Unilateral uniportal endoscopy with unilateral fixation for lumbar degenerative diseases: A retrospective observational study and technical note

Abstract

Objective

For lumbar degenerative diseases, minimizing surgical trauma while maintaining fusion stability remains challenging. This retrospective observational study and technical note describes a modified unilateral uniportal endoscopic technique combined with unilateral pedicle screw fixation and evaluated preliminary clinical and radiological outcomes, focusing on cage subsidence and migration.

Methods

Eighteen consecutive patients with single-level lumbar degenerative disease who underwent unilateral uniportal endoscopy with unilateral screw fixation between January 2025 and June 2025 were included. Clinical outcomes were assessed using the visual analog scale for back pain and leg pain as well as the Oswestry Disability Index. Radiological outcomes included disc height, disc height loss, cage migration, maximal cage penetration depth, cage subsidence, and Bridwell grading.

Results

The mean age of the study population was 57.8 ± 9.5 years. The operated level was L4/5 in nine patients and L5/S1 in nine patients. The mean operative time was 146.4 ± 26.2 min, and mean blood loss was 87.8 ± 20.7 mL. Visual analog scale scores for back pain and leg pain as well as the Oswestry Disability Index scores improved postoperatively and were maintained at 12 months. Mean disc height increased from 11.4 ± 2.0 mm preoperatively to 14.4 ± 1.9 mm on postoperative day 3 and was 13.6 ± 1.9 mm at 6 months. Radiological measurements at 12 months postoperatively were available for 13 patients, with a mean disc height of 13.3 ± 2.0 mm. Cage subsidence >2 mm occurred in three patients (16.7%) at 6 months and two of the 13 patients (15.4%) at 12 months. Cage migration >3 mm occurred in one patient at both 6 months and 12 months. No screw loosening, screw breakage, or neurological deterioration occurred, and no patient required revision surgery.

Conclusions

Unilateral uniportal endoscopy with unilateral screw fixation may be feasible for carefully selected patients with single-level lumbar degenerative disease. Satisfactory clinical outcomes are achieved; however, cage subsidence and migration may occur. Larger comparative studies with longer follow-up duration are required.

Introduction

The population of individuals aged >60 years has increased substantially with global population aging. ¹ Consequently, the prevalence of lumbar degenerative diseases (LDDs), including lumbar disc herniation and lumbar spinal stenosis, continues to rise.^2,3 LDDs may result in low back pain, leg pain, walking difficulty, and reduced quality of life, imposing a considerable burden on patients, families, and the society.^4,5 For the surgical management of LDDs, transforaminal lumbar interbody fusion (TLIF) has become a conventional and well-established procedure because it can achieve neural decompression, disc-space restoration, interbody fusion, and spinal stabilization.^6–10

In lumbar interbody fusion, bilateral pedicle screw fixation is commonly used to provide rigid internal fixation and promote fusion. ¹¹ However, bilateral fixation may be associated with an increase in soft-tissue injury, paraspinal muscle damage, blood loss, and implant-related cost. Unilateral pedicle screw fixation has therefore attracted attention as a less invasive alternative in carefully selected patients. Kabins et al. first reported unilateral fixation in the early 1990s. ¹² Several studies have suggested that unilateral fixation may reduce operative time, surgical trauma, and blood loss.^12–17 However, this concept has not been widely accepted. The main concern among spine surgeons is that reduced fixation rigidity may increase the risk of cage migration, cage subsidence, and delayed or insufficient fusion.

Spinal endoscopic techniques, such as unilateral biportal endoscopy (UBE), have been widely adopted because of their advantages of small incisions, minimal bleeding, clear visualization, and precise decompression.^18,19 For patients without significant spinal instability, endoscopic decompression alone can prevent fusion and minimize surgical trauma. However, achieving decompression and fusion while further reducing surgical trauma remains technically challenging, despite recent efforts to combine endoscopic techniques with interbody fusion and percutaneous fixation.^20–22

Therefore, our team further modified the conventional UBE technique into a unilateral uniportal endoscopic procedure combined with unilateral pedicle screw fixation (UUE with UF). This technique integrates endoscopic decompression, interbody preparation, cage placement, and unilateral percutaneous screw fixation through a single small incision. This study included 18 patients with LDD treated using UUE with UF. The aims were to describe the modified surgical technique and report preliminary clinical and radiological outcomes, with emphasis on cage subsidence and migration.

