Mechanical Characterization of a Viscoelastic Disc for Lumbar Total Disc Replacement

Abstract

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.

Effect of Axial Load on the Flexural Properties of an Elastomeric Total Disc Replacement

“…It is not part of the routine pre-clinical testing of the device that would be required for regulatory approval. Although the aim is to compare the behaviour of the Cadisc™-L implant with that of the natural disc, it is also compared with published experiments on other metal-elastomeric implants that have been designed to address similar questions to those addressed here 6,12 .…”

“…They attempt to provide motion and material properties that resemble those of the natural disc, more closely than ball and socket designs [3][4][5][6] . Unlike previous articulating TDR implants, elastomeric devices provide the opportunity to restore axial stiffness and flexural stiffness characteristics [3][4][5][6] ; however, little is understood about how the flexural properties of these implants may change under different axial compressive loads. In the natural spine, there is an increase in flexural stiffness and reduction in the range of motion with increasing axial load, resulting in a more "stable" motion segment [7][8][9] .…”

“…The design of these implants is commonly based on a low friction ball and socket articulating against each other (metal-on-metal or metal-on-polymer). More recently, elastomeric TDR implants have been developed, that are intended to mimic the movement of the natural disc [3][4][5][6] . They attempt to provide motion and material properties that resemble those of the natural disc, more closely than ball and socket designs [3][4][5][6] .…”

“…More recently, elastomeric TDR implants have been developed, that are intended to mimic the movement of the natural disc [3][4][5][6] . They attempt to provide motion and material properties that resemble those of the natural disc, more closely than ball and socket designs [3][4][5][6] . Unlike previous articulating TDR implants, elastomeric devices provide the opportunity to restore axial stiffness and flexural stiffness characteristics [3][4][5][6] ; however, little is understood about how the flexural properties of these implants may change under different axial compressive loads.…”

“…The chemical composition of the polyurethane-polycarbonate polymer used to manufacture the device consists of methyl-bis-(4-cyclohextliscocyanate), polycarbonate diol and glycerol 5 . The design of the implant attempts to mimic the mechanical properties of the natural disc more closely [3][4][5][6] .…”