Flatback Syndrome

Flatback syndrome is loss of the normal lumbar lordosis, producing positive sagittal balance: the C7 plumb line falls anterior to the posterosuperior corner of S1 and the patient cannot hold an upright posture without significant compensation. A normal spine carries the head centred over the pelvis with minimal energy expenditure. In flatback the head sits anterior to the pelvis and the posture is held by constant muscular work.

Who gets it. Flatback followed 20-40% of Harrington rod instrumentations; after modern spinal fusion the prevalence is 5-10%. Iatrogenic cases peak at 50-70 years, degenerative cases at a variable age, and women outnumber men about 2:1 because more of them have had fusions for scoliosis.

Risk factors. Four groups:

• Iatrogenic: Harrington rods, long fusion to the sacrum, flat rod contouring
• Degenerative: disc collapse, compression fractures, ankylosing spondylitis
• Post-surgical: pseudarthrosis, junctional kyphosis, hardware failure
• Patient factors: osteoporosis, smoking, obesity, prior laminectomy

The Harrington era. From the 1960s to the 1990s scoliosis was corrected with distraction instrumentation, which straightened the spine in the coronal plane and eliminated lumbar lordosis in the sagittal plane. The result was an epidemic of flatback syndrome. Modern instrumentation preserves lordosis with contoured rods and pedicle screw fixation.

Normal sagittal alignment. The spine balances through reciprocal curves: cervical lordosis 20-40 degrees, thoracic kyphosis 20-40 degrees and lumbar lordosis 40-60 degrees from L1 to S1. When the C7 plumb line falls within 2-5cm of the posterosuperior corner of S1, standing needs minimal muscular effort. That is the cone of economy: the small region in which the trunk can be held with little muscular work, and the region a flat lumbar segment pushes the patient out of.

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Pelvic parameters. Three angles describe the pelvis, and only one of them is fixed:

PI = PT + SS, always, which is a check on the measurement. Lumbar lordosis should approximately equal pelvic incidence, with a mismatch (PI-LL) under 10 degrees. PI cannot change, so once lordosis falls below it the pelvis is the first thing to move.

How imbalance develops. The sequence is the same whatever the cause.

• Stage 1, loss of lordosis. Flat rod contouring or distraction forces in the iatrogenic case, disc collapse or compression fractures in the degenerative one. Lumbar lordosis falls below PI.
• Stage 2, pelvic compensation. The pelvis rotates posteriorly: PT rises and SS falls, bringing the C7 plumb line back over the sacrum at a moderate increase in energy cost.
• Stage 3, hip and knee compensation. Once the pelvis has nothing more to give, the hips extend (limited by the hip flexors) and the knees flex to shift the centre of mass posteriorly. The energy cost is now severe and fatigue comes early.
• Stage 4, decompensation. Every mechanism is exhausted. The trunk lean becomes fixed, the patient cannot stand upright without support, and disability is severe.

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Iatrogenic flatback, the most common cause. The Harrington construct combined distraction without lordosis preservation, long fusions often to L5 or the sacrum, and flat rods with no sagittal profile, and it removed the lumbar lordosis entirely. Modern instrumentation can produce the same deformity:

• Undercorrection of lordosis in long fusions
• Flat rod placement in degenerative fusions
• Pseudarthrosis with rod fracture and kyphosis
• Adjacent segment degeneration above the fusion

Prevention. Contour the rods to restore a lordosis matched to the pelvic incidence, control it with pedicle screw fixation, use interbody cages with lordotic angles of 20-30 degrees, and avoid long fusions unless they are necessary.

Degenerative flatback. Lordosis is lost progressively over time. Multiple-level disc collapse loses height anteriorly and with it the disc lordosis, and facet arthropathy prevents extension. Osteoporotic compression fractures, thoracolumbar Scheuermann kyphosis and the fixed kyphosis of ankylosing spondylitis do the same from the vertebrae. After extensive laminectomy the facets become incompetent and progressive kyphosis follows, especially at the thoracolumbar junction.

Post-traumatic flatback. Alignment changes through bony collapse or malunion: burst fractures with anterior column collapse, compression fractures at several levels, or a fracture-dislocation that has malunited. Pelvic injuries act through the pelvic parameters, a sacral fracture changing the sacral slope and an acetabular fracture altering hip mechanics.

The cardinal symptom is forward trunk inclination. Patients say they cannot stand upright, that they are "looking at the ground" when they walk, that they need to lean on a shopping trolley or walker, and that standing or walking exhausts them. Standing tolerance is often under 15-30 minutes and walking distance is limited by fatigue, not neurogenic claudication. The social cost is real: they cannot make eye contact, and cannot stand long enough to cook, shop or socialise.

