This article was updated on May 11, 2015, because of a previous error. In Figure 4B, the pelvic incidence angle was incorrectly labeled as 36° but actually measured 49°. The figure has been replaced with a new figure in which the angle measures 37°. In addition, the legend for Figure 4B had previously read “Fig. 4B A low pelvic incidence value is associated with a more horizontal upper sacral plate and flatter sagittal curves. LL = lumbar lordosis.” The legend now reads “Fig. 4B A low pelvic incidence value is associated with a more horizontal upper sacral end-plate and flatter sagittal curves. The predicted lumbar lordosis in this case is 37° + 9° = 46°; the actual lumbar lordosis is even further reduced (35°) secondary to disc degeneration with collapse of the disc height. LL = lumbar lordosis.”
➢ Bipedal posture and gait is less energy-efficient than quadrupedal posture and gait.
➢ Spinopelvic balance is critical for energy-efficient upright balance of the head and spine on the lower extremities.
➢ Pelvic incidence is a fundamental anatomical characteristic of the pelvis. It increases in childhood but does not change in adult life. A large body mass index (BMI) may predispose to increased pelvic incidence during the growing period of life. A large pelvic incidence may predispose to both developmental and degenerative spondylolisthesis by increasing sacral slope.
➢ Pelvic tilt and sacral slope determine the orientation of the pelvis on the femoral head in the sagittal plane. The relationship is defined with the equation: pelvic incidence = sacral slope + pelvic tilt.
➢ The sagittal vertical axis and lumbar lordosis describe the sagittal balance of the spine above the pelvis.
➢ Disc degeneration, spondylolisthesis, and adult spinal deformity may lead to sagittal decompensation as the sagittal vertical axis moves forward and lumbar lordosis is diminished.
➢ Retroversion of the pelvis is a compensatory mechanism to restore sagittal balance, leading to increased pelvic tilt. Further compensation may occur with extension of the hips and, finally, flexion of the knees.
➢ A large pelvic tilt, as a compensatory mechanism, may hide the sagittal imbalance; any abnormal increase in pelvic tilt should be recognized and restored to normal values in order to achieve successful operative correction of spondylolisthesis, arthrodesis for the relief of back pain, and spinal osteotomy for the correction of adult spinal deformity.
Spinopelvic balance is of critical importance for understanding the sagittal balance of the spine and the energy efficiency of bipedal stance and gait in humans. In the sagittal plane, the pelvis and spine can be considered as a linear chain linking the head to the pelvis, with the shape and orientation of each anatomic segment being closely related and influencing the adjacent segments to maintain a stable posture with a minimum of energy expenditure. Changes in shape or orientation at one level will have a direct influence on the adjacent segments1. The normal and stable orientation of the pelvis in relation to the spine may be disturbed in patients with spinal disorders such as spondylolisthesis, lumbar disc degeneration, postoperative flat back, spinal deformity, and so on. The study of normal posture and spinopelvic orientation may be traced back in the literature to the late nineteenth century, but substantial progress was made during the past decade. The current review analyzes the present understanding and knowledge of the spinopelvic balance as applied to various clinical conditions of the spine.
Evolution of Human Upright Posture, Bipedal Locomotion, and Energy Efficiency
The imperfect upright posture and bipedal gait of the early prehuman primates required high energy consumption compared with that in quadrupedal animals2-5. Millions of years of selection and evolution of bipedalism in humans reduced the energy cost of walking compared with that in our ape-like ancestors4. The decreased cost of walking experienced by humans as compared with apes was attributed to more extended hips and longer hindlimbs. With erect posture, the body weight acts on the ground at the point of contact (i.e., the feet) and is in equilibrium with an equal and opposite force exerted by the ground on the body at the feet, described as the ground reaction force. The bent-hip, bent-knee gait of chimpanzees positions the center of mass of the body anterior to the hip joint and increases the moment arm of the ground reaction force vector. This posture generates large external flexion moments. In contrast, the upright posture of humans places the ground reaction force vector nearer to the hip and knee joints, keeping the moment arm of the ground reaction force smaller and thereby reducing the energy cost of stance and gait6 (Fig. 1). A kyphotic spinal deformity in humans forces the center of gravity of the body mass to move anterior to the hips and knees, increasing the energy cost of standing and walking, similar to a bipedal chimpanzee gait. However, despite improvements in energy efficiency compared with that in chimpanzees, the net costs of running and walking in modern humans are still high for an animal of our size. An optimized modern human bipedality is still far from being achieved7.
