Dr. Michael O. Lagrone

Flat-back syndrome is a recognized complication of scoliosis surgery. It is a postural disorder, primarily caused by loss of normal lumbar lordosis. This results in forward inclination of the trunk, back pain, and inability to stand erect without flexing the knees (Fig 1). This article reviews the diagnosis, pathogenesis, management, and prevention of flat-back syndrome.

Figure 1. Clinical appearance of a patient with flat-back syndrome. The lumbar spine is flattened and the patient's trunk is pitched forward. To stand erect with a straightforward gaze, the knees are flexed and the neck is hyperextended.

This postural disorder is a syndrome and therefore represents a spectrum of etiologic and aggravating factors. Those factors that have been identified include

Clearly, the most important causative factor responsible for flat-back syndrome is loss of lumbar lordosis. Distraction instrumentation extending into the lower lumbar spine or sacrum has been identified as the most frequent cause of loss of lordosis. With the introduction of distraction instrumentation for the correction of scoliosis, attention was too often focused on the frontal plane without adequate appreciation of the effects of instrumentation on the sagittal plane. When a straight distraction rod is used to correct scoliosis in the lumbar spine, lordosis is diminished. The severity of loss of lumbar lordosis increases as the level of instrumentation extends caudally.

The loss of lumbar lordosis with the use of distraction rods is not only a function of a straight rod in the lumbar spine, but also a function of distraction forces that produce flattening of the lumbar spine (Fig 2).

Fig. 2. The effects of a distraction rod in the lumbar spine. Despite contouring for lordosis, the lumbar spine is flattened and the kyphosis above the instrumented area is increased.

Other Factors

The thoracolumbar junction often plays a significant role in the pathogenesis of flat-back syndrome. If a thoracolumbar kyphosis is present and the lumbar spine is instrumented without inclusion of the thoracolumbar junction, two things may happen. One, the lumbar lordosis can be flattened by the mechanisms described previously. Two, the thoracolumbar kyphosis will often progress proximal to the instrumented area. This combination often leads to the development of flat back syndrome. In addition to loss of correction, pain from a pseudarthrosis may compound the symptoms of flat-back syndrome.

Pseudarthrosis should be suspected if lower back pain is a major complaint in a patient with flat-back syndrome. Hip hyperextension is the favored compensatory mechanism for loss of lumbar lordosis. With flexion contractures of the hip, this ability to compensate is lost.

Patients may be able to compensate for loss of lumbar lordosis with extension of the thoracic spine. With a fixed thoracic kyphosis from either a prior fusion or ankylosis, the patient may be more susceptible to developing flat back syndrome.

Flattening of the lumbar spine is the common denominator in flat back syndrome, but one or more of these aggravating factors is often involved. All factors must be considered when planning surgical treatment for flat-back syndrome.

Clinical Presentation

Patients with flat-back syndrome present with fixed forward inclination of the trunk and inability to stand erect with their knees fully extended (Fig 1). In order to maintain an erect posture, the patient will flex the knees and extend the upper thoracic and cervical spine. Most patients complain of back pain. Typically, the pain is in the upper back and is described as a fatigue-type pain secondary to efforts to hyperextend their thoracic spine in order to stand erect. Most patients report that the sensation of leaning forward and associated upper back pain increases as the day progresses. Many patients also report lower back pain. This may be secondary to a pseudarthrosis in the lumbar spine or to degenerative changes below the previous fusion. Lower cervical pain is another frequently reported complaint. Patients will often hyperextend the cervical spine in order to see straight ahead. This can lead to muscular fatigue and pain in the cervical region.

Radiographic Assessment

The most useful radiograph in evaluating patients with sagittal plane deformities is a full-length (36 in), standing lateral radiograph of the entire spine. The patient should be instructed to stand with the knees fully extended. Thoracic kyphosis, lumbar lordosis, and thoracolumbar junction measurements can be determined. The absolute values for kyphosis and lordosis are not as important as the overall sagittal balance. This is determined by dropping a plumb line from the center of the seventh cervical vertebra and measuring the distance from the anterior aspect of the sacrum to this line. This measurement has been termed the C7-S1 distance1 or, more appropriately, the sagittal vertical axis. Normally, the SVA falls over the sacrum and should not fall more than 2 cm anterior to the sacrum.

Other radiographic studies are often needed, particularly when planning surgical treatment. These may include hyperextension radiographs to assess flexibility, oblique radiographs to assess for pseudarthroses, and magnetic resonance imaging or computed tomography / myelography to evaluate the spinal canal.

