MRI for axial SpA: Diagnosis, disease activity assessment, and recent advances.

Magnetic resonance imaging (MRI) is a sensitive imaging modality to detect early inflammatory changes in axial spondyloarthritis (SpA). Over a decade has passed since the inclusion of MRI assessment in the 2009 Assessment of SpondyloArthritis International Society (ASAS) classification criteria for axial SpA. Evidence and clinical experience of MRI in axial SpA have accumulated rapidly since. This has led to a better understanding of the clinical utility of MRI in early diagnosis, disease activity assessment, and monitoring of treatment response in axial SpA. Furthermore, technological advancements have paved the way for the development of novel MRI sequences for the quantification of inflammation and image optimization. The field of artificial intelligence has also been explored to aid medical imaging interpretation, including MRI in axial SpA. This review serves to provide an update on the latest understanding of the evolving roles of MRI in axial SpA.

Assessment of axial spondyloarthritis activity using a magnetic resonance imaging-based multi-region-of-interest fusion model.

BACKGROUND: Identifying axial spondyloarthritis (axSpA) activity early and accurately is essential for treating physicians to adjust treatment plans and guide clinical decisions promptly. The current literature is mostly focused on axSpA diagnosis, and there has been thus far, no study that reported the use of a radiomics approach for differentiating axSpA disease activity. In this study, the aim was to develop a radiomics model for differentiating active from non-active axSpA based on fat-suppressed (FS) T2-weighted (T2w) magnetic resonance imaging (MRI) of sacroiliac joints. METHODS: This retrospective study included 109 patients diagnosed with non-active axSpA (n = 68) and active axSpA (n = 41); patients were divided into training and testing cohorts at a ratio of 8:2. Radiomics features were extracted from 3.0 T sacroiliac MRI using two different heterogeneous regions of interest (ROIs, Circle and Facet). Various methods were used to select relevant and robust features, and different classifiers were used to build Circle-based, Facet-based, and a fusion prediction model. Their performance was compared using various statistical parameters. p < 0.05 is considered statistically significant. RESULTS: For both Circle- and Facet-based models, 2284 radiomics features were extracted. The combined fusion ROI model accurately differentiated between active and non-active axSpA, with high accuracy (0.90 vs.0.81), sensitivity (0.90 vs. 0.75), and specificity (0.90 vs. 0.85) in both training and testing cohorts. CONCLUSION: The multi-ROI fusion radiomics model developed in this study differentiated between active and non-active axSpA using sacroiliac FS T2w-MRI. The results suggest MRI-based radiomics of the SIJ can distinguish axSpA activity, which can improve the therapeutic result and patient prognosis. To our knowledge, this is the only study in the literature that used a radiomics approach to determine axSpA activity.

Medicare Reimbursement Rates and Utilization Trends in Sacroiliac Joint Fusion.

INTRODUCTION: Sacroiliac joint (SIJ) fusion is a surgical treatment option for SIJ pathology in select patients who have failed conservative management. More recently, minimally invasive surgical (MIS) techniques have been developed. This study aimed to determine the trends in procedure volume and reimbursement rates for SIJ fusion. METHODS: Publicly available Medicare databases were assessed using the National Summary Data Files for 2010 to 2020. Files were organized according to current procedural terminology (CPT) codes. CPT codes specific to open and MIS SI joint fusion (27279 and 27280) were identified and tracked. To track surgeon reimbursements, the CMS Medicare Physician Fee Schedule Look-Up Tool was used to extract facility prices. Descriptive statistics and linear regression were used to evaluate trends in procedure volume, utilization, and reimbursement rates. Compound annual growth rates were calculated, and discrepancies in inflation were corrected for using the Consumer Price Index. RESULTS: A total of 33,963 SIJ fusions were conducted in the Medicare population between 2010 and 2020, with an overall increase in procedure volume of 2,350.9% from 318 cases in 2010 to 7,794 in 2020. Since the introduction of the 27279 CPT code in 2015, 8,806 cases (31.5%) have been open and 19,120 (68.5%) have been MIS. Surgeon reimbursement for open fusions increased nominally by 42.8% (inflation-adjusted increase of 20%) from $998 in 2010 to $1,425 in 2020. Meanwhile, reimbursement for MIS fusion experienced a nominal increase of 58.4% (inflation-adjusted increase of 44.9%) from $582 in 2015 to $922 in 2020. CONCLUSION: SIJ fusion volume in the Medicare population has increased substantially in the past 10 years, with MIS SIJ fusion accounting for most of the procedures since the introduction of the 27279 CPT code in 2015. Reimbursement rates for surgeons have also increased for both open and MIS procedures, even after adjusting for inflation.