Methods

Study design and ethics

This retrospective observational study included 18 consecutive patients who underwent UUE with UF for single-level LDD at Zhejiang Provincial People's Hospital between January 2025 and June 2025. The study size was determined based on the number of consecutive eligible patients who underwent UUE with UF during the study period. No formal sample size calculation was performed because this was a preliminary retrospective observational study and technical note.

The study was conducted in accordance with the Declaration of Helsinki of 1975, as revised in 2024. The study protocol was approved by the Institutional Review Board of Zhejiang Provincial People's Hospital, Zhejiang, China (approval number: 2025JS032; approval date: 2025-04-21). The reporting of this study conforms to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. ²³

Because this retrospective study used deidentified clinical and imaging data, the requirement for written informed consent to participate was waived by the Institutional Review Board. All patient details were deidentified before analysis and presentation. The representative radiological images were fully deidentified and contained no names, hospital identification numbers, facial features, or other directly identifiable patient information.

Inclusion and exclusion criteria

The inclusion criteria were as follows: (a) diagnosis of single-level lumbar disc herniation with unilateral low back pain and/or unilateral lower limb symptoms; (b) symptoms refractory to conservative treatment for at least 3 months; (c) consistency between clinical symptoms, physical examination, and imaging findings in identifying the responsible level; (d) single-level pathology located at L4/5 or L5/S1; (e) treatment using unilateral uniportal endoscopy with unilateral pedicle screw fixation; and (f) availability of complete preoperative and 6-month clinical and radiological follow-up data.

The exclusion criteria were as follows: (a) bilateral lower limb neurological symptoms; (b) bilateral lateral recess stenosis or complete central canal stenosis on imaging; (c) spondylolysis, lumbar infection, tumor, fracture, congenital lumbar spinal stenosis, or spinal deformity; (d) severe scoliosis, defined as a lumbar coronal Cobb angle >20° or any coronal or sagittal deformity requiring corrective surgery; (e) previous lumbar spine surgery; (f) osteoporosis, defined as a bone mineral density (BMD) T-score ≤−2.5, or the presence of osteoporotic vertebral fracture; and (g) severe systemic comorbidity that could affect surgical tolerance or follow-up evaluation.

Data collection

Baseline data included age, sex, symptom side, disease duration, body mass index (BMI), BMD of L1–L4, comorbidities, operated level, lesion classification, Pfirrmann disc degeneration grade, Schmorl's nodes, and preoperative clinical scores.

Lesion classification and disc degeneration grading were evaluated using preoperative magnetic resonance imaging and computed tomography when available. Lesion type was classified according to the main imaging findings and the corresponding symptomatic side. Disc degeneration was evaluated according to the Pfirrmann grading system. ²⁴ Imaging evaluation was performed by two spine surgeons, and disagreements were resolved via consensus.

Surgical technique

All surgeries were performed by the same experienced senior surgeon. An additional video file shows this surgical technique in more detail (see Additional file 1).

Patient preparation

After induction of general anesthesia, patients were placed in a standard prone position on a radiolucent operating table. The chest and abdomen were padded to reduce intra-abdominal pressure, thereby minimizing bleeding from the epidural venous plexus during the surgery.

Surgical portal localization and percutaneous guidewire placement for screws. The target level was localized under anteroposterior and lateral fluoroscopy guidance with a C-arm. After routine disinfection and draping, a long needle was used to reconfirm the position of the pedicle. Percutaneous guidewires for unilateral pedicle screws were inserted under two-dimensional or three-dimensional fluoroscopic guidance (Figure 1(a) and (b)). The screw trajectory was adjusted so that the screw tails were brought closer to the midline, allowing the subsequent endoscopic procedure and unilateral screw fixation to be performed through the same small incision. Hemostatic forceps were used to fix the tail of the guidewires for later use while avoiding contamination.

Figure 1.

