Adult spinal deformity (ASD) encompasses a number of pathologies with a wide variety of radiographic and clinical presentations. Furthermore, it carries an estimated prevalence of 60%–70% in the general population of adults age 60 years and older.1 Adult idiopathic scoliosis (AIS) is one such form of ASD and is characterized by the progression of spinal deformity that begins during adolescent years. As such, AIS typically features deformity in the coronal, sagittal, and axial planes, and often involves both the thoracic and lumbar spines. This is not to be confused with degenerative scoliosis, another common form of ASD, which occurs due to asymmetric aging of the intervertebral disks and facet joints. Although these 2 common subcategories of ASD hold a number of shared features, AIS, in particular, presents a unique challenge to surgeons and patients alike, as it features a prominent rotational deformity, and commonly involves both the thoracic and lumbar spines.
Widely utilized idiopathic scoliosis classification systems by Ponseti and Friedman,2 King et al,3 and Lenke et al4 have focused primarily on the character and degree of coronal deformity, which is generally indicative of the degree of pathology. However, AIS is undeniably a 3-dimensional process that also involves a deformity of the spinal column in the sagittal and axial planes. More recently popularized classification systems by Nash and Moe,5 and the Perking Union Medical College6 have emphasized the clinical importance of rotational (axial) deformity. Fewer studies and/or classification systems, however, have highlighted the role of sagittal balance in AIS. As novel surgical techniques and technologies have allowed for full or near-full correction of coronal and rotational alignment, attention has increasingly shifted toward the sagittal plane.7
A selective review of the literature was performed to identify studies that analyzed the role of spinal alignment, particularly sagittal alignment, in ASD surgery. Studies were included in this review if they included patients 18 years of age or older, with a minimum of 2 years of postsurgical follow-up. Studies pertaining to degenerative scoliosis were included in the review if the treatment principles were described as being applicable to both degenerative scoliosis and AIS. There was no limitation on the magnitude or flexibility of the patients’ deformities. The references of all pertinent studies were reviewed to identify potential additional papers of value to the reader. In addition, we reviewed a number of studies pertaining to the surgical techniques for the treatment of these patients, including the method of correction, the role of additional procedures such as osteotomies or facetectomies, the type of construct, and the type of materials utilized. Finally, we highlight unique strategies employed by our group and report a number of pertinent examples. There was no statistical analysis or meta-analysis performed, as it is beyond the scope of this review.
Sagittal alignment has long been suggested to carry considerable clinical importance for patients with ASD, including those with AIS.8–12 Studies have established this association through analysis of both global measures of sagittal alignments, such as sagittal vertical axis (SVA) or T1 spinopelvic inclination, and regional measures such as thoracic kyphosis (TK), lumbar lordosis (LL), or pelvic incidence (PI). In fact, Lafage et al11 analyzed the clinical value of multiple radiographic parameters in 125 patients with either AIS or degenerative scoliosis, finding that T1 spinopelvic inclination had the strongest correlation with health-related quality of life (HRQOL) measures, followed closely by SVA and pelvic tilt (PT). Glassman et al10 also studied the association between SVA and HRQOL measures in 352 patients with ASD, finding that an increasing SVA was directly associated with adverse HRQOL scores. In regards to more regional measures, Schwab et al8 performed a prospective radiographic analysis of 95 patients with ASD, reporting that the degree of thoracolumbar kyphosis was directly associated Visual Analogue Scale pain scores, while the degree of LL was inversely associated with Visual Analogue Scale pain scores. A later prospective radiographic analysis by Schwab et al9 of 947 patients with ASD similarly suggested that an increasing degree of LL was associated with both adverse Oswestry Disability Index (ODI) scores, and adverse Scoliosis Research Society pain and function scores.
