Abstract
Background This study aims to determine whether single-level lateral lumbar interbody fusion (LLIF) with unilateral pedicle screw fixation (UPSF) might offer advantages over bilateral pedicle screw fixation (BPSF) in terms of radiation emission, cost, and outcomes.
Methods The records of 101 patients who underwent single-level LLIF with percutaneous pedicle screw fixation from September 2017 to August 2024 were analyzed. Patients were divided into 2 groups: 42 with UPSF and 59 with BPSF. Demographic data, social history, comorbidities, surgical characteristics, costs (based on manufacturer prices), and radiation metrics (radiation emitted, fluoroscopy time, number of images, and magnification mode used) were collected. Clinical outcomes were assessed using the Numeric Rating Scale (NRS), the Oswestry Disability Index, and procedure satisfaction, while radiographic evaluation employed a novel fusion classification system.
Results There were no significant differences in age, body mass index, social history, comorbidities, or operative level. However, the BPSF group included significantly more women (P = 0.002) and a higher proportion of spondylolisthesis cases (P < 0.001). Oswestry Disability Index and NRS scores were similar, except for greater improvements in NRS back pain at 1 year in the BPSF group (−4.0 vs −1.75, P = 0.008). While the total fluoroscopy time, number of images, and Mag 1 usage were greater in the BPSF group (all P < 0.001), the average radiation emitted did not significantly differ (39.38 milligray for UPSF vs 50.75 milligray for BPSF, P = 0.211). Fusion grades were comparable (P = 0.478), and UPSF costs were 27.7% lower.
Conclusions Our study found that when used according to clinical indications, UPSF results in similar radiation emission and radiographic outcomes, while being 27.7% less expensive than BPSF for single-level LLIF. Additionally, while BPSF was associated with greater improvement in 1 year NRS back scores, no other significant differences in patient-reported outcome measures were observed between the 2 groups.
Clinical Relevance This study provides clinically relevant insights for selecting between UPSF and BPSF in single-level LLIF when both are considered appropriate.
Level of Evidence 3.
- lateral lumbar interbody fusion
- unilateral pedicle screw fixation
- radiation exposure
- cost analysis
- radiographic outcomes
- clinical outcomes
- bilateral pedicle screw fixation
Introduction
Lateral lumbar interbody fusion (LLIF) has become an increasingly utilized technique owing to its ability to achieve high fusion rates through a minimally invasive approach and given the potential to accomplish indirect decompression.1,2 Because of the large footprint of the interbody device and the added support derived from engagement with the stronger apophyseal ring of the endplates, the stability achieved with posterior fixation has been demonstrated to improve upon interbody constructs.3 While bilateral pedicle screw fixation (BPSF) is more commonly utilized for posterior interbody constructs due to its greater biomechanical stability and higher fusion rates, the application of this historic data to the more stable lateral lumbar interbody devices may not be relevant.4 Pimenta et al demonstrated that a larger LLIF cage (26 mm, anterior-posterior width) with unilateral fixation can actually provide greater biomechanical stability compared with standard posterior interbody techniques with bilateral fixation.5,6
While the use of unilateral pedicle screw fixation (UPSF) and BPSF has been well studied for posterior lumbar interbody fusion, there have been very few clinical studies for their use in LLIF.7,8 Notably, these studies only reported clinical outcomes up to 1 year after the operation, and none of them evaluated for cost utility/effectiveness or radiation exposure in these 2 scenarios.
Minimally invasive procedures for the spine are typically associated with high doses of ionizing radiation and prolonged exposure time due to decreased direct visualization.8,9 This is particularly problematic for the surgical team because of the substantial occupational exposure that occurs when performing these surgeries routinely. Much of the radiation exposure occurs during the application of posterior fixation through fluoroscopic guidance. If posterior fixation density could be reduced to unilateral vs bilateral placement, there may be a commensurate reduction in radiation exposure from the overall study, in addition to the decrease in cost of the entire construct.
To our knowledge, no study has yet quantified this difference in radiation exposure when performing UPSF and BPSF. Additionally, while UPSF has been shown to reduce direct costs in comparison to BPSF, no study, to our knowledge, has analyzed these differences in the setting of LLIF while also considering radiation and patient outcomes.10,11 Therefore, we aimed to determine the cost and radiation exposure impact of utilizing UPSF with LLIF when clinically appropriate, in comparison with deploying BPSF, along with characterizing the fusion rates and clinical outcomes in both conditions.
