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Lumbar Pseudoarthrosis
Evaluation and Diagnostic Work-Up
Lumbar arthrodesis is a commonly employed treatment strategy in numerous spinal conditions, including degenerative pathologies, infection, tumor, trauma, and deformity. As the relative proportion of older individuals in the United States has steadily increased, there has been a similar trend in the volume of elective lumbar fusions performed, which has increased by 62.3% from 2004 to 2015.1 Despite improvements in surgical techniques, lumbar pseudoarthrosis, often defined as a failure to achieve osseous fusion by 1-year postoperatively, remains a common reason for revision surgery. 2 The rates of pseudoarthrosis after lumbar spine fusions range from 5% to 35% and are dependent on several factors, including patient comorbidities, operative techniques, and diagnostic methods.3-6 While pseudoarthrosis may be asymptomatic in some patients, others can experience significant pain and disability that requiring surgical intervention.7 Given the diagnostic and technical challenges in the management of these patients, careful preoperative planning and intraoperative attention to detail are essential to optimize outcomes. The purpose of this review is to discuss current methods for the evaluation and diagnosis of lumbar pseudoarthrosis.
In cases of persistent axial low back or radicular pain, a thorough history and physical examination should be performed. Other potential causes of persistent pain, including infection, implant failure, or adjacent seg- ment disease, should be ruled out clinically and radiographically. 2 Several diagnostic modalities are available to guide clinicians with a suspicion of pseudoarthrosis.
While the utility of static radiographs is limited in diagnosing lumbar pseudoarthrosis, indirect signs include screw loosening (demonstrated by lucency or haloing around the screws), implant breakage, or progression of deformity across the fused segments.8 However, without these findings, the diagnosis of pseudoarthrosis cannot be definitively made, as static radiographs have a low sensitivity/specificity compared to other imaging modalities.9
In the setting of pseudoarthrosis, there will be angular motion across the construct in the sagittal plane, which may be evident on the flexion/extension radiographs.8 While theoretically no angular motion should exist after a solidly fused construct, in actuality, there is some residual motion, which is attributed to the elastic properties of bone and limits the diagnostic utility of dynamic radiographs.10
To better understand this phenomenon, Bono et al conducted a cadaveric study and demonstrated that up to 5 degrees of motion in the sagittal plane is acceptable in a solidly fused construct, and values exceeding this number suggest a pseudoarthrosis.10 Additional methods have been described to help guide clinicians in making a diagnosis. The Simmons method involves the identification of two landmarks on the anterior portion of the superior and inferior vertebral body, with lines drawn through them.11 When the angle subtended by the two lines increases by greater than 2 degrees with extension, non-union is presumed.11 Nevertheless, several studies have demonstrated poor interobserver agreement in determining pseudoarthrosis with flexion/ extension radiographs, as there remains no established criteria.12,13
With a high degree of clinical suspicion, thin-section computed tomography (CT) can be a reliable option to diagnose pseudoarthrosis, with advantages over traditional radiographs.2 Although there is no universally accepted criterion, common findings include a complete absence of continuous bony trabeculation between adjacent vertebrae or peri-implant lucency that may not be readily apparent from radiographs.14 CT demonstrates a superiority in visualizing early signs of arthrodesis compared to radiographs, which often underestimate the fusion rate in the initial 6 to 9 months postoperatively.8 Furthermore, thin-section CT offers clinicians the ability to closely evaluate the facets along with the posterolateral gutter for a solid fusion. Carreon et al conducted a retrospective review of 93 prerevision thin-section CT scans over 163 fused levels among patients who had revision surgery to assess the facet joints and posterolateral gutters for fusion.15 The study noted that the probability of a solid fusion intraoperatively was higher when both posterolateral gutters were fused on CT scan (89%) compared to when both facets were fused (74%).15 When bilateral facets and posterolateral gutters were fused on CT scan, the probability of a solid fusion on exploration was 96%.15 The evaluation of precise anatomic detail offered by thin-section CT is also beneficial in the setting of interbody fusion assessment, where it can aid in diagnosing a “locked pseudoarthrosis,” a condition that occurs when the graft has fused to the superior and inferior endplates but remains unfused to the cage.2