Global Sagittal Balance Parameters
On the basis of evidence such as the studies listed above, in addition to their own clinical experience, Schwab et al13 offered radiographic parameters to guide surgical realignment in the sagittal plane for all ASD patients, including those with AIS. They proposed a goal for global sagittal realignment of an SVA measure of <5 cm, finding that achievement of this goal was associated with significantly better ODI scores. This classification system was subsequently validated by 9 independent senior-level spine surgeons.14 Similarly, Protopsaltis et al15 and the International Spine Study Group performed a prospective multicenter analysis of patients with ASD to determine the utility of different measures of global sagittal alignment, including the T1 pelvic angle. They found that categorizing patients by increasing the T1 pelvic angle resulted in a significant and progressive worsening in HRQOL scores. Furthermore, they proposed that a goal T1 pelvic angle of ≤20 degrees would result in a clinically significant reduction in disability. Obeid et al16 and the European Spine Study Group described an additional measure of global sagittal alignment known as global tilt (GT), which is the sum of the PT and C7 vertical tilt. They found that, although SVA and PT were significantly influenced by patient positioning, GT was consistent amongst a variety of positions and may, therefore, be the most reliable measure of global sagittal balance. Banno et al17 subsequently found that a GT of ≤33.7 degrees was associated with a clinically significant reduction in disability.
Regional Sagittal Balance Parameters Including TK
The Schwab et al’s13 study group referenced above also proposed regional sagittal balance parameters than surgeons should aim to achieve. These included a goal PT of <20 degrees and LL within 9 degrees of PI. Similar to their global sagittal goal, they found that achievement of these regional measures were also independently associated with significantly better ODI scores.
The degree of TK is a regional measure of sagittal alignment that is of critical importance for the successful treatment of patients with AIS. It has been theorized that, as the spinal column grows during the development of idiopathic scoliosis, the anterior portion grows most rapidly. This discrepancy in growth velocity may, therefore, contribute to hypokyphosis of the thoracic spine.18 Normative values for TK have been reported to range from 40 to 50 degrees, with the term hypokyphosis describing curves <25 degrees.19 When unaddressed, this thoracic hypokyphosis has been suggested to adversely influence pulmonary function,20 degree of concurrent LL,10 likelihood for cervical decompensation,21 patient perception of his/her deformity,22 and junctional alignment.23,24
Junctional alignment, specifically the propensity for proximal junctional kyphosis (PJK) and distal junctional kyphosis (DJK), has been linked to reoperation and increased costs and represents a growing area of research emphasis.25 In an analysis of 851 patients with idiopathic scoliosis, Lonner et al26 reported an overall PJK incidence of 7.05%, with incidence varying according to Lenke type (Lenke I, 6.35%; Lenke II and IV; 4.39%; Lenke III and VI, 11.64%; Lenke V, 8.49%). For Lenke II and IV patients, loss of kyphosis and more lordotic rod contour angles were risk factors for PJK. For Lenke III and VI patients, larger preoperative T5–T12 kyphosis was a risk factor for PJK. In regards to DJK, Lowe et al27 analyzed 375 patients with idiopathic scoliosis, reporting a DJK incidence of 7% following anterior fusion and 14.6% after posterior fusion. Risk factors identified for this DJK included surgical approach, residual kyphosis at T10–L2, and failure to include the junctional level in the instrumentation. Given the frequent involvement of the thoracic spine and hypokyphosis in patients with AIS, it is critical to properly address any thoracic deformity in the sagittal plane before adverse clinical effects are realized.
The primary indication for deformity surgery is a loss of function attributable to the curve, whether that is related to back pain, neurological symptoms, or curve progression.28 If symptoms persist despite extensive conservative management, surgical intervention is warranted. Because of altered anatomy related to the deformity, patients often present with a variety of neurological complaints. Smith et al29 suggested that, of patients presenting to a surgical clinic for adult scoliosis, ∼8% report weakness due to radiculopathy, 9% report claudication, 1% have myelopathy, and 3% may even report bladder or bowel dysfunction. Radicular symptoms more commonly occur on the concavity of the deformity, but symptoms on the convex side of the curve should heighten the surgeon’s willingness to recommend operative treatment. Surgical indications for these patients are the same as for the general population and typically serve as a component in the correction procedure.
