Interbody Spacer Material Properties and Design Conformity for Reducing Subsidence During Lumbar Interbody Fusion

Anatomically and mechanically conforming patient-specific spinal fusion cages designed by full-scale topology optimization

Cage subsidence after instrumented lumbar spinal fusion surgery remains a significant cause of treatment failure, specifically for posterior or transforaminal lumbar interbody fusion. Recent advancements in computational techniques and additive manufacturing, have enabled the development of patient-specific implants and implant optimization to specific functional targets. This study aimed to introduce a novel full-scale topology optimization formulation that takes the structural response of the adjacent bone structures into account in the optimization process. The formulation includes maximum and minimum principal strain constraints that lower strain concentrations in the adjacent vertebrae. This optimization approach resulted in anatomically and mechanically conforming spinal fusion cages. Subsidence risk was quantified in a commercial finite element solver for off-the-shelf, anatomically conforming and the optimized cages, in two representative patients. We demonstrated that the anatomically and mechanically conforming cages reduced subsidence risk by 91% compared to an off-the-shelf implant with the same footprint for a patient with normal bone quality and 54% for a patient with osteopenia. Prototypes of the optimized cage were additively manufactured and mechanically tested to evaluate the manufacturability and integrity of the design and to validate the finite element model.

Advances in implants and bone graft types for lumbar spinal fusion surgery

The increasing prevalence of spinal disorders worldwide necessitates advanced treatments, particularly interbody fusion for severe cases that are unresponsive to non-surgical interventions. This procedure, especially 360° lumbar interbody fusion, employs an interbody cage, pedicle screw-and-rod instrumentation, and autologous bone graft (ABG) to enhance spinal stability and promote fusion. Despite significant advancements, a persistent 10% incidence of non-union continues to result in compromised patient outcomes and escalated healthcare costs. Innovations in lumbar stabilisation seek to mimic the properties of natural bone, with evolving implant materials like titanium (Ti) and polyetheretherketone (PEEK) and their composites offering new prospects. Additionally, biomimetic cages featuring precisely engineered porosities and interconnectivity have gained traction, as they enhance osteogenic differentiation, support osteogenesis, and alleviate stress-shielding. However, the limitations of ABG, such as harvesting morbidities and limited fusion capacity, have spurred the exploration of sophisticated solutions involving advanced bone graft substitutes. Currently, demineralised bone matrix and ceramics are in clinical use, forming the basis for future investigations into novel bone graft substitutes. Bioglass, a promising newcomer, is under investigation despite its observed rapid absorption and the potential for foreign body reactions in preclinical studies. Its clinical applicability remains under scrutiny, with ongoing research addressing challenges related to burst release and appropriate dosing. Conversely, the well-documented favourable osteogenic potential of growth factors remains encouraging, with current efforts focused on modulating their release dynamics to minimise complications. In this evidence-based narrative review, we provide a comprehensive overview of the evolving landscape of non-degradable spinal implants and bone graft substitutes, emphasising their applications in lumbar spinal fusion surgery. We highlight the necessity for continued research to improve clinical outcomes and enhance patient well-being.

In silico medical device testing of anatomically and mechanically conforming patient-specific spinal fusion cages designed by full-scale topology optimisation

A full-scale topology optimisation formulation has been developed to automate the design of cages used in instrumented transforaminal lumbar interbody fusion. The method incorporates the mechanical response of the adjacent bone structures in the optimisation process, yielding patient-specific spinal fusion cages that both anatomically and mechanically conform to the patient, effectively mitigating subsidence risk compared to generic, off-the-shelf cages and patient-specific devices. In this study, in silico medical device testing on a cohort of seven patients was performed to investigate the effectiveness of the anatomically and mechanically conforming devices using titanium and PEEK implant materials. A median reduction in the subsidence risk by 89% for titanium and 94% for PEEK implant materials was demonstrated compared to an off-the-shelf implant. A median reduction of 75% was achieved for a PEEK implant material compared to an anatomically conforming implant. A credibility assessment of the computational model used to predict the subsidence risk was provided according to the ASME V&V40-2018 standard.

Tomographic Assessment of Fusion Rate, Implant-Endplate Contact Area, Subsidence, and Alignment With Lumbar Personalized Interbody Implants at 1-Year Follow-Up

BACKGROUND

Incongruity between irregularly shaped vertebral endplates and the uniform surfaces of stock interbody fusion cages has been identified as contributing to cage subsidence, pseudarthrosis, and unpredictable alignment. Advances in manufacturing techniques have driven the development of personalized interbody cages (PICs) that can match individual endplate morphology and provide the exact shape and size needed to fill the disc space and achieve the planned correction. This study used computed tomography (CT) imaging to evaluate the implant-endplate contact area, fusion, subsidence, and achievement of planned alignment correction in patients receiving PIC devices.

METHODS

This retrospective study included patients treated for adult spinal deformity at a single site and implanted with PIC devices at L4 to L5 or L5 to S1 for segmental stabilization and alignment correction, who received 1-year postoperative CT images as part of their standard of care. An evaluation using 3-dimensional thin-section scans was conducted. Implant-endplate contact and signs of fusion were assessed in each CT slice across both endplates. The degree of subsidence as well as measures of segmental and global lumbar alignment were also assessed.

RESULTS

Fifteen patients were included in the study, with a mean age of 68.2 years. Follow-up ranged between 9 and 14 months. Twenty-six total lumbar levels were implanted; 20 with PIC devices via the anterior lumbar interbody fusion approach, 2 with stock cages via the anterior lumbar interbody fusion approach, and 4 with PIC devices via the transforaminal lumbar interbody fusion approach. CT analysis of PIC-implanted levels found an overall implant-endplate contact area ratio of 93.9%, a subsidence rate of 4.5%, a fusion rate of 100%, and satisfactory segmental and global lumbar correction compared with the preoperative plan.

CONCLUSIONS

PIC implants can provide nearly complete contact with endplate surfaces regardless of the individual endplate morphology. Subsidence, fusion, and alignment assessments in this tomographic study illustrated results consistent with the benefits of a personalized interbody implant.

LEVEL OF EVIDENCE: 4

Biomechanical Comparison of Subsidence Between Patient-Specific and Non-Patient-Specific Lumbar Interbody Fusion Cages

STUDY DESIGN

Biomechanical study.

OBJECTIVES

Several strategies to improve the surface of contact between an interbody device and the endplate have been employed to attenuate the risk of cage subsidence. 3D-printed patient-specific cages have been presented as a promising alternative to help mitigate that risk, but there is a lack of biomechanical evidence supporting their use. We aim to evaluate the biomechanical performance of 3D printed patient-specific lumbar interbody fusion cages in relation to commercial cages in preventing subsidence.

METHODS

A cadaveric model is used to investigate the possible advantage of 3D printed patient-specific cages matching the endplate contour using CT-scan imaging in preventing subsidence in relation to commercially available cages (Medtronic Fuse and Capstone). Peak failure force and stiffness were analyzed outcomes for both comparison groups.

RESULTS

PS cages resulted in significantly higher construct stiffness when compared to both commercial cages tested (>59%). PS cage peak failure force was 64% higher when compared to Fuse cage (P < .001) and 18% higher when compared to Capstone cage (P = .086).

CONCLUSIONS

Patient-specific cages required higher compression forces to produce failure and increased the cage-endplate construct' stiffness, decreasing subsidence risk.

Feasibility of Non-window Three-Dimensional-Printed Porous Titanium Cage in Posterior Lumbar Interbody Fusion: A Pilot Trial

Background

The commercially available design of a three-dimensional (3D)-printed titanium (3D-Ti) cage can be divided into two types according to the presence of a window: a cage with a window that allows filling of bone graft materials and a non-window cage for stand-alone use. This prospective observational case series study aimed to explore the clinical feasibility of using a non-window type 3D-Ti cage in cases of combined window and non-window cage implantation. Furthermore, we evaluated the bone in growth patterns of non-window cages and their correlation with published fusion grading systems.

