16 Sagittal Plane Malalignment and Degenerative Scoliosis in the Aging Spine
Abstract
Degenerative deformity in the aging spine has become a prevalent issue in medicine due to the increasing aging population. This chapter describes the etiology and biomechanics of degenerative scoliosis, as well as appropriate goals and strategies for managing spinal deformity. Treatment options discussed include nonoperative management, limited decompression, focal stabilization, and deformity correction. The reader will then learn the benefits and risks of surgery associated with degenerative scoliosis. Surgical considerations to avoid complications described in this chapter include the use of multiple surgeons and tranexamic acid to reduce blood loss. Another common complication discussed is proximal junctional kyphosis. Understanding the aging spine and the importance of sagittal balance correction in degenerative scoliosis will help providers execute decisions for management and surgical considerations for our aging population.
Key Points
Sagittal alignment is a key component in the evaluation and treatment of deformity in scoliosis.
The primary goal for management of these patients is to relieve pain, improve function, halt curve progression, and perhaps improve cosmetic appearance.
Nonoperative management should be considered for all patients. If nonoperative modalities fail, surgery may be considered with careful patient selection and attention to modifiable risk factors.
Implementing a shared decision-making model among providers can decrease unnecessary surgeries.
Once the patient and their providers have agreed that the benefits of surgery outweigh the risks, careful surgical planning is necessary to optimize surgical outcomes and reduce risks.
Surgical risks highlighted in this chapter include proximal junctional kyphosis and blood loss, with evaluation of research and guidelines to minimize these risks.
Understanding the changes that occur in the aging spine will help surgeons and other providers execute decisions for management and surgical considerations for our aging population.
16.1 Epidemiology
In the aging population, one of the most prevalent causes of spinal deformity is degenerative (de novo) scoliosis. 1 The reported incidence of adult degenerative scoliosis varies greatly, with reports reaching 64 to 68% in the elderly, 2 and is most commonly seen in patients older than 40 years of age. 3
The disease process is not entirely understood, but it is widely believed to be a result of accumulative degenerative changes that result from an aging spine. These changes include asymmetric disc degeneration, dehydration, and collapse along with ligament laxity and facet joint degeneration, leading to spinal column laxity. 1 Asymmetric degeneration, along with the subsequent unequal loading of the spinal column, initiates a dynamic and possibly synergistic pattern of curve progression and 3-dimensional deformity. 4 Vertebral column fractures due to osteoporosis, an all-but-inevitable consequence of aging, is a major risk factor for development of sagittal plane deformity. Iatrogenic spinal deformity as a result of lumbar fusion and simple decompression (laminectomy) can lead to a loss of lumbar lordosis (LL) known as flat back syndrome. This is a significant risk factor for sagittal malalignment and further spinal deformity in the aging spine. In those with degenerative scoliosis, the spinal curvature tends to progress at a rate of 1–6° per year with an average of 3° per year. 5
The clinical presentation of adult-onset degenerative scoliosis varies widely, with back and leg pain being the most commonly reported complaints in spinal deformity. Aside from pain and deformity, patients will often present with a previous spinal surgery history or have failed nonoperative management. An appropriate physical exam will include a full neurologic exam and assessment of deformity in supine, sitting, standing, and ambulatory positions.
Back pain has been reported in 40 to 90% of patients with degenerative scoliosis, 4 with an etiology that is often multifactorial. Back pain can be attributed to muscle fatigue, likely due to a compensatory mechanism to malalignment as a result of degenerative disk disease. Facet arthropathy is also a common pain generator in the aging spine. 5 The amount of back pain patients experience is not directly related to the size of the curvature; however, apical rotation and sagittal imbalance can exacerbate back pain. 6
Leg pain usually presents as radiating pain or intermittent claudication as a result of spinal stenosis, which is observed in around 90% of degenerative scoliosis. 7 , 8 , 9 Foraminal stenosis is more likely to affect the nerve roots on the concave side of the curve as a result of compression,although neurological symptoms may develop on the convex side as a result of the overstretching of nerve roots.
