In present study, we introduced a modified foraminoplasty using trephine combined with anchor drill under fluoroscopy guidance, and compared it with routine foraminoplasty in which trephine was used alone. Patients included in our study had significant symptom relief after surgeries and no severe complications occurred during the whole follow-up period. Compared with using trephine alone, combination of trephine with anchor drill had a lower bone residual incidence, lower incidence of neural irritation and shorter endoscopic operation time, all with statistically significant differences. Nevertheless, no significant differences were observed in fluoroscopy time, duration of foraminoplasty and VAS and JOA scores of each follow-up visit between two cohorts.
As was known, initial endoscopic transforaminal approach was based on the “inside-out” technique focusing on intradiscal decompression which allowed very limited observation of neural elements[3]. For patients with migrated or sequestered disc, lateral recess or foramen stenosis, the “inside-out” technique encountered great difficulty[17]. According to imaging studies, foraminoplasty can improve cross-section area of foramen considerably[18, 19]. Therefore, foraminoplasty is regarded as a critical technique in transforaminal approach, generating enough and safe space for working cannula and endoscopy by enlarging foramen, and then facilitating to perform exiting nerve decompression and epidural space exploration[15, 20–22].
Well-performed foraminoplasty facilitates subsequent decompression and discectomy. Conversely, poor-performed foraminoplasty will be of no help to surgery and even has defects of more bleeding and pain, longer operation time, higher risk of neural injury and longer rehabilitation time[15, 23–25]. Besides, normal annulus fibrosus will probably be injured additionally under inadequate foraminoplasty or foraminoplasty is absent. As a consequence, spinal surgeons have been exploring safe, efficient and precise foraminoplasty techniques[17, 26–28]. Nevertheless, each technique may have its merits and demerits. Foraminoplasty under the surveillance of C-arm fluoroscopy is thought to be efficient and time-saving, but it may put surgeons and patients in high radiation exposure and is associated with iatrogenic injuries of vascular, neural elements and articular facet due to the uncertainty without visualization[15, 17, 29, 30]. On the contrary, foraminoplasty under endoscopic visualization is better controllable, more reliable and less harm from radiation exposure, but the low efficiency, high expense are the major concerns[17, 28]. In addition, endoscopic visualized foraminoplasty and discectomy have a steep learning curve and a learning course and personal experience are necessary[31, 32]. Moreover, muscle, ligaments and facet cysts are difficult to identify under endoscopic view for beginners and are at risk of iatrogenic injury[32].
Generally, foraminoplasty is technically demanding and challenging, especially for beginners. This study focused on one problem we encountered in practice and tried to propose countermeasures, that was it was not always successful to take out the target bone structure of SAP together with trephine in routine foraminoplasty. The reasons may include the following: First, the volume of SAP is too small relative to the inner diameter of trephine. Second, the trephine protective cannula is not pressed down sufficiently so that the amount of target bone is too little to be taken out. Third, location of foraminoplasty is near to the tip of SAP and the amount of target bone is too little to be taken out. In conditions of failure, the residual bone could only be taken out with forceps under endoscopic view. The residual bone remaining in foramen or spinal canal may compress or irritate neural elements.
Therefore, anchor drill was designed and applied to assist trephine in foraminoplasty under fluoroscopy guidance. By anchoring the distal end of anchor drill into the target bone through the hollow trephine makes the cut bone expand inside the trephine, increasing the friction with the inner wall and reducing the incidence of bone dislodgement, are the rationale of anchor drill. Figure 5 illustrates a schematic diagram of the combination of trephine with anchor drill and trephine alone in foraminoplasty. In present study, among the 55 patients in combination group, only 5 patients were failure in foraminoplasty with a success rate of 90.91%, which was significantly higher than trephine group with a success rate of 66.67%. However, there were still a few failure cases in combination group. We consider the main reasons may include the following: First, the volume of SAP is too small relative to the size of tip of anchor drill. Second, the tip of anchor drill is not anchored stably with the cut bone.
Previous studies have reported that neurological complications after foraminoplasty and discectomy in PETD occurred in approximately 0–12.4% cases[14, 15, 33–37]. The total neurological complication rate of present study was 12%, among which neural irritation was 3.64% and 17.78% in combination group and trephine group respectively, and neural injury was 0% and 4.44% in combination group and trephine group respectively. The higher incidence of neural complications in trephine group may be close related to the higher bone residual incidence due to unsuccessful foraminoplasty. However, two patients in combination group also experienced symptoms of neural irritation. By reviewing their intraoperative fluoroscopic images, we believe that the two adverse events were both associated with the locations of foraminoplasty which were at the tip of SAP. It has also been reported that foraminoplasty at the tip of SAP was more likely to occur neurological complications than at the base of SAP.[14] Nevertheless, it cannot be ruled out that some slight neurological symptoms may be just a normal neurological phenomenon after nerve root decompression instead of neural irritation during foraminoplasty[17]. Although twelve patients experienced neural irritation or injury in our series, it had minimal influence on function and daily life of them. According to previous studies, minority neurological complications with various degrees did exist after foraminoplasty and discectomy, however, up to date, no serious cases were reported and all obtained recovery spontaneously or with conservative treatment[12, 14, 15, 17, 37–39]. In trephine group, one patient suffered local paraspinal muscle hematoma postoperatively, which might be attributed to blood vessel injury around the intervertebral foramen[40]. In our study, no recurrence occurred and no second surgery was required. Notably, care should be taken when manipulation due to this foraminoplasty procedure is still fluoroscopy-dependent and remained blind technique.
There were also some limitations in this study. Firstly, the sample size was small and the follow-up time was relatively short. Secondly, because of the retrospective design, selection and recall bias might exist. Thirdly, only patients with LDH at L4-5 level were included and the conclusions might not be comprehensive. Despite these limitations, this was a preliminary study on evaluation of efficacy and safety of anchor drill in foraminoplasty. Better designed, prospective studies with large sample sizes may be needed to further investigate the advantages and disadvantages of anchor drill in a comprehensive manner. Furthermore, multi-center studies comparing different modified techniques may be conducted to seek optimal foraminoplasty strategy.