Metastatic disease commonly occurs in the spine and incidence is likely to increase secondary to improved survival rates in many cancer patients. Despite published research on instability in patients with metastatic disease of the thoracolumbar spine, controversy exists regarding risk factors for instability and indications for surgical stabilization. The objective of this systematic review was to determine what defines instability and impending instability in patients with metastatic disease of the thoracic and lumbar spine. We systematically reviewed the medical literature in order to identify all the relevant studies concerning patients with metastatic involvement of T1-L5, in the domains of biomechanics, epidemiology, clinical issues, and radiographic parameters. Two independent observers performed study selection, methodological quality assessment, and data extraction in a blinded and objective manner for all the identified studies. We were then able to define the criteria to identify instability of the spine with metastases. A literature search and review identified 14 relevant, good quality studies for inclusion. The predictors of instability included increased tumor size, a larger cross-sectional area of bone defect, increased force of spinal loading, decreased bone density, posterior location of the tumor within the vertebrae, destruction of the costovertebral joint, pedicle destruction in the thoracolumbar spine, increased axial rigidity, and sagittal spinal deformity. Definitive conclusions cannot be reached due to lack of evidence. However, variables such as tumor size, magnitude of spinal loading, bone density, tumor location within the vertebrae and spine, and tumor type are risk factors for instability in spinal metastases. Improved clinical research methodology for this patient population is required.

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