Surgical technique. (a) By adjusting the screw trajectories, the projected screw-tail positions at the skin surface are brought closer together, as indicated by the two short vertical markers, thereby allowing both screws to be inserted through a single incision. (b) Percutaneous unilateral pedicle screw guidewires are placed. (c) A longitudinal midline incision approximately 2–3 cm in length is made along the line connecting the two guidewires (arrows) to serve as the minimally invasive portal. (d) The ipsilateral lamina and facet joint are exposed under endoscopy. (e) Part of the lamina and facet joint is removed using an endoscopic osteotome to expose the ligamentum flavum. (f) The ligamentum flavum is resected using rongeurs. (g) The nerve root is exposed. (h) The herniated nucleus pulposus and degenerative disc material are thoroughly removed. Decompression of the nerve root is achieved. (i) The superior and inferior cartilaginous endplates are thoroughly scraped. (j) The bony endplates are exposed, revealing punctate bleeding. (k) An appropriately sized trial cage is inserted into the disc space. (l) The bone-filled interbody cage is implanted along the working channel. (m and n) Using the preinserted percutaneous guidewires, pedicle screws of appropriate diameter and length are inserted under their guidance.

Creation of the working portal

A longitudinal incision, approximately 2–3 cm in length, was made along the line connecting the two guidewires (Figure 1(c)). Through this incision, two independent channels were established, including one for the endoscope connected to a continuous irrigation system and the other for surgical instruments.

Discectomy

Soft tissue was dissected to expose the ipsilateral lamina and facet joint under endoscopic visualization. Ipsilateral laminotomy and partial facetectomy were performed using an endoscopic osteotome, high-speed burr, or Kerrison rongeur (Figure 1(d)). The ligamentum flavum was then exposed and resected to expose the lateral border of the dural sac and traversing nerve root (Figure 1(e) to (g)). The nerve root was carefully protected and gently retracted medially using a nerve retractor (Figure 1(h)). The posterior annulus fibrosus was exposed and incised. Degenerative disc material and herniated nucleus pulposus were removed, achieving decompression of the nerve root.

Endplate preparation and interbody cage placement

The superior and inferior cartilaginous endplates were prepared using curettes until the bony endplates were exposed and punctate bleeding was observed while preventing excessive violation of the bony endplate (Figure 1(i) and (j)). Autologous bone chips collected from the lamina and facet joint and artificial bone were tightly packed into the disc space using a funnel-shaped bone graft inserter. An appropriately sized trial cage was inserted to confirm cage size, position, depth, and disc height restoration (Figure 1(k)). A bone-filled interbody cage was then implanted through the working channel under endoscopic and fluoroscopic guidance (Figure 1(l)). The largest cage was not forcibly inserted if nerve root safety was a concern.

Percutaneous pedicle screw fixation

After cage placement, pedicle screws of appropriate diameter and length were inserted through the preinserted guidewires (Figure 1(m) and (n)). A prebent connecting rod was inserted percutaneously into the U-shaped screw heads. Finally, locking caps were tightened to complete unilateral fixation. Anteroposterior and lateral fluoroscopy was performed to confirm cage position, screw trajectory, and lumbar alignment.

Closure

All instruments were removed. The surgical field was irrigated using normal saline, and meticulous hemostasis was achieved. The thoracolumbar fascia, subcutaneous tissue, and skin were closed in layers.

Postoperative management

Patients received prophylactic antibiotics for 24 h postoperatively. Lumbar brace protection was recommended during early ambulation. The brace was worn for approximately 3 months. Heavy lifting and excessive lumbar flexion-extension activities were avoided during the early postoperative period.

Clinical evaluation

Clinical outcomes were evaluated using the visual analog scale (VAS) for back pain, VAS for leg pain, and Oswestry Disability Index (ODI).^25,26 Assessments were performed preoperatively, on postoperative day 3, at 6 months, and at 12 months.

Radiological evaluation

Radiological evaluation was performed using preoperative, postoperative day 3, postoperative 6 months, and, where available, postoperative 12 months follow-up radiographs and computed tomography images. Complete 12-month radiographic disc-height measurements were available in 13 of 18 patients.

Disc height was measured on standard lateral radiographs as the average of anterior and posterior disc height at the operated level. Disc height loss was calculated as the disc height on postoperative day 3 minus the disc height at follow-up.

Cage position was assessed by measuring the distance from the posterior margin of the cage to the posterior border of the adjacent vertebral body on lateral radiographs. Cage migration distance was defined as the absolute change in cage position between postoperative day 3 and each follow-up time point. For patients with complete 12-month cage-position measurements, cage migration from postoperative day 3 to 12 months and additional cage-position change from 6 to 12 months were also calculated. Clinically relevant cage migration was defined as cage displacement >3 mm.