Methods
Following Institutional Review Board approval for our retrospective patient medical record review, we studied all consecutive patients who underwent single-level LLIF with percutaneous pedicle screw fixation (PSF) at our institution between September 2017 and August 2024. All surgeries were performed by a single surgeon specializing in lumbar degenerative spinal conditions.
Patients older than 18 were included if they presented with a degenerative indication for fusion: degenerative or isthmic spondylolisthesis or spinal stenosis requiring fusion with indirect decompression. At the time of surgery, all patients presented with persistent axial back pain, radiculopathy, and/or neurogenic claudication. Only patients who underwent surgery between L1 and L5 were included in the study. Exclusion criteria encompassed surgeries involving more than a single level, non-LLIF types of interbody, and adjacent segment disease.
Patients were categorized based on their type of posterior fixation: 1 group received unilateral percutaneous PSF concurrently with LLIF in a single position (SP) laterally, while the other group received bilateral percutaneous PSF in dual-position lumbar surgery after LLIF. Patients requiring foraminal height elevation for indirect decompression and without torsional or translational instability were considered for unilateral posterior fixation. Those patients with unstable spondylolisthesis underwent with bilateral posterior fixation.
We collected demographic data (age, gender, and body mass index [BMI]), comorbidities (diabetes, renal disease, and osteopenia/osteoporosis), and surgical characteristics (presence of spondylolisthesis, levels involved, length of hospital stay, estimated blood loss, and operative time). Additionally, tobacco use within 1 year prior to surgery and ongoing alcohol use at the time of surgery were documented. The cost analysis assessed the percentage differences in total implant and instrumentation costs, as well as operating room (OR) time, valued at $100 per minute.
Clinical and Radiographic Outcomes
Functional outcomes were assessed using the Oswestry Disability Index (ODI) and Numeric Rating Scale (NRS) scores, collected preoperatively and at 1 month, 3 months, 6 months, 1 year, 2 years, and the most recent available follow-up (mean follow-up: 3.41 years). NRS scores were obtained separately for lower back pain and leg pain. Procedure satisfaction was also assessed at postoperative time points.
Fusion classification was determined based on the most recent imaging available, at least 1 year postoperation. Computed tomography images were used for all patients. An independent spine surgeon assessed interbody fusion using a novel classification system that was validated in prior published studies on anterior lumbar interbody fusion and LLIF, across various cage designs and biological materials.12–14 Grade 1 fusions had clear intervertebral connection through and around the implant, as well as cephalad and caudal implant-endplate interface connection with no evidence of lucency interfering (Figure 1). Grade 2 fusions had clear intervertebral bone connection through (Figure 2A and B) and/or around the implant (Figure 2C and D), without any appositional connection to the implant at 1 or both implant-endplate interfaces. Grade 3 fusions had cephalad and caudal endplate bone apposition to a portion of the implant on both surfaces and with no clear intervertebral bone connection through or around the implant (Figure 3). Grade 4 fusions were absent of any clear intervertebral continuous bone connection between the cephalic and caudal vertebral end plates and no appositional connection at both implant-endplate interfaces (Figure 4).
Grade 1 fusion. Clear intervertebral connection is seen through and around the implant, as well as cephalad and caudal implant-endplate interface connection with no evidence of lucency interfering.
Grade 2 fusion. Images show clear intervertebral bone connection through (A and B) and around (C and D) the implant without any appositional connection to the implant at 1 or both implant-endplate interfaces.
Grade 3 fusion. Images show cephalad and caudal endplate bone apposition to a portion of the implant on both surfaces and no clear intervertebral bone connection.
Grade 4 fusion. Images show no clear intervertebral continuous bone connection between the cephalic and caudal vertebral end plates and no appositional connection at both implant-endplate interfaces.
Surgical Technique
LLIF procedures were performed using a minimally invasive technique in the lateral decubitus position via the left or right side. All cases utilized a traditional transpsoas approach that was made through a direct lateral incision. Neuromonitoring was used during all steps of the lateral approach. 3D porous titanium cages were utilized for interbody reconstruction along with ceramic biphasic calcium putty. The paramedian Wiltse approach was used for posterior fixation application. The SP group underwent LLIF with UPSF in the same lateral decubitus position for PSF, utilizing a single titanium alloy rod. For BPSF, patients were repositioned prone for posterior fixation application after performance of the LLIF.
