Introduction

Both adult scoliosis and adolescent idiopathic scoliosis are relatively common conditions cared for by the spinal surgeon. Patients who have large, progressive curves often require large spinal fusions that include parts of the thoracic spine. Instrumentation of the thoracic spine can often be more challenging than instrumentation of the lumbar spine due to the difficulty with visualization, the smaller pedicle size, and the proximity to vital structures. However, improvements in pedicle screw designs have allowed for safer and biomechanically stronger correction of thoracic scoliotic curves. Pedicle screw constructs have recently become the most popular means of fixation, supplanting hook and wire constructs as well as anterior approaches.

Epidemiology and Natural History

Scoliosis can be subdivided into congenital scoliosis, adolescent idiopathic scoliosis (AIS), and adult scoliosis. Congenital scoliosis is a rare condition caused by errors in formation or segmentation of the vertebral elements during development. Progression is based on type of congenital scoliosis [1]. AIS is a diagnosis of exclusion and is identified in 1–3% of children aged 10–16 years [2]. Rate of curve progression is associated with time of diagnosis, magnitude of the curve, and location of the curve apex. The majority of cases are treated with observation and bracing. However, large curves greater than 50° are associated with pulmonary compromise and curves greater than 40° can be associated with body image issues and psychological disturbances. As a result, large curves often require spinal fusion. Finally, adult scoliosis , defined as a coronal Cobb angle >10° on the coronal plane in a skeletally mature patient, can be secondary to untreated adolescent idiopathic scoliosis (AdIS) or can be a de novo adult degenerative scoliosis (ADS) that occurs later in life [3]. It is estimated that the prevalence of scoliosis in patients older than 50 years is between 1.4% and 9%, affecting approximately 500,000 Americans [4]. Patients often present with back pain secondary to muscle fatigue, trunk imbalance, degenerative disc disease, and facet arthropathy. If left untreated, AdIS thoracic curves of greater than 50° will tend to progress by approximately one degree per year while curves less than 30° tend to be stable [4]. ADS curves are thought to develop secondary to loss of intervertebral disc height, leading to facet arthropathy. In conjunction with weakened paraspinal musculature in the elderly population, this leads to axial rotation of the spinal column and stretching of the surrounding ligaments causing laterolisthesis of the vertebral bodies. Larger curves with increased laterolisthesis have the highest rates of progression. [3]

Assessment

The assessment of scoliosis starts with a thorough history and physical exam. The history should focus on location of symptoms, rate of progression of the deformity, and any neurologic or cardiopulmonary symptoms associated with the condition. Other important factors to consider include medical comorbidities, psychosocial comorbidities, and smoking status.

A complete head to toe physical exam should be performed in the scoliotic patient, paying special attention to the neurologic portion. The scoliotic curve should be assessed in the standing and bending position.

Imaging and Classification

The most commonly utilized imaging modality in the assessment and management of scoliosis is plain radiography, typically standing full-length 36-inch cassettes. Bending films are useful in determining the flexibility of the curves. Sequential AP and lateral full-length films are utilized to monitor the progression of the curve and the response to nonoperative interventions. Prior to surgery, CT scans are typically obtained to help with surgical planning and three-dimensional imaging of multiplanar curves. MRI is useful to obtain in cases where there are neurological deficits, neurologic symptoms, or rapidly progressive scoliosis. In cases of large curves exceeding 60° or patients with any cardiopulmonary complaints, pulmonary function tests should be obtained. The most commonly utilized classification system for the selection of fusion levels is the Lenke classification which utilizes the curve(s) location and mobility of the curve on bending films to determine fusion levels [5].

Treatment

Operative Treatments

Non-pedicle Screw Constructs

The anterior approach to the thoracic spine has seen a substantial decline over the last two decades, once accounting for greater than 25% of all instrumented fusions, now seen in less than 5% [11]. Similarly, the use of hook constructs and other instrumentation has seen a rapid decline over the same time period. These approaches and methods have been overtaken by all posterior, all-pedicle-screw constructs [12]. These devices have proven to be safe, have greater biomechanical advantage for curve correction, require fewer fused levels, and have less morbidity. The one major disadvantage of these constructs is cost of implants, though this is expected to decrease with time and is clearly offset by significant advantage [13].

Hook constructs are not used in common practice currently. However, there are instances where hook or hybrid hook-screw constructs may be used or needed as an additional measure. Hooks can be placed in multiple areas of the spine. Pedicle hooks are placed by first removing the inferior articulating process (IAP) of the facet to expose the superior articulating process (SAP). The pedicle hook (with a central notch) can then be engaged directly on the pedicle and buttressed inferiorly by the SAP. A sublaminar hook is placed within the substance of the ligamentum flavum itself. It may require some exposure, either by removal of a portion of the cranial lamina or of the caudal facet. Transverse process (TP) hooks require a wider exposure and cannot be used in isolation; the TP is prone to fracture. The goal of using hooks is to minimize exposure of the dura; however, there is some degree of impingement when placing sublaminar hooks [14]. Hooks can be safely placed beneath the musculature of the spine using a medial or paramedian approach with limited dissection compared to pedicle screws where the facet is typically exposed entirely. Hook constructs are not engaged to the bone outside of the distraction or compressive forces they are creating. They are susceptible to dislodgement as the thoracic curve is corrected and the force profile changes [15]. Multiple hooks can be placed in opposing directions to create a “claw,” creating a compressive force between those levels on one side of the spine – this can correct deformity , but also reduces the likelihood of hook dislodgement. Correction using hooks occurs when the hooks are attached to pre-contoured rods which are rotated into place to produce the final correction – correction with rods is discussed in depth in the pedicle screw construct section. In addition, hooks can be fixed to their rod in a compression or distraction mode depending on orientation to achieve correction [14].

