they are the same thing.
If you mean SSI, maybe not. If SSDI is denied based on lack of work credits, you might be eligible to apply for SSI.
but is sounds like you were denied based on disability, so SSI will also be denied.
realize that to get SSDI/SSI you have to be unable to do ANY work for at least a year and earn at least 1170/month. So if you can scrub toilets near full time, you don't qualify,
if you can be a cashier at a dollar store full time, you don't qualify.
and once target raised minimum wage to $15/hour, if you can work there part time--about 18 hours per week you don't qualify.
if you are at least 55, you only have to be unable to do jobs similar to what you are qualified for, you are not expected to retrain for new types of work.
I knew a woman who claimed she couldn't work..didn't have work credits for SSDI as she had been a housewife..husbands unemployment disqualified her from SSI....she was not eligible to apply...2 weeks later she had 2 jobs..1 full time/1 part time...and has been working like that for a few years now, after trying to claim disability
sciatica is treatable...been there done that, know someone temporarily disabled by it...needed surgery...you need to be disabled DESPITE treatment....but those are only a couple examples, many options of easy light work..you have to be unable to do ANY work