In the aging patient, it is important to assess quality of life, as aging patients are more likely to have comorbidities such as depression. 10 , 11 , 12 , 13 Measuring a patient’s health-related quality of life (HRQOL) can give a base line of a patient’s health and the resulting degree of satisfaction following surgery.
Comorbid conditions that are common in the aging include diabetes and osteoporosis. Uncontrolled diabetes has been linked to poor wound healing and a higher risk of infection. 14 A hemoglobin A1c over 8 has shown a significantly slower wound-healing rate in diabetic patients. 15 A dual-energy X-ray absorptiometry (DEXA) scan should also be considered in the aging, as they are at an increased risk of osteoporosis. Asymptomatic women > 65 years old and men > 70 years old, along with any symptomatic patients should be tested. Other patients for whom a DEXA scan should be considered are those with known endocrine or medical conditions affecting bone mineral density and risk factors such as cigarette and glucocorticoid use, a low BMI, estrogen deficiency, loss of height, or history of amenorrhea. Obesity and smoking have also shown a correlation with lower back pain, which should also be addressed in this population. 16
Radiographic imaging is required to properly evaluate patients with degenerative scoliosis. Standing frontal (anterior-posterior (AP) or posterior-anterior (PA)) and sagittal (lateral) whole spine, along with supine (nonweight-bearing) imaging characterize the deformity and give information on the flexibility of the deformity. As previously mentioned, a full-body sagittal radiograph is also useful to evaluate compensatory mechanisms.
Adult degenerative scoliosis (ADS) is fundamentally defined as an abnormal curvature of the spine with a coronal deviation greater than 10°. However, characterizing scoliosis is far more difficult. Although some studies have reported characteristic parameters, there has been no large-scaled studies or comprehensive description of ADS, which is essential to understanding and treatment of ADS. 2 A number of studies in recent years have looked at asymptomatic patient spines in relation to age to develop baseline parameters. 17 , 18 , 19 Among the most important radiographic parameters of interest in degenerative scoliosis include pelvic incidence (PI), lumbar lordosis (LL), pelvic incidence minus lumbar lordosis (PI-LL), pelvic tilt (PT), sacral slope (SS), sagittal vertical axis (SVA) (Fig. 16‑1), coronal Cobb angle, number of segments involved, apex rotation, and thoracic kyphosis (TK) (Fig. 16‑2). Among all radiographic measurements associated with degenerative scoliosis, the most concerning characteristics are global imbalance in the sagittal and coronal planes, measured with lateral and AP/PA films respectively.
16.2 Biomechanical Considerations
Sagittal malalignment plays a role in most, if not all spinal pathologies. Depending on age, the spine will also have a differing range of what is considered normal, making the benefit of analyzing sagittal alignment not strictly limited to patients with deformity. 18 The literature clearly shows that sagittal alignment is also the radiographic parameter most highly correlated with adverse health status outcomes and quality of life. 6 , 18 , 20 , 21
Sagittal alignment is determined by measuring the distance between a vertical line through the geometric center of C7, also known as the plumb line (C7PL) and the dorsal/rostral corner of the sacrum. 22 This difference, known as SVA, changes significantly with age and is also affected by the patient’s positioning and pelvic rotation (Fig. 16‑1). An increase in the SVA is associated with increased pain and disability. 23 The SVA does not capture the whole picture of sagittal alignment however. Research has shown that the pelvis is a key regulator for sagittal alignment, and its associated parameters (pelvic tilt and pelvic incidence minus lumbar lordosis specifically) also show a strong correlation between pain and disability. 24
As a patient develops sagittal plane malalignment, their bodies recruit mechanisms to compensate. This tends to start in the flexible part of the altered levels and eventually progresses distally to affect the hip and lower extremities. 25 One of the main drivers of sagittal alignment deterioration is the loss of LL as a result of spinal degeneration. 26 To counter their forward translation of their center of mass they straighten the thoracic region, causing muscle exertion. 27 This is usually followed by posterior translation and retroversion of the pelvis, along with flexion of the knees. 25 These compensatory mechanisms are important to evaluate, as they can mask an abnormal SVA if they are not taken into account.
