Higher screw-to-cage length ratios in standalone lateral interbody fusion using integrated, expandable implants provide more predictable outcomes with less variability: A 2-year follow-up study

Abstract

Background Context

Lateral Lumbar Interbody Fusion (LLIF) is widely used for degenerative spinal disorders. Standalone LLIF with expandable cages integrating plate-screw fixation (eLLIFp) has emerged to address disc degeneration and adjacent segment disease while reducing the need for posterior fixation. Biomechanical data suggest greater stability with longer screws, but the clinical impact of screw-to-cage length ratios remains unclear. This study evaluates outcomes following eLLIFp and investigates whether screw-to-cage ratios influence results.

Methods

Eighty-one patients (87 levels) underwent eLLIFp, mean age 63.8±10.8 years (49.4% female), BMI 27.7±4.9 kg/m². Common levels treated were L2/3 (35%) and L3/4 (29%). Mean retractor time was 30.2±5.5 min. Retrospective analysis of prospectively collected data examined integrated screw length (30–60 mm) and cage length (50–65 mm). Screw-to-cage ratios were grouped into tertiles: Low (0.46–0.75, n=36), Medium (0.76–0.82, n=18), and High (0.83–1.0, n=27). Outcomes included pain (VAS), disability (ODI), quality of life (SF-12), and fusion status on 12-month CT. Minimum follow-up was 2 years.

Results

At 12 months, all patients showed significant improvement (p<0.0001) [VAS back: 7.6→0.9; VAS leg: 6.3→0.7; ODI: 25.7→5.1; SF-12 PCS: 29.9→49.2; MCS: 37.3→55.6]. Improvements were comparable across tertiles, though the high-ratio group exhibited less variability in leg pain outcomes. Fusion was achieved in 79/81 (97.5%) patients. Complications occurred in 5 (6.2%): 3 neurological (motor radiculopathy, radicular pain, transient thigh numbness) and 2 symptomatic nonunions requiring posterior fixation, both with lower screw-to-cage ratios (0.7).

Conclusions

Higher screw-to-cage ratios (0.83–1.0) in standalone eLLIFp constructs may enhance consistency in leg pain relief without increasing complications. Maximizing screw length relative to cage dimensions may optimize outcomes and reduce variability in standalone LLIF.

Introduction

Lateral Lumbar Interbody Fusion [LLIF] is a well-established surgical technique for treating degenerative spinal conditions, offering minimally invasive access to the intervertebral disc space [1,2]. Introduced in 2006, LLIF has since become a standard-of-care approach for managing a wide range of thoracolumbar pathologies, including degenerative disc disease, spinal stenosis [3], deformity correction [4], adjacent segment disease [5], traumatic injuries [6], and oncologic conditions [7].

The initial use of static intervertebral spacers in LLIF procedures has been associated with a tendency to subsidence into the vertebral bodies due to repeated trialing risking potential iatrogenic endplate injury [8]. Subsidence compromises the efficacy of indirect decompression of neural structures and limits spinal realignment through lordosis restoration [3,9]. Standalone lateral cages risked nonunion [10,11]. To address these limitations, modern interbody implants have evolved, incorporating additive manufacturing techniques to enhance biomechanical properties and reduce subsidence rates [12].

More recently, expandable interbody cages have been introduced in LLIF [13]. These implants offer multiple advantages, including low implantation heights that preserve endplates, optimized disc height restoration, and improved sagittal alignment through controlled expansion mechanisms [9,14]. Lateral cages with separate lateral plates risked vertebral fracture and lumbar plexus injury [15]. Modular plate fixation to an in-situ lateral cage have been reported [16]. The integration of plate-screw fixation into these expandable cages (eLLIFp) has enabled their use as standalone constructs, eliminating the need for supplemental posterior fixation. This innovation has reduced surgical morbidity while maintaining adequate stability for fusion [17].

Despite the biomechanical advantages of these eLLIFp constructs, clinical evidence remains limited. A recent cadaveric and finite element analysis (FEA) examining the biomechanical impact of different integrated screw lengths compared to the overall length of the expandable lateral interbody fusion device suggested that a higher ratio of integrated screw length to expandable intervertebral body cage length leads to improved biomechanical performance, evidenced by lower resultant range of motion compared to when shorter screws are used relative to the cage length [18]. However, the clinical implications of this relationship have not been explored.

