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Introduction: The aetiology of scoliosis is not known. Many different mechanisms have been suggested as playing a part in the development. Dysfunction of the segmental paravertebral muscles have been suggested to have some impact on the condition. It is known that the injection of botulinum toxin type B will paralyse muscles by blocking of the motor endplate. The effect has been shown to last up to three months. The experiment was designed to study if segmental muscles in the thoracic region of the spine in pigs play a role in the development of the spine.

Materials and Methods: Six seven days old piglets were used in the experiment. In the lower thoracic region in three levels on the left side the paraspinal muscles were infiltrated with botulinum toxin type B. It was used 10 units of Botox® on each level, a total of 30 units were used on each animal. It was taken care to infiltrate the different small muscles as the toxin does not spread readily to adjacent muscles. The pigs were then left for normal care and development. They all were assessed at four weeks intervals until they were sacrificed three months after initial injection. x-ray were then taken of the spine.

Results: During the follow-up there were no visible changes in the alignment of the spine. The piglets developed normally. On x-ray there were no signs of developmental disturbances and we did not see any signs of scoliotic development. If anything, there was the development of a long curvature in the thoracic spine. On examination there was clear atrophy of the segmental muscles in the injected regions.

Discussion: This experiment suggests that the development of the spine is not guided by either the presence or absence of muscle activation. The dose of Botox® applied to these small muscles should be more than adequate to stop nearly all muscle activity. The pig has a rapid growth period from seven days to three months. Any changes caused by muscle activation should have been detected in this period. It could be that the effect on fast growing animals is shorter than three months. Nevertheless we still saw muscle atrophy at time of sacrifice.

Correspondence should be addressed to Jeremy C T Fairbank at The Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX7 7LD, UK