8 Sagittal Balance in the Elderly

10.1055/b-0039-171404

8 Sagittal Balance in the Elderly

Martin Gehrchen

Abstract

The aging spine is a continuum of a degenerative process that affects the sagittal balance in different degrees. The evolution of Roussouly spine shapes in the aging spine and the resulting involvement of the sagittal spine shape is described for each of the Roussouly types.

8.1 Introduction

The aging spine is most often characterized by the degenerative changes of the spine and their extent. ¹ ^, ² ^, ³ ^, ⁴ The pelvic incidence (PI) tends to increase linearly in adolescence with growing age until adulthood, then becomes a constant anatomical parameter. ⁵ ^, ⁶ ^, ⁷ ^, ⁸ The global and local spinal parameters (spinal lordosis [SL] and thoracic kyphosis [TK]) are closely dependent on the PI. ⁹ ^, ¹⁰

To consider sagittal balance and normative shapes of the aging spine, it is important to understand, or at least make assumptions, as the change from the non-aging spine to the aging spine is a continuum with the development and growth of degenerative changes. ¹¹ Thus, there is a gray zone where it can be difficult to differentiate between the young spine and the aging spine; however, degenerative changes of the spine such as disk degeneration and facet joint degeneration are signs that must be considered aging or the onset of aging. We cannot consider the aging process as a pathology; this is the natural evolution of extended life that affects the whole population whatever the initial shape of the global spinopelvic set. This chapter aims to characterize the aging spine and the implications of aging on the balanced spine shape in patients not previously operated on.

8.2 Classical Patterns of Aging Spine: Pelvis Retroversion with High Pelvic Incidence

Many attempts have been made to classify the adult aging spine. ¹² ^, ¹³ ^, ¹⁴ All of them, however, are based on positional parameters and do not take the shape of the spine and its pathological evolution into account. ² ^, ¹⁵ ^, ¹⁶ Lafage et al ¹⁷ showed the forward displacement of the gravity line and C7 plumb line in the aging population. Literature describing sagittal balance in the aging population is generally based on one pattern: loss of lumbar lordosis (LL) and increase of TK compensated for by a highly retroverted pelvis. It is well known that with age we may potentially lose height and the head may displace forward in relation to the pelvis. Loss of balance in the aging spine has several causes and sometimes they exist in combination. The main factor is the degenerative diskopathies that may determine increased kyphosis in the thoracic and the lumbar region (decreased lordosis), although osteoporosis with fractures also induces the same problem as do neuromuscular conditions with a decreased muscular strength. ² ^, ¹⁸ ^, ¹⁹ Scheuermann’s disease and ankylosing spondylitis in variating degrees may, with aging, also impact sagittal balance significantly. ²⁰ ^, ²¹

A classic description of sagittal balance in aging associates forward flexion of the spine, pelvic retroversion (with hip extension), and in more severe situations knee flexion and femoral external rotation. ²² Progressive balance impairment is described over time by increasing muscle tiredness during daily activities inducing fatigue, discomfort, or pain. ²² Compensation of forward unbalance by high pelvic retroversion is a biomechanical property of pelvis with high PI. The act of walking impairs forward balance, thereby forcing the use of helping devices and limiting the ability to walk because of the limitation of hip extension. This situation may be even worse when there is associated arthritis of the hip or knee that compromises the diagnosis.

8.2.1 Spine and Hips Relation in Aging People with Retroverted Pelvis

Relations between hip positioning and worsening spinal shape may explain impairment from hip diseases or late hip prosthesis loosening. The progressive pelvic retroversion changes the acetabulum orientation (vertical positioning: acetabulum anteversion) and reduces the contact surface between femoral heads and acetabulum reduction. ²³ In a normal hip joint, pressure on the cartilage may increase and induce wear on the cartilage and, later, arthritis. By the same mechanism, a total hip prosthesis may be compromised by decreasing contact between the head and acetabulum component, inducing polyethylene wearing and secondary loosening.

