Smallest detectable difference of maximal mouth opening in patients with painfully restricted temporomandibular joint function

Changes in maximal mouth opening reflect the impact of temporomandibular disorders and the effect of a therapeutic intervention. No information about the amount of change in maximal mouth opening with regard to reasoned decision-making is available. The smallest detectable difference, as a measure of reliability assessment, provides this information and is expressed in the unit of the measurement instrument. Twenty-five consecutive patients (5 males, 20 females) with a painfully restricted temporomandibular joint participated in this study. Measurements of maximal mouth opening were performed by two well-trained observers on two separate measurement days, one week apart. The maximal mouth opening measurements were repeated three times. Inter-observer, intra-observer, and test-retest reliability varied between 0.90 and 0.96. Inconsistency in measurement results analyzed in terms of absolute error variance, i.e. the measurement facets plus all the interactions, represented 11% of total variance. The smallest delectable difference of maximal mouth opening varied from 9 to 6 mm. For being successful in painfully restricted temporomandibular joint patients, statistically as well as clinically, the clinician has to measure at least 9 mm of improvement in maximal mouth opening. To reduce the smallest detectable difference from 9 to 6 mm, repeated measurement is necessary.