Side effects of spinal anesthesia

My grandfather just had knee replacement surgery under a spinal. He's doing great, except for the fact that he had not been able to urinate since the surgery. Twice he has had his catheter taken out and had to have it put back in since he could not void on his own. Now they are talking about discharging him with the catheter still in place, which isn't a big deal except that I would like to know what is causing the problem. Is there any chance that it is a result of the spinal???

Hopefully somebody out there will have some information. Thanks for your help!