Abstract
Objective
To perform a repeatable idealized in vitro model to evaluate the effects of key design features and integrated screw fixation on unloaded surface engagement, subsidence, and torsional stability.
Methods
We evaluated four different stand‐alone anterior lumbar interbody fusion (ALIF) cages with two, three, and four screw designs. Polyurethane (saw‐bone) foam blocks were used to simulate the vertebral bone. Fuji Film was used to measure the contact footprint, average pressure, and load generated by fixating the cages with screws. Subsidence was tested by axially loading the constructs at 10 N/s to 400 N and torsional load was applied +/−1 Nm for 10 cycles to assess stability. Outcome measures included total subsidence and maximal torsional angle range.
Results
Cages 1, 2, and 4 were symmetrical and produced similar results in terms of contact footprint, average pressure, and load. The addition of integrated screws into the cage‐bone block construct demonstrated a clear trend towards decreased subsidence. Cage 2 with surface titanium angled ridges and a keel produced the greatest subsidence with and without screws, significantly more than all other cages (P < 0.05). Angular rotation was not significantly affected by the addition of screws (P < 0.066). A statistically significant correlation existed between subsidence and reduced angular rotation across all cage constructs (P = 0.018).
Conclusion
Each stand‐alone cage featured unique surface characteristics, which resulted in differing cage–foam interface engagement, influencing the subsidence and torsional angle. Increased subsidence significantly reduced the torsional angle across all cage constructs.
Keywords: ALIF, Fusion, Integrated, Stability, Stand‐alone
Introduction
The maintenance of rigidity and stability during 3–6 months postoperatively is fundamental in obtaining successful fusion1. Anterior lumbar interbody fusion (ALIF) cage stability relies upon tension on the remaining annulus fibrosis to produce compressive forces2. Non‐fixated implants or grafts are susceptible to failure via collapse, spinal canal impaction, extrusion, pseudoarthrosis, and inadequate immobilization of the intervertebral joints (the primary mechanical cause of non‐fusion)2, 3, thus stimulating the development of supplementary fixation techniques.
Supplementary fixation may simply consist of a single screw in front of an implant migration, although more sophisticated designs exist, including those involving pedicle screw fixation (PSF) reducing inter‐segmental motion, low‐profile anterior plates, trans‐laminar screw systems, and stand‐alone integrated screw cages1, 2. Conversely, additional fixation is associated with increased morbidity, as it involves more extensive surgery4. The introduction of extended surgery and potential screw displacement are associated with significant neurological, vascular and paravertebral musculature complications2. Posterior instrumentation (PI) with rods and screws is the gold standard in facilitating rigidity and is associated with improved fusion rates, although not translating to improvement in clinical efficacy. This may be due to the introduction of a secondary posterior procedure1, 5. Hence the question of a quantifiable yardstick of stability that will facilitate successful fusion arises. The development of a stand‐alone cage with integrated screw fixation has recently gained popularity. It aims to improve segmental stability and reduce construct motion or migration, resulting in increased rates of fusion and potentially better clinical outcomes2, 6.
The standardized stand‐alone cages utilized in clinical practice today are comprised of a polyetheretherketone (PEEK) body with two, three, or four integrated screws threading into the adjacent superior and inferior vertebral bodies7. Their development has stimulated a variety of biomechanical studies comparing their stability to conventional supplementary fixation substantiating a provision of “sufficient” stability, thus the progression to the next phase of clinical trials5, 7, 8, 9. Clinical trials evaluating the efficacy of stand‐alone cages with integrated fixation and supplementary fixation methods have demonstrated comparable postoperative outcomes, yet significant reduction in surgical time and earlier postoperative improvement10, 11. Subsidence of the vertebral endplate due to implant penetration is a frequent clinical occurrence due to the transition in the intervertebral space from a large surface area, high compliance disc to a rigid implant with comparatively small surface area, significantly altering the distribution of stress12.
We have designed an idealized biomechanical testing model and applied it to four currently available stand‐alone cages in an effort to identify the effects of key design features of integrated screw fixation on unloaded surface engagement, subsidence and torsional stability.
Method
We evaluated four different stand‐alone ALIF cages with two, three, and four integrated screw designs (Table 1). The cages had unique surface features, which were identical superiorly and inferiorly (Fig. 1). Polyurethane (Last‐A‐Foam [General Plastics Tacoma, Washington, US], Fr‐3700) blocks (4.5‐cm width, 4.5‐cm length, and 3.5‐cm depth) were used to simulate cranial and cuadal vertebrae. Two new uniform saw bone blocks were used per cage per test (n = 6 tests), minimizing confounding variables that would be introduced with re‐use.
