The study was conducted in accordance with the Declaration of Helsinki and the study protocol was approved by the Institutional Review Board of our medical center. Preoperative demographic data including age, gender, and body mass index (BMI) were collected. All patients gave their informed consent before surgery. Clinical records of consecutive patients with degenerative lumbar disease, including lumbar degenerative spondylolisthesis, lumbar foraminal stenosis, spondylolytic spondylolisthesis, lateral lumbar disc herniation, lumbar vertebral stenosis, and lumbar facet joint cysts, who have had a short-segment TLIF⁹ (Fig. 1) at one or two levels of our hospital between August 2010 and November 2016 were reviewed retrospectively. All surgeries were performed by four spine surgeons, each with over 10 years of clinical experience. Patients with a history of other fusion procedures (such as posterolateral fusion or lateral oblique lumbar interbody fusion combined with TLIF) and patients with any indication of other pathological conditions, such as infectious diseases, malignant neoplasms or significant trauma were excluded and 137 patients were included. During the 5-year follow-up, 6 patients died and 4 patients had diseases affecting activities of daily living, such as Parkinson’s disease (2 cases), cerebral infarction (1 case) or cerebral hemorrhage (1 case). Thirty-six patients failed to complete the 5-year follow-up. The final analysis included a total of 91 patients.

Evaluation of clinical results

Postoperative data were reviewed to determine whether or not patients had postoperative ASD. Radiological ASD (R-ASD) of adjacent cranial and caudal discs was assessed at 5-year follow-up. When there was > 3 mm slippage progression, > 5° posterior opening, and > 3 mm disc height narrowing compared to preoperative lateral flexion-extension radiographs, patients were considered to have an R-ASD, as described in previous studies^14.15. Symptomatic postoperative ASD (S-ASD) was diagnosed when spinal stenosis, foraminal stenosis, disc herniation, or segmental kyphosis (>5°) was present in segments adjacent to the operated segments and the symptoms lower back or lower leg were obviously caused by ASD. Patients were considered S-ASD when symptoms persisted for at least three months and required additional treatment, including medications, epidural blocks, or subsequent surgery. Bone fusion was assessed using radiographs and computed tomography (CT) images 1 year postoperatively, and assessment was performed repeatedly until bone fusion was confirmed or until at the last follow-up. Fusion was defined as (i) the presence of continuous trabecular bone formation through or outside the cages, (ii) movement 50% implant^16,17,18,19. In patients with bilevel fusion, we considered fusion to be complete when both levels reached bony fusion. Clinical outcomes were assessed using (1) visual analog scale (VAS) for low back pain, pain, and lower extremity numbness from 0 (no pain) to 10 (maximum pain); (2) our detailed VAS scoring system originally developed for low back pain in motion, standing and sitting²⁰; (3) Japan Orthopedic Association (JOA) scores for low back pain, lower extremity pain, and intermittent claudication (Table 1); (4) Oswestry Disability Index (ODI); and (5) Nakai’s grading system for surgical outcome assessment, in which scores were categorized as excellent (3), good (2), fair (1), or poor (0) (Table 1)²¹. Clinical scores were assessed preoperatively and 5 years postoperatively. Postoperative improvement in clinical scores was calculated by comparing pre- and postoperative clinical scores. Three patients required revision surgery within 5 years of the TLIF and 14 patients who did not complete the questionnaires either preoperatively or 5 years postoperatively were excluded. Finally, 74 patients were included for the evaluation of clinical scores.

Preoperative radiological assessment

Lumbo-pelvic parameters, such as LL (the angle between the upper plates of L1 and S1) and pelvic incidence (PI, the angle between a line perpendicular to the sacral plate at its midpoint and the line connecting the hip axis that connected the centers of both the femoral heads and the middle of the sacral plateau) were measured using preoperative lateral radiographs obtained with the patient in a standing position (Fig. 1A). Additionally, supine LL was measured using preoperative sagittal reconstruction CT images (Fig. 1B). DiLL between standing and lying was calculated as LL lying-LL standing. Preoperative anteroposterior standing radiographs were used to examine the coronal Cobb angle at levels between T10 and S1.

Demographics and Clinical Outcomes

Patients with DiLL ≥ 0° were defined as DiLL (+), and those with DiLL 10°).

Examining the Correlation Between Preoperative DiLL and Clinical Outcomes

A correlation analysis was performed between preoperative DiLL and R-ASD, S-ASD and bone fusion status at 1 and 5 years, postoperatively, and each clinical score to examine the association between DiLL preoperative and postoperative clinical results. To exclude the influence of age, gender, BMI, presence of scoliosis and number of fused segments, multiple regression analysis was performed after appropriate adjustments.

Review of the influence of DiLL on clinical outcomes in patients with or without preoperative PI-LL mismatch

To examine the influence of DiLL on surgical outcomes in patients with or without PI-LL mismatch, patients were divided into two groups based on preoperative standing radiographs: unpaired (PI-LL > 10°) and paired (PI-LL ≤ 10°) subgroups. Clinical outcomes were compared between DiLL (+) and DiLL (−) patients in each group (misfit and matched).

Data analytics

Continuous data are presented as mean ± standard deviation and categorical variables are presented as numbers. Age, BMI, lumbopelvic parameters, VAS and ODI of the two groups were compared using an unpaired t-test. Pearson’s chi-square test (or Yates’ chi-square test when expected frequencies were

Ethical statements

The study was conducted in accordance with the Declaration of Helsinki and the study protocol was approved by the Institutional Review Board of Sakura Medical Center, Toho University. (No. 2012–071).

Consent to participate/consent to publish

All patients gave their informed consent before surgery.