Like most states, Pennsylvania follows the general rule that an employee who suffers a work-related injury cannot under almost any circumstance file a lawsuit against their employer.  Such an employee is limited to filing a Workers Compensation claim. This result is the same even if the employer knew of the dangerous condition that caused the injury and failed to take measures to prevent it. 

Thus it appears that under Pennsylvania law the former NFL players are limited to a legal "hail Mary " pass in their attempt to recover from the league for their head injuries.