The Anterolateral Transpedicular Transverse Process Screw Technique: Surgical Techniques, Biomechanical study and Preliminary Outcomes

ABSTRACT

Objective

Given the limitations of current oblique lateral lumbar interbody fusion (OLIF) with internal fixation—such as long operation time, high blood loss, and soft tissue injury—this study evaluates the feasibility, preliminary effectiveness, and biomechanical properties of a modified Anterolateral Transpedicular Transverse Process Screw (ALTTS) technique.

Methods

ALTTS screw placement success rate and key vertebral parameters were evaluated via CT‐based simulation in 40 patients (L1–L5). A finite element model of L3‐S1 was constructed to compare three OLIF constructs: ALTTS plus bilateral anterolateral screws (ALTTS + BALS), anterior supplemental fixation (ASF), and bilateral pedicle screw fixation (BPSF). Range of motion (ROM), fixation instrument stress, and cage stress were assessed under varying bone quality. Clinically, 12 patients undergoing OLIF with L5 ALTTS and L4 BALS fixation were evaluated using VAS, ODI, and imaging.

Results

Of 200 vertebral bodies, 81 (40.5%) were suitable for ALTTS. The success rate was 0% at L1–L2, 2.5% at L3 (due to vascular constraints), and 97.5% at L5, indicating ALTTS is primarily suitable for the lower lumbar spine, specifically L5. A pedicle width (PW) > 10.21 mm was a key predictor significantly increasing placement success. Biomechanically, under normal bone density, OLIF with ALTTS + BALS demonstrated comparable ROM and stress distribution to BPSF and outperformed ASF. Under osteoporotic conditions, its stability remained superior to ASF but inferior to BPSF. Clinically, the 12 patients showed significant postoperative improvement in VAS and ODI scores, with 3‐month CT scans indicating intervertebral fusion, confirming preliminary safety and short‐term efficacy.

Conclusions

ALTTS is a lower‐lumbar‐predominant technique, offering the highest safety and placement success rate at L5, particularly when PW > 10.21 mm. Biomechanically, OLIF with ALTTS + BALS significantly outperforms OLIF with ASF. Preliminary clinical results demonstrate the novel technique is safe, feasible, and effective over a six‐month follow‐up.

This study designed a novel anterolateral screw placement technique and, based on this, developed a new anterolateral fixation method. Imaging, biomechanical, and short‐term efficacy analyses revealed that the Anterolateral Transpedicular Transverse Process Screw (ALTTS) technique achieves a high success rate for L5 vertebral fixation (pedicle width > 10.21 mm), and the ALTTS + Bilateral Anterolateral Screw (ALTTS + BALS) construct demonstrated superior stability compared to anterolateral screw fixation (ASF). Short‐term clinical follow‐up confirmed its safety and effectiveness.

1. Introduction

Lumbar degenerative diseases (LDD) are a category of conditions that result from the natural aging and degeneration of lumbar structures, such as intervertebral discs, vertebrae, joints, and ligaments, leading to pain and functional impairments [1]. Lumbar Degenerative Disease (LDD) is frequently associated with Intervertebral Disc Herniation (IDH), Spinal Canal Stenosis (SCS), and Lumbar Instability. The surgical rate of patients with lumbar spine problems is also showing an increasing trend year by year [2, 3]. For example, the incidence rate of IDH is approximately 0.26%–0.33%, and it predominantly occurs in the lumbar spine, with the L4‐L5 segment being the most common. The incidence rate of SCS significantly increases with age, and the overall prevalence rate is 14.1% among individuals aged 70–79 years (16.4% in females and 10.3% in males) [4]. Oblique Lateral Interbody Fusion (OLIF) has been extensively used in the treatment of LDD. This surgical technique enables direct exposure of the anterior and lateral surfaces of the target vertebral body and intervertebral space by virtue of the anatomical interval between the psoas major muscle and major abdominal blood vessels. Consequently, it averts impairment of the posterior lumbar soft tissues and spinal stability, effectively restores the lordosis angle, mitigates bleeding, and curtails both the surgical duration and postoperative recovery period [5, 6]. In clinical practice, OLIF is often combined with internal fixation to enhance segmental stability [7, 8, 9, 10, 11, 12, 13, 14] and reduce the risk of postoperative cage subsidence (CS) [15]. Although conventional bilateral pedicle screw fixation (BPSF) offers biomechanical advantages, its high rigidity may accelerate adjacent segment degeneration (ASD) [14, 16]. Additionally, the need for intraoperative repositioning increases operative time, blood loss, number of incisions, and radiation exposure, whereas also carrying risks of posterior soft tissue injury and persistent back pain [17, 18, 19, 20, 21, 22]. To address these limitations, anterolateral screw fixation (ASF) applied through the same OLIF corridor has been proposed and shows satisfactory clinical outcomes, though its stability remains debated, with higher reported risks of CS and hardware failure compared to BPSF [23]. For patients with Lumbar Degenerative Disease (LDD) accompanied by severe lumbar instability, the majority of experts regard bilateral pedicle screw fixation (BPSF) as the standard procedure for achieving segmental stability.

Based on this, our research team proposes a novel anterolateral lumbar screw placement technique—the Anterolateral Transpedicular Transverse Process Screw (ALTTS). This method builds upon the traditional anterolateral vertebral body screw by optimizing screw diameter, increasing anchoring depth, and enhancing cortical bone penetration. Previous studies have indicated that such modifications contribute to improved screw stiffness and pullout strength [24, 25, 26, 27, 28]. Building on the ALTTS screw, the team has further developed a new anterior fixation strategy for the L4–5 segment: the L5 vertebra is stabilized using an ALTTS, while the L4 vertebra is fixed with two parallel anterolateral vertebral body screws. This configuration is abbreviated as “ALTTS + BALS” (ALTTS + Bilateral Anterolateral Screw).

To address the challenges of simulating lumbar spinal biomechanics in vitro, the finite element (FE) method has been employed for lumbar biomechanical analysis [29, 30, 31, 32]. FE modeling and biomechanical testing not only facilitate the understanding of complex spinal structures but also provide critical references for the design, validation, and clinical application of spinal implants, particularly in early‐stage research [33]. Therefore, the purposes of this study were to: (i) evaluate the biomechanical properties of the novel anterolateral fixation system by comparing the effects of three fixation techniques (OLIF + BPSF, OLIF + ASF, and OLIF + ALTTS + BALS) on L4–5 segmental stability, cage stress, and adjacent segment behavior using an L3–S1 finite element (FE) model; (ii) identify the factors influencing screw placement through a retrospective review of imaging studies from 40 randomly selected patients with lumbar degenerative disease (LDD); and (iii) preliminarily assess the clinical reliability of this fixation system for managing L4‐L5 degenerative diseases through a short‐term outcomes analysis of a cohort comprising 12 patients who underwent OLIF combined with L4–5 ALTTS + BALS fixation.

2. Methods

2.1. Surgical Techniques

2.1.1. Selection of Vertebral Bodies

On CT images, Pedicle Width (PW) ≥ 10 mm and Pedicle Height (PH) ≥ 8 mm have been used as prerequisite conditions for the planning of anterior lateral vertebral pedicle transverse process screws in order to enhance the safety of the surgery [34].

2.1.2. Determination of the Range of the Screwable Plane

The research by Wang Kai et al. has shown that in OLIF combined with ASF surgery, placing the screws parallel to the vertebral endplates effectively reduces the risk of Cage Subsidence (CS) in the OLIF surgery [26]. Accordingly, the plan was to make the plane where the screw's center lies parallel to the upper endplate of L5 as possible. First, we needed to ascertain the plane where the largest height diameter of the pedicle was located in the sagittal image. Subsequently, the horizontal image where the midpoint of the largest height diameter of the pedicle was situated was designated as the starting plane for screw placement planning (Plane 1). At this stage, the distance between the center of the screw path and the upper and lower edges of the pedicle reaches its maximum, ensuring the safest screw insertion in this plane. Thereafter, we identified two planes that were 4 mm away from the upper and lower edges of the pedicle, labeled as the upper (Plane 2) and lower (Plane 3) limit planes, see Figure 1. Thus, the area between Planes 2 and 3 constitutes the range of the planes within which the screw can be placed. Labeling and explanation have been shown in Table 1.

FIGURE 1.

Axial CT planes defining the safe window for ALTTS insertion. Sagittal and axial CT images of L5 demonstrating Planes 1–3 (optimal and 4‐mm superior/inferior limits) used to establish the superior–inferior corridor for screw placement.