Methods

A total of 31 consecutive patients who underwent single-level posterior lumbar interbody fusion surgery were included. Two 3D-Ti cages with different designs were inserted: a non-window cage on the left side and a window cage on the right side. Radiographic fusion was defined by the segmental angle between flexion and extension radiographs (F-E angle) and cage bridging bone (CBB) scores on computed tomography. The association between the F-E angle and osteointegration scoring system including the surface osteointegration ratio (SOR) score was analyzed.

Results

Radiographic fusion was achieved in 27 of 31 patients (87%) at 12 months postoperatively. Among the non-window cages, 23 of 31 (74.2%) had fair SOR scores, while 19 of 31 (61.3%) window cages had fair intra-cage CBB scores. The higher the SOR score was, the smaller the flexion-extension angle (SOR 0 vs. SOR 1: 6.30° ± 2.43° vs. 1.95° ± 0.99°, p < 0.001; SOR 0 vs. SOR 2: 6.03° ± 2.43° vs. 0.99°± 0.74°, p < 0.001).

Conclusions

The clinical feasibility of using a non-window 3D-Ti cage during lumbar interbody fusion might be acceptable. Furthermore, a newly suggested fusion criterion for the use of the non-window cage, the SOR score, showed a significant association with the published fusion grading systems, demonstrating its feasibility in determining interbody fusion in lumbar spinal surgery.

Preliminary exploration of the biomechanical properties of three novel cervical porous fusion cages using a finite element study

BACKGROUND

Porous cages are considered a promising alternative to high-density cages because their interconnectivity favours bony ingrowth and appropriate stiffness tuning reduces stress shielding and the risk of cage subsidence.

METHODS

This study proposes three approaches that combine macroscopic topology optimization and micropore design to establish three new types of porous cages by integrating lattices (gyroid, Schwarz, body-centred cubic) with the optimized cage frame. Using these three porous cages along with traditional high-density cages, four ACDF surgical models were developed to compare the mechanical properties of facet articular cartilage, discs, cortical bone, and cages under specific loads.

RESULTS

The facet joints in the porous cage groups had lower contact forces than those in the high-density cage group. The intervertebral discs in all models experienced maximum stress at the C5/6 segment. The stress distribution on the cortical bone surface was more uniform in the porous cage groups, leading to increased average stress values. The gyroid, Schwarz, and BCC cage groups showed higher average stress on the C5 cortical bone. The average stress on the surface of porous cages was higher than that on the surface of high-density cages, with the greatest difference observed under the lateral bending condition. The BCC cage demonstrated favourable mechanical stability.

CONCLUSION

The new porous cervical cages satifies requirements of low rigidity and serve as a favourable biological scaffold for bone ingrowth. This study provides valuable insights for the development of next-generation orthopaedic medical devices.

Three-Dimensional-Printed Titanium Versus Polyetheretherketone Cages for Lumbar Interbody Fusion: A Systematic Review of Comparative In Vitro, Animal, and Human Studies

Interbody fusion is a workhorse technique in lumbar spine surgery that facilities indirect decompression, sagittal plane realignment, and successful bony fusion. The 2 most commonly employed cage materials are titanium (Ti) alloy and polyetheretherketone (PEEK). While Ti alloy implants have superior osteoinductive properties they more poorly match the biomechanical properties of cancellous bones. Newly developed 3-dimensional (3D)-printed porous titanium (3D-pTi) address this disadvantage and are proposed as a new standard for lumbar interbody fusion (LIF) devices. In the present study, the literature directly comparing 3D-pTi and PEEK interbody devices is systematically reviewed with a focus on fusion outcomes and subsidence rates reported in the in vitro, animal, and human literature. A systematic review directly comparing outcomes of PEEK and 3D-pTi interbody spinal cages was performed. PubMed, Embase, and Cochrane Library databases were searched according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines. Mean Newcastle-Ottawa Scale score for cohort studies was 6.4. A total of 7 eligible studies were included, comprising a combination of clinical series, ovine animal data, and in vitro biomechanical studies. There was a total population of 299 human and 59 ovine subjects, with 134 human (44.8%) and 38 (64.4%) ovine models implanted with 3D-pTi cages. Of the 7 studies, 6 reported overall outcomes in favor of 3D-pTi compared to PEEK, including subsidence and osseointegration, while 1 study reported neutral outcomes for device related revision and reoperation rate. Though limited data are available, the current literature supports 3D-pTi interbodies as offering superior fusion outcomes relative to PEEK interbodies for LIF without increasing subsidence or reoperation risk. Histologic evidence suggests 3D-Ti to have superior osteoinductive properties that may underlie these superior outcomes, but additional clinical investigation is merited.

Biomechanical Evaluation of Lateral Lumbar Interbody Fusion with Various Fixation Options for Adjacent Segment Degeneration: A Finite Element Analysis

OBJECTIVE

Adjacent segment degeneration (ASD) is a common phenomenon after lumbar fusion. Lateral lumbar interbody fusion (LLIF) may provide an alternative treatment method for ASD. This study used finite element analysis to evaluate the biomechanical effects of LLIF with various fixation options and identify an optimal surgical strategy for ASD.

METHODS

A validated L1-S1 finite element model was modified for simulation. Six finite element models of the lumbar spine were created and were divided into group 1 (L4-5 posterior lumbar interbody fusion [PLIF] + L3-4 LLIF) and group 2 (L5-S1 PLIF + L4-5 LLIF). Each group consisted of 1) cage-alone, 2) cage + lateral screw fixation (LSF), and 3) cage + bilateral pedicle screw fixation (BPSF) models. The range of motion, intradiscal pressure, and facet loads of adjacent segments as well as interbody cage stress were analyzed.

RESULTS

The stress on the LLIF cage-superior endplate interface was highest in the cage-alone model followed by the cage + LSF model and cage + BPSF model. The increase in range of motion, intradiscal pressure, and facet loads at the adjacent segment was highest in the cage + BPSF model followed by the cage + LSF model and cage-alone model. However, the biomechanical effect on the adjacent segment seemed similar in the cage-alone and cage + LSF models.

CONCLUSIONS

LLIF with BPSF is recommended when performing LLIF surgery for ASD after L4-5 and L5-S1 PLIF. Considering cage subsidence and biomechanical effects on the adjacent segment, LLIF with LSF may be a suboptimal option for ASD surgery.

Recent advancement in finite element analysis of spinal interbody cages: A review

Finite element analysis (FEA) is a widely used tool in a variety of industries and research endeavors. With its application to spine biomechanics, FEA has contributed to a better understanding of the spine, its components, and its behavior in physiological and pathological conditions, as well as assisting in the design and application of spinal instrumentation, particularly spinal interbody cages (ICs). IC is a highly effective instrumentation for achieving spinal fusion that has been used to treat a variety of spinal disorders, including degenerative disc disease, trauma, tumor reconstruction, and scoliosis. The application of FEA lets new designs be thoroughly "tested" before a cage is even manufactured, allowing bio-mechanical responses and spinal fusion processes that cannot easily be experimented upon in vivo to be examined and "diagnosis" to be performed, which is an important addition to clinical and in vitro experimental studies. This paper reviews the recent progress of FEA in spinal ICs over the last six years. It demonstrates how modeling can aid in evaluating the biomechanical response of cage materials, cage design, and fixation devices, understanding bone formation mechanisms, comparing the benefits of various fusion techniques, and investigating the impact of pathological structures. It also summarizes the various limitations brought about by modeling simplification and looks forward to the significant advancement of spine FEA research as computing efficiency and software capabilities increase. In conclusion, in such a fast-paced field, the FEA is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons' learning costs for new cage products, and probably assisting them in determining the best IC for patients.