A deep learning model for the diagnosis of sacroiliitis according to Assessment of SpondyloArthritis International Society classification criteria with magnetic resonance imaging.

PURPOSE: The purpose of this study was to develop and evaluate a deep learning model to detect bone marrow edema (BME) in sacroiliac joints and predict the MRI Assessment of SpondyloArthritis International Society (ASAS) definition of active sacroiliitis in patients with chronic inflammatory back pain. MATERIALS AND METHODS: MRI examinations of patients from the French prospective multicenter DESIR cohort (DEvenir des Spondyloarthropathies Indifférenciées Récentes) were used for training, validation and testing. Patients with inflammatory back pain lasting three months to three years were recruited. Test datasets were from MRI follow-ups at five years and ten years. The model was evaluated using an external test dataset from the ASAS cohort. A neuronal network classifier (mask-RCNN) was trained and evaluated for sacroiliac joints detection and BME classification. Diagnostic capabilities of the model to predict ASAS MRI active sacroiliitis (BME in at least two half-slices) were assessed using Matthews correlation coefficient (MCC), sensitivity, specificity, accuracy and AUC. The gold standard was experts' majority decision. RESULTS: A total of 256 patients with 362 MRI examinations from the DESIR cohort were included, with 27% meeting the ASAS definition for experts. A total of 178 MRI examinations were used for the training set, 25 for the validation set and 159 for the evaluation set. MCCs for DESIR baseline, 5-years, and 10-years follow-up were 0.90 (n = 53), 0.64 (n = 70), and 0.61 (n = 36), respectively. AUCs for predicting ASAS MRI were 0.98 (95% CI: 0.93-1), 0.90 (95% CI: 0.79-1), and 0.80 (95% CI: 0.62-1), respectively. The ASAS external validation cohort included 47 patients (mean age 36 ± 10 [SD] years; women, 51%) with 19% meeting the ASAS definition. MCC was 0.62, sensitivity 56% (95% CI: 42-70), specificity 100% (95% CI: 100-100) and AUC 0.76 (95% CI: 0.57-0.95). CONCLUSION: The deep learning model achieves performance close to those of experts for BME detection in sacroiliac joints and determination of active sacroiliitis according to the ASAS definition.

Non-radiographic axial spondyloarthritis in South America. Burden of disease and differential features with respect to ankylosing spondylitis at time of diagnosis. A comprehensive analysis with a focus on images.

BACKGROUND: Non-radiographic axial spondyloarthritis (nr-axSpA) data from South America are scarce, especially regarding image features. Objective To estimate the frequency of nr-axSpA and ankylosing spondylitis (AS) in a cohort of Argentinian patients with chronic low back pain (LBP) and to analyze the difference between both, with focus on magnetic resonance imaging (MRI) lesions, at diagnosis. METHODS: Patients with LBP and a diagnosis of axSpA who participated in a reuma-check program were included. All patients with a suspicion of SpA were evaluated using blood analytics, HLA-B27, and images (MRI). Sociodemographic data, SpA features, diagnostic dela,y and clinimetrics were assessed by an operator who was blinded to the patient's test results. On MRI, the presence of SpA lesions was assessed and a concordance exercise was carried out between rheumatologists and radiologist. RESULT: Of 198 LBP patients, 97 had axSpA, 54% of whom were nr-axSpA. A positive MRI was found in 50%. No difference in terms of disease activity, functional impact, laboratory or treatments between nr-axSpA and AS were found. Higher frequencies of male sex and chronic lesions on sacroiliac MRI were found in AS patients. In the logistic regression, an independent association with AS diagnosis was found: male (odds ratio [OR] 4.8), MRI fat replacement (OR 4.6), MRI sclerosis (OR 7.6), and diagnostic delay of more than 2 years (OR 10). The concordance between rheumatologists and radiologists was considered good to very good (κ 0.7-0.8). CONCLUSION: The frequency of nr-axSpA was 54%. We found a higher frequency of being male, more SpA features, and a longer diagnostic delay in patients with AS. Patients with AS had more structural lesions, with a good concordance between rheumatologist and radiologist.

Assessment of interclass and intraclass variability of specific lesions of sacroiliac magnetic resonance imaging.