An electromyographic study by Joseph and Williams in 19578 showed that when the body weight passes slightly behind the femoral head, there is a lack of muscular action potential in the hip extensors in standing posture. No posterior stabilizing forces are required in such conditions, suggesting that this is the most economical standing posture in terms of muscle fatigue and skeletal strain. The conditions required for an economic standing position were further evaluated by Duval-Beaupère et al.9 in a Barycentremetric study of the sagittal shape of spine and pelvis in seventeen human volunteers with full spine radiographs. The Barycentremeter is a gamma ray scanner that provides in vivo measurement of the weight and center of weight supported by each vertebra and the coxofemoral joints. The investigators assumed that the position of the center of weight, in front of or behind the vertebrae or the coxofemoral joints, requires an opposing muscle force to ensure mechanical stability. Their study demonstrated that the axis of gravity of the upper body segment is located behind the lumbar vertebrae and the femoral heads in the presence of normal lumbar lordosis, thus ensuring economy and stability. The sagittal morphology of the pelvis determines the amount of lordosis needed for each individual. The proper harmony of the sagittal spinal curves allows a stable balance that is economical in terms of mechanical effects and muscular energy9,10.
Understanding of Pelvic Inclination and Postural Deformities of the Spine in the 1930s
Wiles11 delivered a Hunterian lecture before the Royal College of Surgeons of England on April 12, 1937, entitled “Postural Deformities in the Anteroposterior Curves of the Spine.” He presented his concept of pelvic inclination (as opposed to pelvic incidence, which will be described later) as the angle between the horizontal plane and the plane of the lines joining the posterior-superior iliac spines to the symphysis pubis. He developed a method to measure the pelvic inclination in living subjects using a so-called pelvic inclinometer (Fig. 2), during a period when a suitable x-ray machine capable of producing a radiograph of the whole spine was not available. With upright posture, the area occupied by the feet and the space between the feet constitutes the so-called base of support; increasing the distance between the feet increases the base of support. Wiles recognized that, in order for the upright position to be maintained, it is essential for the center of gravity of the whole body to fall somewhere within the base of support. His analysis showed that only two components contribute to define the upright posture: (1) pelvic inclination and (2) thoracolumbar kyphosis.
Pelvic inclination, as described by Wiles, identifies the orientation of the pelvis relative to the horizontal plane, which may vary in the same individual, depending on posture. Legaye et al.12 proposed a fundamental morphological pelvic parameter, and some positional pelvic parameters, as described below (Fig. 3).
Pelvic incidence is defined as the angle between a line perpendicular to the sacral plate at its midpoint and a line connecting this point to the center of the femoral heads (bicoxofemoral axis). Pelvic incidence is a fundamental anatomical parameter that is independent of the spatial orientation of the pelvis; i.e., the angular value is unaffected by changes in the human posture and will remain the same whether a subject is standing, sitting, or lying down. It is unique to each individual and is independent of age once growth is completed, with the assumption that there is no substantial motion occurring at the sacroiliac joints.
Sacral slope is defined as the angle between the superior end plate of S1 and a horizontal line. Sacral slope is a positional parameter; its value is low in vertical positions of the sacrum and high in horizontal positions of the sacrum.
Pelvic tilt is defined as the angle between a line connecting the midpoint of the sacral plate to the bicoxofemoral axis and a vertical line. Pelvic tilt is a positional parameter; its value is high in vertical positions of the sacrum, as in an individual with a retroverted pelvis, and low in horizontal positions of the sacrum, as in an individual with an anteverted pelvis.