Imaging and Radiation Exposure
All cases were performed utilizing fluoroscopy imaging intraoperatively and with a GE OEC 9900 Elite C-arm. The radiation emission was measured in milligrays (mGy) by a fluoroscopic imaging system. Fluoroscopy exposure time was measured in minutes and converted to seconds. Radiation emission values were captured for each patient as a total mGy per case. The total number of fluoroscopic shots used for both cage placement and posterior instrumentation was recorded. Additionally, the percentage use of magnification settings (Mag 1 and Mag 2), as well as continuous vs pulsed dosing, was collected.
Statistical Analysis
All data were statistically analyzed using SPSS version 29.0.2.0 (Armonk, NY: IBM Corp.). All variables were tested for normality using Kolmogorov-Smirnov and Shapiro-Wilk tests. A between-group analysis was performed using the Mann–Whitney U test to compare age, BMI, fluoroscopy time, number of fluoroscopic images, magnification percentage, continuous/pulsed mode percentages, radiation dose, operative time, procedure satisfaction, NRS scores, and ODI scores. Categorical variables, including gender, spondylolisthesis diagnosis, tobacco/alcohol use, comorbidities, operative level, and fusion grades, were compared between groups using a χ 2 test. All tests were 2-tailed, and an alpha level of 0.05 was used for statistical significance.
Results
Our study identified a cohort of 101 patients who met inclusion criteria; 42 patients underwent UPSF by SP (mean age, 63.86 years) and 59 patients underwent BPSF by dual-position (mean age, 68.88 years). A total of 53.47% of patients were men. Gender distribution and the presence of spondylolisthesis differed significantly between the BPSF and UPSF groups (P = 0.002 and P < 0.001, respectively), with a higher proportion of women and spondylolisthesis cases in the BPSF group. Operated level also differed significantly between groups (P = 0.009). The most commonly operated level was L4 to L5 in both groups—76.3% in BPSF and 50.0% in UPSF. No significant differences were found in age, BMI, tobacco/alcohol use, or comorbidities, including diabetes, osteoporosis/osteopenia, and renal disease. Operative time was significantly longer in the BPSF group compared with the UPSF group (130.12 minutes vs 86.26 minutes, P < 0.001). A summary of demographic and operative characteristics is provided in Table 1.
Comparison of demographic and operative characteristics.
The implant/instrumentation costs were 27.7% less in the UPSF group than the BPSF group, with costs of $6,160 and $8,520 for unilateral and bilateral PSF, respectively (Tables 2 and 3). With OR costs estimated at $100 per minute, the longer operative time associated with BPSF may result in an additional $4,386 in cost per surgery compared with UPSF.
Cost of XLIF with UPSF.
Cost of XLIF with BPSF.
Although the fluoroscopy time per screw was lower in the BPSF group (32.60 seconds vs 50.54 seconds, P < 0.001), the total fluoroscopy exposure time was significantly higher in the BPSF group (130.39 seconds vs 101.07 seconds, P < 0.001). Similarly, the number of shots for posterior instrumentation per screw was significantly lower in the BPSF group (20.21 vs 28.83, P < 0.001), but the total number of shots was significantly higher in the BPSF group (155.34 vs 129.93, P < 0.001). Additionally, the magnification modes differed significantly, with the BPSF group showing less use of normal mode (P < 0.001) and more use of Mag 1 (P < 0.001). The radiation dose emitted was 39.38 mGy for the UPSF group and 50.75 mGy for the BPSF group (P = 0.211; Table 4).
Radiation metrics.
Both groups demonstrated significant improvement in patient-reported outcome measures at most follow-up time points (Table 5). When comparing outcomes between groups, there were no significant differences in changes in ODI or NRS scores at any follow-up time points (Table 6), except for a greater decrease in NRS scores for back pain in the bilateral group at the 1 year follow-up (P = 0.008). Procedure satisfaction did not differ significantly between the 2 groups at 1 month, 3 months, 6 months, 1 year, 2 years, or the latest follow-up (P = 0.373, 0.781, 0.311, 0.242, 0.310, and 0.504, respectively). At 1 year, 73.3% of patients in the unilateral group and 82.6% in the bilateral group reported being satisfied or extremely satisfied.
Changes in PROMs within groups.
Changes in PROMs between groups.
Notably, there were no differences in fusion grades between the 2 groups (P = 0.478). The unilateral group demonstrated a higher proportion of grade 1 fusions (85.3% vs 76.1%), whereas the bilateral group had a slightly higher rate of grade 2 fusions (15.2% vs 14.7%). No patients in the unilateral group exhibited grade 3 or 4 fusions, while the bilateral group included 3 patients with grade 3 fusion and 1 patient with a nonunion (grade 4; Table 7).