The PT and PI-LL should also be taken into account for a comprehensive analysis on sagittal alignment (Fig. 16‑2). 18 A high PT reflects a compensatory mechanism that can reduce the apparent global sagittal alignment. 24 Patients with greater PT often need larger corrections (osteotomies) to reduce the risk of postoperative failures. 23 PT assesses the degree of retroversion by measuring the angle between the line from midpoint on the sacral end plate to the axis of the femoral head and the vertical axis through the femoral head.
Measuring the PI-LL is important for surgical planning for patients with a small LL relative to their PI to emphasize proper postoperative LL alignment. 23 The LL is the measurement of the sagittal Cobb angle between the superior endplate of S1 and the superior endplate of L1. PI is the angle between the midpoint perpendicular axis of the sagittal end plate and a line from the midpoint of the sagittal end point to the axis of the femoral head. Diebo et al has shown that PT, PI-LL, and SVA all normally increase in the aging spine and the alignment goals should be individualized to the patient. 18 Although not commonly practiced, a full body radiograph is better than a full spinal image to visualize a patient’s alignment and all compensatory mechanisms utilized to maintain an erect posture. 18
In the aging population with spinal deformity, surgery aims to correct sagittal parameters (Fig. 16‑3) more than coronal parameters to relieve associated pain and disability. Coronal curves (Fig. 16‑4a, b) and imbalance can also cause back pain and impaired functioning but is more commonly associated with an unsatisfactory appearance. 20 Biomechanical considerations must include both sagittal and coronal parameters during surgical correction to achieve the highest possible HRQOL.
16.2.1 Classifying Scoliosis
Classification systems are useful tools that allow effective comparison and communication and provide a framework for evidence-based approaches to better understand management and prognosis. 22 In regard to spinal deformity, there are a number of systems with differing degrees of complexity. Simpler classification systems tend to be of more use in clinical practice but lack many of the deformity’s characteristics, and vice versa for more complicated systems.
The most commonly used system is the SRS-Schwab classification system that was designed based on clinical relevance, 23 as the curve types and sagittal modifiers in this system were found to be closely related with quality of life in the initial study. 23 Other classification systems for adult spinal deformity include the Aebi 28 and the Berjano and Lamartina 29 systems. The classification designed by Aebi et al grouped deformity based on etiology, which proved simple to use, but surgical planning and treatment decisions had some difficulty using this system. 23 Berjano and Lamartina’s classification system was designed to assist surgeons with operative planning. 29 The rationale behind this system was to identify selected fusions as to avoid full curve fusion to reduce surgical risk while maintaining a larger range of motion and reducing the risk of junctional disease and decompensation. With the addition of a more recent system, the Comprehensive Spinal Osteotomy classification allows easy communication regarding the varying degrees of bony resections performed in deformity surgery. 30
16.3 Treatment Options
16.3.1 Nonoperative Management
DS is a progressive disease that takes years before it significantly impacts a patient’s quality of life. Nonoperative treatment is the mainstay of treatment to obviate the risks of spinal deformity surgery. The primary goals of both nonsurgical and surgical treatment are the same: to relieve pain, improve function, halt curve progression, and perhaps to improve cosmetic appearance. 22 Even patients with relatively limited disability based on HRQOL measures may benefit from a focused nonoperative treatment regimen. 31
In the case of DS, there is a lack of evidence-based research specific for nonoperative management. In 2010, Glassman et al studied HRQOL outcomes in ADS with the following modalities: physical therapy, exercise, injections, chiropractic care, and pain management. This study showed no significant benefit based on change in HRQOL measures over a 2-year observation period, but it was unclear if the disease progression slowed as a result of treatment. 31 Patients with ADS, regardless of the quantity and severity of their symptoms, use a significant amount of resources aside from surgery to try and manage their disease. 31 , 32
16.3.2 Shared Decision-Making
Most degenerative spinal conditions are managed by a single surgeon. 33 In spinal deformity cases, there is a growing trend to utilize multidisciplinary spine conferences to identify patients most likely to benefit from surgical treatment. Shared decision-making with physiatrists, anesthesia pain specialists, and other nonsurgical providers has demonstrated benefits in the literature while decreasing the amount of unnecessary surgeries. 33 Even a second opinion from another spinal surgeon has shown a decrease in surgical rates. 34 A study by Yanamadala et al showed that when 100 degenerative spinal patients were referred to a multidisciplinary spine center for surgery, 58 of them were instead recommended nonoperative management. 33 Nonoperative interventions commonly employed include physical therapy, epidural steroid injections, weight loss, smoking cessation, and spinal cord stimulation. The long-term benefits of these nonoperative treatments, however, remains debatable.
