Maximal Force Generated by Magnetically Controlled Growing Rods Decreases With Rod Lengthening.

Objectives: To evaluate the maximal force generated by magnetically controlled growing rods (MCGRs) at three different lengthened positions. Summary of background data: The introduction of MCGRs has been met with great enthusiasm by surgeons managing early-onset scoliosis. These devices offer the potential to decrease the cost and morbidity associated with repeated surgeries, compared to traditional growing rods. One potential negative consequences of growing rod treatments is the law of diminishing returns, where the spine length gained decrease with each subsequent lengthening. The cause of this phenomenon is unknown and probably multifactorial, yet it may be affected by the strength of the lengthening mechanism in the MCGRs.

Methods: Twelve MCGRs (90-mm actuator length) were obtained and tested to evaluate the maximal force generated at different lengths. The maximal lengthening force measured in pounds-of-force generated by each rod was recorded at expansion lengths of 0, 25, and 40 mm. Longitudinal analysis was performed using mixed effects linear regression to account for repeated measures and variability between individual implants.

Results: At 0 mm of actuator lengthening, the mean maximum force was 46.8 lb (standard deviation [SD] 2.06, range 43-50). At 25 mm of expansion, the mean maximum force was 44.9 lb (SD 2.48, range 39.4-49.5). At 40 mm of lengthening, the mean maximum force was 43.2 lb (SD 5.56, range 27.3-49.1). In the mixed effects linear model, there was a statistically significant decrease in the maximal force generated with progressive MCGR lengthening, at an average decrease of 0.089 lb of force (95% CI, 0.030-0.148; p = .003) per millimeter of lengthening.

Conclusion: There is a small but statistically significant decrease in the maximal force generated by MCGR as the rods are lengthened. The decrease in force generated may result in diminished spine length gained with each subsequent MCGR lengthening. Level of evidence: Level IV.