Aim

The purpose of the current study was to evaluate the clinical performance measures of patients treated with standalone LLIF using a titanium expandable intervertebral cage with integrated plate-screws [eLLIFp] and to assess whether the ratio between integrated screw length and cage length was associated with patient outcomes.

Methods

An analysis of 81 consecutive patients treated at a single institution in Australia with eLLIFp at 1-2 contiguous lumbar level(s) (ELSA®, Globus Medical, Inc.) was undertaken (Fig. 1). Data were compiled retrospectively from a prospectively collected registry with minimum follow up of 24 months postoperative. Patients were only included if the full follow up protocol was meet. Institutional ethics committee approval was obtained (EH2023-1033).

Fig. 1.

Titanium expandable LLIF interbody spacer with integrated plate-screws device (eLLIFp) (A) photograph; and (B) lateral intra-operative radiograph.

Patient and treatment characteristics

Baseline patient information included demographics, medical comorbidities, indication for surgery and levels treated. All patients underwent multimodal intraoperative neural monitoring (IONM) [19].

Patient-reported outcomes measures (PROMs) were back and leg pain (visual analogue scale (VAS), disability (Oswestry Disability Index (ODI) and quality of life (SF-12 version 1 physical and mental component scores (PCS and MCS)). Each of these PROMs were collected preoperatively and post operatively at 2 days, 6 weeks, 3 months, 6 months, 12 months and 24 months.

Interbody cages and graft material

All patients received an eLLIFp device with fixation into the supra- and infra-adjacent vertebral bodies. Cage lordosis was a fixed 6° lordosis at L1-L2 and L2-L3 and an adjustable 5° to 20° lordosis at L3-L4 and L4-L5. Cage dimensions were 20mm wide and lengths used were 50-65mm (in 5mm increments) and integrated screw lengths used were 30—60mm (in 5mm increments). The cost of cages varying in lordosis and length, and screws of differing lengths were the same. Cages were filled then backfilled with allograft (Australian Biotechnologies, Sydney, NSW, Australia).

Screw-cage ratio analysis

Patients were categorized into 3 approximately equally sized tertiles based on screw-implant ratios: Low (0.46–0.75, n=36), Medium (0.76-0.82, n=18), and High (0.83–1.0, n=27). Association between screw implant ratios and outcome measures were analyzed as change scores (12-month scores minus baseline), with negative changes in VAS and ODI indicating improvement, and positive changes in SF-12 components indicating improvement.

Interbody fusion and subsidence

CT scans (Somatom Definition Flash; Siemens AG, Erlangen, Germany) were performed at 6 and 12 months until solid interbody fusion was confirmed on coronal and sagittal views. Fusion was defined as bridging interbody trabecular bone on coronal and sagittal views [20]. Subsidence was measured radiographically from the vertebral endplate to the caudal or cranial margin of each cage [3]. Independent radiologists reported the CT results.

Statistical methods

Statistical analysis was carried out using Stata version 18 (Stata Corporation, College Station, Texas, USA 2023) and included frequency statistics of all variables, paired t-tests of preoperative to 12 month postoperative time points. To facilitate interpretation, screw-cage ratio tertiles were compared across outcome change scores, using 1-way analysis of variance, testing the assumption of homogeneity or equality of variance using the Levene test [21] based upon the means. If the latter was found to be statistically significant, the robust Brown-Forsythe test [22] was employed. Regression analyses examining the relationship between actual (ie, uncategorized) screw-cage ratio and outcome changes scores were also performed. It should be observed that the nonparametric analog of the 1way ANOVA, the Kruskal-Wallis test [23], also assumes equal of variance across groups [24]. Statistical significance was set at p = 0.05, 2-tailed. 95% confidence intervals were reported wherever appropriate.

Results

Patient characteristics

A total of 81 consecutive patients treated with eLLIFp with at least 12 months postoperative follow up were included. All 81 patients completed the full follow-up protocol and PROMs questionnaires at all designated time points.

Mean age was 63.8 years, standard deviation (SD) 10.8, range 34–82 (49.4% female) and mean body mass index (BMI) was 27.7 kg/m², SD 4.9, range 19-41. Most common levels treated were L2/3 (43%) then L3/4 (35%)(Table 1). Mean retractor time for eLLIFp placement was 30.2 minutes (SD 5.5, range 15–40 minutes).

Table 1.

Summary of baseline patient demographics, diagnostic distribution, and operative details for the cohort (N=81 patients, 87 levels).

