TLIF surgery allows for circumferential decompression of the neural elements with relatively small injury compared with PLIF surgery (Posterior lumbar interbody fusion), and has been widely used in treatment of lumbar spinal disorders19. Dissection of bilateral muscles is widely used to access the posterior column of the vertebral body during the TLIF surgery, while it is regarded as a paraspinal iatrogenic injury. Therefore, many minimal invasive approaches have been proposed to avoid bad clinical outcomes related to the dissection of bilateral muscles, such as the Wiltse approach, the oblique trajectory anterior to psoas (ATP) approach, minimally invasive transforaminal lumbar interbody fusion (MIS TLIF), and et al. SRT has been verified as a safe screw placement method with good clinical outcomes by our previous study, such as greater reduction in wound pain, reduction of ASD and anatomical structure advantage6,20. To minimise the injury of paraspinal muscles during the TLIF surgery and accelerate patients’ recovery, we combined SRT technique with Wiltse approach, and the clinical outcomes of SRT through Wiltse approach in TLIF surgery were reported as follows:
Less blood loss and better HRQOL
SRT was a relatively easy technique for surgeons to learn and operate since the location of the entry point of SRT was relatively stable, did not change with vertebral degeneration, and was easy to locate in each vertebra (Fig. 1). While in TLIF surgery with tradtional entry points, it is common for the anatomical structures of pedicles in some elderly patients to be severely degenerative, making it harder to find the regular entry point and implant the screws, which might cause nerve injury and other severe surgical complications21. Besides, less exposure of the bilateral muscles, superior facet joint and facet joint capsule was required in TLIF surgery with SRT through Wiltse approach. All these factors made this surgery eariser and safter as well as the shorter operation time. In our surgry, the majority of operation time was used in the depression procedure rather than the implantation of pedicle screws which might be difficult in TLIF surgery with tradtional entry points. Furthermore, with the shift-down of the pedicle screw entry point in SRT, resulting in a shorter wound length and connection rods length in the SRT, which was consistent with our previous study6.
More important, less injury of bilateral muscles, superior facet joint and facet joint capsule in our operation resulted in less intraoperative blood loss, especially during the procedure of exposure of the entry point. In our study, blood loss was observed during the procedure of depression of nerve roots and implantation of interbody cages rather than the implantation of pedicle screws. Consequently, the total postoperative drainage volume before removall and days of removal of drainage were both significantly decreased compared with TLIF surgery with traditional entry point, may leading to shorter hopital days and less economic burdens. In addition, due to the less damage of paraspinal muscles and less intraoperative blood loss, patients felt mild wound pain after the operation with the VAS value of 2.08, 0.95 and 0.25 at 1st day, 2nd day and 3rd day, respectively. Consequently, the less postoperative drainage volume, less days of removal of drainage and mild postoperative wound pain accelerate patients’ recovery and improved patients’ quality of life, which was the most be the most significant advantage of SRT in TLIF surgery through Wiltse approach. Therefore, SRT through Wiltse approach is recommended in TLIF surgery, especially for long fusion levels, since less posterior complex, facet joint, and paraspinal muscles are incised, resulting in greater improvement of wound pain.
Reduction of ASD with SRT through Wiltse approach
Adjacent segment degeneration (ASD) is among the most recognized long-term complications of lumbar surgery for degenerative spine pathologies with a relevant impact in spine surgical and clinical practice22. It has been verified that several factors are associated with ASD. The posterior complex, including the superior facet joint, facet joint capsule, and paraspinal muscles, plays an important role in lumbar stabilization, and intraoperative superior facet joint violation might cause deduced adaptability to biomechanical change and has been verified as an important risk factor for ASD after spine fusion surgery23. We compared the incidence of ASD in PLIF surgery with SRT and regular PLIF surgery with traditional entry points, and our results6 showed that in one-level and three-level fusions, there was a significant difference in the incidence of ASD between SRT and regular PLIF surgery, indicating that applying SRT in PLIF surgery could play a protective role against ASD development. In our opinion, the application of SRT during TLIF surgery could protect the superior facet joint, facet joint capsule, and muscles and could help avoid superior facet joint violation, ultimately decreasing the occurrence of ASD as well.