AIM: Sacroiliac joint (SJ) imaging is the key point in the diagnosis of ankylosing spondylitis (AS). The curved anatomy of the SJ makes the interpretation of imaging difficult. The aim of this study is to evaluate the interclass and intraclass reliability of specific lesions (bone marrow edema [BME], joint space narrowing, erosions, effusion, ankylosis, bridging, sclerosis, fat deposition, and other additional pathologies) on SJ magnetic resonance imaging (MRI). METHOD: In a total of 310 randomly chosen patients, 620 SJs were evaluated by three different radiologists with different radiology experiences of specialties other than musculoskeletal radiology. RESULTS: The agreement between readers for BME was fair to substantial, for active sacroiliitis was moderate to substantial, for sacroiliac narrowing was fair at best, for erosions was fair to moderate, for SJ sclerosis was none to slight, for chronic sacroiliitis was slight to fair, for degenerative sacroiliitis was none to slight, for normal SJ was slight, for SJ effusion was none to slight, and for fatty deposition was none. Intraclass correlation for readers 1 and 3 was usually good to excellent and for reader 2 was poor to fair. CONCLUSION: This study was designed to assess the agreement between radiologists who were not familiar with SJ MRI. The agreement between readers was usually fair to substantial and even intraclass correlation was poor to fair for reader 2. Future studies can be designed for standardization and validation of each MRI lesion for better interpretation of SJ MRI.

Role of macrophage-associated chemokines in the assessment of initial axial spondyloarthritis.

OBJECTIVES: To identify biomarkers that reflect disease activity scores and to investigate the role of macrophage-associated chemokines in initial axial spondyloarthritis (axSpA). METHOD: Patients with axSpA were enrolled. The SpondyloArthritis Research Consortium of Canada (SPARCC) method was used to score bone marrow oedema (BMO) in the inflammatory lesions on magnetic resonance imaging (MRI). Radiographic assessment of the spine was performed using the modified Stoke Ankylosing Spondylitis Spine Score (mSASSS). Clinical variables, including inflammatory markers, serum CC chemokine ligand 2 (CCL2), CCL3, CCL7, CCL8 and C-X3-C motif ligand 1 (CX3CL1), were measured. Correlation analysis between serum levels of these macrophage-associated chemokines and clinical data was performed. RESULTS: There were no significant differences between the axSpA group and the healthy control group in terms of serum levels of CCL2, CCL3 or CCL8. Compared to the healthy control group, the serum levels of CCL7 and CX3CL1 were significantly higher in ankylosing spondylitis (AS) (p = 0.045, p = 0.017, respectively). In the AS subgroup, the serum level of CX3CL1 had a positive correlation with SPARCC scores. CONCLUSIONS: In AS, serum CCL7 and CX3CL1 levels are elevated. The serum level of CX3CL1 is associated with MRI-determined oedema in AS. CX3CL1 may be useful as a biomarker to predict active inflammation in the sacroiliac joint (SIJ) in AS. Key Points • Serum levels of CX3CL1 are associated with MRI-determined oedema in AS. • CX3CL1 may be a useful biomarker to predict active inflammation in the sacroiliac joint in AS.

Novel imaging techniques for sacroiliac joint assessment.

PURPOSE OF REVIEW: Imaging of the sacroiliac joints is one of the cornerstones in the diagnosis and monitoring of axial spondyloarthritis. We aim to present an overview of the emerging imaging techniques for sacroiliac joint assessment and provide an insight into their relevant benefits and pitfalls. RECENT FINDINGS: Evaluation of structural and active inflammatory lesions in sacroiliitis are both important for understanding the disease process. Dual-energy computed tomography (CT) can detect inflammatory bone marrow edema in the sacroiliac joints and provides an alternative for magnetic resonance imaging (MRI). Three-dimensional gradient echo sequences improve the visualization of erosions on MRI. Susceptibility weighted MRI and deep learning-based synthetic CT are innovative MRI techniques that allow for generating 'CT-like' images and better depict osseous structural lesions than routine MRI sequences. SUMMARY: New imaging innovations and developments result in significant improvements in the imaging of spondyloarthritis. Advanced MRI techniques enhance its potential for the accurate detection of structural and active inflammatory lesions of sacroiliitis in one single imaging session.

Detailed bone assessment of the sacroiliac joint in a prospective imaging study: comparison between computed tomography, zero echo time, and black bone magnetic resonance imaging.

