Motor nerve palsy after total hip replacement (THR) is relatively rare. Studies report one to four percent of the cases end up this way. There is some evidence to suggest it's happening more often than it used to.
A recent study from the Mayo clinic thinks this may be related to the newer uncemented implants. Implant without cement requires more forceful pounding on the bone during the operation. The stress may put extra strain on the nearby nerves.
One other possible cause of the more recent rise in nerve palsies after THR may be the amount of leg lengthening that takes place. There aren't any studies to show just how far the leg can be lengthened without problems occurring. The Mayo study showed nerve palsies with more than 3.8 cm of length added.
They suggested the nerve palsy can occur with a change in nerve length or by scar tissue forming in the area. Scarring can keep the nerve from gliding and moving even a little bit so that even minor amounts of lengthening become a problem.