Psychometric Properties of the modified Symptom Severity Index (SSI)

Relationships between various dimensions of pain, such as sensory, temporal and affective dimensions, have not been adequately researched. Objective: This study investigated the psychometric properties of the modified Symptom Severity Index (SSI) because it includes these dimensions allowing for their assessment. Methods: The SSI is composed of 5 Likert scales assessing pain intensity, frequency, duration, unpleasantness, and difficulty to endure. Subjects completed these scales assessing pain in 3 locations (temple, temporomandibular joint, masseter) resulting in a total of 15 items. Test-retest reliability was assessed in 55 TMJD subjects with ratings 2 to 48 hours apart. Cronbach's alpha was used as a measure of internal consistency. Dimensionality was assessed in 106 TMJD subjects at 10 time points, baseline to 60 months, with factor analysis used to assess relationships between items. Results: Test-retest reliability of the SSI was found to be excellent, ICC = 0.97 (95% CI = 0.96 - 0.99). The 15 items correlated highly with one another, with 77 of a possible 106 comparisons being greater than 0.71. Two factors emerged from the factor analysis, termed Temple Pain and Jaw Pain. The Jaw Pain factor comprised of the temporomandibular joint and masseter locations, indicating that subjects did not differentiate between these two locations. With further analysis Jaw Pain could be separated with pain duration and frequency clustering, while pain intensity clustered with the affective dimensions. Temple Pain could not be further reduced. Internal consistency was high with alphas > 0.92 for scales associated with all factors. Conclusions: When assessing TMJD pain with the SSI, the most important characteristic is the location of pain in the temple and jaw. The inability to reduce Temple Pain may have been influenced by concurrent orofacial pains, such as headache. Conversely, duration and frequency are important dimensions of Jaw Pain.

Supported by R29-DE08668, P30-DE09737 and K12-RR23247.