I agree with MrO and njee – SH51 should let you release by twisting in either direction, whereas the other one lets you roll your foot out and some other stuff and basically makes it difficult to stay in if you mash the pedals. It might feel like it is much harder/impossible to clip out in one direction – I reckon this is because you normally unclip in the same direction, the cleat wears until it becomes easy, but the other side stays like new and feels like you can’t unclip. I unclip inwards on my right foot on 520 and the plastic guard 520 pedals and its really hard (but not impossible) to unclip outwards. It may be that the crank was in the way when you crashed. Eggbeaters seem easier to clip out of, though partly as the cleats and pedals wear faster…
Plus – ouch! snapping your leg from not unclipping, shudder!
(ps if you are Dave and Mrs MC is Fran – hi, its Ian C from Oxford! Living in Helsinki now, Tymmo is coming over at the end of the month for a ride round the Baltics…)