The Significance of Clunking in Magnetically Controlled Growing RodDdistractions: A Prospective Analysis of 22 Patients

Abstract

Summary: A prospective study of 22 patients with magnetically controlled growing rods (MCGRs) was performed to determine the significance of clunking and identify possible risk factors. Clunking contributed to reduced length gain achieved with distractions. Patients who are older experienced earlier clunking. Hypothesis: Increased deviation between expected and achieved distraction lengths occur due to frequent clunking. Design: Prospective analysis. Introduction: Ability to achieve successful MCGR distraction is vital for gradual spine lengthening. The phenomenon of “clunking” has been described as a failure of magnet rotation to lead rod lengthening, and is thought to be due to a slippage of the internal mechanism. However, its onset, risk factors and significance are currently unknown. Hence, the aims of this study are to identify the risk factors of early onset clunking and increased clunking frequency, and to determine its effect on achieved length gain. Methods: 22 patients with MCGR implantation with at least 6 distraction episodes were prospectively studied. Parameters including maturity status, age of implantation, total number of distractions, months of distraction from initial implantation, initial and postoperative Cobb angle, T1-T12, T1-S1, T5-T12 kyphosis, fulcrum flexibility, fusion block length, and distance between magnets in dual rods and between the magnets and apex of the curve were studied as risk factors for onset and number of clunks per distraction by regression analysis. Differences between expected and achieved distraction lengths were assessed with regards to clunking episodes. Results: Patients had a mean age of 10.2 years at initial rod implantation, mean follow-up of 49.8 months and mean 32.4 distractions. The mean onset of clunking was the 5th distraction for the right rod and 9th distraction for the left rod. The total number of clunking episodes were 456 and 492 for right and left rods, respectively. Although regression analysis did not yield significant risk factors, those who clunked early (<6 distraction episodes) were idiopathic (p=0.03) and older (p=0.02) (mean 12 years old at initial rod implantation). Expected distraction lengths did not translate to achieved distraction lengths (Figure) and this deviation increased when clunking occurred. Conclusion: This is the first prospective study to specifically analyze the impact of clunking on distraction lengths and the risk factors associated with its onset and frequency. Clunking is an important factor determining continuous spine lengthening. Larger sample sizes are necessary to identify significant risk factors.