TABLE 1.

Planes and auxiliary geometric elements used for CT‐guided ALTTS planning.

Name	Meaning
Plane 1	The plane where the midpoint of the highest diameter of the right pedicle of L5 is located. This plane is the optimal plane where the center of the screw lies.
Plane 2	The screw insertion planning plane that is closest to the upper edge of the right pedicle of L5. This plane is the upper limit plane where the center of the screw is located
Plane 3	The screw insertion planning plane that is closest to the lower edge of the right pedicle of L5. This plane is the lower limit plane where the center of the screw is located.
Circle a	Approximate the cortical bone in the right recess area of the spinal canal to an arc of a circle, and then draw the circle where this arc lies
Circle b	A concentric circle of circle a. The radius of circle b is the radius of circle a plus 10 mm.
Circle c	Approximate the cortical bone at the transitional part from the right pedicle to the transverse process of L5 to an arc of a circle, and then draw the circle where this arc lies.
Line A	The tangent line of circle a.
Line B	The line that is parallel to line A and tangent to circle b.
Line C	The central axis of the path formed by lines A and B.
Line D	A line parallel to the transverse axis of the vertebra
Line E	The line passing through the inner edge of the left L5 pedicle and perpendicular to the D line.
Angle α	The included angle between line C and line D.

Note: This table delineates the safe operative window for L5 anterolateral transpedicular transvertebral screw placement by defining three axial planes and their associated arcs, lines, and angles. Specifically, Planes 1–3 mark the superior and inferior boundaries of the screw pathway center; Circle a–c outline the regions of bony structures; Lines A–E act as auxiliary trajectories; and Angle α represents the safe screw insertion angle.

2.1.3. Determine the Theoretical Screw Placement Areas Within Each Plane

As depicted in Figure 2, circles a, b, and c, and lines A, B, C, D, and E are drawn on the screw placement plane. The details of these steps are presented in Table 1. Line C is the central line of the screw placement path, and lines A and B are employed to define it. These lines are moved until line B is tangent to circle c. Then, line C represents the center of the outermost screw placement path. Angle α is the minimum screw insertion angle in this plane, and the intersection of line C and the anterior vertebral cortical bone marks the outermost screw‐entry point. Given that the OLIF surgery exposes only the left half of the vertebra, the innermost screw entry point is the intersection of the vertebral midline and anterior vertebral cortical bone. After determining the path and its center according to the above steps, angle α becomes the maximum screw placement angle in this plane, and the anterior vertebral cortical bone between the centers of the innermost and outermost screw placement paths constitutes the theoretical screw placement area in this plane.

FIGURE 2.

Geometric construction of theoretical screw‐entry area. This schematic is based on the 3D reconstructed CT image of the L5 vertebra from a 57‐year‐old female patient with lumbar degenerative disease and is illustrated for Plane 1 of this vertebra. (a, b) The auxiliary circles and lines referenced in Table 1 are depicted, using the outermost screw trajectory as an example. (c–e) Diagrams showing the insertion points and trajectories for both the outermost and innermost screws. (f) Schematic of the theoretical safe zone for screw placement on this plane. Abbreviations: BIA, bilateral iliac arteries; IVC, inferior vena cava.

2.1.4. Determination of the Actual Screw Entry Point

First, we identified the theoretical screw placement area on Plane 1. The optimal screw entry point within this area is the outermost point. Because a screw inserted through this point can engage the maximum number of cortical bone layers and have the longest insertion path, it ensures the greatest anchoring strength. Subsequently, based on the size of the screw tail in use, there is a region devoid of blood vessels and muscle attachments that can accommodate the screw tail. If the optimal screw entry point met the criteria, it was selected as the actual screw entry point. Otherwise, choose the point closest to the optimal point that satisfies the requirements. If neither option meets the criteria, re‐define another screw placement plane within the defined range and then determine the actual screw entry point as described above. If no suitable screw entry point is found across all the screw placement planes, this screw cannot be inserted into this vertebral body. After determining the actual screw insertion point, its position on the anteroposterior diameter of the vertebral body (APD) was determined via CT. In addition, distances to line E and the vertebral superior endplate were measured. Then, this point was located on the X‐ray film, and the actual screw insertion point was planned according to the steps. The screw insertion angle at this point was the maximum insertable angle. The trajectory was rotated clockwise around the insertion point until line B is tangent to circle C. The angle obtained at this time was the minimum angle for this point. See Figure 3.

FIGURE 3.

Determination of actual screw‐entry point and angular range. This schematic is based on the 3D reconstructed CT images and intraoperative fluoroscopic views of the L5 vertebra from a 57‐year‐old female patient with lumbar degenerative disease. (a, b) Determination of the actual screw insertion point on CT imaging: The red solid line indicates the distance from the insertion point to the E‐line (medial border of the left pedicle); the blue solid line marks the anteroposterior position of the point within the vertebral body; the yellow solid line shows the distance from the point to the superior endplate. (c, d) Localization of the insertion point under intraoperative C‐arm fluoroscopic guidance, with solid lines defined as above. (e) Determination of the screw trajectory angle range, using the innermost insertion point as an example.

2.1.5. Surgical Details

Following the standard OLIF surgical procedure, a working channel was inserted into the patients. Discectomy was performed. Based on preoperative planning, an appropriately sized ALTTS was implanted into the L5 vertebral body, and two parallel appropriately sized vertebral screws were implanted into the L4 vertebral body. Next, the CAGE was implanted following the traditional OLIF surgical procedure. An appropriately long titanium rod was selected to connect and fix the three screws on the L4 and L5 vertebral bodies. The titanium rod was then connected to the CAGE using a connector and locked in place. Finally, the incision was closed in layers.

2.2. Preliminary Clinical Validation

2.2.1. Patients

This retrospective study included LDD patients treated at our institution between December 2023 and August 2024, based on the following inclusion criteria. Ethical approval was granted by the Ethics Committee of The Second Hospital of Dalian Medical University (IRB/IEC No. XJS2024‐110‐01). The study was conducted in strict compliance with local regulatory guidelines and the ethical principles of the Declaration of Helsinki, and written informed consent was obtained from all participants prior to enrollment. Inclusion Criteria:

1. Patients with a confirmed clinical diagnosis of lumbar degenerative disease (LDD) who meet the radiological and symptomatic indications for oblique lateral interbody fusion (OLIF);

2. Age ≥ 18 years, with no severe dysfunction of vital organs (heart, lungs, liver, or kidneys) that would preclude CT imaging or surgery, as determined by preoperative evaluation;

3. Lumbar CT imaging must demonstrate intact vertebral anatomy from L1 to L5, without severe vertebral fractures, osteolytic destruction, or spinal tumors. For Surgical Candidates: Additional preoperative CT angiography (CTA) or MRI is required to confirm vascular safety for anterolateral access (e.g., left iliac vessels ≥ 5 mm from L5 vertebral body);

4. Patients or their legal representatives must fully understand the study protocol and voluntarily provide written informed consent for imaging analysis, surgery, or both as applicable.

2.2.1.1. Exclusion Criteria

1. Previous lumbar surgery, especially OLIF, ALIF, or posterior lumbar instrumentation, which may alter anatomical landmarks or compromise fixation stability;

2. Severe spinal deformities (e.g., congenital spina bifida, scoliosis > 20° Cobb angle) that would impede surgical access, screw placement, or accurate imaging analysis;

3. Coagulopathy (INR > 1.5, platelets < 100 × 10⁹/L) or active hematologic disorders associated with a prohibitive risk of perioperative bleeding;

4. Severe psychiatric or cognitive conditions precluding cooperation with preoperative assessment, surgical procedures, or postoperative follow‐up (e.g., dementia, uncontrolled schizophrenia);

5. Left‐sided iliac vessels positioned < 5 mm from the L5 vertebral body (confirmed by CTA/MRI); T‐score < −3.5 without planned cement augmentation; BMI> 35 kg/m², which may increase surgical difficulty or anesthesia risk.

This study has been approved by the ethics committee (ethical approval number: XJS2024‐110‐01) and informed consent was obtained from all included participants.

