ASTM biomechanical study comparing the AxioMed lumbar viscoelastic disc to human lumbar disc data.

BACKGROUND

Artificial disc replacements aim to preserve motion in patients with lumbar disc degeneration, but most do not replicate the natural stiffness of the healthy human lumbar disc. Existing ball-and-socket designs often permit excessive motion and fail to provide the nonlinear, load-dependent stiffness that characterize native spinal biomechanics. To date, no in vitro study has directly compared the stiffness of a viscoelastic total disc replacement (VTDR) to that of the natural lumbar disc under physiologic conditions.

METHODS

Ten AxioMed® lumbar VTDRs were tested using standardized ASTM protocols in a physiologic environment (PBS at 37 ± 3 °C). Axial compression, flexion-extension, axial rotation and compressive shear stiffness were measured using servohydraulic test systems. An additional five implants underwent static axial loading up to 20,000 N. All values were compared to published stiffness ranges for the healthy human lumbar disc.

FINDINGS: Axial stiffness ranged from 2.56 to 3.48 kN/mm, overlapping the reported native range of 0.5 to 2.5 kN/mm. Flexion-extension stiffness (1.69-2.14 Nm/deg) matched the physiologic range (0.8-2.5 Nm/deg). Rotation stiffness (0.79-0.83 Nm/deg) was lower than native values (2.0-9.6 Nm/deg), resulting in greater rotational mobility. Compressive shear stiffness (0.49-0.59 kN/mm) fell within the native lumbar disc range (0.4-0.7 kN/mm). All implants withstood static compression to 20,000 N without structural failure.

INTERPRETATION: These findings show that the AxioMed® VTDR reproduces lumbar disc stiffness more closely than prior designs. The ability to replicate both compliant and stiff loading zones suggests improved biomechanical performance and segmental stability, supporting its use as a potential alternative to spinal fusion.