Facet Joint Signal Change on MRI at Levels of Acute/Subacute Lumbar Compression Fractures

Patient Selection

Approval of this Health Insurance Portability and Accountability Act–compliant study was obtained from the institutional review board. We searched the institutional vertebral augmentation data base for patients who underwent neuroradiologic evaluation for potential vertebral augmentation for painful lumbar osteoporotic compression fractures from May 2008 through June 2011. Fractures that occurred with absent or only minor trauma without evidence of pathologic vertebral body lesions were considered osteoporotic, even if a formal bone mineral attenuation was not available. This classification is consistent with criteria for a clinical diagnosis of osteoporosis, which can be made with either an osteoporotic fracture or with bone mineral attenuation testing.⁷ To be included, a patient had to be deemed an appropriate candidate for vertebral augmentation after a formal consult with an experienced neuroradiologist, had to have documented pain and tenderness at the level of vertebral augmentation, and then had to undergo the vertebral augmentation procedure in at least 1 lumbar level. These inclusion requirements were instituted to ensure that the selected patients indeed had a clinical pain syndrome that corresponded to the site of a compression fracture and that the patient group was one in which the potential findings of this study could most likely apply in clinical practice. An MR imaging including fat-suppressed T2-weighted images and/or fat-saturated gadolinium-enhanced T1-weighted images covering all facet joints from T12/L1 through L5/S1 at the time of fracture evaluation was also required. For patients who had undergone vertebral augmentation procedures on multiple days, only the most recent vertebral augmentation evaluation and/or procedure and the corresponding closest preceding MR images were analyzed to avoid redundancy.

Exclusion Criteria

Patients with traumatic nonosteoporotic vertebral body compression fractures or with pathologic fractures, such as those due to metastatic disease or multiple myeloma, were excluded. Patients with lumbar spine surgery within 6 months of the MR imaging examination were excluded to prevent confounding findings due to postoperative change. All patients with metallic hardware in the lumbar spine were excluded to avoid the effects of artifacts.

Imaging

At our institution, patients routinely underwent MR imaging examinations before evaluation for potential vertebral augmentation. Our vertebroplasty protocol included either sagittal fat-saturated T2-weighted images or sagittal STIR images, depending on the specific MR imaging scanner used and the radiologist's preference. In certain settings such as radiologist's preference, prior lumbar surgery, or evaluation for a possible vertebral body mass, gadolinium was administered. All MR imaging examinations were performed on a 1.5T scanner.

Image Analysis

All images were reviewed at an electronic PACS workstation by 3 neuroradiologists (C.P.W., K.R.T., C.H.H.) and a neuroradiology fellow (V.T.L.) with 21 years, 16 years, 3 years, and 1 year of experience, respectively, for consensus diagnosis. At the time of review, readers were blinded to the original radiology report and contents of the neuroradiology vertebroplasty consult. Consensus review was performed to ensure that the interpretation generated was representative of a typical group of neuroradiologists. All lumbar levels were evaluated in each patient.

Sagittal fat-suppressed T2-weighted and/or gadolinium-enhanced T1-weighted images were the primary ones used to assess vertebral body edema and facet joint signal change. However, these images were reviewed in conjunction with each patient's entire examination, which typically included sagittal T1-weighted images; sagittal T2-weighted images without fat-suppression; and any available axial T2-weighted images, axial T1-weighted images, or axial gadolinium-enhanced T1-weighted images.

The vertebral bodies were evaluated for the presence or absence of an acute/subacute compression fracture, indicated by T2 hyper/T1 hypointense edema signal or a discrete visible fracture line, and for a chronic compression fracture, indicated by height loss without edema signal or a fracture line. Vertebral body edema is not a precise measure of compression fracture age because the duration after an osteoporotic compression fracture is not known with certainty. It is thought to typically resolve within 1–3 months, though edema in vertebral bodies treated with vertebral augmentation can remain for >12 months in a relative minority of cases⁸ and can even progress.⁹ Because the precise date of compression fracture for each patient was not known, vertebral body edema was considered a reasonable indicator of acute/subacute status.

