The Size of Simulated Lytic Metastases Affects the Strain Distribution on the Anterior Surface of the Vertebra

Metastatic lesions of the vertebra are associated with risk of fracture, which can be disabling and life-threatening. In the literature, attempts are found to identify the parameters that reduce the strength of a metastatic vertebra leading to spine instability. However, a number of controversial issues remain. Our aim was to quantify how the strain distribution in the vertebral body is affected by the presence and by the size of a simulated metastatic defect. Five cadaveric thoracic spine segments were subjected to non-destructive presso-flexion while intact, and after simulation of metastases of increasing size. For the largest defect, the specimens were eventually tested to failure. The full-field strain distribution in the elastic range was measured with digital image correlation (DIC) on the anterior surface of the vertebral body. The mean strain in the vertebra remained similar to the intact when the defects were smaller than 30% of the vertebral volume. The mean strains became significantly larger than in the intact for larger defects. The map of strain and its statistical distribution indicated a rather uniform condition in the intact vertebra and with defects smaller than 30%. Conversely, the strain distribution became significantly different from the intact for defects larger than 30%. A strain peak appeared in the region of the simulated metastasis, where fracture initiated during the final destructive test. This is a first step in understanding how the features of metastasis influence the vertebral strain and for the construction of a mechanistic model to predicted fracture.