The issue of airline obligations to passengers who have a fused or immobilized leg (cannot bend knee) came up quite a bit in comments on the the most recent round of modifications to the Air Carrier Access Act (the US disability regulation that applies to air travel, and is binding on all carriers, foreign and domestic, with flights that originate or terminate in the US).

As things stand today, the airline has no legal obligation to accommodate these passengers, besides providing "a bulkhead seat or other seat that provides greater legroom".

Part of the problem is conflicts with FAA regulations that require that nothing (including legs) be sticking out into the aisles during take off and landing.

If you are requesting accommodations under the Air Carrier Access Act, it's logical for the airline to not permit you to sit in an exit row - can't have both.