2.2.2. Imaging Studies

Preoperative CT images (Siemens Healthineers) of these patients were collected, covering the sagittal sections of each lumbar vertebra and the axial sections parallel to the upper endplate of the lumbar vertebrae. According to the above‐mentioned method, the pedicle height (PH) was first measured [35, 36]. Subsequently, PW was calculated [35]. The pedicle length (PL) [37], transverse pedicle angle (TPA) [35, 36], anteroposterior diameter of the vertebral body (APD) [38] and interpedicular distance (IPD) [39] of each patient were measured within the image of plane 1. Simultaneously, the minimum and range of insertable angles of theoretically implantable vertebrae in Plane 1 (a vertebra fulfilling the prerequisite conditions, with its outermost screw‐entry point at or left of the vertebral midline, is defined as a theoretically implantable vertebra). Then, we planned these vertebrae using the aforementioned method and counted the number of actually implantable ones. See Figure 4.

FIGURE 4.

Measurement methodology for vertebral parameters. Illustration of how pedicle width (PW), height (PH), length (PL), transverse pedicle angle (TPA), anteroposterior diameter (APD), and interpedicular distance (IPD) were obtained from axial CT images.

2.2.3. Preliminary Clinical Outcomes Assessment

A 6‐month postoperative short‐term follow‐up was conducted. The following information was collected: (1) Demographic data: age and sex; (2) Surgical parameters: operative time and intraoperative blood loss; (3) Radiographic outcomes: ①Fusion rate at 12 months postoperatively, evaluated by two independent researchers using the Bridwell fusion classification criteria [40] based on lumbar X‐ray and 3D‐CT scans taken preoperatively, at 1 day, and 6 months postoperatively. Bridwell grades I and II were defined as fusion; ② CS rate at 6 months, determined on lateral lumbar radiographs with subsidence > 3 mm defined as positive [41]. Visual Analogue Scale (VAS) and Oswestry Disability Index (ODI) scores were assessed preoperatively and at 3 and 6 months postoperatively to evaluate pain and functional status.

2.3. Statistical Analysis

Statistical evaluation was performed using the SPSS software (version 29.0; IBM Corp). Categorical variables were described as frequencies. Statistical significance was set at p < 0.05, and all data were independently measured twice, and the average values were used for analysis. The reliability of the two sets of data was evaluated using the intraclass correlation coefficient (ICC). All continuous variables, including PW, PH, PL, TPA, APD, and IPD, were examined for normality using the Shapiro–Wilk test. Variables that satisfied the normality assumption were expressed as mean ± SD and analyzed with the paired‐samples t‐test; non‐normally distributed variables were reported as median (IQR) and analyzed using the Mann–Whitney U test. Compute the proportion of theoretically implantable vertebrae per lumbar spinal segment. Subsequently, the success rate of the actual planning for each segment is determined. Employ differential analysis to screen vertebral parameters influencing screw implantation and conduct binary logistic regression on the screened factors to identify the most valuable ones. The cutoff value of the determining factors was ascertained through ROC analysis in accordance with the Youden index. Therefore, the proportion of vertebral bodies within each lumbar spine segment that satisfied this threshold was calculated. Finally, Spearman's correlation analysis was performed to examine the relationship between the most valuable vertebral parameters and both the minimum screw insertion angle and angle range of the theoretically implantable vertebrae in plane 1.

Clinical outcomes: Statistical analysis was performed using SPSS 29.0. Continuous variables were expressed as mean ± standard deviation or median (interquartile range) based on distribution normality, which was verified by the Kolmogorov–Smirnov test. One‐way ANOVA (for normally distributed data) or non‐parametric rank‐sum tests (for non‐normal data) were used to compare differences among the three groups and changes within each group across time points. A p value < 0.05 was considered statistically significant.

2.4. Biomechanical Study

2.4.1. Construction of the Finite Element Lumbar Spine Model

A healthy 30‐year‐old male volunteer (height: 172 cm, weight: 75 kg, no history of lumbar disease) was selected. The lumbar spine was scanned using a 64‐slice spiral CT scanner (Siemens Healthineers). The obtained images were imported into Mimics 21.0 (Materialize Technologies, Leuven, Belgium) for segmentation of the L3–S1 segments. The segmented model was then smoothed and converted into NURBS surfaces and a cancellous bone model using Geomagic Wrap 2021 (Geomagic Inc., North Carolina, USA). Solid models of the cortical bone, cancellous bone, intervertebral disc, and endplates were reconstructed and assembled with screw fixation systems in SolidWorks 2024 (Dassault Systèmes SolidWorks Corporation, USA). To focus on the L4–5 segment, the model was simplified as follows: only cancellous bone was included in S1, and no cancellous bone was modeled in the posterior elements of L3. The complete 3D model was imported into Ansys 2022 (Ansys Mechanical, USA), where material properties were assigned, contact pairs and ligaments were defined based on anatomical relationships, resulting in a functional spinal finite element model for surgical simulation (Figure 5).

FIGURE 5.

An ANSYS modeative Complications in 155 Patiel of a healthy adult lumbosacral spine.

An OLIF procedure was simulated on this model: the annulus fibrosus, nucleus pulposus, and partial cartilaginous endplate were removed at the L4–5 level while preserving the posterior structures. A cage (50 × 18 × 15 mm, 12° angle; Double Medical, China) filled with allograft cancellous bone was positioned in the anterior third of the disc space. Three fixation techniques were simulated using the same cage placement: (1) ALTTS + BALS group: Two screws (45 mm × 6.5 mm) were placed in L4 parallel to its inferior endplate. One screw (55 mm × 7.5 mm) was inserted into L5 parallel to its superior endplate. The L5 screw entry point was at the left anterolateral pedicle level, traversing the right pedicle and penetrating the posterior cortex (Figure 6A). (2) ASF group: One screw (45 mm × 6.5 mm) each was placed in L4 and L5 parallel to their inferior and superior endplates, respectively (Figure 6B). (3) BPSF group: Two pedicle screws (45 mm × 6.5 mm each) were placed in both L4 and L5 (Figure 6C). In all models, the rod diameter was set to 5.5 mm.

FIGURE 6.

Anteroposterior and lateral views of the finite element models representing OLIF combined with three fixation techniques: (A) Anterolateral Screw Fixation (ASF); (B) Anterolateral Transpedicular Transverse Process Screw + Bilateral Anterolateral Screw (ALTTS + BALS); (C) Bilateral Pedicle Screw Fixation (BPSF).

2.4.2. Material Properties, Parameter Settings, and Loading Conditions

All components in the L3–S1 finite element model were defined as isotropic homogeneous elastic materials, except for the annulus fibrosus of the intervertebral disc. Specific parameters were set as follows: (1) Vertebral structures, including cancellous bone, cortical bone, endplates, and articular cartilage: The cortical bone thickness was set to 1.0 mm and endplate thickness to 0.5 mm [42]. Contact between articular cartilage surfaces was modeled as no‐separation constraint. (2) Intervertebral disc: Composed of the nucleus pulposus (40% of disc volume) and annulus fibrosus (60%) [16]. The nucleus was modeled as an isotropic homogeneous elastic material. The annulus ground substance was simulated using a hyperelastic constitutive law, with fiber bundles represented by nonlinear springs oriented at approximately 30°; coefficients for the fifth‐order Ogden hyperelastic model were derived from experimental data [43] (Table 2 for parameters). (3) Implants: The Young's modulus of screws and rods was set to 110,000 MPa; the PEEK cage was assigned a modulus of 3600 MPa. (4) Ligaments: Modeled as tension‐only spring elements, including the anterior longitudinal, posterior longitudinal, ligamentum flavum, supraspinous, interspinous, and intertransverse ligaments. All material properties were adopted from previously validated models [44, 45], detailed parameters are listed in Table 3. To simulate osteoporotic bone conditions, the Young's modulus and density of cancellous bone were reduced by 66%, and those of cortical bone and posterior vertebral elements by 33%, based on bone density measurements. These values were derived from calculations based on bone mineral density (BMD) measurements and Young's modulus in healthy individuals versus osteoporosis patients. This difference results in an approximately 35% reduction in the overall compressive stiffness of functional spinal units (FSUs) compared to that of normal FSUs [46]. Identical boundary and loading conditions were applied to all three fixation models: the sacroiliac joints were fixed in all degrees of freedom. A preload of 500 N (approximately 2/3 of body weight, 50 kg) was applied to the superior surface of L3. Additionally, a 7.5 N m pure moment was imposed along anatomical planes as described in [35] to simulate six physiological motions: flexion, extension, left/right lateral bending (LB, RB), and left/right axial rotation (LR, RR). Biomechanical performance was evaluated by measuring the range of motion (ROM), equivalent stress peak of fixation instruments (ESPFI), and equivalent stress peak of the cage (ESPC) within the internal fixation system.