Evaluation of the contact surface between vertebral endplate and 3D printed patient-specific cage vs commercial cage

Biomechanical study. To evaluate the performance of the contact surface for 3D printed patient-specific cages using CT-scan 3D endplate reconstructions in comparison to the contact surface of commercial cages. Previous strategies to improve the surface of contact between the device and the endplate have been employed to attenuate the risk of cage subsidence. Patient-specific cages have been used to help, but only finite-element studies have evaluated the effectiveness of this approach. There is a possible mismatch between the CT-scan endplate image used to generate the cage and the real bony endplate anatomy that could limit the performance of the cages. A cadaveric model is used to investigate the possible mismatch between 3D printed patient-specific cages and the endplate and compare them to commercially available cages (Medtronic Fuse and Capstone). Contact area and contact stress were used as outcomes. When PS cage was compared to the Capstone cage, the mean contact area obtained was 100 ± 23.6 mm² and 57.5 ± 13.7 mm², respectively (p < 0.001). When compared to the Fuse cage, the mean contact area was 104.8 ± 39.6 mm² and 55.2 ± 35.1 mm², respectively(p < 0.001). Patient-specific cages improve the contact area between the implant and the endplate surface, reducing the contact stress and the risk of implant subsidence during LIF surgeries.

Subsidence after lateral lumbar interbody fusion using a 3D-printed porous titanium interbody cage: single-institution case series

OBJECTIVE

Cage subsidence is a well-known phenomenon after lateral lumbar interbody fusion (LLIF), occurring in 10%-20% of cases. A 3D-printed porous titanium (pTi) cage has a stiffness that mimics the modulus of elasticity of native vertebrae, which reduces stress at the bone-hardware interface, lowering the risk of subsidence. In this study, the authors evaluated their institutional rate of subsidence and resultant reoperation in patients who underwent LLIF using a 3D-printed pTi interbody cage.

METHODS

This is a retrospective case series of consecutive adult patients who underwent LLIF using pTi cages from 2018 to 2020. Demographic and clinical characteristics including age, sex, bone mineral density, smoking status, diabetes, steroid use, number of fusion levels, posterior instrumentation, and graft size were collected. The Marchi subsidence grade was determined at the time of last follow-up. Outcome measures of interest were subsidence and resultant reoperation. Univariable logistic regression analysis was performed to assess the extent to which clinical and operative characteristics were associated with Marchi grade I-III subsidence. Significance was assessed at p < 0.05.

RESULTS

Fifty-five patients (38 with degenerative disc disease and 17 with adult spinal deformity) were treated with 97 pTi interbody cages with a mean follow-up of 18 months. The mean age was 63.6 ± 10.1 years, 60% of patients were female, and 36% of patients had osteopenia or osteoporosis. Patients most commonly underwent single-level LLIF (58.2%). Sixteen patients (29.1%) had posterior instrumentation. The subsidence grade distribution was as follows: 89 (92%) grade 0, 5 (5%) grade I, 2 (2%) grade II, and 1 (1%) grade III. No patients who were active or prior smokers and no patients with posterior instrumentation experienced graft subsidence. No clinical or operative characteristics were significantly associated with graft subsidence. One patient (1.8%) required reoperation because of subsidence.

CONCLUSIONS

In this institutional case series, subsidence of pTi intervertebral cages after LLIF occurred in 8% of operated levels, 3% of which were grade II or III. Only 1 patient required reoperation. These reported rates are lower than those reported for polyetheretherketone implants. Further studies are necessary to compare the impact of these cage materials on subsidence after LLIF.

Biomechanical comparison of subsidence performance among three modern porous lateral cage designs

BACKGROUND

Cage subsidence remains a major complication after spinal surgery. The goal of this study was to compare the subsidence performance of three modern porous cage designs.

METHODS

Three porous cages were evaluated: a porous titanium cage, a porous polyetheretherketone cage and a truss titanium cage. Mechanical testing was performed for each cage per the American Society for Testing and Materials F2077 and F2267 standards to evaluate cage stiffness and block stiffness, and per a novel clinically relevant dynamic subsidence testing method simulating cyclic spine loading during 3-months postoperatively to evaluate the subsidence displacement.

FINDINGS

The porous polyetheretherketone cage demonstrated the lowest cage stiffness (21.0 ± 1.1 kN/mm), less than half of both titanium cages (truss titanium cage, 49.1 kN/mm; porous titanium cage, 43.6 kN/mm). The block stiffness was greatest for the porous titanium cage (2867.7 ± 105.3 N/mm), followed by the porous polyetheretherketone (2563.4 ± 72.9 N/mm) and truss titanium cages (2213.7 ± 21.8 N/mm). The dynamic subsidence displacement was greatest for the truss titanium cage, which was 1.5 and 2.5 times the subsidence displacement as the porous polyetheretherketone and porous titanium cages respectively.

INTERPRETATIONS

Specific porous cage design plays a crucial role in the cage subsidence performance, to a greater degree than the selection of cage materials. A porous titanium cage with body lattice and microporous endplates significantly outperformed a truss titanium cage with a similar cage stiffness in subsidence performance, and a porous polyetheretherketone cage with half of its stiffness.

Biodegradable interbody cages for lumbar spine fusion: Current concepts and future directions

Lumbar fusion often remains the last treatment option for various acute and chronic spinal conditions, including infectious and degenerative diseases. Placement of a cage in the intervertebral space has become a routine clinical treatment for spinal fusion surgery to provide sufficient biomechanical stability, which is required to achieve bony ingrowth of the implant. Routinely used cages for clinical application are made of titanium (Ti) or polyetheretherketone (PEEK). Ti has been used since the 1980s; however, its shortcomings, such as impaired radiographical opacity and higher elastic modulus compared to bone, have led to the development of PEEK cages, which are associated with reduced stress shielding as well as no radiographical artefacts. Since PEEK is bioinert, its osteointegration capacity is limited, which in turn enhances fibrotic tissue formation and peri-implant infections. To address shortcomings of both of these biomaterials, interdisciplinary teams have developed biodegradable cages. Rooted in promising preclinical large animal studies, a hollow cylindrical cage (Hydrosorb™) made of 70:30 poly-l-lactide-co-d, l-lactide acid (PLDLLA) was clinically studied. However, reduced bony integration and unfavourable long-term clinical outcomes prohibited its routine clinical application. More recently, scaffold-guided bone regeneration (SGBR) with application of highly porous biodegradable constructs is emerging. Advancements in additive manufacturing technology now allow the cage designs that match requirements, such as stiffness of surrounding tissues, while providing long-term biomechanical stability. A favourable clinical outcome has been observed in the treatment of various bone defects, particularly for 3D-printed composite scaffolds made of medical-grade polycaprolactone (mPCL) in combination with a ceramic filler material. Therefore, advanced cage design made of mPCL and ceramic may also carry initial high spinal forces up to the time of bony fusion and subsequently resorb without clinical side effects. Furthermore, surface modification of implants is an effective approach to simultaneously reduce microbial infection and improve tissue integration. We present a design concept for a scaffold surface which result in osteoconductive and antimicrobial properties that have the potential to achieve higher rates of fusion and less clinical complications. In this review, we explore the preclinical and clinical studies which used bioresorbable cages. Furthermore, we critically discuss the need for a cutting-edge research program that includes comprehensive preclinical in vitro and in vivo studies to enable successful translation from bench to bedside. We develop such a conceptual framework by examining the state-of-the-art literature and posing the questions that will guide this field in the coming years.

