10.1 Introduction

The sagittal balance of the spine has been recognized as one of the new pillars in spine surgery. The analyses of pelvic parameters, spinal parameters, and the assessment of the global balance of the spine are providing new perspectives on the surgical treatment of spinal pathologies. Recognized since Hippocrates, the sagittal spinal segmentation has limited the lumbar lordosis (LL) between T12 and S1. More recently, Stagnara et al ¹ and Berthonnaud et al ² have redefined the functional borders of human distal lordosis with respect to an inflection point where lordosis transitions into kyphosis. Spinal curvatures are thereafter characterized by their length and angle, where among the humans there are shorter lordoses and longer lordoses with higher and lower angles. The distribution of angles throughout the lordotic curve is, however, not always homogeneous and has led to the classification of four spinopelvic morphotypes.

By employing the sacral slope (SS) and the pelvic incidence (PI) on lateral radiographs, Roussouly et al have classified four spinopelvic morphotypes according to four lordosis types. ³ These authors observed a trend in patterns of spinal degeneration according to the type of presenting lordosis. It has been suggested that differences in spinal degeneration are also dependent on different patterns of spinal architecture, as exerted gravitational and muscle forces apply different patterns of mechanical stresses on spinal articulations. The classification of these degeneration patterns detailed in this chapter provides the specialist a better understanding of the natural history of spinal degeneration for each specific spinopelvic morphotype and, therefore, serves as an impetus to promote more appropriate treatments.

10.2 Distal Spinal Lordosis and Pelvic Incidence as Sagittal Balance Modifiers

As described in Chapter 6, the authors emphasize herein differences in nomenclature: LL and distal spinal lordosis (DSL). Based on functional segmentation (Berthonnaud et al ² ), the new term DSL is defined as the part of the spine in extension between the S1 endplate and the inflection point where lordosis transitions into kyphosis (Fig. 10‑1). The classic usage of LL remains, that being, lordosis between T12-L1 and the S1 plateau.

LL and SS are integrally related to pelvic orientation (r = 0.85), which is strongly influenced by the PI (r = 0.83). ⁴ Stagnara found a strong correlation between LL and SS. ¹ Likewise, an important correlation was found between LL and PI ^{1 ,​ 5} in asymptomatic lower back pain (LBP) individuals. ⁶ Usually, the presence of a small PI equates to a smaller SS. The wide variability of SS in accordance with PI values was demonstrated by Roussouly, which led to his classification of four types of spinopelvic morphologies. ⁷

10.3 Distal Spinal Lordosis Geometrical Analysis ⁸

The radiological quantification of lordosis has been studied and demonstrated through a variety of ways. The arc of circle system of Berthonnaud et al ² demonstrated that lordosis could be mathematically expressed as having two contiguous arcs of circle tangents in the horizontal line drawn from the apex of the lordosis (apex level is determined by the vertical line drawn tangentiating the anterior-most part of the convex side of lordosis). ² The horizontal line crossing the apex of lordosis creates two arcs of a circle: the upper arc of the lordosis (from the apex horizontal line to the inflection point where lordosis bends into kyphosis) and the lower arc of the lordosis (from the apex horizontal line to the sacral endplate). The angle of the lower arc and the SS are the same angle. ³

Based on spinal lordosis/kyphosis segmentation, Roussouly et al developed a classification of four lordosis types, according to the concordances of the SS and the PI (Chapter 6). The main distinction among lordotic curves is the magnitude of the lower arc of lordosis. From geometric constructions, the lower arc of DSL is equal to SS. In agreement with the same geometric principle, the upper arc of DSL is equal to the lower arc of spinal kyphosis. DSL analysis thus consisted of the analyses of both the lower arc (equal to SS) and the upper arc. To segment it, SS was divided into three categories: low (<35°), average (35°<SS<45°), and high (>45°) SS.

10.3.3 High SS (>45°)

Type 4 Lordosis (Hypercurved)

Also known as the greater lordosis, SS is high, with the lower arc also being large (>45°). The distal spinal lordosis is long composed of more than five vertebrae included in the curve. The apex of lordosis is high (above L4) and the inflection point is also high, beyond the classic T12-L1 limits. The toggle angle is generally from zero to negative. The TK is generally much curved in correspondence with higher lordosis.

Roussouly et al observed that types 1 and 2 lordoses usually have lower PI values, while types 3 and 4 generally have higher PI values. ⁹ The authors have also noticed subjects with a very small or even negative pelvic tilt (PT) of <10° in very anteverted pelvises. This situation may allow for SS >40° even with a small PI, recognized as an anteverted type 3 with a small PI. ¹⁰ The PI value suggests a tendency for LL morphotypes; however, to extract an LL value from PI is an inexact extrapolation (Fig. 10‑2).

10.4 Lumbar Lordosis and Thoracic Kyphosis Angle

Spinal parameters LL and TK are interdependent. Jackson and McManus ¹¹ observed a significant correlation between LL and TK. ⁶ The method of tangent arcs of circle segmentation applies to the thoracic spine. There is a direct relation between the upper arc of LL and the lower arc of TK. A change in one induces a change in the reciprocal segment of the other depending on the flexibility of the spine. This is more relevant in type 1 (too-small lower LL arc) where the total LL depends mostly on the upper arc of LL, which is forced to compensate for the higher, lower arc of TK in the thoracolumbar area.

TK may extend beyond the thoracolumbar area. Sometimes, both LL and TK curves are separated by a straight segment composed of a variable number of vertebrae. The importance of this disposition has not been well validated and requires further study.

10.5 Contact Forces: Resultant from Gravity and Muscular Action

We postulate a correlation between the spinopelvic morphotypes and the degenerative patterns found in computed tomography (CT) scans and magnetic resonance imaging (MRI). In the nonbalanced spine, there is a mechanical trend for forward gravitational forces to prevail over counteracting back muscle forces. These contact forces, the sum of both gravitational and muscular action, may load more strongly on the anterior parts of the spine (i.e., vertebrae and disks or the posterior elements of the spine, that is, zygapophyseal facet joints), depending on the morphology of one’s spinopelvic structure. Individuals with flat spine or very poor lordosis curvature tend to present contact force loading predominantly in the anterior elements: disks and vertebrae. Inversely, a prominently curved lordosis produces higher loading in the posterior facet joints. Futhermore, the greater the tilt of the sacral plate, the greater are the resultant shear-slipping forces.

In Chapter 4, a spinal model using finite elements confirms this variation of loading action in agreement with the spinopelvic types.

In the “flat back,” that is, type 2, the resultant forces are located forward in the lumbar spine, promoting high disk pressures. In type 1, combining both short hypercurved DSL and thoracolumbar kyphosis (TLK), load stress acts predominantly posteriorly on facet joints in the distal spinal lordosis area, but acts more anteriorly in TLK, creating a forward hyperpressure on disks in the TLK area.

In cases of greater lordosis, as in type 4, the resultant forces are displaced posteriorly over the posterior facet joints. This sagittal orientation contributes to bone remodeling in the facet arthrosis, lumbar narrow canal, and olisthesis (degenerative spondylolisthesis) in the distal end of the curve where the disks are more tilted. It becomes clear that the sagittal spinopelvic structure defines which specific alterations will occur through aging and natural degenerative processes. Each of the four spinopelvic morphotypes have a trend to produce specific outcomes in CT scans and MRIs ^{7 ,​ 8} (Fig. 10‑3)