Flatback Syndrome: Can Lumbar Flatback Syndrome Be Treated Adequately with Minimally Invasive Techniques?

10 Flatback Syndrome: Can Lumbar Flatback Syndrome Be Treated Adequately with Minimally Invasive Techniques?

MIS: Navid R. Arandi and Gregory M. Mundis Jr.
Open: Randa El Mallah and Ahmad Nassr

10.1 Introduction

In 1973, Doherty 1 first outlined sagittal plane deformities in patients with adult spinal deformity (ASD) with thoracolumbar scoliosis. He noted postural complications and loss of lumbar lordosis (LL) in patients following posterior spinal fusion and Harrington’s instrumentation. Subsequently in 1977, Moe and Denis² popularized the term “flatback syndrome” and reported satisfactory short-term results with closing wedge osteotomies in 16 patients. Flatback is characterized by a decline in LL, which leads to a fixed sagittal plane deformity, failure to stand upright without knee flexion and hip hyperextension, stooped forward posture of the torso, and pain. Patients often try to compensate by hyperextending their hips and flexing their knees in order to maintain their gaze at level with the horizon. The constant strain required to maintain sagittal equilibrium leads to pain and fatigue along the spine, thighs, and buttocks.

Flatback syndrome has been extensively studied over the last decade and has evolved to a clinical diagnosis known as sagittal imbalance. This has been defined as a distinct clinical entity that has strong correlations with spino-pelvic radiographic parameters. As such, they warrant careful consideration, as treatment recommendations are made and operative plans are created and executed. Glassman et al demonstrated that adult deformity patients with a positive sagittal vertical alignment (SVA > 5 cm) have worse health-related quality of life (HRQOL) scores and that a linear relationship exists between function scores and increasing magnitude of sagittal imbalance.^3,4 Pelvic tilt (PT), pelvic incidence (PI), and sacral slope are also essential preoperative parameters to consider. PI is a morphologic parameter that remains constant once skeletal maturity is achieved. Its value is fundamental in the understanding of an individual’s ideal physiologic LL. HRQOL has been found to significantly worsen when PI–LL is greater than 10°. PT is the major compensatory mechanisms of the pelvis to maintain horizontal gaze with worsening sagittal balance. A PT greater than 20° has also been established as an independent predictor of HRQOL and may even be a primary indication for surgery.^5,6 Schwab et al⁵ recently validated a comprehensive and clinically based classification system for ASD ( Fig. 10.1). The Scoliosis Research Society (SRS)-Schwab classification for ASD characterizes four different curve types along with three sagittal and pelvic modifiers and was developed using radiographic and HRQOL outcomes. Their classification system should be considered in all deformity cases for preoperative planning to achieve optimal deformity correction.

10.2 Indications

A thorough preoperative history is necessary to gauge the duration and progression of deformity, prior treatment, and the extent of leg and back pain. Physical exam should assess the flexibility of the deformity and the integrity of the extensor spinal musculature. Hip and knee flexion contractures should be identified as they can undermine surgical outcomes if left untreated. Patient’s general health along with their clinical presentation and ability to withstand extensive reconstructive surgery must also be considered when deciding on a treatment plan. Although debatable, nonoperative treatment plays only a minor role in the management of a patient with sagittal imbalance including medications for symptomatic pain relief, physical therapy to strengthen core musculature, and bracing. Prolonged bracing, however, can contribute to atrophy of the paraspinal musculature and does not play a significant role in the definitive treatment of sagittal imbalance.

Surgery is the only definitive form of treatment for flatback. Surgical goals are not only to reduce pain, improve function, and achieve arthrodesis but also to restore sagittal balance with PI–LL ratio within 10°, PT less than 20°, and SVA < 5 cm.5,6 Traditional methods for the treatment of flatback syndrome have been open surgical osteotomies including Smith-Petersen osteotomy (SPO), pedicle subtraction osteotomy (PSO), and vertebral column resection (VCR). Although these techniques have a proven track record, the associated morbidity is often a deterrent for patients to subject themselves to reconstruction.

Recent technological advancements in conjunction with efforts to decrease perioperative morbidity, hospital costs, and recovery times have led to heightened interest in minimally invasive techniques as viable modalities of treatment.

