Flat Back Syndrome (FBS) is defined by the loss of the natural inward curve, or lordosis, in the lower spine (lumbar spine). This flattening creates a spinal deformity that disrupts the body’s alignment in the front-to-back plane, known as sagittal imbalance. When the lumbar spine becomes too straight, the body’s center of gravity shifts forward, causing the individual to lean forward. The chronic effort required to maintain an upright posture often leads to severe back pain, muscle fatigue, and difficulty standing for extended periods.
Understanding Normal Spinal Curvature
The human spine possesses a series of alternating curves that absorb shock and maintain balance. This side-view geometry includes an inward curve in the neck (cervical lordosis) and the lower back (lumbar lordosis), balanced by an outward curve in the mid-back (thoracic kyphosis). The coordinated harmony of these curves ensures the head is positioned directly over the pelvis, minimizing the muscular effort required to stay upright.
The lumbar lordosis aligns the upper body’s weight over the hips and provides necessary shock absorption. A healthy lumbar spine typically exhibits an inward curve measuring between 40 and 60 degrees. This curvature is a fundamental component of sagittal balance, allowing the spine to distribute forces evenly and maintain a stable posture. When this inward curve is reduced or eliminated, the spinal column’s ability to maintain a balanced, vertical line is compromised.
Flat Back Syndrome Caused by Previous Surgery
Historically, a common cause of FBS was iatrogenic, meaning it was an unintended consequence of earlier spinal corrective surgeries. This issue is primarily associated with Harrington rods, which were standard for scoliosis treatment from the 1960s through the early 1990s. These rigid, straight stainless steel rods were designed to straighten the spine in the coronal plane.
The mechanical flaw was that these straight rods were often extended into the lumbar spine, forcing it to conform to the rod’s straight shape. This technique eliminated the patient’s natural lumbar lordosis, which the instrumentation could not preserve or recreate. While unfused segments could temporarily compensate for this flatness immediately after surgery, long-term issues arose.
Over years or decades, the spinal segments below the rigid, fused area began to degenerate under excessive stress. The discs and joints below the fusion wore out prematurely, leading to a forward tilt and loss of height in the lower discs. This progressive degeneration resulted in the pronounced forward-leaning posture characteristic of Flat Back Syndrome.
Modern spinal fusion techniques, common after the Harrington era, mitigate this risk by using pedicle screws and segmental fixation to restore lumbar lordosis during the procedure. However, patients who received the older instrumentation decades ago continue to present with this delayed, surgery-induced form of the syndrome.
Underlying Degenerative and Inflammatory Conditions
Flat Back Syndrome is also caused by non-surgical, progressive conditions that gradually erode the lumbar spine’s inward curve. Degenerative Disc Disease (DDD) is a frequent culprit, as intervertebral discs lose water content and height over time. When multiple lumbar discs collapse and flatten, the wedge shape contributing to lordosis is lost, diminishing the overall curvature.
Another common cause involves vertebral compression fractures, often resulting from osteoporosis. When the front part of a vertebral body fractures and collapses, it creates a wedge shape that reverses the normal lumbar curve. This wedging effect reduces the spine’s overall lordosis and pushes it toward a more kyphotic, or outward, curvature, contributing to the flat back deformity.
Certain inflammatory conditions can also lead to the syndrome by causing the spine to fuse in a straightened position. Ankylosing Spondylitis, a chronic inflammatory arthritis, causes vertebral bodies to progressively fuse together. If this natural fusion occurs with reduced lordosis, the resulting spinal rigidity permanently locks the patient into a flat or forward-stooped posture.
Modern Surgical Failure
Even modern spinal fusions can progress to FBS if the original surgery fails to achieve the specific amount of lordosis needed to keep the patient balanced. This failure can potentially lead to adjacent segment disease above or below the fused area.