Masseter muscle fibre changes following reduction of masticatory function

This study evaluated histological changes in masseter muscle fibres following reduced masticatory function by injection of botulinum toxin type A (BTX). Sixty 30-day-old Long-Evans male rats were randomly separated into four groups (15 per group): group I BTX masseter, 25 U/ml (0.04 ml each muscle) BTX was injected in bilateral masseter muscle whilst bilateral temporalis muscles received an equal amount of normal saline; group II BTX temporalis, 25 U/ml (0.04 ml each muscle) BTX was injected in bilateral temporalis muscle whilst bilateral masseter muscle received an equal amount of normal saline; group III BTX temporalis and masseter, bilateral temporalis and masseter were given 25 U/ml (0.04 ml each muscle) BTX; group IV normal saline (control), bilateral temporalis and masseter were given normal saline (0.04 ml each muscle). After 45 days, the rats were killed, the muscles dissected and mean muscle mass recorded. The superficial masseter muscles were immunohistochemically analysed. Fibre sizes in group III were bigger than those in other groups. There was a small percentage of type IIa fibres in group III. Reduction in muscle fibre size and transition of muscle fibre subtypes from type IIa to IIx or IIb fibres may occur due to reduced masticatory function.