Buchowski et al conducted a prospective study of 14 patients who underwent anterior cervical fusions and found that CT most closely agrees with intraoperative findings of pseudoarthrosis when compared to radiographs or MRI ( p < 0.05).16 Nevertheless, CT does have some limitations, particularly in instrumented fusions, where the metallic artifact can obscure the image quality, leading to some studies demonstrating only moderate interobserver reliability.2
In MRI, solid arthrodesis is validated by high signal intensity on T1-weighted imaging and low signal intensity on T2-weighted imaging. 8 On the contrary, pseudoarthrosis is associated with edema, inflammation, and hyperemic changes within the construct and is resultantly exhibited by low T1 and high T2 signal intensities.8 The primary disadvantages of an MRI are the expense and the significant metal artifact that disrupts the overall image quality.2 As a result, MRI is typically considered only when other imaging modalities prove inconclusive.8
Bone scintigraphy (bone scan) uses radioactive isotopes, commonly 99mTc-labeled diphosphonate, which enhances tissue with high metabolic activity.14 While there remains a role for this imaging modality in diagnosing bone infections, neoplasms, and occult fracture, its value is limited in pseudoarthrosis, with low specificity and sensitivity reported by multiple studies.2,8,17
While further validation is warranted, recent studies have demonstrated the utility of integrated single-photon emission computed tomography (SPECT/CT). In some instances, CT can overestimate the lucency in the interbody space, which may not correspond to intraoperative findings.18 Rager et al conducted a retrospective review of SPECT/CT and CT scans alone of 10 patients with recurrent back and/ or leg pain with suspicion for pseudarthrosis.18 All patients with screw loosening on CT alone demonstrated an abnormal uptake on SPECT/ CT. However, there were three discordant cases where there was lucency around the interbody cage on the CT scan but no significant uptake on SPECT/CT. All three patients improved after surgery without revising the cage, which may suggest that SPECT/CT can guide surgeons in determining which component of the construct may be contributing to the patient’s symptomatology.18 Further studies are warranted, as it remains unclear whether tracer uptake necessarily reflects instability due to pseudoarthrosis or physiologic remodeling after surgery. Additionally, the optimal time to obtain a SPECT/CT postoperatively is uncertain as current studies report heterogeneous data on the timeline of expected postsurgical uptake.18
Most argue that surgical exploration remains the gold standard in assessing pseudoarthrosis, as it offers the ability to directly inspect and determine the solidity of the fusion mass. However, these days, routine exploration is performed less frequently due to the advent of noninvasive imaging modalities. Exploration is often reserved for patients who have a high clinical suspicion and inconclusive imaging studies.8,14
Despite extensive reports on pseudoarthrosis as a complication of lumbar spine fusions, it continues to pose diagnostic and technical challenges to surgeons. The cause of a failed lumbar fusion may be multifaceted in nature as a result of significant pathology or the psychological outlook of the patient. Other causes of pain, including infection, implant failure, or adjacent segment disease, must be ruled out prior to considering pseudoarthrosis as a diagnosis. In some cases of pseudoarthrosis, radiographs demonstrate significant instability and radiolucency of the hardware. However, radiographs are often unreliable, and other imaging modalities, such as thin-section CT, are employed (Figure 1). A thorough understanding of the imaging modalities available and their relative advantages and limitations is essential to help guide clinicians in the evaluation and diagnosis of pseudarthrosis.
References
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18. Rager O, Schaller K, Payer M, Tchernin D, Ratib O, Tessitore E. SPECT/CT in differentiation of pseudarthrosis from other causes of back pain in lumbar spinal fusion: report on 10 consecutive cases. Clin Nucl Med. 2012;37(4):339-343.
Contributors:
Neil Mohile, MD
Gregory Lopez, MD
From the Department of Orthopaedic Surgery at Rush University Medical Center in Chicago, Illinois.
