Ossification of the posterior longitudinal ligament (OPLL) is rare, especially in the thoracic spine. However, for patients with OPLL and thoracic spinal stenosis, surgery is often the only choice to solve the clinical symptoms. At present, there are a variety of surgical schemes for the treatment of T-OPLL. There are also many literature reports on the surgical methods for the treatment of T-OPLL, and they have a certain curative effect. Including anterior direct decompression, posterior direct decompression, posterior indirect decompression, anterior-posterior decompression, 360 ° circular decompression, "culvert collapse method" and "sliding osteotomy", but these technologies are not enough in terms of surgical technical difficulty, spinal cord safety, incidence of intraoperative and postoperative complications and improvement rate of postoperative symptoms.
As a new surgical method, thoracic column antedisplacement and fixation (TCAF) achieves direct decompression of the spinal cord while avoiding direct removal of ossification, which greatly reduces the difficulty of the operation and the risk of complications [10]. However, it is not the case that a greater antedisplacemental distance results in a better decompression without producing spinal cord injury. Because antedisplacing the vertebrae can lead to nerve root retraction and can involve the spinal cord at the same time, excessive antedisplacement can lead to mechanical spinal cord injury. This study investigated the safety of the spinal cord by thoracic column antedisplacement and fixation in goat models.
Ten healthy goats were selected in this experiment. Because their structural composition and biomechanics of thoracolumbar spine are similar to those of humans, there are 13 thoracic vertebrae and 6 lumbar vertebrae in goats, and the spinal cord terminates near the level of L6 [16–17], whereas in humans it terminates near the L1-2 level. Further, the T12-L1 segment, which matches the thoracolumbar level in humans, were selected as the antedisplacemental segment in our study. It is representative and feasible in investigating the ultimate antedisplacemental distance of TCAF and the safety of spinal cord.
Inserting 3.5mm*25mm pedicle screws on both sides of the T12-L1 so as to reserve enough thread length to antedisplace the vertebrae. Inserting 3.5*20mm pedicle screws on both sides of T10-11 and L2-3 so as to lock the nuts after implanting the connecting rod. Observing 10 minutes after completing the procedure of antedisplacement. When the SSEP monitoring just did not show a positive result, the length of sunken thread of T12-L1 was the ultimate antedisplacemental distance, and then locking the nut and finishing the experiment. Combined with postoperative imaging measurement, the ultimate antedisplacemental distance was about 8.0 ± 0.2mm. In the procedure of goat animal experiment, the monitoring of motor evoked potential was greatly disturbed by ultrasonic bone knife, electric knife instrument, attractor and other instruments. The monitoring system was unstable and couldn’t continuously evaluate the motor pathway. Therefore, SSEP was used to continuously evaluate sensory nerve pathways throughout the experiment, and some literatures showed that the use of muscle relaxants during operation would not affect the monitoring results of SSEP and reduced the experimental errors caused by other factors [18–19].
SSEP monitoring is a common method for the function of spinal cord through sensory nerve pathway in animal spinal surgery [9]. A delay in the amplitude loss of SSEP from baseline of more than 50% and / or relative to baseline peak latency of more than 10% is generally considered a positive result [13], and the amplitude is more sensitive than the peak latency of SCI. However, during SSEP monitoring, a series of false negative results closely related to hypotension, hypothermia and inhalation anesthesia will appear [20–21]. Therefore, in our experiment, the arterial pressure and temperature of goats were closely monitored to keep normal. In addition, we used intravenous anesthesia instead of inhalation anesthesia. Finally, in order to minimize the incidence of false negative SSEP results, wake-up test was used to evaluate the function of spinal cord when SSEP did not show positive results.
We found that the average anteroposterior diameter of T12-L1 spinal canal was about 9.6 ± 0.1mm, and the average width of T12-L1 vertebrae was 14.6 ± 1.2mm. During the goat experiment, we found that when three vertebrae of T12-L1 were antedisplaced and there was no positive result in intraoperative SSEP monitoring, the ultimate antedisplacemental distance measured by postoperative X-ray and CT was 8.0 ± 0.2mm. Due to the different position and direction of the needle entry point, the length of the reserved thread during the operation can be different, and because the direction of the pedicle screws forms a certain angle with the sagittal position of the spine, the length of the reserved thread measured during the operation was longer than the ultimate antedisplacemental distance measured by postoperative imaging. Therefore, in order to ensure the unity and accuracy of the results, the distance measured by postoperative imaging was taken as the ultimate antedisplacemental distance (Fig. 7).
The linear regression analysis obtained the linear equation as Y=-7.769 + 1.648X. This equation can be used to preliminarily calculate the estimated antedisplacemental distance based on the relevant parameters of the vertebrae. During the operation, the length of the reserved thread did not exceed this ultimate antedisplacemental distance, because the screw placement direction had a certain angle with the sagittal position of the spine, then the actual antedisplacemental distance was less than the reserved thread length, which could ensure the safety of the spinal cord. Whether the linear equation can help in the development of clinical TCAF surgery remains to be tested. If the ultimate antedisplacemental distance, the ratio of the anteroposterior diameter of the antedisplacemental spinal canal or the ratio of the average width of the antedisplacemental vertebrae can be clearly quantified, it will greatly promote the development of clinical TCAF surgery. The ossifications have a risk of continued growth, so the TCAF surgery requires adequate decompression to prevent them from growing and recompressing the spinal cord. Thus, if the decompression is sufficient but the antedisplacemental distance less than the ultimate antedisplacemental distance, which means that we can antedisplace a certain distance to reserve more safety space.
Finally, since the basic data of this experiment was based on the goat model, which differed from humans in the anteroposterior diameter of the spinal canal, spinal cord accommodation, and tolerance of the spinal cord, they could only be informative and not definitive, so they could only be of reference rather than decisive significance to humans. However, our study showed that there was a linear relationship between the ultimate antedisplacemental distance and the anteroposterior diameter of the spinal canal, and there may be some kind of linear relationship in humans. However, the analysis of this linear relationship in humans should be more rigorous, since spinal cord injuries are often prone to irreversible consequences. If we can explicitly quantify this value, it will be of great help in the development of clinical TCAF surgery.