Lengthening of Dual Growing Rods and the Law of Diminishing Returns

Study Design. A retrospective multicenter study. Objective. To evaluate the effect of repeated surgical lengthenings and time on spinal growth and Cobb angle in children with early onset scoliosis and dual growing rods. Summary of Background Data. Previous studies have established the effectiveness of dual growing rods for controlling spinal deformity and promoting spinal "growth." Although anecdotal experience suggests that the effectiveness of repeated lengthenings decreases over time, this has not been previously studied. Methods. Medical records from five different centers were reviewed to identify children treated with dual growing rods for early onset scoliosis who had a minimum of 2-year follow-up and at least three lengthening procedures. Initial radiographs, postimplantation radiographs, and radiographs from before and after each lengthening were measured for T1-S1 distance and Cobb angle. Linear regression and analysis of variance were used for statistical analysis. Results. Thirty-eight patients from five centers met the inclusion criteria. The average age of our patients was 5.7 years (range 1.7-8.9 years); mean follow-up was 3.3 years (range 2-7 years). The average interval between lengthenings was 6.8 months. Cobb angle decreased from a mean value of 74 degrees preoperatively to 36 degrees after the primary implantation and did not change significantly with repeated lengthenings (P = 0.96). After initial implantation, the average annual T1-S1 gain was 1.76 +/- 0.71 cm/year. The T1-S1 gain after a given lengthening, however, decreased significantly with repeated lengthenings (P = 0.007). When the effect of time was considered, there was also a significant decrease in T1-S1 gain over time (P = 0.014). Conclusion. There seems to be a "law of diminishing returns" with repeated lengthenings of dual growing rods. Repeated lengthenings still result in a net T1-S1 increase; however, this gain tends to decrease with each subsequent lengthening and over time. This phenomenon may be due to autofusion of the spine from prolonged immobilization by a rigid device.