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

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.