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Spinal orthoses have been traditionally used in the management of thoracolumbar injuries treated with or without surgical stabilization. However, the orthotic treatment modality in the management of spinal fractures remains subjective since few objective data are available on the effectiveness of orthoses in stabilizing injured segments. This study used a finite element model of the spine to evaluate the effectiveness of a hyperextension orthosis in controlling the progression of deformities at the injury site under gravitational and flexion loads. Two types of injuries were simulated: a single-level injury at T12-L1 and a two-level injury at T11-T12-L1 segments. An injury of increasing severity was simulated by progressively reducing the bending stiffness of the affected segments relative to the normal values in flexion-extension mode. The interaction of a three-point hyperextension orthosis with the spine was simulated using experimentally measured stiffness properties of the orthosis. The authors' results suggest that in single-level injuries that cause up to 50% loss of segmental stiffness, the orthosis can restore normal resistance to deformity at the injured segments, under gravitational as well as large flexion loads. In loss of stiffness between 50% and 85% of normal, such as severe two-column disruptions, the orthosis can restore resistance to deformity under restricted patient activity level in the brace (low-flexion moment). Beyond 85% loss of segmental stiffness, such as three-column injuries, the orthosis alone appears to be ineffective in preventing progression of deformity.(ABSTRACT TRUNCATED AT 250 WORDS)