INTRODUCTION
Transforaminal lumbar interbody fusion (TLIF) is a widely adopted surgical technique for treating various degenerative conditions of the lumbar spine, including spondylolisthesis, disc degeneration, and spinal instability [1,2]. The procedure aims to restore disc height, decompress neural elements, and achieve segmental stability. However, one of the major long-term complications following lumbar fusion is adjacent segment degeneration (ASD), defined as radiographic degenerative changes at levels adjacent to the fused segment. The reported incidence of ASD varies from 5% to 49%, depending on diagnostic criteria, patient characteristics, and duration of follow-up [3-5].
The pathophysiology of ASD is multifactorial, involving biomechanical stress concentration at adjacent segments, altered motion dynamics, and progressive degenerative aging [6] . In response to concerns about iatrogenic tissue damage and postoperative muscular atrophy, minimally invasive TLIF (MIS-TLIF) was developed to minimize paraspinal muscle disruption and preserve posterior spinal structures. By using smaller incisions, tubular retractors, and targeted decompression, MIS-TLIF aims to reduce intraoperative blood loss, postoperative pain, and recovery time, while maintaining comparable fusion rates [7]. From a biomechanical perspective, reduced muscle trauma and preservation of the posterior tension band may decrease stress on adjacent levels, potentially lowering the risk of ASD.
Nevertheless, the long-term radiographic benefit of MIS-TLIF over the conventional open approach remains controversial. While short-term studies have demonstrated reduced early degeneration with minimally invasive techniques, several long-term investigations have reported that the incidence of ASD increases with time, regardless of surgical approach [4,8]. Jeong et al. [9] reported that the prevalence of radiographic adjacent segment disease did not differ significantly between MIS and open lumbar fusion. These findings suggest that the protective effects of MIS techniques may diminish over time, with natural aging and sagittal imbalance playing larger roles in late-onset ASD. A direct long-term radiographic comparison between MIS-TLIF and open TLIF using well-matched patients remains essential. The present study therefore aimed to compare the long-term incidence and progression of radiographic ASD between MIS-TLIF and open TLIF (O-TLIF) in a matched-pair cohort with over 8 years of follow-up, and to identify radiographic factors particularly changes in the Cobb angle that may predispose to kyphosis-type ASD. We hypothesized that MIS-TLIF would show a lower incidence of early ASD compared with O-TLIF, but that this difference would diminish over time due to the multifactorial nature of spinal degeneration.
MATERIALS AND METHODS
1. Study Design and Ethics Approval
This retrospective matched-pair cohort study was conducted at a single tertiary academic institution. Institutional Review Board (IRB) approval was obtained prior to the initiation of the study (IRB No. 2023/2214). The study was designed to compare the long-term radiographic incidence of ASD between patients who underwent MIS-TLIF and those who underwent O-TLIF.
2. Patient Selection and Matching Criteria
All patients who underwent single-level TLIF between January 2004 and December 2009 were screened from a prospectively maintained institutional database. The inclusion criteria were age ≥ 18 years; single-level TLIF at L3–4, L4–5, or L5–S1. Complete preoperative and postoperative radiographs are available and a minimum radiographic follow-up of 8 years. Exclusion criteria included: Multilevel fusion procedures, Previous lumbar fusion., Spinal trauma, tumor, or infection. Incomplete baseline imaging or medical records. Follow-up duration <8 years. After applying the criteria, 60 patients met the eligibility requirements and were included in the matching process. Pair matching was performed in a 1:1 ratio (MIS-TLIF: O-TLIF) based on: Age, within ±3 years; sex; body mass index (BMI), within ±2 kg/m²; operative level: (L3–4, L4–5, or L5–S1). The final matched cohort consisted of 60 patients, divided into 30 who underwent MIS-TLIF and 30 who underwent O-TLIF. Two patients (one from each group) required revision surgery for symptomatic ASD during follow-up; these cases were retained in the analysis according to the intention-to-treat principle, with data included up to their prerevision radiographs. A simplified flowchart is provided to illustrate patient selection and the matched-pair design (Figure 1).
3. Follow-up Protocol
All patients were managed according to a standardized institutional postoperative protocol. The following included clinical visits at 6 weeks, 3 months, 6 months, 1 year, 2 years, 5 years, and subsequently on an annual basis. Standing anteroposterior and lateral lumbar radiographs were obtained at each visit. In addition, patients underwent independent evaluation by certified physiotherapists at the Orthopaedic Diagnostic Centre, where functional assessments and rehabilitation progress were documented using institutional spine outcome forms.
