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646 radiology.rsna.org n Radiology: Volume 278: Number 3—March 2016

1 From the Morsani College of Medicine, University of South Florida, 12901 Bruce B. Downs Blvd, Tampa, FL 33612 (R.D.M., J.U.); and Miller School of Medicine, University of Miami, Miami, Fla (R.M.Q.). Received October 20, 2014; revision requested December 12; final revision received April 8, 2015; accepted April 28; final version accepted June 30. Address correspondence to R.D.M. (e-mail: [email protected] ).

q RSNA, 2016

Ryan D. Murtagh, MD, MBA Robert M. Quencer, MD Juan Uribe, MD

Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery: How I Do It1

Surgeons and radiologists have traditionally focused on frontal radiographs and the measurement of scoliosis curves as important tools in the management of spinal deformity. It has become evident, however, that the man-agement of spinal deformity should use a multidimen-sional approach with an increased emphasis on standing lateral radiographs and the sagittal position of the spine. Furthermore, they have come to realize the critical role that the pelvis plays in the maintenance of posture. Failure to recognize pelvic compensation can lead to under-treat-ment and poor postoperative outcomes.

© RSNA, 2016

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HOW I DO IT: Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery Murtagh et al

Radiology: Volume 278: Number 3—March 2016 n radiology.rsna.org 647

Scoliosis is increasingly recognized as an important health concern in the adult population, with

prevalence as high as 60% of the el-derly population (1,2). In addition to cosmetic concerns, patients with scoli-osis can present with substantial pain and disability (2). Historically, surgeons have focused on coronal alignment and planned surgery accordingly to correct scoliotic deformity (3). In this setting, the goal of surgery is to reduce the de-gree of coronal deformity by using rods held in place by hooks, screws, or a combination of both.

The spine, however, does not func-tion in a single dimension, and while many surgical procedures were able to successfully reduce or alleviate coronal deformity, they neglected or even ex-acerbated the sagittal deformity, the result of which was persistent pain, limited physical function, and adverse

Published online10.1148/radiol.2015142404 Content code:

Radiology 2016; 278:646–656

Abbreviation:SRS = Scoliosis Research Society

Conflicts of interest are listed at the end of this article.

Essentials

n Scoliosis and loss of normal tho-racic kyphosis and lumbar lor-dosis can substantially affect neutral, or balanced, standing posture.

n When standing posture is outside of the normal comfort range, humans will use compensatory measures like flexion of the knees and pelvic retroversion in an effort to return to this more comfortable, balanced position.

n The pelvis plays an important role in compensation, and failure to recognize the presence of compensatory measures on radiographs can lead to an under-estimation of the severity of the deformity and subse-quently lead to more conserva-tive treatment than may be needed.

n It is important for the radiologist to understand the concept of bal-ance and the role that the pelvis plays in the maintenance of normal balance so as to correctly recognize when pelvic compensa-tory measures are being used.

self-image, even in the setting of suc-cessful radiologic outcomes in the coro-nal plane (4,5).

It is apparent that sagittal align-ment correlates highly with quality of life scores and that failure to address sagittal alignment, in addition to the co-ronal deformity, can lead to persistent pain and deformity (3). Specifically, the failure to create the appropriate degree of lumbar lordosis at surgery through osteotomies and/or intervertebral cag-es can lead to a persistent sagittal im-balance in the postoperative patient. In addition, it is understood that anatomic segments of the spino-pelvic axis act as a continuum and share a degree of interdependence in an effort to main-tain a stable posture with a minimum of energy expenditure (6). Surgeons have long understood that changes in biomechanics of one segment of the spine can affect the biomechanics in adjacent segments. The pelvis, in par-ticular, plays an important role in global coronal and sagittal balance, and pelvic morphology and position are important in the biomechanics of the spine (3). Failure to address pelvic positioning and morphology as part of the preop-erative strategy can substantially affect postsurgical outcomes; therefore, the pelvis is increasingly scrutinized in pre-surgical planning.

Imaging of scoliosis is no longer just measuring Cobb angles on roller boards full of “stitched” frontal radio-graphs using measurement tools on a picture archiving and communication system. The radiologist needs to un-derstand the concepts of coronal and sagittal deformity, should appreciate the role that the pelvis plays in global balance, and be able to recognize com-pensatory measures on imaging.

