Syndesmophytes A Superior Imaging Marker of Spinal Fusion than ZJ Fusion

Syndesmophytes frequently appear earlier in thoracolumbar vertebral levels than indications of zygapophyseal joint (ZJ) fusion in patients with ankylosing spondylitis (AS), according to recent research published in the Journal of Rheumatology.

“To our knowledge, ours is the first study to investigate [zygapophyseal joint] ZJ fusion and syndesmophytes simultaneously in patients with AS using CT [computed tomography] scanning, and the first longitudinal study of ZJ fusion,” Sovira Tan, PhD, from the Intramural Research Program and National Institute of Arthritis and Musculoskeletal and Skin Diseases at the National Institute of Health (NIH) in Bethesda, Maryland, and colleagues wrote in their study.

Dr Tan and colleagues enrolled 55 patients at the NIH and Johns Hopkins Medical Institutions in Baltimore, Maryland, who underwent CT scans of the thoracic (T10-T11) and lumbar spine (L3-L4). A rheumatologist and a musculoskeletal radiologist read and scored anonymized scans for ZJ fusion on the sagittal and axial planes, and these scores were compared with the extent of bridging and syndesmophyte height in each scan.

A subset of patients were available for repeat CT scans at 2 years and 4 years after baseline. The researchers used multivariate analysis to determine the effect of syndesmophytes and ZJ fusion with the modified Schober test and lateral thoracolumbar flexion.

They found ZJ fusion in at least 1 vertebral level in 51% of patients, with 129 of 652 individual ZJs analyzed showing fusion. There were 216 of 326 levels (66%) with syndesmophytes and bridging in 102 vertebral levels (31%), with 44% to 45% of vertebral levels showing syndesmophytes but not ZJ fusion, according to the 2 readers.

“ZJ fusion was more common at the thoracolumbar junction than in the lower lumbar spine, as are syndesmophytes,” Dr Tan and colleagues wrote. “However, syndesmophytes were often present at vertebral levels without ZJ fusion, while ZJ fusion was rarely present without syndesmophytes. This pattern suggests that syndesmophytes most often develop before ZJ fusion occurs in the same vertebral level. Structural spinal damage would therefore be detected earlier by syndesmophyte growth than by ZJ fusion in most cases.”

There was a higher level of vertebral levels containing ZJ fusion and bridging (55.9%) compared with ZJ fusion and syndesmophytes (34%). In a multivariate analysis, there were similar associations in patients with syndesmophytes and ZJ fusion in the modified Schrober test, and a strong association between patients with syndesmophytes and decreased lateral thoracolumbar flexion.

Summary & Clinical Applicability

“Thoracolumbar ZJ fusion is common in AS and is an important contributor to restricted spinal motion,” Dr Tan and colleagues wrote. “However, based on their higher prevalence, earlier appearance, greater rates of change, and associations with lumbar motion, syndesmophytes are a more useful imaging marker of spinal fusion in AS.”

Limitations

The researchers reported the study’s modest size, lack of finer gradation in joint scoring, lack of information on the association between syndesmophytes and ZJ damage outside fusion, lack of cervical or upper thoracic spine analysis, and lack of lumbar oblique films for comparison of plain radiography with CT as limitations in the study.

Disclosure

The researchers report grants from the Intramural Research Program, US National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH, the NIH Clinical Center, and the Johns Hopkins University School of Medicine General Clinical Research Center.

Reference

Tan S, Yao J, Flynn JA, et al. Zygapophyseal joint fusion in ankylosing spondylitis assessed by computed tomography: associations with syndesmophytes and spinal motion [published online May 15, 2017. J Rheumatol. doi:10.3899/jrheum.161462

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