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Postoperative spinal deformity manifests as an inability of the spine to maintain an adequate loadbearing capacity or withstand dynamic forces, which may occur because of iatrogenic reasons intraoperatively (decompression, instrumentation), pseudarthrosis, or the inevitable phenomenon of rigid posterior spinal fusion (i.e., adjacent segment disease).
During spinal decompressions, prophylactic fusion should be performed after multilevel cervical laminectomy to decrease the risk of postlaminectomy kyphosis, whereas overresection of the lateral pars interarticularis and true pars in the lumbar spine should be avoided to prevent iatrogenic pars fractures and spinal instability.
To minimize the risk of creating an iatrogenic flatback after short posterior lumbar spinal fusions, a patient’s legs should be extended while positioning them to maximize lumbar lordosis, and straight rods should be avoided between L4 and S1.
For long posterior spinal fusions, distal fixation should include sacral and iliac fixation with unilateral or bilateral iliac screws. S2 alar-iliac screws are alternatives to iliac fixation and are ideal in revision surgeries for lumbosacral pseudarthrosis or instability.
Appreciation of lordotic shape variation according to pelvic incidence, as described by Roussouly, is important as it emphasizes not only angular measurements, but also lordosis curve organization and distribution of lumbar lordosis and thoracic kyphosis.
Acute complications at the junction of long posterior thoracolumbar constructs are commonly a result of a fracture at the proximal vertebrae, instrumentation pullout, or failure of the soft-tissue posterior ligamentous structures. The incidence of these may be minimized by augmenting the posterior ligamentous structures or the anterior column’s loadbearing capacity (i.e., vertebral augmentation of the last instrumented vertebra and the vertebra one level proximal to the last instrumented vertebra).
Surgical strategies to address postsurgical spinal deformity are dictated by patient age, medical comorbidities, degree of debilitation, and the unique characteristics of each deformity.
Preoperative evaluation for a patient with a postoperative spinal deformity should include a diagnostic assessment of metabolic bone disease (dual-energy x-ray absorptiometry scan, vitamin D, calcium, and parathyroid hormone levels), full-length, standing lateral and anteroposterior spinal radiographs, and advanced imaging (i.e., computed tomography, magnetic resonance imaging) as indicated.
The ideal radiographic goals of deformity correction are to bring the pelvic incidence (a fixed value) and lumbar lordosis (a dynamic value) to within 10 degrees of one another, to improve pelvic tilt to less than 20 degrees, and to achieve a less than 4 cm coronal and sagittal imbalance.
A thoughtful and integrated evaluation of spinal mobility, the deformity’s apex, previous fusion/instrumentation, and neurological status will help shape the surgical plan for deformity correction, which may involve multilevel or three-column osteotomies.
Spinal stability and maintenance of horizontal gaze are important to humans. Stability is defined as the spine’s ability to maintain its pattern of displacement such that no neurological deficit, major deformity, or incapacitating pain occurs under physiological loads. Spinal stability is necessary for the transfer of forces between the upper and lower limbs, to prevent the mechanical deterioration that occurs as a result of local compensatory mechanisms, and to protect neural structures. It is maintained through a complex multiarticular system comprised of vertebral bodies, intervertebral discs, posterior osseous elements, facet joints, interspinous ligaments, supraspinous ligaments, and paravertebral muscles. Gradual loss of stability, or in rare cases acute mechanical failure, results from disruption or degeneration of one or more of these structures. Loss of stability may occur because of several etiologies, including congenital anomalies, biological pathologies (i.e., tumor, infection, inflammation), degeneration, or for iatrogenic reasons. This chapter focuses primarily on spinal deformities that occur postoperatively. The etiologies of these deformities are diverse and include flatback syndrome, junctional kyphosis, postlaminectomy deformities, adjacent-level degeneration, postoperative instability, and pseudarthrosis. Additionally, other factors such as advancing age compromise bony structures and soft tissue, making these problems particularly challenging for the spine surgeon. Although postsurgical spinal deformities may be a consequence of the unavoidable natural history of spinal fusion or decompression, the majority of these deformities may be prevented by a fundamental understanding of sagittal balance parameters and surgical techniques to preserve spinal stability intraoperatively and to ensure proper alignment. Thus surgical strategies to avoid destabilizing the spine intraoperatively, as well as prevent intraoperative malalignment, are also addressed in detail in this chapter.
Loss of spinal stability manifests as an inability of the spine to maintain an adequate loadbearing capacity. Malalignment of the spine is a common manifestation of spinal instability and the hallmark of spinal deformity. Neglected spinal malalignment often progresses to marked functional limitation. Severe sagittal imbalance may render individuals socially and functionally debilitated, as it compromises maintenance of horizontal gaze. Thus, performing activities of daily living, including driving, swallowing, speaking, and upkeep of personal hygiene may become more difficult with worsening kyphosis. Glassman and colleagues demonstrated that quality of life is negatively affected by mild sagittal imbalance and is significantly compromised as sagittal imbalance increases. Progressive kyphosis is better tolerated in the thoracic spine and poorly tolerated in the cervical and lumbar spine. Although coronal imbalance is better tolerated than sagittal imbalance, coronal malalignment greater than 5 cm is associated with worsening pain, increased disability, and worse function. In addition to pain and poor general health, postoperative spinal deformity may result in acute or progressive neurological injury. , For example, Watanabe and colleagues reported that the neurological status of 20% of patients who sustained an acute proximal junctional fracture deteriorated from Frankel E to Frankel B. After proximal extension of the instrumentation and revision arthrodesis, these patients recovered to Frankel D. Thus all attempts should be made intraoperatively to both avoid destabilizing the spine and avoid instrumenting and fusing the spine in a misaligned position.
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