Sacrolumbar Interbody Fusion (SLIF): Feasibility, Technical Nuances, Biomechanical Assessment, and Clinical Outcomes

Abstract

Introduction

S1-L5 transdiscal screw fixation is a direct stabilization technique used for surgical treatment of high-grade (III-IV) L5-S1 spondylolisthesis. It has not been used for nonspondylolisthetic cases or in combination with an interbody cage (IC). This study aimed to develop a novel, direct S1-L5 sacrolumbar interbody fusion (SLIF) technique, a combination of IC and sacrolumbar transdiscal screw.

Methods

SLIF was tested in cadaveric, clinical, and finite element analysis settings. Three cadaveric lumbar spines were used to test the SLIF procedure before clinical application. Eight patients underwent the SLIF procedure. Clinical outcomes were evaluated by visual analog score for leg and back pain, short form 36, Oswestry disability index, and neurological examination. CT scans of the lumbar spine were used to assess the hardware placement and subsequent fusion. Finite element analysis was performed on a healthy human CT-based L5-S1 model. Intact segment, unilateral facetectomy and discectomy, SLIF, and transforaminal lumbar interbody fusion (TLIF) procedures were compared in terms of the range of motion (ROM), von Mises stress on hardware, and shear-induced directional deformity. Additionally, the same set of tests were conducted in an osteoporotic model.

Results

Excellent hardware placement was feasible in three cadavers and eight patients. Preoperative neurological deficits improved in all patients. Statistically significant improvements were obtained on all self-reported questionnaire scores. All patients developed solid, Bridwell grade I fusions. Biomechanical testing revealed similar outcomes for TLIF and SLIF regarding the ROM. However, the screw's von Mises stress and shear-induced directional deformity were low for SLIF of healthy and osteoporotic bone.

Conclusions

SLIF is a feasible, safe, and effective L5-S1 fusion option suitable for all clinical scenarios. It provides several biomechanical advantages, yielding excellent clinical outcomes.

Introduction

Lumbar interbody fusion is one of the most commonly performed and highly successful spinal procedures¹⁾. It includes discectomy, placement of an interbody cage and bone grafts into disc space, and supplemental stabilization. Bilateral pedicle screw system (BPSS) is the most frequently used stabilization method due to its robustness. Meanwhile, pedicle screw-rod fixation is an indirect and composite construct, i.e., separate screws are implanted into each vertebra and then connected by rods and nuts. For example, one disc segment stabilization requires 4 screws, 4 screw tulips, 2 rods, and 4 nuts, making 14 solid parts. These parts are tightened and held by friction force. However, BPSS tends to fail under multiaxial repetitive physiological load.

Direct stabilization by screws traversing between vertebrae is a plausible idea. It was first reported in 1982 for the treatment of L5-S1 spondyloptosis using transdiscal S1-L5 fibular bone grafts²⁾. The technique later became obsolete due to poor clinical results. In 1999, Abdu et al.³⁾ reported stabilization using titanium transpedicular sacrolumbar transdiscal screws (SLTS) with rod-connected L4 pedicle screws. This technique (and its modifications) was adopted by other authors and has shown encouraging results^4-10). Additionally, several studies demonstrated similar biomechanical profiles of transdiscal and classical pedicle screw-based constructs^11-13). In cadaveric biomechanical testing, Minamide et al. demonstrated that L5-SLTS pedicle screw-rod system provided a 1.6-1.8 stiffer construct than conventional L5-S1 BPSS. Furthermore, this stiffness equals that of BPSS+interbody cage¹⁴⁾. However, neither Minamide et al. nor any other study tested SLTS in combination with an interbody cage (IC), nonspondylolisthetic L5-S1 segment, or as a stand-alone stabilization system (i.e., without additional pedicle screws and rods).

ICs with bone grafts provide anterior support, yield high fusion rates, and deliver excellent clinical outcomes^1,15). This study primarily aimed to develop a novel interbody fusion technique based on SLTS and IC. SLTS and IC combination is called sacrolumbar interbody fusion (SLIF) like other interbody fusion techniques (TLIF, ALIF, etc.). Currently, SLTS transdiscal transpedicular screw stabilization is used in combination with rod-connected upper-level pedicle screws^4-10). We hypothesized that SLTSs are self-sufficient to achieve robust stabilization without requiring additional pedicle screws, rods, and nuts. The main obstacle associated with transdiscal screws is technical difficulty. High-grade spondylolisthesis creates a straightforward route for transpedicular-transdiscal trajectory. In such cases, reaching L5 vertebral body by directing a standard S1 pedicle screw slightly cephalad is possible. However, placing SLTS in low-grade spondylolisthesis and nonspondylolisthesis cases appears unfeasible and has yet to be attempted. We hypothesized that SLTS can be safely implanted by moving the S1 pedicle screw using a steeper cephalad angle, starting caudally. This study will show that all hypotheses were confirmed experimentally and clinically, proving that SLIF is technically feasible and effective.

