Spinal fusion surgery is a common and effective procedure used to treat various spinal conditions, including degenerative disc disease, spinal stenosis, and instability. While successful in stabilizing a painful segment of the spine, this procedure can sometimes lead to long-term complications. One such complication is Adjacent Segment Disease (ASD), which can present years after the initial surgery. Understanding ASD involves recognizing the underlying changes in spinal mechanics that follow a fusion and how those changes cause new symptoms.
Defining Adjacent Segment Disease
Adjacent Segment Disease (ASD) refers to the development of new degenerative changes in the vertebral segment immediately above or below the area that was surgically fused. This condition is a clinical syndrome, meaning it includes structural deterioration alongside symptoms like pain and neurological deficits. A diagnosis of ASD requires both radiographic changes and the onset of new patient complaints that correlate with degeneration at that specific level.
It is important to distinguish ASD from Adjacent Segment Degeneration (ASDeg), which describes structural changes visible on imaging without the patient experiencing new symptoms. Radiological evidence of degeneration, such as disc height collapse or facet joint arthritis, is frequently observed post-fusion. However, only a fraction of patients with ASDeg develop the symptomatic condition. The failure of the fusion itself, known as pseudoarthrosis, is a separate complication and is not considered part of ASD.
The Biomechanical Mechanism
Spinal fusion eliminates motion at a specific level, inherently altering the natural mechanical forces across the rest of the spine. The unfused segments must compensate for this loss of movement. This compensation translates into a redistribution of load and an accelerated rate of wear on the adjacent intervertebral discs and facet joints. The rigid fusion construct effectively transfers the forces of bending, twisting, and compression to the next mobile segment.
This increased mechanical stress causes adjacent segments to absorb a disproportionately greater range of motion and pressure. Research indicates that the range of motion and stress in the adjacent discs and facet joints increase following a fusion. This biomechanical overload can lead to microdamage in the disc’s outer layer, which accelerates the breakdown of disc tissue. This increased cyclic axial loading reduces the strength of the disc’s structural components, explaining how degeneration progresses.
The long-term consequence of this chronic stress is the premature development of degenerative conditions, such as disc herniation, spinal stenosis, or instability. Computer simulations suggest that the overall range of motion of the spine decreases after fusion, forcing adjacent segments to take on more load to maintain function. The number of fused levels significantly impacts this mechanism, as longer fusion constructs redistribute the load across fewer remaining mobile segments, leading to a higher risk of accelerated degeneration.
Recognizing Symptoms and Confirmation
ASD often manifests with symptoms similar to those that prompted the original fusion surgery, but localized to a new spinal level. Patients commonly report new onset of pain in the back or neck, often near the site of the previous operation. This localized pain can be accompanied by symptoms of nerve compression, known as radiculopathy, which is pain that radiates into the arms or legs.
In more advanced cases, degeneration can lead to spinal cord compression, causing myelopathy. Symptoms of myelopathy include weakness, numbness, or tingling in the extremities, difficulty with balance, or fine motor skills. Patients might also experience stiffness and reduced mobility due to the new degenerative changes. These new symptoms typically emerge gradually, often several years after the initial successful fusion procedure.
Diagnosing ASD requires a comprehensive evaluation combining clinical symptoms with objective imaging findings. Standard X-rays assess spinal alignment and stability, revealing structural changes like disc space narrowing. Detailed imaging, such as MRI or CT scans, confirms degeneration, identifies instability, and determines if there is compression of the spinal cord or nerve roots. A confirmed diagnosis is established when these imaging findings correlate directly with the patient’s new, localized symptoms.
Comprehensive Treatment Strategies
The management of ASD typically begins with non-surgical, conservative treatment options, mirroring the approach for many other spinal conditions. These treatments aim to alleviate pain and inflammation, allowing the patient to maintain function and potentially avoid further surgery. Non-steroidal anti-inflammatory drugs (NSAIDs) and other pain medications are commonly used. Physical therapy focuses on improving body mechanics, strengthening core muscles, and restoring flexibility to reduce stress on the spine.
For targeted pain relief, image-guided steroid injections can be administered into the affected area, such as the epidural space or facet joints, to reduce local inflammation. While conservative methods cannot reverse structural degeneration, they often provide sufficient relief for many patients with early-stage ASD. The decision to progress beyond these initial treatments is based on the severity of the patient’s pain and whether neurological function is compromised.
If conservative treatments fail, or if there is progressive neurological compromise or significant spinal instability, surgical revision may be necessary. The most common surgical approach is an extension of the original spinal fusion, which involves fusing the newly symptomatic adjacent segment to the existing instrumentation. This procedure stabilizes the segment and decompresses any pinched nerves or the spinal cord.
Alternatively, motion-sparing devices, such as artificial disc replacement, may be considered, particularly in the cervical spine. Artificial disc replacement preserves motion at the affected level, potentially reducing mechanical stress on the next adjacent segment compared to traditional fusion. The choice between additional fusion and motion preservation depends on the specific pathology, the extent of degeneration, and the surgeon’s assessment.
Reducing the Risk
Proactive measures and careful surgical planning can help mitigate the likelihood of developing ASD after spinal fusion. Patient-related factors are a significant area for modification, both before and after the initial surgery. Maintaining a healthy body weight is important, as excess weight increases mechanical loading and pressure on the entire spine, accelerating degeneration in the adjacent discs.
Quitting the use of tobacco products, including smoking and vaping, is strongly recommended, as nicotine accelerates disc degeneration throughout the spine. Post-surgery, focusing on core strength and physical conditioning improves body mechanics and provides better support for the spinal column, reducing stress on the segments above and below the fusion.
From a surgical perspective, optimizing spinal alignment, particularly the natural forward curvature (lordosis), during the initial fusion procedure is beneficial. Choosing the appropriate fusion length is also a factor, as limiting the number of fused levels may reduce the stress transferred to adjacent segments. When feasible, motion-preserving techniques, such as artificial disc replacement, may be considered as an alternative to fusion to lessen the biomechanical impact on adjacent levels.