Your mother has been taking Piroxicam for 3 years for oestoarthritis. I do not know from the history if she had any impairment of kidney function prior to surgery. Normal urea and creatinine at or before surgery does not always mean normal renal function. It seems, however, your mother developed acute renal failure post operatively most likely secondary to multiple factors - volume depletion, hypotension, analgesics, anaesthesia and infection. All these factors may contribute in a variable degree to the development of renal failure. It seems your mother is on her way to recovery. If there has been no effect of analgesics on her kidney function prior to surgery, she will recover her kidney function completely without any residual effects. It might take 2-6 weeks for her to regain her full function. Your mother needs to avoid nephrotoxic drugs like routine pain-killers called non-steroidal anti inflammatory drugs (NSAIDs) and aminoglycoside antibiotics (like Gentamycin etc.)