Clinical Characteristics and Surgical Outcomes of Thoracolumbar Ossification of the Ligamentum Flavum with Concomitant Thoracolumbar Scheuermann’s Disease: A Retrospective Study

Technical Summary: Clinical Characteristics and Surgical Outcomes of Thoracolumbar Ossification of the Ligamentum Flavum with Concomitant Thoracolumbar Scheuermann's Disease

Problem Statement
Thoracic spinal stenosis is frequently caused by thoracic ossification of the ligamentum flavum (TOLF), a condition where fibrous tissue transforms into bone within the ligamentum flavum. While TOLF is a known cause of thoracic myelopathy, clinical observations at the study center revealed a significant subset of patients with thoracolumbar TOLF (TLOLF) exhibiting concurrent wedge-shaped deformities of continuous vertebral bodies, suggestive of thoracolumbar Scheuermann's disease (TLSD). The specific clinical characteristics, sagittal alignment parameters, and postoperative prognoses of TLOLF patients with concomitant TLSD had not been systematically investigated. This study aims to fill that gap by comparing TLOLF patients with TLSD against a control group of TLOLF patients without the deformity.

Methodology
This was a single-center, retrospective study conducted between 2012 and 2022 involving 100 patients who underwent surgery for TLOLF (defined as ossification located between T10 and L2).

• Grouping: Patients were divided into a TLSD group ($n=50$) and a control group ($n=50$). TLSD was strictly defined as the presence of at least three consecutive thoracolumbar vertebral bodies with wedge angles greater than 5°.
• Exclusion Criteria: Patients were excluded if they had other thoracic spinal lesions (e.g., ossification of the posterior longitudinal ligament, disc herniation, tumors), coexisting cervical or lumbar spondylosis, or a history of prior spinal surgery.
• Surgical Intervention: All subjects underwent posterior en bloc resection of the lamina and instrumented fusion performed by a senior surgical team.
• Data Collection: Researchers collected demographic data, sagittal spinal alignment parameters (including SVA, CL, TK, TLK, LL, PI, SS, PT), TOLF morphology classifications (Sato and Kuh), spinal canal occupying rates (sagittal and cross-sectional), and postoperative neurological recovery rates (calculated using the modified Japanese Orthopaedic Association [mJOA] score).
• Statistical Analysis: Quantitative variables were analyzed using independent-sample t-tests or Wilcoxon rank-sum tests, while categorical variables were analyzed using chi-square tests.

Key Contributions
This study represents the first investigation specifically targeting the clinical profile and surgical outcomes of TLOLF patients with concomitant TLSD. It provides a comparative analysis of:

1. Demographic and Clinical Profiles: Establishing a distinct male predominance in the TLSD subgroup.
2. Sagittal Alignment: Quantifying the specific impact of TLSD on thoracolumbar kyphosis (TLK) versus other global spinal parameters.
3. Prognostic Correlation: Linking the presence of TLSD and associated mechanical stress to symptom duration and postoperative neurological recovery.
4. Morphological Impact: Determining whether the underlying skeletal deformity alters the specific morphology or canal occupation of the ossified ligament.

Results
The comparison between the TLSD group and the control group yielded the following statistically significant findings:

• Demographics: The TLSD group had a significantly higher proportion of males (76% vs. 32%, $P < 0.001$).
• Symptom Duration: Patients in the TLSD group reported a significantly shorter duration of symptoms prior to surgery (median 7 months vs. 18 months, $P = 0.006$).
• Sagittal Alignment: The TLSD group exhibited significantly greater thoracolumbar kyphosis (TLK) ($14.69^\circ \pm 9.66^\circ$ vs. $7.91^\circ \pm 5.43^\circ$, $P < 0.001$). No significant differences were found in other sagittal parameters (SVA, CL, TK, LL, pelvic parameters) or overall sagittal balance.
• Postoperative Recovery: The TLSD group demonstrated a significantly higher neurological recovery rate (66.7% vs. 41.4%, $P = 0.003$).
• Morphology and Canal Occupation: There were no significant differences between the groups regarding TOLF classification (Sato or Kuh types), spinal canal signal changes, or spinal canal occupying rates in either sagittal or cross-sectional planes.
• Surgical Outcomes: No significant differences were observed in operation duration, blood loss, or the incidence of complications such as cerebrospinal fluid leakage or epidural hematoma.

Significance and Claims
The authors conclude that the presence of TLSD in TLOLF patients is associated with greater thoracolumbar kyphosis, which likely exerts increased mechanical stress and tensile force on the ligamentum flavum. The study posits that this mechanical environment may accelerate the ossification process, leading to a shorter disease course and necessitating earlier surgical intervention. Consequently, these patients achieve better postoperative neurological recovery, likely because earlier intervention prevents irreversible spinal cord damage associated with prolonged symptom duration.

The paper explicitly states that TLSD does not influence the morphological pattern of the ossification (TOLF type) or the degree of spinal canal invasion. The authors maintain a modest stance, acknowledging limitations such as the retrospective nature of the study and the inability to pinpoint the exact onset of vertebral wedge deformation relative to ossification. They recommend that TLOLF patients with TLSD be closely monitored due to the potential for rapid disease progression, but they do not propose new surgical techniques or future experimental protocols beyond the scope of the observed clinical data.