TABLE 2.

Parameter of annulus fibrosus.

Mu1	Alpha1	Mu2	Alpha2	Mu3	Alpha3	D1	D2	D3
−126.22	24.81	123.78	25.00	2.75	11.66	1.42	0	0

TABLE 3.

Material properties of the finite element model.

Component	Young's modulus (MPa)	Poisson ratio	Cross‐sectional area (mm2)
Vertebral body
Cortical bone	12,000 (osteoporosis: 8040)	0.3	/
Cancellous bone	100 (osteoporosis: 34)	0.2	/
Endplate	1000 (osteoporosis: 670)	0.3	/
Screws and rods	110,000	0.29	/
PEEK cage	3600	0.49	/
Nuclear pulposus	1	0.45	/
Facet Cartilage	24.3	0.4	/
Ligaments
ALL	20	0.3	63.7
PLL	20	0.3	20
ISL	11.6	0.3	40
SPL	15	0.3	30
ITL	58.7	0.3	3.6
FL	19.5	0.3	40

Abbreviations: ALL, anterior longitudinal ligament; FL, flavum ligament; ISL, interspinal ligament; ITL, intertransverse ligament; PLL, posterior longitudinal ligament; SSL, suprsaspinal ligament.

2.4.3. Model Validation

The model consisted of 2,797,001 elements and 4,127,346 nodes. The validity of the intact spinal finite element model was confirmed by comparing its predictions with previously published finite element models from Vena et al. [47], Biswas et al. [48], and Pearcy and Tibrewal et al. [49], as well as experimental data from cadaveric studies by Shim et al. [50] (Figure 7).

FIGURE 7.

The predictions of the constructed finite element model were compared with those from studies by Kamal et al. Biswas et al. Pearcy et al. Shim et al. and Yamamoto et al. for validation.

3. Results

3.1. Imaging Results

3.1.1. General Characteristics

The imaging study enrolled a total of 40 eligible patients (19 men and 21 women), with an overall age range of 39 to 81 years (mean age: 63.98 ± 7.56 years). Specifically, the male patients were aged 39–75 years (mean: 63.15 ± 8.98 years), while the female patients were aged 56–81 years (mean: 64.7 ± 6.12 years).

3.1.2. Reliability of Imaging Measurements and Variations in Vertebral Segment Parameters

The results of the intraclass correlation coefficient (ICC) demonstrated excellent consistency between the two sets of data (ICC > 0.75, p < 0.05), thus fulfilling the requirements for subsequent analysis. In total, 200 vertebral bodies (40 L1–L5) were measured in this study. The results indicated that PW, TPA, and IPD of the vertebral bodies from L1 to L5 gradually increased, whereas the PH and PL gradually decreased. In the study, the theoretical screw placement area was successfully determined for 40.5% of the vertebral bodies. The success rates of the vertebral bodies from L1 to L5 were 0%, 0%, 25%, 77.5%, and 100%, respectively. During the process of determining the actual planned screw insertion points, the success rates of the vertebral bodies in each segment from L3 to L5 were 2.5%, 17.5%, and 97.5% respectively, which decreased by 90%, 77.4%, and 2.5% compared to the theoretical screw placement success rates. All the collected measurement values and the screw placement success rates for each lumbar spine are recorded in Table 4.

TABLE 4.

Radiographic parameters and ALTTS placement success rates across lumbar segments L1–L5.

Total number of vertebral bodies collected (n = 200)	L1 n = 40	L2 n = 40	L3 n = 40	L4 n = 40	L5 n = 40
PW (mm)	7.07 ± 1.39	7.84 ± 1.40	9.41 ± 1.61	11.32 ± 1.62	15.14 ± 1.87
PH (mm)	15.0 ± 1.49	13.95 ± 1.72	13.82 ± 1.41	12.18 ± 1.74	12.08 ± 1.66
PL (mm)	15.5 ± 1.54	14.49 ± 1.18	13.19 ± 1.53	12.16 ± 1.72	10.33 ± 2.20
TPA (°)	9.23 ± 4.66	10.86 ± 5.33	15.34 ± 4.82	18.07 ± 5.15	27.71 ± 6.92
APD (mm)	30.0 ± 3.15	31.39 ± 3.19	32.31 ± 3.11	33.06 ± 3.14	32.27 ± 2.73
IPD (mm)	22.3 ± 1.95	22.88 ± 1.74	23.85 ± 1.96	25.60 ± 2.52	31.03 ± 3.62
Planning success rate considering only vertabral condition (%)	0	0	25	77.5	100
Planning success rate considering vascular and vertabral condition (%)	0	0	2.5	17.5	97.5

Note: Mean ± SD (or median [IQR]) values of pedicle width (PW), pedicle height (PH), pedicle length (PL), transverse pedicle angle (TPA), anteroposterior diameter of the vertebral body (APD), and interpedicular distance (IPD) are presented for 200 vertebrae (40 per level). Theoretical success rates reflect vertebral morphology alone; actual success rates additionally consider vascular safety.

3.1.3. Comparison of Parameters Between Successfully and Unsuccessfully Planned Vertebrae

The study analyzed the differences in the overall data of successful and failed theoretical planning. The results showed significant differences in PW, PH, PL, TPA, APD, IPD, and vertebral body serial numbers between the two groups (p < 0.001). A segment‐based difference study was conducted. It was found that planning was unfeasible for the L1 and L2 segments, whereas all planning was successful for the L5 segment. Thus, only data of L3 and L4 segments were used for subsequent difference analysis, and the results indicated significant differences in PW (p < 0.001) and PH (p = 0.007) in the L3 group and in PW (p < 0.001), PH (p = 0.025), and APD (p = 0.002) in the L4 group. The results of the independent t‐test and non‐parametric test are shown in Table 5.

TABLE 5.

Comparison of radiographic parameters between vertebrae with successful and unsuccessful ALTTS planning.

	Planning successful	Planning failure	T	Z	p
General data n = 200 Successful n = 81 Failure n = 119
PW (mm)	13.50 (11.36 ~ 14.88)	7.92 ± 1.50		−11.935	< 0.001
PH (mm)	12.58 ± 1.79	13.97 ± 1.87	5.266		< 0.001
PL (mm)	11.36 ± 2.23	14.35 ± 1.77	10.096		< 0.001
TPA (°)	22.48 ± 8.03	11.99 ± 5.72	−10.143		< 0.001
APD (mm)	33.04 ± 2.79	30.33 (28.21 ~ 33.01)		−4.693	< 0.001
IPD (mm)	26.89 (24.91 ~ 31.31)	22.65 (21.71 ~ 23.61)		−9.23	< 0.001
L3 data n = 40 Successful n = 10 Failure n = 30
PW (mm)	11.46 ± 0.97	8.73 ± 1.12	−6.879		< 0.001
PH (mm)	14.83 ± 1.18	13.48 ± 1.33	−2.844		0.007
PL (mm)	12.74 (12.18 ~ 13.31)	13.27 ± 1.63		−0.625	0.532
TPA (°)	15.17 ± 4.86	15.39 ± 4.89	0.123		0.902
APD (mm)	33.84 ± 2.94	31.80 ± 3.04	−1.857		0.071
IPD (mm)	24.40 ± 1.70	23.67 ± 2.03	−1.018		0.315
L4 data n = 40 Successful n = 31 Failure n = 9
PW (mm)	11.34 (10.88 ~ 12.84)	9.36 ± 1.05		−4.13	< 0.001
PH (mm)	12.51 ± 1.57	11.05 ± 1.91	−2.342		0.025
PL (mm)	12.18 ± 1.87	12.10 ± 1.15	−0.164		0.872
TPA (°)	18.10 ± 5.33	17.96 ± 4.79	−0.071		0.943
APD (mm)	33.77 ± 2.65	29.34 (28.63 ~ 31.02)		−3.061	0.002
IPD (mm)	25.71 ± 2.39	25.22 ± 3.08	−0.502		0.619

Note: Data are shown as mean ± SD or median [IQR] for the overall cohort (n = 200) and for the L3 and L4 segments separately. Significant differences were observed in pedicle width (PW), pedicle height (PH), pedicle length (PL), transverse pedicle angle (TPA), anteroposterior diameter (APD), and interpedicular distance (IPD) between successfully and unsuccessfully planned vertebrae. Bold the font, statistical significance between two groups.