Characteristic	Statistic/n (%)
Demographics
Age (years)	Mean 63.8±10.8 (range 34–82)
Female	40 (49.4%)
Body mass index (kg/m²)	Mean 27.7±4.9 (range 19–41)
Tobacco use (current or former)	11 (13.9%)
Prior lumbar fusion	12 (17.9%)
Diagnoses
Degenerative disc disease	36 (54%)
Lumbar spinal stenosis	37 (55%)
Spondylolisthesis	15 (22%)
Adjacent segment disease	8 (12%)
Treatment characteristics
Total levels treated	87
Single-level procedures	75 (93%)
Two-level procedures	6 (7%)
Levels treated
― L1/2	9 (11%)
― L2/3	31 (35%)
― L3/4	24 (29%)
― L4/5	11 (13%)
Mean retractor time (minutes)	30.2±5.5 (range 15–40)
Implant details
Cage length (mm)
― 50	18 (21%)
― 55	23 (28%)
― 60	33 (40%)
― ≥65	9 (11%)
Integrated screw length (mm)
― 35–40	14 (17%)
― 42.5–47.5	34 (42%)
― 50–60	33 (41%)
Screw-to-cage ratio tertiles
― Low (0.46–0.75)	36
― Medium (0.76–0.82)	18
― High (0.83–1.0)	27

Current or former tobacco use was evidenced in eleven (13.9%) patients while 12 (18%) of patients have prior lumbar spine fusions. Diagnoses (cumulative) included stenosis in 37 (55%), degenerative disc disease in 36 (54%), spondylolisthesis in 15 (22%) and adjacent segment disease in 8 (12%) patients (Table 1).

Treatment characteristics

A total of 87 spinal levels were treated across the 81 patients, with 75 undergoing single-level eLLIFp and 6 undergoing 2-level procedures. The most treated levels were L3-4 [29%] and L2-3 [35%] (Table 2). Mean total psoas retractor time for placement of an eLLIFp was 30.2 minutes per patient (SD 5.1 minutes, range 15–40 minutes) (Table 1). Ratio of Cage-to-screw length was deduced after surgery with n=44 having a high ratio (1–0.8), n=31 medium (0.79–0.6) and n=12 low (<0.59).

Table 2.

Radiographic outcomes, postoperative complications, and a summary of overall improvement in patient-reported outcome measures (PROMs) from baseline to 12 months.

Parameter	Statistic/n (%)
Radiographic outcomes
Fusion achieved on CT at 12 months	79 (97.5%)
Cage subsidence	None observed
Postoperative complications (Total)	5 (6.1%)
Symptomatic nonunion (requiring revision)	2 (2.5%)
Persistent motor radiculopathy (quadriceps weakness)	1 (1.2%)
Radicular pain (resolved)	1 (1.2%)
Transient anterior thigh numbness	1 (1.2%)
Patient-reported outcomes (PROMs)	Preoperative (Mean±SD)	12-months (Mean±SD)	Change (Mean)	p-value
VAS back pain	7.6±1.5	0.9±1.0	-6.7	<.001
VAS leg pain	6.3±2.8	0.7±1.2	-5.6	<.001
Oswestry disability index (ODI)	25.7±6.2	5.1±3.5	-20.6	<.001
SF-12 physical component (PCS)	29.9±6.2	49.2±7.7	+19.4	<.001
SF-12 mental component (MCS)	37.3±6.5	55.6±6.5	+18.3	<.001

Radiographic outcomes

On serial CT assessment, 79 (97.5%) patients achieved fusion by 12 months. No cases of measurable eLLIFp cage subsidence.

Complications

Postoperative complications occurred in 5 patients (6.1%). One patient suffered a persistent motor radiculopathy with quadriceps weakness at 12 months, 1 patient had radicular pain for 8 months and 1 patient had transient anterior thigh numbness that resolved after 2 weeks. Two patients (67 years at L3/4 and 69 years at L2/3, both female, nonsmokers, normal bone density) early in our series with lower screw-to-implant ratios (0.7) (Fig. 2) suffered symptomatic nonunions needing supplemental posterior fixation (8 and 10 months postoperatively) (Table 2).

Fig. 2.

Titanium expandable LLIF interbody spacer with integrated plate-screws device (eLLIFp) (A) AP radiograph with low screw to cage ratio of 0.7; and (B) AP radiograph with high screw to cage ratio of 0.9.