10.3 Advantages of Minimally Invasive Surgery

Despite the reliability of conventional open techniques at correcting sagittal deformity, they carry a high complication profile and can result in significant perioperative morbidity, and require supraphysiologic demands to recover.7,8,9 Recovering from a three-column osteotomy can last as long as a year or more with significant convalescence and need for prolonged perioperative skilled nursing. The rehabilitation process is demanding not only of the medical community at large but also of the patient’s family and caregivers. Minimally invasive techniques reduce the morbidity of the surgery itself without affecting the long-term outcome that can be achieved with open techniques. Minimally invasive surgery (MIS) accomplishes this by reducing intraoperative blood loss, decreasing transfusion rates, and inflicting less collateral damage to surrounding tissues, thereby subsequently incurring fewer complications and ultimately reducing cost. Cost is further driven down by decreasing the need for ICU care and shortening the postoperative convalescence and need for prolonged skilled nursing and rehab.^{10,11,12,13,14,15,16,17,18,19,20}

Fig. 10.1 Scoliosis Research Society (SRS)-Schwab classification for adult spinal deformity. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; SVA, sagittal vertical axis; TL, thoracolumbar. From Akbarnia B, Mundis G, Moazzaz P, et al. Anterior Column Realignment (ACR) for Focal Kyphotic Spinal Deformity Using a Lateral Transpsoas Approach and ALL Release. J Spinal Disord Tech. 2014 Feb;27(1):29-39, used with permission.

A decade ago, the lateral retroperitoneal approach was reintroduced to the spine community using a mini-open approach and minimally invasive retractors to gain access to the anterior spinal column and perform a minimally invasive lateral inter-body fusion (LLIF). While initially used for single-level degenerative disorders, it quickly found significance in spinal deformity surgery by decreasing the perioperative morbidity and enhancing recovery time compared to traditional anterior open techniques.10,12,14,17,18,19 Akbarnia et al recently introduced anterior column realignment (ACR) as an evolution of LLIF to treat focal kyphotic deformity by adding a complete anterior release to the traditional LLIF procedure.¹⁰ Early results are promising; however, larger cohorts with longer follow-up and direct comparisons to open techniques are needed to gauge its true potential.

10.4 Advantages of Open Surgery

Open techniques for the treatment of flatback syndrome and fixed sagittal deformities have been the mainstay of treatment for these disorders. While technically challenging, they have been shown to result in excellent outcomes for patients with significant sagittal plane deformity.21,22,23,24,25 Open surgery offers direct visualization of the neural elements and allows for powerful reduction techniques to be employed to achieve correction of the deformity. The open techniques also expose the entirety of the posterior elements, allowing for a large surface area to achieve fusion. While complication rates are still significantly high in this patient population, many studies have demonstrated durable results with these techniques.^{23,25,26,27,28}

Minimally invasive techniques being applied to the treatment of flatback deformities is a relatively new concept. Although these techniques may provide short-term benefits such as reduced intraoperative blood loss and faster postoperative recovery, there are many factors keeping them from being widely adopted. These techniques often rely on the use of inter-body grafting techniques to achieve both reduction and fusion. The current surgical tools to achieve reduction are still not optimized for the treatment of deformity due to their limited ability to perform compression and distraction maneuvers.^12,14 The associated learning curve and high implant costs associated with these techniques are also other impediments to their widespread adoption.

10.5 Case Illustration

A 66-year-old man with a history of progressive back and leg pain radiating to the upper thighs. He has suffered a significant decline in overall function, with numerous attempts at conservative nonsurgical management proving unsuccessful at alleviating his symptoms or restoring function. On physical exam, he exhibits grossly positive sagittal and coronal imbalance with no hip flexion contractures ( Fig. 10.2a,b). Scoliosis radiographs show an LL of + 2°, thoracolumbar kyphosis of 66° (T10–L3), T1 sagittal pelvic inclination (T1SPI) of 4°, SVA of + 18 cm, and PT and PI of 48° and 60°, respectively ( Fig. 10.2c,d). There is a 62° PI–LL mismatch. Multilevel reconstructive surgery was mutually agreed upon.

Fig. 10.2 (a) Anterior clinical image showing a right coronal imbalance and (b) a lateral image demonstrating the gross sagittal imbalance with a stooped forward posture. (c) Posteroanterior and (d) lateral X-rays further elucidating the severity of the sagittal and coronal imbalance. From Akbarnia B, Mundis G, Moazzaz P, et al. Anterior Column Realignment (ACR) for Focal Kyphotic Spinal Deformity Using a Lateral Transpsoas Approach and ALL Release. J Spinal Disord Tech. 2014 Feb;27(1):29-39, used with permission.

10.6 Surgical Technique in Minimally Invasive Surgery

Selecting the right patient is crucial and depends on the severity of deformity, region and number of involved levels, prior surgery, and flexibility. The lateral retroperitoneal transpsoas approach is best utilized for treating segmental and global kyphotic deformities from T12 to L5 with or without anterior longitudinal ligament (ALL) release. For more severe regional deformities, multilevel ACRs can be supplemented with posterior osteotomies (i.e., PSO) in order to achieve the desired correction.