OBJECTIVES: To compare the value of zero echo time (ZTE) and gradient echo "black bone" (BB) MRI sequences for bone assessment of the sacroiliac joint (SI) using computed tomography (CT) as the reference standard. MATERIALS AND METHODS: Between May 2019 and January 2021, 79 patients prospectively underwent clinically indicated 3-T MRI including ZTE and BB imaging. Additionally, all patients underwent a CT scan covering the SI joints within 12 months of the MRI examination. Two blinded readers performed bone assessment by grading each side of each SI joint qualitatively in terms of seven features (osteophytes, subchondral sclerosis, erosions, ankylosis, joint irregularity, joint widening, and gas in the SI joint) using a 4-point Likert scale (0 = no changes-3 = marked changes). Scores were compared between all three imaging modalities. RESULTS: Interreader agreement was largely good (k values: 0.5-0.83). Except for the feature "gas in SI joint" where ZTE exhibited significantly lower scores than CT (p < 0.001), ZTE and BB showed similar performance relative to CT for all other features (p > 0.52) with inter-modality agreement being substantial to almost perfect (Krippendorff's alpha coefficients: 0.724-0.983). When combining the data from all features except for gas in the SI joint and when binarizing grading scores, combined sensitivity/specificity was 76.7%/98.6% for ZTE and 80.8%/99.1% for BB, respectively, compared to CT. CONCLUSIONS: The performance of ZTE and BB sequences was comparable to CT for bone assessment of the SI joint. These sequences may potentially serve as an alternative to CT yet without involving exposure to ionizing radiation.

Incidence, pain, and mobility assessment of pregnant women with sacroiliac dysfunction.

OBJECTIVES: To determine the incidence of sacroiliac dysfunction in pregnancy and assess its progress during the course of the pregnancy. METHODS: The present prospective cohort study, performed between April 1, 2013, and May 31, 2016, enrolled primigravidae aged 25-35 years before 13 weeks of pregnancies who were experiencing back pain and did not have prior symptoms of sacroiliac dysfunction. Participants attended regular follow-up over 6 months and clinical functional tests were used to diagnose sacroiliac dysfunction. Women with sacroiliac dysfunction were assessed at 3-week intervals with a numeric pain rating scale (NPRS) and the pregnancy mobility index (PMI). RESULTS: Among 1500 women who fulfilled the inclusion criteria, 1181 (78.7%) were diagnosed with sacroiliac dysfunction and 1143 completed all follow-up. Pain assessed by the NPRS gradually worsened from the first toward the third trimester (P<0.001). The level of disability assessed by the PMI also increased from the beginning to the end of pregnancy (P<0.001). CONCLUSION: Sacroiliac dysfunction represents an important problem during pregnancy; pain severity and mobility problems increased during the course of pregnancy in the present study. AUSTRALIAN NEW ZEALAND CLINICAL TRIALS REGISTRY: ACTRN12613000246785.

MRI assessment of bone marrow oedema in the sacroiliac joints of patients with spondyloarthritis: is the SPAIR T2w technique comparable to STIR?

OBJECTIVE: To compare short tau inversion-recovery (STIR) with another fat saturation method in the assessment of sacroiliac joint inflammation. METHODS: This prospective cross-sectional study comprised 76 spondyloarthritis (SpA) patients who underwent magnetic resonance imaging of the sacroiliac joints in a 1.5-T scanner, using STIR, spectral attenuated inversion recovery (SPAIR) T2w and spectral presaturation with inversion recovery (SPIR) T1w post-contrast sequences. Two independent readers (R1 and R2) assessed the images using the Spondyloarthritis Research Consortium of Canada (SPARCC) score. We assessed agreement of the SPARCC scores for SPAIR T2w and STIR with that for T1 SPIR post-contrast (reference standard) using the St. Laurent coefficient. We evaluated each sequence using the concordance correlation coefficient (CCC). RESULTS: We observed a strong agreement between STIR and SPAIR T2w sequences. Lin's CCC was 0.94 for R1 and 0.84 for R2 for STIR and 0.94 for R1 and 0.84 for R2 for SPAIR. The interobserver evaluation revealed a good CCC of 0.79 for SPAIR and 0.78 for STIR. CONCLUSION: STIR technique and SPAIR T2w sequence showed high agreement in the evaluation of sacroiliac joint subchondral bone marrow oedema in patients with SpA. SPAIR T2w may be an alternative to the STIR sequence for this purpose. KEY POINTS: • There are no studies evaluating which fat saturation technique should be used. • SPAIR T2w may be an alternative to STIR for sacroiliac joint evaluation. • The study will lead to changes in guidelines for spondyloarthritis.

