Key points

Introduction: nature of the problem

The principal goal of adult scoliosis surgery is obtaining both sagittal and coronal balance of the spine. However, traditional scoliosis correction has been associated with high-volume blood loss and significant medical complications. Given this situation, minimally invasive spinal surgery (MISS) for the treatment of adult scoliosis is particularly attractive. MISS techniques have been used for the treatment of lumbar degenerative scoliosis, iatrogenic scoliosis, and adult idiopathic scoliosis. Theoretically, blood loss can be limited, and medical complication rates can possibly be reduced with less invasive procedures. Nevertheless, clinical and radiographic outcomes of MISS scoliosis correction need to be comparable with open surgery before recommending widespread adoption of these techniques for the treatment of adult scoliosis. MISS principles and surgical techniques used in MISS scoliosis correction are reviewed in this article, as well as outcomes, complications, and limitations of this rapidly evolving area of spinal surgery.

Introduction: nature of the problem

The principal goal of adult scoliosis surgery is obtaining both sagittal and coronal balance of the spine. However, traditional scoliosis correction has been associated with high-volume blood loss and significant medical complications. Given this situation, minimally invasive spinal surgery (MISS) for the treatment of adult scoliosis is particularly attractive. MISS techniques have been used for the treatment of lumbar degenerative scoliosis, iatrogenic scoliosis, and adult idiopathic scoliosis. Theoretically, blood loss can be limited, and medical complication rates can possibly be reduced with less invasive procedures. Nevertheless, clinical and radiographic outcomes of MISS scoliosis correction need to be comparable with open surgery before recommending widespread adoption of these techniques for the treatment of adult scoliosis. MISS principles and surgical techniques used in MISS scoliosis correction are reviewed in this article, as well as outcomes, complications, and limitations of this rapidly evolving area of spinal surgery.

Therapeutic options or surgical technique(s)

Indications for adult scoliosis correction include deformity progression, sagittal or coronal imbalance with unremitting back pain, radiculopathy on the side of the concavity of the curve caused by foraminal stenosis, lumbar hyperlordosis, patients with a history of flat-back syndrome and back pain, fixed lateral listhesis within the degenerative curve when motion is present on side-bending films, and when extensive decompression including facetectomy or the violation of the pars is planned. A relative indication is progressively worsening deformity with pain as the rib cage abuts the pelvis.

In our practice, adult patients who undergo MISS scoliosis surgery are typically being treated for symptomatic back and leg pain ( Fig. 1 ). These patients include those with adult idiopathic scoliosis, iatrogenic scoliosis, and lumbar degenerative scoliosis. Patients have tried numerous conservative measures, including physical therapy and epidural and facet injections, before being considered for surgery. The main indication for correction of adult scoliosis is mechanical low back pain. This pain is typified by stiffness in the morning, with progressive worsening of pain with activity that increases throughout the day. Often, but not always, this pain may be accompanied by radiculopathy or claudication.

( A, B ) Anteroposterior and lateral 91 cm (36-inch) films of a 68-year-old man with a history of back pain and leg pain refractory to conservative measures. He was noted to have lumbar degenerative scoliosis with a curve measuring 37° from L1 to L5, with the apex to the left.

The main principle of adult scoliosis correction is achieving a balanced spinal alignment and addressing symptomatic levels. Radiographic evaluation of the patient with adult deformity, whether being treated with traditional open correction or MISS, involves measurement of the Cobb angle in the coronal plane, the amount of correction on side-bending films, and the amount of deviation of the apical vertebrae to the central sacral vertical line. In the sagittal plane, a plumb line is drawn from the center of the C7 vertebra to the sacrum. Normally, this line should be within 5 cm of the posterior aspect of the sacrum. In addition, regional alignment and pelvic parameters, such as pelvic incidence and pelvic tilt, are calculated. In planning for adult scoliosis, the patient’s symptoms, stenosis, and disk degeneration must be considered.

Interbody fusion techniques are used to improve lordosis, help correct lateral listhesis, and, potentially, increase fusion rates. For lumbar degenerative scoliosis, proximal fusions are typically stopped at a stable vertebra. Others have advocated stopping at T10. In terms of where to begin and end a fusion, this topic has been discussed in detail elsewhere and is not the focus of this article. If a thoracolumbar fusion is extended to the sacrum, interbody fusion and pelvic fixation should be considered.

Segmental pedicle screw fixation allows for greater pullout strength than previous generation instrumentation systems (ie, hooks, cables). Pedicle screws may allow for shorter fusion length and less operative blood loss than hooks. Traditional scoliosis surgery allows for various open corrective maneuvers, such as derotation, vertebral coplanar alignment, and in situ rod bending. In addition, open surgery allows for both structural interbody techniques and osteotomies to assist in further deformity correction, with the creation of lumbar lordosis as needed.