Rapid Personalised Virtual Planning and On-Demand Surgery for Acute Spinal Trauma Using 3D-Printing, Biomodelling and Patient-Specific Implant Manufacture

Three-dimensional printing is a rapidly growing field, with extensive application to orthopaedics and spinal surgery. Three-dimensional-printed (3DP) patient-specific implants (PSIs) offer multiple potential benefits over generic alternatives, with their use increasingly being described in the spinal literature. This report details a unique, emergency case of a traumatic spinal injury in a 31-year-old male, acquired rurally and treated with a 3DP PSI in a tertiary unit. With increasing design automation and process improvements, rapid, on-demand virtual surgical planning (VSP) and 3DP PSIs may present the future of orthopaedics and trauma care, enabling faster, safer, and more cost-effective patient-specific procedures.

Subsidence and fusion performance of a 3D-printed porous interbody cage with stress-optimized body lattice and microporous endplates - a comprehensive mechanical and biological analysis

BACKGROUND CONTEXT

Cage subsidence remains a serious complication after spinal fusion surgery. Novel porous designs in the cage body or endplate offer attractive options to improve subsidence and osseointegration performance.

PURPOSE

To elucidate the relative contribution of a porous design in each of the two major domains (body and endplates) to cage stiffness and subsidence performance, using standardized mechanical testing methods, and to analyze the fusion progression via an established ovine interbody fusion model to support the mechanical testing findings.

STUDY DESIGN/SETTING

A comparative preclinical study using standardized mechanical testing and established animal model.

METHODS

To isolate the subsidence performance contributed by each porous cage design feature, namely the stress-optimized body lattice (vs. a solid body) and microporous endplates (vs. smooth endplates), four groups of cages (two-by-two combination of these two features) were tested in: (1) static axial compression of the cage (per ASTM F2077) and (2) static subsidence (per ASTM F2267). To evaluate the progression of fusion, titanium cages were created with a microporous endplate and internal lattice architecture analogous to commercial implants used in subsidence testing and implanted in an endplate-sparing, ovine intervertebral body fusion model.

RESULTS

The cage stiffness was reduced by 16.7% by the porous body lattice, and by 16.6% by the microporous endplates. The porous titanium cage with both porous features showed the lowest stiffness with a value of 40.4±0.3 kN/mm (Mean±SEM) and a block stiffness of 1976.8±27.4 N/mm for subsidence. The body lattice showed no significant impact on the block stiffness (1.4% reduction), while the microporous endplates decreased the block stiffness significantly by 24.9% (p<.0001). All segments implanted with porous titanium cages were deemed rigidly fused by manual palpation, except one at 12 weeks, consistent with robotic ROM testing and radiographic and histologic observations. A reduction in ROM was noted from 12 to 26 weeks (4.1±1.6° to 2.2±1.4° in lateral bending, p<.05; 2.1±0.6° to 1.5±0.3° in axial rotation, p<.05); and 3.3±1.6° to 1.9±1.2° in flexion extension, p=.07). Bone in the available void improved with time in the central aperture (54±35% to 83±13%, p<.05) and porous cage structure (19±26% to 37±21%, p=.15).

CONCLUSIONS

Body lattice and microporous endplates features can effectively reduce the cage stiffness, therefore reducing the risk of stress shielding and promoting early fusion. While body lattice showed no impact on block stiffness and the microporous endplates reduced the block stiffness, a titanium cage with microporous endplates and internal lattice supported bone ingrowth and segmental mechanical stability as early as 12 weeks in ovine interbody fusion.

CLINICAL SIGNIFICANCE

Porous titanium cage architecture can offer an attractive solution to increase the available space for bone ingrowth and bridging to support successful spinal fusion while mitigating risks of increased subsidence.

Mesh Ti6Al4V Material Manufactured by Selective Laser Melting (SLM) as a Promising Intervertebral Fusion Cage

Intervertebral cages made of Ti6Al4V alloy show excellent osteoconductivity, but also higher stiffness, compared to commonly used polyether-ether-ketone (PEEK) materials, that may lead to a stress-shielding effect and implant subsidence. In this study, a metallic intervertebral fusion cage, with improved mechanical behavior, was manufactured by the introduction of a three-dimensional (3D) mesh structure to Ti6Al4V material, using an additive manufacturing method. Then, the mechanical and biological properties of the following were compared: (1) PEEK, with a solid structure, (2) 3D-printed Ti6Al4V, with a solid structure, and (3) 3D-printed Ti6Al4V, with a mesh structure. A load-induced subsidence test demonstrated that the 3D-printed mesh Ti6Al4V cage had significantly lower tendency (by 15%) to subside compared to the PEEK implant. Biological assessment of the samples proved that all tested materials were biocompatible. However, both titanium samples (solid and mesh) were characterized by significantly higher bioactivity, osteoconductivity, and mineralization ability, compared to PEEK. Moreover, osteoblasts revealed stronger adhesion to the surface of the Ti6Al4V samples compared to PEEK material. Thus, it was clearly shown that the 3D-printed mesh Ti6Al4V cage possesses all the features for optimal spinal implant, since it carries low risk of implant subsidence and provides good osseointegration at the bone-implant interface.

Biomechanical Evaluation of an Oblique Lateral Locking Plate System for Oblique Lumbar Interbody Fusion: A Finite Element Analysis

OBJECTIVE

The oblique lateral locking plate system (OLLPS) is a novel internal fixation with a locking and reverse pedicle track screw configuration designed for oblique lumbar interbody fusion (OLIF). The OLLPS is placed in a single position through the oblique lateral surgical corridor to reduce operative time and complications associated with prolonged anesthesia and prone positioning. The purpose of this study was to verify the biomechanical effect of the OLLPS.

METHODS

An intact finite element model of L1-S1 (intact) was established based on computed tomography images of a healthy male volunteer. The L4-L5 intervertebral space was selected as the surgical segment. The surgical models were established separately based on OLIF surgical procedures and different internal fixations: 1) stand-alone OLIF (SA); 2) OLIF with a 2-screw lateral plate; 3) OLIF with a 4-screw lateral plate; 4) OLIF with OLLPS; and 5) OLIF with bilateral pedicle screw fixation (BPS). After validation of the intact model, physiologic loads were applied to the superior surface of L1 to simulate motions such as flexion, extension, left bending, right bending, left rotation, and right rotation. The evaluation indices included the L4/5 range of motion, the L4 maximum displacement, and the maximum stresses of the superior and inferior end plates, the cage, and the supplemental fixation.

RESULTS

During OLIF surgery, the OLLPS provided multiplanar stability similar to that provided by BPS. Compared with 2-screw lateral plate and 4-screw lateral plate, OLLPS had better biomechanical properties in terms of enhancing the instant stability of the surgical segment, reducing the stress on the superior and inferior end plates of the surgical segment, and decreasing the risk of cage subsidence.

CONCLUSIONS

With a minimally invasive background, the OLLPS can be used as an alternative to BPS in OLIF and it has better prospects for clinical promotions and applications.

CT Hounsfield Units as a predictor of reoperation and graft subsidence following standalone and multi-level lateral lumbar interbody fusion

INTRODUCTION

Standalone single and multi-level lateral lumbar interbody fusion (LLIF) are increasingly being applied to treat degenerative spinal conditions in a less invasive fashion. Graft subsidence following LLIF is a known complication and has been associated with poor bone mineral density (BMD). Previous research has demonstrated the utility of CT Hounsfield Units (HU) as a surrogate for BMD. This study aims to investigate the relationship between CT HU and subsidence and reoperation after standalone and multi-level LLIF.