Cone of Economy

In 2011, Dubousset introduced the con-cept of “cone of economy” (Fig 1) (7). When standing upright, there is mini-mal energy and maximal comfort when C7 is centered over S1. Normally the spine is straight in the coronal plane, and this position is maintained with minimal effort. In the sagittal plane,

C7 is positioned comfortably over S1 as a result of a series of lordotic and kyphotic curves. Malalignment in ei-ther plane, through scoliosis or loss of the normal sagittal curves, can disrupt this balance, requiring more energy ex-penditure to compensate for posture. While the compensated patient may appear balanced at physical examina-tion, the result of this increased energy expenditure to maintain comfortable positioning can be fatigue, pain, and persistent disability (3,7).

The cone of economy theory pro-vides the basic concept for a multidi-mensional approach to the correction of spinal deformity. While the concepts of sagittal and coronal balance are not novel, the role of the pelvis in balance has been largely neglected. The goal of this article is to familiarize the radiol-ogist with the concepts of coronal and sagittal balance, with an emphasis on pelvic imaging parameters.

Imaging and Measurement Technique

A thorough discussion of coronal and sagittal balance should be predicated on the assumption that radiography is performed with the patient in his or her own neutral standing position (Movie [online]). The knees should be straight and the patient should stand without use of orthotics or special shoes. The arms are placed on adjust-able supports to remove arms from the field of view and provide support while standing. Imaging is ideally acquired by using a three-station, or level, digital technique extending from the cranio-cervical junction through the femoral heads. Each of the three levels that are imaged is digitally fused to form a con-tiguous 36-inch field of view, seen as the final composite image. Because this type of digital imaging is not universally



available, a full-length 36-inch cassette or combining “stitching” of smaller images can be performed. Stitching using conventional radiography is sim-ilar in concept to the digital technique in which images acquired at three different levels are attached to form one larger image. It should be noted, however, that stitching is less optimal and must be done in a controlled en-vironment with close attention to de-tail (specifically, the patient must not move between acquisitions). Whether a full-length cassette or stitching is used, the radiographs must extend from the level of the skull base through the femoral heads.

Figure 1: Drawing of the concept of “cone of economy “introduced by Du-bousset. Printed, with permission, from Kenneth X. Probst. Originally published in reference 3.

Figure 1

Figure 2: Coronal balance. (a) Frontal standing radiograph in a 28-year-old woman with normal coronal alignment. The C7 plumb line (black line) intersects the midpoint of the superior endplate of S1 (central sa-cral vertical line) demonstrating neutral coronal balance. (b) Standing frontal radiograph in a 67-year-old woman with lumbar levoscoliosis and thoracic dextroscoliosis. There is 6.5 cm of coronal plane decompen-sation (CPD) to the patient’s right, demonstrated as the distance between the C7 plumb line (black line) and the central sacral vertical line (red line).

Figure 2

The majority of radiographs in pa-tients with scoliosis will be stored and viewed on one of the many commer-cially available picture archiving and communication systems. Many of them provide measurement tools for the cal-culation of distance and angles, allow-ing the radiologists and surgeons to perform relevant measurements at the time of interpretation. Other robust, spine-dedicated software programs are available from third-party vendors.

This article emphasizes parameters obtained from standing radiographs. Radiography has the advantage of being readily available, inexpensive, and fast

while exposing the patient to relatively little ionizing radiation compared with standard computed tomographic (CT) technique. Perhaps most importantly, radiographs are obtained with the pa-tient in standing position, providing a view of the anatomy in upright, weight-bearing position. The limitations of ra-diography include relatively poor spatial resolution and the inability to visualize the spine in three dimensions, limiting evaluation of axial rotation and other abnormal axial morphology (8).

Scoliosis is a complex three-dimen-sional deformity, and therefore the ability to evaluate the spine in three



dimensions is useful in the evaluation of the deformity and planning of sur-gery. CT provides excellent spatial resolution of the bone structures, and modern scanners allow for volumetric acquisition with the ability to recon-struct in multiple planes. This comes at the expense of increased cost and radiation exposure relative to radiogra-phy. Recent advances have substantially decreased radiation exposure relative to standard technique and this is par-ticularly appealing in the pediatric population (9,10). Low-dose digital stereoradiography is a technique that uses biplanar x-ray technique to create three-dimensional images of the spine with lower radiation exposure than is traditionally seen with standing frontal and lateral radiography (8,11). The ma-chinery required to obtain these images is costly relative to traditional radiogra-phy and is therefore seen predominately in a few facilities that do a large volume of scoliosis imaging.

Finally, magnetic resonance (MR) imaging can play a role in the work up of scoliosis. Limitations of MR imag-ing in the work up of scoliosis include cost, MR imaging contraindications, and artifacts created by any hard-ware. While upright MR imaging is available, the majority of MR imaging is performed with the patient in su-pine position. Advantages of MR im-aging include excellent spatial resolu-tion of soft-tissue structures, with an important role in the evaluation of the spinal canal and intradural structures in the adult population, and the de-tection of any associated neural axis anomalies in patients with idiopathic scoliosis (10).

