1 Introduction

Sacral insufficiency fractures after lumbosacral fusion are a rare complication after surgery and can be misdiagnosed and underreported due to inconclusivity of plain radiographs.¹ Risk factors for these fractures are well established and include, elderly females, osteoporosis, and corticosteroid therapy.³ Symptoms are often nonspecific, typically with new onset lower back or sacral pain, months after surgery and can lead to delayed diagnosis in the unsuspecting clinician. Because plain radiographs are not sensitive enough for detection, it is imperative to order advanced imaging such as computerized tomography (CT) or magnetic resonance imaging (MRI. Treatment varies based on the patient presentation and imaging findings, but includes the full spectrum of treatment from non-operative modalities to surgical fixation. Historically, there has been controversy with the way these fractures were managed with both operative^2,20 and non-operative^13–16 interventions proving to be successful (see Fig. 1).

Given the presence of literature published to support both operative and non-operative management, the primary goal of this systematic review was to examine treatment options for sacral insufficiency fractures after lumbosacral fusion in order to improve clinical practice and management. This systematic review of the literature regarding treatment of sacral insufficiency fractures will assist clinicians in making the accurate diagnosis and devise a strategic treatment plan for patients with sacral insufficiency fractures after spinal instrumentation.

2 Methods

2.2

2.2 Search methodology

A search strategy was designed to identify relevant publications in the PubMed database (Fig. 1). The following terms were included in our search: sacral insufficiency fracture treatment lumbosacral fusion OR sacral insufficiency fracture posterior spinal instrumentation. Search filters used included English language and human subjects. No limitation was put on the date of publication.

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Fig. 1 PRISMA Flow diagram demonstrating screening process of articles.

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3 Results

This search strategy identified 32 publications from the PubMed database for literature review. After evaluating the inclusion and exclusion criteria, a total of 17 articles were included in the review. We reviewed each article and extracted data regarding patient demographics, initial surgery and its corresponding indications, number of levels fused, chief complaint surrounding the insufficiency fracture, diagnostic imaging used, fracture pattern, number of weeks after surgery symptoms developed, treatment, and outcomes. Table 1 summarizes the demographics of the patients studied. A total of 48 patients were reviewed with 22.4% of patients being male and 77.6% being female. The mean age of males was 69.7 and the mean age of females was 65.5 while the overall mean patient age was 67.6. Common pertinent medical history in many of the patients included osteoporosis, hypothyroidism, and corticosteroid use. The average patient BMI was 28.06. The most common surgical indications (Table 2) for the index surgery were spinal stenosis (36.7% of cases), degenerative disc disease (22.4%), spondylolisthesis (20.4%), and adult scoliosis (16.3%). The aforementioned percentages include patients that had more than 1 indication for surgery and may have had overlapping pathologies. The average and median number of levels fused was 4.91 and 3 respectively. The most common chief presenting complaint was new onset lower back or sacral pain with or without radicular symptoms at an average and median of 14 and 4 weeks respectively after index surgery. CT was the most common diagnostic imaging used (Table 3), and was the imaging of choice for sacral insufficiency fractures in 89% of cases reported in the literature.

Treatment of sacral insufficiency fractures leaned towards operative management with 65% of patients receiving revision surgery and 35% of patients proceeding with non-operative, conservative management only (Table 4). Revision surgery always involved sacropelvic fixation which typically led to immediate resolution or reduction of symptoms, with the exception of 2 cases that did not receive adequate reduction of symptoms. Five cases reported failed non-operative management that subsequently responded to revision surgery. Nonoperative management was not always specified throughout the literature, however typically involved physical therapy, bracing, pain control, and medical treatment of osteoporosis if applicable. In the nonoperatively managed cases, there was usually a gradual relief of symptoms as the fracture healed varying from as low to 8 weeks and up to 1.6 years before symptom resolution to; however 3 cases continued to have chronic lower back pain, though fortunately still being able to tolerate ambulation.

4 Discussion

Sacral insufficiency fractures (SIF) are a recognized complication of spinal instrumentation and are estimated to complicate lumbosacral fusion in approximately 1.3–6.1% of cases.^1–3 The incidence however, is more than likely underestimated due to the vague presenting symptoms, inconclusiveness of plain radiographs, and low index of clinical suspicion for these fractures. Yoder et al. reported that plain radiographs accurately diagnose SIF in only 31.4% of cases, which corroborates the presumption that this diagnosis is missed more often than not.³ CT scans on the contrary, accurately diagnose SIF in 97.7% of cases whereas a bone scan or MRI detects SIF accurately 100% of the time, and thus are the preferred diagnostic imaging modality.³ Given the substantial cost of MRI's and the insignificant differences in accuracy, CT scans should be ordered to confirm the diagnosis of SIF. The typical patient at risk for SIF is an elderly, postmenopausal female with osteoporosis. Other commonly cited risk factors reported by Yoder et al. include pelvic radiation therapy, corticosteroid use, long distance running, rheumatoid arthritis, and osteopenia.³ These risk factors should alert the clinician to the risk of SIF after lumbosacral surgery and are important factors to discuss regarding the risks and benefits before undergoing initial surgery to guide expectations. Furthermore, careful attention to these risk factors can alert the practitioner to the possibility of SIF when patients present with new onset buttock or sacral pain after spinal instrumentation.

