17 Advantages and Limitations of the SRS-Schwab Classification for Adult Spinal Deformity

10.1055/b-0039-171413

17 Advantages and Limitations of the SRS-Schwab Classification for Adult Spinal Deformity

Advantages and Limitations of the SRS-Schwab Classification for ASD

Hideyuki Arima, Leah Y. Carreon, and Steven D. Glassman

Abstract

Adult spinal deformity (ASD) is a term that encompasses various primary and degenerative conditions that contribute to an altered three-dimensional structure of the spine. ASD may influence the progression and impact on otherwise normal aging changes in the spine. This complexity makes it difficult to easily describe or classify diagnosis and treatment of adult spinal deformity. Aided by research efforts examining sagittal spinopelvic alignment and balance, our understanding of the pathology of spinal deformity has greatly advanced. Importantly, we have learned that positive sagittal spinal alignment is strongly associated with poor health-related quality of life and low back pain. The Scoliosis Research Society (SRS)-Schwab classification of adult spinal deformity has focused the attention of surgeons and researchers on global sagittal imbalance; and this has contributed to improvement of treatment outcomes. As a result, global research efforts regarding adult spinal deformity has grown, and are helping to improve clinical practice. However, there are also limitations to a radiographic classification that only indirectly assesses clinical symptoms. This is particularly relevant, as adult deformities mainly occur in middle-aged and elderly people, in whom neurological symptoms caused by spinal stenosis are a critical element of diagnosis and treatment. In this chapter, we review the importance of sagittal balance in adult spinal deformities, regarding the advantages and limitations of using the SRS-Schwab classification.

17.1 Introduction

Adult spinal deformity is a broad diagnostic classification that encompasses a complex group of spinal pathologies with various clinical presentations. With longer life expectancy and an increasing proportion of healthy older individuals, there is an increasing population presenting with adult spinal deformity. Schwab and colleagues reported a prevalence of adult spinal deformity as high as 68% in those older than 60 years. ¹ Although some cases are asymptomatic or can be treated nonsurgically, a substantial number of patients present for corrective surgery because of pain and disability. To optimize surgical treatment, it is necessary to understand the existing pathology, the surgical indications, have a comprehensive preoperative plan to restore both spinal sagittal and coronal plane alignment, and spinopelvic alignment, as well as address neurologic compromise. In adult spinal deformity, a lack of understanding of the underlying pathology and poor surgical planning leads to poor outcomes or revision surgery. ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ ^, ⁷ Historically, restoration of the coronal plane was considered of prime importance in the correction of spinal deformities. However, several studies have demonstrated that, in adult spinal deformity, sagittal spinal imbalance, a deformity of the spine in the sagittal plane, has a greater impact on health-related quality of life than the spinal deformity in the coronal plane. ⁸ ^, ⁹ Moreover, patients with adult spinal deformity often have other issues associated with their deformity such as spinal stenosis with radicular or claudication-type pain and osteoporosis. ¹⁰ ^, ¹¹ ^, ¹² ^, ¹³ ^, ¹⁴ ^, ¹⁵

The Scoliosis Research Society-Schwab (SRS-Schwab) adult spinal deformity classification gives weight to sagittal plane evaluation by using the pelvic tilt (PT), sagittal vertical axis (SVA), and the mismatch between pelvic incidence (PI) and lumbar lordosis (PI minus lumbar lordosis [LL]). ¹⁶ This classification is very useful in terms of raising awareness of the importance of spinopelvic alignment among surgeons and researchers. In this chapter, we discuss the advantages and limits of using SRS-Schwab classification in surgical planning in patients with adult spinal deformity.