Furthermore, since the protection of paraspinal muscles has been verified to play important role in reduction of ASD during the spine surgery, Wiltse approach has been proposed and used in the operation, and good clinical outcomes have been reported24. We combined SRT and Wiltse approach in TLIF surgery, hoping to avoid the occurrence of ASD as far as possible. In our study, only 2 patients suffered from ASD with low back pain and leg pain at final follow-up according to the criteria of ASD. MRI showed disc herniation with nerve root compression at upper adjacent segment above the first surgery. Consequently, these two patients received revision surgery, and their symptoms were significantly relieved. Interestingly, obesity was found in both patients with BMI of 28.5 and 28.3 kg/m2, as well as sever osteoporosis with BMD of -2.6 and − 2.5 (T value). Our results were consistent with Cannizzaro et al’s study22 which reported that obesity were relevant risk factors for developing lumbar adjacent segment degeneration, indicating that SRT through Wiltse approach was recommended in obese patients.
The incidence of ASD in our study was 3.12%, whch was significantly lower than it reported in previous study22,23, which might be contributed the protection of superior facet joint, facet joint capsule, and paraspinal muscles in both SRT technique and Wiltse approach. Therefore, the application of SRT through Wiltse approach as a noval surgical method, is recommended in TLIF surgery to reduce the occurrence of ASD. However, smple size, measurement erors and selection biases might influence our results, and extensive multicenter studies will be needed.
Restoration of Sagittal Parameters
Lumbar lordosis was restored in our study as a result of cage implantation, which has been verified as a primary contributor25. Combined with the significant change of LL and the lower incidence of ASD in the SRT group through Wiltse approach, we speculated that better postoperative sagittal alignment might play an important role in the development of ASD, which was consistent with Wang et al.’s meta-analysis25. Interestingly, the reason why the SRT technique could increase lumbar lordosis could be due to the direction of SRT’s projection, as verified in our previous study5. In SRT, the screws are implanted in the direction of 10-20o to the endplate in the sagittal plane (Fig. 1C), which might result in more compression pressure, leading to greater lumbar lordosis5. Besides, the protection of paraspinal muscles in Wiltse approach might provide sufficient lumbar stabilization, leading to the stable lumbar lordosis.
Pelvic incidence (PI) is a key morphological parameter that reflects the relation between the sacrum and iliac wings. It is well accepted that PI remains constant after reaching maturity26. PI remained constant in our study no matter after operation or at final follow-up, which was constient with previoud study6. Pelvic tilt (PT) and sacral slope (SS) are pelvic compensatory parameters which paly important role in the sagittal balance, which is widely evaluated by SVA (Sagittal Vertical Axis). Lumbar lordosis decreased with aging possibly due to disc degeneration, whereas PT increases to restore sagittal balance as a compensatory mechanism. In our study, due to the restoration of lumbar lordosis and sagittal balance (SVA), PT decreased significantly after the operation with increased SS.
Wang et al.27 speculated that abnormal sagittal alignment could negatively affect the adjacent segments, and our findings also indicated that restoring sagittal alignment, especially for the lumbar lordosis, is the key to reducing ASD. Notably, SRT could provide more compression pressure during the sagittal restoration procedures, leading to lower rates of ASD as well as the providence of lumbar stabilization by Wiltse approach.
Limitation
In this study, we present for the first time a novel entry point of pedicle screws (Short Rod Technique, SRT) through Wiltse approach in TLIF surgery. Although good clinical outcomes have been reported, some limitations in this study should not be neglected. First, First, all the patients recruited in our study were outpatients of a single center in China, which may result in selection bias and compromise the statistical power. Second, the sample size and the average follow-up time was relatively small. Thus, future studies with longer follow-ups and larger sample sizes should be conducted. Besides, only 2 patients suffered from ASD and were analyzed in our study. Larger number of ASD should be recruited and studied to detect whether SRT with through Wiltse approach could reduce the occurrence of ASD in TLIF surgery.