4. Radiographic Evaluation of ASD
All radiographs were independently reviewed by a board-certified spine surgeon, blinded to the surgical technique. The following points were evaluated: preoperative, immediate postoperative (within 1 week), 6 months, 2 years, 5 years, and the most recent follow-up (more than 8 years). ASD was defined radiographically as present if any of the following criteria were met at the adjacent level (either superior or inferior to the fused segment) by loss of more than 50% disc height compared to immediate postoperative imaging, listhesis more than 3 mm. (anterior or posterior displacement) and kyphotic deformity >10° in Cobb angle between adjacent vertebrae. Kyphotic alignment at the adjacent segment was assessed using the Cobb angle measured on standing lateral radiographs. The Cobb angle was defined as the angle between the inferior endplate of the vertebral body immediately superior to the fused segment and the superior endplate of the fused vertebral body. All Cobb angle measurements were performed using standardized digital radiographic software. Positive values indicated kyphotic alignment, whereas negative values indicated lordotic alignment. For statistical analysis, absolute Cobb angle values were used to allow consistent comparison between patients. All adjacent segments demonstrated lordotic alignment preoperatively, and no patients presented with preexisting kyphotic alignment at the adjacent level. Therefore, no data transformation for negative values was required in the analysis. To improve measurement accuracy, disc height and vertebral body alignment were assessed at 3 sagittal points (anterior, middle, and posterior) and averaged. Imaging was reviewed using standardized digital radiographic software, calibrated to ensure reproducibility. A musculoskeletal radiologist reviewed the magnetic resonance imaging (MRI) evaluation with preoperative lumbar spine MRI scans (T2-weighted image). Adjacent segment disc condition was graded according to the Pfirrmann classification system [10]. A Pfirrmann grade ≥3 was considered indicative of preexisting disc degeneration. These findings were later analyzed in relation to ASD development during follow-up.
5. Surgical Techniques
All TLIF procedures followed established interbody fusion protocols. For the MIS-TLIF group, a paramedian muscle-sparing approach with sequential tubular dilators and retractors was used. A working channel endoscope or microscope was employed, and unilateral facetectomy was performed to access the disc space. In contrast, the O-TLIF group underwent a traditional midline posterior approach with subperiosteal dissection of paraspinal musculature. In both groups, decompression, discectomy, and placement of a single interbody cage (titanium or polyetheretherketone) with autologous bone graft were performed. Segmental stability was achieved via bilateral pedicle screw fixation.
6. Outcomes and Variables
The primary outcome was the incidence of radiographic ASD at predefined follow-up intervals. Secondary outcomes included operative duration, length of hospital stay, and the association between ASD and risk factors such as age, BMI, surgical approach, level of surgery, and preexisting disc degeneration on MRI.
7. Statistical Analysis
All analyses were performed using IBM SPSS Statistics ver. 23.0 (IBM Corp., USA). Continuous variables were expressed as mean±standard deviation. Normality testing was performed using the Shapiro-Wilk test. For normally distributed variables, comparisons between groups were performed using the independent-samples t-test. For nonnormally distributed data, the Mann-Whitney U-test was applied. Categorical variables were compared using the chi-square or Fisher exact test, as appropriate. Logistic regression analysis was conducted to identify potential predictors of ASD. A p-value <0.05 was considered statistically significant for all tests.
RESULTS
1. Patient Demographics and Radiologic Characteristics
Among the 100 single-level TLIF cases screened between January 2004 and December 2009, 60 patients fulfilled all eligibility criteria and were successfully pair-matched 1:1 (MIS-TLIF=30; O-TLIF=30). The cohort included 52 females (86.7 %) and 8 males (13.3 %), with a mean age of 58.8±9.5 years and mean BMI of 25.4±3.9 kg/m². The L4–5 level was most frequently operated (86.7 %), followed by L3–4 (6.7 %) and L5–S1 (6.7 %). Degenerative lumbar disease was the primary diagnosis in 93.3% of cases. No significant differences were observed between groups regarding age (p=0.557), BMI (p=0.691), or operative time (p=0.636). However, the hospital stay was significantly shorter for MIS-TLIF (3.41±1.67 days) than O-TLIF (6.03±2.30 days, p<0.001). The mean follow-up period was 8.9±1.2 (range, 8–12) years. Baseline MRI assessment revealed Pfirrmann grade ≥ 3 degeneration in 62.7 % of superior and 80.9 % of inferior adjacent discs, without intergroup difference (p>0.05) (Table 1).
