new headrests=better crash ratings

it may not be as simple as redesigning the head restraint. honda may have also redesigned the entire seatback mechanism, into which the headrest fits.

my guess as to what happened is that the MDX's didn't do well in the "dynamic" test, which is the actual rear-similating crash test. there is another test called the "geometry" test in which they measure how well the headrest is positioned at rest, with respect to the dummy's head. but the dynamic test is the test of how well the dummy's head *moves* into the headrest and is protected during the collision. inevitably, there is a lot of movement and "giving" of the car's seatback (notice how much is bends just pushing your weight against it???). this has a lot to do with how well your head moves into the headrest during a rearend crash. IIHS has some pretty cool videos showing this.

so unless you can do some digging around and find out exactly what honda did to improve the headrest performance, it may not make any sense to just buy the updated headrests alone.

ETA: yep, i just checked IIHS and sure enough, the headrest did fine in the geometry test, but "marginal" in the dynamic test, earning it the marginal score.

Keep in mind that the head restraint rating only really applies directly if you are of a typical dummy height and adjust your seat angle and head restraint height to typical levels. In reality, the protection you get may be much better or much worse, depending on how you adjust your seats and how well they fit you.

Testing all trim levels and seating options for various passenger heights and settings is simply too expensive, even though it is only done on a sled simulation and is not a full crash test like the frontal and side tests. Better head restraint designs probably do perform better in many cases, but the test has a lot more variables than the other crash tests.

For kids, the tests are irrelevant if they are in a harnessed child seat or high back booster that provides its own head support.

My advice is to take the rear crash ratings with a grain of salt. If all else is equal, certainly get a model with a better head restraint rating. Otherwise, it's probably enough to make sure that you can at least get a safe adjustment of your seat and head restraint that is also comfortable for driving.

This is a good link on how to make sure your seat is adjusted correctly:

http://www.safety.ed.ac.uk/resources/General/Driving_posture.shtm

http://www.consumerreports.org/cro/2012/12/how-to-save-your-neck-in-a-rear-end-crash/index.htm