Creation and analysis of the animal models with chronic compression myelopathy.

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To determine the functional changes occurring in the early period after the development of chronic cord compression, we created an ossification of the yellow ligament(OYL)model, which is a model of chronic cord compression, in domestic rabbits using bone morphogenetic protein(BHP), and monitored evoked spinal cord potentials(ESCP). Sixty-seven rabbits were used for these experiments. Myelography, ESCP measurements, and histological examinations were performed in 16 of the OYL and 6 of the healthy control rabbits.At an average of 40.6 weeks after creitio, of the OYL model, a mean cord compression ratio of 15.3% was found. When the sciatic nerve was stimulated, no significant differences were found in the latency of the ESCP first wave between the OYL and control rabbits, whereas with stimulation delivered to the motor area the first wave latency differed significantly(P<0.005)between the 2 groups when the cord compression ratio in the OYL rabbits was *10%. Histological changes consisting primarily of a reduction in the number of large fibers in the dorsal column were found in 3 of the 4 OYL rabbits showing a cord compression ratio of *20%.Our results demonstrate that(1)mild degrees of cord compression produce greater ESCP alterations in chronic than in acute cord compression, (2)chronic cord compression from the dorsal side produces injury in the superficial layer of the dorsal column and dorso-lateral column, and(3)changes in ESCP precede the development of paralysis and histological changes.