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.
Pathogenesis 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
loss of lumbar lordosis
pseudarthrosis with loss of sagittal plane correction
fixed thoracic hyperkyphosis
hip flexion contractures.
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
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.
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.
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.
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
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
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
Instrumentation and Fusion to the Sacrum Versus Stopping Short of the
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
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.
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.
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
It is clear that incomplete correction of the sagittal plane imbalance
often leads to pseudarthrosis, progressive loss of correction, and
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
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.
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
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
LaGrone, M.O., Bradford, D.S., Moe, J.H. et al: Treatment
of symptomatic flat back after spinal fusion. J. Bone Joint Surg. Am.
LaGrone, M.O. Loss of Lumbar Lordosis: A complication of
spinal fusion for scoliosis. Orthop Clin North Am 19:383-393, 1988.
LaGrone, M.O.: Flatback Syndrome: Avoidance and
Treatment. Seminars in Spinal Surgery 10(4): 328-338, 1998.