A systematic review of adult scoliosis surgery showed that adult scoliosis surgery is associated with long-term improvement in patient radiographic and clinical outcomes. At a mean follow-up of 3.6 years, average curve reduction was noted to be 40.7% and mean Oswestry Disability Index (ODI) was reduced by 15.7. In terms of scoliosis deformity correction, sagittal balance improvement seems to be the strongest predictor of improved clinical outcomes, with correction of coronal balance being a lesser factor.

Open adult scoliosis correction has certain limitations. Medical complication rates associated with open adult deformity correction may be as high as 70%. Large volume blood loss is not unusual in these complex procedures. Seo and colleagues reported outcomes in 152 patients older than 20 years undergoing open adult scoliosis correction. These investigators noted a mean blood loss of 2855.8 mL ± 1822.9 mL. Transfeldt and colleagues noted a mean blood loss of 1538 mL in patients undergoing full fusion and decompression of their degenerative scoliotic curves. The population undergoing adult scoliosis surgery is often elderly, with significant medical comorbidities and high cardiac risk. Given this situation, the decision to proceed operatively with adult surgical deformity correction in the older population must be made carefully. Considering these limitations, MISS options may be attractive if similar results can be obtained with less blood loss and less tissue trauma.

MISS Scoliosis Correction

A portion of the morbidity associated with traditional spinal surgery occurs because of muscle damage associated with exposure and retraction and subsequent muscular devascularization and denervation. Tubular approaches for diskectomy, decompression, and minimally invasive surgery posterior fusion were developed to minimize these complications. MISS interbody fusion techniques followed. This was an important development, because interbody fusion may have higher fusion rates than posterolateral fusion techniques. In addition, diskectomy and graft placement may allow the achievement of anterior deformity release and alignment. Current options for MISS interbody fusion techniques include transforaminal lumbar interbody fusion (TLIF), miniopen and MISS anterior lumbar interbody fusion (ALIF), lateral transpsoas interbody fusion and the presacral approach for interbody fusion (AxiaLIF) ( Table 1 ). All of these options have subsequently been used in MISS correction of adult scoliosis.

Table 1

MISS interbody fusion technique

Technique	Advantages	Disadvantages	Sagittal and Coronal Plane Correction with Technique
TLIF	Posterior approach for diskectomy and interbody fusion Reduced risk for neurologic injury/durotomy when compared with posterior lumbar interbody fusion	Potential for neurologic injury and durotomy Time consuming	In 1 large deformity series, change in local lordosis ranging from –1.7° to 4° depending on level treated. May have superior results to ALIF in correction of AP lumbar curve and fractional curves. Mean correction of AP lumbar curve reported at 22.9° and AP fractional lumbosacral curve of 10.3°
ALIF	Large grafting surface, indirect neuroforaminal decompression Avoidance of spinal canal	Potential viscous/vascular injury, often requires approach surgeon, potential sympathetic dysfunction/retrograde ejaculation	Superior sagittal correction when compared with TLIF. In 1 large deformity series, increase in local lordosis ranging from 2.5° to 5.5° was noted with ALIF, depending on level treated. Reduced correction of AP lumbar curve and fractional curve was noted when compared with TLIF; 9.9° for AP lumbar curve and 3.3° AP correction for fractional lumbar curve
Transpsoas interbody fusion	Efficient method of achieving diskectomy, deformity release and interbody fusion; reduced risk of vascular/viscus injury when compared with ALIF, large graft surface, indirect foraminal decompression	Usually cannot be performed at L5-S1, potential for thigh dysesthesias, leg weakness	Mean gain of 2.8° lordosis at each level of transpsoas interbody fusion Controversial as to whether global coronal alignment is improved or not Global sagittal balance seems not improved with this technique
AxiaLIF	Minimally invasive corridor to L5-S1	Cannot be performed in cases of prerectal scarring or aberrant vasculature	Data not available

Abbreviation: AP, anteroposterior.

The combined use of 3 techniques to facilitate circumferential MISS scoliosis correction was reported in 2008. These techniques included the transpsoas approach for diskectomy and interbody fusion, the presacral approach for L5-S1 fusion, and percutaneous pedicle screw and rod placement. Percutaneous screw and rod placement has proved to be a major determinant of correction of apical vertical translation, even beyond diskectomy and interbody fusion. Subsequently, other series have reported outcomes using a combination of MISS techniques for adult scoliosis.