Maximal cage penetration depth was defined as the greatest distance that the cage sank into either adjacent vertebral endplate at each specific follow-up time point. Cage subsidence was defined as time-point-specific maximal cage penetration into the adjacent vertebral body >2 mm. Disc height loss and cage subsidence were evaluated separately because disc height loss may have been influenced by postoperative disc-space settling, patient positioning, radiographic projection, or segmental remodeling and may not necessarily indicate direct cage-endplate penetration.

Screw loosening was defined as a radiolucent zone >1 mm around the pedicle screw or obvious screw displacement during follow-up. Early fusion status was evaluated according to the Bridwell grading system. ²⁷ Grade I indicated fusion with remodeling and trabecular formation; Grade II indicated an intact graft without complete remodeling and incorporation and without lucency; Grade III indicated an intact graft with definite lucency at the top or bottom of the graft; and Grade IV indicated absence of fusion with graft resorption and collapse. Because the follow-up duration was relatively short, Bridwell grading at 6 months was interpreted as early fusion status rather than final solid fusion.

Complete preoperative and 6-month clinical and radiological data were available for all included patients. Complete standardized 12-month radiological measurements were available for 13 patients and were analyzed descriptively on an available-case basis. No imputation was performed for missing 12-month radiological measurements.

Statistical analyses

Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) software (version 28.0, SPSS Inc., Chicago, IL, USA). Continuous variables were expressed as mean ± SD, and categorical variables were expressed as numbers and percentages. The Shapiro–Wilk test was used to assess normality. For paired comparisons of preoperative and postoperative continuous variables, the paired t-test was used for normally distributed data, and the Wilcoxon signed-rank test was used for nonnormally distributed data. This was a small preliminary retrospective study without a control group; therefore, statistical results were interpreted descriptively and cautiously. A two-sided p-value <0.05 was considered statistically significant.

Results

Baseline characteristics

In total, 18 consecutive patients were enrolled, including 11 men and seven women. The mean patient age was 57.8 ± 9.5 years, and the mean disease duration was 13.4 ± 6.2 months. The mean BMI was 20.7 ± 2.5 kg/m², and the mean BMD T-score of L1–L4 was −0.7 ± 1.3. Nine patients had normal BMD, and nine had osteopenia. No patient met the diagnostic threshold for osteoporosis according to the BMD T-score.

The operated level was L4/5 in nine patients and L5/S1 in nine patients. Symptoms were located on the right side in 11 patients and on the left side in seven patients. All patients had lumbar disc herniation with unilateral symptoms, and six patients had concomitant foraminal stenosis. Pfirrmann disc degeneration was classified as grade 2 in three patients, grade 3 in nine patients, and grade 4 in six patients. Schmorl's nodes were observed in five patients. Baseline characteristics are summarized in Table 1.

Table 1.

Baseline demographic and imaging characteristics of the included patients.

Variable	Value
Number of patients	18
Age, years	57.8 ± 9.5 (41–73)
Sex	11 male, seven females
Disease duration (months)	13.4 ± 6.2 (6–24)
BMI, kg/m2	20.7 ± 2.5 (16.8–24.3)
BMD T-score, L1–L4	−0.7 ± 1.3 (−2.4 to 1.6)
Operated level	L4/5 (9 patients), L5/S1 (9 patients)
Symptom side	11 (right side), 7 (left side)
Lesion classification	12 (66.7%): LDH with unilateral symptoms; 6 (33.3%): LDH with unilateral symptoms and foraminal stenosis
Pfirrmann disc degeneration grade	Grade 2: 3 (16.7%); Grade 3: 9 (50.0%); Grade 4: 6 (33.3%)
Schmorl's node	Present: 5 (27.8%); Absent: 13 (72.2%)
Comorbidities	Diabetes mellitus: 5 (27.8%); hypertension: 6 (33.3%)
Any listed comorbidity	7 (38.9%); both diabetes mellitus and hypertension: 4 (22.2%)

BMI: body mass index; BMD: bone mineral density; LDH: lumbar disc herniation.

Surgical technique–related outcomes

All 18 surgeries were completed successfully. The mean operative time was 146.4 ± 26.2 (range: 110–195) min, and mean intraoperative blood loss was 87.8 ± 20.7 (range 55–130) mL. The average postoperative hospital stay was 5.1 ± 1.5 (3–8) days.

