Dispersion of Intradiscally Injected Particles is Limited by Size

Due to its avascular nature, treatment of degeneration of the intervertebral disc (IVD) is typically carried out via needle injection into the inner nucleus pulposus (NP) of the disc. Proposed treatments range from small peptides with nanometer scale diameters to hydrogel encapsulated stem cell constructs in the hundreds of nanometers. While low permeability of the NP has been implicated as a limiting factor in fluid injection capacity, the effect on suspended particles is largely unknown. This study tested the hypothesis that increasing particle size decreases intradiscal injectate dispersion by injecting saline carrying fluorescent markers representing small molecules (SM: 1nm diameter) along with either small beads (SB: 45-53µm diameter) or large beads (LB: 125-150µm diameter) into bovine caudal IVDs. Following injection, the discs were transected and imaged. While SM dispersion was consistent and covered a median of 26% of the NP area, bead behavior was varied. While discs in the SB group showed dispersion to a median of 3.8% of the NP area, LB beads were localized mostly to the needle track and covered only 0.66% of NP area. In 20% of the SB injected discs, and 30% of LB, no fluorescent beads were found in the NP. These results suggest that while small molecule treatments may be practically delivered to the NP by injection, larger (>50µm) particles are restricted by NP pore size and will remain localized to the track left by the delivery needle.