The effects of needle puncture injury on microscale shear strain in the intervertebral disc annulus fibrosus

Abstract

Background context

Needle puncture of the intervertebral disc (IVD) is required for delivery of therapeutic agents to the nucleus pulposus and for some diagnostic procedures. Needle puncture has also been implicated as an initiator of disc degeneration. It is hypothesized that needle puncture may initiate IVD degeneration by altering microscale mechanical behavior in the annulus fibrosus (AF).

Purpose

Quantify the changes in AF microscale strain behavior resulting from puncture with a hypodermic needle.

Study design

Cadaveric IVD tissue explant study.

Methods

Annulus fibrosus explants from bovine caudal IVDs that had been punctured radially with hypodermic needles were loaded in dynamic sinusoidal shear while being imaged with a confocal microscope. Digital image analysis was used to quantify local tissue strain and damage propagation with repeated shearing.

Results

Needle puncture changed the distribution of microscale shear strains in the AF under load from homogenous (equal to far field) to a distinct pattern of high (4× far field) and low (0.25× far field) strain areas. Repeated loading did not cause further growth of the disruption beyond the second cycle.

Conclusions

Needle puncture results in a drastic alteration of microscale strain behavior in the AF under load. This alteration may directly initiate disc degeneration by being detrimental to tissue-cell mechanotransduction.

Introduction

The intervertebral disc (IVD) is the largest avascular organ in the body, making it susceptible to degeneration and slow to repair [1]. Needle injection into the IVD is commonly used in discography for diagnostic purposes [2], [3] and is important for therapeutic [4], [5], [6], [7], [8] procedures, including growth factor injection and cell therapies. However, there is somewhat of a paradox as needle punctures are also commonly used to induce degeneration in the IVD. In animal models, these injuries affect both annulus integrity and nucleus pressurization [9] and can result in an acute loss of disc height [4], [10], [11], [12], [13], axial stiffness [12], [14], [15], [16], and rupture pressure [17] as well as progressive structural changes consistent with degenerative disc disease [13], [18], [19], [20]. More recently, discography procedures performed on nondegenerative discs have been shown to increase the risk of later degeneration [21]. It has been suggested that small relative needle sizes (ie, needle diameter relative to total IVD height) will result in a negligible effect on IVD mechanics, whereas large relative needle sizes have greater effects [14]. To date, there has been no investigation into microscale mechanics surrounding needle punctures in the IVD, which is essential for developing techniques to minimize or repair these injuries.

There is a reason to believe that microscale structural disruption after a needle puncture of the annulus fibrosus (AF) plays a role in initiating the degenerative cascade. In bovine IVDs cultured under axial compression, localized cell death has been observed in the AF in the proximity of a needle puncture [12]. Classical elasticity theory proposes that if an object with a focal defect is placed under load, the material surrounding that defect will be subjected to local strains different from those far away from the defect [22]. Intervertebral disc cells are known to be metabolically sensitive to tissue strain conditions, with low levels promoting matrix protein production but high levels leading to apoptosis [23], [24], [25], [26], [27], [28]. Taken together, this suggests a scenario where altered strain patterns in the AF at the site of a needle puncture lead to structural disruption and altered cell metabolism or death.

It has been suggested that a primary cause of IVD degeneration is the accumulation of microfailure damage [29]. More recent research into interlamellar connectivity, however, indicates that the AF structure has complex interlamellar connectivity, making it particularly robust and likely effective at arresting the propagation of injuries under physiological loading [30], [31], [32]. Additionally, there is some indication that chemical cross-linking agents are able to restore mechanical function to damaged discs [33], [34], [35], although it is unclear whether these results reflect changes in all parts of the tissue [36] or a targeted repair. Furthermore, without a clear indication of how injury disrupts microscale fiber mechanics, it is difficult to design optimally effective repair techniques.

Knowledge of how the AF structure responds mechanically to injury at the microscopic level is essential to developing both effective repair strategies and less invasive diagnostic and therapeutic procedures. Based on our current understanding of AF tissue mechanics, we hypothesize that needle puncture will result in altered microscale shear strains under tissue loading; chemical cross-linking will inhibit some of this alteration; and a puncture injury will not propagate under physiological levels of applied shear strain. These hypotheses were tested using a combination of dynamic shear loading of punctured AF tissue explants, confocal microscopy, and image processing techniques, including Radon transform and feature tracking algorithms.