Compensatory Mechanisms Contributing to the Maintenance of Sagittal Balance in Degenerative Diseases of the Lumbar Spine

Fig. 48.1

Evaluation of global sagittal alignment using the spinosacral angle (a) and the SVA/SFD ratio (b). The SSA is defined as the angle between the sacral plate and the line connecting the centroid of C7 vertebral body and the midpoint of the sacral plate [17]. Sacro-femoral distance (SFD) is the horizontal distance between the vertical bicoxofemoral axis and the vertical line passing through the posterior corner of the sacrum. The horizontal distance between C7 PL and the posterior corner of the sacrum (i.e., SVA) was also measured. Then we calculated the SVA/SFD ratio corresponding to the ratio between SVA distance and SF distance [3]. This ratio is equal to zero, when C7 plumb line projects exactly on the posterior corner of the sacrum, and to one, when C7 plumb line projects exactly on the bicoxofemoral axis. It is negative when C7 plumb line projects posteriorly to the sacrum and more than one when C7 plumb line projects from anterior to the femoral heads [3] (From Barrey et al. [3])

The method to measure SVA/SFD ratio is presented in Fig. 48.1b. This ratio is equal to zero, when C7 plumb line projects exactly on the posterior corner of the sacrum, and to one, when C7 plumb line projects exactly on the bicoxofemoral axis. It is negative when C7 plumb line projects posteriorly to the sacrum and more than one when C7 plumb line projects from anterior to the femoral heads. In the normal population, the value of this ratio is −0.9 ± 1 [3].

The spinosacral angle and the C7/SFD ratio facilitate the evaluation of the global sagittal alignment of the spine above the pelvis. According to the severity of the imbalance, we propose the identification of four different stages (from I to IV): balanced, compensated, partially compensated, and imbalanced (Fig. 48.2). In the last stage, the compensatory mechanisms are not efficient enough to maintain the sagittal balance and C7 plumb-line falls in front of the femoral heads (SVA/SFD ratio >1). An illustration of each situation is presented in Fig. 48.3.

Fig. 48.2

Classification of global sagittal alignment in 4 stages with respect to the severity of the imbalance. Balanced: C7 plumb line falls close to the posterior corner of the sacrum, SVA/SFD ratio is close to 0, there is no compensation. Compensated: C7 plumb line is closer to the posterior corner of the sacrum than to the femoral heads, SVA/SFD ratio is <0.5, compensation is present. Partially compensated: C7 plumb line is closer to the femoral heads than to the posterior corner of the sacrum, SVA/SFD ratio is >0.5, compensation is present. Imbalanced: C7 plumb line is placed in front of the femoral heads, SVA/SFD ratio is >1, compensation is present but not sufficient to keep the balance

Fig. 48.3

Illustrations showing the four situations of sagittal balance

48.3 Compensatory Mechanisms

Compensatory mechanisms can be observed in the spine, the pelvis, and/or the lower limb areas and are summarized in Fig. 48.4. Although these mechanisms are rarely observed all together in the same patient, they are usually associated at different degrees, depending mainly on the stiffness of the spine, the musculature status, painful phenomenon, and the severity of the imbalance.

Fig. 48.4

Sagittal imbalance and the different compensatory mechanisms in the spine, pelvis, and lower limb areas (From Barrey et al. [18])

Their basic concept is to extend adjacent segments of the kyphotic spine, facilitating the acquisition of compensated spine. Most of these mechanisms result from muscle action, thus exposing the subject to chronic pain and muscle fatigue.

To understand the variations of positional parameters such as sacral slope (SS), pelvic tilt (PT), LL, and TK in the patients’ population, we previously published values of six different classes of pelvic incidence in a normal control group of 154 subjects [4]. Values of positional parameters for each class of PI (from I to VI corresponding to a progressively increase of the PI value) are summarized in Table 48.1. Theoretical normal values for spinopelvic parameters (i.e., theoretical PT and theoretical LL) may also be estimated from mathematical relations (Table 48.2). Otherwise, to analyze segmental changes, we have to keep in mind that the L4–S1 segment provides the two third of the total lumbar lordosis [6, 11, 19].