The Concept of Coronal Balance and Important Radiologic Parameters

Humans are most comfortable in a neutral, midline posture in the upright position (ie, not leaning to the left or right). Coronal spinal deformity (scoli-osis), abnormal pelvic tilt, and even leg length discrepancy can affect the neu-tral position, causing the individual to lean to the left or right. For example, severe dextroscoliosis will cause a shift

of the more cranial neural axis to the left. In this setting, the patient will at-tempt to compensate by tilting the pel-vis to the right to restore neutral mid-line positioning.

The unintended consequences of this compensation are both cosmetic and physiologic. From a cosmetic standpoint, these patients can present with a rib hump and shoulder asym-metry. From a physiologic standpoint, these patients must expend additional energy to return to and maintain neu-tral balance. Compensating patients, whether they have had surgery or not, often present with poor self-image and often complain of pain and fatigue (3).

Coronal balance plays an important role in the multidimensional approach to deformity surgery and, if not suffi-ciently addressed, persistent deformity, pain, and fatigue can result in what is perceived to be a failed surgery. As a result, it is important that the radiolo-gist, surgeon, and other treating phy-sician understand the concepts of co-

ronal balance and compensation. The most important measurements in the interpretation of coronal balance from the standpoint of the spine (exclusive of the pelvis at this point) are coro-nal plane decompensation and Cobb angles.

Coronal Plane DecompensationCoronal plane decompensation is the most useful tool in assessment of coro-nal balance, and an assessment of co-ronal positioning should be mentioned in the interpretation of all full frontal, standing radiographs (Fig 2). In neutral position, the midpoint of the inferior endplate of C7 is directly superior to the midpoint of the superior endplate of S1. The coronal plane decompen-sation is calculated first by drawing a plumb line (which is a line drawn per-pendicular to the floor) from the infe-rior midpoint of C7. The central sa-cral vertical line is then identified. The central sacral vertical line is a plumb line that passes through the midpoint

Figure 3: Standing frontal radiograph of the lumbar spine in a 62-year-old woman with dextroscoliosis. The Cobb angle is calculated as the angle created by a line drawn along the superior endplate of the most superior vertebral body of the curve (superior terminal vertebral body) and a line drawn along the inferior endplate of the most caudal involved segment (inferior terminal verte-bral body).

Figure 3



Figure 4: Sagittal balance. (a) Image shows neutral sagittal balance in which the plumb line (yellow line) from the midpoint of the inferior endplate of C7 (red dot) passes through the posterior superior corner of S1 (black dot). (b) Image shows 4.5 cm of positive sagittal balance calculated as the distance between the plumb line from the midpoint of the inferior endplate of C7 (yellow line) and the plumb line through the pos-terosuperior corner of S1 (green line). SVA 5 sagittal vertical alignment.

Figure 4

Figure 5: Standing frontal radiograph in a 70-year-old man demonstrates pelvic obliquity, which is calculated as the angle between a line connecting the most superior margins of the iliac wings (pelvic coronal reference line) and a horizontal reference line (line parallel to the floor). There is substantial pelvic obliquity to the right (7°) but only minimal coronal plane decompensation from midline (red line shows coronal plane decompensation). Findings are consistent with pelvic compensation to correct coronal plane decompensation.

Figure 5

of the superior endplate of the sacrum (12). Coronal plane decompensation is the horizontal difference between these two lines. Coronal plane decompen-sation is described as being “right” or “left” depending if the shift is to the pa-tient’s right or left. Coronal plane de-compensation is most often the result of scoliosis but can result from any ab-normality (eg, a leg length discrepancy) that shifts the C7 plumb line to the right or left of the central sacral vertical line. Patients with coronal plane de-compensation greater than 4 cm have been shown to report poor function and increased pain relative to those with less than 4 cm of coronal plane decompensation (5).

Cobb AnglesThe Cobb angle (Fig 3) is a well-estab-lished technique for measuring scoliotic curvature. The curve is calculated by identifying the vertebral bodies at the superior and inferior margins of the curve (also known as the terminal ver-tebral bodies) (12). The terminal ver-tebral bodies are the cranial and cau-dal vertebral bodies with the greatest degree of tilt. Once identified, a line is drawn along the superior endplate of the most cranial terminal vertebral body, and another line is drawn along the inferior endplate of the caudal ter-minal vertebral body. The resultant an-gle is the Cobb angle. In the adolescent population, progressive scoliosis with a

Cobb angle between 25° and 45° will be managed conservatively, while Cobb angle greater than 50° is typically treat-ed surgically (13).

