The Clunking Phenomenon in Magnetically-controlled growing rods:

Possible risk factors

James Tan, Dino Samartzis, Cora Bow, Jason Pui Yin Cheung, Kenneth Cheung

Department of Orthopaedics and TraumatologyThe University of Hong Kong

Disclosure

Professor Kenneth Man Chee Cheung was a consultant and received research support for Ellipse Technologies Inc.

Aim: To assess the occurrence and potential determinants associated with “distraction failure” or slippage of the MCGR during distraction.

Lancet. 2012

Distraction of rodsStandard

Offset

Methods• 5 EOS patients with dual rods inserted

• Congenital, CHARGE syndrome, Ehlers-Danlos syndrome, JIS, neurofibromatosis

• Single surgeon – Dec 2009 - Oct 2012• 12 months minimum FU

• Monthly distractions ~2mm

• Standard rods distracted first

• Definition of “Distraction Failure”– Rod slippage of the magnetic mechanism during the

distraction procedure– Recognize via a “clunk” sound & feeling

• Demographic• Age at surgery and first clunking, number and frequency of clunking,

BMI

• Radiologic• Side of curve convexity, Cobb angle, sagittal profile, curve flexibility,

distance of rod to apex of curve, distance between the two magnets

Magnet to apex

Between two magnets

Results

• One patient excluded due to faulty rod design (EDS)

• Mean age 12.7y at initial surgery• 1st clunk within 1 year from initial surgery

• No difference in BMI, Cobb angle and flexibility assessments

Total failure episodes in Standard Rod

Time period to 1st failure (months)

Total failure episodes in Offset Rod

Time period to 1st failure (months)

Idiopathic 0 - 17 16Syndromic 0 - 10 12Congenital 28 12 27 8Neurofibromatosis 6 5 0 -Total 34 54

Results (n=4)Standard Rod Offset Rod

Idiopathic 0/23 5/23Syndromic 0/24 9/24Congenital 3/16 0/16

Neurofibromatosis 11/21 15/21Total 14/84 (17%) 29/84 (35%)

Distraction failure more prevalent in off-set rod

Distraction failure occurs within 1st year

Frequency of Distraction Failure

• Rods that clunked were nearer to the apex of the major curve

• Distance between internal magnets– Single rod clunking (56mm)

• Offset rod (68mm)• Standard rod (31mm)

– Dual rod (33mm)

• First study to identify “distraction failure” or rod slippage - may help predict distraction potential

– can occur within the first year since initial rod implantation may continue thereafter

– more prevalent to occur in the offset rod

• Potential complication or risk factor ? – rod breakage, revision surgery, loss of correction, improper spinal growth, etc

• Naturally occurring ?– physiological fail-safe mechanism that benefits the patient ?

• Larger studies are needed to further validate our findings and elaborate

Conclusions