Table 48.1

Classes of pelvic incidence and corresponding values of spinopelvic positional parameters from a group control of 154 subjects [4]

	n	PI	PT	SS	LL	TK
I 28° < PI < 37.9°	12	35.4 ± 1.3 [33.7–37.9]	3.9 ± 4.5 [−1.5–13.3]	31.5 ± 5.2 [21.2–38.5]	53.3 ± 6.6 [41.2–62]	43.8 ± 9.1 [22.5–51.5]
II 38° < PI < 47.9°	44	42.7 ± 2.8 [37.9–47.6]	8.9 ± 4.8 [–5.1–18.2]	33.8 ± 4.8 [23.1–48.4]	55.5 ± 8 [41.5–76.5]	48 ± 8.8 [24–64.7]
III 48° < PI < 57.9°	59	52.6 ± 2.8 [48.2–57.4]	12.5 ± 5.6 [−1.2–23.2]	40.1 ± 5.5 [28.2–52.9]	61.5 ± 8.4 [43.1–81.9]	47.4 ± 10.7 [24–70.3]
IV 58° < PI < 67.9°	26	62.6 ± 2.8 [58.2–67.6]	15.8 ± 4.3 [7.1–26.8]	46.8 ± 4.2 [37.9–58.5]	68.3 ± 5.1 [60.9–76.3]	47.6 ± 7.8 [34.7–64.7]
V 68° < PI < 77.9°	11	72.6 ± 2.8 [69.6–77.4]	19.7 ± 5.5 [12.6–27.9]	52.9 ± 5.2 [46.2–59.6]	74.9 ± 6.8 [62.2–81.6]	46 ± 10.2 [29.7–62]
VI 78° < PI < 87.9°	2	81.4 ± 3.3 [79.1–81.4]	21.9 ± 12.3 [13.2–30.6]	59.5 ± 9 [53.1–65.9]	76 ± 8.3 [70.1–81.9]	44.6 ± 12.2 [36–53.3]

Table 48.2

Theoretical pelvic tilt and lumbar lordosis according to the pelvic incidence [4]

PI class	PI (°)	Theoretical PT (°)	Theoretical LL (°)
I	<38	4	PI + 18
II	38–47	8	PI + 13
III	48–57	12	PI + 9
IV	58–67	16	PI + 6
V	68–77	20	PI + 2
VI	>78	24	PI – 5

As examples, for PI measured to 40°, expected PT should be 8° and LL should be 53°; for PI measured to 52°, expected PT should be 12° and LL should be 61°; and for PI measured to 64°, expected PT should be 16° and LL should be 70°

48.3.1 Spine

48.3.1.1 Cervical Hyperlordosis

Although in most cases the cervical spine is not well evaluated on full spine radiographs, it should be included in the sagittal balance assessment since compensatory curvature can be observed at this level. Hyperextension of the cervical spine is a typical compensatory mechanism above a thoracic hyperkyphosis in order to maintain the horizontality of the gaze. Inconvenience related to this hyperlordosis is not negligible – resulting in acceleration of degenerative changes in the cervical spine (i.e., hypertrophic facet joints arthritis and kissing spinous processes), presence of axial neck pain, foraminal stenosis, and the risk to develop spondylotic myelopathy.

48.3.1.2 Reduction of Thoracic Kyphosis

Reduction of thoracic kyphosis permits the limitation of anterior translation of the axis of gravity and is typically observed in young patients with a flexible spine (Fig. 48.5). It is the consequence of active muscle actions and therefore implies a good quality of erectors muscles of the spine. Takemitsu et al. described this mechanism for patients with lumbar kyphosis [20]. In a previous work, we also found that patients with degenerative disk disease and disk herniation were characterized by flat spine with significant reduction of both lumbar lordosis and thoracic kyphosis. This profile was more marked for patients with disk diseases below 45 years old [4]. Our findings were concordant with those reported by Rajnics et al. through a similar study [21]. When the spine is rigid (aging of the spine is kyphosis and ankylosis) or in case of atrophy of spinal erectors, it is not possible for the patient to reduce the magnitude of the thoracic curve.

Fig. 48.5

Patient with lumbar kyphosis due to severe multilevel degenerative disease stenosis from L2 to S1. The patient is unbalanced (C7PL/SFD >1). Thoracic spine is clearly flattened (thoracic curve measured to only 21°). PI was measured to 26°, PT was 12°, and SS was 16°. Compared to group control from normal and asymptomatic population, we should expect value of PT around 4°. After corrective surgery in the lumbar spine and restoration of good sagittal balance (C7PL/SFD <0.5), we noted the reappearance of the thoracic kyphosis and reduction of PT (from 12 to 6°)

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