The Concept of Sagittal Balance and Important Radiologic Parameters

The principles of sagittal balance are similar to those of coronal balance: Humans are most comfortable in a neutral standing position and will ex-pend effort to maintain this position when acted upon by internal defor-mity or outside forces. For example, loss of lumbar lordosis can cause the patient to lean forward (increased or “positive” sagittal balance). As a re-



sult, the subject will expend additional energy attempting to compensate for the imbalance, including retroversion of the pelvis and flexion of the knees. Glassman et al and others have shown that sagittal balance is the single most important and consistent radiologic predictor of clinical outcomes as de-termined by self-assessment surveys including the Scoliosis Research So-ciety (SRS)-22 questionnaire, Short Form 12-item survey, and Oswestry Disability Index profiles (14,15). This applies to nonoperated deformities, as well as to patients with persis-tent deformity after surgery (16). The presence of sagittal imbalance is more likely to predict persistent dis-ability and pain than the size of the curve, location of the curve, or the presence of coronal plane decompen-sation (5,14). The end result after surgery can be markedly impaired

Figure 6: (a, b) Standing lateral radiographs of the pelvis and lumbar spine in a 30-year-old woman. The pelvic incidence is the angle created by a line drawn from the midpoint of the femoral heads (black dot) to the midpoint of the superior endplate of S1 and a line drawn perpendicular to a line drawn parallel to the supe-rior endplate of S1 (red line). The normal lumbar lordosis (calculated by using the Cobb angle technique, yellow lines) should be within 10° of the pelvic incidence as can be seen on b (lumbar lordosis 5 60° and pelvic incidence 5 51°). (c) Standing lateral radiograph in a 75-year-old woman demonstrates a substantial differ-ence between lumbar lordosis and pelvic incidence, thereby suggesting that the patient would benefit from osteotomy and/or cage placement to increase lordosis. PT 5 pelvic tilt, PI 5 pelvic incidence.

Figure 6

health status measures (manifested as persistent pain, limited function, and poor self-image) if this imbalance is not properly addressed. Relevant radiologic parameters in the under-standing of sagittal balance include spinal vertical alignment, thoracic ky-phosis, and lumbar lordosis.

Spinal Vertical AlignmentSpinal vertical alignment (Fig 4) is measured as the distance between a plumb line through the midpoint of the inferior endplate of C7 and a plumb line through the posterosupe-rior corner of S1. In neutral position, the plumb line of C7 will intersect with the posterosuperior corner of S1. The mean spinal vertical alignment in asymp-tomatic adults is 0.5 cm 6 2.5 (standard deviation) and increases with normal aging (17,18). In 2012, the SRS pub-lished the SRS-Schwab Classification

as a guideline for the interpretation and treatment of adult deformity. By using this classification system, a spi-nal vertical alignment of less than 4 cm is graded as “0” or “non-pathological” sagittal alignment, that 4 cm to 9.5 cm is graded as “1” or “moderate” defor-mity, and that greater than 9.5 cm is graded as “11” or “marked” deformity (17–19).

Thoracic KyphosisThoracic kyphosis is the Cobb angle created by drawing a line across the su-perior endplate of T2 and the inferior endplate of T12 (3). It is often difficult to visualize the superior endplate of T2 on lateral views, and the superior endplate of T5 is often used in lieu of T2. The normal thoracic kyphosis is be-tween 30° and 40° in men aged 50–80 years old and increases with normal ag-ing. Average thoracic kyphosis is closer



to 40° in women older than 50 years of age (20).

Lumbar LordosisThe lumbar lordosis is the Cobb angle resulting from intersecting lines drawn across the superior endplate of T12 and S1 (3). The average lumbar lordosis in adults is 33.2° 6 12.1 and increases with aging (21). Decreased lumbar lor-dosis is correlated with pain and loss of function (22). Flatback deformity is a term applied to patients with severe loss of both lumbar lordosis and tho-racic kyphosis, effectively giving the spine a flat appearance on lateral views.

The Role of the Pelvis in Maintenance of Sagittal and Coronal Balance and Important Radiologic Parameters

The spine and pelvis effectively act as a continuum in the maintenance of neutral coronal and sagittal balance. Abnormal positioning or biomechanics seen in one segment inherently affect the adjacent segment. For example, changes in the position or biomechan-ics of the lumbar spine can produce changes in both the position of the pel-vis and the thoracic spine secondary to compensatory mechanisms.

