A biomimetic artificial intervertebral disc system composed of a cubic three-dimensional fabric

In the quest for clinically functional artificial intervertebral discs (AIDs), multidisciplinary technologies have been employed. Existing solid mobile AIDs essentially consist of the superposition of solid plates and core materials; however, it is thought that an ideal surgical AID technology has not yet been developed. To overcome the limitation of these existing AIDs, we developed a unique flexible AID disc system on the basis of our original biomimetic concept. The AID is composed of a cubic three-dimensional fabric (3DF) with a triaxial fiber alignment, which offers biomimetic long-term dynamic mechanical behavior along with durability.

This article substantiates the potential clinical use of the 3DF disc system that quite differs from existing ones.

We designed the lumbar and cervical 3DF discs that improved the structural weaknesses caused by the collagenous fiber alignment of biological intervertebral disc. Bioresorbable hydroxyapatite particles were deposited on the surface layer of the 3DF disc to promote new bony ingrowth and to ensure secure binding at the interface of the contacting vertebral bodies. A stand-alone system was devised for surgical reliability in terms of both positioning and fixation, allowing tight press fitting with the vertebral bodies. Bioactive and bioresorbable pins were penetrated through the 3DF disc body and projected from the surface to allow ideal insertion and fixation to the disc space, preserving the precise position during dynamical movement. In vitro endurance of the 3DF disc was examined under long-term alternating stresses, and the in vivo animal tests were conducted in the intervertebral lumbar discs at L5-L6 excised from baboons and replaced with the lumbar 3DF disc.

The static mechanical endurance was assessed through a creep test. In vitro endurance of the 3DF disc under repetitive stresses including axial compressing, flexion-extension, torsional twisting, and lateral bending were applied to the 3DF disc for a long-term for up to 105 million stresses, which is roughly equivalent to exposure of natural biological movement for more than 50 years. In the animal test, eight baboons were euthanized 6 months postoperatively. To their extracted spines, six pure moments (flexion and extension, left and right lateral bending, and left and right torsion) were applied vertically to the superior end of the specimen and then values of range of motions (ROMs) were calculated. Histological analyses were conducted on 12 reticuloendothelial and systemic tissues.

The 3DF disc retained its biomimetic "J-shaped" stress-strain behavior without generating wear debris for up to 105 million stresses. A 130-N loading for the creep test decreased the height of 0.3mm during 80 to 1,000 hours. In the biomechanical test, ROM values of axial rotation and flexion-extension showed no significant difference from the intact excluding that of lateral bending because the location of each pin to stand alone certainly controlled the bending behavior only. The histological analysis indicated no significant pathologic changes induced by the 3DF disc.

The 3DF disc system is clinically suitable for human disc replacement arthroplasty based on the findings of long-term durability with dynamic motion in vitro and effective animal tests in vivo. This system surely overcomes the limitations of existing solid AIDs, and the clinical potential of the biomimetic 3DF discs has been verified. This new biomaterial technology delivers most of the functions and characteristics required by a clinically available AID if applied correctly by surgeons.