11 Sagittal Imbalance Compensatory Mechanisms

10.1055/b-0039-171407

11 Sagittal Imbalance Compensatory Mechanisms

Martin Gehrchen

Abstract

The compensatory mechanisms are described for both flexible and stiff spines and in relation to Roussouly types with examples, including illustrative figures. Examples of decision making where the surgeon can easily make the wrong decision are included in this chapter.

11.1 Introduction

This chapter will focus mainly on the compensatory mechanisms of keeping an upright position and a horizontal gaze when balance is affected. This natural adaptation to any kyphosing event is the body’s effort at self-preservation. So, whatever the pathologies involved in the kyphosing event, the body composition’s self-preservation, to keep upright gait and horizontal gaze, is the driver in these compensatory mechanisms. ¹ ^, ² ^, ³ ^, ⁴ A description of this is partly found in Chapter 9, but a more systematic description will be given in this chapter (with examples) of the causal pathology.

The kyphosing event can occur in any part of the spine, but this chapter focuses on the thoracolumbar (TL) spine. In general, kyphosing events may occur over many segments as in multilevel degenerative processes or, for example, in Scheuermann’s disease or, more acutely, osteoporosis, fractures, or short segmental degeneration. ⁵ ^, ⁶ ^, ⁷ ^, ⁸ In addition, there is a significant number of iatrogenic causes for kyphosing events such as postlaminectomy syndrome (flat back) or a lumbar spine fusion in relative kyphosis. ⁹ ^, ¹⁰ Less known, hyperlordosis may bring specific compensations to counteract paradox imbalance. We shall not treat specific situations of hyperlordosis in neuromuscular diseases here. Natural hyperlordosis may accompany a pelvis with very high pelvic incidence (PI). A less recognized but worse situation is iatrogenic hyperlordosis caused by surgical overcorrection of lumbar lordosis.

11.2 Compensatory Mechanism of a Kyphosing Event

11.2.1 Pelvic Retroversion

The first well-known and described compensatory mechanism for a sagittal imbalance is pelvic retroversion. ¹¹ First described in severe kyphosis of ankylosing spondylitis (AS), pelvic posterior rotation around the femoral head axis was constant with a decreasing sacral slope (SS) frequently calculated as close to 0° (close to horizontal). Later, using the Duval-Beaupère parameters—PI, pelvic tilt (PT), and SS—a relation was established between PT and SS. ¹² At that time, the reciprocal positioning of the hips and knees was not clear, and the hip position was frequently considered in flexion but, in reality, they were in extension. The last frequent feature of severe kyphosis in AS was the knee position in flexion.

What Is the Mechanical Effect of the Pelvic Retroversion?

The posterior pelvis rotation around the femoral heads induces a physical effect and a shape transformation.

Physical effect: A kyphosing event wherever the place in the spine displaces forward the gravity of the body above. To counteract this gravity force displacement, the pelvis acts like a reverse pendulum around the femoral heads and moves the body back to ensure the gravity line position is inside the feet area.
Shape transformation: We have seen previously that the lumbar lordosis is closely linked with SS and that a small lordosis accompanies a small SS. This is the same in pathology in the case of decreasing lordosis, by degeneration, for example, SS decreases. If SS decreases, PT increases. There is a coupling mechanism between pelvic retroversion, decreasing SS, and decreasing lordosis.

How the Pelvic Shape Influences the Mechanism of Retroversion

To know the different pathways of compensation, it is necessary to fully understand the limitations and implications of the PI and the implications of the magnitude of the PI. ¹³ A patient with a high PI may have more pelvic retroversion for compensation than a patient with low PI. This is geometrically explained by the angular relation: PI = PT + SS, where PT indicates the magnitude of rotation of the pelvis around the femoral heads and the SS, the slope of the sacral plateau. ¹⁴ If we consider that, in the standing position, the lower virtual value of SS that the system can reach is zero in low PI, there is a lesser ability of pelvic retroversion than with a high PI. In other words, to reach a high PT (strong retroversion), a patient must have a high PI. For the same quantity of kyphosis, in AS, the global balance (C7 plumb line) appears in a better position in a higher PI where the compensatory mechanism of retroversion is stronger than in a patient with a lower PI (Fig. 11‑1). A high level of retroversion is always a specificity of a pelvis with high PI. Following Roussouly’s classification, a highly retroverted pelvis (PT >25°) characterizes the degenerative evolution of types 3 and 4. Types 1 and 2 have a poor ability for retroversion.