The pelvis plays a critical role in spi-no-pelvic alignment, yet recent studies have shown that the position of the pel-vis has long been neglected in the work up of scoliosis patients (3). Failure to evaluate the pelvic parameters in defor-mity surgery can result in postoperative misalignment and subsequent treatment failure. The pelvis has increasingly been shown to play an important role in up-right sitting and standing postures, and as a result presurgical planning now re-quires evaluation of pelvic parameters. The relevant parameters include pelvic obliquity, pelvic incidence, pelvic tilt, sacral slope, and the T1 pelvic angle.

Pelvic ObliquityPelvic obliquity (Fig 5) refers to the position of the pelvis relative to a line drawn parallel to the floor. Pelvic obliq-uity is calculated as the angle between the pelvic coronal reference line and a horizontal reference line (a line drawn

Figure 7: Pelvic incidence (PI) on standing lateral radiographs (a) before and (b) after surgery in a 78-year-old woman. In a there is substantial loss of lumbar lordosis, resulting in 19° difference between lumbar lordosis (Angle 1 ) and pelvic incidence. Surgery (b) incorporated a combination of osteotomies and anterior cages to increase lordosis, now nearly equal to the pelvic incidence (lumbar lordosis 5 40°, pelvic incidence 5 39°). PT 5 pelvic tilt.

Figure 7

parallel to the floor) (3). The pelvic co-ronal reference line can be identified in a number of ways, most commonly as a line connecting the two iliac crests.

Pelvic obliquity plays an important role in the coronal correction strategy. As stated previously, humans are most comfortable in neutral alignment (little or no coronal plane decompensation), and the pelvis can serve as a compensa-tory mechanism when there is substan-tial right or left coronal plane decompen-sation resulting from scoliosis. Tilting of the pelvis to the right or left in the coronal plane can substantially affect co-ronal plane decompensation by moving the C7 plumb line toward midline. While pelvic obliquity can serve as an indicator of compensation in a patient with scoli-osis, it can also signify the presence of other important underlying physiologic abnormalities, such as leg length dis-crepancy, that should be sought out and

addressed, if present. Ideally, correction of the coronal deformity should reduce the need for compensation, thereby cor-recting the pelvic obliquity.

Pelvic IncidenceThe pelvic incidence is the angle cre-ated by intersecting lines drawn from the midpoint of the femoral heads to the midpoint of the superior endplate of the sacrum and a line perpendicular to the superior endplate of the sacrum as measured on lateral images (5,17). This parameter describes the morphology or shape of the pelvis. It is fixed and does not change with posture or positioning (6). The pelvic incidence is a value that stays nearly constant throughout life, except for a slight change at puberty.

The pelvic incidence (Figs 6, 7) is perhaps most important for its relation-ship to the lumbar lordosis. Ideally, the lumbar lordosis is within 9° of the pel-



vic incidence such that lumbar lordosis equals pelvic incidence 6 9° (17–20). By using the SRS-Schwab Classification, pelvic incidence-lumbar lordosis of less than 10° is graded as “0” or “non- pathologic,” that of 10°–20° is graded as “1” or “moderate” deformity, and that greater than 20° is graded as “11” or “marked” deformity (17–20). Pelvic in-cidence effectively describes the natural shape of a patient’s sacrum and, from this, provides a baseline reference for the calculation of the optimal degree of lumbar lordosis to be introduced at

Figure 8: (a) Pelvic tilt on standing lateral radiograph of the pelvic and lumbar spine in a 30-year-old woman. Pelvic tilt is the angle created between a line drawn from the midpoint of the femoral heads (black dot) to the midpoint of the superior endplate of S1 (yellow dot) and a plumb (vertical) line through the midpoint of the femoral heads (red line). (b) Lateral radiograph in a 75-year-old man shows an increased pelvic tilt, which is an indicator of compensatory retroversion. There is min-imal, nonpathologic, positive sagittal balance (nearly neutral) in the setting of markedly elevated pelvic tilt. The pelvis is retroverted as a compensatory means by which to restore neutral sagittal balance. (c) Image in an 83-year-old woman also demonstrates marked pelvic retroversion, but in this case it is not enough to over-come the markedly positive sagittal balance (sagittal vertical alignment [SVA], 10.7 cm). Note the severe loss of thoracic kyphosis and lumbar lordosis with resultant flatback deformity. PT 5 pelvic tilt, PI 5 pelvic incidence.

Figure 8

surgery. Surgeons can increase lordosis by taking away height posteriorly in the lumbar spine (such as through Smith Peterson or pedicle subtraction oste-otomy) or by adding height anteriorly (with interbody cages).

