Understanding the Recovery of the Intervertebral Disc: A Comprehensive Review of In Vivo and In Vitro Studies

Within the consistent daily rhythm of human life, intervertebral discs endure a variety of complex loads beyond the influences of gravity and muscle forces, leading to significant morphological changes (in terms of volume, area, and height) as well as biomechanical alterations, including an increase in disc stiffness and a decrease in intradiscal pressure. Remarkably, the discs demonstrate an ability to regain their original morphological and biomechanical characteristics after a period of nocturnal rest. The preservation of normal disc function is critically dependent on this recovery phase, which serves to forestall premature disc degeneration. This phenomenon of disc recovery has been extensively documented through numerous in vivo studies employing advanced clinical techniques such as Magnetic Resonance Imaging (MRI), stadiometry, and intradiscal pressure measurement. However, the findings from in vitro studies present a more complex picture, with reports varying between full recovery and only partial recuperation of the disc properties. Moreover, research focusing on degenerated discs in vitro has shed light on the quantifiable impact of degeneration on the disc ability to recover. Fluid dynamics within the disc are considered a primary factor in recovery, yet the disc intricate multiscale structure and its viscoelastic properties also play key roles. These elements interact in complex ways to influence the recovery mechanism, particularly in relation to the overall health of the disc. The objective of this review is to collate, analyze, and critically evaluate the existing body of in vivo and in vitro research on this topic, providing a comprehensive understanding of disc recovery processes. Such understanding offers a blueprint for future advancements in medical treatments and bionic engineering solutions designed to mimic, support, and enhance the natural recovery processes of intervertebral discs.