Critical length of fusion requiring additional fixation to prevent nonunion of the lumbosacral junction.

STUDY DESIGN: A retrospective study. OBJECTIVE: To determine the critical length of fusion that warrants additional stronger fixation in lumbosacral (L-S) fusion, and to analyze the risk factors of nonunion at the L-S junction. SUMMARY OF BACKGROUND DATA: Long lever arm fusion down to S1 requires stronger fixation than short lever arm fusion. However, no published criteria are available about the critical length of fusion requiring stronger fixation to the ilium or S2 to obtain adequate stability for union at the L-S junction. METHODS: A total of 327 adult patients with degenerative lumbar disease, who were treated with instrumented fusion, including the L-S junction, were included in this study. Mean patient age was 59.7 (20-79) years and the minimum follow-up was 12 months. Union rates were compared using univariate and multivariate logistic regression analysis. Length of fusion, age, sex, lumbar lordosis at preoperative, early postoperative and final follow-ups, BMD, smoking history, associated morbidities, fat content of paraspinal muscle, methods of fusion, and levels of intercristal line were examined as independent variables to identify factors that affect union rate at the L-S junction. RESULTS: Of the 327 patients, 47 (14.4%) had nonunion at the L-S junction. Union rate of the L-S junction at the single level, and at 2, 3, 4, 5, and more than 5 levels were 96.6%, 92.9%, 87.4%, 64.7%, 66.7%, and 58.0%, respectively. A significant difference of union rate was found between less than 4 levels and 4 or more levels of fusion (P < 0.05). The factors found by multivariate analysis to significantly affect union rate at the L-S junction were fusion length and fat content of paraspinal muscle. CONCLUSION: The risk of nonunion at the L-S junction was found to increase significantly for more than 3 levels of fusion. We advise that additional stronger fixation is needed in such cases.