Pelvic TiltThe pelvic tilt is the angle created be-tween a line drawn from the midpoint of the femoral heads to the center of the superior endplate of the sacrum and a vertical plumb line through midpoint of femoral heads (Fig 8) (3,17,18). Pelvic

tilt is dependent on the position of the patient. As humans progressively lose lumbar lordosis with age or degenera-tive disk disease, there is a steady in-crease in the spinal vertical alignment. Patients instinctively will seek to restore neutral sagittal balance and can achieve this through a combination of pushing the pelvis posteriorly (“retroversion”) and with flexion the knees (3,17,18). Compensatory pelvic retroversion is re-flected on images as an increase in the pelvic tilt. Pelvic retroversion requires additional work and energy expenditure,



Figure 9: Drawing shows the relationship between spinal vertical alignment and pelvic tilt. In, A, there is substantial positive sagittal balance with relatively small pelvic tilt. Pelvic retroversion is reflected as an increase in pelvic tilt (seen in, B, and then, C ). The result is a progressive decrease in sagittal vertical alignment and return to neutral sagittal balance. (Original artwork, printed with permission from Kenneth X. Probst.)

Figure 9

Figure 10: Sacral slope in a 30-year-old woman. The sacral slope is the angle created by a line drawn along the superior endplate of S1 and a horizontal reference line (line parallel to the floor). Sacral slope plus pelvic tile equal pelvic incidence.

Figure 10

and the presence of pelvic retroversion in the postoperative patient is correlated with poor clinical outcomes (3,17).

As a general rule, the ideal pelvic tilt is greater than 10° and less than 20°. Pelvic tilt greater than 20° indi-cates compensatory pelvic retroversion. Studies have shown that as the spinal vertical alignment increases there is a compensatory increase in pelvic tilt (Fig 9). A study by Schwab et al (18) showed that for a negative, neutral (0–5 cm), and positive (. 5 cm) spinal ver-tical alignment, the average pelvic tilt is 10°, 16°, and 21°, respectively. By using the SRS-Schwab Classification, a pelvic tilt of less than 20° is graded as “0” or “non-pathologic,” a pelvic tilt of 20°–30° is graded as “1” or “moderate” deformity, and pelvic tilt of greater than 30° is graded as “11” or “marked” de-formity (17–20).

Sacral SlopeThe sacral slope, like pelvic tilt, is in-dicative of pelvic position and can be used to identify pelvic retroversion (Fig 10). The sacral slope is defined as the angle between a line drawn parallel to the superior endplate of S1 and a

horizontal reference line or line drawn parallel to the floor. Sacral slope plus pelvic tilt equals pelvic incidence, and therefore changes to sacral slope are inversely proportional to changes in the pelvic tilt (3). While sacral slope and pelvic tilt are complimentary measure-ments, the pelvic tilt is more often used in treatment planning.

T1 Pelvic AnglePelvic tilt and spinal vertical alignment are essential to a basic understand-ing of a patient’s sagittal balance and compensatory mechanisms used by the patient to maintain neutral balance. The T1 pelvic angle (Fig 11) is a newer measurement technique that takes into consideration the combined effect of both increased sagittal balance and pel-vic retroversion (23,24). The T1 pelvic angle is calculated as the angle between a line drawn from the midpoint of the femoral heads to the midpoint of the su-perior endplate of S1 and a line drawn from the midpoint of the femoral heads to the center of the T1 vertebral body. The T1 pelvic angle effectively incorpo-rates both the spinal vertical alignment and pelvic tilt measurements and has

been shown to correlate strongly with clinical outcomes (23,24). According to Ryan et al, the goal of surgery should be a T1 pelvic angle of around 10°. Pa-tients with T1 pelvic angle greater than 20° are considered to have severe de-formity (23).

Bringing It All Together

A thorough understanding of scoliosis imaging is one way in which the radi-ologist can add value to what has tradi-tionally been seen as a mundane imag-ing modality. Simply stating that there is “S-shaped scoliosis” or providing simple Cobb angle measurements on the frontal view is no longer sufficient because surgeons view deformity as a three-dimensional process. The radiol-ogist should understand the basic con-cepts of sagittal and coronal balance, understand and be able to calculate the basic metrics relevant to deformity sur-gery, and be able to synthesize these data to form a relevant and practical overview for each individual that will help the surgeon determine the ap-



propriate course of treatment (Table). In particular, it is imperative that the radiologist understands the role of the pelvis in compensation and accurately

Overview of Spinopelvic Parameters

Deformity Normal According to SRS-Schwab Classification

Spinal vertical alignment 6 2.5 cm in asymptomatic adults ,4 cm: non-pathologic 4–9.5 cm: moderate deformity .9.5 cm: marked deformityPelvic incidence Normally within 10° of lumbar lordosis ,10°: non-pathologic 10°–20°: moderate deformity .20°: marked deformityPelvic tilt Normally between 10° and 20° ,20°: non-pathologic 20°–30°: moderate deformity .30°: marked deformityT1 pelvic angle Approximately 10° Severe deformity greater than 20°