Materials and Methods

This study followed the ethical standards of the Declaration of Helsinki and was conducted in three phases. The preclinical or experimental phase aimed to simulate the SLIF procedure in cadaveric lumbosacral spines for feasibility assessment. The biomechanical performance of SLIF and transforaminal lumbar interbody fusion (TLIF) were compared using finite element analysis. Lastly, the clinical part was conducted to test SLIF in a real-life scenario.

Experimental phase

Three formalin-fixed cadaveric lumbosacral spines without a history of disease or previous lumbar surgery were used in the preclinical phase. Transforaminal lumbar interbody cages (TLIF-IC) or anterior lumbar interbody cages (ALIF-IC) were conventionally implanted into the L5-S1 disc spaces. The cage was carefully positioned in the anterior half of the disc space's midline. Thereafter, SLTS was implanted using a percutaneous technique. The transdiscal route was established using Jamshidi needles under fluoroscopy. Fig. 1 schematically demonstrates the optimal screw trajectory. The starting point was caudal to that of a typical S1 pedicle screw. The direction was inferosuperior, lateromedial, and posteroanterior, aiming at the anterosuperior part of the L5 vertebral body (for maximal engagement). Guidewires were inserted through Jamshidi needles and cannulated 6.5 or 7.5 mm tulipless titanium screws were implanted over the guidewires (Medtronic, Dublin, Ireland). Fig. 2 demonstrates the sequence of SLIF steps. Following the implantation, CT scans were obtained for every specimen to assess the hardware.

Figure 1.

Schematic representation of SLIF.

A. Lateral view. B. Anteroposterior view. The optimal cage position is shown by the blue rectangle. Note that the cage should ideally be in the anterior half of the disc space and centered over the midline (dashed line). The optimal screw trajectory (black line) passes through the middle of the S1 pedicle (red asterisk). The exit point from the sacrum (yellow asterisk) should be at the top of the S1 pedicle (AP view) and ventral to the spinal canal (lateral view). The optimal end-point (green asterisk) of the transdiscal trajectory should be in or near the midline.

Figure 2.

Step-by-step sacrolumbar screw insertion process in a cadaveric specimen.

A. ALIF cage has been implanted into L5–S1 disc space, and the Jamshidi needle has been docked on the sacrum (anteroposterior view).

B. ALIF cage has been implanted into the L5–S1 disc space, and the Jamshidi needle has been docked on the sacrum (lateral view).

C. Needle has been advanced into the S1 pedicle; note that the tip of the needle did not breach the medial wall of the S1 pedicle (anteroposterior view).

D. Needle has been advanced into the S1 pedicle (lateral view).

E. The transdiscal path is drilled (anteroposterior view).

F. The first screw has been implanted (anteroposterior view).

G. Both screws have been implanted (anteroposterior view).

H. Both screws have been implanted (lateral view).

I. SLTS and TLIF interbody cage combination (lateral view).

J. SLTS with TLIF interbody cage combination (anteroposterior view).

Finite element analysis (FEA)

Set up

Computer simulations were conducted on a 3D mesh L5-S1 model derived from a healthy human CT scan. Mimics software (Materialise, Leuven, Belgium) was used for finite element analysis. Bone and soft tissue characteristics were taken from previous publications^16-18), which Table 1 presents. The model was validated with experimental results¹⁹⁾ comparing the angular range of motion (ROM) for each level of the lumbar spine for all six loading conditions, such as flexion, extension, left and right lateral bending, and left and right axial rotation. Biomechanical tests were performed according to previously published protocol²⁰⁾. The L5-S1 model was tested in four conditions: intact, unilateral facetectomy and discectomy, SLIF, and TLIF.

Table 1.

Material Properties of Finite Element Model. EM-H Elastic Modulus of Healthy Bone. EM-O Elastic Modulus of Osteoporotic Bone. PR-H Poisson’s Ratio of a Healthy Bone. PR-H Poisson’s Ratio of Osteoporotic Bone.