Fusion classification grades.
Discussion
The objective of our study was to compare radiation emission, radiographic outcomes, clinical outcomes, and cost—based on direct implant expenses and OR time—between UPSF and UPSF following single-level LLIF. We found no significant differences in radiation emission or radiographic and clinical outcomes between the 2 groups, except for greater improvement in NRS back pain at 1 year in the BPSF group. There was also a significant cost reduction (nearly 30%) when utilizing unilateral posterior fixation in comparison with bilateral posterior fixation. Notably, these direct cost comparisons are further impacted by operative time reductions with SP surgery, given the significant time value of OR services.
Unilateral posterior fixation was applied in specific clinical scenarios, such as cephalocaudal collapse of the neuroforamen resulting in refractory radiculopathy, for which facet resection and restoration of the foraminal volume would be required. It was not utilized in the setting of translational or torsional instability. Nonetheless, as demonstrated within our data, unilateral fixation in appropriate clinical settings can lead to equivalent fusion and clinical outcomes. Therefore, with a striking cost differential, unilateral posterior fixation should be considered in some clinical scenarios within which LLIF has been utilized for reconstruction.
Reducing radiation exposure when performing spinal surgery has become of increasing significance given the advent of minimally invasive approaches and the reliance on imaging guidance, at least in part.15,16 Striano et al demonstrated that performing LLIF was associated with higher radiation emission than with performing TLIF and PLIF, even after controlling for covariates such as sex, BMI, surgeon, surgical time, and age.17 So, it is likely of increased importance to find means of reducing radiation exposure when applying LLIF techniques.
With the increased popularity of minimally invasive spine surgery such as LLIF, certain strategies have been recommended to reduce surgeon exposure, such as the use of lead aprons, thyroid shields, and lead gloves. Additionally, technicians have been recommended to use pulsed doses, rather than continuous mode, during image capture, along with lower dose shots when feasible (resolution degradation may limit these approaches). It is also of critical importance to avoid the proximity of the surgeon's limbs/body to the fluoroscopy beam.18
To our knowledge, no previous study has examined differences in radiation emitted between unilateral and bilateral fixation for lumbar surgery. Hypothetically, unilateral PSF could additionally reduce radiation exposure in comparison to bilateral PSF due to decreased instrumentation. Although the total fluoroscopy time, number of images, and Mag 1 usage were significantly higher in the BPSF group, these differences did not result in a significantly greater amount of radiation emitted. Interestingly, both the radiation time and the number of posterior instrumentation shots per screw were significantly lower in the BPSF group (P < 0.001). This may be because fluoroscopy shots can often be utilized concurrently for each side during BPSF and thus be more efficiently used to deploy screws at each vertebra. Additionally, unilateral fixation application in the lateral decubitus position differs ergonomically and may therefore result in a higher number of images to successfully and accurately deploy posterior screws.
For LLIF, clinical outcomes comparing UPSF and BPSF have shown mixed results. One study reported no significant differences in NRS scores, while another found greater improvements in the unilateral group based on the Zurich Claudication Questionnaire and Japanese Orthopedic Association score.7,8 However, these studies only assessed outcomes up to 1 year postoperatively. In contrast, the present study evaluated clinical outcomes up to an average of 3.4 years postoperatively. We found no significant differences in ODI, NRS, or procedure satisfaction at any follow-up time point—except for greater improvement in NRS back pain at 1 year in the BPSF group. However, this difference was no longer observed at the 2-year follow-up.
Importantly, and consistent with previous research, our study demonstrated that operative time was significantly shorter in the UPSF group.7 This is intuitive and not surprising given that the UPSF group underwent fixation and LLIF in an SP and thus avoided the requisite repositioning that the BPSF group endured. While performing BPSF could be accomplished in an SP, the application of fixation is typically more difficult to accomplish in as efficient a manner (simultaneous left/right screw placement at each vertebral level), largely due to poorer ergonomics. The reduction in operative time has implications for surgical and overall episode of care costs, in addition to the direct implant costs.