17.2 Sagittal Spinopelvic Alignment

17.2.1 History

Before the 1980s, many studies focused mainly on thoracic kyphosis and/or lumbar lordosis. ¹⁷ ^, ¹⁸ ^, ¹⁹ ^, ²⁰ In 1991, Itoi ¹⁵ investigated for the first time the relationship between postural deformities to include both the spine and lower extremities. In 1994, Jackson ²¹ reported the measurement method of sagittal balance using C7 sagittal plumb line. At the same time, research on sagittal morphology of pelvis was being actively conducted in Europe. ²² ^, ²³ ^, ²⁴ ^, ²⁵ In 1998, Legaye et al ²⁶ proposed PI as a key anatomic pelvic parameter that is predictive of the sagittal balance of the spine. Since then, the importance of spinal global sagittal alignment and balance including the pelvis has been extensively studied. ²⁷ ^, ²⁸ ^, ²⁹ ^, ³⁰ ^, ³¹ This has revealed that the sagittal morphology and inclination of the pelvis are important components in human standing alignment. ³¹

17.2.2 Imaging Studies

Standard evaluation of spinopelvic alignment is performed by using full-length 36-inch standing posteroanterior and lateral radiographs of the spine and pelvis. The SRS recommends that patients should stand with their knees locked, feet shoulder-width apart, looking straight ahead with their elbows bent and knuckles in the supraclavicular fossa bilaterally. ³² This will place the patient’s arms at approximately a 45° angle to the vertical axis of the body. The position of the arms at the time the radiograph is taken can have a bearing on sagittal alignment. ³³ If a leg length discrepancy of greater than 2 cm is present, a shoe lift should be used. ³² The lateral radiograph should include C0 and both femoral heads at a minimum, and sagittal alignment and balance should be measured with the patient ideally standing fully erect with the knees and hips in full extension to counteract compensatory mechanisms. ³⁴ Because of the importance of sagittal alignment of lower extremities including the hip joint and knee joints has been recently clarified, full-length standing lateral radiographs of lower extremities should also be evaluated when available.

17.2.3 Sagittal Spinopelvic Parameters

Pelvic Parameters

Three pelvic parameters have been defined: PI is an individual-specific morphologic pelvic parameter that has been demonstrated to define lumbar alignment. ²⁶ PI is a shape parameter that is not affected by changes in the alignment of the lower extremities. ³⁵ The PT is a positional and dynamic pelvic parameter that measures pelvic retroversion. In some cases, this parameter has been demonstrated to act as a compensatory mechanism to maintain an upright posture. The sacral slope (SS) quantifies the sagittal sacral inclination and completes the geometric relationship “PI = PT + SS” ³⁶ (Fig. 17‑1a).

Global Sagittal Alignment

SVA is usually used to determine the global sagittal spinal alignment (Fig. 17‑1b). ²¹ ^, ³⁷ This is identified by a plumb line dropped from C7 to the anterior edge of the sacral endplate. If compensated by posture, such as knee flexion and pelvic retroversion, SVA may be underestimated. ³⁸ ^, ³⁹ Recently, T1-pelvic angle (TPA) and global tilt (GT) have been proposed as novel global spinopelvic parameters (Fig. 17‑1c). ³⁸ ^, ⁴⁰ TPA is defined as the angle subtended by a line from the femoral heads to the center of the T1 vertebral body and a line from the femoral heads to the center of the superior sacral endplate. GT is the superior sacral endplate to the center of the C7 vertebral body and a line from the femoral heads to the center of the superior sacral endplate. Both novel parameters combine trunk anteversion and pelvic retroversion as one parameter to assess global spinal deformity. ³⁹

17.3 Sagittal Imbalance

17.3.1 Cone of Economy

Abnormal posture in the sagittal plane has been demonstrated to cause significant impairments in the elderly. ⁸ ^, ¹⁴ Sagittal balance in normal asymptomatic individuals is a state of congruence among the spine, pelvis, and lower extremities to achieve an economic posture, placing the axis of gravity in a physiologic position. ²⁶ ^, ⁴¹ ^, ⁴² ^, ⁴³ ^, ⁴⁴ This concept is clearly illustrated by Dubousset’s “cone of economy.” ⁴³ Increasing positive sagittal imbalance causes the body to assume a position toward the periphery of the cone, which results in increased muscular effort and energy expenditure causing pain, fatigue, and disability. If the body is shifted beyond the periphery of the cone, external supports such as a cane, crutch, or walker may be required to maintain balance. ¹¹ ^, ⁴³

