clavicle position. ³ Vedantam et al ⁴ found that elevating the arms from 30° to 90° in standing lateral radiographs shifted the sagittal vertebral axis (SVA) 10 mm posteriorly in subjects with previous spinal fusion, which was not significant in subjects without spinal fusion. They also found that thoracic and lumbar curvatures were not affected by arm position in both subjects. For some weak patients (aging, neuromuscular), prolonged immobility without support is difficult to obtain and may induce artifacts. Marks et al ⁵ affirmed that shoulder flexion of 45° is the best position to use when a lateral radiograph is made for repeated SVA measuring.

Recently, the EOS system has been developed, which allows for full-spine standing imaging with minimal irradiation. When using EOS, the sliding sources system needs only 6 to 10 seconds for image capture but requires a highly stable patient positioning during the full procedure to avoid artifacts.

The main rule for positioning the patient is the standardization of the radiography procedure to allow for the comparison two different images. The radiograph must show the femoral heads (FHs) and at least the skull base. Radiographs need be digitized, and all measurements should be performed using specialized software, such as Surgimap or KEOPS (SMAIO, Lyon, France). Such software permits rapid and precise measurements of all angular parameters on digitized radiographs. Using a computerized software, the intraobserver and interobserver reliability is very high and the results are similar to those obtained by manual measurement. ⁶ ^, ⁷ Other reports have evaluated the intra- and interobserver reliability of measuring pelvic incidence (PI) using manual methods on digitized radiographs. ⁸ ^, ⁹ Dimar et al ⁹ obtained low intraclass correlation coefficients (ICCs) of intra- and interobserver reliability for manual measurements of PI even among experienced spinal surgeons (0.69 and 0.41, respectively). Dimar et al recommended computer-assisted method for measuring PI with high reliability. ⁹ Yamada et al ¹⁰ used a computer-assisted method obtaining exceptionally high agreement of intra- and interobserver ICC of Full Spine (FS)-PI (0.84 and 0.79, respectively) and correlation coefficient of 0.81 between FS-PI and Computed Tomography (CT)-PI. These authors ¹⁰ noted that the error of measuring PI is not mainly a result of the difficulty in precisely identifying the bicoxofemoral axis but a result of the difficulty in identifying the sacral endplate in cases with lumbar scoliosis, obesity, and elderly patients with possible osteoporosis. Legaye et al ¹¹ observed, especially in situations with a dome-shaped sacrum, that the inaccurate visualization of the superior plate of S1 does not allow an exact measurement of PI.

Whatever the employed technique, it is mandatory and essential to obtain good radiographs with perfectly readable landmarks. In case of doubtful situations where radiological landmarks are missing or are not visible, the analyzed case must be rejected.

5.2.1 Pelvic Parameters

Central to the concept of global spinal balance is the idea that the sacrum, as the basis of spine building, is almost a contiguous and fully mobile element of the spine: the “pelvic vertebra” as it is referred to by Jean Dubousset (Fig. 5‑1). The sacrum is part of the pelvic ring through the sacroiliac joints (SIJs), a diarthrodial joint with minimal movement thus comprising a single functional unit with the pelvis, articulating with the FHs through the acetabulum. The relationship between the sacrum and the acetabulum is thought to remain stable during most of an adult’s life, with changes usually as a consequence of trauma or surgery with consequent alterations to the SIJ or a suggested very slow and small increase as a result of aging. ¹² Effective hip and pelvic balance are necessary to provide the spinal and pelvic muscles an optimal alignment that effectively supports the spinal column, which consequently is crucial to maintain a standing erect posture.

**Fig. 5.1** **(a)** Geometrical construction of the pelvic parameters described by Duval-Beaupère: pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). **(b)** Sagittal reconstruction of the spine using KEOPS software.

These concepts came to light in the late 1980s not only as a result of an enhanced understanding of the normal aging of the spine but also of increased perception of iatrogenic spinal deformities following spinal fusions. ¹¹ ^, ¹³ Although some anatomists such as Delmas in France had previously described variations in the sacral anatomy, it was only in 1986 that During et al ¹⁴ presented a new pelvis morphometry that demonstrated a variability in the pelvis anatomy that influenced the pelvis orientation and the spinal shape. During et al proposed a skeletal outline of the spinopelvic complex defining morphological and positional angles. It was suggested that “aberrations of posture” might cause low back pain as a result of patterns of stress concentrations. This concept was very advanced for the time and even the term “sagittal balance” was never mentioned. During’s system of arcs of circle were to become the cradle to the classification of lordosis types by Roussouly et al. ¹⁵ Duval-Beaupère et al ¹⁶ clarified a pelvic morphological evaluative method defining the PI as the angle linking the anatomical and functional relations between the sacrum and the hips joints. These relationships between hip-pelvis and spine may be exemplified mathematically and radiologically by three angles, collectively termed pelvic parameters.

