Minimally Invasive Techniques for Thoracolumbar Deformity

Summary of Key Points

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The prevalence of adult spinal deformity is increasing due to an aging U.S. population.
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Open surgical correction has been shown to lead to significantly less pain, better quality of life, and greater satisfaction in adults, but it has reported complication rates of 20% to 80%.
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Literature on minimally invasive surgery (MIS) for adult spinal deformity correction is sparse but suggests there is an appropriate population to which it may be applicable.
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Emerging data show that the application of current MIS technology and techniques to adult spinal deformity correction is limited due to the inability to attain an adequate degree of correction to result in acceptable sagittal balance.
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MIS techniques are generally not sufficient for treatment of sagittal imbalances greater than 10 cm, curves with a Cobb angle greater than 90 degrees, rigid kyphotic deformities, deformities with fused spinal segments, and osteoporosis with T scores of less than −2.
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Only recently have data comparing MIS versus open surgery techniques to adult spinal deformity become available. Direct comparisons are sparse and often difficult to interpret due to inconsistent reporting.

Spinal deformity is a broad term that describes numerous heterogeneous pathologies affecting both the pediatric and adult populations. Spinal deformity is becoming more prevalent in the United States due to an increase in the aging population. It occurs in up to 60% of the older population. Many of these deformities occur in the thoracolumbar region. Thoracolumbar deformity in the adult is most commonly caused by asymmetric disc degeneration, osteoporosis, vertebral body compression fractures, and persisting childhood deformity. In the pediatric population, thoracolumbar deformity can be idiopathic, congenital, neuromuscular, neural (e.g., Chiari, syringomyelia), or syndromic in nature.

Spinal deformity is a dynamic process that involves coronal, sagittal, and rotational plane distortion. Deformity in the coronal plane is termed scoliosis. Sagittal plane deformity results in abnormal kyphosis or hyperlordosis. Axial plane alterations result from vertebral body rotation. Due to its complexity, a satisfactory comprehensive classification system for spinal deformity does not exist. However, two commonly used classification systems include Aebi’s and Schwab’s.

The Aebi classification consists of four categories based on the cause of the deformity. These include primary degenerative scoliosis (type 1) also termed “de novo” scoliosis caused by asymmetric disc and facet joint degeneration, progressive idiopathic adolescent scoliosis (type 2), degenerative scoliosis secondary to progression of an adjacent curve to one treated in adolescence or in the setting of an oblique pelvis or lumbosacral transitional anomaly (type 3a), and degenerative scoliosis secondary to metabolic bone disease such as osteoporosis combined with asymmetric arthritic disease or vertebral fractures (type 3b).

The Schwab classification establishes criteria determining the severity of a curve. It takes into account curve type and three sagittal modifiers ( Fig. 157-1 ). A curve type T describes patients with a thoracic major curve of greater than 30 degrees in the coronal plane. A curve type L describes patients with a thoracolumbar or lumbar curve of greater than 30 degrees in the coronal plane. A curve type D describes patients with a double curve (both thoracic and thoracolumbar/lumbar) with Cobb angles greater than 30 degrees. A curve type N describes patients with no major coronal deformity (all curves with angles less than 30 degrees). The three sagittal modifiers are pelvic incidence (PI) minus the lumbar lordosis (LL) modifier where this value is 0 if PI − LL is less than 10 degrees, + if between 10 and 20 degrees, and ++ if greater than 20 degrees. The second modifier is a global alignment modifier with a value of 0 if the sagittal vertical axis (SVA), or C7 plumbline, is less than 4 cm, + if the SVA is between 4 and 9.5 cm, and ++ if the SVA is greater than 9.5 cm. The third and final modifier is a pelvic tilt (PT) modifier with a value of 0 if PT < 20 degrees, + if PT is between 20 and 30, and ++ if PT > 30 degrees.