A diffusion-based quantification technique for assessment of sacroiliitis in adolescents with enthesitis-related arthritis.

OBJECTIVE: To investigate the use of a quantitative diffusion-weighted imaging (DWI) tool for measuring inflammation of the sacroiliac joints (SIJs) in enthesitis-related arthritis (ERA). METHODS: A retrospective study was performed with institutional review board approval. Subjects were adolescents who had undergone MRI of the SIJs since January 2010. 10 patients with a clinical diagnosis of ERA and 10 controls with a clinical diagnosis of mechanical back pain were assessed. Axial T1 weighted, short tau inversion recovery (STIR) and DWI (b-values 0, 50, 100, 300 and 600 mm(2) s(-1)) images were acquired. Apparent diffusion coefficient (ADC) maps were generated using a monoexponential fit. On each of four slices, two to three linear regions-of-interest were placed on each joint. Normalized ADC (nADC) values were defined as joint ADC divided by a reference ADC derived from normal sacral bone. STIR images were scored using a modification of an established technique. The correlation between nADC values and STIR scores was evaluated using Spearman's rank correlation. RESULTS: Mean nADC values were significantly higher in cases than in controls (p = 0.0015). There was a strong correlation between STIR scores and nADC values (R = 0.85). CONCLUSION: ADC values are significantly increased in inflamed SIJs compared with controls. There is a good correlation between this diffusion-based method and STIR scores of inflammation. ADVANCES IN KNOWLEDGE: We have described and provisionally validated a method for quantifying the severity of inflammation in the SIJs in ERA using ADC measurements. This method is quick, is reproducible and could potentially be automated.

The ASAS Health Index (ASAS HI) - a new tool to assess the health status of patients with spondyloarthritis.

Within the variable course of ankylosing spondylitis (AS), peripheral arthritis, enthesitis, and involvement of other organs can add to the burden of the disease. The primary complaints of patients with spondyloarthritis (SpA) are pain, stiffness, fatigue, and limitation in activities and social participation. Instruments currently available for the assessment of patients with SpA focus predominantly on specific aspects of health such as pain, disease activity, and physical function and measure specific concepts like physical function and health-related quality of life (HR-QoL). However, the overall picture of impairments, limitations and restrictions in activities or social participation of patients with AS is not adequately assessed in SpA-specific questionnaires. Most of the existing questionnaires are not conceptualised with regard to their underlying construct. The International classification of functioning, disability and health (ICF) Core Set for AS may serve as an appropriate model and underlying construct to develop a health index, since the whole range of functioning and disability of patients with AS is captured. Based on these assumptions, ASAS developed for patients with SpA an instrument assessing health as operationalised by the ICF. The questionnaire was developed by preparing an item pool, linkage of the items to the comprehensive ICF core set for AS and test of the item pool in two cross-sectional studies. The analysis of the questionnaire and the response scale were done with Rasch analysis. Emphasis was on optimal targeting, the capacity of items to differentiate between different levels of health, and optimal coverage of items to the spectrum of ICF categories, so that the final questionnaire could represent as much of the entire range of difficulty levels as possible. The ASAS HI is a linear composite measure and includes 17 items which cover most of the ICF core set. Preliminary validity has been confirmed in a field test in 4 English-speaking countries. The ASAS HI should soon be used in clinical trials and in clinical practice to test its real life performance and to confirm that this new composite index captures relevant information on functioning and health of patients with AS.

Assessment of sacroiliitis by radiographs and MRI: where are we now?

PURPOSE OF REVIEW: Both MRI and plain radiography are used to assess sacroiliitis. A weakness of radiography - apart from its inability to detect early disease - is reader variability. On the contrary, experience with MRI is relatively limited by comparison. RECENT FINDINGS: This review summarizes recent advances in sacroiliac joint imaging using radiography and MRI in spondyloarthritis. SUMMARY: Observer variation in reading radiographs of sacroiliac joints remains an unresolved issue. In recent years, more studies on MRI in the diagnosis of axial spondyloarthritis have become available. Incorporating structural lesions in the sacroiliac joint and spine and inflammatory lesions in the spine in the definition of a positive MRI are hot topics in research.

Similarities and differences between nonradiographic and radiographic axial spondyloarthritis: a clinical, epidemiological and therapeutic assessment.

