Strain distribution on metastatic spine segments before and after intervertebral disc degeneration

Intervertebral disc degeneration causes anatomical and mechanical changes to the spine, leading to increased flexibility and consequent instability. Simultaneously, the presence of a metastatic vertebrae also poses challenges to spinal stability. Analyzing how the coexistence of these issues might prevent patients from undergoing therapies to address disc degeneration, which could partially alleviate the instability problem, is essential. The aim of this study was to evaluate the superficial strain field in spinal segments affected by metastasis before and after disc degeneration to assess the impact of disc degeneration. Three-dimensional surface strain distributions were measured and analyzed using Digital Image Correlation (DIC). Three human spine segments consisting of one vertebra with metastases, one control, and other two vertebrae embedded at the extremities were prepared. Specimens were tested in three different loading conditions (pure compression, flexion, and torsion), before (intact) and after (degenerated) enzymatic disc degeneration. Mechanical tests were performed using a spine simulator equipped with 6-degree freedom load cell in displacement control. The strain filed was measured with the DIC in the intact and degenerated condition. After disc degeneration, the stiffness of the entire spine segment decreased, resulting in greater flexibility of the structure. Both major and minor principal strains in the intervertebral discs tended to increase under all three loading conditions, while in vertebrae there was not a uniform trend. This resulted to be consistent with the stiffness results, highlighting the higher mobility of the spinal segment after disc degeneration. This study is as a first attempt to analyze how the effects of disc degeneration interact with those of metastatic vertebrae and can completely alter the outcomes on degenerated metastatic spines.