A geometric construction with use of complementary angles reveals that the anatomical parameter of pelvic incidence is the algebraic sum of sacral slope and pelvic tilt (pelvic incidence = sacral slope + pelvic tilt). The dispersion of the positional parameters of sacral slope and pelvic tilt for any particular pelvic morphology (i.e., pelvic incidence) vary according to the orientation of the pelvis in the sagittal plane. In fact, sacral slope more strongly influences spinal balance above the level of the pelvis (i.e., spinal curves). In contrast, pelvic tilt influences the balance under the level of the pelvis (i.e., the angle of the coxofemoral joints in upright posture)13. The distribution of the pelvic incidence angle into sacral slope and pelvic tilt will be discussed further under the section on spondylolisthesis. Lumbar lordosis is a spinal parameter. There is a close relationship between pelvic incidence and sacral slope that strongly determines lumbar lordosis, as described later12,14.
Following the introduction and initial studies by Legaye et al.12, the pelvic parameters have been extensively studied by multiple authors13,15,16. In a large, multicenter study involving 149 healthy adults with no spinal disorders (including seventy-eight men and seventy-one women with ages ranging from nineteen to fifty years), Boulay et al.13 presented a reference basis for pelvic parameters based on full-spine lateral radiographs made with the patients in a standardized upright position (Table I).
Pelvic incidence plays a key role in the regulation of positional pelvic parameters (pelvic tilt and sacral slope) and spinal parameters (lumbar lordosis). A regression coefficient established a strong influence of pelvic incidence on sacral slope, pelvic tilt, and lumbar lordosis13. A high pelvic incidence value (≥62°) is associated with increased sacral slope and more pronounced lumbar lordosis (Fig. 4). Conversely, a low pelvic incidence value (≤44°) is associated with decreased sacral slope and flattening of lumbar lordosis. Because of such relationship, pelvic incidence may predict the expected normal lumbar lordosis. The mean lumbar lordosis may be estimated on the basis of the mean pelvic incidence in an asymptomatic adult individual with the formula: lumbar lordosis = pelvic incidence + 9° (±9)17. When the lumbar lordosis is flattened by various pathological conditions like spondylolisthesis or spinal deformity, the ideal lumbar lordosis in that individual, and the magnitude of the surgical restoration that is required, may be estimated on the basis of pelvic incidence with use of the above formula as pelvic incidence remains unaffected17.
Relationship Between Age, Sex, and Pelvic Incidence
Boulay et al.13 found no relationship between sex and pelvic incidence. However, they found that pelvic incidence can increase with growth in childhood or adolescence, reaching a constant value in adulthood. Mangione et al.16 measured pelvic incidence on radiographs for thirty fetuses, thirty children, and thirty adults and observed that pelvic incidence considerably increases during the first months, continues to increase during early years, and stabilizes around the age of ten years. On the basis of this relationship, the authors concluded that pelvic incidence is a mark of bipedalism. In another age-related study on pelvic parameters involving 180 normal subjects between the ages of four and eighteen years, Mac-Thiong et al.15 observed that pelvic incidence remains relatively constant during childhood, increases during adolescence, and reaches maximum constant value in adulthood.
Pelvic Incidence and BMI
Boulay et al.13 found a strong positive correlation between body mass index (BMI) and pelvic incidence as well as between BMI and lumbar lordosis. The correlation could be explained by the effects of a large BMI on the ossification of the sacrum. This ossification may continue until late, even after the age of twenty years. Therefore, the biomechanical constraints can deform the sacrum beyond the end of osseous growth and until the age of twenty years. This finding may raise an important question, namely, whether childhood obesity may alter the shape of the pelvis, leading to an increased pelvic incidence and sacral slope, which in turn may lead to an eventual increase in lumbar lordosis and make the child vulnerable to the development of back pain and disc degeneration with eventual spondylolisthesis in adult life.
Spinopelvic Balance in Sitting versus Standing Posture
Lazennec et al.18 evaluated the changes in lumbar lordosis, pelvic tilt, and sacral slope in twenty-four normal patients in different postures. They observed that the sitting position is normally characterized by a decrease in lumbar lordosis and a shift of the sacrum toward a more vertical position, resulting in pelvic retroversion. In contrast, during standing, the sacrum moves to a more horizontal position, resulting in pelvic anteversion. Therefore, lumbar lordosis is reduced in the sitting position compared with the standing position in the same individual. There is a corresponding reduction of the sacral slope (and increase in the pelvic tilt) in the sitting position (i.e., with the pelvis in a more vertical orientation) as compared with the standing position. This relationship between sacral slope in sitting versus standing posture may have an important bearing on patients with persistent back pain despite spinal fusion, as will be discussed later.