METHODS

A prospectively-maintained single-institution database was retrospectively reviewed for LLIF patients from 2017-2020 including single and multi-level standalone cases with or without supplemental posterior fixation. Data on demographics, graft parameters, BMD on DEXA, preoperative mean segmental CT HU, and postoperative subsidence and reoperation, were collected. Three-foot standing radiographs were used to measure preoperative global sagittal alignment and disc height, and subsidence at last follow-up. Subsidence was classified using the Marchi grading system corresponding to disc height loss: Grade 0: 0-24%; I: 25-49%; II: 50-74%; III: 75-100%.

RESULTS

Eighty-nine LLIF patients met study criteria, with mean follow-up 19.9 ± 13.9 months. Among the 54 patients who underwent single-level LLIF, mean segmental HU was 152.0 ± 8.7 in 39 patients with Grade 0 subsidence, 136.7 ± 10.4 in nine with Grade I subsidence, 133.9 ± 23.1 in three with Grade II subsidence, and 119.9 ± 30.9 in three with Grade III subsidence (p=0.032). In the 96 instrumented levels in 35 patients who underwent multi-level LLIF, 85 had Grade 0 subsidence, 9 Grade I, 1 Grade II, and 1 Grade III, with no differences in HU. In multivariate logistic regression, increased CT HU was independently associated with a decreased risk of reoperation in both single-level and multi-level LLIF (OR:0.98, 95%CI:0.97-0.99, p=0.044; and OR:0.97, 95%CI: 0.94-0.99, p=0.017, respectively). Overall BMD on DEXA was not associated with graft subsidence nor reoperation. Using a receiver-operating-characteristic curve to establish separation between patients requiring reoperation and those that did not, the determined threshold HU for single-level LLIF was 131.4 (sensitivity 0.62, specificity 0.65), and for multi-level was 131.0 (sensitivity 0.67, specificity 0.63).

CONCLUSIONS

Lower CT HU are independently associated with an increased risk of graft subsidence following single-level LLIF. In addition, lower CT HU significantly increased the risk of reoperation in both single and multi-level LLIF with a critical threshold of 131 HU. Preoperative CT HU may provide a more robust gauge of local bone quality and the likelihood of graft subsidence requiring reoperation following LLIF, than overall BMD.

Biomechanical analysis of stand-alone lumbar interbody cages versus 360° constructs: an in vitro and finite element investigation

OBJECTIVE

Low fusion rates and cage subsidence are limitations of lumbar fixation with stand-alone interbody cages. Various approaches to interbody cage placement exist, yet the need for supplemental posterior fixation is not clear from clinical studies. Therefore, as prospective clinical studies are lacking, a comparison of segmental kinematics, cage properties, and load sharing on vertebral endplates is needed. This laboratory investigation evaluates the mechanical stability and biomechanical properties of various interbody fixation techniques by performing cadaveric and finite element (FE) modeling studies.

METHODS

An in vitro experiment using 7 fresh-frozen human cadavers was designed to test intact spines with 1) stand-alone lateral interbody cage constructs (lateral interbody fusion, LIF) and 2) LIF supplemented with posterior pedicle screw-rod fixation (360° constructs). FE and kinematic data were used to validate a ligamentous FE model of the lumbopelvic spine. The validated model was then used to evaluate the stability of stand-alone LIF, transforaminal lumbar interbody fusion (TLIF), and anterior lumbar interbody fusion (ALIF) cages with and without supplemental posterior fixation at the L4-5 level. The FE models of intact and instrumented cases were subjected to a 400-N compressive preload followed by an 8-Nm bending moment to simulate physiological flexion, extension, bending, and axial rotation. Segmental kinematics and load sharing at the inferior endplate were compared.

RESULTS

The FE kinematic predictions were consistent with cadaveric data. The range of motion (ROM) in LIF was significantly lower than intact spines for both stand-alone and 360° constructs. The calculated reduction in motion with respect to intact spines for stand-alone constructs ranged from 43% to 66% for TLIF, 67%-82% for LIF, and 69%-86% for ALIF in flexion, extension, lateral bending, and axial rotation. In flexion and extension, the maximum reduction in motion was 70% for ALIF versus 81% in LIF for stand-alone cases. When supplemented with posterior fixation, the corresponding reduction in ROM was 76%-87% for TLIF, 86%-91% for LIF, and 90%-92% for ALIF. The addition of posterior instrumentation resulted in a significant reduction in peak stress at the superior endplate of the inferior segment in all scenarios.

CONCLUSIONS

Stand-alone ALIF and LIF cages are most effective in providing stability in lateral bending and axial rotation and less so in flexion and extension. Supplemental posterior instrumentation improves stability for all interbody techniques. Comparative clinical data are needed to further define the indications for stand-alone cages in lumbar fusion surgery.

Early Outcomes of 3D-printed Porous Titanium versus Polyetheretherketone (PEEK) Cage Implantation for Standalone Lateral Lumbar Interbody Fusion in the Treatment of Symptomatic Adjacent Segment Degeneration

OBJECTIVE

This study compared outcomes of 3D-printed porous titanium (Ti) versus polyetheretherketone (PEEK) cage implantation for standalone lateral lumbar interbody fusion (SA-LLIF) in the treatment of symptomatic adjacent segment degeneration (ASD).

METHODS

44 patients (59 levels) underwent SA-LLIF with Ti or PEEK cages between 10/2016 and 07/2020. The primary outcome was cage subsidence according to Marchi et al. Secondary outcomes included revision/recommendations for revision surgery, back/leg pain severity, changes in disc/foraminal height and global/segmental lumbar lordosis.

RESULTS

44 patients (21 female) were included with a mean age at surgery of 61.8±11.5 years, average radiological follow-up of 12.5±8.2 and clinical follow-up of 11.0±7.1 months. The overall subsidence rate was significantly less in the Ti versus PEEK group (20% vs. 58.8%; p=0.004). Revision was recommended to none of the patients in the Ti and 3 in the PEEK group (p=0.239). Furthermore, patients in the Ti group showed significantly better improvement in back pain NRS score (p=0.001). Disc height (p<0.001) and foraminal height restoration (p=0.011) were statistically significant in the Ti group, whereas only disc height restoration was significant in the PEEK group (p=0.003).

CONCLUSION

In patients undergoing SA-LLIF for ASD treatment, 3D-printed Ti cages had significantly lower overall subsidence rate compared to PEEK cages. Furthermore, Ti cages resulted in fewer recommendations for revision surgery. Whether greater pain reduction in the Ti group is associated with earlier or higher fusion rates needs to be further elucidated.

Development of a decision-making pathway for utilizing standalone lateral lumbar interbody fusion

PURPOSE

To develop a decision-making pathway for primary SA-LLIF. Furthermore, we analyzed the agreement of this pathway and compared outcomes of patients undergoing either SA-LLIF or 360-LLIF.

METHOD

A decision-making pathway for SA-LLIF was created based on the results of interviews/surveys of senior spine surgeons with over 10 years of experience. Internal validity was retrospectively evaluated using consecutive patients undergoing either SA-LLIF or 360-LLIF between 01/2018 and 07/2020 with 3D-printed Titanium cages. An outcome assessment looking primarily at revision surgery and secondary at cage subsidence, changes in disk and foraminal height, global and segmental lumbar lordosis, duration of surgery, estimated blood loss, and length of stay was carried out.

RESULTS

78 patients with 124 treated levels (37 SA-LLIF, 41 360-LLIF) were retrospectively analyzed. The pathway showed a direct agreement (SA-LLIF) of 100.0% and an indirect agreement (360-LLIF) of 95.1%. Clinical follow-up averaged 13.5 ± 6.5 months including 4 revision surgeries in the 360-LLIF group and none in the SA-LLIF group (p = 0.117). Radiographic follow-up averaged 9.5 ± 4.3 months, with no statistically significant difference in cage subsidence rate between the groups (p = 0.440). Compared to preoperative images, patients in both groups showed statistically significant changes in disk height (p < 0.001), foraminal height (p < 0.001), as well as restoration of segmental lordosis (p < 0.001 and p = 0.018). The SA-LLIF group showed shorter duration of surgery, less estimated blood loss and shorter LOS (p < 0.001).

