The Role of Bone Mineral Density in a Successful Lumbar Interbody Fusion: A Narrative Review

BACKGROUND

The incidence of osteoporosis is a prime concern, especially in parts of the world where the population is aging, such as Europe or the US. Many new therapy strategies have been described to enhance bone healing. Lumbar interbody fusion (LIF) is a surgical procedure that aims to stabilize the lumbar spine by fusing two or more vertebrae using an interbody cage. LIF is a standard treatment for various spinal conditions, such as degenerative disc disease, spinal stenosis, and spondylolisthesis. However, successful fusion is challenging for patients with osteoporosis due to their reduced bone mineral density (BMD) and increased risk of cage subsidence, which can lead to implant failure and poor clinical outcomes.

METHODS

 A comprehensive literature search yielded 220 articles, with 16 ultimately included. Keywords included BMD, cage subsidence, osteoporosis, teriparatide, and lumbar interbody fusion.

RESULTS

 This review examines the relationship between BMD and LIF success, emphasizing the importance of adequate bone quality for successful fusion. Preoperative assessment methods for BMD and the impact of low BMD on fusion rates and patient outcomes are discussed. Additionally, techniques to improve fusion success in patients with weakened bone density, such as biological enhancement and BMD-matched interbody cages, are explored. However, consensus on the exact BMD threshold for a successful outcome remains elusive.

CONCLUSION

 While an apparent correlation between BMD and fusion rate in LIF procedures is acknowledged, conclusive evidence regarding the precise BMD threshold indicative of an increased risk of unfavorable outcomes remains elusive. Surgeons are advised to exercise caution in surgical planning and follow-up for patients with lower BMD. Furthermore, future research initiatives, particularly longitudinal studies, are encouraged to prioritize the examination of BMD as a fundamental risk factor, addressing gaps in the literature.

Resisting subsidence with a truss Implant: Application of the "Snowshoe" principle for interbody fusion devices

The primary objective was to compare the subsidence resistance properties of a novel 3D-printed spinal interbody titanium implant versus a predicate polymeric annular cage. We evaluated a 3D-printed spinal interbody fusion device that employs truss-based bio-architectural features to apply the snowshoe principle of line length contact to provide efficient load distribution across the implant/endplate interface as means of resisting implant subsidence. Devices were tested mechanically using synthetic bone blocks of differing densities (osteoporotic to normal) to determine the corresponding resistance to subsidence under compressive load. Statistical analyses were performed to compare the subsidence loads and evaluate the effect of cage length on subsidence resistance. The truss implant demonstrated a marked rectilinear increase in resistance to subsidence associated with increase in the line length contact interface that corresponds with implant length irrespective of subsidence rate or bone density. In blocks simulating osteoporotic bone, comparing the shortest with the longest length truss cage (40 vs. 60 mm), the average compressive load necessary to induce subsidence of the implant increased by 46.4% (383.2 to 561.0 N) and 49.3% (567.4 to 847.2 N) for 1 and 2 mm of subsidence, respectively. In contrast, for annular cages, there was only a modest increase in compressive load when comparing the shortest with the longest length cage at a 1 mm subsidence rate. The Snowshoe truss cages demonstrated substantially more resistance to subsidence than corresponding annular cages. Clinical studies are required to support the biomechanical findings in this work.

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.

Improving the Management of Patients with Osteoporosis Undergoing Spinal Fusion: The Need for a Bone Mineral Density-Matched Interbody Cage

With an increasingly aging population globally, a confluence has emerged between the rising prevalence of degenerative spinal disease and osteoporosis. Fusion of the anterior spinal column remains the mainstay surgical intervention for many spinal degenerative disorders. However, decreased vertebral bone mineral density (BMD), quantitatively measured by dual x-ray absorptiometry (DXA), complicates treatment with surgical interbody fusion as weak underlying bone stock increases the risk of post-operative implant-related adverse events, including cage subsidence. There is a necessity for developing cages with advanced structural designs that incorporate bioengineering and architectural principles to tailor the interbody fusion device directly to the patient’s BMD status. Specifically, lattice-designed cages that mimic the web-like structure of native cancellous bone have demonstrated excellent resistance to post-operative subsidence. This article provides an introductory profile of a spinal interbody implant designed intentionally to simulate the lattice structure of human cancellous bone, with a similar modulus of elasticity, and specialized to match a patient’s bone status across the BMD continuum. The implant incorporates an open pore design where the degree of pore compactness directly corresponds to the patient’s DXA-defined BMD status, including patients with osteoporosis.

Truss implant technology™ for interbody fusion in spinal degenerative disorders: profile of advanced structural design, mechanobiologic and performance characteristics

Introduction: Interbody fusion devices are customarily used in fusion of the anterior spinal column for treatment of degenerative disc disease. Their traditional role is to reestablish and maintain intervertebral disc height, contain bone graft and provide mechanical support for the spine while osseointegration takes place. Utilizing the principles of mechanobiology, a unique biokinetic interbody fusion device has been developed that employs an advanced structural design to facilitate and actively participate in the fusion consolidation process.Areas covered: This article profiles and characterizes 4WEB Medical's Truss Implant Technology™ which includes a range of 3D-printed titanium spinal interbody implants and non-spinal implants whose design is based on truss structures enabled by advances in additive manufacturing. Four main areas of the implant design and functionality are detailed: bio-architecture, mechanobiologic underpinnings, bioactive surface features, and subsidence resistance. Pre-clinical and clinical examples are provided to describe and specify the bioactive roles and contributions of each design feature.Expert opinion: The distinct and unique combination of features incorporated within the truss cage design results in a biokinetic implant that actively participates in the bone healing cascade and fusion process.

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.