Original Research

Is a Persistent Vacuum Phenomenon a Sign of Pseudarthrosis After Posterolateral Spinal Fusion?

Am J Orthop. 2016 July;45(5):E249-E253

The purpose of this study is to evaluate whether the vacuum phenomenon (VP) resolves after posterolateral lumbar fusion, and whether persistence of VP is indicative of failed fusion. We retrospectively reviewed patients with degenerative lumbar spinal stenosis with instability with a positive VP on preoperative computed tomography (CT) who underwent posterolateral lumbar spinal fusion. Lumbar CT and radiographs were evaluated for the presence of VP and fusion at each level. Thirty-six positive VP levels were identified on the preoperative lumbar CT at the levels in the fusion in 18 patients. The mean age at surgery was 67.6 ± 9.4 years and mean follow-up was 1.6 ± 0.86 years. Fusion was seen at 32 levels (88.9%). Of the 15 levels where VP persisted, evidence of fusion was seen in 13 levels and pseudarthrosis was seen at 2. Of the 21 levels where VP disappeared, fusion was seen at 19 levels and pseudarthrosis was seen at 2 .There was no significant difference between the 2 groups (P > .05). We did not find an association between persistence of VP and pseudarthrosis. Persistence of VP after spinal fusion may not be an indicator of pseudarthrosis, and should not be misinterpreted as an indication for additional surgery.

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References

1. Knutsson F. The vacuum phenomenon in the intervertebral discs. Acta Radiol. 1942;23:173-179.

2. Resnick D, Niwayama G, Guerra J Jr, Vint V, Usselman J. Spinal vacuum phenomenon: anatomical study and review. Radiology. 1981;139(2):341-348.

3. Lardé D, Mathieu D, Frija J, Gaston A, Vasile N. Spinal vacuum phenomenon: CT diagnosis and significance. J Comput Assist Tomogr. 1982;6(4):671-676.

4. Stallenberg B, Madani A, Burny F, Gevenois PA. The vacuum phenomenon: a CT sign of nonunited fracture. AJR Am J Roentgenol. 2001;176(5):1161-1164.

5. Martel W. Spinal pseudarthrosis: a complication of ankylosing spondylitis. Arthritis Rheum. 1978;21(4):485-490.

6. Chan FL, Ho EK, Chau EM. Spinal pseudarthrosis complicating ankylosing spondylitis: comparison of CT and conventional tomography. AJR Am J Roentgenol. 1988;150(3):611-614.

7. Edwards CE, Antonoiades SB, Ford L, Crabster E. CT vacuum disc sign: a highly specific predictor of lumbar nonunion. Poster presented at: 41st Annual Meeting of the Scoliosis Research Society; September 2006; Monterey, CA.

8. Carreon LY, Djurasovic M, Glassman SD, Sailer P. Diagnostic accuracy and reliability of fine-cut CT scans with reconstructions to determine the status of an instrumented posterolateral fusion with surgical exploration as reference standard. Spine. 2007;32(8):892-895.

9. Goobar JE, Pate D, Resnick D, Sartoris DJ. Radiography of the hyperextended lumbar spine: an effective technique for the demonstration of discal vacuum phenomena. Can Assoc Radiol J. 1987;38(4):271-274.

10. Grenier N, Grossman RI, Schiebler ML, Yeager BA, Goldberg HI, Kressel HY. Degenerative lumbar disk disease: pitfalls and usefulness of MR imaging in detection of vacuum phenomenon. Radiology. 1987;164(3):861-865.

11. Wang HJ, Chen BB, Yu CW, Hsu CY, Shih TT. Alteration of disc vacuum contents during prolonged supine positioning: evaluation with MR Image. Spine. 2007;32(23):2610-2615.

12. D’Anastasi M, Birkenmaier C, Schmidt GP, Wegener B, Reiser MF, Baur-Melnyk A. Correlation between vacuum phenomenon on CT and fluid on MRI in degenerative disks. AJR Am J Roentgenol. 2011;197(5):1182-1189.

13. Mirovsky Y, Anekstein Y, Shalmon E, Peer A. Vacuum clefts of the vertebral bodies. AJNR Am J Neuroradiol. 2005;26(7):1634-1640.

