This week’s research spotlight focuses on a recently completed in vitro assessment comparing the AxioMed Freedom Lumbar Disc (FLD)—a one-piece viscoelastic total disc replacement (VTDR)—with 2 well-established articulating TDR systems (CHARITÉ and prodisc L). The findings highlight compelling advantages of viscoelastic, non-articulating spinal disc technology, with potential implications for long-term implant survival and reduced biological complications.

Background

Wear particle generation remains one of the most persistent and clinically significant limitations of orthopaedic arthroplasty. In large joints, submicron polymer and metal debris is known to trigger macrophage-mediated inflammation, osteolysis, and eventual implant loosening. Although spinal disc arthroplasty was originally expected to avoid these issues, growing evidence shows that articulating lumbar TDRs can generate biologically active particulate debris and even systemic metal ion release.

To address this risk, the AxioMed FLD utilizes a single-piece, viscoelastic silicone-polycarbonate urethane core, eliminating metal-on-metal or metal-on-polymer interfaces. The device deforms internally to allow motion rather than relying on mechanical articulation.

This week’s featured study puts that design to the test.

Study Overview

Five FLD devices underwent 30 million motion cycles—equivalent to ~240 years of lumbar bending activity—under ASTM-standard flexion-extension, lateral bending, and axial rotation conditions.
Wear debris was sampled every 5 million cycles and analyzed using scanning electron microscopy and laser diffraction.

For comparison, particle size and wear rate data for CHARITÉ and prodisc L were sourced from their FDA SSED reports.

Key Results

1. Dramatically Lower Wear Rate

• AxioMed FLD: 1.7 mg per million cycles (mg/MC)

• prodisc L: 5.7 mg/MC

The FLD produced ~70% less wear than the articulating prodisc L. CHARITÉ reported a lower overall wear rate but produced extremely small, biologically active particles.

2. Significantly Larger Particle Sizes

Wear particle size is perhaps the most clinically relevant finding:

AxioMed FLD

Mean Particle Diameter: 1.9 μm (number-average)

Typical Range: 0.8–6.9 μm

prodisc L

Mean Particle Diameter: 0.44 μm

Typical Range: 0.08–2.29 μm

CHARITÉ

Mean Particle Diameter: 0.2 μm

Typical Range: 0.08–16.3 μm

Particles greater than 1 μm are substantially less likely to trigger inflammatory cascades, macrophage activation, or osteolysis.
The FLD consistently produced larger, less bioactive debris.

3. Mechanical Durability and Stability

• No mechanical failures across 30 million cycles.

• Only minor, nonprogressive smoothing at the polymer core’s posterior flash ring.

• Dimensional changes were minimal and within expected viscoelastic tolerance.

This suggests extremely high structural resilience even under exaggerated, long-term loading.

Why These Findings Matter

1. Reduced Risk of Osteolysis

Submicron particles are the principal driver of:

• macrophage activation

• cytokine release (TNF-α, IL-1β)

• periprosthetic bone resorption

• implant instability

By generating larger particles and lower total debris volume, the FLD may significantly reduce osteolytic risk, a problem reported in 8–64% of cervical TDR cases with articulating designs.

2. Improved Long-Term Implant Survival

The data support the concept that:

• viscoelastic motion preservation,

• non-articulating design, and

• chemically bonded endplates

work synergistically to reduce wear and mechanical fatigue.

3. Clinical Advantage for Younger Patients

Younger, active patients place higher lifetime motion demands on implants.
A device producing:

• fewer particles

• larger, less inflammatory debris

• zero structural failures at 240 simulated years

may provide a meaningful long-term advantage.

Limitations & Next Steps

The study is robust but does have typical preclinical limitations:

Limitations

• in vitro testing cannot fully replicate in vivo biological conditions

• PBS medium lacks proteins that may influence wear

• particle number and composition were not quantified

• small sample size (n = 5)

• comparator data from FDA reports lacked detailed variability metrics

Recommended Future Work

1. Particle composition analysis (chemical characterization).

2. Particle burden quantification (number per unit volume).

3. Serum-based wear testing to simulate biological lubrication.

4. Viscoelastic fatigue testing under long-term cyclic loading.

5. Clinical retrieval analyses of explanted devices.

6. Longitudinal clinical trials comparing VTDRs and ball-and-socket arthroplasties.

Conclusion

This week’s analysis highlights strong evidence that the AxioMed viscoelastic TDR model produces substantially lower wear rates and significantly larger, less reactive wear particles than traditional articulating disc replacement systems.

Combined with exceptional mechanical durability at extreme cycle counts, these findings support VTDR technology as a promising, biologically safer alternative for motion-preserving lumbar spine surgery.