Adding to this site’s archives canvassing safety studies in combat sports, a study was recently published in The Clinical Journal of Sports Medicine comparing boxing and MMA injury rates over a 13 year period.
The study, titled Combative Sports Injuries: An Edmonton Retrospective, reviewed post fight medical exams for combat sports bouts in Edmonton from 2000-2013. The study reports that while MMA athletes suffer a greater overall injury rate than boxers, the injuries tend to be more trivial (ie contusions and bruising) whereas boxers suffer a higher incidence of serious injuries such as brain trauma and eye injuries.
The study’s Abstract reads as follows –
Objective: Mixed martial arts (MMA) is an increasingly popular combative sport involving aggressive techniques that present substantial injury risk. We examined the incidence and types of injuries sustained in MMA fights and compared this with injuries sustained in boxing matches.
Design: Consecutive Case Series.
Setting: We used data from post-fight medical examinations on all bouts in Edmonton, Canada, between 2000 and 2013.
Participants: The participants were 1181 MMA competitors and 550 boxers.
Main Outcome Measures: The attending physician conducted a mandatory post-fight examination of all fighters and documented the nature of injuries sustained.
Results: Boxers were significantly more likely not to experience injury (49.8% vs 59.4%, P < 0.001), whereas MMA fighters were significantly more likely to experience 1 injury (typically contusion/bruising, P < 0.001). Boxers were more likely to experience loss of consciousness (7.1% vs 4.2%, P = 0.01) and serious eye injury (1.1% vs 0.3%, P = 0.02).
Conclusions: The overall injury incidence in MMA competitors appears slightly higher than for boxers, but MMA fighters experience more minor contusion/bruising injuries. Boxers are more likely to experience serious injury such as concussion/head trauma involving loss of consciousness or eye injury such as retinal detachment.
Combat Sports Law 
And I would comment that boxers experience head trauma and concussions due to the rule the allows them to be concussed and continue fighting. When a boxer is downed from a head blow and suffers a short term loss of consciousness (aka a mini concussion) he is given a mandatory eight count and permitted to continue fighting if he recoups quickly enough. In MMA bouts a downed fighter is not given this chance to recuperate. He is stopped by the referee or the opponent. There is no “break” in the action given him time to recuperate and fight on thus risking more head trauma.