A thorough radiographic evaluation is necessary to fully categorize the thoracolumbar deformity as well as to determine an appropriate treatment plan. Standing 36-inch radiographs should be performed with the hips and knees fully extended to negate hip extension and knee flexion as compensatory mechanisms. It is important to determine if pelvic obliquity is present due to limb length inequality. If this is the case, the shorter limb should be equalized. Shoulder position should also be evaluated on coronal standing films to determine if any discrepancy is present. Supine radiographs can be performed with and without bolster placement to help determine the amount of change in LL possible with positioning on the operating room table. Similarly, standing dynamic films including side-bending and flexion-extension radiographs can help determine curve flexibility in the coronal and sagittal planes. A computed topography (CT) scan of the thoracolumbar spine, a CT-myelogram of the thoracolumbar spine, or magnetic resonance imaging (MRI) scans are the final imaging modalities needed to fully work up a thoracolumbar deformity.

Coronal Plane Deformity

Coronal plane deformity is termed scoliosis when the curve has a Cobb angle greater than 10 degrees ( Fig. 157-2 ). Curves may be structural and nonstructural. It is important to correctly determine which curves are structural and which are nonstructural, or compensatory, as this affects the levels of surgical inclusion. In a patient with a single curve, that curve is automatically deemed structural. In a patient with multiple curves, the curve with the largest Cobb angle is considered structural. Minor curves are also deemed structural if their Cobb angles are greater than or equal to 25 degrees on side bending films (i.e., the curve does not correct) and if there is focal kyphosis present in the portion of the deformity that is at least 20 degrees in curvature.

Lumbar scoliotic curves can be further described by the relationship of the lumbar curve to the center sacral vertical line (CSVL) on coronal imaging. The CSVL is a vertical line drawn superiorly from the midsacrum. The most rostral vertebral body (thoracic or upper lumbar) whose interpedicular distance is most closely bisected by this line is named the stable vertebra. This vertebra is important in planning the construct length necessary for curve correction. The relationship between a lumbar curve (if present) and the CSVL is also helpful in determining the extent of the lumbar deformity.

In severe scoliosis, coronal imbalance can occur. Coronal imbalance is defined as lateral displacement of the SVA more than 4 cm from the midsacral vertical line (see Fig. 157-2 ).

Spondylolisthesis

Patients with spondylolisthesis are classified with the Meyerding grading scale, which takes into account the amount of displacement of the upper vertebral body over the lower vertebral body on lateral imaging films. Grade I is translation of the upper vertebra of up to 25%. Grade II is translation of between 26% and 50%. Grade III is translation of between 51% and 75%. Grade IV is translation of between 76% and 99%. Grade V (100%) is complete spondyloptosis of the upper vertebral body ( Fig. 157-3 ). Spondylolisthesis is most commonly caused by degenerative disease but may also be due to spondylolysis (a defect in the pars interarticularis) causing isthmic spondylolisthesis.

Spondylolisthesis and scoliosis can be seen in association with each other in patients of all ages. There are three categories that describe this association: scoliosis completely unrelated to the spondylolisthesis, a neural arch defect or facet hypoplasia allowing asymmetric forward displacement and rotation of the vertebral body causing scoliosis (olisthetic type), and deformity resulting from muscle spasm caused by the neural compression of a spondylolisthesis (sciatic type). Curves due to the olisthetic or sciatic types should theoretically correct on side-bending films. Therefore, these films should be evaluated before deciding whether to treat both pathologies concurrently or just to treat the spondylolisthesis and evaluate for the autocorrection of the scoliotic deformity over time. In addition, spinopelvic parameters should be evaluated to assess spondylolisthesis prior to surgical intervention.

Sagittal Plane Deformity

Sagittal plane deformity is the result of abnormal lumbar spine kyphosis, decreased thoracic spine kyphosis, abnormal thoracic spine lordosis, or decreased lumbar spine lordosis. Ideal global spine alignment, and, most important, sagittal balance, will allow for minimal muscular energy expenditure upon standing. Sagittal plane balance is portrayed radiographically on standing scoliosis films by determining the position of the SVA with respect to the posterior edge of the sacral 1 (S1) end plate. This line is defined as the vertical line drawn inferiorly from the center of the C7 vertebral body ( Fig. 157-4 ). If it is less than 5 cm, the patient is deemed to be in sagittal balance. Sagittal imbalance is defined as anterior (or less commonly posterior) displacement of the SVA of greater than 5 cm. Positive sagittal balance results in increased muscular effort causing pain, fatigue, and disability. Therefore, it is of utmost importance to plan deformity correction that results in sagittal balance.