Spinopelvic Balance and Spondylolisthesis
An association between pelvic incidence and spondylolisthesis has been reported in many publications14,19,20. In a recent study by the Spinal Deformity Study Group (SDSG) in which 240 subjects with spondylolisthesis were compared with 160 normal, asymptomatic young adults, the authors found that pelvic incidence, sacral slope, pelvic tilt, and lumbar lordosis were significantly greater (p < 0.01 for all four parameters), whereas the thoracic kyphosis was significantly lower (p < 0.01), in the subjects with developmental spondylolisthesis as compared with the normal reference population21. There is common agreement that greater pelvic incidence and sacral slope could predispose to spondylolisthesis. Furthermore, the differences between the two populations increases in a direct linear fashion as the severity of the spondylolisthesis increases, suggesting that pelvic anatomy has a direct influence on the development of spondylolisthesis14,21.
The association of a large pelvic incidence and sacral slope with developmental spondylolisthesis may be explained by an increased shear force across the lumbosacral disc. However, there may be other reasons to explain such an association. The pelvic incidence angle remains constant after adolescence in normal subjects, but it may be altered by any pathologic process that modifies the shape of the sacrum or the position of the acetabulum within the pelvis. Morphologic changes such as doming or anterior lipping of the sacral end plate can be encountered in patients with high-grade developmental spondylolisthesis. These changes also make it difficult to perform a precise geometric measurement of pelvic incidence10.
Classification of Spondylolisthesis on the Basis of Spinopelvic Sagittal Balance
Not all patients with L5-S1 spondylolisthesis present with a large pelvic incidence. On the basis of a cluster analysis of a large population of patients (n = 540) with low-grade spondylolisthesis in the SDSG database, Labelle et al.22 observed three distinct subgroups: a smaller subgroup with low pelvic incidence (mean and standard deviation, 42° ± 5°) and low sacral slope (35° ± 4°); a larger subgroup with normal pelvic incidence (54° ± 4°) and normal sacral slope (45° ± 4°), with a similar spinopelvic alignment compared with the normal population; and a third subgroup with high pelvic incidence (74° ± 10°) and high sacral slope (53° ± 7°).
Roussouly et al.23 noted that low-grade spondylolistheses (grade 2 or less, with <50% displacement of L5 on S1) are often associated with low pelvic incidence and sacral slope and may have a different pathogenesis. The authors proposed that patients with large pelvic incidence and sacral slope may have a so-called shear-type mechanism and tension on the L5 pars, as opposed to patients with a low pelvic incidence and a smaller sacral slope, who may have impingement of the posterior elements of L5 between the L4 and S1 facets during extension, thereby causing a so-called nutcracker effect, with compression on the L5 pars.
In contrast to patients with low-grade spondylolisthesis, almost all subjects with high-grade slips (grade 3 or more, with ≥50% displacement) have above-average pelvic incidence values (≥60°). Using cluster analysis, Hresko et al.24 identified two subgroups of spinopelvic alignment in patients with high-grade spondylolistheses according to the pelvic version. The “balanced pelvis” version group included patients who stood with a high sacral slope but normal or low pelvic tilt, a posture similar to that in a control subgroup with a high pelvic incidence but no evidence of spondylolisthesis. The other group, with a low or normal sacral slope but high pelvic tilt, represented patients with a retroverted pelvis with a vertical sacrum. Labelle et al.22 designated this group as the “unbalanced pelvis” group. The retroversion of the pelvis represents a compensatory mechanism to restore the global spinal balance, which is defined by the sagittal vertical axis (measured as the offset between the C7 plumb line and the posterosuperior corner of S1). Labelle et al.22 further subclassified patients with high-grade spondylolisthesis and an unbalanced pelvis into a “balanced spine” group, in which the retroversion of the pelvis adequately restores the C7 plumb line to normal (falling behind the hips), and an “unbalanced spine” group, in which there is a persistent anterior shift of the C7 plumb line (falling in front of the hips, with a sagittal vertical axis of ≥3 cm) despite retroversion of the pelvis. On the basis of the above studies, the SDSG developed a system for the classification of developmental spondylolisthesis (Fig. 5)22.
