It's both, I guess; technically, it's a comma splice.
http://grammar.ccc.commnet.edu/grammar/runons.htm
The sentence you provided, "He underwent a left ankle block, Esmarch tourniquet was used to just above his ankle," is a comma splice. A comma splice occurs when two independent clauses (complete sentences) are joined together with only a comma. To correct this, you can either separate the two independent clauses into separate sentences or use a coordinating conjunction (such as "and," "but," or "so") to join them properly.
Based on the information provided, the given statement is a fragment, as it does not form a complete sentence. A complete sentence should contain both a subject and a predicate that expresses a complete thought. In this case, the statement appears to be missing crucial information, such as what "it" refers to and what the main subject or action is. To determine whether the statement is a run-on sentence or a comma splice, we would need more information or context.
