Temporal stability of the Minnesota Multiphasic Personality Inventory (MMPI) in patients undergoing lumbar fusion: a poor predictor of surgical outcome.

In this prospective study, the Minnesota Multiphasic Personality Inventory (MMPI) was administered to patients before and after lumbar spines fusion to investigate the stability of MMPI scores after surgical intervention and to attempt to correlate MMPI scale scores with outcome data. Sixty-eight patients were included. Testing was performed before surgery and at a mean of 1.5 years after surgery. Clinical outcome ratings were assigned by using criteria of pain relief and analgesic use. In addition, demographic variables known to affect outcome were analyzed. Sixty percent of the patients had a successful clinical outcome. Positive outcome correlated with the demographic factors of occupation (homemaker) and solid fusion. MMPI scales were stable across time, with no difference between groups. Independent t tests were used to study preoperative MMPI scores with respect to clinical outcome. Unsatisfactory outcomes were associated with higher scores on scales 1, 3, and 10 before surgery. Postoperative testing revealed significant outcome correlations--higher scores on scales 1, 2, 3, 5, 7, and 8 associated with an unsatisfactory outcome. However, discriminant function analysis of preoperative MMPI data was able to classify outcomes correctly in only 58.8% of the cases. The utility of the MMPI as a predictor of outcome after surgical intervention is quite limited. Use of group data and testing before and after surgery does not appear to influence this conclusion. Although the scores as a group were stable across time, the amount of variance in outcome that could not be accounted for by using MMPI scales as predictors was unacceptably large.