*2.5.2. Pelvic incidence and its relationship to LL and sagittal balance*

A mean value PI was documented in 2011 to be 55 ± 10° [24], and a mean value of LL and thoracic kyphosis (TK) was documented in 1989 to be 44 and 36° [26]. These mean values do not imply ideal but simply a fixed angle providing anatomical characteristics of the pelvis and lumbar spine. Ranges of value are more appropriate for describing normal, but in this section, the mean values give an easier way of understanding the concept of match versus mismatch between the two. A ±10° difference between PI and LL was documented as an ideal match for optimal maintenance of sagittal balance [27]. A *match* occurs when both PI and LL are within the margin of 10° difference (known as PI-LL = 10 or PI − LL = 10). PI and LL can be high or low in degrees and still be considered a match. For example, a PI of 70° and an LL of 65° would be considered a high degree. A PI of 30° and an LL of 35° would be considered a low degree. In both cases, the difference between them is 5° and thus considered a match and a harmonious sagittal plane alignment (**Figure 7**).

**Figure 7.** Representative drawing of harmonious spine in the sagittal plane: high PI and LL (left) and low PI and LL (right). Both are a match between PI and LL of two possible separate individuals.

A *mismatch* occurs when there is a greater than 10° difference between PI and LL and can lead to a disharmonious sagittal plane alignment. A mismatch can be presented by high PI and low LL (PI-LL >10) or the opposite (**Figure 8**).

The most documented [24–26] types of mismatches in adolescent idiopathic scoliosis (AIS) and ASD include those with PI-LL >10° where the PI is high and the LL is low (**Figure 9**).

**Figure 8.** Representative drawing of match (left) and mismatch (right) PI and LL.

**Figure 9.** Mismatch PI-LL in adolescent (left) and older adults (middle and right).

**Figure 7.** Representative drawing of harmonious spine in the sagittal plane: high PI and LL (left) and low PI and LL

alignment of the gravity line or the horizontal gaze. Compensatory mechanisms need active mus-

deformity along with compensatory thoracic hyper-kyphosis may commonly be observed [23].

ideal but simply a fixed angle providing anatomical characteristics of the pelvis and lumbar spine. Ranges of value are more appropriate for describing normal, but in this section, the mean values give an easier way of understanding the concept of match versus mismatch between the two. A ±10° difference between PI and LL was documented as an ideal match for optimal maintenance of sagit-

as PI-LL = 10 or PI − LL = 10). PI and LL can be high or low in degrees and still be considered a

would be considered a low degree. In both cases, the difference between them is 5°

), a regional hyper-lordosis lumbar

[24], and a mean value of LL and thoracic

would be considered a high degree. A PI of 30°

[26]. These mean values do not imply

difference (known

and

and

cle contraction by the subject [24, 25]. When the SS is high (>45°

*2.5.2. Pelvic incidence and its relationship to LL and sagittal balance*

tal balance [27]. A *match* occurs when both PI and LL are within the margin of 10°

and an LL of 65°

thus considered a match and a harmonious sagittal plane alignment (**Figure 7**).

A mean value PI was documented in 2011 to be 55 ± 10°

122 Innovations in Spinal Deformities and Postural Disorders

match. For example, a PI of 70°

an LL of 35°

kyphosis (TK) was documented in 1989 to be 44 and 36°

(right). Both are a match between PI and LL of two possible separate individuals.

### *2.5.3. Clinical implications*

Radiological parameters that most highly correlate with pain, disability, and low quality of life are sagittal vertical axis, pelvic tilt, and the PI-LL relationship and are thus key components of the SRS-Schwab ASD classification [20, 27–29].

In a multi-center, prospective cohort trial in 2013 related radiological parameter thresholds to clinical findings as being predictive of worse clinical symptoms and poorer quality of life [28]. They proposed and concluded that a PI-LL of 10° or less, global alignment (positive SVA) of less than 4 cm, and PT of less than 20° were the ideal spinopelvic alignment for reducing operative intervention procedures and postoperative pain and disability [28].

Overall, literature demonstrates increased surgical complexity with increased severity of sagittal deformity modifiers. A significantly higher osteotomy rate was reported with increasing positive sagittal malalignment and a PI-LL mismatch [29–31]. Iliac fixation was more commonly used as global alignment became increasingly positive. Berjano and Aebi reported the value of the pedicle subtraction osteotomy (PSO), a procedure to restore lumbar lordosis in patients with lumbar/thoracolumbar scoliosis, concurrent loss of lumbar lordosis, and PI-LL mismatch [32]. This procedure has been demonstrated to restore sagittal alignment and improve patient self-reported pain and function.

The incorporation of spinopelvic parameters into surgical decision making has provided greater insight into the relationship between spine deformity and compensatory strategies to attempt to maintain upright alignment. It is theorized, for example, that a person with a low PI may adapt well to a situation if their lumbar lordosis is reduced (due to degenerative changes or scoliosis) because their sagittal alignment, based on the existence of low PI, may more readily "accept" changes that will cause hypolordosis in the lumbar spine and still maintain a match between PI and LL. Conversely, an individual with a high PI and low or loss of lumbar lordosis (secondary to lumbar degenerative changes or scoliosis or to compensate for a decreased thoracic kyphosis) will likely not adapt as well as the one described first. The high PI may not allow the individual to adapt well to the low lumbar lordosis, and after exhausting compensatory strategies at the pelvis (through pelvic tilt) and lower extremities, they may tend toward a positive sagittal balance (positive SVA) and potential compensations in the thoracic spine as well.

