The influence of the donor nerve on the function and morphology of a mimic muscle after cross innervation: an experimental study in rabbits

In the present study we used the scutuloauricularis muscle in the rabbit to investigate the functional and morphometric alterations in the mimic-muscle system after cross-reinnervation. The scutuloauricularis muscle is the first experimental model that allows functional assessment of a mimic muscle by force measurements. A total of 36 rabbits were separated into three groups. In group 1 the scutuloauricularis nerve was cut and re-sutured to itself to achieve self-reinnervation; in group 2 the buccal nerve was used to cross-reinnervate the fast scutuloauricularis muscle and in group 3 the slow buccinator muscle was cross-reinnervated by the scutuloauricularis nerve. After a period of 6 months the maximal tetanic tensions of the reinnervated scutuloauricularis muscles were determined and histomorphometric examinations of muscle and nerve biopsies were carried out. Force measurements showed no loss of muscle force after self- and cross-reinnervation. The normal scutuloauricularis muscle contained 33%, and the buccinator muscle 46%, slow type I fibres. After self-reinnervation of the scutuloauricularis muscle the fibre-type composition remained unchanged. After cross-reinnervation we saw a significant fast-to-slow transformation of the scutuloauricularis muscle and a significant slow-to-fast transformation of the buccinator muscle. The number of myelinated nerve fibres in the scutuloauricularis nerve increased after cross-reinnervation from 1531 to 4077 (group 2) and to 3813 (group 3). The number of nerve fibres in the buccal nerve (3209) was unchanged after cross-reinnervation. The results of the present study might be relevant in the treatment of irreversible facial palsy by functional muscle transplantation and cross-face nerve grafting. The facial nerve branch used for cross-reinnervation seems to determine the functional outcome.