Pain. It follows the compensation, so it is felt where the work is being done:

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Inspection. The trunk leans forward and the patient cannot stand upright without support. The knees are flexed in compensation, hip extension is limited by tight hip flexors, and the gait is shuffling, antalgic and requires an assistive device.

Palpation and movement. The paraspinal muscles are in spasm and tender, the lumbar lordotic curve is gone, there is a step-off if a spondylolisthesis is present, and hardware is palpable after prior surgery. Lumbar extension is severely limited or absent while flexion is often preserved, and hip extension is limited, with a positive Thomas test.

Neurological examination. Usually normal: this is not a neurological condition. Look for radiculopathy if there is foraminal stenosis, and for myelopathy if the cervical spine is compensating.

Quality of life. The Oswestry Disability Index correlates strongly with SVA, SF-36 physical function is severely impaired, and depression and anxiety are common with the chronic pain and disability.

The standing full-length lateral radiograph. Everything in this topic is measured on one film, and the film is only valid if it is taken properly:

• True standing position, no leaning, no support
• Arms positioned with the fists on the clavicles or grasping horizontal bars, to clear the thoracic spine
• Field from the skull to the femoral heads
• Cassette at least 36 inches long

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Sagittal vertical axis. Drop a plumb line from the centre of the C7 vertebral body and measure its horizontal distance from the posterosuperior corner of S1. A positive value means C7 sits anterior to S1, the flatback direction. Under 5cm is normal and goes with minimal disability; 5-10cm is abnormal, with moderate disability; over 10cm is severe.

The pelvis and the lordosis on the flatback film. The parameters are defined in Pathophysiology; on the film of an established flatback PT has risen above 25 degrees as compensation is spent, SS has fallen below 30 degrees, and lumbar lordosis, measured as the Cobb angle from the superior endplate of L1 to the superior endplate of S1, is under 30 degrees. Subtract that lordosis from the pelvic incidence: a PI-LL under 10 degrees is normal, over 10 degrees is symptomatic, and over 20 degrees is severe.

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CT is the planning study: bony anatomy and pedicle size, the prior fusion mass, the hardware (loosening, fracture) and the choice of osteotomy level. MRI answers the neurological and soft-tissue questions: neural compression from stenosis or foraminal narrowing, disc degeneration and how much disc health remains, fluid signal at a fusion site suggesting pseudarthrosis, and cord integrity if myelopathy is suspected. Flexion-extension radiographs show whether the adjacent segments move, pick up motion through a pseudarthrosis, and tell you which segments are mobile and which are fused when the osteotomy is being planned.

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Ames-ISSG classification. The most used grading of flatback deformity, built from SVA and PT, and it maps onto management:

What conservative treatment can do. Its efficacy in established flatback is limited: it may relieve symptoms, and it does not correct the deformity or restore lost lordosis. It is still the first step.

Physiotherapy. The goals are to strengthen the core, lengthen the hip flexors and keep whatever lumbar motion remains:

Medication. Nothing here treats the mechanics:

Injections. An epidural steroid injection is for radiculopathy from foraminal stenosis and gives temporary relief, weeks to months; it does nothing for the mechanical back pain of imbalance. Facet injections are for facet-mediated pain, with variable and temporary effect, and are not a treatment for sagittal imbalance.

Assistive devices. A walker with forearm supports unloads the spine and permits the forward lean, at the cost of dependence and social stigma. A cane gives minimal support and is insufficient for severe flatback. A corset or brace may reduce pain and does not correct the deformity.

Natural history if untreated. SVA and PT worsen progressively, pain and disability increase, the segment above a prior fusion degenerates, and the compensating hips and knees develop arthritis.

Indications for surgery. Any of:

• Failed conservative management after a 3-6 month trial
• Severe disability, ODI over 40
• SVA over 5cm with symptoms
• PI-LL mismatch over 10 degrees with symptoms
• Progressive deformity

Goals. Radiographically the operation aims for an SVA under 5cm, ideally 0-3cm, a PI-LL mismatch under 10 degrees, a lumbar lordosis restored to match the pelvic incidence, and coronal balance maintained. The SRS-Schwab targets quoted in the guidelines section are tighter, SVA under 4cm and PT under 20-25 degrees, and age-adjusted targets deliberately leave older patients in mild positive balance. Clinically the aims are an upright posture without compensation, less pain and disability, better walking tolerance and daily function, and the fewest complications.

Choosing the osteotomy. Three operations, in ascending order of power and risk. The choice turns on how much correction is needed and whether the anterior column is mobile or fused.

