Introduction: The Evolution of Spinal Fusion Implants

Spinal fusion is one of the most frequently performed orthopedic procedures, with hundreds of thousands of surgeries carried out annually worldwide to treat conditions such as degenerative disc disease, spinal instability, scoliosis, and trauma. The fundamental goal of fusion is to create a solid bony bridge between adjacent vertebrae, eliminating painful motion and restoring structural alignment. To achieve this, surgeons rely on internal fixation devices—typically screws, rods, and interbody cages—that immobilize the motion segment while the bone graft integrates.

For decades, titanium alloys and stainless steel have been the standard materials for these implants, prized for their high strength, fatigue resistance, and biocompatibility. However, metal hardware comes with trade-offs. It remains permanently in the body, can cause stress shielding (where the implant bears load instead of bone, weakening the fusion mass), and may produce artifacts on advanced imaging. In response, researchers have developed biodegradable — also called bioresorbable — screws and cages made from polymers that gradually dissolve and are replaced by living bone. This article examines the science, benefits, drawbacks, and clinical considerations of biodegradable devices in spinal fusion, helping surgeons and patients weigh the evidence.

What Are Biodegradable Screws and Cages?

Biodegradable spinal implants are constructed from materials that undergo hydrolysis and enzymatic degradation in the physiological environment, breaking down into harmless by-products (typically carbon dioxide and water) that are metabolized or excreted. The most commonly used polymers include poly(L-lactic acid) (PLLA), polyglycolic acid (PGA), poly(DL-lactic acid) (PDLLA), and their copolymers such as poly(lactic-co-glycolic acid) (PLGA). These materials have a long history of safe use in absorbable sutures, bone pins, and interference screws in sports medicine.

The devices are manufactured into screws for pedicle fixation or interbody cages (placed between vertebrae to restore disc height and promote fusion). They provide temporary mechanical stability — typically lasting 6–24 months, depending on polymer composition, molecular weight, and implant design — before losing structural integrity. As the implant degrades, new bone grows into the space, gradually assuming load-bearing function. Eventually, the implant is fully resorbed, leaving behind a fully remodeled bony fusion mass.

Advantages of Biodegradable Spinal Implants

Elimination of Hardware Removal Surgery

One of the most compelling benefits is the avoidance of a second operation to retrieve metal hardware. While many patients live comfortably with permanent implants, a subset experiences late complications such as screw breakage, prominence, adjacent-level irritation, or infection. A 2019 retrospective analysis reported that symptomatic metal implants requiring removal occur in 2–8% of spinal fusion cases. Biodegradable devices obviate this risk entirely, reducing cumulative surgical morbidity, healthcare costs, and recovery time.

Improved Imaging Compatibility

Titanium and stainless steel create significant susceptibility artifacts on MRI and CT scans, obscuring adjacent anatomy and making postoperative assessment of fusion challenging. Biodegradable polymers are radiolucent and produce minimal MRI signal interference. This allows higher-quality imaging to evaluate bone healing, detect nonunion, and survey for complications such as stenosis or adjacent segment degeneration — a distinct advantage in the long-term follow-up of young or active patients who may require future spinal imaging.

Reduced Stress Shielding and Enhanced Bone Remodeling

Rigid metal implants carry the majority of physiological loads, shielding the underlying bone from mechanical stimulation. This phenomenon, known as stress shielding, can delay bony consolidation and lead to osteopenia in the fused segment over time. Biodegradable implants gradually transfer load back to the spine as they degrade, mimicking the natural mechanical environment and potentially stimulating osteoblast activity. Animal studies have demonstrated more robust bone formation and remodeling around resorbable cages compared with permanent metal cages, though human data remain limited.

Lower Risk of Long-Term Systemic Effects

Although titanium alloys are generally well tolerated, concerns persist about metal ion release and potential hypersensitivity. Nickel in stainless steel has been linked to allergic reactions in susceptible individuals. Biodegradable polymers degrade into biological metabolites (lactic and glycolic acid) that are safely cleared. For patients with known metal allergies or young adults facing decades of metal residence, biodegradable alternatives offer a biologically appealing solution.