Clinical outcomes

At the 3-day, 6-month, and 1-year postoperative follow-ups, the back pain VAS scores were 2.9 ± 0.8, 1.5 ± 1.2, and 0.9 ± 0.6, respectively, all lower than the preoperative score (5.4 ± 0.9). Compared with preoperative scores, the back pain VAS score decreased by 46% at 3 days, 72% at 6 months, and 83% at 1 year postoperatively.

The leg pain VAS scores were 3.3 ± 0.9 at 3 days, 0.7 ± 0.7 at 6 months, and 0.6 ± 0.6 at 1 year postoperatively, all lower than the preoperative score (6.4 ± 1.1). Compared with preoperative scores, the leg pain VAS score decreased by 48% on postoperative day 3, 89% at 6 months, and 90% at 1 year postoperatively.

The ODI scores were 27.7 ± 5.1 at 3 days, 18.4 ± 7.5 at 6 months, and 16.6 ± 4.0 at 1 year postoperatively, all lower than the preoperative score (50.4 ± 9.9). Compared with preoperative scores, the ODI score decreased by 45% at 3 days, 63% at 6 months, and 67% at 1 year postoperatively.

Interbody height and fusion

Mean disc height increased from 11.4 ± 2.0 mm preoperatively to 14.4 ± 1.9 mm on postoperative day 3 and 13.6 ± 1.9 mm at 6 months postoperatively. Disc height at 3 days and 6 months postoperatively was greater than the preoperative value. Partial disc height loss was observed during follow-up, with a mean disc height loss of 0.8 ± 0.8 mm from postoperative day 3 to postoperative 6 months.

Data on 12-month clinical outcomes were available for all 18 patients. Complete standardized 12-month radiological measurements, including disc height, cage position, cage penetration depth, and Bridwell grading, were available for 13 patients. In these patients, the mean disc height was 13.3 ± 2.0 mm at 12 months, and the mean disc height loss from postoperative day 3 to postoperative 12 months was 1.2 ± 0.7 mm.

The mean cage migration distance from postoperative day 3 to postoperative 6 months was 1.4 ± 1.4 mm. Cage migration >3 mm was observed in one patient (5.6%) at 6 months postoperatively. This patient exhibited a cage migration distance of 6.5 mm and concomitant cage subsidence; however, no neurological deterioration was observed, and revision surgery was not required during follow-up.

Among the 13 patients with complete 12-month cage-position measurements, the mean cage migration distance from postoperative day 3 to 12 months was 1.4 ± 1.7 mm. Cage migration >3 mm was observed in one patient (7.7%) at 12 months postoperatively; this was the same patient who had exhibited cage migration at 6 months. In this patient, the cage migration distance was 6.5 mm at 6 months and 6.6 mm at 12 months postoperatively, indicating no obvious progression between 6 and 12 months postoperatively (Figure 2). The mean additional change in cage position from 6 to 12 months postoperatively was 0.2 ± 0.2 mm, and no new clinically relevant cage migration was observed between 6 and 12 months postoperatively.

Figure 2.

Representative case with cage subsidence and cage migration.

A 57-year-old male patient presented with low back pain radiating to the left lower limb. BMD T-score of L1–L4 was 0.0. (a) Preoperative anteroposterior and lateral radiographs, (b) oblique radiographs, (c) flexion-extension radiographs, (d) computed tomography, and (e) magnetic resonance imaging demonstrated L5/S1 disc herniation without obvious instability, Schmorl's node or bony destruction. The patient underwent unilateral uniportal endoscopy with unilateral fixation. After guidewire placement, (f) a single 2–3 cm incision was made for both endoscopic manipulation and instrumentation. (g) Under endoscopic visualization, (h) discectomy and ipsilateral nerve root decompression were performed. (i) Postoperative day 3 radiographs show satisfactory cage and screw positions. At 6 months, (j) radiographs show cage migration, and (k) computed tomography demonstrates cage subsidence into the inferior endplate of L5, with early graft incorporation changes but persistent radiolucency consistent with Bridwell Grade III early fusion status. (l) Magnetic resonance imaging confirms satisfactory neural decompression. At 1 year, (m) anteroposterior and lateral radiographs and (n) computed tomography show no obvious progression of cage migration or subsidence compared with the 6-month images. The cage migration distance was 6.5 mm at 6 months and 6.6 mm at 12 months. Despite these radiological changes, the patient showed improvement in preoperative symptoms, with no neurological deficit, neurological deterioration, or need for revision surgery during follow-up.