As previously suggested, functional deficits are often attributed to the increased workload and fatigability associated with positive SVA. The increased energy expenditure required to maintain upright posture will lead to early fatigue and a progressive limitation in mobility. Coronal alignment and prognosis can be considered together. Although there is no firm guideline regarding Cobb angles and indications for surgery, a Cobb angle of <30 degrees is usually stable, while >50 degrees almost always progress.30,31 In the 30–50-degree range, clinical presentation, and close radiographic monitoring can help to determine the clinical path. Other factors predictive of curve progression include lateral listhesis of >6 mL, apical rotation >30 degrees, and disk degeneration at L5–S1.32 Regardless, curve progression should be documented on serial radiographs taken several months apart.
When discussing surgical timing for adults in middle age, the decision for the timing of surgery must be balanced between the current quality of life, work-related productivity, and personal commitments. As patients present later in their seventh and eighth decades of life, the balance shift to a discussion between the quality of life and the ability to tolerate a large deformity-correcting surgery. Extensive medical comorbidities may represent relative contraindications to surgery, such as diabetes mellitus, congestive heart failure, and chronic obstructive pulmonary disease. Other relative contraindications include psychiatric disease, osteoporosis (due to poor bone quality and difficulty with fixation), and poor social support.33 Many patients with AIS have multiple medical comorbidities and are at risk for decompensation perioperatively. Therefore, a thorough preoperative workup should be performed once a patient is deemed appropriate for surgical correction.
Even as the discussion shifts to operative intervention, there is still a significant challenge in determining the most beneficial procedure for the patient. A thorough discussion should be had between the surgeon and patient as to what the exact symptoms are, and what the postoperative expectations will be. Comorbidities and the ability to tolerate large surgery are also major considerations when discussing the extent of coronal and sagittal balance correction versus neurological decompression. A thorough radiographic workup should be completed before the decision to proceed to the surgery along with a comprehensive evaluation that includes patient presentation, alignment, prognosis, comorbidities, and home support system.
Once the surgery has been planned and the patient has been cleared by the medical personnel, routine setup should include neuromonitoring, specifics which are dependent on whether or not the patient is myelopathic, the levels involved, and the extent of the surgical procedure planned. In addition, blood conservation methods should be utilized due to the heavy blood losses associated with several of these procedures. Among these methods are the use of Cell Saver, hemostatic agents such as Gelfoam, electrocautery, and bipolar cautery. Tranexamic acid, which is used commonly in total joint surgery, is beginning to gain favor as a hemostatic agent with high expected blood loss.34 An open-frame Jackson table should be used routinely to prevent abdominal compression. This allows for lower venous pressure in the inferior vena cava which decreases overall venous bleeding throughout the case.33
Spinal Fusion Construct
Regarding surgical constructs, Harrington35 first introduced a means to reduce spinal curvature through distraction and compression with rods and screws. Subsequent advancements in design by Cotrel and Dubousset provided adequate fixation with the ability to improve TK.36 In recent years, pedicle screw constructs have gained favor over hook constructs for restoring TK (Fig. 1). Suk et al37 analyzed 51 patients with AIS and thoracic hypokyphosis treated with either a hook construct or pedicle screw construct. They found that patients treated with pedicle screws had roughly 13 degrees of improvement in thoracic hypokyphosis relative to those treated with a hook construct.
There are a number of materials that have been utilized for fusion constructs. Titanium alloy (Ti) rods have increasingly replaced stainless steel, with significant improvements in reducing imaging artefact and reducing the postoperative infection rate.38 However, the elasticity of Ti has encouraged researchers to consider other materials for achieving optimal and lasting correction of spinal deformity. Cobalt-chromium (CoCr) in particular has recently garnered interest, also carrying reduced artefact but mechanical properties comparable to stainless steel.39 Lamerain et al40 reported that CoCr rod constructs can be effective in restoring normal sagittal alignment in patients with idiopathic scoliosis, including those with thoracic hypokyphosis. In a comparative effort, Angelliaume et al41 analyzed 70 patients with Lenke I or II idiopathic scoliosis who underwent a posterior fusion with either Ti or CoCr rods, with a minimum of 2-year follow-up. They found that CoCr allowed for greater restoration of TK and no difference in complication rates, with these results remaining significant at final follow-up. Rod and screw density have also been suggested to play critical roles. Liu et al42 analyzed 77 patients with Lenke I idiopathic scoliosis who underwent a posterior spinal fusion with a high or low screw density, and high or low rod density (Ti 5.5 vs. 6.35 mm, respectively). They found that the high rod stiffness and high screw density on the concave side combination resulted in the greatest correction of TK. In regards to shape, traditional circular rods have long been considered the standard of care. However, Gehrchen et al43 recently compared the use of beam-shaped CoCr rods to circular CoCr rods for 129 patients with idiopathic scoliosis. Although there was no difference in the amount of or maintenance of sagittal correction, the beam-shaped group experienced a significantly greater correction in the coronal plane. Further research is needed before noncircular rods are widely adopted.
