Are head injuries in MMA common?

When knockouts and technical knockouts are combined in mixed martial arts (MMA), match-ending head trauma occurs in 31.9% of matches. 90% of technical knockouts are the result of repetitive strikes to the head, often with the losing competitor sustaining a flurry of blows to the head that increase in frequency. The incidence of head trauma in MMA (15.9 per 100 athlete-exposures) is higher than the reported incidence of concussions in professional football (8.08 per 100 game positions), boxing (4.9 per 100 AEs), and ice hockey (2.2 per 100 AEs).

Source: American Journal of Sports Medicine, Published online before print March 21, 2014