Study design: Experimental dog model of spinal cord shortening. Objectives: To clarify the relationship between the amount of shortening of the spinal cord and the degree of injury it may induce, and to determine the safe range of the shortening. Setting: Xi'an Jiaotong University, China. Methods: Thirty adult dogs were randomly allocated to five groups. Dogs in Group A (sham operation control) underwent spondylectomy to have two-thirds of the thirteenth thoracic segment (T13) resected, without bone-to-bone contact of the adjacent vertebral bodies. Those in Group B, C, D and E had one-third, half, two-thirds and total of their T13 resected, respectively, with boneto-bone contact. Somatosensory-evoked potentials (SEP) and spinal cord blood flow (SCBF) were detected. The histopathologic changes of spinal cord tissue were observed by hematoxylin and eosin stain and electron microscope. Results: The shortening of the spinal cord o half of a vertebral segment height caused a reversible change of SEP. Whereas, the changes resulted from the shortening of more than two-thirds of a vertebral segment height did not return to the normal level. SCBF increased temporarily when the shortening was within two-thirds of a vertebral segment height; whereas, it decreased progressively when the length of the shortening was equal to one vertebral segment height. More serious hemorrhage occurred as the shortening increased. INTRODUCTIONVertebrectomy is commonly performed in treating spinal deformity, tumors of the spinal column and other abnormal spinal conditions. [1][2][3] In this process, spinal shortening is a basic and commonly performed operation for the correction of spinal deformity and the fusion of osteotomy surface. 2,4,5 However, there exists the risk of neurologic complications induced by spinal cord injury (SCI) due to overshortened spinal cord. 2 These complications can be catastrophic for the patients and are very costly. Therefore, it is of great importance to clarify the pathophysiological changes of spinal cord shortening and determine the safe range of the shortening so as to prevent SCI.However, the safe range of shortening of the spinal column is still in debate. Two previous studies recognized the association between SCI and the range of spinal column shortening, but reached different conclusions in the safe range. 6,7 It seems that further research is necessary to confirm the safe shortening range. Besides, we have found no report about the serial changes of somatosensory-evoked potentials (SEP) and spinal cord blood flow (SCBF) over time with different amount of spinal shortening. The present study was therefore conducted, in which a modified animal model was used and detailed variations of SCBF and SEP were quantitatively recorded over a longer period of time at each stage of shortening. The obtained