The mean maximal cage penetration depth was 1.0 ± 1.0 mm at 6 months postoperatively. Cage subsidence >2 mm was observed in three patients (16.7%) at 6 months postoperatively. Among the 13 patients with complete 12-month cage-penetration measurements, the mean maximal cage penetration depth was 1.0 ± 1.0 mm at 12 months postoperatively. Cage subsidence >2 mm was observed in two patients (15.4%) at 12 months postoperatively; both had already exhibited cage subsidence at 6 months. No new cage subsidence was observed between 6 and 12 months postoperatively in patients with available 12-month postoperative radiological follow-up data.

At the 6-month follow-up, early fusion status was classified as Bridwell Grade II in 15 patients (83.3%) and Grade III in three patients (16.7%). Among the 13 patients with available 12-month Bridwell grading, Grade II was observed in 11 patients (84.6%) and Grade III in two patients (15.4%). No patient was classified as Bridwell Grade IV at either 6 or 12 months postoperatively.

No screw loosening, screw breakage, neurological deterioration, or revision surgery occurred during follow-up. Radiological outcomes are summarized in Table 2. Representative cases are shown in Figures 2 and 3.

Table 2.

Radiological outcomes and early fusion status.

Variable		Value
Disc height (mm)	Preoperative	11.4 ± 2.0
	Postoperative day 3	14.4 ± 1.9
	Postoperative 6 months	13.6 ± 1.9
	Postoperative 12 months, n = 13	13.3 ± 2.0
Disc height loss (mm)	Day 3 to 6 months postoperatively	0.8 ± 0.8
	Day 3 to 12 months postoperatively, n = 13	1.2 ± 0.7
Cage migration (mm)	Cage migration >3 mm at 6 months postoperatively	1/18 (5.6%)
	distance from postoperative day 3 to 6 months postoperatively	1.4 ± 1.4
	Cage migration >3 mm at 12 months postoperatively, n = 13	1/13 (7.7%)
	Change in distance from postoperative day 3 to 12 months postoperatively, n = 13	1.4 ± 1.7
	distance from postoperative 6 months to 12 months, n = 13	0.2 ± 0.2
Cage subsidence (mm)	>2 mm at 6 months postoperatively	3/18 (16.7%)
	>2 mm at 12 months postoperatively, n = 13	2/13 (15.4%)
	Maximal cage penetration depth at 6 months postoperatively	1.0 ± 1.0
	Maximal cage penetration depth at 12 months postoperatively, n = 13	1.0 ± 1.0
Bridwell grade	at 6 months postoperatively	Grade II 15, Grade III 3
	at 12 months postoperatively, n = 13	Grade II 11, Grade III 2

Figure 3.

Representative case. A 50-year-old female patient presented with right-sided symptoms caused by single-level L4/5 lumbar disc herniation. The BMD T-score was −1.1. (a) Preoperative computed tomography shows degenerative changes at L4/5. (b) Preoperative magnetic resonance imaging demonstrates L4/5 disc herniation corresponding to the patient's right-sided symptoms without Schmorl's node. (c) Postoperative day 3 radiograph shows appropriate cage placement and unilateral pedicle screw fixation. (d) Six-month computed tomography shows maintained cage position and no cage subsidence. (e) One-year computed tomography shows preserved cage-endplate support without progressive cage penetration. The patient showed improvement in preoperative symptoms, with no neurological deficit, neurological deterioration, or need for revision surgery during follow-up.

Early fusion status and Bridwell Grade III cases

At the 6-month follow-up, early fusion status was classified as Bridwell Grade II in 15 patients (83.3%) and Grade III in three patients (16.7%). No patient was classified as Bridwell Grade IV. Among the 13 patients with available 12-month Bridwell grading, 11 patients were classified as Grade II and two as Grade III. No patient was classified as Grade IV.

