I. Inclusion and exclusion criteria
The inclusion criteria were as follows: having received a clinical diagnosis of single-segment OLF, having received indications for surgery, having complete clinical data and imaging examination data, and having been informed of the study details and signed informed consent forms.
The exclusion criteria were as follows: severe internal medical diseases such as cardiovascular and cerebrovascular diseases, respiratory diseases, and liver and kidney dysfunction preventing the patient from tolerating surgery; other severe degenerative diseases; cognitive impairment or mental illness; and incomplete clinical and radiological data.
II. General information
This study included 16 patients. The T11/12 segments were affected in 6 patients, the T10/11 segments were affected in 6 patients, and the T9/10 segments were affected in four patients. All of the cases involved the lower thoracic vertebrae. Of these patients, 7 were men, and 9 were women; the patients were aged 46-68 years, with an average age of 54.31 years. The disease duration was 1-24 months, with an average duration of 7.1 months. The clinical manifestations of the disease were sensory disorders in 14 patients, lower limb muscle loss in 9 patients, sphincter dysfunction in 0 patients, chest and waist band sensation in 1 patient, intermittent claudication in 6 patients, lumbar and leg pain in 11 patients.
III. Surgical methods
All patients were anesthetized with tracheal intubation. After anesthesia, the patient was placed in the prone position. Preoperatively, a C-arm X-ray machine was used for visualization, and the projection of the diseased vertebrae and the adjacent upper and lower vertebrae were marked on the body surface. Routine disinfection was performed, and aseptic towels were spread. A puncture was made by a Kirschner needle at the point 1 cm outside the posterior median line to locate the lateral lamina behind the lesion segment. With the guidance of C-arm visualization, the Kirschner needle was confirmed to be located on the posterior osseous wall of the lamina, and its axis was extended to the ossified ligamentum flavum (Fig. 1a). An incision measuring approximately 7 mm was made on the skin, the Kirschner needle was inserted into the extension rod, and the working catheter was inserted into the posterior wall of the spinal canal. A ring saw was used to remove a piece of the lamina to make a tunnel (fig. 1b), the guide rod, laminar bone piece and ring saw were removed(fig. 1c), and endoscopic system was placed. The ossified ligamentum flavum was then resected by endoscopic rongeur to decompress the spinal cord and nerve(fig. 1d & e). To explore the dural sac, radiofrequency ablation of surrounding soft tissue was performed(fig. 1f). The catheter was removed after saline irrigation, and the skin was sutured. The operative time and the amount of bleeding were recorded, and the changes in symptoms and signs were observed after the operation.
IV. Imaging measurements
A thoracic computed tomography (CT) plain scan was performed before the operation, and thoracic CT scans were performed 3 days and 1 year after the operation. SURGIMAP software was used to measure the area and proportion of the OLF and the proportion of vertebral canal encroachment.
Methods of measurement:
1. The sagittal area of OLF on the CT scans (As) was measured as follows:
For the preoperative measurements, the sagittal CT images with the largest magnitude of OLF forward protrusion were selected to mark the upper and lower ends of the ossified ligamentum flavum (the junction of the segment ligamentum flavum and the upper and lower lamina), and the anterior wall of the spinal canal was intersected at two points perpendicular to the posterior wall of the spinal canal. Two straight lines were made with the anterior and posterior walls of the spinal canal to make rectangular regions; that is, the sagittal area. The high-density shadow in the sagittal spinal canal area was outlined, and the area of the ossified ligamentum flavum was measured (fig. 2a & b).
Sagittal OLF area =As (OLF)
For the postoperative measurements, the same level of the sagittal CT sagittal images was selected, and the same rectangular area was selected as the sagittal spinal canal area. The residual OLF area was measured (fig. 2c & d).
2. The proportion of OLF on the sagittal CT scans (Ps) was measured as the ratio of the ossified ligamentum flavum area to the spinal canal area.
Ps= As (OLF)/As (Canal)
3. The cross-sectional area of OLF on the CT scans (Ac) was measured as follows:
For the preoperative measurements, the segment with a lesion on the transected CT image with the largest magnitude of OLF protrusion to the front was selected, and SURGIMAP software was used to outline the spinal canal wall contour, including the posterior wall of the vertebral body, the lamina and the articular process joint. The surrounding area was the Ac area of the canal on cross-section. The high-density shadow in the area of the cross-sectional canal, as the area of OLF on the transected scan, was outlined and measured (fig. 3a & b):
Cross-sectional area of OLF= Ac(OLF)
For the postoperative measurements, the same location on the image was selected as before the operation in the cross-sectional CT image, the same spinal canal area was selected as before the operation, and the contour of the area was outlined. The residual OLF area was measured (fig. 3c & d):
Cross-sectional area of OLF= Ac(OLF)
4. The cross-sectional proportion of OLF on the CT scans (Pc) was measured at the above level as the ratio of the ossified ligamentum flavum area to the spinal canal area.
Pc= Ac (OLF)/Ac (Canal)
V. Clinical Assessment:
The clinical symptoms were evaluated by the modified Japanese Association of Orthopedics scale(mJOA), visual analog scale for pain(VAS), Oswestry lumbar dysfunction index(ODI) and Macnab evaluation before the operation, 3 days after the operation and 1 year after the operation. The Hirabayashi recovery rate of all patients were calculated using the preoperative and final follow-up mJOA scores[18]