Surgical Decision-Making in the Treatment of High-Grade Spondylolisthesis
In a retrospective study of seventy-three patients with developmental spondylolisthesis who were managed with operative reduction and fusion with instrumentation, Labelle et al.25 observed that while the shape of the pelvis (i.e., pelvic incidence) is unaffected by attempts at surgical reduction (as expected), proper repositioning of L5 over S1 improves the sagittal spinopelvic relationship and the shape of the lumbar spine, with significant changes (p < 0.01) in terms of pelvic tilt, sacral slope, and lumbar lordosis, and with improvement in the clinical outcome. The need for reduction is still debated because of the risks and complications associated with extensive surgical procedures. The authors recommended22 that patients with a high-grade spondylolisthesis may be managed with arthrodesis in situ when the pelvis is balanced, that is, when pelvic tilt is low or normal despite a large sacral slope (type-4 spondylolisthesis). The risky reduction maneuver may be reserved for patients with an unbalanced pelvis (a large pelvic tilt) and an unbalanced spine with a sagittal vertical axis of ≥3 cm (type-6 spondylolisthesis). It is also important to attempt a reduction maneuver for patients in whom spinal balance is corrected by retroversion of the pelvis (as indicated by a sagittal vertical axis of ≤3cm) but the pelvis remains unbalanced (as indicated by a large pelvic tilt) (type-5 spondylolisthesis). Hresko et al.24 emphasized that the failure to analyze spinopelvic balance, and therefore to distinguish between a balanced and an unbalanced pelvis, could account for the variability in the literature regarding the outcome of reduction for high-grade spondylolisthesis (Fig. 5).
Spinopelvic Balance and Low Back Pain
A review of literature suggests that spinopelvic balance may have an important influence on disc herniation, degenerative disc disease, and degenerative spondylolisthesis. Barrey et al.26, in a retrospective study, compared spinopelvic alignment in eighty-five patients who had lumbar degenerative disc disease (disc herniation, degenerative disc disease, or degenerative spondylolisthesis) with that in a control population of 154 asymptomatic adults. Among patients who were forty-five years old or more, the average pelvic incidence (representing the shape of the pelvis) was 49.8° and 51.6° in the disc herniation and degenerative disc disease groups, respectively, compared with 52° in the control group; these differences were not significant. Among younger patients (less than forty-five years old) who had disc herniation or degenerative disc disease, the pelvic incidence was significantly lower (average, 48.3°) than that in the control group (p < 0.05). In contrast, pelvic incidence was significantly greater in the degenerative spondylolisthesis group than in the control group (60° compared with 52°; p < 0.0005). The orientation of the spine in all three groups of patients showed a loss of sagittal balance compared with the control group; after matching according to pelvic incidence, the lumbar lordosis was less for the disc herniation, degenerative disc disease, and degenerative spondylolisthesis groups than for the control group (p < 0.0005 for all comparisons), and there was an anterior translation of the C7 plumb line (sagittal vertical axis) (p < 0.005 for the disc herniation group, p < 0.05 for the degenerative disc disease group, and p < 0.05 for the degenerative spondylolisthesis group). There was a compensatory pelvic retroversion in all three groups of patients, as indicated by a decrease of the sacral slope, after matching according to pelvic incidence (p = 0.001 for the disc herniation group, p < 0.0005 for the degenerative disc disease group, and p < 0.0005 for the degenerative spondylolisthesis group), and an increase of the pelvic tilt (p = 0.017 for the disc herniation group, p < 0.0005 for the degenerative disc disease group, and p < 0.0005 for the degenerative spondylolisthesis group).
Schuller et al.27 compared the spinopelvic balance in patients with back pain due to degenerative spondylolisthesis (n = 49) with that in a reference group of patients with back pain due to disc degeneration (disc herniation and degenerative disc disease) without spondylolisthesis (n = 77). The authors found that the degenerative spondylolisthesis group had significantly larger pelvic incidence (66.2° compared with 54.2°; p = 0.001), pelvic tilt (25.6° compared with 21.0°; p = 0.046), and sacral slope (42.3° compared with 33.4°; p = 0.002) compared with the reference group. In addition, the degenerative spondylolisthesis group also had a significantly higher BMI (p = 0.030) and larger lumbar lordosis (p = 0.045) compared with the reference group. The authors suggested that the anatomic orientation of the pelvis in patients with a high pelvic incidence and sacral slope seems to represent a predisposing factor for degenerative spondylolisthesis. The increase of pelvic tilt in the degenerative spondylolisthesis group may suggest a compensatory pelvic retroversion to address the large pelvic incidence. The association of a large BMI and a relatively vertical inclination of the S1 end plate (large sacral slope), together with sagittal orientation of the facet joints, are the anatomical predisposing factors for an anterior translation of L4 on L5.
