CSFL is a common complication of spinal surgery, and the incidence of different sites varies greatly. Hannallah  reported that the incidence of CSFL after cervical surgery was only 1%. Marcelo  reported that the incidence of CSFL after lumbar vertebrae surgery was 3.2%. The incidence of CSFL after thoracic surgery was much higher than those of other parts of the spine, which was reported to be about 20%ཞ40% [,,]. Moreover, the operative approach differed significantly. Hu  counted 362 cases of thoracic surgery, and the overall incidence of CSFL rate was 32.3%. The incidence of anterior decompression (AD), posterior decompression (PD) and circumferential decompression (CD) were 20.5%, 31.1% and 41.8%, respectively. The incidence of CSFL of CD was much higher than other surgical approaches. Takahara  reported that the incidence of CSFL after CD was as high as 40%. The posterior approach of CD was a direct decompression, which required the removal of ventral compression of the spinal cord from the rear. As a result, spinal cord occlusion led to narrow space and limited visual field, which was the reason why the incidence of CSFL of circular decompression surgery was higher than that of other approaches. The surgical method in this study is CD, and the incidence of CSFL was 28.0%, which was slightly lower than the previous result. The improvement of surgical instruments is an important factor. During the operation, we used the piezosurgery to assist in spinal cord decompression. The safety and efficiency of piezosurgery in the removal of bone structure have been recognized by academic circles. Compared with the traditional bone knife or high-speed drill, high-energy, high-frequency but low-amplitude characteristics can achieve basically no vibration when removing ossified blocks, and the characteristics of “cutting soft and not cutting hard” can help to avoid dural injury. As a result, the rational use of piezosurgery can effectively reduce the incidence of CSFL. But it must be admitted that even if the incidence of CSFL could be reduced through operator’s surgical techniques and improved surgical instruments, nearly a third of cases were diagnosed with CSFL. Therefore, predicting the related factors that can easily lead to CSFL before surgery and screening out the high risk population in thoracic OPLL circumferential decompression was helpful to warn the operator and inform patients of the risk in advance.
At present, there are few reports of exploring predictors of CSFL after thoracic OPLL circumferential decompression, and there are no universally-acknowledged conclusions. Sun  found that patients with more than 3-segment laminectomy (odds ratio = 2.4, P odds ratio = 0.01) have higher incidence of CSFL (odds ratio = 2.4, P < 0.01). In this study, although the number of removed vertebral plates in the CSFL group was significantly higher than in the non-CSFL group (5.9 VS 3.7, P = 0.01), there was no statistical difference in logistic regression analysis. This was related to differences in the etiology and surgical methods of the two studies: Sun studied the cases with OLF, the surgical method was posterior approach laminectomy and decompression, and the dural injury occurred only in the dorsal spinal cord. By contrast, in this study, the included case was OPLL, the surgical method was circumferential decompression. Dural injury occurred more often in the ventral spinal cord. As a result, the factors leading to CSFL were more complex. Nicholas  found that patients with diabetes (OR = 2.3, P = 0.04) or smoking history (OR = 3.4, P = 0.02) were more likely to develop CSFL in the study of dural injury during lumbar surgery. The smoking history in this study is also a risk factor for the CSFL after thoracic OPLL circumferential decompression (OR = 30.1, P = 0.003), which was consistent with the study by Nicholas. It is widely believed that smoking can lead to thinning of the fascia and decreased toughness . The dura mater, though different from the abdominal pelvic fascia, has a similar composition and structure. It can therefore partly explain why smoking increases the risk of CSFL. Hu  found that CSFL is more likely to occur in T5-7 circular decompression surgery than other segments. In our study, the probability of CSFL occurring in the upper thoracic segment (OR = 188.0, P = 0.002) and the middle thoracic segment (OR = 57.4, P = 0.005) was also significantly higher than that of the lower thoracic segment. This is mainly because the middle and upper thoracic segments were narrower and had limited surgery space, so they were likely to cause dural injury.
The shape of OPLL itself may also be the cause of postoperative CSLF. The sagittal and transverse diameters of OPLL were used to evaluate the occupying ratio and OPLL base ratio. It was found that the vertebral occupying ratio and OPLL base ratio in the CSFL group were higher than those in the non-CSFL group. The multi-factor regression analysis showed that the OPLL base ratio was a predictor of postoperative CSFL. The wider the OPLL base, the more likely postoperative CSFL (or = 1.3, P = 0.007) will happen. Du  came to a similar conclusion in studying the risk factors of CSFL after anterior approach cervical OPLL decompression. Du collected the data of 90 patients who used anterior approach cervical decompression to treat cervical OPLL, and found that CSLF is more likely to happen during resection of wide-base OPLL (OR = 1.09, P = 0.012) than that of narrow-base OPLL. The reason was that the base at both head and tail of OPLL was cut off in advance in order to take out the OPLL ossific block. As a result, the wide base of OPLL will increase the difficulty of resection, cause more dural invasion, and develop CSLF. After cutting the base at both head and tail of OPLL, we used the “collapse method” to push the free OPLL ossific block directly to the anterior vertebral gap without directly contacting the final part of the OPLL ossific block near the midline. Therefore, the OPLL sagittal diameter had little effect on the formation of CSFL, which was consistent with the results of regression analysis in this paper.
As far as I know, there is no literature on the predictive model of CSFL after thoracic circumferential decompression. This study attempted to establish a predictive model of CSFL probability after thoracic OPLL circumferential decompression. The Logistic regression analysis found that smoking history, segment of circumferential decompression and OPLL base were statistically significant, which can be used to predict the occurrence probability of postoperative CSFL. In general, the area under the ROC curve > 0.6 indicates that the model has good predictability. The ROC of the model was 0.955, indicating that the model can well predict the risk of CSFL after thoracic OPLL circumferential decompression. There are still some shortcomings in this study: (1) Because the focus of this study is to explore the predictive model of CSFL, patients were not followed up for long periods. So the long-term effects of CSFL on patients, such as pseudocyst formation, nerve function recovery and so on, require further follow-up studies; (2) The number of cases was limited, which was limited by the lower incidence of thoracic OPLL, and not all cases require circumferential decompression, so long-term accumulation of cases is required.