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Management of Degenerative Scoliosis
Scoliosis refers to an abnormal curvature of the spine due to developmental, degenerative, idiopathic, or iatrogenic changes of subaxial biomechanics in the coronal, sagittal, and axial planes. Over much of this past century, the term scoliosis has generally referred to young patients with congenital or idiopathic curves that occurred during childhood or adolescence. A more comprehensive term, known as spinal deformity , likely encompasses the breadth of spinal curves that occur from congenital, idiopathic, degenerative, syndromic, neuromuscular, traumatic, and iatrogenic causes. In this chapter, the focus is adult degenerative spinal deformity. Although curve characteristics and patient complaints may be similar in some respects, management of adult scoliosis and goals of treatment often differ vastly from the approach to other etiologies of spinal deformity.
Terminology
The incidence of degenerative scoliosis has increased significantly in the past decade, and it is estimated that up to 30% of adults will be diagnosed with this condition in their lifetime. The increased incidence of this disease is due to the increased number of aging patients, along with increased incidence of obesity, osteoporosis, and number of spinal surgeries performed worldwide. Adult patients presenting with spinal deformity in the absence of previous surgery, trauma, or syndromic or neuromuscular causes are classically placed into one of two categories. The first main category of adult scoliosis involves patients who previously possessed a spinal deformity at a much younger age in the form of infantile, childhood, or adolescent idiopathic scoliosis. Such patients become symptomatic during adulthood likely from a progression of these curves with or without superimposed degenerative changes. The second major category of adult scoliosis involves patients without preexisting scoliosis who develop spinal deformity as a result of adult-onset degenerative changes (e.g., disc degeneration, facet joint arthropathy, osteoporosis, and vertebral body compression fractures). , This latter condition is classically referred to as de novo or adult degenerative scoliosis. Differentiating between these two is often determined based on history, symptoms, and imaging.
With regard to history, patients with preexisting curves might acknowledge the presence of their curve since adolescence or young adulthood, which may be corroborated by past photographs. In terms of presenting symptoms, patients with de novo scoliosis typically complain of low back pain, neurogenic claudication, and/or radiculopathy in keeping with most patients with degenerative spine pathology. They might also present with progressive deformity and spinal imbalance leading to severe axial back pain, but such symptoms occur more commonly in patients with preexisting scoliosis. Finally, with regard to imaging, patients with previous idiopathic curves often exhibit larger curves that span more spinal segments, often extending over the thoracic and lumbar regions. These curves may also be associated with significant rotational components. With de novo patients, curves more commonly center on the lumbar region, focally span fewer segments, involve listhesis or focal rotation of one vertebral body over another, and involve degeneration of the lumbosacral junction. Although these generalizations are associated with each type of adult scoliosis, such characteristics are not specific ( Table 170.1 ). For the remainder of the chapter, a general view of de novo scoliosis is discussed.