Figure 11: Standing lateral radiograph in a 71-year-old woman showsT1 pelvic angle. The T1 pelvic angle is a global measurement that effectively incorporates both the pelvic tile and the sagittal vertical alignment into a single measurement tech-nique. T1 pelvic angle values are correlated with both pelvic tilt and sagittal vertical alignment values, as well as with patient outcomes. T1 pelvic angle is the angle created by a line drawn from the midpoint of the femoral heads (black dot) to the midpoint of the superior endplate of S1 (yellow dot) and a line drawn from the central aspect of the T1 vertebral body and the midpoint of the femoral heads.

Figure 11

recognizes when there is pelvic com-pensation. Failure to do so may lead to an underestimation of the degree of sagittal and/or coronal deformity and subsequently lead to more conservative treatment than may be required.

On the frontal radiographs the radi-ologist should be able to identify the in-dividual scoliosis curves, describe the af-fected levels, and provide accurate Cobb angle measurements. More importantly, the radiologist should be able to recog-nize and measure coronal plane decom-pensation and recognize if the patient is attempting to compensate for any clin-ically important coronal plane decom-pensation evidenced by pelvic obliquity.

On the lateral radiographs it is not sufficient to simply recognize an in-crease or decrease in thoracic kypho-sis or lumbar lordosis. The radiologist should understand the concept of in-terdependence between segments of the spino-pelvic axis and appreciate the compensatory capability of the pelvis. Standing lateral radiographs should in-clude a description of any substantial positive sagittal balance (. 4 cm) and, if present, recognition of any compen-satory pelvic retroversion (pelvic tilt . 20°). Patients with positive sagittal bal-ance and pelvic retroversion have been shown to have poor outcomes if not ad-dressed during surgery, and therefore it is imperative that this be recognized in the planning stages.

Standing lateral radiographs provide important information to the calcula-tion of the optimal degree of lumbar lordosis to be introduced at the time of surgery. The normal lumbar lordosis is within 10° of pelvic incidence, and it is important to recognize when the pa-tient would benefit from increased lor-dosis introduced through osteotomies, cages, or both. As a result, a greater

than 10° difference between lumbar lordosis and pelvic incidence should be mentioned in the report. Likewise, a pelvic tilt of greater than 20° suggests that there is compensatory pelvic ret-roversion, and this should also be indi-cated in the report.

Finally, it should be noted that the emphasis of this review is on the inter-dependence of the pelvis and the tho-racolumbar spine. Recently, and not surprisingly, studies have shown that the positioning of the cervical spine can play an important role in sagittal balance. Specifically, patients with sub-stantial cervical sagittal deformity can compensate with pelvic retroversion, thoracic hypokyphosis, and lumbar hy-perlordosis (25). A detailed discussion of cervical coronal and sagittal param-eters is beyond the scope of this review article but remains an important con-sideration in the assessment of global balance.

Summary

In the interpretation of scoliosis imag-ing, the radiologist should:

1. Measure Cobb angles on the frontal view.

2. Determine if there is any coro-nal plane decompensation and, if so, how much. Is there compensatory pel-vic obliquity that may cause the reader to underestimate the true amount of coronal plane decompensation?

3. Determine if there is clinically important (. 4 cm) positive sagittal balance. Is the patient compensating for this with pelvic retroversion (pelvic tilt . 20°)?

4. Determine if there is lumbar lordosis within 10° of pelvic incidence. Is there room to introduce more lum-bar lordosis at surgery?



Disclosures of Conflicts of Interest: R.D.M. disclosed no relevant relationships. R.M.Q. dis-closed no relevant relationships. J.U. Activities related to the present article: disclosed no rel-evant relationships. Activities not related to the present article: reports grant and personal fees from Nuvasive. Other relationships: disclosed no relevant relationships.

References 1. Schwab F, Dubey A, Pagala M, Gamez L,

Farcy JP. Adult scoliosis: a health assessment analysis by SF-36. Spine 2003;28(6):602–606.

2. Schwab FJ, Lafage V, Farcy JP, Bridwell KH, Glassman S, Shainline MR. Predicting out-come and complications in the surgical treat-ment of adult scoliosis. Spine 2008;33(20): 2243–2247.

3. Ames CP, Smith JS, Scheer JK, et al. Impact of spinopelvic alignment on decision making in deformity surgery in adults: a review. J Neurosurg Spine 2012;16(6):547–564.