To determine factors predictive of the preoperative to postoperative change in thoracic hypokyphosis, Monazzam et al7 performed a prospective analysis of 280 patients with AIS and preoperative thoracic hypokyphosis (5–20 degrees) treated with posterior spinal fusion. They found that, among the degree of postoperative kyphosis, treating surgeon, rod material, implant density, and use of ≥1 Ponte osteotomies, only the treating surgeon was a significant predictor of restoring normal kyphosis. Surgeon, and therefore surgical technique, may, therefore, play the most critical role in achieving the correction of TK.
Overview of Surgical Approaches
The correction of spinal deformity for patients with AIS historically involved a combined posterior and anterior approach.44,45 This method is highly effective in altering and maintaining the desired alignment of the spinal column. Over the past several decades, the use of an isolated posterior approach and pedicle screw fixation has increased and has been shown to successfully address deformity in the coronal and axial planes with less morbidity.46 However, correction in the sagittal plane with this technique has proven to be more difficult. Schmidt et al,47 for example, compared anterior dual rod instrumentation with posterior pedicle screw fixation in 42 patients with AIS and thoracic hypokyphosis. They found that, although considerably more invasive, the anterior instrumentation method resulted in an additional 7 degrees of improvement in TK when compared with posterior pedicle screw placement. As the emphasis on patient safety and reducing the incidence of complications has grown, researchers have turned their focus to achieving correction in the sagittal plane while minimizing the morbidity associated with multiple surgical approaches.24
Most Commonly Practiced Techniques
Simultaneous Translation Over 2-Rod (ST2R)
Today, a majority of surgeons utilize pedicle screw fixation constructs and a single posterior approach. Clement and colleagues performed a retrospective review of 62 consecutive patients with idiopathic scoliosis treated by a single surgeon with posterior spinal fusion and an ST2R technique. In the ST2R method, 2 rods are bent to the desired sagittal alignment, with minimal to no bend in the coronal plane. These rods are placed in cranial and caudal anchorages and are oriented in the desired sagittal plane without any reduction maneuver. The proximal bolts are then tightened on the anchorages to lock the rotation of the construct. The remaining bolts are then simultaneously tightened bilaterally at the same level moving in a caudal direction, thereby resulting in the translation maneuver. For the 27 patients with preoperative hypokyphosis, the average kyphosis angle was improved from 9 to 29 degrees and maintained during follow-up.48 In a similar study focusing solely on 24 consecutive patients with thoracic hypokyphosis, Clement et al49 found that the ST2R technique resulted in the improvement of kyphosis from 9 to 30 degrees, again maintained during follow-up.
Simultaneous Double Rod Rotation
Another commonly used method is the simultaneous double rod rotation technique.50 Pedicle screws are first placed on both sides at each desired vertebra using polyaxial screw heads. The contoured rods are then placed within the polyaxial screw heads without any rod rotation in the sagittal plane. The rods are then sequentially rotated beginning on the concave side and followed by the convex side, first to 45 degrees, then 60 degrees, and finally 90 degrees. The screw heads are then tightened and locked. Sudo et al51 analyzed the effectiveness of this technique in 32 patients with AIS and hypokyphosis, finding that TK improved from an average of 11.9 degrees preoperatively to 20.5 degrees postoperatively. This method can also be used in situ, by which the rods are progressively bent at sequential levels to correct a deformity after they are secured within the screw heads. Charles et al52 analyzed 54 patients treated with this technique over an 8-year follow-up period, finding that this method may be particularly effective at achieving sagittal balance over desired short stretches of vertebral segments.
