Pseudarthrosis/Nonunion

Introduction

Degenerative disease of the lumbar spine is a common cause of disability and pain, with lumbar spondylolisthesis affecting 11.5% of the US population and lumbar stenosis affecting more than 200,000 adults in the United States. Spinal fusion surgery is indicated in select patients who have failed conservative medical management of symptomatic degenerative disease of the lumbar spine. The ultimate goal of surgery is to relieve pain, improve disability, and provide stability via bony fusion to prevent motion at the degenerated segment(s). Rates of lumbar fusion surgeries in the United States are rising substantially, with a 220% increase in the number of lumbar fusion surgeries performed from 1991 to 2001. Although later analyses show a slight plateau in rates of lumbar fusion since the 1990s, complex fusion procedures are becoming more frequent, with a 15-fold increase in complex lumbar fusions from 2002 to 2007.

Pseudarthrosis, also known as nonunion, is a relatively common complication encountered after lumbar fusion surgery that can be catastrophic to the integrity of the fusion construct. Complications including mechanical instability, instrumentation failure, and proximal and distal junctional failure/kyphosis may occur as a result of nonunion. Furthermore, patients with pseudarthrosis may experience further pain and disability in addition to their presenting symptoms that can lead to the progression of the disease and the need for reoperation.

In this chapter, the authors review the definition of lumbar pseudarthrosis, risk factors for developing pseudarthrosis, methods of prevention, diagnostic methods for evaluation, and the mechanical biology of bone healing.

Lumbar Pseudarthrosis Definition, Epidemiology, and Diagnosis

Pseudarthrosis, or nonunion, is defined as the failure of postoperative bony fusion after spinal surgery, resulting in the potential for mechanical instability. Pseudarthrosis may or may not be symptomatic. Symptoms, when they do occur, primarily consist of low back pain and spinal deformity. Fig. 23.1 demonstrates an example of a pseudarthrosis. Heggeness and Esses proposed a posterior lumbar pseudarthrosis classification system in 1991, with four different morphologies depending on bony fusion construct geometry: atrophic, transverse, shingle, and complex, with atrophic being the most common subtype.

The true incidence of pseudarthrosis is difficult to properly assess because of the number of patients who may be asymptomatic or have undiagnosed failure of bony fusion. Martin et al. reported that 471 of 2345 patients who underwent lumbar fusion in one series from 1990 to 1993 required reoperation after fusion, and 111 patients (23.6% of 471 patients who required reoperation or 4.7% of 2345 total) underwent reoperation for pseudarthrosis. In a systematic review of lumbar pseudarthrosis, Chun et al. reported an incidence of 5% to 35% after lumbar fusion surgeries and noted that the incidence increased as the number of levels fused increased.

Several randomized trials have been completed to discern whether pseudarthrosis rates/rates of fusion are lower in patients who undergo instrumented fusion versus decompression and fusion alone and to discern whether the choice of surgical approach affects lumbar fusion rates. Fischgrund et al. randomized patients to decompression and posterolateral fusion alone or to decompression and fusion with instrumentation; they found that arthrodesis occurred by 2 years in 82% of the instrumented cases versus 45% of the noninstrumented cases ( P =.0015). At follow-up of at least 5 years, the patients with solid fusions had better pain/clinical outcomes than the patients who did not.

Regarding fusion rates by surgical approach, Christensen et al. reported that patients randomized to circumferential fusion (anterior lumbar interbody fusion and posterolateral fusion) had higher rates of solid fusion (92% vs. 80%, P < .04) than patients who underwent posterolateral fusion. However, Lee et al. performed a systematic review of randomized trials comparing fusion rates of different surgical approaches and found that no firm conclusions could be drawn from the available data on surgical approaches and lumbar fusion rates.