CONCLUSION

The proposed decision-making pathway provides a guide to adequately select patients for SA-LLIF. Further studies are needed to assess the external applicability and validity.

LEVEL OF EVIDENCE III

Diagnostic: individual cross-sectional studies with consistently applied reference standard and blinding.

Evaluation of cage subsidence in standalone lateral lumbar interbody fusion: novel 3D-printed titanium versus polyetheretherketone (PEEK) cage

PURPOSE

This study aims to compare the early subsidence rate (6-12 months) of standalone novel 3D-printed titanium (Ti) versus polyetheretherketone (PEEK) interbody cages after lateral lumbar interbody fusion (LLIF).

METHOD

A retrospective study of 113 patients (186 levels) who underwent LLIF surgery with Ti or PEEK cages was conducted. Early subsidence was measured in each treated level using the Marchi et al. classification in radiographs or CT scans acquired at 6-12 months follow-up. Multivariate logistic regression analyses with generalized mixed models, setting subsidence as the outcome variable and including cage type (Ti vs PEEK) as well as significant and trending variables (p < 0.10) in univariate analyses, were conducted.

RESULTS

In total, 51 female and 62 male patients were analyzed. The median [IQR] age at surgery was 60.0 [51.0-70.0] years. Of the 186 levels, 119 levels were treated using PEEK and 67 levels with Ti cages. The overall subsidence rate for Grades I-III was significantly less in the Ti versus the PEEK group (p = 0.003). For high-grade subsidence (Grade II or III), Ti cages also demonstrated a subsidence rate (3.0%) that was significantly less compared to PEEK cages (18.5%) (p = 0.002). Multivariate analysis showed that patients treated with Ti cages were less likely to develop severe subsidence compared to those treated with PEEK (OR = 0.05, 95% CI = 0.01, 0.30) (p = 0.001).

CONCLUSION

Our study demonstrated that 3D-printed novel Ti cages had a significantly lower early subsidence rate compared to PEEK cages in standalone LLIF patients.

The biomechanical effects of Ti versus PEEK used in the PLIF surgery on lumbar spine: a finite element analysis

Titanium (Ti) and polyetheretherketone (PEEK) are commonly used in posterior lumbar interbody fusion (PLIF). The study investigated biomechanical effects of Ti versus PEEK used as materials of cage and rods on the lumbar spine. Four different configurations of PLIF were constituted. Stiff Ti rods provided satisfactory initial stability but increased the stress on rods significantly under simulated physiological load conditions. Ti cage increased the stress on bone endplates significantly. Materials of cage and rods had insignificant effects on the nucleus pressure and facet joint force of non-instrumented segments. Further clinical studies and follow-up observations are essential for corroborating these findings.

Current Concepts of Contemporary Expandable Lumbar Interbody Fusion Cage Designs, Part 2: Feasibility Assessment of an Endplate Conforming Bidirectional Expandable Interbody Cage

BACKGROUND

Expandable cages that allow for bidirectional expansion, in both height and width, may offer benefits over traditional expandable cages or static cages. Effective stiffness must also be considered, as implants with exceedingly high stiffness may increase subsidence risk and reduce graft loading.

METHODS

A retrospective case series of 7 patients were assessed with computed tomography (CT) scan at the final 1-year follow-up to evaluate the interbody fusion and configuration of the expandable cage related to the endplates within the intervertebral space. CT scans were reformatted using cage's tantalum markers as fiducials for single-plane orientation for each intervertebral cage. Device height and width at maximum in situ expansion was measured at its anterior and posterior aspects to evaluate implant deformation. The new bone volume within each cage was measured from the same CT scan data sets and by the Bridwell classification of interbody fusion.

RESULTS

The average difference between medial and lateral height measurements was 1.82 mm (±1.08) at the device's anterior aspect and 1.41 mm (±0.98) at the posterior aspect. The average difference between medial and lateral heights was 18.55% (±9.34) anteriorly and 15.49% (±9.24) posteriorly. There was a successful fusion in all 7 patients, as evidenced by measurable bone volume in the center of each interbody cage with an average of 586.42 mm3 (±237.06).

CONCLUSION

The authors demonstrated the feasibility of successfully using bidirectionally expandable multimaterial cages to achieve interbody fusion. These composite open-architecture cages were found to conform to each patient's endplate configuration. The authors' observations support the concept of material selection impacting the effective construct stiffness. The design investigated by the authors provided sufficient anterior column support and successful fusion in all patients.

LEVEL OF EVIDENCE

4.

Current Concepts of Contemporary Expandable Lumbar Interbody Fusion Cage Designs, Part 1: An Editorial on Their Biomechanical Characteristics

BACKGROUND

Bidirectional expandable designs for lumbar interbody fusion cages are the latest iteration of expandable spacers employed to address some of the common problems inherent to static interbody fusion cages.

OBJECTIVE

To describe the rationales for contemporary bidirectional, multimaterial expandable lumbar interbody fusion cage designs to achieve in situ expansion for maximum anterior column support while decreasing insertion size during minimal-access surgeries.

METHODS

The authors summarize the current concepts behind expandable spinal fusion open architecture cage designs focusing on advanced minimally invasive spinal surgery techniques, such as endoscopy. A cage capable of bidirectional expansion in both height and width to address constrained surgical access problems was of particular interest to the authors while they analyzed the relationship between implant material stiffness and geometric design regarding the risk of subsidence and reduced graft loading.

CONCLUSIONS

Biomechanical advantages of new bidirectional, multimaterial expandable interbody fusion cages allow insertion through minimal surgical access and combine the advantages of proven device configurations and advanced material selection. The final construct stiffness is sufficient to provide immediate anterior column support while accommodating reduced sizes required for minimally invasive surgery applications.

LEVEL OF EVIDENCE

7.

Load Sharing and Endplate Pressure Distribution in Anterior Interbody Fusion Influenced by Graft Choice

BACKGROUND

Cage subsidence is a known complication of spinal fusion. Various aspects of cage design have been investigated for their influence on cage subsidence, whereas the potential contribution of graft material to load sharing is often overlooked. We aimed to determine whether graft in the aperture affects endplate pressure distribution.

METHODS

The pressure distributions of a polyetheretherketone interbody cage with 3 different aperture graft conditions were evaluated: empty, demineralized bone matrix, and supercritical CO₂-treated allograft bone crunch (SCCO₂).

RESULTS

Graft materials contributed as much as half the load transmission for SCCO₂, whereas demineralized bone matrix contributed one third. Endplate areas in contact with the cage demonstrated decreased areas within the highest-pressure spectrum with SCCO₂ graft materials compared with empty cages.

CONCLUSIONS

Graft choice plays a role in reducing peak endplate pressures. This finding is relevant to implant subsidence, as well as graft loading and remodeling.

Comparison of dynamic response of three TLIF techniques on the fused and adjacent segments under vibration

To explore which TLIF techniques are advantageous in reducing the risk of complications and conducive to bone fusion under the vibration. The L1-L5 finite element lumbar model was modified to simulate three different TLIF techniques (a unilateral standard cage, a crescent-shaped cage, and bilateral standard cages). The results showed that the crescent-shaped cage may reduce the risk of subsidence and provide a more stable and suitable environment for vertebral cell growth under the vibration compared to the other TLIF techniques. Unilateral cage may increase the risk of adjacent segment disease and cage failure including fatigue failure under vibration.