17.3.2 Cause of Sagittal Malalignment

The causes of sagittal spine imbalance are multifactorial and range from iatrogenic causes to genetic and metabolic causes. ³⁴ Sagittal imbalance of the spine is mainly related to any underlying pathology causing loss of LL such as multilevel degenerative disk disease, ankylosing spondylitis, diffuse idiopathic skeletal hyperostosis, flat back syndrome following spinal fusion surgery, osteoporosis, tumor, trauma, or infection. ² ^, ¹¹ ^, ³⁴ ^, ⁴⁵ ^, ⁴⁶ Bridwell and colleagues classified sagittal imbalance into primary or secondary causes. The most common primary cause of sagittal imbalance is multilevel disk degeneration. Secondary causes are iatrogenic in nature and are related to previous spinal fusion surgeries. The classic secondary type is a flat back syndrome following the Harrington operation for adolescent idiopathic scoliosis. ⁴⁷ ^, ⁴⁸ However, the most frequent cause of secondary sagittal imbalance is that following lumbar spinal fusion (e.g., posterior lumbar interbody fusion, transforaminal lumbar interbody fusion, and posterolateral fusion) with inadequate restoration of sagittal alignment. ¹¹ ^, ¹² ^, ⁴⁹ Other secondary causes of sagittal plane deformity include posttraumatic kyphosis that can be divided into the following two types: fixed kyphosis following major spinal fractures/dislocations that occurs in relatively young individuals; and vertebral collapse or pseudarthrosis that occurs in elderly with severe osteoporosis. ¹¹

17.3.3 Sagittal Imbalance and Health-Related Quality of Life

In sagittal imbalance, spinal malalignment generally causes back pain and postural abnormalities. Patients may also present with difficulty in maintaining a standing posture caused by fatigue of the back muscles, neurologic symptoms caused by coexisting spinal stenosis, gastrointestinal symptoms caused by a decrease in the abdominal cavity volume, and mental health issues resulting from anxiety about pain and deformity.

In the assessment of health-related quality of life for patient with adult spinal deformity, surgeons use a common patient-reported outcome such as 36-Item Short Form Survey (SF-36), ¹ ^, ⁵⁰ ^, ⁵¹ ^, ⁵² the Short Form-12 (SF-12), ⁸ ^, ⁹ EuroQol 5D (EQ-5D), Oswestry Disability Index (ODI), ⁹ ^, ⁵¹ ^, ⁵³ and the SRS-22r instrument. ⁵⁴ The SRS-22r has been found to be reliable, valid, and responsive to change in patients undergoing surgery for patients with adult spinal deformity. ⁵⁴ ^, ⁵⁵ ^, ⁵⁶ These clinical studies revealed that health-related quality of life in patients with adult spinal deformity is lower than normal controls ¹ ^, ⁵² or patients with other common chronic conditions. ⁵⁷ It is very important to evaluate the clinical symptoms of adult spinal deformity patients with the health-related quality of life questionnaire, but it is equally important to objectively evaluate the spine deformity when considering surgical treatment. To establish objective surgical criteria, the relationship between radiographic parameters and clinical outcome has been extensively studied. As a result, previous studies demonstrated that the global positive balance in the sagittal plane rather than imbalance in the coronal plane was significantly associated with deterioration of the health-related quality of life. ⁸ Among various spinopelvic parameters, PI minus LL, SVA, and PT were detected as key parameters related to pain and low back disability. ⁵⁸

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Tags: Neurosurgery, Orthopaedics and Trauma Surgery, Part V The Non Scoliotic Spine, Sagittal Balance of the Spine

May 11, 2020 | Posted by drzezo in NEUROSURGERY | Comments Off

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17 Advantages and Limitations of the SRS-Schwab Classification for Adult Spinal Deformity