Background/Purpose: Erosions, sclerosis and squaring are included in the modified Stoke Ankylosing Spondylitis (AS) Spine Score (mSASSS). However, their value in predicting the development of new syndesmophytes has not been established, and their value in the mSASSS has been argued. We aimed at analysing the effect of erosions, sclerosis and squaring on the development of new syndesmophytes over 12 years.

Methods: Biannual radiographs from patients with AS (mNY criteria) followed in OASIS (up to 12 years) were included. Two readers (R1 and R2) independently scored the x-rays according to the mSASSS and separately registered all the items with a score of 1 (erosions, sclerosis and squaring, or any combination) per vertebral corner (VC). The progression from erosions, sclerosis and squaring to new (bridging) syndesmophytes (either score of 2 or 3 in the mSASSS) in a 2-year period and over the 12 years was investigated by means of a multilevel (adjusted for time, reader and cervical/lumbar spinal region) auto-regressive and time-lagged longitudinal model, using generalized estimating equations. Interactions with reader and spinal region were investigated.

Results: 211 patients were included in this analysis (mean age 43 (SD 13) years old, 71% male, 85% HLA-B27 positive, 11.5 (SD 9.1) years since diagnosis and 20.5 (SD 11.6) years since symptom onset, 4.4 (SD 1.8) radiographs per patient). A total of 921 radiographs were included in this analysis, with a total of 20509 VCs (R1) and 20568 VCs (R2) evaluable.  Of these, erosions were scored in 1% for R1 (2.5% R2), sclerosis in 0.3% for R1 (1.7% R2), squaring in 6.5% for R1 (5.1% R2) and syndesmophytes in 25% for R1 (27% R2). The presence of erosions, sclerosis and squaring together had, compared with a normal VC, a risk for the development of new syndesmophytes over a 2-year period of 2.0 (Table). There was a significant interaction with ‘spinal region’ (OR cervical spine: 3.1; OR lumbar spine: 1.3). There was no significant difference between the readers. Separately, the presence of erosions or the presence of sclerosis, but not the presence of squaring, gave an increased risk for new syndesmophytes, Squaring was predictive of the development of new syndesmophytes in the cervical spine only (Table).

Conclusion: Erosions and sclerosis occur rarely on X-rays, but if so, are predictive for the development of new syndesmophytes. Therefore, these lesions not only seem to precede the development of syndesmophytes, but are also correctly included in the mSASSS. The possibility of excluding squaring from the mSASSS in the lumbar spine may be considered.

Table – Longitudinal effect (GEE) of erosions, sclerosis and squaring on the development of new syndesmophytes over 12 years (adjusted for spinal region and reader)

		Multivariable GEE models with syndesmphytes as the outcome* OR (95% CI)	Same model, but with results stratified for spinal region or reader if significant interactions OR (95% CI)
	Factor: Erosion, sclerosis and/or squaring (score of 1 in mSASSS)	2.0 (1. 7 – 2.3)
Stratification	– Cervical spine		3.1 (2.5 – 3.9)
	– Lumbar spine		1.3 (1.0 – 1.6)
	– Reader 1		No significant interaction
	– Reader 2
	Factor: Erosions	2.1 (1.6 – 2.8)
Stratification	– Cervical spine		No significant interaction
	– Lumbar spine
	– Reader 1		No significant interaction
	– Reader 2
	Factor: Sclerosis	5.1 (3.9 – 6.8)
Stratification	– Cervical spine		No significant interaction
	– Lumbar spine
	– Reader 1		15.3 (9.2 – 25.5)
	– Reader 2		3.9 (2.8 – 5.5)
	Factor: Squaring	1.3 (1.0 – 1.6)
Stratification	– Cervical spine		8.5 (2.1 – 34.7)
	– Lumbar spine		1.1 (0.8 – 1.4)
	– Reader 1		1.6 (1.2 – 2.1)
	– Reader 2		1.0 (0.6 – 1.4)

* Adjusted for spinal region and reader (each row reflects the OR for main predictor of the model, all different models).

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/erosions-and-sclerosis-but-not-squaring-predict-the-development-of-new-syndesmophytes-a-12-year-longitudinal-analysis-oasis/