Table 1.
ALIF cage reference numbers, names, manufacturing, and descriptions
| No | Name and manufacturing | Cage description |
|---|---|---|
| 1 | Synfix‐ LR (Synthes, West Chester, PA) | A PEEK cage with an integrated anterior Ti stabilization plate, which incorporates the four angle‐fixated divergent locking screws and three central apertures. The surface of the cage is slightly roughened and covered with rows of pyramidal teeth or spikes (1.5‐mm height) with beveled rims |
| 2 | Redmond Lumbar cage (A‐spine, Taipei City, Taiwan) | A titanium (Ti) and polyetherketone (PEEK) composite design, consisting of a Ti end‐plate inlay, structurally fixated in a PEEK body and two anterior non‐angle fixated locking screws. The Ti endplate surface features include 18 angulate ridges (0.6‐mm height) spanning the horizontal width of the implant and a central beveled keel (2.3‐cm long, 1.5‐mm height, and 0.25‐mm width) dividing the central aperture symmetrically |
| 3 | Midline STALIF (Centinel Spine, West Chester, PA) | A PEEK semicircular cage with three centrally converging non‐angle fixated screws, with a unique screw head design producing a “lag effect.” The observable surface features are the three continuous angulated ridges (0.9‐mm height) and a single central aperture |
| 4 | PILLAR SA PEEK spacer system (Orthofix, Lewisville, TX) | An ovoid shaped cage with four medially‐orientated non‐angle fixated converging screws. There are 16 continuous ridges (0.4‐mm height) spanning across the entire surface and a single central ovoid aperture |
Figure 1.
(A) Cage 1 (Synfix ‐ Synthes Spine, West Chester, Pennsylvania, US) is featured by rows of pyramidal teeth or spikes with beveled rims; (B) Cage 2 (REDMOND lumbar cage ‐ A‐Spine, Taiwan) is featured by 18 angulate ridges and a central beveled keel; (C) Cage 3 (Midline STALIF ‐ Centinel Spine, West Chester, Pennsylvania, US) is featured by three continuous angulated ridges and a single central aperture; (D) Cage 4 (PILLAR SA PEEK spacer system – Orthofix, Lewisville, Texas, USA) is featured by 16 continuous ridges and a single central ovoid aperture; (E–H) lateral view of the cages.
Surface Pressure Generation: Fuji Film
Medium pressure Fuji Film (Fujifilm, Tokyo, Japan) was used to measure the contact footprint, average pressure and load generated by fixating the cages with screws13. Preliminary testing of cages on the spectrum of seven different pressure ranges indicated that the film (9.8–49 MPa) was ideally suited. Film was cut into 4.5‐cm square sheets. Holes were placed to allow the screws to penetrate.
The film was placed in between the cage and bone block and the appropriate screws were inserted through the cages and tightened. The construct was disassembled after 2 min. The color developed Fuji Film layer was scanned and the density of color change was translated to a numerical pressure reading using the FPD‐8010E software (Fuji Film, Tokyo, Japan) generating pressed area/contact footprint (mm2), average pressure (MPa) and load for each sample. A sample size of six Fuji Film sheets per cage was used.
Subsidence and Axial Stability
Vertebral body (VB)/Cage/VB constructs were assembled with and without screws for assessment of subsidence and axial stability on a calibrated 858 Mini Bionix (MTS Systems, Eden Prairie, Minnesota, US) servo hydraulic testing machine. A total of eight groups were tested (n = 6 tests per group). The system was calibrated for compliance using an intact polyurethane foam block, allowing for correction of data to account for only motion between the cage and the foam and eliminating PU foam deformation.
Each construct was firmly locked into vice grips, which were fixed on one side to the load cell, and on the superior side to an x‐y bearing and the MTS actuator (Fig. 2). This setup corrected for any off‐center mounting. For subsidence testing, the constructs were axially loaded at 10 N/s to 400 N. Torsional load or torque was applied +/−1 Nm for 10 cycles to assess stability. Displacement, load, torque and angle were recorded at 100 Hz throughout. Outcome measures included total subsidence and maximal torsional angle range.
Figure 2.
858 Mini Bionix MTS System and mechanical construct designed to test the saw bone blocks under 400 N load and angular rotation.
