You will need to try to recall what you were doing that might have caused you to sprain your knee. If not they will deny it and will not pay for treatment. Even with the explanation it could be denied since you did not recall how it was injured at the time it happened. In that case you may need an atty to talk to about it.
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You need to get the assistance of an attorney. If you were in the course and scope of your employment when you suffered the sprain, it should be a compensable injury.
You need to fill out a first report of injury and turn it in to the Division of Workers' Compensation DWC-41 form. This will start the process.
Your case will likely have to be litigated. You need to file a claim and get the process started. The best way to do that is to hire an attorney. There are other cases like yours. The Appeals Panel of the Division of Workers' Compensation has held that an injury sustained while walking on the employer's premises is covered under workers' comp. Your case is no different. The problem is that adjuster's don't know or don't care about the law. That's why the law allows you to get your case in front of a judge so that you can get the benefits you deserve. Hire a good lawyer though because workers' comp is a legal process and the hearing is a legal hearing in front of a judge. The insurance company will have its lawyer, so you must have yours.