3.1.4. Multivariate Regression Analysis Results

The study included the statistically significant variables from the differential analysis in the regression model; only PW had a predictive value (OR: 802.182, 95% CI: 6.373–100975.819), and the data are recorded in Table 6. Furthermore, the predictive ability of PW was evaluated by plotting an ROC curve. Its AUC was 0.998 (95% CI: 0.994–1.000). The PW value corresponding to the maximum Youden index was 10.21 mm (Youden index: 0.958, sensitivity: 100%, specificity: 95.8%), and the image was recorded in Figure 8.

TABLE 6.

Multivariable analysis of statistically significant variables.

Factor	OR	95% CI	p
PW (mm)	802.182	6.373 ~ 100975.819	0.007
PH (mm)	1.841	0.448 ~ 7.572	0.397
PL (mm)	0.815	0.299 ~ 2.222	0.69
TPA (°)	1.382	0.87 ~ 2.197	0.171
APD (mm)	0.92	0.572 ~ 1.481	0.731
IPD (mm)	1.226	0.538 ~ 2.794	0.628

Note: Bold the font, statistical significance between two groups.

Abbreviations: APD, anteroposterior diameter; IPD, interpedicular distance; PH, pedicle height; PL, pedicle length; PW, pedicle width; TPA, transverse pedicle angle.

FIGURE 8.

ROC analysis and correlations of pedicle width with screw angles. (a) ROC curve for PW predicting ALTTS feasibility; (b) inverse correlation between PW and minimum entry angle; (c) positive correlation between PW and available angular range.

3.1.5. ROC Curve Validation and Correlation Analysis Results

The study analyzed the correlations between PW and the minimum screw insertion angle and the range of available screw insertion angles. The r value of PW with the former was −0.894 (p < 0.001). The value of the latter was 0.908 (p < 0.001). These images are presented in Figure 8.

3.2. Results of the Preliminary Clinical Outcomes Assessment

3.2.1. Baseline Characteristics of the Clinical Surgical Cohort

This study enrolled 12 patients who underwent OLIF with ALTTS + BALS, with a mean age of 64.83 ± 5.72 years (6 males and 6 females). Preoperative primary diagnoses included 6 cases of degenerative/spondylolytic spondylolisthesis (Grade I) and 6 cases of degenerative lumbar spinal stenosis. Procedural data demonstrated a mean blood loss of 115 ± 38.49 mL and an average operative time of 142.08 ± 14.22 min.

3.2.2. Changes in Function and Pain at 6 Months Post‐Operation

Regarding the ODI scores, the 12 patients improved from a preoperative value of 67.42 ± 7.39 to 27.59 ± 4.40 at 3 months and 20.92 ± 4.38 at 6 months postoperatively. For the VAS‐low back pain scores, patients improved from 7.83 ± 0.94 preoperatively to 2.42 ± 0.51 at 3 months and 1.75 ± 0.45 at 6 months postoperatively. Similarly, the VAS‐leg pain scores improved from 7.25 ± 1.14 preoperatively to 2.42 ± 0.67 at 3 months and 1.67 ± 0.65 at 6 months postoperatively. All three scores showed significant improvement compared to preoperative values (all p < 0.05), and the pairwise comparisons at each follow‐up time point are presented in the Table 7.

TABLE 7.

The changes in ODI and VAS before and after surgery.

	Pre‐	3 months Post‐	6 months Post‐	H	p
ODI (%)	67.42 ± 7.39	27.59 ± 4.40†	20.92 ± 4.38†	27.86	< 0.001
VAS‐lower back	7.83 ± 0.94	2.42 ± 0.51†	1.75 ± 0.45†	28.5	< 0.001
VAS‐leg	7.25 ± 1.14	2.42 ± 0.67†	1.67 ± 0.65†	27.35	< 0.001

Note: Data presented as mean ± standard deviation (SD). All non‐parametric pairwise comparisons were corrected using the Bonferroni method. Bold the font, statistical significance between three groups.

Abbreviations: Post, postoperative; Pre, preoperative.

^† p < 0.05 compared to Pre.

3.2.3. Typical Case

A 55‐year‐old male with L4–5 LDD and L4–5 spondylolisthesis underwent treatment with OLIF with L5 anterolateral transpedicular transverse process screw combined with bilateral L4 anterior vertebral screws for internal fixation. Stable intervertebral fusion was noted at the 6‐month follow‐up. See Figure 9.

FIGURE 9.

Representative case—55‐year‐old male with L4–5 spondylolisthesis. A 55‐year‐old male with L4–5 LDD and L4–5 spondylolisthesis underwent treatment with OLIF with L5 anterolateral transpedicular transverse process screw combined with bilateral L4 anterior vertebral screws for internal fixation. (a, b) Pre‐operative anteroposterior and lateral X‐rays. (c, d) Anteroposterior and lateral X‐rays 3 days post‐operation. (e, f) Anteroposterior and lateral X‐rays 3 months post‐operation. (g, h) Pre‐operative and 3‐day post‐operative MRI. (i, l) Pre‐operative, 3‐day post‐operative and 3‐month post‐operative CT images. (j) shows the axial CT image 3 months after surgery. At the 3‐month follow‐up, the internal fixation was in a good position. Stable intervertebral fusion was noted at the 6‐month follow‐up.

A 65‐year‐old male with L4–5 LDD and osteoporosis underwent OLIF with L5 anterolateral transpedicular transverse process screw combined with bilateral L4 anterior vertebral screws for internal fixation, along with bone cement reinforcement. The internal fixation was in a good position. See Figure 10.

FIGURE 10.

Representative case—65‐year‐old osteoporotic male with cement‐augmented fixation. A 65‐year‐old male with L4–5 LDD and osteoporosis underwent OLIF with L5 anterolateral transpedicular transverse process screw combined with bilateral L4 anterior vertebral screws for internal fixation, along with bone cement reinforcement. (a, b) Pre‐operative anteroposterior and lateral X‐rays. (c, d) Anteroposterior and lateral X‐rays 3 days post‐operation. (e, f) Anteroposterior and lateral X‐rays 3 months post‐operation. (g, h) Pre‐operative and 3‐day post‐operative MRI. (i‐l) Pre‐operative, 3‐day post‐operative and 3‐month post‐operative CT images. Figure j shows the axial CT image 3 months after surgery. No CS was seen during the follow‐up, and the internal fixation was in a good position.

3.3. Results of the Finite Element Research

3.3.1. Range of Motion

The ROM values at the L4–5 segment under different bone conditions are summarized in Table 8 and Figure 11. Under normal bone conditions, the ROM at L4–5 in all three surgical models decreased by > 75% compared to the intact model. Specifically, in the OLIF + ASF group, ROM decreased by 89.55% in flexion, 79.41% in extension, 96.20% in left lateral bending (LB), 95.80% in right lateral bending (RB), 97.22% in left rotation (LR), and 96.93% in right rotation (RR). Except in RB and RR, where ROM was similar to the OLIF + ALTTS + BALS group, the OLIF + ASF group exhibited greater ROM than the other two groups in all other directions, with significantly higher values in flexion and extension compared to both the OLIF + ALTTS + BALS and OLIF + BPSF groups. Except for extension (with an 86.04% decrease in ROM), the ROM of the OLIF + ALTTS + BALS group was comparable to that of the OLIF + BPSF group in all other motion directions (Figure 11).

TABLE 8.

L4‐5 rang of motion and percentage reduction in motion.

	Flexion	Extension	LB	RB	LR	RR
L4‐5 rang of motion (°)
Normal	4.4	5.73	5	4.76	3.24	3.26
ALTTS + BALS	0.23	0.8	0.18	0.2	0.08	0.1
ALTTS + BALS osteoporotic	0.25	1.2	0.26	0.33	0.1	0.13
ASF	0.46	1.18	0.19	0.2	0.09	0.1
ASF osteoporotic	0.58	1.79	0.27	0.34	0.12	0.13
BPSF	0.13	0.1	0.14	0.16	0.07	0.09
BPSF osteoporotic	0.16	0.14	0.22	0.23	0.08	0.13
Percentage reduction in motion (%)
ALTTS + BALS	94.77%	86.04%	96.40%	95.80%	97.53%	96.93%
ALTTS + BALS osteoporotic	94.31%	79.06%	94.80%	93.07%	96.91%	96.01%
ASF	89.55%	79.41%	96.20%	95.80%	97.22%	96.93%
ASF osteoporotic	86.82%	68.76%	94.60%	92.86%	96.30%	96.01%
BPSF	97.05%	98.25%	97.20%	96.64%	97.84%	97.24%
BPSF osteoporotic	96.36%	97.56%	95.60%	95.17%	97.53%	96.01%

Abbreviations: ALTTS + BALS, the anterolateral transpedicular transverse process screw + bilateral anterolateral screw; ASF, anterolateral screw fixation; BPSF, bilateral pedicle screw fixation; LB, left bending; LR, left rotation; RB, right bending; RR, right rotation.

