"Yesterday I went to see an extremely good specialist in Cardiff where my X-ray results have shown that I've fractured my clavicle," Frost said in a blog for betting company Matchbook.
"I suffered a fracture previously which healed well under pressure. My body's response from that fracture makes me positive for when I go back for my assessment in a fortnight's time and a swift return."
The Grand National meeting will be held at Aintree from April 4-6. Devon-born Frost had finished fifth on Milansbar in last year's National.
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(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Toby Davis)