4. Emami A, Deviren V, Berven S, Smith JA, Hu SS, Bradford DS. Outcome and complica-tions of long fusions to the sacrum in adult spine deformity: luque-galveston, combined iliac and sacral screws, and sacral fixation. Spine 2002;27(7):776–786.

5. Glassman SD, Berven S, Bridwell K, Horton W, Dimar JR. Correlation of radiographic parameters and clinical symptoms in adult scoliosis. Spine 2005;30(6):682–688.

6. Berthonnaud E, Dimnet J, Roussouly P, La-belle H. Analysis of the sagittal balance of the spine and pelvis using shape and ori-entation parameters. J Spinal Disord Tech 2005;18(1):40–47.

7. Dubousset J. Reflections of an orthopaedic surgeon on patient care and research into the condition of scoliosis. J Pediatr Orthop 2011;31(1 Suppl):S1–S8.

8. Glaser DA, Doan J, Newton PO. Comparison of 3-dimensional spinal reconstruction accu-racy: biplanar radiographs with EOS versus computed tomography. Spine 2012;37(16): 1391–1397.

9. Kalra MK, Quick P, Singh S, Sandborg M, Persson A. Whole spine CT for evaluation of scoliosis in children: feasibility of sub-milliSievert scanning protocol. Acta Radiol 2013;54(2):226–230.

10. Qiao J, Zhu Z, Zhu F, et al. Indication for preoperative MRI of neural axis abnormal-ities in patients with presumed thoracolum-bar/lumbar idiopathic scoliosis. Eur Spine J 2013;22(2):360–366.

11. Ilharreborde B, Sebag G, Skalli W, Mazda K. Adolescent idiopathic scoliosis treated with posteromedial translation: radiologic evaluation with a 3D low-dose system. Eur Spine J 2013;22(11):2382–2391.

12. Malfair D, Flemming AK, Dvorak MF, et al. Radiographic evaluation of scoliosis: review. AJR Am J Roentgenol 2010;194(3 Suppl): S8–S22.

13. El-Hawary R, Chukwunyerenwa C. Update on evaluation and treatment of scoliosis. Pe-diatr Clin North Am 2014;61(6):1223–1241.

14. Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F. The im-pact of positive sagittal balance in adult spinal deformity. Spine 2005;30(18): 2024–2029.

15. Mac-Thiong JM, Transfeldt EE, Mehbod AA, et al. Can c7 plumbline and gravity line predict health related quality of life in adult scoliosis? Spine 2009;34(15):E519–E527.

16. Booth KC, Bridwell KH, Lenke LG, Baldus CR, Blanke KM. Complications and pre-dictive factors for the successful treatment of flatback deformity (fixed sagittal imbal-ance). Spine 1999;24(16):1712–1720.

17. Schwab F, Patel A, Ungar B, Farcy JP, Laf-age V. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? an overview of key parameters in assessing alignment and planning corrective surgery. Spine 2010;35(25):2224–2231.

18. Schwab F, Lafage V, Boyce R, Skalli W, Farcy JP. Gravity line analysis in adult vol-unteers: age-related correlation with spinal parameters, pelvic parameters, and foot po-sition. Spine 2006;31(25):E959–E967.

19. Schwab F, Ungar B, Blondel B, et al. Scoliosis Research Society-Schwab adult spinal defor-mity classification: a validation study. Spine 2012;37(12):1077–1082.

20. Fon GT, Pitt MJ, Thies AC Jr. Thoracic ky-phosis: range in normal subjects. AJR Am J Roentgenol 1980;134(5):979–983.

21. Lin RM, Jou IM, Yu CY. Lumbar lordosis: normal adults. J Formos Med Assoc 1992; 91(3):329–333.

22. Kostuik JP, Hall BB. Spinal fusions to the sacrum in adults with scoliosis. Spine 1983;8(5):489–500.

23. Ryan DJ, Proptopsaltis TS, Ames CP, et al. T1 Pelvic angle (TPA) effectively evaluates sagittal deformity and assesses radiographi-cal surgical outcomes longitudinally. Spine 2014.39(15):1203-1210

24. Protopsaltis TS, Schwab FJ, Bronsard N, et al. The T1 pelvic angle (TPA), a novel radiographic measure of global sagittal de-formity, accounts for both pelvic retrover-sion and truncal inclination and correlates strongly with HRQOL. Presented at the Annual Meeting of the Scoliosis Research Society, Lyon, France, 2013.

25. Scheer JK, Tang JA, Smith JS, et al. Cervi-cal spine alignment, sagittal deformity, and clinical implications: a review. J Neurosurg Spine 2013;19(2):141–159.

Pelvic Evaluation in ˜ ˚˜ Thoracolumbar Corrective REVIEWS ...HOW I DO IT: Pelvic Evaluation in...

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