PURPOSE OF REVIEW: The concept of axial spondyloarthritis with two forms or subtypes (nonradiographic and radiographic) has been established over the last few years. However, debates concerning especially the nonradiographic form of the disease are still ongoing. Here we summarise recent data on similarities and differences (and their possible explanations) between nonradiographic axial spondyloarthritis and radiographic axial spondyloarthritis (ankylosing spondylitis). RECENT FINDINGS: Nonradiographic and radiographic forms are about equally frequent among patients first diagnosed with axial spondyloarthritis and have in general similar clinical characteristics, especially related to clinical signs of disease activity and similar rates of treatment response. Nonradiographic axial spondyloarthritis is characterised by a higher prevalence of females and lower percentage of patients with elevated C-reactive protein that might reflect the presence of a certain proportion of patients who develop structural damage in the axial skeleton very slowly or do not develop it at all. Elevated C-reactive protein and active sacroiliitis on magnetic resonance imaging are strongest predictors of structural damage development in the sacroiliac joints and, therefore, of progression from nonradiographic to radiographic stage. The same parameters predict a good clinical response to therapy with tumour necrosis factor alpha blocking agent in axial spondyloarthritis, but especially if used in nonradiographic disease. SUMMARY: Currently available data support the concept of axial spondyloarthritis as one entity. Nonradiographic axial spondyloarthritis seems to be, however, more heterogeneous than ankylosing spondylitis because of the presence of patients with a self-limiting disease or a slow disease course.

Nonoperative care to manage sacroiliac joint disruption and degenerative sacroiliitis: high costs and medical resource utilization in the United States Medicare population.

OBJECT: Low-back pain (LBP) is highly prevalent among older adults, and the cost to treat the US Medicare population is substantial. Recent US health care reform legislation focuses on improving quality of care and reducing costs. The sacroiliac (SI) joint is a recognized generator of LBP, but treatments traditionally have included either nonoperative medical management or open SI joint fusion, which has a high rate of complications. New minimally invasive technologies have been developed to treat SI joint disruption and degenerative sacroiliitis, so it is important to understand the current cost impact of nonoperative care to the Medicare program. The objective of this study was to evaluate the medical resource use and associated Medicare reimbursement for patients managed with nonoperative care for degenerative sacroiliitis/SI joint disruption. METHODS: A retrospective study was conducted using claim-level data from the Medicare 5% Standard Analytical Files (SAFs) for the years 2005-2010. Included were patients with a primary ICD-9-CM (International Classification of Diseases, Ninth Revision, Clinical Modification) diagnosis code for degenerative sacroiliitis/SI joint disruption (ICD-9-CM diagnosis codes 720.2, 724.6, 739.4, 846.9, or 847.3) with continuous enrollment for at least 1 year before and 5 years after diagnosis. Claims attributable to degenerative sacroiliitis/SI joint disruption were identified using ICD-9-CM diagnosis codes (claims with a primary or secondary ICD-9-CM diagnosis code of 71x.xx, 72x.xx, 73x.xx, or 84x.xx), and the 5-year medical resource use and Medicare reimbursement (in 2012 US dollars) were tabulated across practice settings. A subgroup analysis was performed among patients who underwent lumbar spinal fusion. RESULTS: Among all Medicare patients with degenerative sacroiliitis or SI joint disruption (n = 14,552), the mean cumulative 5-year direct medical costs attributable to degenerative sacroiliitis/SI joint disruption was $18,527 ± $28,285 (± SD) per patient. The cumulative 5-year cost was $63,913 ± $46,870 per patient among the subgroup of patients who underwent lumbar spinal fusion (n = 538 [3.7%]) and $16,769 ± $25,753 per patient among the subgroup of patients who had not undergone lumbar spinal fusion (n = 14,014 [96.3%]). For the total population, the largest proportion of cumulative 5-year costs was due to inpatient hospitalization (42.1%), outpatient physician office (20.6%), and hospital outpatient costs (14.9%). The estimated cumulative 5-year Medicare reimbursement across practice settings attributable to SI joint disruption or degenerative sacroiliitis is approximately $270 million among these 14,552 Medicare beneficiaries ($18,527 per patient). CONCLUSIONS: In patients who suffer from LBP due to SI joint disruption or degenerative sacroiliitis, this retrospective Medicare claims data analysis demonstrates that nonoperative care is associated with substantial costs and medical resource utilization. The economic burden of SI joint disruption and degenerative sacroiliitis among Medicare beneficiaries in the US is substantial and highlights the need for more cost-effective therapies to treat this condition and reduce health care expenditures.