Effects of Level, Loading Rate, Injury and Repair on Biomechanical Response of Ovine Cervical Intervertebral Discs

Abstract

A need exists for pre-clinical large animal models of the spine to translate biomaterials capable of repairing intervertebral disc (IVD) defects. This study characterized the effects of cervical spinal level, loading rate, injury and repair with genipin-crosslinked fibrin (FibGen) on axial and torsional mechanics in an ovine cervical spine model. Cervical IVDs C2-C7 from nine animals were tested with cyclic tension-compression (- 240 to 100 N) and cyclic torsion (± 2° and ± 4°) tests at three rates (0.1, 1 and 2 Hz) in intact, injured and repaired conditions. Intact IVDs from upper cervical levels (C2-C4) had significantly higher torque range and torsional stiffness and significantly lower axial range of motion (ROM) and tensile compliance than IVDs from lower cervical levels (C5-C7). A tenfold increase in loading rate significantly increased torque range and torsional stiffness 4-8% (depending on amplitude) (p < 0.001). When normalized to intact, FibGen significantly restored torque range (FibGen: 0.96 ± 0.14, Injury: 0.88 ± 0.14, p = 0.03) and axial ROM (FibGen: 1.00 ± 0.05, Injury: 1.04 ± 0.15, p = 0.02) compared to Injury, with a values of 1 indicating full repair. Cervical spinal level must be considered for controlling biomechanical evaluations, and FibGen restored some torsional and axial biomechanical properties to intact levels.

Morphological and biomechanical effects of annulus fibrosus injury and repair in an ovine cervical model

“…A repeated measures study design found a 5 to 10% reduction in axial range of motion and torque range from an identical injury on ovine cervical samples . Here, we observed torque range values for Injured and FibGen samples which were beyond the range of similarly aged level matched intact samples, suggesting the high interspecimen variability seen in this model may be due to fibrotic healing or boney changes occurring in vivo. Interestingly, the Wilke group observed decreased range of motion from nucleotomy after 12 weeks in vivo in contrast to increased range of motion seen in vitro, suggesting an in vivo stiffening process, such as the irregular bone formation seen here .…”

“…Relatively high variance in the biomechanical data could be attributed to vertebral body morphology changes and osteophyte formation. A repeated measures study design found a 5 to 10% reduction in axial range of motion and torque range from an identical injury on ovine cervical samples . Here, we observed torque range values for Injured and FibGen samples which were beyond the range of similarly aged level matched intact samples, suggesting the high interspecimen variability seen in this model may be due to fibrotic healing or boney changes occurring in vivo.…”

“…No differences were detected for torsional or axial biomechanical behavior of FibGen and Injured IVDs, as seen in representative torque‐rotation and force‐displacement curves from Injured (level C2‐C3) and FibGen (level C3‐C4) treated IVDs (Figure A,F). Previous results suggest intact IVDs from C2 to C3 and C3 to C4 have comparable mechanical behavior . There were no detected differences in torque range (Figure B,D), neutral zone length (Figure C,E), axial range of motion (Figure G), axial neutral zone length (Figure H), torsion stiffness nor axial compliance (Table ).…”

“…For the surgery, the sheep were placed in dorsal recumbency and, under aseptic technique, the cervical levels were exposed through a ventral approach from the right side . A longitudinal incision and blunt preparation between the sternocleidoideus and longus colli muscles/carotid sheath (lateral) and the trachea (medial) exposed the anterior aspect of the IVD (C2/3‐C6/C7).…”

“…The biopsy punch injury was used because it is repeatable, clinically relevant and has precedent in the literature. 31,32 IVDs in the treatment groups were repaired by either injection of FibGen (n = 4), injection of FibGen and suture of PU membrane onto the defect (n = 4), or implantation of PTMC scaffold with sealing by FibGen and suturing of PU membrane (n = 4) ( Figure 1G). Treatments were rotated systematically by level (C2-C5) to avoid potential level bias.…”