The Lesk measure works by finding overlaps in the extended definitions of the two concepts. The relatedness score is the sum of the squares of the overlap lengths. For example, a single word overlap results in a score of 1. Two single word overlaps results in a score of 2. A two word overlap (i.e., two consecutive words) results in a score of 4. A three word overlap results in a score of 9.
The vector measure works by forming second-order co-occurrence vectors from the UMLS extended definitions of concepts. The relatedness of two concepts is determined as the cosine of the angle between their vectors.