Ex vivo organ model and sample holder for intervertebral disc studies in a new generation of multiaxial bioreactors

A novel ex vivo intervertebral disc (IVD) organ model and corresponding sample holder were developed according to the requirements for six degrees of freedom loading and sterile culture in a new generation of multiaxial bioreactors. We tested if the model can be maintained in long-term IVD organ culture and validated the mechanical resistance of the IVD holder in compression, tension, torsion, and bending. An ex vivo bovine caudal IVD organ model was adapted by retaining 5-6 mm of vertebral bone to machine a central cross and a hole for nutrient access through the cartilaginous endplate. A counter cross was made on a customized, circular IVD holder. The new model was compared to a standard model with a minimum of bone for the cell viability and height changes after 3 weeks of cyclic compressive uniaxial loading (0.02-0.2 MPa, 0.2 Hz, 2h/ day; n= 3 for day 0, n= 2 for week 1, 2, and 3 endpoints). Mechanical tests were conducted on the assembly of IVD and holder enhanced with different combinations of side screws, top screws, and bone adhesive (n=3 for each test). The new model retained a high level of cell viability after three weeks of in vitro culture (outer annulus fibrosus 82%, inner annulus fibrosus 69%, nucleus pulposus 75%) and maintained the typical values of IVD height reduction after loading (≤ 10%). The holder-IVD interface reached the following highest average values in the tested configurations: 320.37 N in compression, 431.86 N in tension, 1.64 Nm in torsion, and 0.79 Nm in bending. The new IVD organ model can be maintained in long-term culture and when combined with the corresponding holder resists sufficient loads to study IVD degeneration and therapies in a new generation of multiaxial bioreactors.