2. Incidence and Temporal Progression of Radiographic ASD
Radiographic ASD was detected in 28 patients (46.7%) at final follow-up. At 2 years, MIS-TLIF demonstrated a significantly lower ASD rate (1 case [3.3%]) than O-TLIF (7 cases [23.3%], p=0.023). However, this early advantage disappeared over time (5 years: 16.7 % vs. 30.0 %, p=0.222; ≥8 years: 43.3 % vs. 50.0 %, p=0.605). The most frequent degenerative pattern was disc-height loss >50 % (43.3%), followed by antero-posterior listhesis>3 mm (25%) and segmental kyphosis>10° (6.7%). Listhesis occurred predominantly at the superior adjacent segment.
Two patients (one in each group) underwent revision surgery for symptomatic ASD; both were analyzed up to their last prerevision radiographs according to the intention-to-treat protocol (Table 2).
3. Preoperative MRI Findings
Preoperative MRI evaluation demonstrated moderate-to-severe disc degeneration (Pfirrmann≥3) in the majority of adjacent segments. However, no significant relationship was found between baseline Pfirrmann grade and subsequent ASD development (superior p=0.135; inferior p=0.100), indicating that preexisting disc degeneration alone did not predict future ASD.
4. Risk-Factor Analysis
Univariate analysis revealed no significant association between ASD and demographic or perioperative variables including age, BMI, operative time, hospital stay, or Pfirrmann grade (p>0.05). (Table 3). Neither superior nor inferior adjacent disc degeneration (Pfirrmann grade≥3) was significantly associated with the development of radiographic ASD.
5. Kyphosis-Type ASD and Cobb Angle Progression
Among patients with ASD, 4 (14.3%) developed kyphosis-type ASD at the superior adjacent level. These patients exhibited significantly higher Cobb angles at all measurement points, suggesting that sagittal malalignment predisposes to kyphotic degeneration: preoperative: 11.44°±5.09° versus 7.51°±3.41° (p=0.036), postoperative: 11.98°±4.09° versus 7.21°±3.34° (p=0.009) and ≥ 8-year follow-up: 13.17°±6.12° versus 8.84°±4.01° (p=0.017) (Table 4).
DISCUSSION
This study is a matched-pairs cohort study that compares the long-term radiographic outcomes of MIS-TLIF and O-TLIF, with a particular focus on the development and progression of ASD. The results demonstrated that MIS-TLIF offered a short-term radiographic advantage by reducing the early incidence of ASD within 2 years; however, this benefit was not sustained beyond 5 years of follow-up. At long-term follow-up of more than 8 years, the incidence of ASD was similar between the 2 surgical techniques. These findings suggest that while MIS-TLIF may delay the onset of radiographic degeneration by minimizing paraspinal muscle and posterior element injury, the eventual degenerative process is primarily influenced by patient-specific and biomechanical factors rather than surgical exposure alone.
In comparison with previous studies, the early reduction in ASD incidence following MIS-TLIF in this study aligns with reports highlighting the short-term benefits of minimally invasive approaches in reducing mechanical stress and soft-tissue trauma at the adjacent level [8,9,11]. By preserving the posterior ligamentous complex and paraspinal musculature, MIS-TLIF theoretically maintains spinal stability and reduces shear stress on adjacent segments. However, long-term results from the present cohort indicate convergence in ASD rates between MIS and open TLIF, consistent with the findings of Jeong et al. [9], who reported no significant difference in ASD after long-term of follow-up. This convergence supports the concept that ASD is a multifactorial and time-dependent process, with ageing, disc dehydration, and progressive segmental instability contributing more substantially over time than the surgical approach itself [12]. Similarly, Peng et al. [11] reported that although MIS-TLIF reduces early postoperative morbidity, it does not alter the long-term risk of radiographic degeneration.