Lamartina and Berjano [24] describe a comprehensive classification of sagittal imbalance. In their classification, two compensatory mechanisms occur in response to reduced lumbar lordosis (i.e., lumbar kyphosis). First, local lumbar kyphosis may be compensated for by thoracic lordosis (**Figure 10**). In this situation, the thoracolumbar junction is normal or lordotic. Second, lumbar kyphosis may not be compensated by thoracic lordosis but by thoracic hyperkyphosis causing global kyphosis. In this case, the TLJ is in kyphosis, and the whole spine demonstrates an anterior loss of sagittal balance. In both situations, there will be additional compensations, including pelvic retroversion and knee flexion to maintain upright posture. The reason for the differences between the two is not yet understood but the most accepted theory is that the differences in compensation in the thoracic region are based on the preexisting alignment of the thoracic (being originally hypo-kyphotic vs. normal or hyper-kyphotic)

**Figure 10.** (A) Lumbar kyphosis with a compensatory thoracic lordosis, pelvic retroversion (increased pelvic tilt), and knee flexion. (B) global kyphosis—lumbar kyphosis is not compensated by thoracic lordosis. The TLJ is kyphotic. Compensatory pelvic tilt and flexed knees are present.

and the TLJ. Lumbar kyphosis differs from global kyphosis in that in *lumbar kyphosis*, there is a local kyphosis (at the lumbar region) with compensatory thoracic hypo-kyphosis (**Figure 10A**). In this alignment strategy, thoracic extensor muscles are active, and patients may benefit from remodeling of the lumbar lordosis conservatively or by selective lumbar osteotomy and fusion where the patient may regain a balance with reversal of the compensatory mechanism of the thoracic spine. In *global kyphosis*, the thoracic spine fails to compensate to the lumbar kyphosis and the whole trunk becomes kyphotic (**Figure 10B**).

#### *2.5.4. Summary*

*2.5.3. Clinical implications*

124 Innovations in Spinal Deformities and Postural Disorders

nents of the SRS-Schwab ASD classification [20, 27–29].

improve patient self-reported pain and function.

in the thoracic spine as well.

Radiological parameters that most highly correlate with pain, disability, and low quality of life are sagittal vertical axis, pelvic tilt, and the PI-LL relationship and are thus key compo-

In a multi-center, prospective cohort trial in 2013 related radiological parameter thresholds to clinical findings as being predictive of worse clinical symptoms and poorer quality of life [28]. They proposed and concluded that a PI-LL of 10° or less, global alignment (positive SVA) of less than 4 cm, and PT of less than 20° were the ideal spinopelvic alignment for reducing

Overall, literature demonstrates increased surgical complexity with increased severity of sagittal deformity modifiers. A significantly higher osteotomy rate was reported with increasing positive sagittal malalignment and a PI-LL mismatch [29–31]. Iliac fixation was more commonly used as global alignment became increasingly positive. Berjano and Aebi reported the value of the pedicle subtraction osteotomy (PSO), a procedure to restore lumbar lordosis in patients with lumbar/thoracolumbar scoliosis, concurrent loss of lumbar lordosis, and PI-LL mismatch [32]. This procedure has been demonstrated to restore sagittal alignment and

The incorporation of spinopelvic parameters into surgical decision making has provided greater insight into the relationship between spine deformity and compensatory strategies to attempt to maintain upright alignment. It is theorized, for example, that a person with a low PI may adapt well to a situation if their lumbar lordosis is reduced (due to degenerative changes or scoliosis) because their sagittal alignment, based on the existence of low PI, may more readily "accept" changes that will cause hypolordosis in the lumbar spine and still maintain a match between PI and LL. Conversely, an individual with a high PI and low or loss of lumbar lordosis (secondary to lumbar degenerative changes or scoliosis or to compensate for a decreased thoracic kyphosis) will likely not adapt as well as the one described first. The high PI may not allow the individual to adapt well to the low lumbar lordosis, and after exhausting compensatory strategies at the pelvis (through pelvic tilt) and lower extremities, they may tend toward a positive sagittal balance (positive SVA) and potential compensations

Lamartina and Berjano [24] describe a comprehensive classification of sagittal imbalance. In their classification, two compensatory mechanisms occur in response to reduced lumbar lordosis (i.e., lumbar kyphosis). First, local lumbar kyphosis may be compensated for by thoracic lordosis (**Figure 10**). In this situation, the thoracolumbar junction is normal or lordotic. Second, lumbar kyphosis may not be compensated by thoracic lordosis but by thoracic hyperkyphosis causing global kyphosis. In this case, the TLJ is in kyphosis, and the whole spine demonstrates an anterior loss of sagittal balance. In both situations, there will be additional compensations, including pelvic retroversion and knee flexion to maintain upright posture. The reason for the differences between the two is not yet understood but the most accepted theory is that the differences in compensation in the thoracic region are based on the preexisting alignment of the thoracic (being originally hypo-kyphotic vs. normal or hyper-kyphotic)

operative intervention procedures and postoperative pain and disability [28].

The literature review of this section demonstrates that the sagittal plane is vital to understanding pain and disability in patients with ASD, and that SVA, PT, and PI-LL mismatch are the main drivers that affect disability and decreased function [27–32]. Becoming proficient in defining radiographically the above parameters, values and limits, can help guide a better therapeutic decision-making process conservatively and operatively with the aim and focus on maintaining or creating the best sagittal alignment for the individual that will improve function, as well as decrease pain and disability.
