You may have a case. The issue is whether 1) they did the tests or evaluations they should have done; and 2) whether based on your symptom reports and the tests they did, their conclusion was reasonable. That is, malpractice requires more than simply a negative consequence for the patient; for there to be malpractice, the medical care provider must have failed to have provided care that meets current accepted standards, such as by not doing the right tests, or misinterpreting them in a way no "reasonable" doctor should.It is certainly possible that a failure to diagnose a broken knee was malpractice, but such a failure isn't always malpractice; in a related case, as I've learned from personal experience, it is often impossible to tell that a foot is broken until long after the fact--some breaks do not even show on X-rays. So if the ER did the proper tests, and based on those test results and what you told them, came to a reasonable (even if incorrect) conclusion, there would be no malpractice.Another issue: even if there is malpractice, unless the malpractice made your injury worse or caused you to injur significant medical costs or other costs which  you would not have incurred with a prompt correct diagnosis, there is no point in suing. The legal system only provides compensation for actual costs or significant pain and suffering caused by malpractice. So if the outcome would have been the same or nearly the same had they gotten the diagnosis right, there is likely no point in suing--you could probably not recover enough to justify the cost of the lawsuit. On the other hand, if you have  incurred significant extra costs, pain, or disability, a lawsuit may well be worth it.A good idea would be to contact a medical malpractice attorney, tell him or her what has happened, and see if he or she feels  you have a viable case.