Functional limitation is a construct distinct from disability, but the distinction between these constructs is often blurred when measuring jaw status because of content overlap in measurement instruments that include items pertaining to mechanical jaw functions, jaw functions that have obvious social implications, and only social functions. We describe analyses in support of assessing functional limitation as distinct from disability by using both factor analysis and the Rasch measurement model in reanalyzing two existing instruments and then create a preliminary version of a new instrument that has excellent psychometric properties.
STUDY DESIGN AND SETTING:
Subjects in three separate studies from two university settings completed two existing self-report instruments (Research Diagnostic Criteria for Temporomandibular Disorders and Mandibular Functional Impairment Questionnaire). Rasch methodology, factor analysis, and the combination of both were used in these analyses. Unidimensionality, in support of an instrument assessing a single construct, was also assessed.
Problems identified in the existing instruments included scaling, internal reliability, length, unidimensionality, and content validity; the preliminary version of the Jaw Functional Limitation Scale (JFLS) resolves these problems critical to delineating functional limitation and it exhibits internal reliability coefficients of 0.82 for persons and 0.99 for items. Construct validity, via convergent and discriminant validity with other associated constructs, of the JFLS was established via low correlations with depression, anxiety, somatization, pain interference, pain-free opening, and palpation sensitivity, and via moderate correlations with pain and jaw symptoms.
While areas of further development are outlined, we conclude that this preliminary version of the JFLS is a valid instrument for the measurement of temporomandibular disorder (TMD)-related functional limitation, and consequently delineates functional limitation from disability.