Rod contouring and bending are also key aspects of achieving the desired alignment. The degree of rod deformation that occurs in response to a rigid curve or during a reduction maneuver must be considered for accurate preoperative planning. Salmingo and colleagues analyzed the magnitude of rod deformation in the sagittal plane following surgery using computed tomography imaging in 20 patients with AIS. They utilized 6.0-mm Ti rods for both the concave and convex curves, with a double rod rotation technique. They found that, although rods on the convex side did not deform significantly, the angle of curvature for the concave rods decreased from 33.6 to 17.8 degrees in the sagittal plane.53 Over bending or use of a stiffer material, such as CoCr, should, therefore, be strongly considered for the concave aspect of the curve.
Osteotomies and/or Facetectomies
For particularly large or rigid curves, the addition of ≥1 osteotomies or facetectomies has been suggested to increased flexibility of the spinal column.54 A Ponte osteotomy, for example, consists of removal of the inferior aspect of the spinous process, interspinous ligament, ligamentum flavum, and the bilateral facet joints at a given level.55 In regards to restoring TK in patients with preoperative hypokyphosis, however, this benefit has remained unclear. Pizones et al56 analyzed 73 patients with Lenke I through IV curves, on the relationship between Ponte osteotomies with sublaminar wires and degree of deformity correction. Overall, there was no difference in the change in TK when comparing the osteotomy cohort and the nonosteotomy cohort. However, patients with extreme thoracic hypokyphosis (<10 degrees) and hyperkyphosis (>40 degrees) both experienced significant improvements in the restoration of normal TK through the use of Ponte osteotomies. Sudo et al57 analyzed 64 patients with Lenke I idiopathic scoliosis curves undergoing posterior spinal fusion to assess the effect of multilevel facetectomy on the sagittal correction. For patients with thoracic hypokyphosis (<15 degrees), they found that the number of levels correlated directly with the degree of sagittal correction. These data support the consideration of facetectomy and osteotomy for patients with more severe thoracic hypokyphosis, but research is needed to more accurately guide treatment principles.
Unique Surgical Strategies Based on the Authors’ Clinical Experience
Rod Derotation Technique
We practice a derotation surgical technique for addressing sagittal imbalance and thoracic hypokyphosis. A stiff, CoCr rod or rail is chosen for the concave side of the curve, with a rail being reserved for patients with more rigid deformities. A rod of similar size is also selected for the convex side of the curve, with the composition based on the need for a step-down to a less stiff material, such as Ti. The surgeon bends the CoCr rod for the concave side of the curve to ∼20 degrees greater than the measured degree of TK on preoperative radiographs. This method expects the rod to flatten during the correction based on the rigidity of the deformity, rod or rail geometry, and the composition of the metal component (Ti vs. CoCr).
Initially, the underbent, and potentially less stiff rod is placed on the convex side of the deformity, allowing for partial correction of the coronal deformity (Fig. 2). The Cricket Reducers (K2M, Leesburg, VA) are subsequently advanced but not fully tightened. This provides a cantilever push against the apex of the curve while allowing for rotation to occur around the convex rod. This maneuver decreases the rib hump deformity by derotation of the spinal deformity.
Again, using Cricket Reducers, we slowly reduce the coronal deformity using the overbent concave rod/rail (Fig. 3). During this time, the spinal deformity is derotated by pulling up the concave rotated side of the spine slowly advancing toward the apex. We utilize a differential tightening strategy of the Crickets back and forth from outside to the apex, creating a zipper effect to correct the spinal curvature in all 3 planes while introducing minimal stress.
