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

Area of Science:

• Spinal surgery
• Orthopedic biomechanics
• Pediatric orthopedics

Background:

• Magnetically controlled growing rods (MCGRs) are used for early-onset scoliosis, offering an alternative to traditional growing rods with fewer surgeries.
• A potential limitation of growing rod treatments is the law of diminishing returns, where lengthening efficacy decreases over time.
• The maximal force generated by MCGRs may influence the effectiveness of spinal lengthening.

Purpose of the Study:

• To evaluate the maximal force produced by magnetically controlled growing rods (MCGRs) at various stages of lengthening.
• To determine if the force output of MCGRs changes as they are expanded.

Main Methods:

• Twelve 90-mm actuator length MCGRs were tested to measure maximal force at 0, 25, and 40 mm of expansion.
• Force measurements were recorded in pounds-of-force.
• Longitudinal analysis using mixed effects linear regression was employed to analyze the data.

Main Results:

• The mean maximal force at 0 mm lengthening was 46.8 lb.
• At 25 mm lengthening, the mean maximal force decreased to 44.9 lb.
• At 40 mm lengthening, the mean maximal force further decreased to 43.2 lb, showing a statistically significant decline of 0.089 lb per millimeter of lengthening.

Conclusions:

• A statistically significant, albeit small, decrease in maximal force occurs as MCGRs are lengthened.
• This force reduction may contribute to diminished spine length gains in subsequent lengthenings.
• Further research is needed to understand the clinical implications of this finding.