gms | German Medical Science

Objectives: Recently a tissue engineered intervertebral disc (IVD)-like construct has been evaluated in rat tail and beagle cervical spine models in vivo. While biological capabilities were encouraging, the biomechanical properties were inferior to native IVDs and implant displacement occurred occasionally. The aim of this study was to investigate the biomechanical responses of TE-IVDs combined with a resorbable stabilization system (BSS) to static compression in a canine ex vivo model.

Methods: Cervical IVDs from beagles were obtained and dissected into annulus fibrosus (AF) and nucleus pulposus (NP) tissues by macroscopic appearance. Then the tissues were digested in collagenase Type II at 37°C for 12 hours prior to being filtered and centrifuged. NP and AF cells were cultured individually to confluence. Alginate (3% weight/volume) was then seeded with the cultured NP cells (2.5 × 10⁶ cells/ml) and injected into predesigned molds. Each of the molded NPs was placed in a well of a culture plate as a neutralized collagen gel solution (4 mg/ml seeded with 3 × 10⁶ cells/ml) was pipetted around the NP. All components were gelled at 37°C and TE-IVDs were cultured for 4 weeks. Cervical spinal motion segments from beagles were dissected and the mechanical compatibility was assessed for each motion segment at different time points: (1) intact, (2) after discectomy, (3) after implantation of the TE-IVD and (4) after implantation of the TE-IVD and application of the BSS ventrally. Unconfined stress relaxation tests were performed up to 15% strain and their instantaneous and equilibrium modulus were calculated and normalized to their corresponding intact motion segment (N=12). One-way ANOVA and Tukey HSD were used to determine significant differences at p<0.05.

Results and Conclusion: Intact motion segments revealed an equilibrium and instantaneous modulus of 174 ± 36 kPa and 1760 ± 430 kPa, respectively. Mechanical properties from motion segments after discectomy (Dx), TE-IVD implantation without BSS (IVD -), and TE-IVD implantation with BSS (IVD +) were all significantly lower than the intact motion segments (p<0.05). Dx motion segments resulted in 14 ± 6% and 13 + 4% of the intact equilibrium and instantaneous modulus. Application of the BSS (IVD +) revealed a 2-fold increase in the equilibrium and instantaneous modulus (p<0.05) over the groups with TE-IVDs implantation only (IVD -).

The present study showed that a resorbable plate can be used in combination with a TE-IVD to stabilize a canine cervical motion segment, while allowing load sharing to the TE-IVD. Implantation of the TE-IVD resulted in similar mechanical properties to those of the discectomized segments suggesting low magnitudes of loads are shared by the construct. The significant increase in mechanical properties of the motion segment with a plate suggests that the BSS increases the stability of TE-IVD construct and helps reduce implant displacement outside of the disc space.