I am sorry that you were injured at Walmart's. Did you provide Walmart with a report of the incident? If so, you should immediately request a copy of the report. You should also consider going to the doctor for treatment as you were injured and then contact a personal injury attorney to discuss the matter with. Many personal injury attorneys offer a free consultation. Take advantage of that free consultation. Good luck.
The above is general legal and business analysis. It is not intended nor construed to be "legal advice" but rather it is analysis, and different lawyers may analyze this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I have been licensed to practice law in California since 1978. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Get a claim number from Walmart. If it turns out you truly injured your knee, retain a local personal injury lawyer to investigate.
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To be successful in any claim versus walmart you would have to show that they acted wrongfully in some way: failed to protect you from a known hazard for example. Your note is a little short on information about their behavior.
But you should also know that walmart very likely has a medpay provision in their insurance that covers medical care up to a set limit (say, $10,000) regardless of fault, for people injured on their property. Make sure the store has a full report on the incident, and ask them for a claim form for your medicals.