	Nodes	Elements	EM-H	EM-O	PR-H	PR-O
Cortical	362,440	182,977	12,000	8,040	0.3	0.3
Cancellous	2,510,390	1,844,131	100	33	0.25	0.25
Endplate	120,914	61,152	1,000	670	0.4	0.4
Cage	43,067	231,830	4,340		0.40
Screw(s)			110,000		0.3

Osteoporosis is a prevalent problem in patients undergoing fusion surgery. Therefore, the performance was also tested in an osteoporotic model. To simulate osteoporotic bone, the Young's modulus of the cortical and cancellous bone was reduced by 33% and 66%, respectively, and the cortical shell's thickness was reduced by 40%.

Testing

S1 was fixed, and 500 Newton (N) compressive preload was applied to L5. A 7.5 Nm moment was applied to the upper endplate of L5. The model was tested for flexion-extension, lateral bending, and rotation. Range spinal segment motion and von Mises stress on screws and cage were obtained as outcomes. Additionally, SLIF and TLIF constructs were tested under shear forces. Under 500 N compressive preload, 100 N translational force was applied to the upper endplate of the L5 vertebral body in the sagittal plane. Directional deformations were obtained and compared.

Clinical phase

All patients enrolled in this study were informed about the SLIF and other standard interbody fusion options. Potential advantages and pitfalls were thoroughly discussed with patients and their families. All patients in this study consented to use their medical information for scientific purposes. Eight patients were operated using SLIF technique between June 2022 and June 2023. Clinical, radiological, and follow-up data were collected and used for analysis.

Surgical procedure

All patients underwent surgery by single surgeon (K.A.). Under general endotracheal anesthesia, the patients were placed in a prone position and were prepped and draped. Intraoperative fluoroscopy was brought into the surgical field. TLIF-IC (Medtronic, Dublin, Ireland) was implanted into the anterior half of the L5-S1 disc space through unilateral facetectomy and discectomy. Autologous bone grafts were packed into the disc space to promote fusion. Two 6.5-8.5 mm tulipless pedicle screws (Medtronic, Dublin, Ireland) were implanted as SLTSs percutaneously from separate 1-cm incisions under fluoroscopic control.

Clinical outcomes assessment

The outcomes were evaluated clinically and radiologically. Neurological examination, visual analog scores for leg and back, short form 36, and Oswestry disability index were obtained pre- and postoperatively. Paired Student t-test was used for parametric data comparison. Statistical analysis was performed using Numbers Software (Apple, One Apple Park Way; Cupertino, CA 95014, USA). A p-value <0.05 was considered statistically significant. Radiological evaluation included a lumbar CT scan on postoperative day one and at six months follow-up. Bridwell grading system²¹⁾ was used for fusion assessment.

Results

Cadaveric

A successful SLIF procedure was achieved in all three cadavers. No interference/collision between SLTSs and ICs was encountered. ALIF-ICs and TLIF-ICs were compatible with SLTSs (Fig. 2G-H). CT scans demonstrated that screws did not infringe spinal canal, S1, or L5 nerve roots (Video 1-2).

Finite element analysis

Unilateral facetectomy increased ROM in all motion modalities. SLIF and TLIF significantly reduced ROM and demonstrated comparable values for healthy and osteoporotic models (Table 2, Fig. 3, 4). Screw von Mises stress was lower on SLIF than on TLIF in all motions for healthy and osteoporotic models (Table 3, Fig. 4). Osteoporosis increased von Mises stress on screws, which was more in the TLIF model. One exception was on SLIF screw von Mises stress on extension, which was less for osteoporotic bone (Table 3). Cage von Mises stresses were similar between SLIF and TLIF in healthy models (Table 3, Fig. 4). In an osteoporotic model, stresses on cage increased. This increase was more for SLIF than for TLIF in flexion-extension and lateral bending, yet rotation stress was less for SLIF than for TLIF (Table 3, Fig. 4). Shear force assessment demonstrated a threefold increase in directional deformation on TLIF in comparison with SLIF on healthy bone. The similar effect was present on osteoporotic bone, although the difference was twofold.

Table 2.

Range of Motion Angles for Various Treatment Scenarios.

Motion	Intact	Facetectomy	SLIF	TLIF
Healthy bone
Flexion	5.19	8.97	0.57	0.56
Extension	3.84	5.12	0.19	0.15
Left bending	3.65	3.78	0.19	0.41
Right bending	3.71	5.67	0.41	0.31
Left rotation	2.3	2.67	0.39	0.38
Right rotation	2.54	3.21	0.38	0.33
Osteoporotic bone
Flexion	4.37	8.6	1.27	1.3
Extension	2.91	5.73	0.22	0.11
Left bending	2.94	3.54	0.81	0.73
Right bending	3.13	3.78	0.68	0.67
Left rotation	2.24	5.44	0.76	0.68
Right rotation	2.39	5.8	0.73	0.33

SLIF – sacrolumbar interbody fusion

TLIF – transforaminal interbody fusion

Figure 3.

