Bone Healing Strategies for Lumbar Interbody Fusion

Overview

Bone grafts play a crucial role in spinal fusion procedures, with autogenous iliac crest bone grafting (ICBG) being the gold standard. This method is highly favored due to its ability to promote new bone growth and its non-immunogenic properties. However, harvesting bone from the iliac crest can result in pain and complications, which has driven the exploration of alternative techniques.

Bone Grafts

Bone grafts, including autogenous iliac crest bone grafting, are frequently utilized in spinal fusion procedures to promote bone healing. ICBG is the preferred bone grafting method for spinal fusion due to its ability to promote new bone growth and non-immunogenic properties.

However, harvesting bone from the iliac crest can result in pain and complications. The negative effects of harvesting bone from the iliac crest for spinal fusion highlight the importance of exploring alternative techniques.

Bone Biomaterials

Bone biomaterials refer to either synthetic or naturally occurring materials utilized to facilitate bone repair in instances of bone defects. An ideal bone biomaterial for repairing bone defects should possess characteristics such as biocompatibility, moldability, absorbability, radiographic identifiability, sterilizability, and accessibility.

Numerous biomaterials, such as polyesters, collagen scaffolds, hydroxyapatite, and β-tricalcium phosphate, have been employed for spinal fusion. Combining bone biomaterial with osteogenic cells and/or growth factors is more effective in achieving favorable spinal fusion outcomes.

Bone biomaterials aid in spinal fusion by serving as a scaffold for cell migration, proliferation, and osteogenic differentiation. They also function as a carrier for osteogenic proteins.

Osteoinductive Protein‑Based Osteobiologics

It has been reported in literature that the use of demineralized rabbit bone can induce the formation of new bone. The discovery of demineralized rabbit bone’s ability to induce new bone formation led to the production of recombinant human BMP-2 (rhBMP2), which has been extensively used as an osteoinductive growth factor in spinal fusion procedures.

RhBMP2 has been demonstrated to have better fusion results than the conventional ICBG harvesting technique, resulting in FDA approval for anterior lumbar fusion in 2002. Its applications have expanded to various on and off-label uses.

However, there are conflicting reports on the potential of high doses of rhBMP2 to cause tumorigenesis or life-threatening complications. Therefore, further research with different doses and long-term follow-up is needed to clarify these concerns.

Improvement Of Interbody Cages And Surgical Techniques

Various materials, including titanium, PEEK, carbon fiber, bioabsorbable, and composite implants, are used as interbody options for spinal fusion. These have its unique set of benefits and drawbacks.

There is a growing body of clinical evidence, but there are varying opinions on the optimal shape for interbody cages for spinal fusion. This includes considerations such as the construction material, shape, biomechanical properties, and whether to use expandable or fixed height cages.

Advantages And Disadvantages Of Peek, Titanium, And Composite Cages

The interbody implants that are commonly used in spinal fusion procedures include titanium, PEEK, and composite implants that incorporate both materials.

Titanium cages have the potential for osseointegration and biocompatibility, but they also have increased stiffness and radiopacity. PEEK is radiolucent and has a stiffness similar to cortical bone, but lacks osteoconductivity and may impact cage stability and fusion rates.

Spinal implant companies have developed proprietary processing techniques to address the limitations of interbody implants, such as surface modifications and composite implants, including PEEK with titanium coating.

Recent research has shown that composite implants, such as PEEK with titanium coating, have significant potential for bone ongrowth. In addition, a novel PEEK interbody cage with impactionless insertion technology has been shown to improve lumbar bony fusion and provide relief from back pain and ASD.

Expandable Versus Fixed Height Cages

An important design consideration is whether to use expandable or fixed interbody implants. Fixed interbody implants usually have a larger bone graft window, whereas expandable interbody implants provide improved bony endplate contact and disc height restoration.

Recent research has demonstrated that using expandable interbody implants can result in a greater improvement in disc height and foraminal height than fixed implants. 3D-printed dynamically semi-crystalline liquid crystal elastomer (LCE) for an expandable endplate-conforming interbody fusion cage may have improved mechanical properties compared to other expandable interbody implants.

However, studies have yielded inconsistent findings on the efficacy of expandable interbody implants compared to fixed implants. More extensive and long-term studies are needed to assess the clinical efficacy of different types of interbody fusion cages.

Improvement Of Surgical Techniques

There is ongoing progress in surgical techniques to enhance the effectiveness of spinal fusion procedures and improve patient outcomes. The use of cortical bone trajectory (CBT) screw insertion through a caudomedial starting point is a surgical technique that offers benefits such as reducing the need for dissection of the superior facet joints and muscles.

As a result, it has been found to have lower rates of early cephalad adjacent segment degeneration (ASD) after posterior lumbar interbody fusion (PLIF) compared to the traditional trajectory screw fixation (TT-PLIF). However, it is necessary to conduct further studies with a larger patient population and a longer follow-up period to make accurate comparisons of novel surgical techniques and fusion cages.

Conducting future studies with a larger patient population and a longer follow-up period can offer greater insights into the strategies for enhancing fusion rates, alleviating back pain and neurological symptoms, and mitigating the risk of ASD.

Development Of More Effective Osteobiologic Products And Bone‑forming Stimulators

Spinal fusion outcomes are dependent on both the surgical procedure and the use of effective bone grafting or osteogenic products. Despite the advancements made in the development of osteoinductive proteins and osteoconductive biomaterials, there is currently no agreement on the specific type or quantity of osteogenic products required for successful spinal fusion.

Discovering Novel Osteogenic Proteins

Literature has reported that AB204, an activin A/BMP2 chimera, had better bony fusion than rhBMP2 alone at a low dose in a rat model of posterolateral spinal fusion. However, additional studies are necessary to explore other osteogenic proteins for spinal fusion.

Developing Novel Biomaterials And Controlled Release Techniques

Different types of biomaterials, such as collagen, ceramics, and synthetic polymers, have been employed for delivering proteins to promote bone healing or fusion. However, these biomaterials differ in their ability to undergo resorption and remodeling.

Using delivery materials with low affinity for rhBMP2 can result in a burst release, which may cause supraphysiological doses and associated complications. To solve these problems, it is necessary to develop nanomaterials that can limit the movement of proteins, contain them in particular areas, and gradually release them over time.

Defining The Role Of Mesenchymal Stem Cells In Spinal Fusion

Recent research has indicated that MSC-based therapies have the potential to be effective for spinal fusion in both animal models and human clinical trials. However, there is still a need for further investigation to gain a better understanding of the mechanisms of action and the long-term efficacy of these therapies.

Mesenchymal stem cells (MSCs) have been demonstrated to be safe and effective for intervertebral disc repair and spinal fusion, although, in some studies, they did not show better results compared to current treatments.

Electrical Stimulation

Both preclinical and clinical studies suggest that electrical stimulation technologies, particularly direct current stimulation (DCS), have the potential to increase the rate of spinal fusion.

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