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Terrell Owens went under the knife for a stem cell treatment to help repair his joints, tendons and ligaments ... and TMZ Sports was invited into the operating room for the procedure!!!

Break out your scalpel and your O.R. scrubs -- it's about to get ALL SURGICAL UP IN HERE!!!

Here's the deal ... Owens visited Dr. Raj from the Beverly Hills Orthopedic Institute for a treatment he essentially described as bone marrow-derived stem cell injections.

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So, what does that all mean? He takes stem cells from the hip bone and injects them into damaged areas of the body.

"I'm utilizing regenerative cells to regenerate and repair joints, tendons, and ligaments," Raj tells us.

"It's minimally invasive. In T.O.'s case, he damaged himself from years and years of football. He had both knees, both shoulders, both elbows (triceps tendonitis) and right hip done."

Owens says the move isn't part of an NFL comeback -- he just wants to stay healthy and feeling good as he hits his mid-40s.

The video of the procedure is super interesting -- especially when the doc hammers the syringe into T.O.'s hip for the extraction!