In evaluating direct costs, it has been shown that the screws, rods, interbody cages, and bone graft/alternatives make up approximately 37% of the total cost of performing LLIF surgery.19 The cost comparison between UPSF and BPSF has also been studied previously for transforaminal lumbar interbody fusion. Awad et al showed that the implant cost for single level UPSF with TLIF was 25.9% less than the BPSF group, and Han et al showed that UPSF was 32.3% less than the BPSF group in 2-level TLIF.20,21 Xue et al reported that the mean implant cost in UPSF and BPSF with TLIF was $3,943.3 ± $19.5 and $6,765.3 ± $50.8, respectively.22 Our study demonstrated that the direct implant cost reduction was approximately 30% when applying UPSF, in comparison with BPSF. This reduction in direct costs, along with time savings potentially available by performing surgery in an SP, may have important implications for the reduction of costs in caring for patients with these advanced minimally invasive techniques.
Some previous studies have highlighted some disadvantages of UPSF. Specifically, UPSF has been shown to be less stable than BPSF during axial rotation, lateral bending, and contralateral rotation in TLIFs.23–27 This reduced stability may contribute to increased cage migration and lower fusion rates observed with UPSF in TLIFs and PLIFs.28–30 However, a recent study indicated that using larger cages, positioned symmetrically beyond the dorsal margin of the endplates, may prevent cage migration and improve fusion rates for UPSF in TLIFs.31 Interestingly, these disadvantages of UPSF have not been observed with LLIF. In short-segment LLIF, studies have shown no significant differences in fusion rates between UPSF and BPSF.7,8 Consistent with these findings, we found similar levels of fusion, based upon our bioactive fusion classification scheme, when comparing UPSF and BPSF after LLIF with a porous 3D-printed titanium cage.12
Limitations
While our study is the first to report on more than 3 years of follow-up data regarding clinical outcomes, radiation exposure, and cost differences between UPSF and BPSF for LLIF, it has some limitations. First, the population was somewhat heterogeneous in terms of gender and diagnosis, with significantly more women (P = 0.018) and spondylolisthesis cases (P < 0.001) in the BPSF group. This variation in baseline pathology may have impacted clinical outcomes and influenced our results. Second, postoperative clinical outcomes were recorded when available, as some patients lacked data due to the timing of their surgery or missed follow-up appointments. Despite these limitations, our study offers valuable insights and potential benefits in comparing UPSF and BPSF for single-level LLIF, when clinically warranted.
Conclusion
The use of unilateral pedicle screw instrumentation or bilateral pedicle screw instrumentation has long been debated in the context of spinal fusion surgery. However, there has been limited research specifically comparing BPSF and UPSF for LLIFs. Our study found that when used according to clinical indications—BPSF for patients with unstable spondylolisthesis and UPSF for those without significant torsional or translational instability—UPSF results in similar radiation emission and radiographic outcomes, while being 27.7% less expensive than BPSF for single-level LLIF. Additionally, while BPSF was associated with greater improvement in 1 year NRS back scores, no other significant differences in patient-reported outcome measures were observed between the 2 groups.
Footnotes
Funding This study was supported in part by a grant from the Scripps Clinic Medical Group.
Declaration of Conflicting Interests B.A.A. receives IP royalties from Globus, Stryker and DePuy Spine. A.K. is paid consultant for Medtronic (local), ChM (regional); serves as PI for conducting clinical trials for Sail-Fusion Bow-Tie and SST LTD; and serves as chairman for the Uzbekistan Association of Spine Surgery. R.K.E. holds stock or stock options in Aclarion, Alphatec Spine, Orthofix, Nuvasive, and Spine Innovations; IP royalties from Aesculap/B.Braun, Globus Medical, Nuvasive, Seaspine, and SI Bone; is a paid consultant for Aesculap/B.Braun, Amgen Co., Johnson & Johnson, Kuros, Medtronic, Neo Spine, NuVasive, Silony, Spinal Elements, and Seaspine; receives research support from Medtronic, Sofamor Danek, Nuvasive, and Seaspine; and serves as a board and committee member for the San Diego Orthopaedic Research Society, San Diego Spine Foundation, and Scoliosis Research Society. G.M.M. holds stock or stock options in Alphatec Spine, Nuvasive, and Orthofix; receives IP royalties from NuVasive, Seaspine, and Stryker; is a paid consultant for Nuvasive, Seaspine, and SI-Bone; receives research support from Medtronic, Sofamor Danek, Globus, and Orthofix; and is a board or committee member for the Scoliosis Research Society, Society of Minimally Invasive Spine Surgery, San Diego Orthopaedic Society, Global Spine Outreach, and San Diego Spine Foundation.
Ethical Approval The study was conducted in accordance with the Declaration of Helsinki and was approved by Scripps IRB (13-6297) on 26 October 2023, with the need for written informed consent waived.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2025 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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