TABLE 170.1
Factors That Help Differentiate De Novo or Adult Degenerative Scoliosis From Preexisting Idiopathic Scoliosis Presenting in the Adult
| Factor | De Novo or Adult Degenerative | Previous Idiopathic Curve |
|---|---|---|
| History of previous curve | No | Yes |
| Age | 60s | 40s |
| Sex | Male > female | Female > male |
| Stenosis | Common | Uncommon |
| Curve magnitude | Smaller (15–50 degrees) | Larger (35–80 degrees) |
| Location | Lower lumbar | Thoracolumbar or lumbar |
| Length (in spinal segments) | <5 levels | >5 levels |
| L5–S1 involvement | Yes | No |
| Neurological dysfunction | 50%–90% | 7%–30% |
| Imbalance, coronal | No | Yes |
| Imbalance, sagittal | Yes | No |
Clinical Presentation
In the younger populations with scoliosis, progressive deformity and cosmetic concerns are often the reasons for clinical presentation, but in the adult population, pain is the most common reason for presentation (up to 90%). , This latter group of patients might complain of radicular pain due to neural compression, often on the side of the lumbar concavity. However, complaints of axial pain are common, believed to result from muscle fatigue and spasm over the convexity of the curve. Pulmonary compromise can occur, but only with severe thoracic curves that are more than 80 or 90 degrees in the sagittal or coronal plane. Current studies have shown that the location of thoracic kyphotic apex plays a role in determining which patient will experience pulmonary compromise as their curves progress. More commonly patients with thoracolumbar deformity complain of early satiety with eating because the rib cage can severely indent the abdomen, preventing gastric filling. When evaluating patients, it is important to timeline the velocity of curve progression to establish the etiology of the disease and recommend appropriate treatment. The natural history of degenerative curves is a relatively slow process and is caused by bony resorption and remodeling in the facet joints and degeneration in the intervertebral discs. Contrarily, patients presenting with rapid changes in a previously stable curve should be evaluated for more insidious neurologic condition such as a spinal tumor, enlarging syrinx, etc. It is also important to understand that, although the incidence of adult degenerative scoliosis increases with age, progression is not universal. Therefore it is reasonable to follow patients symptomatically before offering surgical intervention. Most degenerative curves progress at an average rate of 3 degrees per year; however, curves greater than 30 degrees, increased rotation at the apex of the curve, and lateral slippage tend to progress at faster rates.
Radiographic Evaluation
Plain Films
It is important that the work-up of a patient with spinal deformity include a 3-foot (36-inch) standing posteroanterior (PA) and lateral radiographs at the very minimum. On PA films, Cobb angles are used to measure curves in the coronal plane. First, curves should be measured so as to create the greatest curve angle, and the levels at the top and bottom of such curves should be noted. These curves are used as the foundation to define clinical curve progression. Second, the relationship of the center sacral line to C7 should be documented so as to determine the presence of coronal imbalance or trunk shift ( Fig. 170.1 ). This line bisects a line passing through both iliac crests and ascends perpendicularly. Emami and colleagues have defined coronal imbalance as greater than 25 mm of lateral offset of this plumb line. Finally, bending films, traction films, or push-prone films can be used to explore the flexibility of the curves. Such maneuvers can help to identify the primary structural curves of the scoliosis, because smaller curves can flatten out on such imaging. In addition, these images might yield information on the flexibility of such curves so that surgeons can quantify the ease or difficulty with which curves may be corrected by surgery.
On the lateral films, thoracic kyphosis, lumbar lordosis, and pelvic parameters (especially pelvic incidence) should be measured first. Second, the C7-plumb line and/or T1 tilt should be measured to help determine the patient’s overall sagittal balance ( Fig. 170.2 ). Such measurements are exceedingly important because multiple studies have shown that the quality of life in patients with adult degenerative spinal deformities is highly correlated with sagittal balance, both preoperatively and following spine surgery. , “Normal” sagittal balance is classically defined as a C7 plumb line that passes 2 to 4 cm posterior to the ventral S1 vertebra or 1 cm posterior to the L5–S1 disc space. , An offset of greater than 40 mm has been defined as sagittal imbalance. There is increased sagittal balance with age that is not necessarily pathologic, and must be kept in mind during patient evaluations. As with the coronal films, bending films can be taken from the lateral view. Such images are often referred to as extension films or supine films, and these can be taken with a bump under the patient’s kyphosis while lying supine. The goal is for the surgeon to estimate how much flexibility there is in the sagittal plane, once again to provide information preoperatively regarding the potential ease or difficulty the surgeon might face in the operating room during deformity-correction maneuvers.
Although all the radiographic parameters just outlined can be obtained for all patients with degenerative scoliosis, the imaging of most patients with adult deformity is focused on a lumbar rotational deformity involving a lumbar or thoracolumbar coronal curve that is hypolordotic in the sagittal plane. Such deformity is often associated with a relatively flexible thoracic compensatory curve typically less than 30 degrees. In addition, rotatory subluxation or lateral translation at L3–L4 and obliquity at L4–L5 are the most commonly affected areas. Although not necessarily obvious on plain films, it must be noted that many patients with long-standing deformities in the sagittal plane develop hip and knee flexion contractures. Such problems can limit the patient from obtaining a balanced posture (head over pelvis and over feet) even following an apparent surgical correction of spinal imbalance. In these patients, further evaluation of hip pathology may be required prior to attempting spine surgery.