Among the three patients with Bridwell Grade III early fusion status, two had cage subsidence >2 mm, and one had cage migration >3 mm. Clinically, all three patients showed improvement in VAS scores and ODI values compared with preoperative values. During follow-up, no screw loosening, screw breakage, or neurological deterioration occurred in any patient, and no patient required revision surgery. The follow-up duration was relatively short, and the study population was small; therefore, Bridwell Grade III was interpreted as indicative of delayed or uncertain early fusion status rather than definitive nonunion.

Discussion

Given the limited number of clinical studies on unilateral fixation in endoscopic lumbar interbody fusion, this study reported a modified UUE with UF and its preliminary outcomes. The main technical modification was that the percutaneous screw trajectory was adjusted to bring the screw tails closer together, allowing the endoscopic working channel and unilateral pedicle screw fixation to be performed through a single small incision. This design may further reduce the number of incisions and soft-tissue dissection.

In this preliminary retrospective study, all surgeries were completed successfully, and no severe intraoperative complications such as dural tear, vascular injury, or nerve injury occurred. VAS scores for back and leg pain as well as ODI values improved postoperatively and were maintained during follow-up. These findings suggest that the technique provides adequate decompression and satisfactory short-term symptom relief in carefully selected patients. However, this was a retrospective study with only 18 patients and no control group; therefore, the clinical results should be interpreted cautiously and should not be regarded as evidence of superiority over conventional unilateral biportal endoscopic fusion, minimally invasive TLIF, or bilateral fixation.

LDDs predominantly affect middle-aged and older patients.^1–5 Older patients often have higher surgical risk and greater risk of postoperative complications.^28–31 Therefore, minimally invasive techniques that reduce surgical trauma while maintaining clinical effectiveness are clinically important. The single-incision technique in the present study was developed based on arthroscopic-assisted uniportal spinal surgery and UBE. Previous studies have suggested that uniportal or endoscopic techniques reduce tissue injury and facilitate postoperative recovery.^32,33 In the present study, the mean operative time and blood loss were acceptable; however, direct comparisons with other techniques were not performed because of the small sample size and lack of a control group.

For lumbar interbody fusion, bilateral pedicle screw fixation remains the standard method for performing rigid fixation. However, bilateral fixation may cause greater soft-tissue and paraspinal muscle injury and affect adjacent segment mechanics.^34,35 Unilateral fixation aims to reduce surgical trauma while maintaining sufficient stability. Previous studies on unilateral fixation in TLIF have reported acceptable clinical outcomes.^{12–17,36–41} However, concerns remain regarding cage migration, cage subsidence, and fusion quality. Previous studies have suggested that cage migration is more common after unilateral fixation than after bilateral fixation.^14,41 Therefore, cage-related radiological events are particularly important when unilateral fixation is performed.

Endoscopic lumbar interbody fusion has been attempted for treating LDDs with favorable outcomes.^42–45 However, unilateral pedicle screw fixation under a single-incision uniportal endoscopic technique remains rarely reported. Unilateral pedicle screw fixation under unilateral biportal endoscopic technology has been recently reported.^20–22 The far-lateral approach in endoscopic fusion allows relatively extensive endplate preparation and foraminal decompression, providing space for cage implantation. ⁴⁵ However, when the space between the traversing and exiting nerve roots is not wide enough, implantation may be challenging. The technique reported in the present study still employs a conventional single cage. Therefore, cage-related mechanical behavior remains an important issue. Further biomechanical studies and modifications to cage designs may help improve cage stability in endoscopic fusion combined with unilateral fixation.

In the present study, disc height was restored postoperatively and remained greater than the preoperative value at 6 months postoperatively. However, partial disc height loss occurred during follow-up, and cage subsidence >2 mm was observed in three patients. Cage subsidence is a multifactorial event related to bone quality, cage size, cage footprint, cage position, endplate preparation, postoperative loading, and fixation rigidity. Previous studies have suggested that BMD, disc height, and cage position may be associated with cage subsidence after minimally invasive TLIF. ⁴⁶ Unilateral fixation provides less mechanical reserve than bilateral fixation, especially against axial rotation and lateral bending; therefore, stress at the cage-endplate interface may increase in some patients. Therefore, cage subsidence should be carefully monitored when this technique is used.

