Pseudarthrosis (Spinal Nonunion)

Overview

Pseudarthrosis, also called spinal nonunion or failed arthrodesis, occurs when a surgically attempted spinal fusion does not achieve solid bony union within the expected healing window, generally defined as the persistent absence of bridging bone twelve months or more after the operation. A spinal fusion is designed to convert a mobile segment of the spine into a single solid bony mass by stimulating bone growth across prepared surfaces, an interbody space, or a posterior gutter, usually supplemented with hardware such as pedicle screws, rods, plates, or interbody cages, along with bone graft or biologic materials. When the normal healing sequence is disrupted, fibrous or fibrocartilaginous tissue fills the intended bone bridge instead, leaving a residual mobile segment. That persistent micromotion imposes abnormal load on the hardware and surrounding bone, which can lead to screw loosening, rod fracture, cage subsidence, and recurrent nerve root compression.

What causes it

Several patient-related and technical factors raise the likelihood that a fusion will not consolidate. The dominant modifiable risk factor is nicotine use: smoking directly impairs the vascular ingrowth and osteoblast activity that bone healing requires, and the effect is substantial. Other modifiable contributors include diabetes mellitus, obesity, osteoporosis or poor bone quality, chronic use of corticosteroids or NSAIDs, malnutrition, and chronic systemic illness. From a surgical and mechanical standpoint, multilevel constructs, revision surgery, inadequate preparation of bone surfaces, insufficient or poorly placed graft, failure to achieve rigid fixation, long unsupported segments, and fusions at mechanically demanding junctions, such as the cervicothoracic or lumbosacral junction, all increase risk. Osteobiologic agents such as bone morphogenetic protein are sometimes used in higher-risk patients to improve the biological environment for fusion.

Symptoms and warning signs

The classic presentation is a recurrence or persistence of axial back or neck pain after an initial period of postoperative improvement, sometimes following several months without symptoms. The pain is typically mechanical (worse with activity, loading, and position changes) and may be accompanied by recurrent arm or leg radiculopathy when residual segment motion or hardware migration irritates a nerve root, producing dermatomal pain, tingling, numbness, or weakness. Physical examination may reveal focal tenderness over the operated levels and reproduction of symptoms with provocative loading.

Certain features should prompt urgent evaluation rather than watchful waiting. Fever, night sweats, unexplained weight loss, or escalating rest pain may point to a deep surgical site infection or, rarely, malignancy. Sudden saddle numbness, loss of bowel or bladder control, or rapidly progressive leg weakness are warning signs of cauda equina syndrome or spinal cord compression and require emergent imaging and intervention rather than continued conservative management.

How it's diagnosed

Because radiographic nonunion is not always symptomatic, and reported pain is not always caused by nonunion, diagnosis requires careful correlation of symptoms with imaging. Plain radiographs, including flexion-extension views, are the first step and may reveal motion across the segment, screw lucency or loosening, hardware breakage, or absent bridging trabeculae. Thin-slice computed tomography (CT) with multiplanar reconstruction is the most accurate noninvasive test for confirming or excluding solid bony union and is generally considered the reference standard. MRI is most useful for evaluating recurrent nerve compression, adjacent segment disease, and possible infection or tumor, though metal artifact can limit its clarity. Laboratory markers such as C-reactive protein and erythrocyte sedimentation rate help screen for infection; image-guided aspiration or biopsy is occasionally needed when infection or malignancy cannot be excluded. Symptoms that seem disproportionate to imaging findings should also prompt consideration of other causes, including hip osteoarthritis, shoulder pathology, and peripheral neuropathy.

Treatment options

In patients with tolerable, stable symptoms, management begins without surgery. Conservative measures include activity modification, smoking cessation (especially critical for those still smoking), optimization of bone health with calcium and vitamin D supplementation and treatment of underlying osteoporosis, physical therapy, and analgesics. Electrical bone growth stimulators (such as pulsed electromagnetic field or direct-current devices) are sometimes used as an adjunct to encourage bone formation at the fusion site. Low-intensity ultrasound devices are used for long-bone fracture nonunion, but their use for spinal fusion is investigational and not well established. Selective image-guided injections can help identify the pain source and provide temporary relief while conservative management continues.

When surgery is considered

Surgical revision is indicated for symptomatic confirmed pseudarthrosis when disabling pain persists, progressive deformity develops, hardware fails, or recurrent neurologic compromise does not respond to conservative care. Surgery becomes urgent when there is significant neural compression, spinal instability, or proven deep infection. The revision strategy is individualized and typically involves re-exploration, revision or supplemental instrumentation, more aggressive bone grafting, and in many cases the addition of anterior or interbody support to create a more favorable biomechanical and biological environment. Addressing modifiable factors, particularly stopping smoking and treating metabolic bone disease, before or at the time of revision is an important part of the overall plan, as outcomes improve when these contributors are corrected and when surgery is selected for patients whose radiographic nonunion clearly accounts for their clinical syndrome.