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Spine (Phila Pa 1976). 2019 Sep 30. doi: 10.1097/BRS.0000000000003268. [Epub ahead of print]

The Crooked Rod Sign: A New Radiological Sign to Detect Deformed Threads in the Distraction Mechanism of Magnetically Controlled Growing Rods and a Mode of Distraction Failure.

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Department of Orthopaedics and Traumatology, The University of Hong Kong, Pokfulam, Hong Kong, SAR China.
Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong SAR, China.



Case report OBJECTIVE.: To report a unique mechanical failure of magnetically controlled growing rods (MCGR) that is related to continuous rod slippage.


Despite the growing popularity of MCGR in the management of patients with early onset scoliosis, postoperative complications and reoperations are not uncommon. Unique complications or unplanned reoperations are observed in patients with MCGRs that are not seen in traditional growing rods. The complications include rod slippage, mismatch between targeted and achieved distraction length, metallosis and actuator pin fracture. We have identified an unreported failure mechanism whereby deformed threads occur in the internal distraction mechanism of the MCGR. This phenomenon may indicate increased MCGR internal screw friction and increased distraction resistance. This complication ultimately led to distraction failure and revision surgery.


A girl with early onset scoliosis (EOS) was treated by dual MCGRs. She developed proximal junctional kyphosis (PJK) and continuous rod slippage. Distractions in the out-patient clinic and under sedation did not achieve successful rod lengthening. A tell-tale "crooked rod" radiological sign was identified by angulation between the actuator and the extendable portion of the rod.


Rod exchange was performed and she is now 2 years after revision surgery with successful lengthening episodes. Upon review of the extracted MCGR, distraction was not possible even after rod removal and was only resumed after the screw was manually reinserted along its correct threads.


Deformed threads is a complication that cannot be reverted without rod removal and exchange. Users should be aware of this potential failure mechanism and proceed to early rod exchange rather than attempting any further distractions.



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