S-ASD has been found more frequent than used to be after spinal fusion so that clinicians should be alert about the potential probability of ASD occurrence in patients who need to undergo lumbar spinal fusion preoperatively.
Risk factors of ASD after spinal fusion have been evaluated extensively but the reported results are not consistent. Zhong et al[1] analyzed risk factors of ASD after lumbar fusion in terms of adult lumbar spondylolisthesis and found that simultaneous decompression and preexisting spinal stenosis at the unfused adjacent level were significantly related to ASD, but patient-related factors, fused levels, and sagittal alignment did not seem to contribute to ASD. Kyeong Hwan Kim et al[2] evaluated the incidence of clinical and radiologic ASD and precipitating factors for clinical ASD in 69 patients with lumbar spondylolisthesis who underwent instrumented single-level interbody fusion at the L4–L5 level at more than 5 years after surgery, and reported that the occurrence of R-ASD and S-ASD was 84.0% and 24.0% respectively. Compared with patients with asymptomatic ASD, patients with S-ASD showed a significantly smaller lordotic angle at the L4–L5 level after operation. Maintaining the postoperative L4–L5 segmental lordotic angle at about 20° or more is beneficial for prevention of S-ASD. Hikono Aiki et al[3] conducted a study including 117 patients who underwent posterior lumbar fusion and followed up for at least 2 years. They found that the re-operation rate was 7.7% in ASD cases associated with multilevel fusion. Georgios et al[5] performed a retrospective cohort study among patients undergoing instrumented lumbar fusion for degenerative disorders (spondylolisthesis, stenosis, or intervertebral disc degeneration) with a minimum follow-up of 6 months, and found beyond fused level (OR=2.6) contributing to ASD.
Wang et al[6] summarized that higher BMI, preoperative disc degeneration at the adjacent segment and intra-operative superior facet joint violation were risk factors for ASD.
The lumbar lordosis (LL) angle is also considered as a risk factor of ASD after lumbar spinal fusion. Matsumoto et al[12] conducted a retrospective 1:5 matched case-control study including 20 patients who underwent revision surgery for symptomatic ASD after L4–5 PLIF and 100 patients who underwent L4–5 PLIF during the same period, and found no sign of symptomatic ASD, suggesting that pre- and postoperative lower LL were significantly associated with ASD. The above results indicate that the achievement of the appropriate LL may prevent ASD after lumbar spinal fusion. Ou et al[20] stressed that BMI was a risk factor (OR=1.68) for ASD in patients undergoing lumbar fusion for degenerative spine disorders. An increase of 1 mean value in BMI would increase the ASD incidence rate by 67.6%. Controlling body weight before or after surgery may help reduce the occurrence of ASD.
Clinicians have developed various surgical techniques to deal with degenerative lumbar spinal stenosis for preventing ASD, such as application of Wallis interspinous implants and dynamic internal fixation systems in spinal surgery, but high medical costs limit their applications.
In the present study, we evaluated the radiographic and clinical results of extended fusion surgery using two different pedicle screw-insertion and fusion techniques. Based on the above mentioned factors of ASD, less trauma, favorable improvement of lumbar lordosis, and satisfactory clinical outcomes were considered as key indicators for superior surgical technique.
As showed in Table 2-3, TPS-Domino-TLIF offered a shorter operative duration, smaller intraoperative EBL, a lower frequency of intra-operative fluoroscopy, and superior restoration of radiographic parameters compared with CBT-PLIF. The bone fusion rate at the operated level was similar between the two groups.
In our study, the LL angle obtained optimized the improvement rate in TPS-Domino-TLIF group due to the correcting spinal sagittal alignment ability of the pedicle screws and rod system. It is worth noting that prebending of the rods is crucial for restoring the sagittal alignment. We feel that CBT screws and rods may not show superior correction potential for spinal sagittal alignment.
Many studies in recent years have addressed the comparison between CBT and TPS fixation in spinal fusion. Lee and Ahn[21] conducted a comparative study of CBT vs. TPS in single level PLIF, and reported that the fusion rate and clinical outcomes were similar between the two groups. Keorochana et al[22] conducted a systematic review and meta-analysis based on eight studies to compare the outcomes of CBT and TPS in lumbar spinal fusion and found that CBT was comparable to TPS in terms of the clinical outcomes and fusion rates, but offered a lower incidence of complications and caused less trauma.