Patient reported outcome measures

Although the Medium tertile exhibited the greatest mean improvement in VAS back pain, differences among the 3 tertiles narrowly failed to reach statistical significance (F(2,77)=2.54, p=.086). Levene’s test based upon deviations from group means, did not find the group (tertile) variances to be significantly different from each other (p=.415)

All tertiles demonstrated similar mean improvements in VAS leg pain scores, with no statistically significant differences between them (F(2,77)=0.44, p=.646). However, the High tertile exhibited significantly less variability in outcomes compared to the Low and Medium tertiles, as confirmed by Levene’s tests for equality of variances based on variations from means (p=.003) and from the Brown-Forsythetest based upon 10% trimmed group means (ie the largest and smallest 10% of observations excluded) (p=.019) although not the Brown-Forsythe test based upon medians (or 50% trimmed means) (p=.122) . Notably, all patients in the High tertile demonstrated some degree of improvement, with no cases showing worsened symptoms at 12 months, unlike the Low and Medium tertiles where some patients experienced no improvement or worsening of symptoms (Fig. 3).

Fig. 3.

Variability in leg pain improvement by screw-to-cage ratio. The change in VAS leg pain scores from baseline to 12 months for each screw-to-cage ratio tertile.

As illustrated in Fig. 4, patients within the High screw-to-cage ratio tertile (0.83–1.0) demonstrated a visibly narrower distribution in VAS leg pain improvement at 12 months compared to the Low and Medium tertiles. While mean improvements were similar across all groups, outcomes in the High tertile were more consistent, with all patients showing postoperative leg pain reduction. This contrasts with the greater variability observed in the Low and Medium tertiles, where several patients experienced limited or no improvement. The graphical trend mirrors the statistical findings of Levene’s and Brown–Forsythe tests, which confirmed significantly lower variance in leg pain improvement in the High tertile (p=.003 and p=.019, respectively), suggesting greater predictability of outcomes when longer integrated screws are utilized.

Mean ODI improvements were similar across all tertiles. The Medium tertile showed marginally better improvements compared to the High and Low tertiles, however there was no statistically significant overall difference in means (F(2,77)=2.02, p=.140). Levene’s test suggested comparable variability in outcomes across all tertiles (p=.152).

The High tertile demonstrated marginally better improvements in physical health scores compared to the Medium and Low tertiles, although these differences were not statistically significant (F(2,77)=1.22, p=.302). Standard deviations were comparable across tertiles, Levene’s test p=.318.

All tertiles showed similar improvements in mental health scores, with the High tertile demonstrating marginally better outcomes, though differences were not statistically significant (F(2,77)=0.92, p=.403). Outcome variability was comparable across tertiles (Levene’s test p=.251).

Discussion

The present study specifically aimed to determine whether the ratio of screw-to-cage length influences variability and predictability of postoperative outcomes following standalone eLLIFp procedures. While lateral interbody fusion is a well-established technique for treating lumbar spinal disease [25], there is a paucity of literature on novel modern interbody technologies and indications for use in the evolution of these procedures. There is a need to evaluate the more recently adopted standalone (interfixated intervertebral implants certified for use without supplemental internal fixation) options for lateral interbody fusion.

Standalone devices are commonly used in the cervical spine for anterior cervical discectomy and fusion (ACDF) and in the lumbar spine for anterior lumbar interbody fusion (ALIF). Interfixated lateral intervertebral fusion devices have been available for over a decade. However, such devices have only been available in expandable designs and for on-label use without supplemental fixation recently.

