When it comes to field turf most of the studies I have seen are inconclusive if it does measurable damage to the knees or ankles over grass. What was originally thought to be an issue with these surfaces is the amount of staff infections that occur when the base material gets into an open wound. Studies found out this was not the case and most of the cases that were thought to be from the surface were actually from the locker rooms.
Skin infections associated with contact with synthetic turf have received national attention in recent years, but there is no scientific evidence to support concern that the surfaces of infilled synthetic turf (the kind containing crumb rubber found in
www.momsteam.com
Much of what has come out in the last 4 years shows an unequivocal link between lower extremity injuries occurring more frequently on synthetic turf. The most important study during this time is linked below:
These results support the biomechanical mechanism hypothesized and add confidence to the conclusion that synthetic turf surfaces have a causal impact on lower extremity injury.
pubmed.ncbi.nlm.nih.gov
The caveat is this data comes from the most well-maintained football fields in the world. Take this down to a high school level and the maintenance on both synthetic turf and natural grass is severely lacking and is ultimately impacting player safety.
One of the arguments that you'll often hear is a poorly maintained natural grass field is less safe than a poorly maintained synthetic turf field. While there is no definitive data (yet), the anecdotal evidence suggests they both are very unsafe but in different ways.
Big picture, schools need to do a much better job of taking care of the fields they have, regardless of whether they are synthetic turf or natural grass. The safety of our players depends on it.