Evolution of sagittal spinal shape for the development of thoracic ossification of ligamentum flavum

Thoracic ossification of the ligamentum flavum (TOLF) is a rare pathology for which limited research exists. While it is known that mechanical factors play a role in the development of TOLF, little is currently understood about the sagittal alignment and related mechanical stress involved in its development. This study aims to describe the sagittal alignment of patients with TOLF based on the pathologic evolution of the Roussouly classification. The current study evaluated the preoperative Roussouly type in consecutive patients who underwent posterior decompressive laminectomy with or without posterior screw fixation for TOLF between January 2015 and December 2021. The post-evolution sagittal alignments were analyzed using the classic Roussouly classification based on sacral slope (SS). To determine the pre-evolution Roussouly type, the patients were retrospectively classified using their individual PI and PT values. Lumbopelvic parameters and morphological index including inflection point (IP), lumbar apex (LA), and lordosis distribution index (LDI) were also evaluated. Forty-three patients (21 women and 22 men) were included; their mean age was 64.21 ± 11.01 years (range 43–81). The most affected level was T10-11 (48.83%). The mean PI was 50.81 ± 9.56°, the mean SS was 33.11 ± 8.61°and the mean PT was 17.69 ± 7.89°. According to the post-evolution Roussouly classification, type 2 shape was the most frequently observed type (n = 23, 53.5%) in the post-evolution classification while type 3 was the most common type observed in the pre-evolution classification (n = 22, 51.5% and P = .00). The level of IP and LA in type 3 moved caudally (around L2 and L4/5 level, respectively) and the LDI increased (77.98 ± 8.08%) than the normal standard value. The authors found that the majority of the patients had a false type 2 spine, which had evolved pathologically from Roussouly type 3 and exhibited increased LDI, a lowered level of IP, and a lowered level of LA. These changes of spinal shape, including the transition to long hypolordosis and increased length of the thoracic kyphosis, may have accentuated tensile stress at the lower thoracic spine and contributed to the development of TOLF.