Porous fusion cage design via integrated global-local topology optimization and biomechanical analysis of performance

Porous fusion cage is considered as a satisfactory substitute for solid fusion cage in transforaminal lumbar interbody fusion (TLIF) surgery due to its interconnectivity for bone ingrowth and appropriate stiffness reducing the risk of cage subsidence and stress shielding. This study presents an integrated global-local topology optimization approach to obtain porous titanium (Ti) fusion cage with desired biomechanical properties. Local topology optimizations are first conducted to obtain unit cells, and the numerical homogenization method is used to quantified the mechanical properties of unit cells. The preferred porous structure is then fabricated using selective laser melting, and its mechanical property is further verified via compression tests and numerical simulation. Afterward, global topology optimization is used for the global layout. The porous fusion cage obtained by the Boolean intersection between global structural layout and the porous structure decreases the solid volume of the cage by 9% for packing more bone grafts while achieving the same stiffness to conventional porous fusion cage. To eliminate stress concentration in the thin-wall structure, framework structures are constructed on the porous fusion cage. Although the alleviation of cage subsidence and stress shielding is decelerated, peak stress on the cage is significantly decreased, and more even stress distribution is demonstrated in the reinforced porous fusion cage. It promises long-term integrity and functions of the fusion cage. Overall, the reinforced porous fusion cage achieves a favorable mechanical performance and is a promising candidate for fusion surgery. The proposed optimization approach is promising for fusion cage design and can be extended to other orthopedic implant designs.

Development and Validation of a Computationally Efficient Finite Element Model of the Human Lumbar Spine: Application to Disc Degeneration

INTRODUCTION

This study develops and validates an accurate, computationally efficient, 3-dimensional finite element model (FEM) of the human lumbar spine. Advantages of this simplified model are shown by its application to a disc degeneration study that we demonstrate is completed in one-sixth the time required when using more complicated computed tomography (CT) scan-based models.

METHODS

An osseoligamentous FEM of the L1-L5 spine is developed using simple shapes based on average anatomical dimensions of key features of the spine rather than CT scan images. Pure moments of 7.5 Nm and a compressive follower load of 1000 N are individually applied to the L1 vertebra. Validation is achieved by comparing rotations and intradiscal pressures to other widely accepted FEMs and in vitro studies. Then degenerative disc properties are modeled and rotations calculated. Required computation times are compared between the model presented in this paper and other models developed using CT scans.

RESULTS

For the validation study, parameter values for a healthy spine were used with the loading conditions described above. Total L1-L5 rotations for flexion, extension, lateral bending, and axial rotation under pure moment loading were calculated as 20.3°, 10.7°, 19.7°, and 10.3°, respectively, and under a compressive follower load, maximum intradiscal pressures were calculated as 0.68 MPa. These values compare favorably with the data used for validation. When studying the effects of disc degeneration, the affected segment is shown to experience decreases in rotations during flexion, extension, and lateral bending (24%-56%), while rotations are shown to increase during axial rotation (14%-40%). Adjacent levels realize relatively minor changes in rotation (1%-6%). This parametric study required 17.5 hours of computation time compared to more than 4 days required if utilizing typical published CT scan-based models, illustrating one of the primary advantages of the model presented in this article.

CONCLUSIONS

The FEM presented in this article produces a biomechanical response comparable to widely accepted, complex, CT scan-based models and in vitro studies while requiring much shorter computation times. This makes the model ideal for conducting parametric studies of spinal pathologies and spinal correction techniques.

Dynamically Crystalizing Liquid-Crystal Elastomers for an Expandable Endplate-Conforming Interbody Fusion Cage

Degenerative disc disease (DDD) is the leading cause of low back pain and radiating leg pain. DDD is commonly treated surgically using spinal fusion techniques, but in many cases failure occurs due to insufficient immobilization of the vertebrae during fusion. The fabrication and demonstration of a 3D-printed semi-crystalline liquid crystal elastomer (LCE) spinal fusion cage that addresses these challenges in particular subsidence are described. During implantation of the fusion cage, the LCE is rubbery and capable of deforming around and conforming to delicate anatomy. In the hours following implantation, the device crystallizes into a rigid, structural material with the modulus increasing tenfold from 8 to 80 MPa. In the crystalline regime, a 3D-printed prototype device is capable of enduring 1 million cycles of physiologic compressive loading with minimal creep-induced ratcheting. Effects of LCE molecular architecture on the rate and magnitude of modulus increase, material processability, and mechanical properties are explored. This fundamental characterization informs a proof-of-concept device-the first bulk 3D printed LCE demonstrated to date. Moreover, the novel deployment strategy represents an exciting new paradigm of spinal fusion cages, which addresses real clinical challenges in expandable interbody fusion cages.

Novel Titanium Cages for Minimally Invasive Lateral Lumbar Interbody Fusion: First Assessment of Subsidence

Introduction

Implant subsidence is a potential complication of spinal interbody fusion and may negatively affect patients subjected to procedures relying on indirect decompression such as minimally invasive transpsoas lateral lumbar interbody fusion (LLIF). The porous architecture of a recently developed titanium intervertebral cage maximizes bone-to-implant contact and minimizes stress shielding in laboratory experiments; however, its subsidence rate in patients has not yet been evaluated. The goal of this current study was to evaluate implant subsidence in patients subjected to LLIF.

Methods

Our institutional review board-approved single-center experience included 29 patients who underwent 30 minimally invasive LLIF from July 2017 to September 2018 utilizing the novel 3D-printed porous titanium implants. Radiographs, obtained during routine postoperative follow-up visits, were reviewed for signs of implant subsidence, defined as any appreciable compromise of the vertebral endplates.

Results

Radiographic subsidence occurred in 2 cases (6.7%), involving 2 out of 59 porous titanium interbody cages (3.4%). Both cases of subsidence occurred in four-level stand-alone constructs. The patients remained asymptomatic and did not require surgical revision. Ten surgeries were stand-alone constructs, and 20 surgeries included supplemental posterior fixation.

Conclusions

In our patient cohort, subsidence of the porous titanium intervertebral cage occurred in 6.7% of all cases and in 3.4% of all lumbar levels. This subsidence rate is lower compared to previously reported subsidence rates in patients subjected to LLIF using polyetheretherketone implants.

Effect of porous orthopaedic implant material and structure on load sharing with simulated bone ingrowth: A finite element analysis comparing titanium and PEEK

Osseointegration of load-bearing orthopaedic implants, including interbody fusion devices, is critical to long-term biomechanical functionality. Mechanical loads are a key regulator of bone tissue remodeling and maintenance, and stress-shielding due to metal orthopaedic implants being much stiffer than bone has been implicated in clinical observations of long-term bone loss in tissue adjacent to implants. Porous features that accommodate bone ingrowth have improved implant fixation in the short term, but long-term retrieval studies have sometimes demonstrated limited, superficial ingrowth into the pore layer of metal implants and aseptic loosening remains a problem for a subset of patients. Polyether-ether-ketone (PEEK) is a widely used orthopaedic material with an elastic modulus more similar to bone than metals, and a manufacturing process to form porous PEEK was recently developed to allow bone ingrowth while preserving strength for load-bearing applications. To investigate the biomechanical implications of porous PEEK compared to porous metals, we analyzed finite element (FE) models of the pore structure-bone interface using two clinically available implants with high (> 60%) porosity, one being constructed from PEEK and the other from electron beam 3D-printed titanium (Ti). The objective of this study was to investigate how porous PEEK and porous Ti mechanical properties affect load sharing with bone within the porous architectures over time. Porous PEEK substantially increased the load share transferred to ingrown bone compared to porous Ti under compression (i.e. at 4 weeks: PEEK = 66%; Ti = 13%), tension (PEEK = 71%; Ti = 12%), and shear (PEEK = 68%; Ti = 9%) at all time points of simulated bone ingrowth. Applying PEEK mechanical properties to the Ti implant geometry and vice versa demonstrated that the observed increases in load sharing with PEEK were primarily due to differences in intrinsic elastic modulus and not pore architecture (i.e. 4 weeks, compression: PEEK material/Ti geometry = 53%; Ti material/PEEK geometry = 12%). Additionally, local tissue energy effective strains on bone tissue adjacent to the implant under spinal load magnitudes were over two-fold higher with porous PEEK than porous Ti (i.e. 4 weeks, compression: PEEK = 784 ± 351 microstrain; Ti = 180 ± 300 microstrain; and 12 weeks, compression: PEEK = 298 ± 88 microstrain; Ti = 121 ± 49 microstrain). The higher local strains on bone tissue in the PEEK pore structure were below previously established thresholds for bone damage but in the range necessary for physiological bone maintenance and adaptation. Placing these strain magnitudes in the context of literature on bone adaptation to mechanical loads, this study suggests that porous PEEK structures may provide a more favorable mechanical environment for bone formation and maintenance under spinal load magnitudes than currently available porous 3D-printed Ti, regardless of the level of bone ingrowth.