On the other hand, hip arthritis may reduce hip extension and worsen spinal imbalance by pelvic retroversion limitation. In this case, a decision regarding hip prosthesis has to be made before spinal surgery. ²⁴ Sometimes, imbrication of hip arthritis and lumbar stenosis may result in diagnosis error between thigh pain as a result of the hip and neurological femoral pain. ²⁵ ^, ²⁶ ^, ²⁷

8.3 Aging Spine Evolution in Relation with Pelvic Incidence

If the previous description generally fits well with the balance of the features of aging, it describes mainly patients with higher PI and an increased tendency for more retroversion of the pelvis. This classic evolution of an aging person with high PI has led some authors to consider high PI as a specific pattern of aging people and a change in the position of sacroiliac joints as a cause of increasing PI with age. The poor angle variation related in those studies (3° to 4°) was sufficient to induce balance changes but remained within the limits of measurement error.

Relating the aging spine shape to the normal spine shape and looking at the different spine types according to Roussouly’s classic four types ²⁸ applied to the aging process, we can analyze the effect of aging on the shape of the spine. The contact forces in the posterior elements such as the facet joints increase with increased SL. The less the SL (kyphosis or flat back), the more impact there is on the disks. The classification thus can be helpful in determining where the high local stress zone is located in the spine in different types. The classification was updated recently with anteverted types 3 and 4 characterized with high SS and low PI in the normal population ²⁹ (Fig. 8‑1).

**Fig. 8.1** The Roussouly classification integrating the anteverted pelvis shape. PI, Pelvic incidence.

In general, three courses are in play regarding degenerative changes and aging of the spine depending on whether changes are occurring in the TK, SL, or a combination of both. If the occurrence is in the transition zone between TK and SL, both zones can potentially be affected.

Increasing spinal kyphosis caused by aging will move the head anteriorly in relation to the pelvis. The first changes will most likely be mild, and it will be very difficult to confirm it clinically; only magnetic resonance imaging (MRI) may confirm it in the very early stages. With increasing spinal kyphosis, extension of the spine will happen above and/or below and/or may be accompanied by retroversion of the pelvis (decreasing the lower SL) and increase of the upper SL until a point where no compensation above and below exists, and the spinal kyphosis is increased in general.
Decreasing lower SL caused by aging will also tend to move the head anteriorly; however, an extension of the adjacent levels will happen first, if possible. Here too, in the earlier stages, it may also be impossible to confirm it clinically, and only an MRI may document the early degenerative changes. ² ^, ³⁰ An increase in upper SL will accompany these changes as well.
Increasing spinal kyphosis and decreasing lower SL in combination naturally will affect the position of the head faster, as the combination of changes has the same effect as the above-mentioned consequences.

The smaller the PI, the lower the potential for balance compensation of the spine by pelvis retroversion (increasing PT) and vice versa for higher PI. As a general biomechanical behavior, when the spine is flexible around a kyphosing event, an increased extension of the flexible spine above and/or below the local kyphosis will be seen. ² But we must consider that in aging persons, the lack of muscular strength forbids this mechanism or at least is rapidly overpassed with muscle weakness. On the contrary, the second compensation mechanism is not compromised by muscles weakness. With progressive kyphosis, if the spine is rigid, the gravity line moves forward, and the pelvis rotates backward (retroversion), inducing a decrease in sacral slope (SS). ² ^, ³¹ ^, ³² ^, ³³ ^, ³⁴ It thus seems both logical and crucial to implement Roussouly’s four classic types (i.e., shape of spine) when evaluating the aging spine and its evolution to improve decision-making when surgical treatment is indicated. ²⁸ Thus, the evolution of the sagittal alignment of the degenerative spine can be described based on the different types as proposed. ³⁵ (For a description of the types in the normal population, see Chapter 7). For the degenerative spine, several types were identified such as “classic” type 1–4, anteverted types 3 and 4, false type 2, false type 2 + TK, false type 3, and the “kyphotic” group (lumbar and global).

The evolution of the aging spine based on the Roussouly classification of the normal spine can be described as follows depending on PI. ²⁸ ^, ²⁹

8.3.1 Poorly Retroverted Pelvis (Low Pelvic Incidence <50°)

Type 1

Compensation is developed low in the lumbar spine by increasing the LL if a kyphotic event occurs. If the spinal compensation mechanisms are consumed as a result of stiffness/degeneration, SL disappears, and if spine extension in the usually kyphotic thoracolumbar area is insufficient, then there will be a “global kyphosis” with a small PI (Fig. 8‑2). Degenerative evolution of previous idiopathic thoracolumbar scoliosis may turn progressively the scoliosis deformity into a thoracolumbar kyphoscoliosis with small PI and sagittal patterns of type 1.