I am going to have to point something out regarding this.
A. MMA fighters in the larger events (UFC/Strikeforce, etc) are considered professionals. All participants on that level are also legal adults. I do not know if the underground events are sanctioned or insured by anybody.
B. MMA today compared to what it was when it first came out is completely different. MMA was based off of Vale Tudo, and there were no weight classes, you just went at it. There have been multiple techniques/maneuvers that have been outlawed over the years such as small joint manipulation and direct strikes from elbows, and weight classes have been added.
C. MMA fighters have to undergo pre and post fight MRI's and complete physical exams prior to fighting. There are also multiple physicians on site at each fight. There is no chance that Bobby from Lazy's MMA gym is goint to enter that ring, as you have to be invited to participate in that level of fighting and work your way up to fighting the best in the division. If a fighter has a concussion, they need to sit out for a length of time- I believe it is at least a couple months of NO TRAINING, let alone fighting.
The results have been that no athlete has sustained a serious, life altering injury in the UFC.