Stand-alone lumbar cage subsidence: A biomechanical sensitivity study of cage design and placement

BACKGROUND AND OBJECTIVE

Spinal degeneration and instability are commonly treated with interbody fusion cages either alone or supplemented with posterior instrumentation with the aim to immobilise the segment and restore intervertebral height. The purpose of this work is to establish a tool which may help to understand the effects of intervertebral cage design and placement on the biomechanical response of a patient-specific model to help reducing post-surgical complications such as subsidence and segment instability.

METHODS

A 3D lumbar functional spinal unit (FSU) finite element model was created and a parametric model of an interbody cage was designed and introduced in the FSU. A Drucker-Prager Cap plasticity formulation was used to predict plastic strains and bone failure in the vertebrae. The effect of varying cage size, cross-sectional area, apparent stiffness and positioning was evaluated under 500 N preload followed by 7.5 Nm multidirectional rotation and the results were compared with the intact model.

RESULTS

The most influential cage parameters on the FSU were size, curvature congruence with the endplates and cage placement. Segmental stiffness was higher when increasing the cross-sectional cage area in all loading directions and when the cage was anteriorly placed in all directions but extension. In general, the facet joint forces were reduced by increasing segmental stiffness. However, these forces were higher than in the intact model in most of the cases due to the displacement of the instantaneous centre of rotation. The highest plastic deformations took place at the caudal vertebra under flexion and increased for cages with greater stiffness. Thus, wider cages and a more anteriorly placement would increase the volume of failed bone and, therefore, the risk of subsidence.

CONCLUSIONS

Cage geometry plays a crucial role in the success of lumbar surgery. General considerations such as larger cages may be applied as a guideline, but parameters such as curvature or cage placement should be determined for each specific patient. This model provides a proof-of-concept of a tool for the preoperative evaluation of lumbar surgical outcomes.

Biological evaluation and finite-element modeling of porous poly(para-phenylene) for orthopaedic implants

Poly(para-phenylene) (PPP) is a novel aromatic polymer with higher strength and stiffness than polyetheretherketone (PEEK), the gold standard material for polymeric load-bearing orthopaedic implants. The amorphous structure of PPP makes it relatively straightforward to manufacture different architectures, while maintaining mechanical properties. PPP is promising as a potential orthopaedic material; however, the biocompatibility and osseointegration have not been well investigated. The objective of this study was to evaluate biological and mechanical behavior of PPP, with or without porosity, in comparison to PEEK. We examined four specific constructs: 1) solid PPP, 2) solid PEEK, 3) porous PPP and 4) porous PEEK. Pre-osteoblasts (MC3T3) exhibited similar cell proliferation among the materials. Osteogenic potential was significantly increased in the porous PPP scaffold as assessed by ALP activity and calcium mineralization. In vivo osseointegration was assessed by implanting the cylindrical materials into a defect in the metaphysis region of rat tibiae. Significantly more mineral ingrowth was observed in both porous scaffolds compared to the solid scaffolds, and porous PPP had a further increase compared to porous PEEK. Additionally, porous PPP implants showed bone formation throughout the porous structure when observed via histology. A computational simulation of mechanical push-out strength showed approximately 50% higher interfacial strength in the porous PPP implants compared to the porous PEEK implants and similar stress dissipation. These data demonstrate the potential utility of PPP for orthopaedic applications and show improved osseointegration when compared to the currently available polymeric material.

STATEMENT OF SIGNIFICANCE

PEEK has been widely used in orthopaedic surgery; however, the ability to utilize PEEK for advanced fabrication methods, such as 3D printing and tailored porosity, remain challenging. We present a promising new orthopaedic biomaterial, Poly(para-phenylene) (PPP), which is a novel class of aromatic polymers with higher strength and stiffness than polyetheretherketone (PEEK). PPP has exceptional mechanical strength and stiffness due to its repeating aromatic rings that provide strong anti-rotational biaryl bonds. Furthermore, PPP has an amorphous structure making it relatively easier to manufacture (via molding or solvent-casting techniques) into different geometries with and without porosity. This ability to manufacture different architectures and use different processes while maintaining mechanical properties makes PPP a very promising potential orthopaedic biomaterial which may allow for closer matching of mechanical properties between the host bone tissue while also allowing for enhanced osseointegration. In this manuscript, we look at the potential of porous and solid PPP in comparison to PEEK. We measured the mechanical properties of PPP and PEEK scaffolds, tested these scaffolds in vitro for osteocompatibility with MC3T3 cells, and then tested the osseointegration and subsequent functional integration in vivo in a metaphyseal drill hole model in rat tibia. We found that PPP permits cell adhesion, growth, and mineralization in vitro. In vivo it was found that porous PPP significantly enhanced mineralization into the construct and increased the mechanical strength required to push out the scaffold in comparison to PEEK. This is the first study to investigate the performance of PPP as an orthopaedic biomaterial in vivo. PPP is an attractive material for orthopaedic implants due to the ease of manufacturing and superior mechanical strength.

L5 En-Bloc Vertebrectomy with Customized Reconstructive Implant: Comparison of Patient-Specific Versus Off-the-Shelf Implant

BACKGROUND

Spine surgery has the potential to benefit from additive manufacturing/3-dimensional printing (3DP) technology with complex anatomical pathologies requiring reconstruction, with the potential to customize surgery to reduce operative times, reduce blood loss, provide immediate stability, and potentially improve fusion rates. We report a unique case of intraoperative trial placement of a custom patient-specific implant (PSI) versus the final implantation of a customizable off-the-shelf (OTS) implant. Data collected for comparison included time to implant, ease of implantation, firmness of press-fit, and fixation options after implantation.

CASE DESCRIPTION

A 64-year-old man presented with low back pain. Computed tomography and magnetic resonance imaging revealed a solitary lesion in the L5 vertebral body, confirmed by positron emission tomography scan. Removal of the L5 vertebral body was performed, and reconstruction was achieved with an expandable cage. The time of implant insertion was minimal with the PSI (90 seconds) versus the OTS (>40 minutes). Immediate press-fit and "firmness" of implantation was clearly superior with the PSI, although this was an intraoperative subjective assessment. Other benefits include integral fixation that is predetermined with the PSI, reduced time and blood loss, and ease of bone grafting with a PSI.

CONCLUSIONS

Use of 3DP has been able to reduce operative time significantly. Surgeons can train before performing complex procedures, which enhances their presurgical planning, with the goal to maximize patient outcomes. When considering implants and prostheses, the use of 3DP allows a superior anatomical fit for the patient, with the potential to improve restoration of anatomy.