There are several different classification systems described in the literature in regards to sacral fractures. The Denis classification system divides the sacrum into three zones (Zone I, II, III), based on the location of the fracture. Zone I, or the alar zone, classifies fractures isolated to the ala without damaging the surrounding foramina or the central sacral canal. Zone II, or the foraminal zone, refers to fractures involving the foramina; however without impingement of the central sacral canal, finally, zone III is the central zone that involves fractures through the central sacral canal.⁴ The Roy-Camille classification system classifies sacral fractures into 3 different types based on the displacement of the upper sacrum fragments. A type 1 fracture is a flexion fracture with anterior bending of the upper sacrum fragments. Type 2 fractures refer to flexion fractures with posterior displacement of the upper fragment, and type 3 fractures refer to an extension fracture with anterior displacement of the upper fragment.⁵ The Strange-Vognsen and Lebech classification of sacral fractures is similar to the Roy-Camille classification; however with the addition of a type 4 fracture, which refers to a non-displaced, neutral fracture with total comminution of the upper sacrum without displacement.⁶

A proposed treatment algorithm by Buell et al. begins with a trial of conservative management for up to 6 months using bracing, activity modification, pain control, and physical therapy in patients with less severe fractures and symptoms.⁷ However, non-operative management has shown varying degrees of success with recovery timelines ranging from 8 weeks to 2 years for complete resolution of symptoms.^{1,10,13–17,22,23} One noted disadvantage of conservative treatment is residual pain¹⁷ and the need for chronic pain management. In the aforementioned situations clinical judgement is needed to determine if the patient should proceed with surgery instead. Indications for surgery are multifactorial including: failed non-operative management, neurologic deficits, significant pain, decreased activity, and/or prolonged immobilization. Further surgical indications should be considered through radiological assessment. Parameters such as lumbosacral kyphosis with sagittal imbalance, pelvic incidence-lumbar lordosis mismatch, pseudarthrosis involving the lumbosacral junction, and fracture displacement/angulation should all be recognized and considered in conjunction with the patient's presentation when determining treatment.^3,23 Meredith et al. suggested that sacral fractures with anterolisthesis >2 mm and kyphotic angulation >5° were more likely to fail conservative management, and fusions greater than four levels were significantly associated with an increased incidence of sacral insufficiency fractures.⁴ Similarly, a retrospective study by Klineberg et al. concluded that the instability of SIF along with kyphosis and anterolisthesis of the upper fragment may contribute to persistent symptoms of pain and postural malalignment.² Meanwhile, another retrospective study by Odate et al. demonstrated that a high pelvic incidence-lumbar lordosis mismatch were risk factors for SIF and even suggested prophylactic iliosacral fixation in patients with a high pelvic incidence planning on undergoing lumbosacral fusion.²³ Regardless, revision surgery should extend fixation to the pelvis, either with S2AI screws or iliac screws. The majority of patients in our review treated with surgery experienced immediate resolution of symptoms regardless of the technique used for sacropelvic fixation. Our results show that 65% of SIF were treated with sacropelvic fixation and 94% of cases lead to symptom resolution or minimal pain. These results encourage operative fixation as a reliable treatment for SIF.

There were several limitations to this study. One limitation is the inclusion of only case reports and case series in order to individualize and accurately extract data. These articles are generally considered to be low-level evidence. Larger studies such as the retrospective studies by Odate et al. and Klineberg et al. were not included in this systematic review due to the inability to individualize the data. Odate et al. evaluated 116 patients that we were unable to individually extract and Klineberg et al. evaluated 9 patients that were not described individually, and thus unable to extract. Another limitation to this study is the heterogeneity in the reports of the fracture patterns. Classification systems were not consistently used when describing the treatment approach. One final limitation is our strict search protocols may have excluded other relevant articles surrounding this topic.

5 Conclusion

Sacral insufficiency fracture is a potential complication after spinal instrumentation. New onset buttock or sacral pain without an inciting event should raise suspicion of this complication and a CT scan should be used to appropriately diagnose or rule out SIF. Non-operative management including bracing, physical therapy, and pain control may lead to eventual relief; however if symptoms fail to improve after 6–8 weeks, operative management, involving extension of fusion into the pelvis, should be considered. Revision surgery with spinopelvic fixation typically leads to immediate resolution of symptoms. Fractures with anterolisthesis >2 mm and kyphotic angulation >5° should undergo extension of fusion surgery.

Authors contributions

oCaleb P Shin: Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND drafting the work or revising it critically for important intellectual content; AND Final approval of the version to be published; AND Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoLuke D Mascarenhas: Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND drafting the work or revising it critically for important intellectual content; AND Final approval of the version to be published; AND Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoBrendan M Holderread: Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND drafting the work or revising it critically for important intellectual content; AND Final approval of the version to be published; AND Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoMatthew Awad: Substantial contributions to the conception or design of the work and the acquisition of data and interpretation-analysis. Assisted in drafting the manuscript and provided final approval of the version to be published; Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoDavid Botros: Substantial contributions to the conception or design of the work and the acquisition of data and interpretation-analysis. Assisted in drafting the manuscript and provided final approval of the version to be published; Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoIoannis Avramis: Substantial contributions to the conception or design of the work and the acquisition of data and interpretation-analysis. Assisted in drafting the manuscript and provided final approval of the version to be published; Agreement to be accountable for all aspects of the work in ensuring that questions relatedoIshaq Syed: Substantial contributions to the conception or design of the work; interpretation of data for the work; A Revising the work critically for important intellectual content; Final approval of the version to be published; Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedoJames R Rizkalla: Substantial contributions to the conception or design of the work; interpretation of data for the work as well as revising the work critically for important intellectual content. Author provided final approval of the version to be published and agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved

Consent

Patient consent or IRB approval was not required for this study.