FIGURE 11.

ROM at the L4/5 surgical segment in the three fixation models under normal and osteoporotic bone conditions.

Under osteoporotic conditions, the ROM at L4–5 in all three models decreased by > 80% compared to the intact model, except for the OLIF + ALTTS + BALS group in extension (79.06% decrease) and the OLIF + ASF group in extension (68.76% decrease). Compared to normal bone conditions, ROM increased significantly in all three groups under osteoporosis. The OLIF + ASF group showed the most pronounced increase in ROM during extension, which was the highest among the three groups. The OLIF + BPSF group still demonstrated the smallest ROM. The OLIF + ALTTS + BALS group showed a noticeable increase in ROM during extension compared to normal bone conditions (even after the increase, it remained comparable to the extension ROM of the OLIF + ASF group under normal bone conditions), whereas its ROM in other directions remained similar to that of the OLIF + BPSF group (Figure 11).

Adjacent Segment ROM (L3–L4 and L5–S1): At the L3–L4 segment, under both normal and osteoporotic conditions, all three surgical models showed a significant increase in ROM across all six directions compared to the intact model (Figure 7). At the L5–S1 segment, under both bone conditions, a slight increase in ROM was observed during flexion and extension in all three surgical models compared to the intact model. During LB, RB, LR, and RR, ROM values were similar to those of the intact model under normal bone conditions but slightly increased under osteoporotic conditions (Figure 12).

FIGURE 12.

Range of motion (ROM) at the L3/L4 and L5/S1 segments in the three OLIF models under normal and osteoporotic bone conditions.

3.3.2. Equivalent Stress Peak of Fixation Instruments (ESPFI)

The Equivalent stress peak of fixation instruments (ESPFI) of the three surgical constructs under varying bone conditions are compared in Figure 13 and Table 9. Under normal bone conditions, the OLIF + ASF group exhibited the lowest ESPFI value (52.52 MPa) during LR, but the highest values among the three groups in flexion (238.26 MPa), extension (435.47 MPa), LB (195.46 MPa), and RR (182.90 MPa). The ESPFI in extension was significantly higher than that of the other two groups, with maximum stress concentrated at the tail of the screw. The OLIF + BPSF group showed the lowest ESPFI values in flexion (82.52 MPa), extension (166.40 MPa), and LB (94.40 MPa), but the highest in right lateral bending (RB, 172.03 MPa) and LR (162.46 MPa). The OLIF + ALTTS + BALS group demonstrated the lowest values in RB (95.92 MPa) and RR (97.79 MPa). Except in LR, its ESPFI values were lower than those of the OLIF + ASF group, with reductions of 30.97% in flexion, 60.76% in extension, 39.83% in LB, 2.62% in RB, and 43.53% in RR. The ESPFI of the OLIF + ALTTS + BALS group was only slightly higher than that of the OLIF + BPSF group in flexion, but comparable or superior in all other directions (Figure 14).

FIGURE 13.

Equivalent Stress Peak of Fixation Instruments (ESPFI) in the three models under normal and osteoporotic bone conditions.

TABLE 9.

ESPFI and ESPC at the L4–L5 level.

	Flexion	Extension	LB	RB	LR	RR
The stress of the fixation instruments (MPa)
ALTTS + BALS	164.48	170.88	117.60	95.92	104.56	97.79
ALTTS + BALS osteoporotic	211.48	227.31	157.60	103.75	133.41	90.01
ASF	238.26	435.47	195.46	98.50	52.52	182.90
ASF osteoporotic	254.39	459.55	205.09	130.70	69.49	179.92
BPSF	82.52	166.40	94.40	172.03	162.46	100.07
BPSF osteoporotic	104.52	185.53	110.14	224.28	199.54	110.80
The stress of the cage (MPa)
ALTTS + BALS	24.68	47.67	38.61	24.07	24.63	22.04
ALTTS + BALS osteoporotic	29.22	50.21	41.78	28.99	25.78	25.56
ASF	35.51	47.96	33.15	37.44	32.82	21.05
ASF osteoporotic	42.84	53.69	31.84	47.26	37.16	26.64
BPSF	23.97	10.85	25.71	19.96	16.27	23.35
BPSF osteoporotic	29.87	12.37	26.50	26.05	18.37	25.79

Abbreviations: ALTTS + BALS, the anterolateral transpedicular transverse process screw + bilateral anterolateral screw; ASF, anterolateral screw fixation; BPSF, bilateral pedicle screw fixation; LB, left bending; LR, left rotation; RB, right bending; RR, right rotation.

FIGURE 14.

Schematic representation of stress distribution in the three internal fixation systems under normal bone conditions.

Under osteoporotic conditions, ESPFI values increased in almost all directions compared to normal bone, except for RR in the OLIF + ALTTS + BALS and OLIF + ASF groups. The OLIF + ASF group generally exhibited the highest ESPFI values among the three groups in all directions except LR, indicating inferior mechanical performance. The OLIF + BPSF group showed the lowest overall ESPFI values, demonstrating the best stress distribution. The OLIF + ALTTS + BALS group performed better than the OLIF + ASF group in all directions except LR. Its ESPFI values were higher than those of the OLIF + BPSF group in flexion and extension, but comparable or superior in the remaining directions (Figure 13).

3.3.3. Equivalent Stress Peak of the Cage (ESPC)

The Equivalent Stress Peak of the Cage (ESPC) values for the three surgical models under varying bone conditions are shown in Figure 15 and Table 9. Under normal bone conditions, the OLIF + ASF group exhibited the poorest stress performance, showing the highest ESPC values among the three groups in flexion (35.51 MPa), extension (47.96 MPa), RB (37.44 MPa), and LR (32.82 MPa), whereas demonstrating the lowest value only in RR (21.05 MPa). The OLIF + BPSF group demonstrated the optimal stress performance, with the lowest ESPC values in all directions except RR (23.35 MPa), where it was the highest. Specifically, its values were 23.97 MPa in flexion, 10.85 MPa in extension, 25.71 MPa in left LB, 19.96 MPa in RB, and 16.27 MPa in LR. The OLIF + ALTTS + BALS group showed the highest ESPC value in LB (38.61 MPa), but its values in flexion, extension, RB, and LR were significantly lower than those of the OLIF + ASF group. Its overall stress performance was intermediate between the OLIF + BPSF and OLIF + ASF groups (Figure 16).

FIGURE 15.

Peak equivalent stress in the cage across the three OLIF models under normal and osteoporotic bone conditions.

FIGURE 16.

Schematic representation of cage stress in the three OLIF models under normal bone conditions.

Under osteoporotic conditions, the ESPC values increased in all directions compared to normal bone conditions, except for LB in the OLIF + ASF group. The OLIF + ASF group continued to demonstrate relatively poor overall stress performance, with higher ESPC values in all directions except LB. The OLIF + BPSF group maintained the best overall performance with the lowest ESPC values among the three groups. The OLIF + ALTTS + BALS group exhibited lower ESPC values than the OLIF + ASF group in all directions except LB, indicating superior stress performance compared to OLIF + ASF (Figure 15).

4. Discussion

4.1. Main Findings of This Study

This study systematically evaluated the feasibility, biomechanical properties, and preliminary clinical outcomes of a novel anterolateral screw fixation technique—the Anterolateral Transpedicular Transverse Process Screw (ALTTS)—for use in OLIF surgery. The main findings are as follows. First, CT‐based simulation of 200 vertebral bodies from 40 patients demonstrated that the ALTTS technique is primarily suitable for the lower lumbar vertebrae, with an overall screw placement success rate of 40.5%. Notably, the L5 vertebra achieved the highest success rate (97.5%), whereas L1 and L2 were unsuitable (0%). Pedicle width (PW) was identified as the most valuable predictive parameter, with a threshold of > 10.21 mm significantly increasing implantation feasibility. Second, finite element analysis of the L3–S1 segment revealed that, under normal bone conditions, OLIF combined with ALTTS + BALS provided comparable segmental stability (ROM), internal fixation stress (ESPFI), and cage stress (ESPC) to the gold‐standard BPSF construct, and was biomechanically superior to OLIF with ASF. Under osteoporotic conditions, the ALTTS + BALS construct maintained greater stability than ASF, although it was inferior to BPSF. Third, preliminary clinical assessment in 12 patients who underwent OLIF with L5 ALTTS and bilateral L4 anterolateral screw fixation demonstrated significant improvements in VAS and ODI scores at 6 months postoperatively, with no serious vascular complications and radiographic evidence of intervertebral fusion. Collectively, these findings suggest that the ALTTS technique is a feasible and effective anterolateral fixation option for lower lumbar degenerative diseases, particularly at the L4–L5 segment.