ACR is a modification of the minimally invasive LLIF technique using the lateral retroperitoneal approach, hyperlordotic interbody cages, and complete anterior release, including release of the ALL, for the treatment of sagittal plane deformity. There have been considerable advancements in the technique since its introduction in 2006 by Ozgur et al 19 and the recently described ACR technique published by Akbarnia et al.¹⁰

Work-up of the patient for their sagittal plane deformity includes obtaining 36-inch radiographs that must include the femoral heads and the cervical spine. Supine hyperextension lateral radiographs are utilized to assess flexibility at the apical intervertebral disc, the site of the desired ACR. Magnetic resonance imaging (MRI) and/or a computed tomography (CT) myelograms are also used preoperatively to localize the psoas muscle and lumbar plexus, visualize the vascular anatomy, and to assess the posterior bony structures for facet arthropathy and fusion, and the anterior disc space for ankylosis and large osteophytes. The MRI will also help determine if patients may need a direct posterior decompression in addition to their realignment procedure. Dual-energy X-ray absorptiometry (DEXA) scans are obtained on all patients, and if the T-score is less than –2.5, the patient must be treated for their osteoporosis before the spine is reconstructed. Consideration should be given to vitamin D and calcium supplementation among all patients.

After the administration of anesthesia, the patient is placed in a lateral decubitus position on an operating table that has a break in it. The patient is padded appropriately to ensure that all pressure points are cushioned. Anteroposterior and lateral fluoroscopy is used to locate the intervertebral disc space. Excessive flexion of the operating table should be avoided in order to prevent undue tension on the psoas muscle and lumbar plexus. A lateral retroperitoneal approach is made to the disc space with concurrent directional eletromyographic (EMG) neuromonitoring to ensure safe passage through the psoas. The target of the first dilator and guidewire is the posterior one-third of the disc space to ensure a complete anterior release and to facilitate the cage placement. Many transpsoas exposure systems and monitoring systems exist. The surgeon must be familiar with the particular characteristics of each. The ACR technique was developed in conjunction with Nuvasive XLIF-ACR (eXtreme lateral interbody fusion [XLIF]; Nuvasive Inc., San Diego, CA) as the procedure requires specialized instruments designed specifically for this procedure.

Following sequential dilations, the retractor is secured to the operating table with a mounting bracket. A shim (retaining pin) is used to maintain the position of the retractor and prevent its anterior migration. The retractor is opened to the margins of the disc space, just enough to perform the initial discectomy. Once the discectomy is complete with release of the contralateral annulus, the disc space is prepared for an appropriate-size implant. Ideally, 24 mm of disc space exposure is obtained to accommodate a 22-mm wide interbody cage used in ACR. The anterior retractor is placed between the ALL and the anterior vasculature. The gentle development of the plane anterior to the ALL is critical for the proper retraction of the vascular structures. Fluoroscopy is used to confirm if the retractor reaches the contralateral pedicle. A sufficiently wide anterior retractor is used to ensure that the anterior retractor does not fall into the disc space after the ALL is released. Any additional disc material behind the anterior annulus and ALL is removed in order to sequester the ALL and ensure its safe release. The ALL is then released using a custom knife. A paddle distractor can also be used to confirm a satisfactory release of the ALL. Incomplete release of the ALL or a partially intact contralateral annulus can result in persistent tension during distraction. One must reassess and assure that these structures are fully released before continuing with trialing.

Sequential trialing is performed with standard 22-mm trials up to an anterior height of 12 mm. The ACR trial implants, which come in 20° and 30°, are then inserted. The amount of lordosis needed should be predetermined from preoperative planning. The appropriate sized cage is then prepared on the back table with the biologic of choice. The implant is then placed while attached to the posterior blade of the retractor to ensure that the implant is placed at the desired location within the disc space. The position of the interbody cage is confirmed with biplanar fluoroscopy. A screw is placed through the cephalad flange abutting the end plate of the cephalad vertebral body. This helps prevent expulsion of the implant and allows for unobstructed placement of pedicle screws.

The wound is closed in a layered technique. A small Hemovac drain is placed overlying the psoas to prevent hematoma formation. It is removed once the patient starts to ambulate the following day.