Statistic Analysis
A multivariate analysis with post‐hoc tests (P < 0.05) was used to investigate the effects of cages and screws on mechanical outcomes (SPSS IBM22, Armonk, NY, US), with correlations examined using Pearson's test. Pressure film data was assessed similarly with respect to cage design and the cage footprint within the block was qualitatively assessed following the completion of testing.
Results
Load Generation (Fuji Film)
Cages 1, 2, and 4 were symmetrical and, thus, produced similar results in terms of contact footprint, average pressure and load (Fig. 3, Table 1). Cage 3 was non‐symmetrical; the contact footprint of the superior side was significantly higher (P < 0.001) than the inferior surface and all other cage surfaces. As such, the results from each surface were analyzed independently. This translated to the generation of a significantly larger load in comparison to the inferior surface of cage 3 (P < 0.001) and all other cage surfaces (P < 0.001). In terms of average pressure, cage 4 differed significantly from cage 2 (P = 0.010); however, no differences were detected between all other cages (P > 0.05, Table 2).
Figure 3.
Row 1 (A–D) is the unloaded pressure film footprint generated by fixating the cages with screws into saw bone blocks. Row 2 (E–H) is the 400 N loaded saw bone cage footprint generated post‐rotational testing.
Table 2.
Interfacial pressures generated by the cages post‐fixation without loading
| Cage | Contact footprint (mm2) | Average pressure (MPa) | Maximum surface load | |||
|---|---|---|---|---|---|---|
| Inferior | Superior | Inferior | Superior | Inferior | Superior | |
| 1 | 105 (23.2) | 103 (18.9) | 13 (0.50) | 12 (0.48) | 1331 (288) | 1263 (200) |
| 2 | 98 (14) | 85 (19) | 12 (0.79)+ | 12 (0.39)~ | 1174 (157) | 1015 (225) |
| 3 | 86 (33) | 149 (16.5)* | 12 (0.83) | 13 (0.53) | 982 (297)^ | 1866 (207)*^ |
| 4 | 91 (16) | 108 (27.9) | 14 (1.30)+ | 13 (0.60)~ | 1210 (152) | 1367 (345) |
Mean values for each cage (six trials) are reported, and the standard deviations (SD) are presented in parentheses; *, difference to all other cages within the column, P < 0.05; +, ~, ^, difference between samples with the same symbol, P < 0.05.
Subsidence and Torsional Stability
The addition of integrated screws into the cage‐bone block construct demonstrated a clear trend towards decreased subsidence. This difference was significant (P = 0.004) for all but cage 1 (P = 0.122) and was most pronounced for cage 2. Cage 2 produced the greatest subsidence with (P < 0.001) and without (P < 0.001) screws. When comparing cages with screws, all cages were significantly different (P < 0.001), except for cages 3 and 4 (P = 0.259). When tested as a stand‐alone cage, cages 3 and 4 were, again, not significantly different (P = 0.550), as was the case for cages 1 and 4 (P = 0.954). All other cages were significantly different (P < 0.020, Fig. 4).
Figure 4.
The subsidence of each cage construct post 400 N loading, with and without screws. Mean and standard deviation illustrated with SD bars. Numbers above each bar represent the corresponding inter‐cage tests that differed significantly (P < 0.05). Significant (P < 0.05) intra‐cage differences with and without screws were indicated by in cage 2, 3, and 4.
Angular rotation was not as clearly effected as subsidence with the addition of screws. With the exception of cage 1, the mean values for angular rotation were higher with the addition of screws; however, none of these individual differences achieved significance (P < 0.066). A statistically significant correlation existed between subsidence and reduced angular rotation across all cage constructs (P = 0.018). Cage 2 produced the least angular rotation, significantly less than cages 1 (P < 0.001) and 4 (P = 0.002) without screws and cage 4 with screws (P = 0.005). When comparing constructs with screws, only cage 2 and 4 were different. When comparing constructs without screws, cages 1 and 4 were different to cages 2 and 3 (P < 0.016, Fig. 5).
Figure 5.
The loaded (400 N) torsional stability of each cage construct, with and without screws. Mean and standard deviation are shown, and numbers above each bar represent the corresponding inter‐cage tests that differed significantly (P < 0.05).
Discussion
In vitro laboratory testing plays a fundamental role in the biomechanical assessment of implants. It facilitates the evaluation of endpoints that cannot be obtained with patient data, and is pivotal in guiding the direction of ensuing animal and clinical trials. We performed an in vitro study to evaluate the impact of stand‐alone ALIF cage surface features and screw configuration on engagement and the resulting relationship with subsidence and torsional stability.
