After a transatlantic flight, my 2 year old artificial hip is sore. what could cause this?

Nothing serious: These plane rides generally are over six or seven hours. Being stationary for that period of time can cause stiffness. It is not uncommon for people with joint replacements that travel for extended periods of time to feel achiness or soreness after these long trips. The pressure changes from high-altitude even though the cabin is pressurized can also cause joint become sore and swelling of the lower extremities.

Ambulate!: Long plane trips, sitting in one position, somewhat cramped frequently exacerbate osteoarthritis. Also not good for circulation. Move about when you can on long flights!