16.3.3 When is Surgery Appropriate
Surgical management of spinal deformity requires diligent planning and careful patient selection. Spinal deformity surgery is often associated with prolonged recovery times, higher complication rates, and significant cost.
The most common indication for surgery in ADS is radiculopathy, neurogenic claudication, or myelopathy refractory to nonoperative procedures and progressive deficit. 7 , 8 Radiating pain in ADS is mostly due to foraminal stenosis. Back pain, although common in ADS is rarely an indication for surgery alone, 7 as is the case with progression of scoliosis without any other symptoms. 8 That being said, following surgery, resolution of back pain is 6 times, and leg pain 3 times, more likely to improve compared to nonoperative treatment. 35 Scheer et al also found that patients with sagittal malalignment and more severe symptoms showed a better improvement in pain and Oswestry Disability Index (ODI) scores postoperatively. 10 , 35 , 36 Other surgical indications include compression of neural structures causing bowel/bladder dysfunction or gait disturbances, instability, flat back deformities, iatrogenic deformities, or pain from progressive degenerative deformity.
Radiographic parameters are useful in characterizing the severity of the deformity but do not always predict resolution of clinical symptoms once corrected. Measurements that favor surgical correction include a pelvic tilt > 20°, a sagittal vertical axis > 5cm, and coronal imbalance > 4cm. 37
A study in 2016 by Chen et al sought to answer the question of the appropriateness of surgical intervention. 38 They conducted a systematically relevant literature review and ran a panel of experts through 260 clinical scenarios. Their results, although subjective, found that surgery is generally appropriate when patients with degenerative scoliosis have at least moderate symptoms and a progressive or larger deformity, sagittal plane imbalance, or moderate stenosis. 38 They also concluded that surgery was generally inappropriate for mild symptoms and smaller deformities, without moderate stenosis or sagittal imbalance, particularly in patients with multiple comorbidities and advanced age. 38
There is no clear consensus on when surgery is appropriate for ADS, given its complexity. A decision must be made between the surgeon and patient regarding the symptom severities, degree of stenosis, degree of deformity, risk factors, and any other additional factors to decide whether the benefits of surgery outweigh the risks. Discussion of complex deformities with a multidisciplinary conference may help to identify patients best suited to surgical intervention.
16.3.4 Mental Well-Being Assessments
Aside from assessing pain, physical symptoms, and the degree of deformity, it is also necessary to evaluate a patient’s overall well-being. In the aging population, this becomes critical, as degenerative scoliosis and sagittal malalignment can negatively impact the patient’s quality of life. This assessment can be accomplished with different methods to calculate an HRQOL. To measure someone’s HRQOL, various tools such as Scoliosis Research Society-22 Patient Questionnaire (SRS-22) 39 instrument, ODI, 40 and the Medical Outcomes Form Short Form–12 (SF-12) are used to better understand someone’s physical, mental, emotional, and social functioning in relation to their quality of life. Improvements in disability scoring systems are a major priority both in the clinical setting and in research to further improve how degenerative scoliosis is managed in the future. Additional work has been done to better understand the psychosocial effects of spinal deformity, and pretreating patients with counseling and therapy may help patients prior to surgery.
