Development of a biomimetic artificial intervertebral disc

Replacing the intervertebral disc (IVD) by a total disc replacement (TDR) is a possible treatment for degenerative disc disease. Current TDRs are ball-and-socket designs, aiming at motion preservation. They provide reasonable clinical results, at least in the short-term, although concerns remain about changes in spinal motion, overloading of the facet joints, adjacent segment disease, and wear. In contrast to these ball-and-socket designs, the IVD is a complex structure, providing an inherent resistance to motion, resulting in a characteristic sigmoid moment-deflection curve in flexion-extension and lateral bending. New, second generation TDRs have been proposed, which deviate from the ball-and-socket design, and mimic some of the salient features of the natural disc. In this thesis, a new biomimetic artificial intervertebral disc (AID) is introduced, mimicking the fiber-reinforced, osmotic, visco-elastic, and deformation properties of the IVD. Its concept is based on the hypothesis that the better the material structure of the IVD is mimicked, the better its functionality is mimicked. Hence, the AID comprises a swelling, ionized, hydrogel core (the nucleus), and a surrounding fiber jacket (the annulus). A first prototype of the biomimetic AID was tested in-vitro in axial compression. The AID remained intact up to 15 kN in quasi-static compression and up to 10 million cycles of fatigue loading, which illustrated that the design is mechanically safe. It was also demonstrated that its axial deformation behavior was similar to that of a natural disc in creep and dynamic loading, although fatigue loading introduced some irreversible changes in behavior. These changes were mainly caused by the settling of the fiber jacket, and this effect should be taken into account in further development. The biomimetic design concept was compared to other TDR designs, using a finite element analysis. The theoretical ability to mimic the non-linear motion patterns of the natural IVD was determined for two elastomeric TDRs, an elastomeric TDR with a fiber jacket, and a TDR consisting of a hydrogel core and fiber jacket. The material properties of the different designs were optimized via a computer algorithm to match as closely as possible the natural disc behavior. It was shown that to mimic the non-linear relationship between moment and deflection, a fiber envelope was necessary. Furthermore, no differences were found between the design with an elastomer core and the design with a hydrogel core. Nevertheless, from the in-vitro creep experiments, the advantages of a hydrogel core over an elastomeric core are obvious. The hydrogel core provides osmotic, creep, and time-dependent behavior, characteristic for the IVD, and the possibility of insertion in a smaller dehydrated state, reducing the invasiveness of the surgery. The last part of this thesis focused on the fixation of the biomimetic design to the vertebrae. A finite element model of a spinal motion segment was developed based on a previous developed model, of which the IVD part was replaced by a model of the biomimetic design. The effect of different fixation methods on spinal behavior was determined. The model including the TDR resulted in similar ROM as the IVD model, and mimicked the non-linear in-vitro spinal behavior, which confirmed that the biomimetic concept is a suitable TDR concept. When bone ingrowth is used for fixation, incomplete bone ingrowth increased ROM and facet forces. When only the peripheral edge of the TDR was fixed to the vertebrae, spinal behavior was maintained, highlighting the vital role of fixation along the annular rim. Adding spikes for fixation improved spinal behavior, which could be considered a good short-term solution until bone ingrowth can occur for more optimal long-term performance. Alternatively, using rigid endplates also maintained spinal behavior. Concerns of correct load distribution favors a ring shaped endplate above a disc shaped one. In conclusion, a new biomimetic AID was proposed. The first prototype was shown to have ample strength and fatigue life, and it was demonstrated that it could mimic the axial creep and dynamic behavior of the IVD. Its motion in six degrees of freedom was simulated numerically and compared to other designs. The inclusion of a fiber jacket is a key factor in mimicking the characteristic sigmoid shape of moment-deflection curves. Fixation to the vertebrae was demonstrated to be a key issue to focus on in future research. Hence, finalizing the endplate design and fixation method, optimizing the properties of the AID, and standardizing the manufacturing procedure, should be followed up by six-degree of freedom testing in vitro. In parallel, animal experiments to test the fixation by bone ingrowth should be tested in vivo and in vitro.