The degree of vertebral body height loss, whether acute/subacute or chronic, was rated on a 1–4 scale: 1 = 1%–25% height loss, 2 = 26%–50% height loss, 3 = 51%–75% height loss, and 4 = >75% height loss. Prior vertebral augmentation procedures were recorded as present or absent at each level. Edema within vertebral bodies that had undergone prior augmentation was not considered evidence of acute/subacute status because vertebral body edema can persist in successfully treated compression fractures.^8,9 The lowest mobile lumbar-type vertebral body was designated L5 in every patient for consistency.

Facet joint signal change was defined as T2 hyperintensity on fat-suppressed T2-weighted images and/or enhancement on fat-suppressed T1-weighted images. Although these signal changes have been previously referred to as inflammation,⁶ the term “signal change” was used because the precise physiologic cause of this signal change is not known with certainty. Facet joint signal change was graded by using an established scale of grades 0–4 based on increasing osseous or soft-tissue signal abnormality at the facet joint (Table 1).⁶ To be consistent with the scale as used by Czervionke and Fenton,⁶ we graded a simple facet joint effusion without any other change zero because an effusion alone was not considered by these authors to necessarily indicate active inflammation or synovitis (personal communication L.F. Czervionke). All facet joints associated with the lumbar vertebral bodies were assessed individually, from the T12/L1 facet joints through the L5/S1 facet joints.

Chart Review

A neuroradiology consult was performed for every patient considered for vertebral augmentation at our institution. The dates of the consults and vertebral augmentation procedures and the levels of vertebral augmentation were recorded for each patient.

Statistics

The primary objective for statistical analysis was to determine whether the presence or degree of facet joint signal change was associated with the presence of an acute/subacute compression fracture. To assess this association, a facet joint signal-change score (Fig 1), which was the sum of the above signal-change grades (0–4) for each of the 4 facet joints associated with a given vertebral body level, was calculated at each level, L1 through L5. This score represented an overall measure of facet joint signal change for an individual vertebral body level and ranged between 0 and 16. Each facet joint is associated with, and potentially affected by, a compression fracture of the vertebral body immediately above and immediately below it. To account for the potential effect of an acute/subacute fracture of an adjacent vertebral body on the shared facet joints, we considered a vertebral body level to be affected by an acute/subacute fracture if that vertebral body or either of the adjacent vertebral bodies had an acute/subacute fracture. For example, the L2 level (L1/L2 and L2/L3 facet joints) was considered to be affected by an acute/subacute fracture if the L1, L2, and/or L3 vertebral body had an acute/subacute fracture.

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Fig 1.

For the L2 vertebral body, the facet joint signal-change score (0–16) is the sum of the facet joint signal-change grades of each of the 4 associated facet joints (0–4). Anatomically, the L2 vertebral body–associated facet joints are shared with the adjacent vertebral bodies. Therefore, the L2-associated facet joints could be subject to stress from compression fractures of the L2 or adjacent L1 and L3 vertebral bodies. For the L2 level, the above facet joint signal-change score for the L1/L2 and L2/L3 facet joints of patients with an acute/subacute compression fracture of L1, L2, and/or L3 was compared with the facet joint signal-change score for the L1/L2 and L2/L3 facet joints of patients without an acute/subacute compression fracture at L1, L2, or L3. This same method was then applied to the remaining lumbar vertebrae.

Wilcoxon rank sum tests were then performed to determine whether these facet joint signal-change scores differed between vertebral body levels with an acute/subacute compression fracture versus those that were normal or had a chronic compression fracture. A level-by-level statistical analysis was performed because the biomechanical properties¹⁰ and baseline rate of facet joint signal change related to chronic arthrosis¹¹ likely vary by level. Because there were few chronic compression fractures in our sample, a separate subgroup statistical analysis of this category was not performed. Finally, the degree of facet joint signal-change score was compared with the degree of height loss at levels with acute/subacute fractures by using the Spearman rank correlation.

Additionally, although the different levels were not combined for the formal statistical analysis, each of the 900 facet joints was also dichotomized into 2 groups: 1) facet joints associated with an acute/subacute compression fracture of either of the adjacent vertebral bodies, for example the L1 and L2 vertebral bodies for an L1/L2 facet joint; and 2) facet joints without an acute/subacute compression fracture of the adjacent vertebral bodies. Signal change was recorded as either present (any grade) or absent for each facet joint. Comparison of these 2 groups was performed to provide a more global view of the results, though this comparison did not account for potential differences in baseline inflammatory osteoarthrosis or biomechanical properties of the different levels.