How should parthood relations be expressed in SNOMED CT?

The Systematized Nomenclature of Medicine, Clinical Terms (SNOMED CT) is a clinical terminology with a broad coverage of health care, which has been developed with the help of a rather inexpressive description logic dialect known as EL [1]. The advantage of using a description logic (DL) for defining a medical ontology is that, instead of error-prone “hierarchy engineering,” where each newly introduced concept needs to be manually positioned at the right place in the concept hierarchy, one adds a definition of the new concept to the knowledge base and the DL reasoner then automatically finds the right position of this concept in the concept hierarchy. The advantage of using an inexpressive DL is that classification (i.e., the computation of the concept hierarchy) is fast even for a very large ontology like SNOMED CT. Efficient reasoners for EL, like Snorocket, which is based on the classification algorithm introduced in [2], can classify SNOMED CT in less than a minute.