The initial segmental stability provided by the cage‐bone engagement or anchorage minimizes excessive interfacial micro‐motion, which is critical for the achievement of fusion14, 15. This has been achieved through the development of supplemental or integrated fixation techniques in combination with unique cage surface features engaging the contacted bone.
Stand‐alone Cage or Supplementary Fixation
Biomechanical Cage Design Comparison Studies
It has been shown that small micro‐motion (up to 28 μm) does not negatively affect bone ingrowth into porous surface implants, whereas large micro‐motion (over 150 μm) could produce a fibrous tissue layer across the contact interface15. The introduction of a stand‐alone cage with integrated fixation screws has prompted a variety of biomechanical studies comparing their stability to conventional supplementary fixation.
Cadaveric studies by Cain et al. 5 and Kornblum et al. 7 compared the stability of integrated screw fixation and traditional fixation (PSF, translaminar facet screws [TS] and anterior plate) in cadaveric lumbar spines, concluding that the integrated cage may provide adequate initial stability to replace the need for supplemental fixation5, 7. Choi et al. 8 and Chen et al. 9 performed 3‐D finite element analysis (FEA) simulating L3–4 segments comparing integrated screw and supplementary fixation (PSF and transpedicular fixation respectively) ALIFs8, 9. Both studies concluded that a stand‐alone cage could provide sufficient stability for clinical application without supplemental fixation.
Clinical Studies
A spurt of clinical trials ensued the favorable biomechanical findings, demonstrating that stand‐alone cages can provide comparable fusion and clinical outcomes to supplementary fixation. Stand‐alone cages additionally reduced operative time and iatrogenic trauma10, 11, 16, albeit one study recommended consideration of additional posterior fusion in cases of obesity and degenerative spondylolisthesis, to optimize fusion and reduce subsidence17.
Number of Screws
The few studies assessing the impact of different screw fixation techniques in stand‐alone cages on biomechanical stability report minimal differences between three and four screws. One study compared the test device SynFix with four divergent fixed angle screws and the STALIF with three convergent non‐angle stable screws, only demonstrating a significant difference between the cages in lateral bending, favoring SynFix4. Another study observed a non‐significant trend towards increased motion with three screws7.
Contrary to expectation, we did not find that the overall number of screws/cage had a significant mechanical effect on the contact footprint, average pressure, load, subsidence or torsional stability. Cage 2 demonstrated the greatest subsidence and torsional stability, though incorporating only two integrated screws. Fuji Film endpoints were similar despite variable screw numbers in cages 1, 2, and 4 (Table 2), although unique cage and screw features (diameter, length and thread) may confound potential differences. Cage 3 was non‐symmetrical; the superior aspect (two screws) produced approximately double the fixation load and contact footprint than its inferior aspect (one screw). This significant difference may be attributed to the convergent anchoring configuration of the screws, favoring two anchor points.
Surface Engagement Features
Cage designs have adapted to feature serrated surfaces, spikes, teeth, keels, threads and ridges in an attempt to improve endplate interdigitation and reduce interfacial migration14, 18, 19. A continuum of fixative strengths subject to implant endplate design and surface features have been demonstrated19, 20. Constructs fixated via four vertebral body screws had the highest pullout/fixation strength, followed by endplates consisting of toothed ridges and keel fixation and, finally, implants consisting of only serrated edges displaying the lowest fixation strength20. Another study found that endplate spikes of sufficient length to penetrate the endplate cortical bone provided the only design with significantly increased torsional rigidity19. Buttermann et al. postulate that the size, number, shape, and intact bony endplate engagement of the spikes in modular spiked implants differentiate them in terms of rotational rigidity comparative to other devices in this and previous studies19. Cages with surface teeth have been shown to increase pull out force shear resistance. They suggested that the surface material that teeth are constructed from might contribute to anchorage; carbon fiber saw teeth produce less resistance to being pulled out in comparison to titanium saw or serrated teeth15.
Our findings reflected a similar trend whereby cage 2 had surface titanium angled ridges and a keel, qualitatively creating the deepest footprint and quantitatively providing the greatest torsional stability. However, this finding is difficult to establish due to the unique combination of a keel and ridges, whereas the PEEK cages did not have a keel.