In addition to sagittal balance, restoring spinopelvic balance has been proposed to be almost as important for good outcomes in deformity correction. Restoring spinopelvic balance is thought to have a synergistic role with correcting sagittal imbalance in the optimization of postoperative patient satisfaction in adult deformity correction. Spinopelvic balance relates to the theory that thoracic spine kyphosis, lumbar spine lordosis, and the pelvis influence one another. Spinopelvic parameters include pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), thoracic kyphosis (TK), and LL ( Fig. 157-5 ). PI is the angle between a line perpendicular to the midpoint of the sacral end plate and a line connecting this point to the center of the femoral head. PI is the only parameter that is independent of posture and does not change postoperatively unless a sacral osteotomy is performed. PT is the angle between a line connecting the midpoint of the sacral end plate to the center of the femoral head and a vertical line at that point. PT is thought to reflect the pelvic compensation for the presence of spinal deformity. SS is the angle between a line parallel to the sacral end plate and a horizontal reference line. PT and SS, unlike PI, are dynamic and measure pelvic version. They help maintain an upright posture when patients have sagittal malalignment. PI is equal to the sum of PT and SS. The average values of PI, PT, and SS are 52, 15, and 30, respectively. For a more in-depth discussion and portrayal of spinopelvic balance and parameters, see Chapter 156 .

These parameters may be applied to preoperative planning for thoracolumbar deformity correction. Postoperatively, LL should ideally equal PI +/− 9 degrees. A SVA of less than 5 cm and a PT of less than 20 degrees should be achieved, as failure to do so is correlated with poor outcomes. Pelvic incidence can also be used to predict the optimal TK/LL combination based on a patient’s age group.

Presentation

Patients with thoracolumbar deformity can present with a constellation of signs and symptoms including back pain, radicular pain, claudication symptoms, curve progression, impaired pulmonary function, and concern regarding cosmesis. The pain is often multifactorial resulting from a combination of muscle fatigue, truncal imbalance, facet arthropathy, and degenerative disc disease. In severe cases, patients may also complain of the inability to stand erect or to maintain horizontal gaze. In adults, deformity can be further complicated by the presence of other age-related conditions such as osteopenia/osteoporosis and medical comorbidities. Thus, it is one of the most challenging spinal disorders to treat. Even though shown to lead to significantly less pain, a better quality of life, and greater satisfaction in adults than patients treated nonoperatively, open surgical correction of these deformities is an invasive endeavor with complication rates between 20% and 80%. Because of the high morbidity and mortality seen in these procedures, a trial of nonsurgical treatment in the absence of neurologic deficit or significant instability is generally recommended prior to consideration of surgical intervention despite a lack of consensus on what the most efficacious nonsurgical treatment methods are.

Nonsurgical Treatment

Nonsurgical treatment of thoracolumbar deformity predominantly involves symptom control. Pain medications including nonsteroidal anti-inflammatory drugs, muscle relaxants, tricyclic antidepressants, anticonvulsants, non-narcotic analgesics, and neuropathic pain agents can be used. The use of narcotic pain medication is not recommended. Physical therapy primarily focused on aerobic conditioning, core strengthening, and pelvic stabilization through extension-based exercise, stretching, and aqua therapy may be helpful. Heat, lumbar traction, and myofascial release has also been used. Hip or knee contractures should be treated. Although a mainstay of nonsurgical adult spinal deformity treatment, there is very weak and inconsistent evidence that supports significant long-term improvement in pain relief and function. Selective nerve root blocks, facet injections, trigger-point injections, and epidural injections can be useful for both evaluation and diagnosis of leg pain (e.g., radiculopathy, claudication). Injections are often used in conjunction with oral analgesics and can reduce the need for pain medication. Unfortunately, conservative management has a low rate of success in treating thoracolumbar deformities.

Bracing has similarly been shown to be ineffective in the long-term treatment of adult spinal deformity. Although the stabilizing effect of bracing may temporarily relieve pain, it does not significantly prevent curve progression. Additionally, when worn for extended periods, braces may result in muscle atrophy and deconditioning. Thus, despite the possibility of pain relief, bracing does not provide spinal deformity correction. Other alternative nonsurgical treatment methods including acupuncture, chiropractic care, yoga, and Pilates are reasonable to consider, but there is a paucity of literature proving true long-term benefit.

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