Management

Nonsurgical
Nonsurgical attempts at symptomatic relief in patients with flat-back syndrome is most often unsuccessful but should be initiated in most patients, especially those with mild deformity. Physical therapy with spinal extension exercises, attempts to increase hip extension, and non-narcotic medication may provide enough symptomatic relief in patients with mild sagittal imbalance to avoid surgical treatment. If anterior displacement of the sagittal vertical axis is greater than 4 cm or the patients have failed conservative measures and remain symptomatic, surgical treatment may be indicated.

Surgical Treatment of Flat-Back Syndrome

Surgical Decision Making
Patients with flat-back syndrome who fail nonsurgical management present a significant challenge to the spine surgeon in both surgical planning and technical skills. The goals of surgical treatment for flat-back syndrome are to obtain a balanced spine in both the coronal and sagittal planes, a solid arthrodesis, and relief of back pain. A multitude of variables are to be considered if one is contemplating surgical correction of sagittal plane imbalance. These include the general health of the patient, the magnitude and location of pain, the severity of sagittal vertical axis displacement, the degree of loss of lumbar lordosis, the status of the thoracolumbar junction, and the coronal plane alignment.

The medical status of the patient has to be evaluated carefully. The patient should be medically fit enough to undergo major reconstructive spine surgery that will frequently require combined anterior and posterior approaches. In addition to addressing known or potential medical problems, the nutritional status of the patient must be considered.

Location of Osteotomies
Surgical correction of flat-back syndrome and other sagittal plane deformities typically involves one or more closing wedge osteotomies through the fusion mass. The decision regarding where to place the osteotomies depends on the site and severity of the deformity. Ideally, the osteotomy or osteotomies should be centered over the principle area of deformity. If the problem is primarily flattening of the lumbar spine without significant thoracolumbar kyphosis, one or more closing wedge osteotomies placed below the level of the conus medullaris will usually provide satisfactory restoration of lumbar lordosis and sagittal balance, with less chance of neurologic injury. If significant thoracolumbar kyphosis (15° or more) exists, however, it should be addressed. If the kyphosis is flexible, as shown by lateral hyperextension radiographs, the patient can be managed by lumbar osteotomies with extension of the instrumentation and fusion proximally into the thoracic spine to obtain correction of the thoracolumbar kyphosis. If the kyphosis is rigid, an osteotomy may be needed at the thoracolumbar junction in addition to the lumbar osteotomies.

The coronal plane requires careful consideration. Care must be taken not to create a coronal imbalance in patients undergoing correction of sagittal plane deformity. If there is a preexisting coronal plane imbalance, it will require correction in conjunction with sagittal plane realignment.

Instrumentation and Fusion to the Sacrum Versus Stopping Short of the Sacrum
The decision whether or not to fuse to the sacrum often difficult and depends on several factors. In patients with mild deformity (SVA less than 4) and two or more "healthy" motion segments distally, it may be reasonable to stop the instrumentation and fusion short of the sacrum. For those patients with more severe deformity, those with symptoms of back pain attributable to the L4-5 or L5-SI disc levels, and those whose previous fusion extended to L5 or the sacrum, the revision surgery should include the sacro-pelvic unit.

Posterior Versus Combined Anterior/Posterior Approach
A posterior-only approach may be appropriate in selected patients with mild to moderate deformity, not requiring fusion to the sacrum. The prerequisites include a relatively young patient, good bone quality, and the ability to obtain satisfactory correction in the sagittal plane with osteotomies at one or more levels in the lumbar spine. Rigid segmental fixation is also required.

As a general rule, most patients with deformity and symptoms severe enough to require surgical treatment are best treated with a combined anterior and posterior approach. An anterior approach can be useful for the following reasons: (I) better surgical correction, (2) improvement of fusion rates, and (3) reconstruction of anterior column defects. A completely corrected spine with a solid arthrodesis has a much better chance of maintaining correction and reducing symptoms.

Each case has to be individualized and a surgical strategy devised that best achieves the goals of completely rebalancing the spine and obtaining and solid fusion with minimal neurologic risk.

Same Day Versus Delayed Staging
Most reconstructive procedures for sagittal imbalance should be performed on the same day. Total blood loss, risk of complications, and costs are significantly reduced. If the planned surgical procedure is anticipated to last more than 10 to 12 hours, however, staging the surgery may be preferred. Patients who may require staging include those with retained segmental instrumentation that is time consuming to remove, as well as the patient who requires a three-stage posterior/anterior/posterior approach.

Fixation Methods
Modern implant systems have many advantages over the older systems. These systems allow the combined use of multiple hooks and pedicle screws. They allow for segmental compression across multiple osteotomies, more stable fixation, better restoration of sagittal contours, and improved methods of sacral-pelvic fixation.