14. Selby MD, Clark SR, Hall DJ, Freeman BJ. Radiologic assessment of spinal fusion. J Am Acad Orthop Surg. 2012;20(11):694-703.

15. Kanayama M, Hashimoto T, Shigenobu K, Yamane S, Bauer TW, Togawa D. A prospective randomized study of posterolateral lumbar fusion using osteogenic protein-1 (OP-1) versus local autograft with ceramic bone substitute: emphasis of surgical exploration and histologic assessment. Spine. 2006;31(10):1067-1074.

The spinal vacuum sign or vacuum phenomenon (VP) is the radiographic finding of an air-density linear radiolucency in the intervertebral disc or vertebral body. The result of a gaseous accumulation, it is often a diagnostic sign of disc degeneration as well as a rare sign of infection, Schmorl node formation, or osteonecrosis.^1,2 Although the VP was first described on plain radiographs, it is better seen on computed tomography (CT).³ Multiple studies have found a possible association between the VP and nonunion in diaphyseal fractures,⁴ ankylosing spondylitis,^5,6 and lumbar spinal fusion.⁷

To our knowledge, no one has studied whether the intervertebral VP resolves after posterolateral lumbar spinal fusion in adults with degenerative spinal pathology, and no one has investigated the association between the persistence of the intervertebral VP and pseudarthrosis after posterolateral spinal fusion.

We conducted a study to determine whether the VP resolves after posterolateral lumbar spinal fusion procedures and whether persistence of the VP after fusion surgery is indicative of pseudarthrosis.

Materials and Methods

After obtaining Institutional Review Board approval for this study, we retrospectively reviewed the medical records of patients who had degenerative spinal stenosis with instability and the intervertebral vacuum sign on preoperative digital lumbar spine CT scans and who underwent posterolateral lumbar spinal fusion with or without instrumentation. Study inclusion criteria were lumbar spine CT at minimum 6-month follow-up after spinal fusion and preoperative and postoperative lumbar spine radiographs. Exclusion criteria were any type of interbody fusion procedure (anterior, posterior, transforaminal, lateral) at a level with the VP, age under 21 years, follow-up of less than 6 months, and incomplete radiographic records. As this was a retrospective study, patient consent was not required.

CT was performed with a 16-, 64-, or 128-slice multidetector CT scanner with effective tube current set at 250 to 320 mA, voltage set at 120 to 140 kV, and pitch set at 0.75 to 0.9. After axial acquisition of 3×3-mm isometric voxels, sagittal and coronal multiplanar images were reconstructed with a slice thickness of 2 mm. Patient demographics, diagnoses, and surgical details were recorded. All digital lumbar spine CT scans and radiographs were initially screened on PACS (picture archiving and communication system) by the orthopedic spine surgery fellow at an academic medical institution; then they were reviewed on a radiology reading room monitor by 3 observers (senior radiologist, senior orthopedic spine surgeon, orthopedic spine surgery fellow). Axial images and sagittal and coronal reconstructed images of the preoperative and postoperative follow-up lumbar CT scans—together with the lateral and anteroposterior lumbar spine radiographs—were evaluated for the intervertebral VP. Mean (SD) follow-up (with CT to assess fusion) was 1.6 (0.86) years (range, 0.75-3.38 years). Fusion at each level was evaluated on the postoperative follow-up CT on axial images and sagittal and coronal reconstructed images; criteria for fusion were continuous bridging bone across posterolateral gutters and facets on one or both sides at each intervertebral level.⁸ Pseudarthrosis was recorded if there was no continuity of bridging bone across both posterolateral gutters and facets, a complete radiolucent line on both sides across a level, or lysis or loosening around screws. All recordings were made by consensus, or by majority decision in case of disagreement.

Presence of the VP at the lumbar levels not included in the fusion was also recorded on the preoperative and follow-up CT scan and radiographs.

Descriptive and inferential statistical tests were performed as applicable. Pearson χ² test and Fischer exact test were used to evaluate if there was a significant association between the groups where the VP disappeared and persisted and fusion and pseudarthrosis. Significance was set at P < .05. Statistical analysis was performed with Stata Version 10.0.