**Fig. 11.1** Pelvic tilt compensation according to pelvic incidence (PI). On the left, with high PI, there is a better ability of pelvis retroversion but with the possibility of reaching the limits of hip extension. On the right, note low PI, with less possibility of pelvis retroversion and less compensation of global imbalance, but no effect on hip limitation.

What Are the Limitations of Pelvic Retroversion Mechanism?

We have seen that the maximal compensation of SS would be 0°, which means a horizontal position of the sacral plateau. Virtually, for a PI of 60°, PT could reach 60°. This mechanism is limited by the ability of the hip extension (HE), ¹⁵ maintaining the femoral shafts strictly vertical, then 60° probably overpasses this possibility. When the maximum HE is reached, the mechanism of pelvis retroversion must use a new and lower axis of rotation. This is why knee flexion occurs inducing a tilt of the femoral shaft. Mangione and Sénégas described an angle between the femoral shaft and vertical axis called the pelvic femoral angle (PFA). ¹⁶ We write the relation: PT = HE + PFA (Fig. 11‑2). As PT considers both HE and PFA, PFA does not have to be taken into account in surgical strategy. PT correction is sufficient to correct both knee flexion and HE. For example, a patient with a severe kyphosis has a PT = 35°, the HE limit is 20°, and the patient needs to tilt the femoral shaft to 15° to reach a PT value of 35°. If simulating the correction, the surgeon tries to obtain PT= 15°; in this situation, hips turn in their normal range of rotation (<20°). Knees are no longer in flexion; the femoral shaft may be vertical without a tilt.

**Fig. 11.2** **(a)** In normal standing position, pelvis rotation is in a neutral position. Femurs are vertical. **(b)** When pelvic tilt (PT) increases, hip extension (HE) increases until its own limit. Femurs stay in a vertical position. **(c)** When hip extension is overpassed, femurs must tilt by knee flexion to increase PT over HE limits. PFA, Pelvic femoral angle.

What Are the Main Clinical Issues of High Pelvic Retroversion?

Muscle action is important for maintaining pelvis retroversion, mainly gluteus and posterior thigh muscle action. This may be misinterpreted as sciatica. However, the most important effect is the alteration of walking. ¹⁷ During gait, when one leg is in the posterior position, the HE must be free to maintain a balanced pelvis. If the hips are locked in extension, the forward femoral tilt induces an anteverted pelvis positioning and a global forward tilt of the spine above. If the retroverted pelvis mechanism seems efficient to correct the sagittal body balance in the standing position, during walking, this mechanism loses its positive effect and strongly impairs the balance (Fig. 11‑3).

**Fig. 11.3** **(a)** When the pelvis is retroverted in a standing position, the patient seems balanced. **(b)** When walking, the pelvis tilts forward driving by the femoral forward tilt during gait, impairing balance.

Different classifications of sagittal balance were based on the retroversion level. Hresko classified the balance of high-grade spondylolisthesis by the level of retroversion; the most retroverted was considered the most unbalanced. The more PT increases, the more sacral plateau positioning moves backward. This may explain the close relation between PT and C7PL forward displacement translated by increasing SVA or Barrey ratio (BR). Based on this, Barrey proposed a sagittal spinal balance classification in degeneration divided into three categories (Fig. 11‑4) ¹⁷ ^, ¹⁸ ^, ¹⁹ :

**Fig. 11.4** Barrey types: balanced, compensated balance, and unbalanced.

Type A or normal spinal balance: Characterized by global balance of the trunk measured BR <100% (C7PL behind femoral heads and posterior edge of the sacral plateau) and 10°<PT<25°; lower limbs are completely extended in the standing position.
Type B or compensated balance: Global balance of the trunk is still normal BR (<100%) but the pelvis is retroverted (PT >25°). The lower limbs show extension of the hips (femur straight) and the knees are in full extension. This means that the HE can reach up to 25°.
Type C or uncompensated balance: Global balance of the trunk shows a positive C7 value that generally falls in front of the femoral heads, and the pelvis shows a PT in retroversion. In the lower limbs, extension of the hips (pelvic retroversion) and flexion of the knees are evident. HE is overpassed.

This classification is dependent on the hips’ range of motion. If the hips are poorly mobile (inflammation, arthritis), the knees’ flexion compensation may occur with a lower value of PT.

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