An important consideration in the surgical correction of spinal sagittal deformity is pelvic parameters and measurements. The sacrum is intimately associated with the pelvic constituents, and thus abnormalities in one must affect the other. A landmark paper by Legaye et al. proposed a measurement classification system for understanding the changes in pelvic parameters associated with sagittal deformity. Pelvic incidence is a fixed angle that occurs naturally after skeletal maturation is complete. This angle does not change based on the position of the pelvis and is highly correlative with lumbar lordosis in normal individuals. The pelvic incidence is calculated by adding the angle of the pelvic tilt and the sacral slope ( Table 170.2 ). Both of these angles change as lumbar lordosis changes, to maintain the head directly over the pelvis. As lumbar lordosis decreases with degenerative or iatrogenic processes, the pelvic tilt increases (retroflexes) and the sacral slope decreases to decrease the PI-LL mismatch. To achieve this, the extensor muscles of the back must contract constantly to maintain this posture. This compensatory mechanism accounts for the axial pain associated with sagittal deformity. The measurement of pelvic incidence is useful clinically to determine how much lordotic correction is needed during surgical intervention. The pelvic incidence can be determined on plain lateral radiographs that include the pelvic components ( Fig. 170.3 ).
TABLE 170.2
Commonly Measured Pelvic Parameters
| Parameter Name | Pelvic Incidence (PI) | Sacral Slope (SS) | Pelvic Tilt (PT) |
|---|---|---|---|
| Derivation | SS + PT | Angle between sacral endplate and horizontal meridian | Angle between midpoint sacral endplate and vertical meridian |
| Normative values | 50–60 degrees | Variable | Variable |
Magnetic Resonance Imaging
In dealing with idiopathic scoliosis in children and adolescents, most deformity surgeons use magnetic resonance imaging (MRI) scans when they suspect an underlying neurologic pathology as the cause for the progressive deformity. Such consideration is clearly indicated with left-sided thoracic curves because most idiopathic curves are right sided. Such a reversal of the normal curve pattern can signify an intrinsic spinal cord tumor, syrinx, or nonspecific compression. In addition, when there is concern for a syndromic (e.g., Chiari malformation, neurofibromatosis) or neuromuscular cause, MRI images may be essential in accurately diagnosing pathology in the neuraxis that is creating or compounding the deformity. However, in patients with adult deformity, MRI scans are routinely used to identify causes of neurologic complaints or symptoms such as radiculopathy from foraminal compromise or myelopathy from canal stenosis. MRI allows for better characterization of soft tissue and disc degenerative changes than computed tomography (CT) or plain films but provides less information on bony anatomy and remodeling.
Bone Quality
Because of the high prevalence of osteoporosis in patients older than 50 years, poor bone quality can affect the surgical approach and instrumentation options. Although plain radiographs and CT imaging can help to predict bone quality, dual-energy x-ray absorptiometry (DEXA) is the “gold standard” for diagnosing osteoporosis. The T-score compares the patient’s bone density to the optimal peak bone density for gender, and it is reported as the number of standard deviations below the average. A T-score greater than −1.0 is considered normal, less than −1.0 is considered osteopenia, and a score of less than −2.5 is diagnostic of osteoporosis. The Z-score is used to compare results with other patients of the same age, weight, ethnicity, and gender. It is important to keep in mind secondary causes of osteoporosis such as thyroid and parathyroid dysfunction, malnutrition, etc. Patients diagnosed with osteoporosis by DEXA scan should have a thorough evaluation by a primary care physician or endocrinologist. Some surgeons suggest performing this study on all women older than 50 years and all men older than 60 years who are considering surgery. However, in addition to age, risk factors for osteoporosis include history of fracture as an adult or fracture in a first-degree relative, white or Asian ethnicity, smoking, low body weight, female sex, dementia, and poor health. ,
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