Anomalous fiber realignment during tensile loading of the rat facet capsular ligament identifies mechanically induced damage and physiological dysfunction

Many pathophysiological phenomena are associated with soft tissue loading that does not produce visible damage or tissue failure. As such, there is an unexplained disconnect between tissue injury and detectable structural damage during loading. This study investigated the collagen fiber kinematics of the rat facet capsular ligament to identify the onset of subfailure damage during tensile loading conditions that are known to induce pain. Quantitative polarized light imaging was used to determine the collagen fiber orientation in the capsular ligament (n=7) under tension, and an alignment vector correlation measurement was employed to identify local anomalous fiber realignment during loading. During the initial portion of loading when tissue stiffness was increasing, anomalous realignment was more likely to be detected than mechanical evidence of structural damage, and as a result, anomalous fiber realignment was identified significantly (p=0.004) before gross failure. The occurrence of anomalous fiber realignment was significantly associated (p=0.013) with a decrease in tangent stiffness during loading (ligament yield), suggesting this optical metric may be associated with a loss of structural integrity. The presence of localized anomalous realignment during subfailure loading in this tissue may explain the development of laxity, collagen fiber disorganization, and persistent pain previously reported for facet joint distractions comparable to that required for anomalous realignment. These optical data, together with the literature, suggest that mechanically induced tissue damage may occur in the absence of any macroscopic or mechanical evidence of failure and may produce local pathology and pain.