Early results of the use of static (nonexpandable) intervertebral cages resulted in relatively high rates of intervertebral subsidence, suggested to be due to iatrogenic endplate injury during repeated trialling and cage impaction, particularly for implants with more narrow (anteroposterior dimension) implants. Le, et al. [8], reported in 2012 on 140 patients treated with lateral interbody fusion utilizing 18 mm or 22 mm wide static, polyetheretherketone (PEEK) intervertebral fusion devices. In this study, the authors found an overall 9% subsidence rate, with a 14% rate of vertebral body subsidence in 18 mm vs. a 2% rate of subsidence in 22 mm wide cages. Additionally, and relevant to the findings of the current study, patients treated with separate lateral plating (for supplemental internal fixation) had higher rates of subsidence compared to those treated with pedicle screw and rod fixation. A study by Frisch, et al. [13] in 2018 reviewed 56 patients treated with lateral interbody fusion and bilateral pedicle screw and rod fixation, 29 with static and 27 with more modern, expandable intervertebral spacers. The expandable spacers were proposed to reduce iatrogenic endplate injury being impacted with low intervertebral height and expanded to desired disc height intraoperatively. In this study, the authors found 16% of the static cages resulted in subsidence, similar to the findings of Le, et al. [8], while none of the expandable spacers had resultant vertebral body subsidence. Huo et al. [9] in 2023 in a prospective evaluation of 98 patients with 169 lateral cages impacted (84 expandable vs 85 static) reported a significant reduced subsidence rates (4% vs 20%) in the expandable cage group at 12 months postoperative. However, a recent systematic review and meta-analysis found no differences in subsidence rates between static and expandable lateral interbody cages [26]. In our cohort, we observed a 97.5% fusion rate with no subsidence the prior findings from Frisch, et al. [13] and Huo et al. [9] where intervertebral subsidence was significantly reduced with the use of 20mm wide expandable cages, by using integrated lateral screw fixation in standalone constructs rather than posterior pedicle screw and rod fixation. As such, data from the current study further validate interfixated expandable cages in reducing vertebral body subsidence even without the use of posterior fixation.

Additional to endplate preservation and avoidance of subsidence, the optimized utilization of lateral expandable standalone interfixated cages has been suggested in biomechanical studies to be related to the use of screw lengths for interfixation near the length of the intervertebral cage [18]. This device optimization theoretically allows for bicortical purchase of the screws (Fig. 2) to maximize the rigidity of the standalone construct compared to when relatively shorter screws are placed. In a study by these authors, Malham, et al., conducted a cadaveric study to assess whether standalone interfixated cages could provide similar mechanical stability compared to a noninterfixated intervertebral device with supplemental posterior screw and rod fixation. In this study, the authors found an inverse relationship between interfixated screw length and segmental range of motion (ROM), with a minimum screw to intervertebral spacer length ratio of 0.9 to achieve comparable ROM when compared to supplemental posterior fixation as a ratio of 0.75 [18]. The purpose of the current study was to test the validity of these biomechanical findings in a prospectively collected cohort of patients treated with a standalone lateral interfixated expandable implant.

Our results demonstrate that while patients across all screw-implant ratio tertiles (Low, Medium, and High) experienced significant improvements in pain, disability, and quality of life measures compared to baseline, the High tertile exhibited notably less variability in VAS leg pain outcomes. Our finding that higher screw-cage ratios were associated with more consistent leg pain improvement aligns with prior biomechanical data from Frisch et al. [13] and Huo et al. [9], who demonstrated that expandable constructs reduce segmental motion and subsidence. This finding is particularly significant, as all patients in the High tertile demonstrated at least some improvement in leg pain at 12 months postoperative, with no cases of worsened symptoms. In contrast, the Low and Medium tertiles contained patients who experienced either no improvement or worsening of leg pain symptoms.

These clinical observations align with the biomechanical findings reported by Malham et al. [18], who demonstrated an inverse relationship between interfixated screw length and segmental range of motion (ROM). Their cadaveric study suggested that a minimum screw to intervertebral cage length ratio of 0.9 was required to achieve comparable ROM to supplemental posterior fixation. Our clinical data supports this theory, as patients with higher screw-implant ratios (0.83-1.0) demonstrated more consistent clinical improvements, particularly in leg pain outcomes.

Fig. 3 further support the hypothesis that higher screw-to-cage ratios contribute to more predictable postoperative outcomes. Although overall mean improvements in leg pain were comparable between tertiles, the reduced variability observed in the high tertile group reinforces the biomechanical advantage of longer screw fixation within the vertebral bodies. This may reflect enhanced construct rigidity and reduced micromotion at the bone–implant interface, leading to more uniform neural recovery. These observations are consistent with prior biomechanical evidence [18], who demonstrated an inverse relationship between screw-to-cage ratio and segmental range of motion. Collectively, these results suggest that maximizing screw length relative to cage dimensions may improve the consistency of patient outcomes without increasing procedural complexity or implant cost.

While mean improvements in VAS back pain, ODI, and SF-12 components were comparable across tertiles without reaching statistical significance, the trend consistently favored the Medium and High tertiles. The consistency of outcomes in the High tertile suggests that maximizing screw-cage ratios may optimize the biomechanical environment for neural recovery and pain relief, particularly for radicular symptoms.