16.3.5 Surgical Strategy
Decision-making with spine deformity is often challenging, particularly with relation to curve correction and level selection. Research has shown that alignment goals need to be age specific due to degenerative changes in the aging. 41 It was once believed that the best practice involved overcorrection to mitigate the loss of correction that occurs in the aging, as alignment deterioration is more common. 42 However, this practice is likely harmful, as less strict alignment goals have been shown to reach population HRQOL. 41 Overcorrection can also lead to complications such as proximal junctional kyphosis. 41 When aging patients receive appropriate correction of LL, they also have modest changes in proximal thoracic kyphosis and improvements in PT and SVA. 43 With age-adjusted alignment goals, patients may have better clinical outcomes and quality of life. 44 With this in mind, there are essentially three broad strategies to consider for treating degenerative scoliosis patients: limited decompression, focal stabilization and deformity correction.
16.3.6 Limited Decompression
For a subset of patients who present with predominantly radicular symptoms from isolated foraminal stenosis, Minimally invasive surgery (MIS) approaches have offered potential options to reduce morbidity in patients with smaller spinal deformities. 45 MIS offers less soft-tissue dissection and easier recovery than open posterior approaches.
Aging patients with degenerative scoliosis may benefit from a minimally invasive laminectomy with or without a foraminotomy. This procedure may allow for symptomatic relief of radicular symptoms while avoiding the risks of a large open fusion. 45 These patients must also be carefully selected, as back pain and sagittal imbalance remain largely unchanged. A potential side effect from this procedure is recurrent radiculopathy and mechanical instability, requiring fusion with posterior multilevel pedicle screw placement.
16.3.7 Focal Stabilization
Focal fusion is often the treatment of choice for single- or two-level degenerative disease. In these scenarios, patients have stenosis with a spondylolisthesis or degenerative disc disease. For patients with mild or moderate deformity, a short fusion construct may be reasonable to restore alignment. For example, a patient with lumbar flat back with an L4-L5 spondylolisthesis may be a candidate for an anterior lumbar interbody fusion of L4-L5 and L5-S1 followed by a posterior fusion L4-S1. Under this patient scenario, degenerative pathology is addressed, while restoring lordosis from L4-S1. Focal fusions can also be addressed with minimally invasive techniques.
Lateral interbody fusion (LIF) is a transpsoas approach that gains access to the anterior column of the spine without disrupting paraspinal musculature, reducing morbidity and postoperative pain. By avoiding the posterior tension band in this approach, it is also hypothesized to protect against adjacent segment disease (ASD). 46 , 47 This approach allows better access to the disc space than posterior approaches, allowing for large cage placement and improved fusion rates. With a resulting increase in disc height, this causes indirect neural decompression. It also negates a traditional anterior approach, reducing the risk to visceral organs. As a consequence, the approach has the potential to injure nerves in the lumbar plexus from the approach.
A meta-analysis by Dangelmajer et al in 2014 found no significant difference in complication rates of MIS vs. open approaches. 45 They did find a significant difference in the age of patients undergoing MIS vs. open surgeries, with older patients being better candidates for MIS due to their poor bone quality and comorbidities. With aging patients who have degenerative scoliosis, MIS has been shown to correct sagittal and predominantly coronal deformities, with the LIF showing the greatest change. 45
16.3.8 Deformity Correction
Correcting deformity is almost exclusively done through a large posterior approach. With access to the vertebrae of interest, whose curves often include a large portion of the lumbar and thoracic spine, a large fusion construct can be built with appropriate osteotomies to allow proper manipulation.