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Planning the correction. The radiographic parameters set the size of the operation, and the patient's bone sets its limits:

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Choosing the level. L3 is the most common PSO level: a large body, safer than L4-5, and the best balance of safety and correction:

The distal foundation. Osteotomy level and correction magnitude get the attention, but the construct fails at its ends, and the distal end of a long lumbar reconstruction is the harder problem. A long fusion stopping at S1 alone loads two S1 screws with the entire cantilever of a corrected lumbar spine, and the predictable results are S1 screw loosening or pull-out and lumbosacral pseudarthrosis. Pelvic fixation exists to protect the S1 screws and the lumbosacral fusion by moving the distal anchor beyond the sacrum, and S2-alar-iliac screws are generally preferred over classic iliac screws because their medial, in-line starting point reduces implant prominence and avoids a separate offset connector.

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Pedicle subtraction osteotomy, step by step. The patient is prone on a Jackson table or radiolucent frame with the hips extended to allow lordosis, the arms abducted to 90 degrees and the neuromonitoring leads placed. A single-level PSO typically takes 4-6 hours.

1. Exposure. Midline incision and subperiosteal dissection
2. Screws. Pedicle screws 3-4 levels above and below the osteotomy
3. Temporary rods. Placed to hold the spine stable while the osteotomy is made
4. Laminectomy. Complete removal of the lamina and spinous process at the osteotomy level
5. Facetectomy. Inferior and superior facets bilaterally
6. Pedicle resection. Both pedicles, identifying the nerve roots
7. Posterior body resection. The posterior third to half of the vertebral body, as a 30-40 degree wedge, tapered laterally so the cavity is egg-shaped
8. Nerve root decompression. Mobilise the roots and protect them through closure
9. Closure. Remove the temporary rods, place the final rods pre-contoured with lordosis, and close the osteotomy by cantilever slowly, 1-2mm at a time, checking the neuromonitoring after each increment
10. Compress the instrumentation, decorticate the fusion bed and apply bone graft and BMP
11. Haemostasis and closure. Large drains, subfascial, with layered closure over them

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Neurological injury. The rate of a new or worsening deficit after PSO is 10-30%, and most recover: 50% have resolved by 1 year and 5-10% are permanent, with motor deficit the most concerning. Prevention is slow closure under continuous neuromonitoring, adequate decompression and no overstretching of the neural elements.

Vascular injury. Prevention is careful anterior dissection, staying midline and palpating the aorta.

Haemorrhage. Average blood loss is 1-3 litres for a PSO and 3-5 litres for a VCR, and 70-90% of patients are transfused. Consider cell salvage and antifibrinolytics (tranexamic acid).

The first six weeks. Epidural haematoma occurs in 2-5% and may need evacuation; nerve root injury in 5-10%, usually transient. Wound problems are superficial infection 5-10%, deep infection 3-5%, seroma or haematoma 5-10% and CSF leak 3-5%; prophylactic antibiotics, meticulous haemostasis and vancomycin powder are the defence against infection. The medical complications have their own prevention:

Proximal junctional kyphosis. Kyphosis over 10 degrees at the upper instrumented vertebra, seen in 20-40% after PSO; 10-15% need revision. The risk factors are stopping the construct at the thoracolumbar junction, osteoporosis and overcorrection, so prevention is to extend the construct to the upper thoracic spine (T9-T10), avoid overcorrection and add prophylactic vertebroplasty at the upper instrumented vertebra. Treatment is extension of the fusion if it is symptomatic or progressive.

Rod fracture. 10-20% overall, higher when the postoperative SVA is over 5cm, and it presents between 6 months and 2 years. Undercorrection, pseudarthrosis and single rods are the risk factors; adequate SVA correction, dual rods and a solid fusion are the prevention, and revision is for symptomatic or progressive deformity.

Pseudarthrosis. 10-20% at the osteotomy site, with smoking, osteoporosis, infection and rod fracture as its risk factors. It is diagnosed on CT, motion or lucency at the site, or declared by a rod fracture, and treatment is revision fusion with BMP and anterior support, needed in 5-10%. BMP and bone graft at the index operation, stopping smoking and optimising nutrition are the prevention.

Implant failure and reoperation. Screw pull-out occurs in 5-10%, from osteoporosis or the forces of overcorrection, screw fracture in 2-5% and cross-link fracture in 5-10%. Reoperation for any reason runs at 20-30% at 2-5 years, driven by rod fracture, PJK and pseudarthrosis.

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The first two days. The patient is monitored in intensive care: haemodynamic stability, because the blood loss is often significant; a neurological examination every 2 hours; drain output; and pain control with a PCA or epidural. Mobilisation starts early, out of bed to a chair on day 1 if stable and walking with the physiotherapist on days 2-3.