The first part of the procedure involves resection of the posterior elements above and below the planned pedicle resection. The transverse processes are then detached from the vertebral body. The posts of the pedicles are left in place to protect the nerve roots and dura. De-cancellation of the vertebral body in a wedge shape is then performed through the pedicles with the use of large curettes. The anterior cortex and ALL are preserved as the hinge for the osteotomy. The pedicles are then resected until they are flush with the posterior vertebrae. Dissection is then carried out lateral to the vertebral body. The lateral vertebral wall is then resected according to the planned osteotomy angle. The posterior vertebral wall is then impacted ventrally to complete the osteotomy. Frequent neurophysiologic monitoring and sometimes an interoperative wake-up test are utilized to ensure adequate space for the neural elements.^22,30

10.7.3 Vertebral Column Resection

In cases of severe rigid deformity, VCR can be utilized to achieve even further correction than is possible with other types of osteotomies. One or more vertebral segments are removed (including the posterior elements, pedicles, entire vertebral body, and discs above and below). A cage is placed anteriorly to act as a fulcrum for deformity correction. This can be performed through a combined anterior and posterior approach; however, all posterior techniques have gained popularity due to the ability to directly control the spine after the osteotomy is created.23,31 Up to 80° of correction can be achieved utilizing this technique.²⁴

10.8 Discussion of Minimally Invasive Surgery

Sagittal imbalance is a major source of pain and dysfunction among patients with ASD.3,4,17,32 Historically, open posterior and/or anterior procedures have been used to reconstruct the spine despite their inherently high complication profile.^7,9 When considering open procedure, due diligence must be paid to patient’s age, comorbidities, potential for blood loss, and complication risk.

The literature on MIS in the treatment of ASD has traditionally been centered around coronal plane correction,^18,33 with incomplete data to truly assess the sagittal plane.^12,34 Direct comparisons with traditional open procedures is also lacking in the current literature. Despite its potential benefits in adult deformity correction, prospective studies with large cohorts and longer clinical and radiographic follow-up are needed in order to characterize the true potential of MIS in the treatment of sagittal plane deformities.

One of the confounding variables in performing ACR is its frequent supplementation with an open posterior release at the ACR level and/or open pedicle screw fixation in an effort to achieve optimal sagittal plane correction. Furthermore, randomization of patients into an MIS or open group is unethical and attempts to gain institutional approval will be futile. Thus, it is challenging to perform quality comparative studies on purely minimally invasive ACR versus open reconstruction for the treatment of sagittal plane deformities.³⁵

10.8.1 Level I and II Evidence in Minimally Invasive Surgery

There are currently no level I or II studies available.

10.8.2 Level III and IV Evidence in Minimally Invasive Surgery

Akbarnia et al 10 were the first to report their midterm experience with ACR for the treatment of focal kyphotic deformity. Everyone in our cohort of 17 consecutive patients underwent ACR procedure followed by open posterior pedicle screw fixation. There were 12 women and 5 men, with average age of 63 years and average follow-up of 24 months. Of the 17 patients, 82% (14) had prior spine surgery, 71% (12) of which were spinal fusions. The procedure was staged with mean intraoperative blood loss of 111 mL for ACR and 1,484 mL during the posterior procedures. Indications for surgery included degenerative scoliosis, progressive focal sagittal plane deformity, instability at the level of the focal deformity, decreased QOL, and pain. Motion segment angle (measured from superior end plate of the upper end vertebrae to the lower end plate of the lower end vertebrae) improved from a mean of 9° preoperatively to –19° (28° change) after ACR and –26° (35° change) after posterior surgery, which are similar to the segmental correction achieved via open methods ( Fig. 10.3). Preoperative LL improved from –16° to –38° and –45° after ACR and posterior instrumentation, respectively, and was maintained at –51° at final follow-up (p < 0.05). PT improved from 34° preoperatively to 24° following ACR and remained stable at 25° at the latest follow-up. The T1SPI (as described by Legaye et al³⁶) was used as a surrogate for sagittal vertical axis to avoid calibration errors. Results were divided into two groups based on baseline preoperative values of T1SPI. Patients with negative preoperative T1SPI averaged –6° and corrected to –0.6° after ACR with posterior instrumentation and –2° at final follow-up. Those with positive or zero T1SPI averaged + 5° and corrected to –0.5° after ACR with posterior instrumentation and –3° at final follow-up. Mean SRS-22 score improved from 2.42 preoperatively to 3.14 (p < 0.05) at final follow-up. Mean visual analog scale improved significantly from baseline to final follow-up (6.8 to 4.1, p < 0.05).

Fig. 10.3 Comparison of single-level ACR vs. single-level PSOs and SPOs in literature. Mean degrees of segmental lordosis correction following single-level ACR was comparable to traditional open procedures such as SPO and PSO. ACR, anterior column realignment; PSO, pedicle subtraction osteotomy; SPO, Smith-Petersen osteotomy. (From Behrooz, A, Mundis, G, Moazzaz, P: Anterior Column Realignment (ACR) For Focal Kyphotic Spinal Deformity Using a Lateral Traspsoas Approach and ALL Release J Spinal Disord Tech. 2014 Feb;27(1):29-39 Wolters Kluwer Heatlh, with permission.)