10yo with knee laceration
Fluid should be injected into the knee joint to determine whether the laceration communicates with the joint. If it does, the patient requires orthopedic consultation and intraoperative joint irrigation to reduce risk of septic arthritis. A common method is to inject saline dyed with methylene blue into the joint – if colored fluid comes out of the laceration, there is communication. Cleanse the skin with antiseptic prior to injection, and use local anesthetic.
Here is an article detailing the process for injecting an elbow joint
Another method is to inject saline (saline load test) and look for saline to extrude from the laceration. The amount of saline required to be sure a test is negative is controversial.