Consider this strategy in the case of a 37-year-old adult female presenting with a Lenke 2A curvature (Fig. 4). The patient reported several months of upper thoracic paraspinal pain and several years of a perceived deformity. She did not have any radicular or myelopathic symptoms. Soft-tissue silhouettes revealed chest and abdominal asymmetry. Preoperatively, her right-sided mid-thoracic curve from T5–T11 was 54 degrees, and her left-sided proximal thoracic curve measured 45 degrees, with shoulder asymmetry. The additional assessment included a PI of 42 degrees, LL of 59 degrees (PI-LL mismatch of 17 degrees), TK of 48 degrees, and SVA of −3.2 cm. In accordance with the rod derotation surgical technique described, we performed a posterior spinal instrumented fusion mainly with pedicle screws, in additional to proximal hooks, from T2–L1. Postoperatively, her coronal cobb angles improved to 25 and 18 degrees, respectively. TK improved to 41 degrees; SVA, to 0.0 cm; LL, to 57 degrees; and PI to 49 degrees (PI-LL mismatch improved to 8 degrees). She appeared to be balanced in the coronal plane. Soft-tissue silhouettes revealed chest and abdominal symmetry. Clinically, the preoperative rib hump was no longer present. Her paraspinal pain had improved, and the patient was pleased with her clinical outcome.
Correction of TK Through LL and PI
During operative planning, it is critical to consider that TK, LL, and PI may be heavily influenced by one another. Addressing TK through thoracic-only instrumentation may result in unwanted consequences and new-onset sagittal deformity in the lumbar and pelvic region. Matsumoto et al58 analyzed 123 patients with idiopathic scoliosis and Lenke I, II, or III curves who underwent a posterior spinal fusion with the lowest instrumented level of L1 or cephalad. They found that the postoperative loss of TK was strongly associated with a reciprocal loss of LL. In a separate study of 155 patients with idiopathic scoliosis and lowest instrumented level between L2 and L5, Matsumoto et al59 found that 38% of patients experienced a decrease in LL at 2 years after surgery compared with their preoperative state. This group that experienced a decrease in LL were more likely to also have a compensatory increase in PT as compared with patients who did not experience a decrease in LL (73% vs. 40%, respectively). Studies like these have highlighted the intimate relationship between TK, LL, and pelvic parameters, and have encouraged surgeons to rethink surgical strategy.
For patients with idiopathic scoliosis and a flatback deformity, in particular, sagittal alignment is characterized by a regional thoracic hypokyphosis.10,60 to maintain a balanced global sagittal alignment, these patients often compensate with a decreased LL. As the above studies have suggested, isolated fusion of the thoracic deformity may lead to further loss of LL or an increase in PT, both of which have been associated with adverse outcomes. For these patients with flatback deformity and symptomatology primarily due to sagittal imbalance, our group focuses on restoration of LL and maintenance of pelvic parameters before consideration of extending fusion cephalad to the mid-thoracic region. Proper surgical alignment of the pelvis, lumbar spine and thoracolumbar junction may result in the correction of thoracic hypokyphosis while eliminating the morbidity associated with instrumentation of the full, or majority of, the thoracic spine.
Consider an example of a 55-year-old female presented with progressive low back pain and a history of scoliosis. Radiographs were remarkable for a Lenke V curve (Fig. 5). The global sagittal balance was maintained, with SVA<5 cm. However, the patient had local kyphosis at L1–L3 with a decrease in overall LL (22 degrees), and a compensatory thoracic hypokyphosis (13 degrees). These 2 extremes allowed for maintenance of global alignment but were thought to be contributing significantly to her symptoms. A posterior spinal fusion from T10 to pelvis using a pedicle screw construct was performed. Postoperative radiographs were remarkable for the restoration of both LL (52 degrees) and TK (40 degrees), with the maintenance of global sagittal alignment. The focus on surgical restoration of normal PI and LL resulted in the harmonious restoration of TK, with only 3 thoracic levels requiring instrumentation. The patient reported improvement in her symptoms and was pleased with the surgical outcome.
Sagittal balance is a critical aspect of the upright adult posture and ASD. Although several studies have offered general guidelines for the ASD population, AIS, in particular, remains a unique challenge for improving alignment in the sagittal plane. An understanding of the diagnostic modalities in addition to surgical strategies, including the rod derotation technique as described by our group, may help physicians to better care for these patients. Further research is necessary to determine the relationship of deformity correction in the sagittal plane to overall functional outcomes. A combination of longer follow-up times and ongoing refining of the current instruments will be critical.
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