The use of expandable cages with allograft in our study resulted in a fusion rate of 97.5% on CT at 12 months postoperative, with minimal subsidence observed across all tertiles. This finding corroborates previous studies by Frisch et al. [13] and Huo et al. [9], which demonstrated significantly reduced subsidence rates with expandable cages compared to static designs. Additionally, these findings were similar to an initial study of ELLIFp for ASD which found no early or delayed cage subsidence and a 94% fusion rate by CT at 12 months [17]. These data corroborate the biomechanical data reported previously by Malham, et al. [18] and provide incremental evidence on clinical decision making to optimize clinical outcomes. The high fusion rates and low subsidence observed in our cohort were likely attributable to multiple factors: (1) the use of expandable technology, which minimizes repetitive endplate damage during insertion; (2) optimal screw placement and length, particularly in the High tertile group; and (3) the biomechanical advantages of bicortical purchase when screw length approaches or exceeds 80% of cage length. These findings align with our initial study of eLLIFp for adjacent segment disease, which reported no cage subsidence and a 94% fusion rate at 12 months [17].

Complications

The total complication rate in our study was 6.1%, similar to initial studies of eLLIFp for ASD reporting a total complication rate of 9% [17]. Our psoas retraction times for eLLIFp (mean 30 minutes) are longer than for impaction of lateral static cages [27], but we found low rates of anterior thigh pain and only 1 patient suffered a persistent motor radiculopathy at 12 months follow-up. Potential contributing factors that may account for our low neurological complication profile include: (1) the majority of our eLLIFp cases were at L2/3 and L3/4 levels, with only 13% at the higher risk L4/5 level [28]; (2) using multimodal IONM to detect lumbar plexus nerve compromise earlier rather than electromyography alone [17]; and (3) the sequential opening of the tubular retractor for discectomy, trials, then eLLIFp impaction and fixation.

The reduced variability in outcomes observed in the High tertile group suggests that optimizing screw-to-cage ratio may contribute to more predictable surgical outcomes, potentially by enhancing construct stability and reducing micromotion that could lead to persistent leg pain or delayed fusion.

Clinical implications

Our findings have several clinical implications for surgeons utilizing standalone integrated lateral cages. First, they corroborate the biomechanical principle that maximizing screw length relative to cage dimensions can optimize clinical outcomes, particularly for leg pain. This suggests that surgeons could consider, when anatomically feasible, to utilize longer screws approaching or exceeding 80% of the cage length at no increased implant cost.

Second, the consistency of outcomes in the High tertile group supports the use of standalone integrated constructs with appropriate screw-cage ratios as a viable alternative to supplemental posterior instrumentation in selected patients. This approach reduces procedural morbidity while maintaining excellent clinical outcomes.

Finally, the significantly lower variability in leg pain outcomes in the High tertile group provides valuable prognostic information for patient counselling. Patients receiving constructs with higher screw-cage ratios may experience more predictable improvements in radicular symptoms compared to those with lower ratios.

Limitations

This study’s retrospective design and single-center setting limit generalizability. The relatively small sample size further emphasizes the need for larger multicenter trials to validate these findings. These weaknesses are offset using prospectively collected consecutive data, the limited scope of the research, specifically designed to assess the potential corroboration of these clinical results with prior biomechanical data. Furthermore, while our study had a high questionnaire completion rate, the retrospective design and single-center setting limit generalizability.

Future directions

Future research should explore the long-term durability of these outcomes beyond the 24-month follow-up period. Larger, multicenter studies with stratification by screw-cage ratios would provide more robust evidence regarding the optimal ratio for clinical outcomes. Additionally, investigation into the integration of adjunctive technologies, such as navigation and robotics, may further optimize screw placement and length selection to maximize the screw-cage ratio while minimizing risks.

Conclusion

Our results support previous biomechanical studies suggesting that higher screw-cage length ratios in standalone lateral interfixated expandable constructs optimize clinical outcomes, particularly regarding the consistency of leg pain improvement. Patients with high, medium and low ratios demonstrated significant improvements compared to baseline, those with high screw-cage ratios of 0.83-1.0 exhibited more predictable outcomes with less variability. These findings suggest that surgeons could consider maximizing screw length relative to cage dimensions when utilizing standalone interfixated devices for lumbar interbody fusion at no increased cost.

Funding

$8,000 was received from Globus Medical for the purpose of this research.

Declaration of competing interest

GMM is a consultant for Australian Biotechnologies, Globus Medical and Medtronic. The remaining authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.