A viscoelastic artificial disc may more closely replicate normal stiffness characteristics of the healthy human disc compared with first-generation total disc replacement (TDR) devices, which do not utilize viscoelastic materials and are based on a ball and socket design that does not allow loading compliance. Mechanical testing was performed to characterize the durability and range of motion (ROM) of an investigational viscoelastic TDR (VTDR) device for the lumbar spine, the Freedom® Lumbar Disc. ROM data were compared with data reported for the human lumbar disc in the clinical literature. Flexibility and stiffness of the VTDR in compression, rotation, and flexion/extension were within the parameters associated with the normal human lumbar disc. The device constrained motion to physiologic ranges and replicated normal stress/strain dynamics. No mechanical or functional failures occurred within the loads and ROM experienced by the human disc. Fatigue testing of the worst case VTDR device size demonstrated a fatigue life of 50 years of simulated walking and 240 years of simulated significant bends in both flexion/extension and lateral bending coupled with axial rotation, with no functional failures. These results indicate that the VTDR evaluated in this mechanical study is durable and has the ability to replicate the stiffness and mechanics of the natural, healthy human lumbar disc.

The FCD performs as expected in patients with single-level and two-level degenerative disc disease.

BackgroundThe purpose of this study is to describe the mechanical durability and the clinical and radiographic outcomes of a viscoelastic total disc replacement (VTDR). The human intervertebral disc is a complex, viscoelastic structure, permitting and constraining motion in 3 axes, thus providing stability. The ideal disc replacement should be viscoelastic and deformable in all directions, and it should restore disc height and angle.MethodsMechanical testing was conducted to validate the durability of the VTDR, and a clinical study was conducted to evaluate safety and performance. Fifty patients with single-level, symptomatic lumbar degenerative disc disease at L4-5 or L5-S1 were enrolled in a clinical trial at 3 European sites. Patients were assessed clinically and radiographically for 2 years by the Oswestry Disability Index (ODI), a visual analog scale (VAS), and independent radiographic analyses.ResultsThe VTDR showed a fatigue life in excess of 50 million cycles (50-year equivalent) and a physiologically appropriate level of stiffness, motion, geometry, and viscoelasticity. We enrolled 28 men and 22 women in the clinical study, with a mean age of 40 years. Independent quantitative radiographic assessment indicated that the VTDR restored and maintained disc height and lordosis while providing physiologic motion. Mean ODI scores decreased from 48% preoperatively to 23% at 2 years’ follow-up. Mean VAS low-back pain scores decreased from 7.1 cm to 2.9 cm. Median scores indicated that half of the patient population had ODI scores below 10% and VAS low-back pain scores below 0.95 cm at 2 years.ConclusionsThe VTDR has excellent durability and performs clinically and radiographically as intended for the treatment of symptomatic lumbar degenerative disc disease.Clinical RelevanceThe VTDR is intended to restore healthy anatomic properties and stability characteristics to the spinal segment. This study is the first to evaluate a VTDR in a 50-patient, multicenter European study.

Background Lumbar disc degeneration (LDD) is one of the most frequently diagnosed spinal diseases. The symptoms these disorders cause are anticipated to increase as the population in Western countries ages.

The purpose of this study was to determine the highest appropriate test frequency for a viscoelastic total disc replacement (VTDR). Natural intervertebral discs display viscoelastic behavior. Viscoelasticity is the time-dependent property of a material to show sensitivity to the rate of loading or deformation, having stress and strain reactions that are out of phase. If frequency is too high during mechanical testing of a viscoelastic polymer or medical device, the specimen is unable to recover fully before the next load application. Polymers absorb energy with each cycle. Since work (or energy utilized) is defined as the area under the force–displacement curve [Giordano, N. J., College Physics: Reasoning and Relationships, Brooks/Cole Publishing, Pacific Grove, CA, 2010], a frequency increase which decreases displacement will by definition also decrease the energy the polymer is using to achieve that decreased displacement. By reducing both total displacement and energy, a high test frequency would “protect” a viscoelastic device. A frequency of 2 Hz was used to determine the expected response of the VTDR during axial compression testing between 400 and 4000N. The response was defined as mean peak-to-peak displacement of five test cycles after 1000 cycles of preconditioning. Comparative data was collected at test frequencies of 3, 6, and 10 Hz. Displacement and energy utilized decreased with increasing test frequency. There were no significant differences between the viscoelastic responses in tests at 2 and 3 Hz. However, there were significant decreases in displacement and energy utilized at 6 and 10 Hz compared to 2 Hz. Over a 10 × 106 million cycle fatigue test, for this device, the total displacement would be 548 000mm less at 6 Hz and 988 000mm less at 10 Hz compared to 2 Hz. By decreasing the displacement, by definition it decreases the amount of overall work the disc has done when tested at these high frequencies [Giordano, N. J., College Physics: Reasoning and Relationships, Brooks/Cole Publishing, Pacific Grove, CA, 2010]. Viscoelastic devices should not be tested at high frequencies which “protect” the device by reducing the energy the device has to use overall by decreasing the total displacement it sees. To accurately evaluate in vivo behavior, fatigue testing should utilize test frequencies which do not significantly change the device’s viscoelastic response from that experienced at a physiologic loading frequency.