Range of motion data obtained from FEA.

A. Healthy bone.

B. Osteoporotic bone.

Figure 4.

Range of motion of SLIF (A) and TLIF (B) models, including von Mises stress distribution on SLTS (C), BPSS (D), SLIF cage (E), and TLIF cage (F).

Table 3.

Von Mises Stresses for Various Treatment Conditions.

	Healthy		Osteoporosis
Motion	SLIF	TLIF	SLIF	TLIF
Screw
Flexion	177	312	166	369
Extension	171	224	133	320
Left bending	164	324	210	453
Right bending	201	213	224	483
Left rotation	206	268	262	309
Right rotation	147	269	279	330
Cage
Flexion	58	53	117	93
Extension	39	32	93	81
Left bending	58	54	97	65
Right bending	39	42	104	70
Left rotation	49	46	61	67
Right rotation	48	40	65	72

SLIF – sacrolumbar interbody fusion

TLIF – transforaminal interbody fusion

Clinical

Table 4 shows the summary of demographic and clinical data. This study included eight patients (3 men, mean age 42.25). Back pain radiating to the lower extremities was the most frequent sign and present in all patients. Preoperative neurologic deficits were present in four patients and disappeared postoperatively. No new neurological deficits developed postoperatively in any patient.

Table 4.

Demographic and Clinical Data of Patients.

N	Age	Sex	Presentation	Procedure	Preoperative neurological deficit	Postoperative neurological deficit
1	37	M	Nonunion, previous failed PLIF	Explantation of previous hardware, SLIF	Absent	Absent
2	37	M	Isthmic spondylolisthesis, grade I	Gill laminectomy, SLIF	Present	Absent
3	55	F	Degenerative, no spondylolisthesis	Unilateral facetectomy, laminectomy, SLIF	Present	Absent
4	44	F	Isthmic spondylolisthesis, grade II	Gill laminectomy, SLIF	Absent	Absent
5	45	F	Degenerative, previous discectomy, no spondylolisthesis	Unilateral facetectomy, laminectomy, SLIF	Present	Absent
6	27	F	Degenerative, no spondylolisthesis	Unilateral facetectomy, laminectomy, SLIF	Absent	Absent
7	52	M	Isthmic spondylolisthesis, grade II	Gill laminectomy, SLIF	Absent	Absent
8	41	F	Isthmic spondylolisthesis, grade II	Gill laminectomy, SLIF	Present	Absent

Subjective pain and quality-of-life assessment questionaries demonstrated statistically significant decrease in pain and an increase in quality-of-life scores (Fig. 5, Table 5).

Figure 5.

Charts summarizing pain and quality-of-life scores comparisons.

Table 5.

Summary of Self-reporting Pain and Quality-of-life Questionary Results.

Parameter	Mean	Standard deviation	Standard error	p-value
VAS leg pre	4.38	±1.19	±0.42	0.000056
VAS leg post	0.75	±1.04	±0.37
VAS back pre	6.38	±1.77	±0.63	0.000078
VAS back post	0.63	±0.92	±0.32
SF 36 pre	58.67	±16.11	±5.69	0.0063
SF 36 post	88.43	±9.30	±3.29
ODI pre	15.38	±7.58	±2.68	0.0034
ODI post	3.38	±2.26	±0.80

CT scans demonstrated optimal hardware placement and robust Bridwell grade I bone fusions at six months in all patients (Fig. 6).

Figure 6.

CT images of the eighth patient A. Axial images at L5–S1 disc level demonstrating relations between screws and cage. B. Screws in the sacrum at S1 lateral recess level. Note the distance between the screws and S1 roots (safety margin). C. Sagittal (slightly oblique, co-planar with screw) image demonstrating screw trajectory. D. Sagittal image at midbody level showing solid bone fusion without lucency around the cage.

