LSTV is a common segmental abnormality in the process of spinal development, which mainly manifested as the enlargement and widening of the unilateral or bilateral transverse process of L5 and/or S1, and joints or fusions can be formed in severe cases [10]. At least 4% of the population are seen radiographic anomalies in L5 and S1 caused by LSTV [11, 12]. LSTV includes many symptoms, which vary from partial to complete transformation into the other vertebra [13]. Many classifications for LSTV are seen in literature [14, 15], and the most used classification was proposed by Castellvi et al. in 1984 [16]. However, these criteria are not perfect. Limited by imaging technology, fine structures in LSTV could not be seen clearly at that time, leading to the disability to counting the number of vertebrae of patients. Also, Castellvi’s classification highlights the relationship between LSTV and DH, and its adaptability to other diseases have not been seriously demonstrated. LSTV is frequently misdiagnosed for the reasons described above. It has been reported that the rate of misdiagnosis can be as high as 23% [2].
There are many studies on the relationship between LSTV and nearby structures. Paraspinal muscle volume has been found to assist in judging the severity of LSTV [17]. Research findings on pedicles of LSTV patients have shown that LSTV can affect the normal state of the pedicle, and the progression of spondylolysis can also adversely affect patients with LSTV and even exacerbate the degeneration of the intervertebral disc [18, 19]. At present, the best imaging technique for the characterization of LSTV is CT, but the diagnosis is often confirmed by accident for the diagnosis purpose of other diseases [10]. In 2013, Mahato et al. proposed a modified classification criterion according to biomechanics studies [20]. In view of shortcomings of Castellvi’s classification, to solve the problem of misdiagnosing and propose a universal imaging method for the diagnosis of LSTV, S-line was proposed based on long-term clinical researches.
Sagittal alignment is a physiological state where muscular forces make the most efficient effort to maintain the normal alignment of the spine and stable standing [21, 22]. Good sagittal alignment provides better biochemical property of spine to keep posture stable and retard the process of disc degeneration [23], and literature has reported sagittal alignment is strongly associated with spinal pathologies such as intervertebral disc degeneration, osteoporosis and low back pain [5]. Lumbar sagittal alignment is defined by several parameters: Lumbar lordosis (LL), Pelvic incidence (PI), pelvic tilt (PT) and sacral slope (SS). Among these, PI refers to the pelvic morphologic angle, which is a fixed parameter unique to each individual and not affected by posture, and PI = PT + SS [6–8]. In recent years, sagittal alignment is a hot topic in research. The impact of sagittal alignment on the pathogenesis of many spinal diseases has been annotated.
According to Table 2, main parameters LL, SS, PI, PT and PI-LL between negative and positive patients are seen statistically significant differences. Among these, PI-LL and PT have been discovered to have definite clinical significance by past researches. It has been demonstrated that LL should match PI within 11 ideally, which means a mismatch greater than 11° is associated with spine degeneration and disability. Meanwhile, a normal PT should be less than 15°, or it will be directly correlated with spinal deformity, leading to pain during walking [5, 24]. In the present study, PI-LL of S-line(-) patients and S-line(+) patients are − 2.07° and 12.30°, respectively, and PT are 13.05°, 16.89°, respectively. It is not difficult to find that the two parameters of S-line(+) patients are abnormal, while those of S-line(-) patients are conversely within normal limits.
For the comparison between S-line(+) I and S-line(+) II patients, no significant differences were obtained between them. As for LL, PI and SS, though they are significantly different between type I and type II, they are either in the normal range (a LL between 20° to 70° is considered normal), or not have clear clinical meaning.
In summary, parameters of sagittal alignment are much more normal in S-line(-) group than in S-line(+) group, while significant differences between S-line(+) I and S-line(+) II were not observed.
According to the results in Table 4 and 5, it is not difficult to see that although S-line (-) and (+) patients all obtained good recovery, S-line (-) group achieved a higher improvement rate. Patients whose S-line is between L4 and L5 are more suitable for TLIF.
In the present study, the instructive significance of S-line is explored, which may contribute to the feasibility analyzing of TLIF performed on LSS patients. S-line has several important strengths to be used as an index for the prediction of results of TLIF. First, the triage of patients according to S-line is straightforward and easy to implement. The approximate situation of sagittal alignment of the patients can be confirmed. S-line is strongly associated with prognosis, patients with S-line (-) will receive better prognosis than patients with S-line (+). Also, JOA scores reflect lighter postoperative pains of patients with S-line (-).
In conclusion, compared to S-line (+) patients, S-line (-) patients have better sagittal alignment, and achieve better correction after TLIF. As for S-line (+) patients whose improvement rates are markedly low, whether to perform surgery needs further discussions. S-line is a simple and effective index for LSS patients’ decision-making regarding TLIF.
Limitation
This study has several limitations. Firstly, this was a retrospective single-center study, and thus the results need to be prospectively verified by multicenter and randomized-control studies in the future. The reason for the normal PI-LL of patients with S-line (+) I has not been fully explained. Thirdly, the sample of patients in this single-center study was relatively small. Further studies with higher level of clinical evidence are warranted to confirm our findings.