Hybrid constructs utilize a combination of hooks and pedicle screws for deformity correction. A particular construct described by Mousny et al. utilizes a proximal cranial claw consisting of three hooks and caudal pedicle screws to anchor and produce sagittal and coronal deformity correction [13, 16]. Numerous other hybrid constructs have been described and are beyond the scope of this text. Relatively newer technologies have replaced other historical constructs like wires with fiber bands which provide fixation without the same risk of catastrophic failure [17]. However, many of these techniques have failed to become popular against the availability, versatility, mechanics, and familiarity of screw constructs [13].

Pedicle Screw Constructs

Biomechanics of Pedicle Screw Constructs

There are two major mechanical advantages to pedicle screw constructs compared to historical treatments. Relative to other constructs, pedicle screws provide three-column stabilization which provides their main mechanical advantage. Second, screws have a significantly greater pullout strength compared to hooks and wires. Of note, in adolescent patients the pedicle screw size can actually be larger than the pedicle itself due to the plastic quality of their bone, increasing stiffness and pullout strength [18].

Safety of Pedicle Screw Constructs

Despite findings of pedicle wall violation on postoperative CT scan , these constructs are safe. Cortical breach has been reported as high as 43%, whereas neurological injury is reported as high as 1.2% [13]. There is a proposed 4 mm “safe zone” for breach in this region composed of 2 mm of epidural and 2 mm of subarachnoid space medial to the pedicle wall [19]. Subsequent cadaveric studies have challenged this finding. Regardless, accurate placement in the thoracic region is paramount with an established criteria of less than 2 mm of breach, but has been shown to be reliably found even in cases of extreme deformity [20]. In cases of congenital deformity in pediatric patients, placement of thoracic pedicle screws presents an additional challenge not only of curve correction but anomalous anatomy of the thoracic pedicles – additional care must be taken in these situations [1].

Pedicle Screw Technique

Selecting levels for fusion is the first and most important step. Goals of the procedure are to correct alignment (in multiple planes), correct deformity , and preserve motion. There are a number of algorithms available for selection of curves, but no single algorithm has been adopted. In general, instrumentation and fusion of the major, structural curve alone is typically enough. Compensatory curves will spontaneously resolve in up to 70% of cases. In some cases, however, there may be two major, structural curves – failure of fixation may be due to insufficient fusion of double major curves [21]. The King classification has been superseded by the Lenke classification as the main guide of fusion levels. The Lenke classification has six major types which are further subdivided into 42 types. This classification first identifies the primary and minor curves and then adds lumbar and thoracic modifiers – it helps to establish which curves require correction and the proximal and distal extent of the fusion required [5]. Of note, in patients with AIS, up to 10% will have an anomalous number of thoracic vertebrae, so special attention must be paid to counting at this step [22].

Once the levels of fusion have been determined and a preoperative plan has been developed, patients can be brought to the operating room for instrumentation and fusion. Various techniques for pedicle screw placement have been described . Kim et al. developed a method utilizing only anatomic landmarks relying on the superior articular facet, lamina, and transverse process to determine screw trajectory (see Fig. 10.1). A curved, blunt probe (gearshift) is used to find the pedicle and is first advanced with the curve facing lateral to prevent medial breach. At about 20 mm of depth, they recommend redirecting medially to obtain purchase within the vertebral body. Progressing distally within the thoracic spine, they recommended probing to 20–25, 25–30, and 30–35 mm in the upper, middle, and lower thoracic segments, respectively. In their original paper, they reported a 6.2% incidence of breech without neurological injury. Fluoroscopy can be used in addition to anatomic landmarks. Use of fluoroscopy requires a skilled technician as it necessitates frequent movement of the fluoroscope in multiple planes. Establishing distinct endplates and symmetric appearing pedicles requires adjustments in the mediolateral and cephalocaudad planes (see Fig. 10.2). Once a distinct image is achieved, landmarks can be easily identified and the probe advanced. This process must be repeated for each vertebral segment [23, 24]. Laminotomy for direct palpation of the medial pedicle wall has also been described, but is generally limited to revision surgeries. Lastly, use of new electronic probes in conjunction with electromyography has been described, but predominantly in lumbosacral instrumentation. This is a growing technique and requires further investigation for use in thoracic instrumentation. Once screws have been placed, they can be stimulated and electromyography recorded to confirm placement. A threshold of 11 mA has been established for the thoracic spine with a 97.5% negative predictive value of pedicle breach [25, 26].

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