| Background and objective The aim of this project is to accomplish the quantitative evaluation of rabbit sciatic nerve constriction injury by diffusion tensor imaging. With the description of the FA-time curve, ADC-time curve, AD-time curve and RD-time curve and the comparison between the DTI parameters and pathologic and behavioral outcomes at specific time, we aim to reveal the dynamic pattern of the DTI parameters of chronic constriction injury, and to explore the clinical value and pathologic basis of the changes of the DTI parameters, and to establish the experimental base for the clinical use of DTI in nerve constriction syndrome.Material and method 20 Japanese white rabbits were selected, whose right sciatic nerves were chosen to have chronic constriction injury with their counterparts sham operation. All the rabbits were randomly divided into two groups. Group A consisted of 6 rabbits with their MR scanning results(including PDWI, T2WI/STIR, DTI) and behavior assessment(including toe reflex, external force confrontation, puncture reaction, gait) recorded at multiple time points including: before the injury and at 3 days, 1, 2, 4, 6, 8, 10 weeks post-injury. The DTI raw data were post-processed for DTI parameters(including FA, ADC, AD and RD), FA-map and DTT. The patterns of changes of DTI parameters and behavior assessments at different time points were statistically analyzed and the features of the injury on T2WI/STIR, PDWI, FA-map and DTT were observationally recorded. Group B consisted of 14 rabbits. Two rabbits were randomly sacrificed for nerve biopsy at every post-injury time point, and both injured nerves and counterparts were taken sample. HE stain, LFB stain and Masson stain were used to evaluate the pathologic feature of the injured nerve.Result DTT: 3 days after constriction, the continuity of the constricted segment was broken down. 1 week after injury, the broken ends of each side got closer to each other. 2-10 weeks after injury, the nerve fibers of injured segment were almost integrate as normal. FA value: 3 days after injury, the FA value of constricted nerve reduced to the minimum(0.316±0.044). 1-10 weeks after injury, the FA value gradually recovered(the values were 0.331±0.018, 0.354±0.044, 0.375±0.015, 0.394±0.020, 0.42±0.03 and 0.464±0.039 respectively). There was significant difference between these values and those of baseline and sham operated side at every time point. ADC value: 3 days after injury, the ADC value of constricted nerve increased to the maximum(1.502±0.126). 1-2 weeks after injury, the ADC values of constricted nerve were 1.462±0.058 and 1.473±0.124 respectively. There was significant difference between these values and those of sham operated side at specific time. 4 weeks after injury, the ADC value decreased to 1.356±0.129, and the ADC values were 1.375±0.107, 1.290±0.064 and 1.298±0.026 at 6-10 weeks respectively. There was no significant difference between the values and those of counterparts. AD value: 3 days after injury, the AD value of constricted nerve decreased to 2.042±0.16. 1-10 weeks after injury, the AD values were 1.19±0.06, 1.181±0.14, 1.071±0.102, 1.068±0.084, 0.961±0.063 and 0.923±0.058 respectively. There was significant difference between these values and those of the sham operated sides at every time points. RD value: 3 days after injury, the RD value of constricted nerve increased to the maximum(1.233±0.152). 1-10 weeks after injury, the RD value gradually decreased(the values were 1.19±0.06, 1.181±0.14, 1.071±0.102, 1.068±0.084, 0.961±0.063 and 0.923±0.058 respectively). There was significant difference between these values and those of the sham operated sides. Limb functional score: 3 days after injury, the score decreased to the minimum(28).1-10 weeks after injury, the limb function of the injured side gradually recovered, the score were 30, 38, 56, 70, 75 and 79 respectively. There was significant correlation between FA value, ADC value, RD value and limb functional score(R=0.805, P<0.001; R=-0.55, P<0.001; R=-0.744, P<0.001). There was no significant correlation between AD value and the score(R=0.281, P=0.053). Pathological changes: The pathological assessment of the injured nerve was based on three kinds of stain including HE stain, LFB stain and Masson stain. During the 10 weeks’ observation, the main pathological presentation of injured nerve were as follows. The changes of myelin sheath included the swelling and loosing of structure, breaking down and demyelination, regeneration and maturing. The main changes of nerve fiber included the transformation from myelinated fiber to swollen fiber and the replacement of swollen fiber by regenerated thinly myelinated fiber. Besides, there were infiltration of acute inflammatory cells and tissue edema at early phase and infiltration of chronic inflammatory cells and epineural fibrosis and endoneural fibrosis at chronic phase.Conclusion 1. The changes of DTI parameters(including FA、ADC、AD and RD) reflect the feature of the natural progression of chronic constriction injury of sciatic nerve. This research offers powerful support for the application of DTI in the diagnosis and monitoring of nerve compression. 2. FA, ADC and RD have significant correlation with the limb function score, which can reflect the changes of limb function. So these parameters are proper to be used to monitor the degeneration and regeneration of peripheral nerve compression. AD significantly decrease after nerve compression and keep stable during the observation. So this parameter may be more proper to be used as a diagnosing indicator. 3. The pathological basis of the changes of DTI parameters in chronic constriction injury model includes demyelination and re-myelination, transformation of nerve fiber, infiltration of inflammatory cells, tissue edema and neural fibrosis. 4. DTT presents the breaking down of nerve tract in the early phase, which may provide some auxiliary diagnosing information. But this presentation mainly attributes to tissue edema rather than axon loss. |
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