Fuji Film
The strength, failure load, and stiffness vary across the endplate surface, the peripheral aspects being the strongest21, 22, 23. Larger contact surface area at the endplate is believed to distribute the stress concentration, preventing subsidence12. This theory persisted at the extremes of the pressed area ranges. Combining inferior and superior areas, cage 3 contact surfaces were the largest (235 mm2) and it had the least subsidence (with screws) and second least subsidence (without screws). Cage 2 had the smallest combined contact area (183 mm2) and resulted in the largest subsidence. However, the trend did not continue with cages 3 and 4. Other surface characteristics are likely playing a role here, such as differences in materials (Ti and PEEK) and surface features (Keel, ridges, and spikes), potentially producing differing contact areas around the features when axially loaded.
Subsidence
Subsidence can be defined as a reduction in the disc space vertical height preceding complete integration of the fusion mass21. Traditionally successful interbody constructs are capable of providing necessary axial support, withstanding the in vivo loads applied during patient activities, to prevent traditionally “undesirable” subsidence or collapse, thus facilitating graft incorporation12, 15.
It is argued that “significant” subsidence is undesirable as endplate penetration is a potential cause of postoperative pain12, 18. Grant et al. 22 performed an indentation study on human cadaveric vertebral endplates and found that the peripheral posterolateral aspects are the strongest, while the center of the bone is the weakest, substantiating the importance of cage design and placement in reducing subsidence22. However, Choi and Sung assessed the correlation between ALIF radiographic fusion and symptom recurrence, with subsidence occurrence, concluding that subsidence had no relationship with the rate of radiographic fusion or recurrence of symptoms. Rather, they described it as a process of incorporation of the cage and the endplates21.
In our study, there existed a correlation between increased subsidence and increased torsional stability in cage‐saw bone constructs with and without screws, thus indicating that to an extent subsidence is fundamental to cage‐endplate engagement, maintaining rigidity and stability of the construct early on, though excess is an adverse event to be prevented.
Interestingly, the integrated screw‐cage constructs consistently demonstrated reduced subsidence in comparison to the cage‐alone constructs without screws. Screw threads potentially increase the shear resistance to subsidence under axial load.
Torsional Stability
We used a compressive preload of 400 N. The stiffness and stability of lumbar segments has been reported to increase with preload23, 24. The reported compressive forces on lumbar segments range from 440 to 1400 N from standing upright to standing with trunk flexed, 100–700 N after a vertebral body replacement when lying down to resistance exercising, though experimentally 400 N is consistently used24, 25, 26. The loaded angular rotation of each cage construct (Fig. 5) was used as a measure of torsional stability. The range of rotation difference observed did not correlate with the number of screws in each construct, nor the presence or absence of screws. We believe the variation is related to the cage‐specific design features. The central beveled Ti keel on the superior and inferior aspect of cage 2 appeared to be the dominant feature in counteracting angular rotation, paired with 18 angulated ridges, demonstrating the greatest stability with and without screws. However, we did not examine other types of motion (flexion, extension, and lateral bending). Cages 2, 3, and 4 showed increased angular rotation with screws in comparison to without screws, potentially attributed to the non‐fixed angle screws, which facilitate a degree of motion in the socket. This was not the case for cage 1, the fixed‐angle screws preventing screw socket motion, and, thus, with screws the angular rotation was less (no significant difference).
Limitations
Polyurethane blocks are a well‐established bone surrogate and are recommended for interbody fusion device testing27, 28. However, they do not include a vertebral cortical shell and endplate, which represents a potential limitation for the clinical extrapolation of results. The pressure applied to facilitate screw fixation was qualitatively determined. This may introduce human error; however, differences between repeats were not significant. Furthermore, the extrinsic preload of 400 N was used throughout the MTS testing to mimic the average pressure experienced in vivo, and allow comparison at a single load. Future work would do well to investigate other preloads. Finally, our experiment is intended as an initial investigation into the effect of cages with incorporated screws. Future work would do well to investigate other planes of motion and performance in a cadaveric model.
Conclusion
Interbody cage surface features have a greater impact on constructing torsional stability than the number of screws alone. Screws do not necessarily reduce axial rotation in this model; however, they consistently reduce subsidence under axial load. There is a relationship between increased subsidence and increased torsional stability in all cages with screws; thus, to a particular extent, subsidence may be favorable.
Acknowledgments
We would like to acknowledge the support and assistance from the staff at SORL and the Neurospine Clinic.
Disclosure: I declare that all authors listed meet the authorship criteria ICMJE guidelines and are in agreement with the manuscript. The authors have no conflicts of interest to declare.
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