Thoracic Ossification of the Posterior Longitudinal Ligament: Thickness Changes After Posterior Decompression and Corrective Fusion in a Longitudinal Study With up to 10-Year Follow-up

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Abstract
Study Design: Retrospective cohort study. Objectives: Thoracic ossification of the posterior longitudinal ligament (OPLL) is traditionally considered progressive, raising concern that the ossified mass left after posterior decompression and instrumented fusion (PDF) may enlarge. We characterized long-term CT-based changes in focal OPLL thickness after PDF and assessed whether the residual mass continued to enlarge. Methods: We reviewed all consecutive adult patients who underwent PDF for thoracic OPLL (2009–2023) with ≥2 years of follow-up. Maximum OPLL thickness was measured on axial CT and tracked longitudinally; a predefined 2-mm threshold, prespecified as the minimal clinically important difference, classified change. Thoracic kyphosis (TK) and modified Japanese Orthopaedic Association (mJOA) scores were also assessed. Results: Twenty-six patients (mean age 49.5 ± 12.4 years; 15 men) were followed 5.4 ± 2.7 years (range 2–11). Mean axial OPLL thickness changed from 7.02 ± 1.58 to 6.90 ± 2.01 mm (Δ −0.12 ± 1.03 mm). By the 2-mm threshold, 92.3% were stable, 3.8% decreased, and 3.8% increased; directional decreases occurred in 69.2% and increased with longer follow-up (37.5%, 77.8%, 88.9%; p = 0.023). TK increased (+6.2°, p < 0.001) but was uncorrelated with thickness change (r = 0.250). mJOA improved from 7.3 to 8.7 (p = 0.0001; recovery rate 34.9%; excellent/good 53.8%). The mean change was equivalent to zero within ±2 mm (two one-sided tests p < 0.001). Conclusions: Focal thoracic OPLL thickness remained predominantly stable after PDF, with threshold-level progression infrequent and neurological improvement maintained. Focal radiographic stability, rather than continued enlargement of the residual mass, was the predominant postoperative course after PDF.

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