Rod lengthening with the magnetically controlled growing rod: factors influencing rod slippage and reduced gains during distractions

Abstract

Introduction: Ability to achieve successful MCGR distraction is crucial for gradual spine lengthening. Rod slippage has been described as a failure of internal magnet rotation leading to a slippage and an inability to distract the rod. However, its onset, significance and risk factors are currently unknown. In addition, how this phenomenon pertains to actual distracted lengths is unknown. Hence, the aims of this study are to identify the factors that are associated with rod slippage and to study the pattern of achieved length gain with a standard distraction methodology. Methods: A total of 22 patients with MCGR implantation with at least 6 distraction episodes were prospectively studied. Patients with rod slippage occurring ≤6 distraction episodes were considered early rod slippage while those >6 or have yet not slipped were grouped as late rod slippage. The association of parameters including body habitus, 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, curve flexibility, fusion block length, and distance between magnets in dual rods and between the magnets and apex of the 2 curve with early or late onset of rod slippage was studied. Differences between expected and achieved distraction lengths were assessed with reference to rod slippage episodes and rod exchanges to determine any patterns of diminishing returns. Results: Patients had mean age of 7.1 years at diagnosis with mean follow-up of 49.8 months. A mean 32.4 distractions were performed per patient. Early rod slippage occurred in 14 patients and late rod slippage occurred in 8 patients. Increased height, weight, body mass index, older age, increased T1-12 and T1-S1 lengths, and less distance between magnets were significantly associated with early rod slippage. Expected distraction lengths did not translate to achieved distraction lengths. Reduced length gains were observed after achieving one-third of the allowable distracted length in the first MCGR implanted. However, this rate returned to its original baseline after rod exchange with another reduction observed after half of the total achievable length was obtained in the second MCGR. Conclusion: This is the first study to specifically analyze the impact of rod slippage on distraction lengths and the risk factors associated with its onset and frequency. Increased body habitus and reduced distance between internal magnets significantly influenced rod slippage events. In addition, mismatching between expected and achieved distraction lengths was illustrated. Diminishing returns in distracted length gains were only observed after a certain usage of the MCGR rather than the number of distractions and overall length gain. * indicated statistical significance Early slippage indicates a slippage ≤6 distractions. Late slippage indicates no slip or slippage >6 distractions BMI: body mass index; DRU: distal radius and ulna classification; FBCI: fulcrum bending corrective in...

3-Year follow-up of a single magnetically controlled growing rod with contralateral gliding system and apical control for early onset scoliosis

“…We found higher distraction rates in the first-year results (13 mm/year) compared to the distraction rate in the period from 1 year until last follow (7 mm/year distraction). This supports our general impression of diminishing returns with the MCGR over time [21,22]. Whether our protocol of distraction every 3 months up to stalling of the actuator is the most optimal to prevent diminishing returns remains to be determined.…”

“…They found that length gains decreased with every additional lengthening over time. More recently, Cheung et al found similar reduced lengthenings with MCGR [22]. We found higher distraction rates in the first-year results (13 mm/year) compared to the distraction rate in the period from 1 year until last follow (7 mm/year distraction).…”

“…Whether our protocol of distraction every 3 months up to stalling of the actuator is the most optimal to prevent diminishing returns remains to be determined. Other publications have done this on a monthly basis or semi-annually [21,22]. The MCGR that were removed typically failed to distract and were send for analysis to the manufacturer.…”

Mechanical wear analysis helps understand a mechanism of failure in retrieved magnetically controlled growing rods: a retrieval study

“…One explanation was that the bending moment on the telescopic bar was increased as the rods extended, leading to severer 'offaxis loading' and material loss. This may explain the frequent 'diminishing returns' or slippage during distraction observed in rods reaching half of the maximum extendable length [19,20] and the angulated telescopic bars noticed on radiographs [21]. Besides, it has been suggested that a single rod construct was subjected to a higher bending moment and therefore at a higher risk of breakage [22].…”

Magnetically controlled growing rods in early onset scoliosis surgery: a review of Englishlanguage literature

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