Spinopelvic Balance and Low Back Pain After Spinal Fusion
Lazennec et al.18 conducted a radiographic analysis of posture before and after lumbosacral fusion to evaluate the influence of spinal alignment on the occurrence and pattern of post-fusion pain or failed back surgery. In the patients with post-fusion pain, the pelvic tilt at the time of the latest follow-up was almost twice the normal value (p = 0.0003), and the sacral slope was low (p < 0.0001), indicating that the sacrum remained abnormally vertical in the subjects with post-fusion pain.
The authors also compared the baseline (preoperative) spinopelvic parameters to predict the outcome of surgery. The group of patients who had a failure of surgery had had a larger pelvic tilt and smaller sacral slope values, even at baseline, compared with those who reported good pain relief after spinal fusion, who had had normal pelvic tilt and sacral slope preoperatively18. It is important to note that pelvic incidence was not significantly different between the groups with and without post-fusion pain.
The results of that study indicate that achieving a solid fusion should not be the only goal. Persistent pelvic retroversion even in the standing posture after spinal fusion replicates the sagittal alignment in the sitting position, which is often a painful position for patients with axial back pain. A common mistake leading to persistent back pain despite solid fusion is the failure to correct concomitant excessive pelvic retroversion with vertical sacrum. If the lumbar lordosis cannot be restored at the time of surgery, the pelvis has to stay rotated backward in order to maintain the sagittal balance as a compensatory mechanism. Persistent pain may arise from the high energy expenses and fatigue of the muscles required to compensate.
Spinopelvic Balance in Adult Spinal Deformity
Glassman et al.28 studied the relationship between spinopelvic balance and health-related quality of life (HRQoL) measures in adult patients with spinal deformity who had had no previous surgery (n = 172) and those who had had prior spinal fusion (n = 126). Using a linear regression model, the authors noted that, in both groups, the patients with positive sagittal balance (a positive sagittal vertical axis) had the most significant compromise in health status when compared with patients who had neutral balance or a negative sagittal vertical axis. In the group of patients who had not had surgery, those with a positive sagittal balance reported greater pain on the Scoliosis Research Society-22 (SRS-22), Short Form-12 (SF-12), and Oswestry Disability Index (ODI) questionnaires (p = 0.01, p = 0.00, and p = 0.00, respectively), diminished physical function on the SRS-22 and SF-12 (p = 0.00 for both), poorer self-image on the SRS-22 (p = 0.03), and poorer social function on the SF-12 (p = 0.02). Similarly, in the group of patients who had surgery, those with a positive sagittal vertical axis had poor outcomes in the pain, function, and self-image domains of the SRS-22 (p = 0.00 for all) as well as in the bodily pain (p = 0.00), physical function (p = 0.00), vitality (p = 0.00), and social function (p = 0.02) domains of the SF-12. They also reported greater pain on the ODI scale (p = 0.00). These findings suggest that restoration of normal sagittal balance is the critical goal in the surgical treatment of adults with spinal deformity and that the magnitude of coronal deformity and the extent of coronal correction are less critical parameters.