5.2.2 The Morphological Pelvic Parameter

PI is the main angle and demonstrates the anatomical relationship between the acetabulum and the S1 endplate. PI is the morphological angle representing the mature morphology of the pelvis. It is considered to grow from childhood until reaching the maximal value. PI is the angle between two lines: the line perpendicular to the midpoint of the sacral endplate and the line drawn by the union of two points: the midpoint of the sacral endplate and the center of the bifemoral axis (or the midpoint of a line connecting both FHs if they are misaligned) (Fig. 5‑1). As mentioned above, PI was once thought to remain stable during adulthood but there is increasing evidence that aging will lead to a gradual increase in PI through modification of SIJ anatomy. ¹⁷ ^, ¹⁸

Anatomists have never described variations in pelvic shape with consideration of PI. In asymptomatic populations, there is a large range of PI values between 35° and 85° with extremes of less than 20° until 95° in pathology. ¹⁵ What is the morphological significance of such variations? Roughly in cases of small PI (<45°), the sacral plateau is just over the FH, the sacrum is long, and the sacral plateau horizontal projection is close to the iliac crest. In cases of high PI, the sacrum is shorter as if a part of the sacral vertebra was missing and is positioned more posteriorly regarding the FH center. The sacral plateau is far below the projection of the iliac crest. A pelvis with small PI is referred to as a narrow pelvis, whereas one with high PI represents a large pelvis. However, it is uncertain whether this hypothesis is correct. It has been supposed that there exist anatomical variations of iliac bone orientation: more frontal in small PI and more sagittal in high PI. This hypothesis needs to be considered in the evolution of the bipedal acquisition, as large apes have a more frontal iliac bone orientation compared to the human pelvis. Pelvic shape in Homo sapiens, on the other hand, has poor variation in the iliac bones, whereas changes in PI are mainly a result of the sacral morphology. Debate exists whether PI is higher in human females or whether there is no difference according to gender. Likewise, there is discussion of whether there are regional or ethnic variations ¹⁹ ^, ²⁰ as well.

To accurately measure PI, the physician may be faced with several difficulties. The sacral endplate with anterior and posterior limits is usually easy to identify. Inversely, determination of the FH center may be confounding because it is difficult to have both FH centers superimposed when using classical radiographs with a fixed source. Two situations may occur:

The FHs are shifted on the vertical axis (Fig. 5‑2). This is because of the X-ray beam inclination to lower limb length difference. Using the midpoint of the line between both centers of FH is therefore acceptable for measuring PI.
The FHs are shifted on the horizontal axis (Fig. 5‑3). This should be unacceptable when this situation interferes with the correct analysis of the sagittal balance. This may happen because of a horizontal rotation of the pelvis. The image is no longer a true lateral view and if the shift is too great, the important PI measurement will be compromised, and the radiograph needs to be rejected.

Fig. 5.2 Vertical femoral head misalignment as a result of inequality of lower limbs: poor change in pelvic incidence (PI) measurement.

Fig. 5.3 Horizontal FH misalignment as a result of pelvic rotation: high change in pelvic incidence (PI) measurement, risk of error in PI evaluation.

When using the EOS system for spinal balance analysis, the perfect horizontal alignment of the beams at each level may allow for the perfect superposition of FH except for lower limb inequality. However, a rotational position of the pelvis with a horizontal shift remains possible and PI measurements have yet to be rejected when the shift overpasses the half diameter of FH.

Sometimes the identification of the sacral endplate may be difficult. In high-grade dysplastic spondylolisthesis, the sacral endplate is no longer flat, and its rounded contour is referred to as “dome”-shaped. In these cases, PI measurement is impossible. In cases of junctional L5-S1 abnormalities, when L5 is sacralized, the L5-S1 vestigial disk must be used for PI measurement when clearly visible. When S1 is part of the lumbar spine, the upper plate of S1 may be used (Fig. 5‑4). Sometimes, in the case of strong ambiguity, the general aspect of the spine may be a guide for the choice of vestigial disk for PI measurement. In every case, when it is impossible to identify the sacral endplate, PI measurement should be rejected.