Most articles reporting MISS scoliosis correction rely heavily on the lateral transpsoas approach. This factor allows the surgeon MISS access to the spine, where diskectomy, deformity release, and interbody fusion can be achieved for multiple levels with minimal tissue disruption. The current technique, as described by Luiz Pimenta and published by Ozgur and colleagues, builds on the experience of other historical approaches in which MISS techniques were used to achieve ALIF. The techniques of Thalgott and colleagues and that of McAfee and colleagues served as precursors to the current technique. The current technique does not rely on endoscopy or laparoscopy. It requires less specialized equipment and theoretically has less of a learning curve. The transpsoas approach has subsequently been widely adopted as a technique to achieve release of scoliotic curves and perform interbody fusion ( Fig. 2 ). However, the lateral approach does place the lumbar plexus and genitofemoral nerve at risk for injury and is not without its own complication profile, reflected in Table 3 .

Anteroposterior fluoroscopic image showing Cobb elevator being used to release contralateral annulus during a direct lateral interbody fusion procedure.

The development of multilevel percutaneously placed screws, with freehand rod passage, allowed for deformity correction and fixation with minimal tissue disruption. Percutaneous rod and screw placement results in substantially less disruption of the thoracolumbar fascia than open techniques. This factor is clinically relevant, because the thoracolumbar fascia may be a major stabilizer of the lumbosacral spine and pelvis. Percutaneous screw and rod placement has proved to be crucial in the correction of apical vertical translation, even beyond diskectomy and interbody fusion. In addition, MISS screw techniques have been developed to allow supplemental iliac fixation, allowing rod insertion without connectors or extensive soft tissue dissection.

In terms of achieving MISS fusion, series to date have relied heavily on interbody grafting and the off-label use of recombinant human bone morphogenetic protein 2 (rhBMP-2) (Medtronic Sofamor Danek, Memphis, TN) ( Table 2 ). Use of rhBMP-2 has facilitated fusion without the need for extensive posterolateral decortication or autogenous bone graft harvesting.

