The 94-year-old actor suffered the painful injury when his equine companion moved too quickly, sending him hurtling to the ground, and though he utilised his stunt experience to help make the fall more bearable, he admitted his age proved another hurdle.
Speaking at the Academy of Science Fiction, Fantasy and Horror Films' 53rd annual Saturn Awards in California last weekend, he said: "I ride the horses that can compete in equine skills, which is fast down and ends on a sliding stop. And the horse that I owned, I came off.
"And she had a habit of going too far, like six inches to the side. And I'm riding it. And I'm ready. And she goes (too fast and sent him flying). I'm not a young stuntman any more. I started to roll but hit the dirt with my shoulder. So, I wrecked my shoulder."
The Star Trek legend explained he was preparing to go through a "new type of shoulder operation called a reverse something or other" this week.
Posting on Instagram on Friday, Shatner dismissed tabloid descriptions of his injury and said he had undergone a "routine" procedure.
"I'm so well that tomorrow I'll be in Cleveland at a convention," he said.
This is not the first hospital visit for Shatner in recent months.
Last September, paramedics attended his house and took him to a medical facility, but he later reassured fans he was "perfectly fine".
Shatner shared a photo of himself on Instagram which was captioned: "Rumors of my demise have been greatly exaggerated.
"I over indulged. I thank you all for caring but I'm perfectly fine. I keep telling you all: don't trust tabloids or AI!"
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