Needle puncture injury of the rat intervertebral disc affects torsional and compressive biomechanics differently

Abstract

Needle puncture is a common method of inducing intervertebral disc (IVD) degeneration in small animal models and may have some similarities to IVD injury conditions such as herniation. Yet, the influence of puncture injuries on IVD biomechanics is not well understood. This study quantified the acute effects of anular injury on the biomechanics of rat caudal IVDs in compression and torsion following puncture with 30, 25 and 21 G needles. In compression, puncture injury reduced elastic stiffness by 20% for all needle sizes, but differences between control and punctured discs did not remain after compressive overload. In contrast, torsional parameters associated with anular fiber tension were affected proportionally with needle size. We conclude that IVD injuries that penetrate through the thickness of the annulus affect IVD biomechanics through different mechanisms for compression and torsion. Anular injuries affect torsional properties in a manner directly related to the amount of fiber disruption and compressive properties in a manner that affects pressurization.

Fig. 1

Left original and modified hypodermic needles. Needles were modified to the shape of a biopsy so that AF penetrating needle injury in the small rat IVD (~2 mm radius) was constrained to a single hole that did not puncture both sides of the AF. Right representative polarized light micrographs of transversely cryosectioned discs of control (a), and specimens with needle injury with 30 G (b), 25 G (c) and 21 G (d). For needle injury groups, arrows indicate injury location

Fig. 2

Schematic representations of loading protocols for compressive test (a), consisting of pre-load (PRE), pre-puncture dynamic test (T1), puncture dwell (DW), post-puncture dynamic test (T2), compressive overload (OL) and post-overload test (T3). Torsional tests (b) consisted of compressive pre-load (PRE), pre-puncture dynamic test (T1), puncture dwell (DW), post-puncture dynamic test (T2) and quasi-static ramp to failure (QS)

Fig. 3

Dynamic compressive tests were used to calculate storage (K′) and loss (K″) stiffnesses (a). Dynamic torsion tests (b) yielded linear region stiffness (K _LR), neutral zone length (L _NZ) and hysteresis area (A _H). Quasi-static stiffness (K _QS), torque and rotation at micro-failure (τ _MF and θ _MF), and torque and rotation at ultimate failure (τ _U and θ _U) were calculated from the ramp to failure (c)

Fig. 4

Rates of micro-failure events by group during quasi-static torsion. Each specimen was classified as experiencing zero, one or multiple micro-failure events

Fig. 5

Torsional failure via rim separation in control (left) and 25 G (right) specimens. Black arrows indicate point of failure and white arrow indicates puncture site