Days 3 to 7. Daily neurological examination, drains out when output is under 30mL per 8 hours, early AP and lateral radiographs to check the alignment, and a transition to oral analgesia. Physiotherapy progresses ambulation, isometric core work, daily-living tasks and stairs before discharge. The patient goes home when haemodynamically stable, neurologically stable or improving, controlled on oral analgesia, ambulating independently or with a walker, and drain-free.

Follow-up. At each visit the standing films are read for the same five things: SVA (goal under 5cm), PI-LL (goal under 10 degrees), hardware integrity, bridging bone across the fusion, and any proximal junctional kyphosis.

Activity.

Bracing. Not usually needed: rigid pedicle screw fixation provides the stability and bracing does not improve fusion rates with modern instrumentation, though a soft lumbar corset may be worn for comfort. The exceptions are osteoporotic bone where screw pull-out is a concern, an extended laminectomy where instability is a concern, and patient preference.

Global Epidemiology

Flatback historically affected 20-40% of patients after Harrington distraction instrumentation; with lordosis-preserving pedicle screw constructs the rate of clinically significant iatrogenic flatback has fallen to roughly 5-10% after long fusion. Adult spinal deformity overall affects up to 60% of people over 60 years on radiographic survey, with a female predominance reflecting the higher rate of scoliosis fusion.

Side-by-Side Society and Group Guidance

Registry and Outcome Notes

• Multicentre deformity databases (e.g. ISSG, European Spine Study Group) report symptomatic rod fracture of 6-8% overall and approximately 16% after PSO, concentrated at the osteotomy site.
• Reoperation rates of 20-30% at 5 years are consistent across large series, driven by rod fracture, PJK and pseudarthrosis.
• Age-adjusted alignment is now standard practice: older patients are deliberately left in mild positive sagittal balance to reduce proximal junctional failure.

High- vs Limited-Resource Variation

• High-resource settings: Routine standing full-length EOS or stitched radiography, intraoperative neuromonitoring (SSEP/MEP/EMG), cell salvage, tranexamic acid, and dedicated spinal rehabilitation units.
• Limited-resource settings: Reliance on long-cassette plain films, selective neuromonitoring, and staged or lower-risk osteotomy choices; PSO/VCR may be deferred or referred to deformity centres given blood-loss and monitoring requirements.

Perioperative Optimisation (Globally Applicable)

• Bone health: DEXA, treat osteoporosis (e.g. teriparatide, antiresorptives) before major instrumented correction.
• Antifibrinolytics (tranexamic acid) and cell salvage to manage 1-3 litre blood loss in PSO.
• Mandatory intraoperative neuromonitoring for all three-column osteotomies.
• Nutritional and cardiopulmonary optimisation; smoking cessation to reduce pseudarthrosis.

Infection Prophylaxis

• Cefazolin 2g IV at induction (3g if over 120kg), re-dosed every 3-4 hours intraoperatively; vancomycin 15mg/kg if beta-lactam allergy.
• Intrawound vancomycin powder is widely used to reduce deep surgical site infection in instrumented deformity surgery.
• Continue systemic prophylaxis for no more than 24 hours postoperatively.

Venous Thromboembolism Prophylaxis

• Mechanical prophylaxis (sequential compression devices) intraoperatively and until mobile.
• Chemical prophylaxis (e.g. low-molecular-weight heparin) typically started 24-48 hours postoperatively once epidural haematoma risk is acceptable, balancing bleeding against thromboembolic risk.
• Early mobilisation is the single most effective measure.

Blood and Bone Management

• Tranexamic acid and cell salvage to manage 1-3 litre blood loss in PSO (3-5 litres in VCR).
• Bone morphogenetic protein and autograft/allograft at the osteotomy and fusion bed; optimise osteoporosis preoperatively.

This topic provides comprehensive coverage of flatback syndrome pathophysiology, radiographic assessment (SVA, PI-LL mismatch), conservative management, and surgical correction with Smith-Petersen, PSO, and VCR osteotomies for global fellowship examination preparation.

Patient-reported outcomes. At 2 years the improvements are large:

Satisfaction. 70-80% of patients are satisfied or very satisfied, 10-20% satisfied with reservations and 5-10% dissatisfied.

Radiographic outcomes. The average SVA falls from 12-15cm to 3-5cm, an 8-10cm gain; the target of under 5cm is achieved in 70-80% and correlates with satisfaction. The average PI-LL falls from 25-35 degrees to 5-15 degrees, a 20-25 degree improvement; the target of under 10 degrees is achieved in 60-70% and correlates with rod fracture risk.

What predicts the result. A postoperative SVA under 5cm, a PI-LL under 10 degrees, a solid fusion at 1 year and no neurological complication predict a good outcome. Undercorrection with an SVA over 5cm, rod fracture, proximal junctional kyphosis, neurological injury and infection predict a poor one.

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