Depending on the type, 30 an osteotomy (Fig. 16‑5) provides a certain amount of curvature correction to help balance the deformed spine. The first two types of osteotomies involve the facet joint. There are six Grades (1-6) that correlate with the degree of bone resection. The first is a partial facet excision, with removal of the inferior facet (Grade 1), and the second type involves both inferior and superior facet excision, along with removal of ligamentum flavum and possibly other posterior elements (Grade 2). Both types of facet osteotomies require a mobile anterior column and provide limited deformity correction. 30 A pedicle subtraction (Grade 3), also known as a closing wedge, is a resection of a wedge portion of the posterior vertebral body, including the pedicles, creating a hinge at the anterior column while sparing the disc. For a larger resection, grade 4 involves a wedge through the vertebral body that the endplate and a portion of one adjacent disc are removed, along with posterior elements with pedicles. With this type of resection, an anterior cage may be placed, particularly in cases with marked shortening.
In certain circumstances, severe inflexible deformities can be managed with a vertebral column resection (VCR). VCR (osteotomy grade 5 and 6) 30 is a powerful operative method, defined as an osteotomy involving the vertebral body, intervening discs, pedicles, and all of the dorsal elements to create a segmental defect requiring provisional instrumentation. 48 , 49 Once dorsal instrumentation is placed, the spine may be manipulated to achieve balance in both the sagittal and coronal planes while shortening the length of the spinal column if necessary. 22 An interbody graft may also be used to augment the anterior column and prevent unwanted shortening of the spine. This technically-difficult procedure has a high associated complication rate, extensive operative times, and significant blood loss. 48 , 50
16.4 Benefits and Risks
Spinal deformity surgery is a potentially life-changing operation. The surgery usually provides more benefit than risk, but it is essential to understand both. Patients who fail conservative therapy, especially those with progressive decline in neurological functioning, intractable pain, significant disability, or deformity can have significant improvements in their quality of life after surgery. 51 , 52 , 53 , 54 Radiographic parameters that are most associated with disability and thus benefit after correction is the global sagittal alignment (SVA) and the spinopelvic parameters PT and PI-LL. 6 , 53 , 55 Studies continue to show a strong correlation between HRQOL and correction of previously mentioned radiographic parameters, particularly the SVA. 56 Yoshida et al found improvement in every category of the HRQOL questionnaires besides lifting and personal care at 1 year post-spinal deformity surgery, although these did improve at the 2-year postsurgery assessment. 56 Looking specifically at degenerative scoliosis, quality of life improved in all domains at 2-years post-surgery.
Determining the causes and rates of morbidity and mortality associated with each surgery is valuable for surgical planning, patient counseling, and efforts to improve patient care. 57 This is increasingly important in aging patients contemplating spinal deformity surgery.
Mortality in spinal surgery is rare, but the risk increases with a patient’s age. In a study by Smith et al looking at the Scoliosis Research Society (SRS) Morbidity and Mortality Database, the authors found a mortality rate of one per 1,000 < 60-year-old patients, with an exponential increase in the aging. In patients older than 90, mortality is 34.4 per 1,000. 57 The majority of deaths in the aging degenerative scoliosis population came from respiratory/pulmonary complications (28%), cardiac complications (22%), sepsis (16%), and stroke (9%). 57
Morbidity in the aging spinal deformity population is common, with rates ranging from 11 to 35% in large-population database studies and showing an increase with age. 58 , 59 A meta-analysis in 2015 reported a complication rate of 55% in adult deformity surgery, although age and etiology was not specified. 52 In Sansur’s study, the complications were broken down, with the majority of them being wound infections (37%), followed by dural tears (29%), neurologic complications (16%), and implant complications (13%). 59 Compared to the younger population, aging patients typically suffer from more postoperative complications due to poorer general health. With age, the spine has increased spondylosis in conjunction with poor bone quality, which reduces the likelihood of a successful fusion without major instrumentation problems. 52 , 60
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