Disadvantages and Challenges of Biodegradable Devices

Reduced Mechanical Strength and Limited Indications

The greatest limitation of biodegradable polymers is their lower initial strength compared with metal. PLLA screws have a tensile strength roughly 50–70% that of titanium; compressive strength of cages is similarly reduced. This restricts their use to relatively stable fusion constructs (e.g., single-level anterior cervical discectomy and fusion, or short-segment posterior lumbar fusion with additional fixation). In complex deformities, high-grade spondylolisthesis, or osteoporotic bone where robust fixation is essential, metal implants remain the standard. Surgeons must carefully assess load demands and host bone quality before selecting a biodegradable device.

Variable and Unpredictable Degradation Kinetics

Degradation rate depends on polymer chemistry, implant geometry, and local tissue factors such as pH, temperature, and vascularity. In some patients, the implant may lose strength too quickly, before solid fusion occurs, leading to construct failure, loss of correction, or pseudarthrosis. Conversely, slow degradation can leave a devitalized polymer mass that impedes bone ingrowth. Clinical studies have reported degradation times ranging from 9 months to over 3 years. This variability makes it difficult to guarantee that the implant will support the fusion process through its critical phases.

Potential for Inflammatory and Immunologic Reactions

As biodegradable polymers break down, they release acidic degradation products (lactic acid, glycolic acid) that can lower local pH. The body responds with an inflammatory cascade involving macrophages and giant cells. While this is usually self-limited, a small percentage of patients develop sterile sinus tracts, osteolysis, or a foreign-body reaction that delays healing or requires intervention. A meta-analysis of absorbable implants in orthopedics found a 3–12% incidence of adverse inflammatory events, with higher rates in weight-bearing applications. The reaction risk must be factored into the risk-benefit analysis for each patient.

Cost and Reimbursement Considerations

Biodegradable screws and cages are generally more expensive than conventional metal implants — sometimes 20–50% higher per unit. In addition, the manufacturing process for bioresorbable polymers is more complex, and sterilization methods (e.g., ethylene oxide) add costs. Although removing the need for a future hardware retrieval surgery may offset some of the upfront expense, the economic analysis depends on institutional contracts, insurance coverage, and the likelihood of revision. In many healthcare systems, reimbursement for biodegradable spinal implants is still evolving, limiting widespread adoption.

Technical Challenges During Surgery

Biodegradable screws have different handling characteristics than metal: they are more brittle and prone to shearing during insertion, especially in dense cortical bone or if the pilot hole is not drilled precisely. Tapping is often required, adding a step to the procedure. The threads may strip if overtightened. Surgeons need familiarity with the specific system and may need to modify their technique. Additionally, radiolucency means the surgeon cannot verify screw position on fluoroscopy after insertion — reliance on tactile feedback and preoperative planning is amplified.

Clinical Evidence: What Does the Research Show?

Clinical data on biodegradable spinal fusion implants have grown considerably over the past decade. In anterior cervical discectomy and fusion (ACDF), several prospective trials have compared PLLA cages with titanium or PEEK (polyetheretherketone) cages. A 2021 systematic review encompassing 438 patients found no significant differences in fusion rates (around 90%) at 12–24 months, but the biodegradable group showed reduced dysphagia and faster resolution of neck pain. The authors attributed this to the lower stiffness of polymer cages decreasing stress on the soft tissues.

In lumbar interbody fusion, the evidence is more mixed. A 2020 multicenter study reported that patients receiving PLGA-based interbody cages had similar fusion rates (87%) to a titanium control group (91%) at 2 years, but the bioresorbable group had a higher incidence of subsidence (14% vs. 8%). Subsidence — where the cage sinks into the vertebral endplate — can lead to loss of disc height and foraminal narrowing. The authors emphasized the importance of careful endplate preparation and avoiding cages that are too stiff for the patient’s bone density.