Cage migration is another important concern in unilateral fixation. In this study, cage migration >3 mm occurred in one patient. This finding does not negate the feasibility of the technique because the patient did not exhibit neurological deterioration and did not require revision surgery during follow-up. However, it indicates that cage positional stability remains a potential limitation of unilateral uniportal endoscopy with unilateral fixation, particularly when a conventional single cage is used. Previous studies have suggested that osteoporosis, pear-shaped disc morphology, posterior cage position, and related mechanical factors increase the risk of cage migration or cage retropulsion after TLIF. ⁴⁷ Careful cage sizing, adequate anterior placement, preservation of the bony endplate, sufficient bone grafting, and strict patient selection are essential.

Bone quality is another key factor when unilateral fixation is selected. Patients with osteoporosis were excluded from this preliminary study. No patient met the BMD criterion for osteoporosis; however, osteopenia was present in 50% of the study cohort. Reduced bone quality may decrease pedicle screw purchase and endplate strength, increasing the risk of cage subsidence, cage migration, screw loosening, or delayed fusion. Therefore, the findings of the present study should not be generalized to osteoporotic patients. In patients with osteoporosis, osteoporotic vertebral fracture, severe endplate weakness, or substantial segmental instability, bilateral fixation may be more appropriate, although the patient with clinically relevant cage migration had a normal BMD T-score, and among the three patients with cage subsidence >2 mm, only one had reduced BMD.

At 6 months postoperatively, 15 patients were classified as Bridwell Grade II and three as Bridwell Grade III. Among the 13 patients with available 12-month Bridwell grading, 11 were classified as Grade II and two as Grade III. A period of 6 months is insufficient to determine final solid fusion. Therefore, the 6-month fusion result was described as early fusion status rather than as final fusion, and the available 12-month Bridwell results were reported descriptively. Nevertheless, postoperative 6-month radiological follow-up remains clinically meaningful for evaluating early cage-related mechanical events, particularly cage subsidence and cage migration. Cage subsidence often develops during the early postoperative period when the operated segment is exposed to initial loading, disc-space settling, and cage-endplate stress. In addition, one of the major concerns regarding unilateral fixation is reduced construct stability, which may increase the risk of cage migration or cage-endplate penetration. Thus, 6-month radiological outcomes were used as the main endpoint for evaluating early cage stability, whereas available postoperative 12-month data were reported descriptively.

Among the three patients with Bridwell Grade III early fusion status, two had cage subsidence, and one had cage migration >3 mm. This observation suggests that cage-related mechanical events are associated with delayed or uncertain early fusion status, although no statistical conclusion can be drawn because of the relatively small number of cases. Continued follow-up is required to determine whether these patients progress to solid fusion or develop persistent nonunion.

This study has certain limitations. First, this was a retrospective preliminary study with only 18 patients and no control group. Therefore, the findings should be interpreted cautiously and cannot be used to establish superiority of the present technique over other endoscopic fusion techniques or bilateral fixation. Second, although 12-month postoperative clinical follow-up data were available for all patients, complete standardized 12-month postoperative radiological measurements, including disc-height, cage-position, cage-penetration, and Bridwell grading assessments, were available for only 13 patients, and the follow-up duration was insufficient to evaluate final solid fusion, long-term cage stability, adjacent segment degeneration, and sustained clinical benefit. Third, formal subgroup analysis was not possible because of the relatively small sample size, particularly for cage subsidence, cage migration, and Bridwell Grade III cases. Fourth, cage-related outcomes may be affected by multiple factors, including cage size, cage position, endplate preparation, bone quality, and postoperative activity, which could not be fully controlled in this retrospective study. Finally, biomechanical testing was not performed. Future prospective comparative studies and biomechanical analyses are needed to validate the stability and indications of this technique.

Conclusions

UUE with UF may be a feasible minimally invasive option for carefully selected patients with single-level lumbar disc herniation-related degenerative disease and unilateral symptoms. This preliminary retrospective study showed satisfactory short-term symptom relief and acceptable early radiological outcomes. However, cage subsidence and cage migration may occur, and the current follow-up is insufficient to determine final fusion or long-term implant stability. Larger prospective comparative studies with longer follow-up are required.

Supplemental Material

Video 1.

Data availability

The datasets generated and analyzed during the current study are not publicly available because they contain potentially identifiable clinical information; however, deidentified data generated and analyzed during the current study are available from the corresponding author upon reasonable request, subject to appropriate ethical and legal considerations.