Table 2

MISS scoliosis series

Author, Year	Number of Patients	Length of Follow-Up	Blood Loss	Radiologic/Clinical Outcomes	Comments	Level of Evidence
Anand et al, 2008	12	75.5 d	164 mL for transpsoas approach; 94 mL for posterior instrumented fusion and AxiaLIF	Decrease in mean Cobb from 18.93° to 6.19° Mean decrease in VAS of 4.8	Retrospective, feasibility study using a combination of the transpsoas lateral approach for diskectomy and interbody fusion, AxiaLIF, and percutaneous pedicle screw and rod placement. Mean segments operated on was 3.64 rhBMP-2 was used for all fusion sites Posterolateral fusion at all fusion sites without interbody fusion and at L5-S1	IV
Anand et al, 2010	28	22 mo	241 mL for transpsoas approach; for posterior procedures (including pedicle screw and rod placement and AxiaLIF) was 231 mL	Decrease in mean Cobb from 22° to 7° ODI improved from 39.13 to 7 VAS improved from 7.05 to 3.03 All patients were noted to maintain correction of their deformity and noted to have arthrodesis on plain radiograph	Retrospective study. MISS correction and fusion over 3 or more levels for adult scoliosis. Combination of the transpsoas lateral approach for diskectomy and interbody fusion, AxiaLIF (if fusing to L5-S1), and percutaneous pedicle screw and rod placement rhBMP-2 was used for all fusion sites. Posterolateral fusion at all fusion sites without interbody fusion	IV
Anand et al, 2013	71	39 mo	Patients with 1-stage same-day surgery had a mean blood loss of 412 mL. Patients with 2-stage surgery had a mean blood loss of 314 mL for transpsoas interbody fusion and 357 mL for posterior instrumentation and axial lumbar interbody fusion	Mean number of levels operated on was 4.4. Mean hospital stay was 7.6 d (mean preoperative Cobb angle was 24.7°, which corrected to 9.5°. Mean preoperative coronal balance was 25.5 mm, which corrected to 11 mm. Mean preoperative sagittal balance was 31.7 mm and corrected to 10.7 mm. Mean preoperative lumbar apical vertical translation was 24 mm and corrected to 12 mm. Fusion rate, as assessed by CT scan was 94%. VAS, ODI, and SF-36 improved significantly: preoperatively, 6.43, 50.3, and 41.8; at last follow-up, 2.35, 41, and 62.7, respectively	Retrospective study. MISS correction and fusion over 2 or more levels for adult scoliosis. Combination of the transpsoas lateral approach for diskectomy and interbody fusion, AxiaLIF (if fusing to L5-S1), and percutaneous pedicle screw and rod placement rhBMP-2 was used for all fusion sites. Posterolateral fusion at all fusion sites without interbody fusion	IV
Caputo et al, 2012	30	1 y	ODI decreasing from 24.8% to 19.0% and VAS leg pain score decreasing from 5.4 to 2.8	Mean Cobb 20.2 preoperatively Postoperatively not reported Mean ODI decreased from 24.8 to 19.0. Mean VAS back decreased; mean VAS leg decreased from 6.8 to 4.6	Patients underwent the transpsoas approach followed by posterior MISS instrumentation for adult degenerative scoliosis. Osteocell used for interbody fusion (Nuvasive, San Diego, CA)	IV
Dakwar et al, 2010	25	80% of patients underwent more than 6 mo of follow-up	53 mL per segment fused	Decrease in mean Cobb from 22.1° to 6.2°. Mean improvement of 5.7 points on the VAS scores and a 23.7% improvement in ODI. All patients with >6 mo follow-up with solid fusion	Patients with adult degenerative deformity. A variety of stabilization techniques were used, including lateral plates, and open and percutaneously placed pedicle screws. At each level, rhBMP-2, tricalcium phosphate, and hydroxyapatite used as fusion material	IV
Isaacs et al, 2010	107	6 wk	Almost two-thirds (62.5%) of patients had a recorded blood loss of 100 mL, and only 9 patients (8.4%) had a 300 mL blood loss	NA	Multicenter prospective, nonrandomized study of transpsoas lumbar interbody fusion procedures in adult degenerative scoliosis. Perioperative outcome study. Fusion material not specified	IV
Karikari et al, 2011	22 patients, of whom 11 had degenerative scoliosis	16.4 mo	227.5 mL	In the subset of patients treated for degenerative scoliosis, the mean preoperative and postoperative coronal Cobb angles were 22° and 14°, respectively	Retrospective study. Most patients without supplemental fixation. Lateral screws in 4 patients. Posterior fixation in 1 patient. All patients treated for scoliosis received rhBMP-2 in their cages	IV
Scheufler et al, 2010	30 patient with thoracolumbar degenerative kyphoscoliosis	19.6 mo	771.7 mL	Fusion rate was 90% (26 patients available for CT scan at a mean of 6 mo). Average segmental correction of 10°–12° in the coronal and sagittal planes. Mean lumbar sagittal Cobb angle correction was 44.8 ± 10.7°, resulting in a mean postoperative lumbar lordosis of −36 ± 6.9°. Mean preoperative sagittal balance of 31.6 ± 15.2 mm (range, 5–96 mm) was reduced by 63.5 ± 30% to a postoperative mean of 8 ± 8.4 mm (range, −4–25 mm). A mean coronal Cobb angle correction of 31.7 ± 13.7° was sufficient to achieve a final mean postoperative coronal Cobb angle of 10.3 ± 7.8° at 12-mo follow-up. Mean preoperative apical vertebral translation was reduced from 22.3 ± 32 mm (range, 17–78 mm) to 9.9 ± 15.6 mm (range, 8–30 mm). Mean VAS reduced from 7.5 to 2.82 at last follow-up; ODI decreased from mean of 57.2 to 23.9	Retrospective series of patients undergoing unilateral MISS TLIFs and percutaneous screw and rod placement for MISS kyphoscoliosis correction using biplanar fluoroscopy or intraoperative CT scanning/navigation. 3 to 8 segments fused using TLIFs and facet fusion. TLIF performed using tubular access. In patients receiving short instrumentation (≤4 segments), TLIF was performed each level, whereas 3–5 segments were treated by TLIF in patients undergoing long instrumentation. Vertebral cement augmentation performed in 10 female osteopenic patients. Autologous bone chips and rhBMP-2 was used for all fusion sites	IV
Sharma et al, 2011	43 patients, 25 with lumbar degenerative scoliosis	1 y	200 mL for transpsoas portion of surgery only	Mean correction of maximal Cobb angle was noted to be 10.4° (43%). VAS, ODI, and SF-12 improved significantly. Preoperatively, VAS and ODI 8.2 and 42.6; at 1 y, 4.6 and 31.5, respectively	Retrospective study. Ten patients in their series were treated with stand-alone transpsoas fusion, 9 with lateral plate and screw fixation, and 24 with pedicle screw fixation. Depending on surgeon preference, autograft or rhBMP-2 was used	IV
Wang and Mummaneni, 2010	23	13.4 mo	477 mL	Decrease in mean Cobb from 31.4° to 11.5° Thoracolumbar lordosis increased by a mean of 8° All interbody fusions with solid arthrodesis as noted on CT scan. Two of 7 cases in which posterolateral fusions were performed alone had pseudarthrosis. Significant improvements in VAS: VAS leg averaged 4.35 and improved to 1.57; VAS back averaged 7.30 and improved to 3.35	Retrospective study. Two centers. Patients with scoliosis >20° or significant sagittal decompensation with loss of sagittal balance. Deformity correction using the transpsoas approach, along with MISS TLIF at L5-S1 if fused to the sacrum. rhBMP-2 was used for all fusion sites. Posterior fixation using percutaneous pedicle screws and rods. Posterolateral fusion at all fusion sites without interbody fusion	IV

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