However, the use of CBT screw fixation technique in the treatment of ASD has only been reported in a limited number of studies. Lee and Shin[23] presented a minimally invasive surgical technique using CBT screw fixation for ASD after lumbar fusion surgery, and compared the postoperative outcomes between CBT and TPS at a 1-year follow-up period. They found that the bone fusion rate in 31 patients with TPS was 90% vs. 91% in 22 patients with CBT. Patient satisfaction at 1 month postoperation and pain intensity within 1 month postoperation were significantly better in CBT group than those in TPS group. In addition, CBT caused less blood loss and offered a shorter operation time and length of postoperative hospital stay. Chen et al[24] also demonstrated the similar results through a technique note with case series in terms of cortical bone trajectory screw fixation in lumbar adjacent segment disease.
Unlike previous studies, our research showed that TPS-Domino-TLIF provided a shorter operative duration and less intraoperative EBL as compared with CBT-PLIF.
We analyzed several factors, including only two pedicle screws in TPS-Domino-TLIF group but four CBT screws needed in CBT-PLIF group, may contribute to less trauma in TPS-Domino-TLIF group.
In addition, the trajectory of CBT screws differs from the TPS trajectory, but preexisting pedicle screws at the index level sometimes impeded smooth implantation of the CBT screws. The relatively complex manipulation technique of the CBT screws prolonged the operation duration and cause larger trauma in CBT-PLIF group.
To surmount the obstruction of the preexisting pedicle screw to current CBT screws at the index level, deliberate preoperative preparation and accurate intra-operative monitoring need to be implemented.
Rodriguez et al[25] introduced CBT screw fixation in the previously instrumented pedicle with the help of intra-operative O-arm guided navigation, which ensured accurate CBT screw placement.
Nevertheless, there was no O-arm imaging system in our institution. CBT screw implantation needs to be accomplished relying on surgeon’s clinical experience, which prolongs the surgical duration, causes more blood loss, and increases the frequency of intra-operative fluoroscopy. On the other hand, improper location of the preexisting pedicle screws and anatomic variances, such as pedicle size and the extent of posterior decompression at the index level, may make it impossible to implant CBT screws adequately at the caudal vertebra of the ASD level. Given the foregoing factors, we performed the surgery using conventional implants such as TPS and the Domino system.
Surgical technique with TPS and the Domino system for ASD has some advantages over CBT-PLIF for ASD. First, our method can be conducted with familiar implants such as TPS and the Domino system, which can be handled conventionally. Second, this manipulation can be performed easily without violating the index segments. Moreover, our outcomes with TPS-Domino-TLIF illustrated the similar fusion rate and clinical outcomes, and provided a shorter surgical duration, less blood loss, and a lower frequency of intra-operative fluoroscopy as compared with CBT-PLIF.
As most ASD patients are elderly with comorbidities that may influence the course of postoperative recovery, seeking a minimal invasive technique is paramount for these patients. Unlike of CBT screw implantation, TPS placement and assembly of the Domino system are easier to master for spine surgeons. TPS placement is a more familiar technique than CBT screw implantation owing to its long history of application in spine surgery. In addition, assembly of the Domino system and rods is a conventional procedure which can be to handled easily. Furthermore, dural tear was found in one case in CBT-PLIF group due to severe dural sac adhesion. Compared with TLIF, manipulations in the spinal canal need to be performed more medially in PLIF at the expense of increasing risk for dural tear.
This study has several limitations. Firstly, it is a retrospective case control design, with a relatively small sample size of 36 patients (16 in CBT-PLIF group and 20 in TPS-Domino-TLIF group). To better establish the safety and efficacy of TPS-Domino-TLIF for ASD, further studies with larger sample sizes under a prospectively randomized design are needed. Secondly, we did not conduct a thorough preoperative evaluation of the radiologic parameters that may affect postoperative outcomes such as sagittal vertical axis (SVA) and pelvic incidence-lumbar lordosis mismatch parameters [7, 12]. Future studies should eliminate the bias regarding preoperative spinopelvic parameters. Thirdly, our study population had limited ranges of such characteristics as age and BMI, so the research results may not apply concordantly to all patient groups. Further studies with larger and more heterogenous patient cohorts are needed.
To the best of our knowledge, this is the first study to introduce and investigate a surgical technique for manipulating ASD after lumbar fusion surgery using TPS-Domino-TLIF. Moreover, our technique has significant advantages over CBT-PLIF for ASD, with a similar fusion rate and clinical outcomes but a shorter surgical duration, less blood loss and lower frequencies of intra-operative fluoroscopy. Thus, this study may serve as a cornerstone for future research to better evaluate the efficacy and safety of a surgical technique using TPS and the Domino system for ASD.