7 Spine Deformations

10.1055/b-0035-106382

7 Spine Deformations

Spinal deformities can be the result of unstable motion segments or, conversely, can cause them. The classification of spinal deformities can be confusing. For example, the use of the long axis of the spine as a reference has traditionally caused the term rotation to be used only for rotation about this axis—that is, rotatory deformations of the spine are traditionally thought of as those deformations that involve rotation, or twisting, of one or more of the vertebrae about the long axis of the spine. Although this use of the term rotation is, for the most part, maintained in this chapter, the term is also used in its more all-encompassing sense (i.e., meaning rotation about any axis). The latter use comprehends flexion, extension, and lateral bending. Translation and rotation can occur, respectively, along and about each of the three axes of the Cartesian coordinate system. Therefore, six fundamental movements can occur. The six fundamental types of spinal deformation are the following: (1) rotation about the long axis of the spine, (2) rotation about the coronal axis of the spine, (3) rotation about the sagittal axis of the spine, (4) translation along the long axis of the spine, (5) translation along the coronal axis of the spine, and (6) translation along the sagittal axis of the spine. Each of these movements or deformations can occur in either of two directions (Fig. 7.1). Each deformity type may involve only one spinal segment or multiple segments. Spinal deformities are most often combinations of two or more of these types. They may result from either acutely or chronically applied loads.

**Fig. 7.1** The six fundamental segmental movements, or types of deformation, of the spine along (straight arrows) or about (curved arrows) the instantaneous axis of rotation are the following: (1) rotation or translation about the long axis of the spine (A); (2) rotation or translation about the coronal axis of the spine (B); (3) rotation or translation about the sagittal axis of the spine (C); (4) translation along the long axis of the spine (A); (5) translation along the coronal axis of the spine (B); and (6) translation along the sagittal axis of the spine (C).

7.1 Rotation Deformations

Rotation deformations are manifestations of the application of an asymmetric load or a rotatory load (torque) to a spinal segment (Fig. 7.2). Rotation deformations about an axially oriented axis (coronal or sagittal) can occur at the level of the vertebral body (via asymmetric loss of vertebral height, as in posttraumatic kyphosis; Fig. 7.2b) or at the level of the disc interspace (via asymmetric disc interspace height loss, as in degenerative scoliosis; see Fig. 7.2). ¹ ^– ⁴ Segmental spinal rotatory deformation can also occur about the long axis of the spine (Fig. 7.3).

**Fig. 7.2** (A) A depiction of the forces and (B) the resultant rotation deformation about a coronally oriented axis of the spine, resulting in a wedgelike deformation. (C) Rotation deformations about an axially (coronally or sagittally) oriented axis can occur at the level of the disc interspace, as well. Curved arrows depict bending moments. Straight arrows depict applied forces.

**Fig. 7.3** (A) A twisting of the spine about its long axis (B) can result in a rotatory deformation about the axis. Curved arrow depicts applied bending moment.

The often-unrecognized coupling phenomenon, whereby one spinal movement or deformation along or about an axis (e.g., lateral bending) obligates another along or about another axis (e.g., rotatory deformation about the long axis of the spine), commonly results in subtle or not-so-subtle rotatory deformities about the long axis of the spine. The concept of spinal coupling is reemphasized here to underscore its importance in complex spinal surgery. As discussed in Chapter 2, the phenomenon of coupling is significant clinically. It plays roles both in the prevention of spinal deformation (by contributing to movement restriction) and in the exaggeration of the complexity of the deformation itself (when a deformity indeed occurs).

7.1.1 Rotation Deformation about the Long Axis of the Spine

The application of a rotatory or torsional load to the spine (either acutely, caused by trauma, or chronically, caused by gradual deformity progression [commonly complicated by the coupling phenomenon]) can cause the spinal segments above the unstable segment to rotate in a direction opposite to the direction of rotation of the segments below the unstable segment. This usually occurs about the long axis of the spine (see Fig. 7.3). In traumatic permanent deformation, ligamentous and bony elements (e.g., facet joints) are often disrupted. Classic examples of such acute injuries are the unilateral cervical locked facet (rotation combined with flexion) and posttraumatic fracture–dislocation with an accompanying rotatory component (see Chapter 6). These two injuries exemplify the fact that rotatory deformation about the long axis of the spine is seldom an isolated entity.

7.1.2 Rotatory Deformation about the Coronal and Sagittal Axes of the Spine

The application of eccentrically placed loads to a spinal segment creates a bending moment. The applied bending moment may result in failure of the spinal segment with accompanying deformation along one or both of the axially oriented axes (see Fig. 7.2). This deformation results in rotation of the segments above and below the involved segment(s). Relatively speaking, the segments above and below rotate toward each other. This rotation can take the form of kyphosis (flexion rotation deformation), lordosis (extension rotation deformation), scoliosis (lateral bending rotation deformation), or a combination of these. A classic rotation deformation about the sagittal or coronal axis, resulting from asymmetric load application, is caused by a wedge compression fracture (see Fig. 7.2 and Fig. 7.4). A ventral wedge compression fracture results in a flexion rotation deformation about an axially oriented axis.

**Fig. 7.4** Rotation deformation can occur about the coronal axis of the spine (ventral wedge compression fracture [see Fig. 7.2]) and about the sagittal axis (lateral wedge compression fracture, as shown here). Curved arrow depicts bending moment. Straight arrows depict applied forces.

It is mainly this type of deformation that leads to aberrant force application to the spine, by creating a moment arm through which externally applied forces can have pathologic effects. In this way, a deformation can cause or create an unstable motion segment (deformation progression) by leading to the application of excessive stresses to the affected segment(s) via the concocted moment arm (see Chapters 3, 4, and 6).

The quantitative assessment of angular deformation in the coronal or axial planes can be accomplished by using the Cobb angle (see Chapter 3). This technique assesses a curve from the neutral vertebrae above to the neutral vertebrae below the deformity. This scheme is not without drawbacks. The Cobb angle is more appropriately used to quantitate multilevel curves, as opposed to a short-segment curve (Fig. 7.5). Regardless, the Cobb angle can be deceptive, even when used in multilevel curves. Its utility in the cervical spine has also been questioned. ⁵

**Fig. 7.5** Deformity begets deformity by increasing the length of the moment arm (d). (A) Determination of the Cobb angle (α) in a spine with a moderate scoliotic deformity is depicted. The Cobb angle is measured from neutral vertebra to neutral vertebra. (B) The neutral vertebrae are located between curves that are concave toward opposite directions, as depicted in Cobb angle β. The radii of curvature of two spinal deformities may be widely disparate despite their having the same Cobb angle. (C) A lesser radius of curvature is observed at the injured segment in a situation in which an acute segmental angulation occurs, as in Cobb angle Δ, compared with (B) less acute multisegmental angulations. Note that each spine has the same Cobb angle (α = β = Δ).

A variety of strategies can be used to assess and objectively quantitate deformities related to posttraumatic fracture (Fig. 7.6). ⁶ The technique associated with the greatest interobserver reliability employs the measurements from the superior endplate of the vertebral body above and the inferior endplate of the vertebral body below the fractured body (Fig. 7.6a).