| Objective: To establish the animal model of spine shortening, and study the effects of spine shortening on the morphous, structure and function of spinal cord. To make contribution to the security of spine osteotomy by researching the safe limits of spine shortening in different spine segments.Materials and methods: The whole study included two parts. In part 1, 15 male rabbits were randomly divided into three groups according to the spine segments in which total vertebrae osteotomy were performed. Group A was in T10, Group B was in L1, and Group C was in L5 (n=5).We Shortened the spinal column gradually after osteotomy and observed the Morphometric changes of spinal cord and dural sac during the surgery. CSEP was used to monitor spinal cord function. The spinal cord was exposed after osteotomy. Then examine and record the CSEP and set it as the standard. When the amplitude of CSEP descended to 50%, we fixed the two broken ends of the spine and sutured the cut. MRI scan was taken to detect the Morphologic change of spinal cord after surgery. Then the spinal cord was extracted and cut into sections after embedded in paraffin for the test of histology with hematoxylin and eosin staining. The safe limits of spinal shortening were determined according to the three results above. In part 2, another 15 rabbits were randomly divided into three groups as parts 1 after spine MRI scan. We measured the spinal canal sagittal diameters on MRI images and performed the total vertebrae osteotomy in the same segments. The difference was this time we distracted the shortening extreme lengthes according to the results of the part 1. Spinal column was gradually shortened until the upper and the lower ends contacted with each other. Then we fixed the two ends with titanium screw and connecting plate to close the vertebral canal. Record the CSEP and compare its amplitude with the one before spinal shortening. Then give the MRI scan and take the histology assessment again.Results: During surgery with the shorten distance of spine growing, we observed that the amplitude of CSEP descended and the latent period prolonged. Morphometric changes occurred in the dural sac and the spinal cord. Dura buckled and spinal cord turned itself. The limits of spine shortening were different in different spine segments. The maximum was 9.8±1.17mm in L5, the minimum was 6.2±0.75mm in T10 and it was 7.6±0.8mm in L1, with statistics significance (P<0.01).When the spine was shortened to the limit, the amplitude of CSEP descended to 50% of its standard. On MRI image we could see dural sac lost its shape. The spinal cord kinked and was compressed. The sign of spinal cord was confusing. The boundary of grey matter and whiter matter was unclear. Histological results demonstrated that spinal cords were edematus. Neurons were swelled. Nerve fibers were unordered. Different spine segments had different spinal canal sagittal diameters with statistics significance (P<0.01). The number was 5.44±0.42mm in T10, 6.16±0.52mm in L1 and 7.48±0.46mm in L5.With the broken ends of spine fixed with titanium screw and connecting plate the SCEP amplitude descended much more, different segments had different change ratios comparing with the amplitude before spinal shortening. It was 1.34mv in T10 with the change ratio of 76.78±2.8%, 1.66mv with the change ratio of 71.22±2.8% in L1, and 2.24mv with the change ratio of 61.18±3.2% in L5, with statistics significance (P<0.01). After osteotomy the spinal cord was kinked, compressed by the buckled dura obviously and had higher sign on MRI image. Spinal cords were bleeding and necrosis observed by microscope. Neurons broken and nerve fibers fractured. The injury in T10 was the most clear.Conclusion: We have established an animal model of spinal shortening successfully with total vertebrae osteotomy and internal fixation. It imitates the clinical process well and is feasible and reasonable. Different spine segments have different shortening safe limits. Shortening the spine to its limits, the main injury of spinal cord is edema, and contusion sometimes. While shorten the same length and close the gap with internal fixation, the main injury will be hemorrhage and necrosis. Different spine segments have different shorten limits. The lumbar limit is the highest and the thoracic limit is the lowest. The SCEP amplitude will descend more with the spinal canal closed. The change ratio has negative correlation with the sagittal diameter of the corresponding vertebral canal. It is the smallest in lumbar spine which has the biggest sagittal diameter.
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