 Patients presenting to the Emergency Department who are 18 years old or older, who do not belong to a vulnerable group, requiring small or medium joint arthrocentesis will be included in this trial. The investigators will randomize each patient into LM vs US. Data collected will include number of attempts to tap the joint, success rate, and time for procedure to be completed. The arthrocentesis will be performed by PGY-3 residents only. A 21 gauge needle will be used for the arthrocentesis. 