Discussion

Currently, there are several available lumbar interbody fusion options, such as anterior, oblique, extreme lateral, posterior, and transforaminal routes. They all comprise interbody cage/graft implantation and supplemental stabilization. Pedicle screw-rod stabilization is considered the strongest and, thus, is the most frequently used option. SLTSs have several advantages over standard pedicle screw-rod systems. First, it is minimally invasive since it requires only two 1-cm stab incisions. In contrast, pedicle screws are implanted via an open approach or percutaneously through separate incisions for each screw. In percutaneous BPSS implantation, additional incisions may also be required for rod passage. Therefore, SLTS is superior to pedicle screws in terms of soft tissue preservation. Another significant advantage of SLIF over conventional PLIF/TLIF is decreased operative time and blood loss. L5 and S1 pedicle screws require extensive tissue exposure/retraction for optimal medialization of the screw tip. In some cases, iliac crests interfere with S1 screws. Thus, some degree of bone resection is required. Therefore, blood loss, operative time, and radiation exposure increase. Meanwhile, SLIF does not require wide tissue exposure or iliac bone resection. Second, SLTS have no moving parts, and there is no risk of hardware dismantlement. Third, the FEA analysis shows that SLIF is biomechanically superior to TLIF. Directional deformation on shear forces was significantly lower for SLIF than for TLIF. This might be due to the screws' orientation. SLTS is almost orthogonal to shear forces, while BPSS are parallel to it. Therefore, SLTS shows high resistance to translational forces. Increased resistance is important for spondylolisthesis cases, where slippage prevention is the principal surgical objective. Another SLIF advantage is decreased von Mises stress on screws in all motion scenarios (Table 3). Such a decrease is essential to prevent common screw failures, like breakage and screw loosening. Lumbar fusion is commonly performed on elderly patients suffering from osteoporosis. Poor bone quality decreases the robustness of the construct, decreasing the chance of a favorable outcome. Therefore, we tested SLIF in an osteoporotic model. Similar to healthy bone, SLIF outperformed TLIF in terms of screw von Mises stress and shear force-induced deformity.

This study demonstrated several innovations. First, it showed that SLIF is a feasible surgical option. SLTS and IC combination has not been reported in the literature. To the authors' knowledge, this study pioneers describing such a technique. Second, this study proved that SLTS can be successfully implanted even in cases with minimal or no spondylolisthesis. So far, the transdiscal technique has been employed exclusively in high-grade spondylolisthesis patients. The reason is trivial: High slippage leads to easy screw placement. Implanting SLTS into nonspondylolisthetic L5-S1 segment may seem technically challenging. However, it was explicitly demonstrated that this innovative method allows SLTS placement even in nonspondylolisthetic L5-S1 segment (the most challenging clinical scenario). Third, this study showed that SLIF yields robust fusion and favorable clinical outcomes. Previous publications described several S1-L5 transdiscal screw combinations with pedicle screw-rod system, sacroiliac screws, and Bohlman and “reverse” Bohlman techniques^{2-4,6,7,9,10,22-25)}. Although authors indicated “excellent” and “good” clinical results, there were also reports of nonunion and screw breakage^5,24). Such nonunions were not encountered in our study, indicating that SLIF is probably more robust than previous techniques. Fourth, this study showed that SLTS can be implanted under standard intraoperative fluoroscopy without requiring image guidance, as was previously proposed²²⁾.

These innovations may profoundly affect clinical practice. L5-S1 is the most frequently affected disc joint by degeneration and spondylolisthesis. SLIF can be applied to all patients requiring L5-S1 fusion, making this procedure suitable for most patients. Additionally, SLIF diversifies surgical options and may be appropriate for patients requiring minimally invasive intervention.

This study has several limitations. First, the patient sample is small. Thus, a large study is required to confirm our findings. Second, this study's design is not a prospective randomized trial. High-quality studies, preferably those comparing fusion techniques, are required to demonstrate the strengths and weaknesses of each technique.

Conclusions

•SLIF is comprising IC and SLTS and is technically feasible and safe.

•SLIF yields excellent clinical outcomes and robust radiological fusion.

•SLIF is suitable for all clinical scenarios requiring L5-S1 fusion.

•SLIF is biomechanically superior to TLIF.

Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.

Sources of Funding: None

Author Contributions: Kamran Aghayev, conceptualization, design, study execution, and article drafting; Utpal Kanti Dhar, design, study execution; Chi-Tay Tsai, design, study execution, supervision; Merdin Ahmedov, article drafting; Frank D. Vrionis, conceptualization, design, article drafting, supervision.

Ethical Approval: This study did not require IRB approval because it used cadaveric specimens and a retrospective clinical series of known techniques, which has been previously published.

Informed Consent: Informed consent for publication was obtained by all participants in this study.

Supplementary Material

Video 1.

Axial postprocedural images of the cadaveric spine following the SLIF procedure.

Video 2.

Sagittal postprocedural images of the cadaveric spine following the SLIF procedure.