4.2. Anatomical Feasibility and Surgical Safety of ALTTS

This study commenced with the design and planning of the ALTTS and its compatible novel internal fixation system. To enhance the stability of the anterolateral fixation system, multiple screws can be inserted into the same vertebral body, thereby improving overall anchorage and rotational resistance. Considering the limited surgical exposure of the OLIF approach and the constraints imposed by the paravertebral anatomical structures, the placement of a dual‐rod system at the anterolateral aspect of the vertebra is challenging. Therefore, this system adopts a “multiple screws with a single rod” configuration. Furthermore, as the lateral aspect of the L5 vertebral body is often obstructed by the iliac crest, placing superior and inferior screws in parallel at this level presents certain difficulties.

Studies by Zhu, Karami, Zhao, et al. have indicated that increasing the number of cortical layers penetrated by pedicle screws contributes to enhanced anchorage strength [25, 27, 28]. Brantley et al. examined the impact of pedicle screw diameter and length on anchoring strength under different bone quality conditions. In normal bone quality, when the cross‐sectional area of the pedicle screw exceeds 70% of the pedicle cross‐section, the deeper the screw penetrates the bone, the greater is the anchoring strength. When the actual penetration depth of the pedicle screw surpasses 80% of the theoretical depth, a larger screw diameter leads to stronger anchoring strength and stiffness. For patients with osteoporosis, longer screws can increase anchoring strength. Fang et al. explored the stability and safety of OLIF combined with the ASF internal fixation method using finite element analysis. Matsukawa et al. demonstrated that larger diameter and longer screws can reduce Von Mises stress around the screws and significantly improve the stability of the internal fixation system [51]. Consequently, the new screw placement scheme proposed in this study, which employs thicker and longer screws (screws with a diameter of 7.0–8.0 mm), exhibits better stability than the traditional ASF internal fixation scheme.

During the actual surgical procedure, it is essential to consider the influence of major prevertebral blood vessels on the screw placement trajectory. Numerous studies have indicated that over half of the abdominal aorta transforms into bilateral iliac arteries at the level of the L4/5 intervertebral space and courses anterior to the vertebral body. Consequently, the risk of injuring the major prevertebral artery during left‐sided approach surgeries at this particular spinal segment is relatively minimal [52, 53, 54, 55].

Meanwhile, this study found that vertebral bodies with PW > 10.21 mm were more suitable for the new screw placement scheme proposed in this study, with a prediction accuracy of 99.75%, sensitivity of 100%, and specificity of 95.8%. Su et al. described the size of the L4 and L5 pedicles in normal individuals. The average width of the L5 pedicle is 20.1 ± 3.7 mm, the average height is 15.2 ± 1.7 mm, and the average minimum pedicle width is 12.4 ± 1.8 mm. The width of the L4 pedicle is 13.6 ± 3.0 mm, the height is 14.9 ± 1.4 mm, and the minimum pedicle width is 10.9 ± 2.0 mm [56]. The study by Goyal et al. described that in patients with lumbar spondylolisthesis, the average width of the L5 pedicle was 15.51 ± 3.4 mm and the average height was 10.19 ± 1.4 mm [35]. The data in this study were similar to those measured in a previous study, both of which support the view that the L5 vertebra is more suitable for fixation using this method and provide evidence for it.

In a further analysis, the study conducted a correlation analysis between PW and the planned minimum screw insertion angle, as well as the range of available screw insertion angles. The results showed that, with an increment in PW, the planned minimum screw insertion angle would correspondingly diminish, and concurrently, the range of available screw insertion angles would increase. The research findings of Kot et al. pointed out that the arterial disc distance (ADD) gradually increases in the L1–L5 segments [54]. The research findings of Kot et al. pointed out that the arterial disc distance (ADD) gradually increases in the L1–L5 segments. This indicates that the pedicle width (PW) of the L5 vertebra is larger than that of other lumbar vertebrae, and the ADD is greater. Consequently, when inserting screws into the L5 vertebra, the traction force on the blood vessels is less than in other vertebral segments. This contributes to higher safety.

During the actual surgery, to minimize the risk of major prevertebral blood vessels being damaged by the internal fixation device, we placed absorbable gelatin sponges on the implant side adjacent to the blood vessels, thus reducing implant‐induced damage to these vessels to the greatest extent. To prevent adverse events due to operational errors, fluoroscopic monitoring of the L5 vertebra's anteroposterior and lateral views was performed during the operation. When the tip of the Kirschner wire reached the level of the posterior edge of the vertebral body without penetration and the distances between the needle tip and the four walls of the right pedicle were all greater than the radius of the selected screw, the path was deemed safe, and screw insertion could commence.

Preliminary results showed ideal short‐term outcomes in all 12 patients, with no recurrence of spondylolisthesis at the surgical site in the 6 affected cases and no serious abdominal vascular complications occurring within 6 months.

4.3. Biomechanical Performance of the ALTTS + BALS Fixation System

Subsequently, this study evaluated the effects of different fixation systems on OLIF by measuring three key biomechanical parameters (ROM, ESPFI, and ESPC). The stability of the surgical segment is a critical indicator of the success of lumbar interbody fusion, as instability may lead to complications such as cage subsidence (CS) and pseudarthrosis, resulting in pain and limited mobility [12, 21]. Lu et al. investigated the biomechanical performance of four LIF techniques (PLIF, TLIF, XLIF, and OLIF) and found that OLIF + BPSF reduced ROM in all directions by 86.9%–89.7% [57]. Chen et al. developed a finite element model of single‐segment crenel lateral interbody fusion (CLIF) and reported a 76.84%–97.97% decrease in ROM across motions compared to the intact model [58]. Oxland et al., in a literature review, noted that lumbar interbody fusion (LIF) can reduce ROM at the target segment by up to 90% [59]. In the present study, under normal bone conditions, the ROM of the surgical segment in the three OLIF models decreased by 79.41%–98.25% (all > 75%) compared to the intact model, which is consistent with previously reported trends [57, 58, 59].

In terms of intergroup comparisons under normal bone conditions, the OLIF + ASF group exhibited the highest surgical segment ROM, whereas the OLIF + BPSF group showed the lowest. The OLIF + ALTTS + BALS group provided better restriction of flexion and extension than the OLIF + ASF group and was comparable to the OLIF + BPSF group in all directions except extension. Under osteoporotic conditions, the OLIF + ALTTS + BALS group overall outperformed the OLIF + ASF group in motion restriction (particularly in flexion and extension). Compared to the OLIF + BPSF group, it was slightly less effective only in limiting extension but performed similarly in other directions.

The OLIF + ASF system uses unicortical screws with a two‐screw‐one‐rod anterolateral configuration, which provides weaker holding strength and less resistance to rotational stresses during flexion and extension, resulting in the poorest performance in restraining these motions. The OLIF + BPSF system employs a four‐screw‐two‐rod posterior fixation, with strong screw purchase and a longer lever arm due to its placement farther from the spinal axis of motion, offering optimal stress resistance. The OLIF + ALTTS + BALS system enhances screw fixation strength and anti‐rotational capacity through larger screw diameter, increased cortical penetration, oblique trajectory design, and multi‐screw configuration, performing better than OLIF + ASF.

Spinal fusion increases rigidity at the surgical segment by immobilizing the motion segment. To maintain overall spinal mobility, compensatory increases in motion occur at adjacent segments, consistent with the findings of this study. Notably, the compensatory increase in ROM was significantly greater at the L3–4 segment than at L5–S1. This aligns with the observation by Maragkos et al. [60] that the incidence of adjacent segment degeneration (ASD) is higher at the cranial than at the caudal segment after L4–5 fusion. This may be because the L5–S1 segment already operates near its physiological ROM limit preoperatively, leaving less compensatory potential, thus requiring the L3–4 segment—which initially has a smaller ROM—to assume a greater compensatory role.