A separate cohort study of biodegradable pedicle screws in adolescent idiopathic scoliosis showed that the screws maintained curve correction until fusion was achieved, with no implant-related failures at a mean follow-up of 4 years. However, the use of biodegradable screws was limited to the apical segments, and thoracic pedicle screws still required metal. The authors concluded that biodegradable screws are feasible for selected applications but not yet a universal replacement.

For a deeper dive into the biomechanics and clinical outcomes, readers may consult the meta-analysis published in Spine (2022) and the review in Archives of Orthopaedic and Trauma Surgery.

Patient Selection: Who Is a Good Candidate?

Patient selection is the single most important factor for success with biodegradable devices. Ideal candidates include:

  • Younger, active patients with good bone healing potential and a long life expectancy, where removal of permanent hardware would be desirable.
  • Single-level or two-level degenerative disease in the cervical or lumbar spine with preserved bone quality (DEXA T-score above -2.0).
  • Non-smokers or those willing to quit — smoking significantly impairs fusion and increases the risk of premature implant degradation without solid union.
  • No history of allergy to polymers or severe autoimmune disorders that might exaggerate the inflammatory response to degradation products.
  • Motivated patients who understand the data limitations and the possibility of slower recovery or the need for conversion to metal if fusion fails.

Contraindications include multilevel (>3 segments) reconstructions, severe osteoporosis, active infection, revision surgery in the presence of pseudarthrosis, and cases requiring bulky, high-load structural support such as corpectomy reconstruction. For these scenarios, metal implants with proven track records remain the standard of care.

Future Directions: Next-Generation Materials and Designs

Research is actively addressing the limitations of current biodegradable polymers. One promising avenue is the development of composite materials — such as hydroxyapatite-reinforced PLA cages — that combine the osteoconductivity of ceramics with the resorbability of polymers. These composites have shown improved compressive strength and more favorable bone-implant interface healing in preclinical models.

Another innovation is the use of polymer blends with tailored degradation profiles that match the bone healing timeline more predictably. For example, coating a fast-degrading PLGA core with a slower-degrading PLLA shell allows spatiotemporal control of strength loss. Additionally, growth factors (e.g., BMP-2) can be incorporated into the polymer matrix to stimulate osteogenesis directly at the fusion site. Early clinical trials of BMP-eluting bioresorbable cages are underway, with encouraging safety profiles.

Advances in additive manufacturing (3D printing) also enable patient-specific implant geometries that optimize load distribution and minimize subsidence risks. A recent proof-of-concept study printed PLLA interbody cages with porous lattice structures that allowed bony ingrowth while the implant resorbed. The FDA has cleared several biodegradable spinal devices, indicating a growing regulatory acceptance of this technology. As manufacturing costs decrease and clinical experience accumulates, the role of biodegradable implants is expected to expand.

Conclusion: Weighing the Evidence for Clinical Practice

Biodegradable screws and cages represent a thoughtful evolution in spinal fusion technology, addressing several well-recognized shortcomings of permanent metal hardware. They eliminate the need for removal surgery, reduce imaging artifacts, and align with the biological principle of gradual load transfer. For carefully selected patients undergoing single- or two-level fusion, current evidence suggests that fusion rates are comparable to those with metal constructs, with the added benefit of fewer hardware-related complications.

However, the technology is not without significant caveats. Lower mechanical strength, unpredictable degradation, the potential for inflammatory reactions, and higher upfront costs limit its role to specific clinical scenarios. Surgeons must evaluate each patient’s anatomy, bone quality, and risk tolerance. Shared decision-making — presenting these pros and cons transparently to the patient — is essential. As material science progresses, we can anticipate biodegradable implants that degrade more reliably, offer stronger initial fixation, and perhaps even deliver therapeutic molecules to accelerate healing. For now, they are a valuable tool in the spinal surgeon’s armamentarium, but not a complete substitute for the proven performance of metal implants in complex reconstructions.