It has to do with the strain, that's how I understand it anyway.
Meh....they do that after most major surgeries.
I've had 5 over the last 5 years....in my mind they were the pleasant part.
Try recuperating after cancer surgery,back surgery and three hip replacements,then you'll know what a challenge is.
Consider yourself lucky.
3 hip replacements?....holy shit!...
All on the same leg.
They botched the first one by leaving it an inch longer than the other. I called the surgeon on it and he said it would be just fine...

Two years later the back failed due to the F'd up hip job.
Off to surgery for the back.
Things were fine for awhile and all the pain came back.
Two back surgeons looked at my MRI and said things were fine there.
Off to a specialist on F'd up hip jobs....he assured me that was where my problem lay.
First surgery he only replaced the femur/ball part since the socket was well set. He warned this could lead to dislocations but it was less intrusive and worth the risk and I agreed.
Two weeks and two dislocations later and he went the distance and replaced the socket as well.
What a joyous couple of years.
Three hip replacements,two dislocations and fusing vertebrae 3 and 4.
Still using the walker but the back pain is gone.
He warned me that rehab from the type of surgery I ended up needing was a mofo...and he wasnt kidding.
They cut me to the bone from 8 inches above the knee all the way to my waist band on the lower quarter of my thigh on circumference.