The Equivalent Stress Peak Fixation Index (ESPFI) is a critical parameter for evaluating the risk of internal fixation failure. According to Chen et al. [61], the yield strength of titanium alloy (a common material in implants) ranges from 897 to 1034 MPa. In this study, the ESPFI values of all three fixation systems under both normal and osteoporotic bone conditions were well below this threshold, indicating favorable anti‐failure safety profiles for all constructs.

Under normal bone conditions, ESPFI values remained below 200 MPa in all directions except flexion (238.26 MPa) and extension (435.47 MPa) in the OLIF + ASF group. Among the groups, OLIF + ASF demonstrated the poorest stress distribution, with significantly higher ESPFI values in flexion and extension, as well as in left lateral bending and right rotation compared to the other two groups. The OLIF + ALTTS + BALS and OLIF + BPSF groups exhibited similar peak stress performance: the former showed slightly higher ESPFI in flexion, extension, and left lateral bending, but lower values in right lateral bending and bilateral rotation. The ESPFI range in the OLIF + ASF group (52.52–435.47 MPa) was notably higher than values reported in previous studies (e.g., 116.45 MPa in the ASF finite element model by Fang et al.; 39.6–145.8 MPa in the lateral plate of the CLIF model by Chen et al. [58]). This discrepancy may be attributed to the oblique screw trajectory used in our ASF model, which predisposes the screw tail to stress concentration during flexion–extension. In contrast, the multi‐screw configuration of the OLIF + ALTTS + BALS group effectively distributed stress across the construct (Figure 9), resulting in lower ESPFI values in all directions except left rotation compared to OLIF + ASF—particularly in extension, where ESPFI decreased by over 60%.

Under osteoporotic conditions, ESPFI values increased in almost all directions compared to normal bone, except for right rotation in both anterolateral fixation groups (OLIF + ALTTS + BALS and OLIF + ASF). This suggests that internal fixation stability is superior in normal bone compared to osteoporotic bone under identical fixation methods, consistent with the findings of Liu et al. [44]. The slight decrease in ESPFI during right rotation in the two anterolateral groups may be related to bone deformation and stress release around screws in osteoporotic bone. Among the groups, OLIF + ASF still exhibited the highest overall ESPFI under osteoporosis. The OLIF + BPSF group showed lower ESPFI in flexion, extension, and left lateral bending, whereas the OLIF + ALTTS + BALS group performed better in the remaining directions. Given the limited number of finite element studies on OLIF fixation under osteoporotic conditions, these findings warrant further validation.

Higher cage stress (ESPC) is associated with an increased risk of cage subsidence (CS) and adjacent segment degeneration over time [62]. Under normal bone conditions, this study found that the OLIF + ASF group had the highest CS risk (reflected by the highest ESPC values), whereas the OLIF + BPSF group had the lowest risk. The OLIF + ALTTS + BALS group showed significantly lower ESPC values than the OLIF + ASF group, indicating a clearly reduced CS risk compared to OLIF + ASF. Under osteoporotic conditions, cage stress increased in all three groups compared to normal bone, except during left lateral bending in the OLIF + ASF group. The OLIF + BPSF group still exhibited the best stress performance (lowest CS risk), whereas the OLIF + ALTTS + BALS group demonstrated lower ESPC values than OLIF + ASF in all directions except left lateral bending, confirming its superior CS resistance relative to OLIF + ASF—though still inferior to OLIF + BPSF. Both ROM and ESPC results indicate that reduced segmental stability correlates with increased cage stress and higher CS risk.

4.4. Clinical Outcomes and Surgical Recommendations

The following clinical recommendations are proposed based on the study results. For L4‐5 degenerative instability in patients with good bone quality, the OLIF + ALTTS + BALS technique is viable, offering comparable fixation strength to BPSF while minimizing iatrogenic injury to the back muscles by avoiding a posterior approach. In osteoporotic patients with lumbar instability, OLIF with BPSF should be prioritized. Alternatively, the OLIF + ALTTS + BALS procedure with bone cement augmentation represents a valid option to enhance fixation stability and reduce the risk of CS.

4.5. Limitations and Strengths

This study has several limitations. First, the clinical sample size (n = 12) was relatively small, and the follow‐up period was short. Although the anatomical study (n = 40) and finite element analysis provided strong theoretical support, the clinical data represents a preliminary pilot study. As ALTTS + BALS is a novel technique, inclusion of early surgical cases may introduce inherent bias in metrics such as operative time and clinical outcomes. We acknowledge that larger‐scale, multi‐center randomized controlled trials with long‐term follow‐up are required to definitively confirm the biomechanical advantages and clinical efficacy of this technique compared to standard fixation methods. Second, the osteoporotic finite element model was constructed from CT images of a young male volunteer rather than actual osteoporosis patients, resulting in morphological differences from clinical reality. Third, the biomechanical model did not incorporate muscle tissues, and ligaments were simplified as tension‐only nonlinear spring elements—a simplification that may affect the accuracy of motion patterns and stress distribution calculations. Fourth, due to software constraints, the complex biomechanical processes in vivo were simplified, and the model could not fully replicate the true stress state of the internal fixation system after lumbar fusion.

Despite these limitations, this study retains academic value: all surgeries were performed by a single surgeon, minimizing the impact of technical variation on outcome evaluation. Furthermore, the validity of the finite element model was verified, and the performance of different fixation devices was compared under consistent conditions. The findings provide valuable reference for clinicians in selecting OLIF‐based fixation strategies. Fourth, regarding material properties, the vertebral structures in this study were defined as isotropic and homogeneous elastic materials. In reality, the transverse process possesses a thin cortical shell that exhibits brittle and anisotropic mechanical behavior, particularly in osteoporotic bone. This simplification may mask stress concentrations at the screw‐bone interface and potentially overestimate the anchorage strength of the ALTTS within the transverse process. Consequently, the model might underestimate the risk of cortical breach or fixation failure in this critical region. Future studies employing micro‐finite element (micro‐FE) models with heterogeneous material assignment could provide a more accurate evaluation of the screw‐bone interaction for this novel technique. Fifth, regarding the failure modes of osteoporotic bone, the current model assumes a continuous material structure and does not account for microscopic structural defects. In osteoporotic vertebrae, the cortical bone—particularly the thin shell of the transverse process—is prone to micro‐fractures and cortical fissures. The FE simulation employed in this study lacks fracture mechanics criteria to model the initiation or propagation of these cracks. Consequently, the model may overestimate the screw‐bone interface strength and the overall stability of the ALTTS construct in osteoporotic conditions, as it does not reflect the risk of cortical shell disruption leading to screw loosening. Furthermore, while the finite element analysis simulated osteoporotic conditions to assess mechanical stability, the clinical cohort excluded patients with severe osteoporosis (T‐score < −3.5). This creates a logical discrepancy, as the biomechanical performance observed in the osteoporotic FE model may not be fully representative of the most severe cases excluded from the clinical arm.

4.6. Conclusion

The ALTTS technique is suitable primarily for the lower lumbar vertebrae. Specifically, when used for anterolateral fixation of the L5 vertebra, it demonstrates the highest screw placement success rate and safety. A pedicle width (PW) of > 10.21 mm can increase the screw placement success rate. Biomechanical studies revealed that, compared to OLIF with ASF, the OLIF with ALTTS + BALS technique provides significantly enhanced biomechanical performance. The novel fixation technique demonstrated satisfactory clinical outcomes over a six‐month follow‐up period.

Author Contributions

Jin Chu: project administration, supervision, data curation, validation, visualization, writing – original draft, funding acquisition. Wei Zhang: methodology, investigation, funding acquisition. Xin Guo: investigation, visualization, funding acquisition, project administration, formal analysis. Zhengwei Li: validation, visualization, investigation, funding acquisition, supervision, data curation. Mingdong Zhang: conceptualization, investigation, validation, visualization. Jiaqi Wang: investigation, funding acquisition, writing – original draft, visualization, formal analysis. Dapeng Feng: writing – original draft, funding acquisition, resources, formal analysis.

Funding

This study was supported by “1 + X” Research Project of the Second Hospital of Dalian Medical University (2022LCJSGC03).

Ethics Statement

This study was approved by the Ethical Committee of The Second Hospital of Dalian Medical University (Ethics Apporval Number: XJS2024‐110‐01).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.