This document describes the work made for obtaining master degree in University Joseph Fourier. This work has been accomplished in the MRIM team of LIG laboratory, and talks about semantic structuring queries in ontology based information retrieval systems applied to medical domain. The objective of this work is to study, in the context of conceptual indexing, the influence of query semantic structuring to the precision of information retrieval system. We study an ontology based matching using concepts by applying some variants of language models. We integrate, with this model, our operation of semantic structuring. A validation of our study is made on the test collection CLEF 2011 and the semantic resource UMLS 2011.