The radiographic analysis included detailed radiologic parameters in this study, such as disc height, listhesis, sagittal Cobb angle, and Pfirrmann grade, which enabled a more comprehensive evaluation of segmental degeneration. The most common finding was disc-height loss of more than 50%, followed by anteroposterior listhesis and segmental kyphosis, reflecting the typical pattern of gradual mechanical deterioration after fusion. Significantly, the Pfirrmann grade of adjacent discs on preoperative MRI did not predict future ASD, suggesting that degenerative potential may be more dependent on dynamic factors, such as sagittal balance and loading distribution, rather than the static degree of preexisting disc degeneration. However, patients who developed kyphosis-type ASD demonstrated significantly greater Cobb angles both preoperatively and postoperatively. This consistent difference implies that sagittal malalignment at the fused and adjacent levels predisposes to kyphotic collapse, confirming the importance of maintaining physiological lumbar lordosis during fusion procedures [13,14]. The strong association between Cobb angle progression and kyphosis-type ASD in this study reinforces the need for precise restoration of sagittal alignment, as emphasized in recent biomechanical literature [15,16]. Our study result presents absolute Cobb angle values at each time point rather than interval changes (Table 4). Patients who developed kyphosis-type ASD consistently demonstrated higher Cobb angles throughout follow-up, reflecting persistent sagittal malalignment rather than paradoxical angular reduction. This finding supports the role of baseline and postoperative sagittal alignment as a predisposing factor for kyphotic ASD rather than a time-dependent reversal phenomenon.
The clinical and pathophysiological aspects of this study primarily evaluated radiographic ASD, the clinical implications of these findings warrant attention. Radiologic degeneration does not always correlate with symptomatic adjacent segment disease, but progressive structural changes may increase the risk of pain, instability, or reoperation over time. The early advantage of MIS-TLIF likely reflects reduced initial iatrogenic damage and improved preservation of paraspinal muscles. However, as biological and mechanical aging continue, the adjacent discs may experience degenerative changes independent of the fusion construct, consistent with the hypothesis of natural disease progression rather than a purely surgical effect [17,18]. In particular, the role of sagittal imbalance and local kyphotic alignment appears critical. Patients with increased Cobb angles exhibited greater stress transfer to the superior segment, potentially leading to accelerated collapse. These findings emphasize the importance of optimal cage positioning, restoration of disc height, and maintenance of segmental lordosis to minimize the risk of kyphotic ASD.
Systemic and metabolic factors are the absence of systemic and metabolic variables such as osteoporosis, diabetes mellitus, smoking, and bone mineral density, all of which are known contributors to ASD [19-21]. The current study design focused primarily on radiographic and surgical factors, but future multicenter prospective research incorporating metabolic and biomechanical parameters could clarify the multifactorial etiology of ASD.
The strengths of our study include a rigorously matched cohort design, long-term follow-up, and comprehensive radiographic evaluation, including Pfirrmann grading and Cobb angle analysis. These factors enhance the validity of the comparison between MIS and open techniques. However, several limitations must be acknowledged. First, the retrospective nature of the study introduces inherent selection bias, despite matched-pair methodology. Second, only radiographic ASD, not symptomatic disease, was analyzed, which may have underestimated the clinical significance. Third, advanced spinopelvic parameters, including pelvic incidence, pelvic tilt, and lumbar lordosis, were not analyzed due to incomplete long-term radiographic data. This limitation restricts differentiation between local segmental kyphosis and global sagittal imbalance as contributors to ASD and should be addressed in future prospective studies.
Future directions and investigations should integrate clinical correlation with radiographic findings and evaluate the impact of sagittal balance correction, cage geometry, and instrumentation design on adjacent segment biomechanics. Long-term, multicenter, prospective studies are essential to delineate the interactions among surgical technique, spinopelvic alignment, and metabolic factors in the pathogenesis of ASD. Finally, our study found that MIS-TLIF significantly reduced the early incidence of radiographic ASD compared to O-TLIF, but long-term rates converged beyond 5 years. Pfirrmann grade did not predict ASD development, highlighting that preexisting disc degeneration is not the sole determinant. Higher Cobb angles and sagittal malalignment were strongly associated with kyphosis-type ASD. Long-term ASD likely represents a multifactorial, age-related process rather than one determined by surgical approach alone.
CONCLUSION
MIS-TLIF offers short-term radiographic advantages by reducing tissue disruption and preserving adjacent segments early. Nevertheless, the long-term risk of ASD remains comparable to open TLIF, underscoring the multifactorial nature of degeneration. Sagittal balance and segmental alignment appear to play pivotal roles in the development of kyphosis-type ASD. Comprehensive preoperative planning and meticulous restoration of spinal alignment are essential to optimize outcomes and minimize long-term adjacent segment changes.
PDF Links
PubReader
ePub Link
XML Download
Full text via DOI
Download Citation 
Print 