Postoperative Management

Patients are kept at bedrest with frequent turning, pulmonary therapy, and mechanical deep venous thrombosis prophylaxis for approximately 48 hours. Typically, chest tubes and suction wound drains are removed after 48 hours. The patient is then mobilized. The decision regarding postoperative bracing depends on the stability of internal fixation and bone quality. The majority of patients are braced for 6 to 9 months postoperatively in a light-weight bivalved body jacket. Patients are discharged when they are ambulatory and no longer require intravenous fluids or parenteral medication. Patients are seen for follow-up at 3 weeks, 3 months, 6 months, and one year after surgery. At each follow-up visit, clinical and radiographic assessment of sagittal balance is made. Evidence of implant failure or loss of sagittal correction should be addressed immediately. The problem will only get worse with time.

Complications

Early complications in the treatment of flat-back syndrome include neurologic injury, dural tears, and medical problems. Late complications include pseudarthrosis, implant failure, and loss of correction. The most frequently reported significant complication is loss of correction or persistent symptoms secondary to inadequate initial correction. Careful attention to medical management and technical details will minimize the complication rate. Pseudarthrosis rates are reduced by restoring normal sagittal balance, the use of rigid internal fixation, and the addition of an anterior fusion. Neurologic injury can be reduced by carefully undercutting the osteotomies, by the use of multiple osteotomies, and the use of spinal cord monitoring.

Results of Surgical Treatment

The results of surgical treatment for flat back syndrome correlate closely with the adequacy of sagittal realignment and the attainment of a solid arthrodesis. Clinical outcome and patient satisfaction will be good in most cases if C7 is centered over the sacrum and the spine is solidly fused.

It is clear that incomplete correction of the sagittal plane imbalance often leads to pseudarthrosis, progressive loss of correction, and clinical failure.

Prevention of Flat-Back Syndrome

The most effective treatment for flat-back syndrome is to prevent its occurrence. Factors that are most important in the prevention of flat back syndrome are avoidance of distraction instrumentation into the lower lumbar spine or sacrum and preservation of lordosis by appropriate patient positioning.

Lumbar fusion should be avoided when possible. For example, selective thoracic fusion in King type II curves allows the maintenance of a mobile lumbar spine and avoids the risk of flat-back syndrome. When the lumbar spine requires instrumentation and fusion, methods are available to preserve lumbar lordosis. In certain patients with lumbar or thoraco-lumbar scoliosis, a short segment anterior fusion with rigid instrumentation is preferred. When combined thoracic and lumbar fusion is required, a multihook double-rod system will provide for satisfactory curve correction and preservation of sagittal contours. Correction of lumbar scoliosis is obtained through compression, translation, and rotation, rather than distraction.

The effect of patient positioning on lumbar lordosis must be considered. A four-poster frame or similar positioning device is recommended for all patients undergoing posterior scoliosis surgery. If the fusion must extend into the lower lumbar spine or sacrum, additional pads are used to fully extend the hips and preserve normal lumbar lordosis. Under no circumstances should a patient be placed in the 90°/90 position for lumbar scoliosis fusion.

Summary

Symptomatic loss of lumbar lordosis is a disabling complication of scoliosis surgery. Loss of sagittal plane balance can produce flat-back syndrome, which is characterized by back pain and an inability to stand erect. Distraction instrumentation into the lower lumbar spine or sacrum is the most frequently identified factor responsible for symptomatic flat back, although loss of lumbar lordosis may occur with modern segmental implant systems if precautions to preserve lumbar lordosis are not taken. Other factors that may aggravate the loss of lumbar lordosis include thoracolumbar kyphosis, fixed thoracic kyphosis, hip flexion contractures, and pseudarthrosis.

There are wide ranges of "normal" for thoracic kyphosis and lumbar lordosis. Absolute values are less important than the overall sagittal balance as determined on a full-length, standing lateral radiograph of the spine. The SVA is determined by measuring the distance from the anterior aspect of the sacrum to a plumb line dropped from the seventh cervical vertebra. The SVA should fall no more than 2 cm anterior to the sacrum.

The surgical treatment of flat-back syndrome is complex, and the risk of complications is great. Correction typically involves posterior closing wedge osteotomies through the previous fusion mass. In most cases, an anterior spinal fusion should be performed in conjunction with posterior osteotomies and instrumentation.

The most important aspect related to this postural disorder is prevention. When fusion to the lower lumbar spine or sacrum is necessary, pay close to the maintenance of balanced sagittal contours. Remember that distraction forces are "kyphosing," and compression forces are "lordosing." Avoid distraction instrumentation into the lower lumbar spine and remember to position the patients prone with the hips extended.

Selected References