In another study, Lafage et al.29 reported that the magnitude of the coronal deformity did not impact pain and disability but that the sagittal vertical axis and the pelvic tilt showed strong correlations with all of the HRQoL measures in adult patients with spinal deformity. The sagittal vertical axis was correlated with the SRS total score (Spearman correlation coefficient [r] = 0.41, p = 0.001). Pelvic tilt was correlated with the SRS total score (r = 0.28, p = 0.001). The mean pelvic tilt in the deformity group (24° ± 12°) was greater than that in normal volunteers (12.1° ± 3.2°) as reported in the literature13,14. The authors classified the patients according to the two key parameters (increasing pelvic tilt and sagittal vertical axis), with Category 1 indicating low pelvic tilt (<25°) low sagittal vertical axis (<50 mm), Category 2 indicating high pelvic tilt (>25°) low sagittal vertical axis (<50 mm), Category 3 indicating low pelvic tilt (<25°) high sagittal vertical axis (>50 mm), and Category 4 indicating high pelvic tilt (>25°) high sagittal vertical axis (>50 mm). The authors observed that the HRQoL scores for pain and disability significantly increased (p < 0.05, analysis of variance) as patients moved from one category to the next.
How Much Lumbar Lordosis Should Be Restored in Adults with Spinal Deformity
Pelvic incidence has been shown to have a positive correlation with lumbar lordosis (r = 0.5867, p < 0.001) and sacral slope (r = 0.7046, p < 0.001)13. A high pelvic incidence causes a high sacral slope, which in turn leads to a high lumbar lordosis in order to maintain the normal sagittal balance with the head over the pelvis10,11,13,27. Schwab et al.17 proposed a simple formula for estimating the mean lumbar lordosis on the basis of the mean pelvic incidence in asymptomatic adults: lumbar lordosis = pelvic incidence + 9° (±9). This formula is helpful for estimating the degree of lordosis that needs to be restored at the time of surgery (e.g., spinal osteotomy) in order to restore harmonious spinopelvic balance.
Schwab et al. found that HRQoL scores were strongly correlated with the sagittal vertical axis (r = 0.47), pelvic tilt (r = 0.38), and lumbar lordosis that was proportional to pelvic incidence (pelvic incidence − lumbar lordosis) (r = 0.45)30. On the basis of these findings, Schwab et al.30 proposed the SRS-Schwab classification system for adult patients with spinal deformity, which includes the spinopelvic parameters that are highly correlated with HRQoL scores (Fig. 6).
The bipedal gait in apes is more energy-consuming than the quadrupedal gait in other animals. In humans, bipedal gait reduces energy cost by adopting a more upright posture, which shifts the axis of gravity posterior to the hips and lumbar spine. Therefore, pelvic morphology and orientation are critical for energy-efficient gait and posture. Pelvic incidence is a fundamental anatomical parameter, as opposed to sacral slope and pelvic tilt, which define the orientation of the pelvis in relation to a vertical or horizontal line the sagittal plane. The relationship between these parameters can be expressed with the formula: pelvic incidence = sacral slope + pelvic tilt. Pelvic incidence gradually increases during childhood, reaching its final shape at around the age of twenty years. Pelvic incidence has a close relationship with lumbar lordosis, as expressed with the formula: lumbar lordosis = pelvic incidence + 9° (±9). A large pelvic incidence is associated with large sacral slope, which is associated with a large lumbar lordosis.
The pelvic parameters (pelvic incidence, sacral slope, and pelvic tilt) and the spinal parameters (lumbar lordosis and sagittal vertical axis) have important bearing on various clinical conditions, including developmental and degenerative spondylolisthesis, back pain and the outcome of fusion surgery, adult spinal deformity, and so on. Pelvic incidence and sacral slope appear to be higher in obese patients with a large BMI, which predisposes to osteolytic and degenerative spondylolisthesis by increasing shear force at the lumbosacral disc. Disc degeneration and/or adult spinal deformity may lead to loss of lumbar lordosis and anterior shift of the sagittal vertical axis; a compensatory pelvic retroversion may restore the sagittal vertical axis to normal, which is reflected in an increase in pelvic tilt. Anterior shift of the sagittal vertical axis and/or increased pelvic tilt are associated with poor HRQoL. Deformity classification and surgical planning therefore require consideration of the pelvic orientation and adequate reduction of pelvic tilt after surgery in addition to restoration of lumbar lordosis and the sagittal vertical axis.
The author acknowledges Meredith A. Arensman, MD, for her help in preparing this manuscript.
Source of Funding: No external funds were received in support of this study.
Investigation performed at the Department of Orthopedics, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire
Disclosure: The author did not receive payments or services, either directly or indirectly (from his institution), from a third party in support of any aspect of this work. The author, or his institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. The author has not had any other relationships, and has not engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
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