The effective volume of the thoracic spinal canal is relatively narrow compared to that of the lumbar spinal canal. If there is anterior or posterior compression, there is almost no buffer space, and the compensatory ability is poor. Additionally, the thoracic spinal cord has a significantly lower blood supply than the cervical and lumbar segments, and less collateral circulation. Postoperative neurological deterioration is related to intraoperative injury of blood vessels around the spinal cord [13]. Owing to these anatomical and blood supply characteristics, the thoracic spine was once considered a restricted area regarding spinal surgery. Reported rates of postoperative neurological deficit range from 5.7 to 33% about thoracic spine surgery [14,15,16].
TOLF mostly occurs in the lower thoracic spine. Low back pain and numbness of the lower limbs can be the main clinical manifestations in early-stage TOLF, which can be mistakenly attributed to lumbar spine disease, thus reducing the chance of early diagnosis. When there is progressive neurological dysfunction, early decompression should be performed to prevent irreversible spinal cord damage. The location of the compression, number of segments involved, and general patient condition should be used to determine an appropriate treatment plan.
PTL is usually used to relieve posterior compression, which is mainly caused by OLF [17, 18]. Although laminectomy achieves good spinal cord decompression, excessive removal of the lamina and facet joint can cause thoracic instability [19]. Therefore, pedicle screw fixation and fusion are sometimes required. Kim et al. [20] reported successful bilateral decompression using unilateral laminectomy to treat 11 cases of TOLF. The recovery rate was 33.2%. Although no fusion was performed, the rates of postoperative thoracic kyphosis and instability did not increase. However, laminectomy inevitably comes at the price of complications such as increased rates of acute neurological deterioration and dura tears. Improper use of a laminar rongeur or an osteotome can also cause spinal cord concussion or other spinal cord injury.
Some researchers have recommended treating TOLF with laminoplasty, which achieves nerve decompression by expanding the volume of the spinal canal without removing the ligamentum flavum. Because most of the posterior structure of the spinal canal is retained, there is little effect on spinal stability. However, it is not recommended for severe OLF because the expandable space is limited and lamina reclosure can occur [21]. The current most widely used surgical technique for treating TOLF remains laminectomy with or without fusion [22].
Given the sensitivity of the thoracic spinal cord and the special anatomy of the thoracic spine, it is necessary to develop an effective surgical technique that is less traumatic, which could ensure fewer complications. In recent years, endoscopic techniques have been used for cervical and lumbar spine surgery, and they have achieved good clinical results. The concept of minimally invasive surgery is not new, but it needs to be emphasized that the surgery involves reduced invasiveness while still ensuring efficacy. Both Jia et al. [8] and Miao et al. [9] reported successful treatment of TOLF using PETD. An et al. [23] performed PETD to treat 18 patients with various types of TOLF. At a mean follow-up point of 17.4 months, the recovery rate was 47.5% and the modified Japanese Orthopedic Association (mJOA) score was obviously improved.
Endoscopic spinal surgery has unique advantages for the treatment of TOLF. It can be performed under local anesthesia. Most traditional surgery uses general anesthesia, and some intraoperative neuromonitoring is performed. However, studies have shown that intraoperative neuromonitoring does not reduce neurological complications [24]. Under local anesthesia, the patient is conscious and able to communicate with the surgeon at any time. When the spinal cord or nerve roots are touched during the operation, the patient may have pain or numbness, and the surgeon can immediately stop the operation to avoid nerve damage. Especially for elderly patients for whom general anesthesia is unsuitable, PETD may be a solution worth considering. In our study, the spinal canal is fully decompressed and the preoperative symptoms are relieved (Figs. 4, 5). No patient discontinued the surgery due to pain or psychological stress. In most cases, intervertebral fusion is not necessary and medical expenses are therefore reduced. To evaluate the approximate decompression range, and especially to determine the removal range regarding the caudal and cranial sides of the lamina, we used a trephine to clearly mark the surface of the lamina under fluoroscopic guidance. This made judging the decompression boundary under endoscopic visualization easier. In addition, the spinal endoscopy technology magnified the field of vision. During the operation, radiofrequency coagulation of small blood vessels and bleeding points was used to ensure a clear field of vision. This helped to accurately remove the lesion and reduce damage to surrounding soft tissues. Using a diamond high-speed drill with continuous saline irrigation reduced local high temperature. After thinning the OLF, it was removed using nucleus pulposus forceps. However, the spinal cord might still be accidentally irritated during this process. In this study, two patients developed transient paralysis in surgery, and recovered after glucocorticoid therapy. Dural adhesion and dural ossification increase the risk of dural tear and the difficulty of surgery [25, 26]. In the PETD group, dural adhesion was found in 3 patients during operation, and one of them had dural tear. One patient was accompanied by dural ossification. After thinning and floating the ossified dural, we did not forcibly remove it, which did not affect the postoperative recovery. Two patients in the PTL group experienced dural tear with cerebrospinal fluid leakage due to dural adhesion.
The use of PETD to treat TOLF requires the surgeon to be skilled in spinal endoscopic decompression, and the learning curve is relatively steep. Regrettably, we were not able to provide quantitative measurements for facet removal between the two groups due to radiographic data was not complete enough. The limitations of this retrospective study are the small sample size and lack of longer follow-up. To objectively and comprehensively evaluate the safety and effectiveness of this method, more patients need to be recruited for multicenter, randomized controlled trials in the future.