Psychometric properties of the modified Symptom Severity Index (SSI)

The psychometric properties of the modified Symptom Severity Index were investigated to assess the relationships among dimensions of pain in temporomandibular disorders (TMD). The 15-item instrument is composed of ordinal scales assessing five pain dimensions (intensity, frequency, duration, unpleasantness and difficulty to endure) as experienced in three locations (temple, temporomandibular joint (TMJ), masseter). In 108 closed-lock subjects, Cronbach's alpha was used to measure internal consistency resulting in 31 of the 105 pair-wise comparisons ≥0·71. Multilevel exploratory factor analysis was used to assess dimensionality between items. Two factors emerged, termed temple pain and jaw pain. The jaw pain factor comprised the TMJ and masseter locations, indicating that subjects did not differentiate between these two locations. With further analysis, the jaw pain factor could be separated into temporal aspects of pain (frequency, duration) and affective dimensions (intensity, unpleasantness, endurability). Temple pain could not be further reduced; this may have been influenced by concurrent orofacial pains such as headache. Internal consistency was high, with alphas ≥0·92 for scales associated with all factors. Excellent test-retest reliability was found for repeat testing at 2-48 h in 55 subjects (Intra-class correlation coefficients = 0·97, 95%CI 0·96-0·99). In conclusion, the modified Symptom Severity Index has excellent psychometric properties for use as an instrument to measure pain in subjects with TMD. The most important characteristic of this pain is location, while the temporal dimensions are important for jaw pain. Further research is needed to confirm these findings and assess relationships between dimensions of pain as experienced in other chronic pain disorders.