Magec Rods Flashcards
Describe Spinal Ligaments
- A ligament connects bone to bone
- Tough fibrous material
- Provides stability and may limit motion
- Some spinal ligaments go from vertebra to vertebra; some run the length of the spine
Issues with traditional growing rods
- An operation to extend the rods every 6-9 months
- Physical trauma for child, psychological issues^, time away from school/friends, parents take time off work
- Cost of multiple operations
- Concerns over effect of repeated general anaesthetics on the growing brain*
Describe Magec Rods
MAGneticExtension Control (MAGEC)
• Use an external magnet to actuate an internal magnet within the rod. When the magnet rotates, it causes the rod to grow
• Child is awake!
• No more operations every 6-9 months
• Reduced trauma, less chance of infection, reduced overall costs
• Cost effective*
• MAGEC rods made from titanium
• Biocompatible and high strength:weightratio
• Can control rod lengthening to less than 1mm
• Available in two lengths (70mm (28mm ext)/ 90mm (48mm ext)) depending on height of child
•Available in two diameters (4.5mm/ 5.5mm) depending on weight of child
What do we define as a broken magec rod?
a) non-functioning internal bearing (so the magnet would struggle to rotate)
b) obvious seal damage (so body fluids could get into the rods and any internal wear debris could escape)
c) broken drive pin (so MAGEC rod might not ‘grow’)
d) internal wear debris
What were the problems with the 34 explanted rods?
- 74% had a non-functional internal bearing
- 53% had badly damaged seals
- 44% had fractured drive pins
- 100% had internal debris
All 34 magec rids showed evidence of wear due to off axis loading
Why do magec rods fail?
- Off-axis loading in vivo
- Causes the rigid extending bar to contact on the internal surface of the outer casing (A), on one side only
- As the rod ‘grows’, so these marks are repeated
- The other end of the extending bar impinges on the internal surface (B) and wears on one side only, at 180°to (A)
-furthermore off axis loading can cause metallosis due to the titanium wear
What % of explanted magec rods produce less force than when new?
Using an External Romote Control (ERC) they measure the force. 49 explanted rods measured:
32 (65.3%) had zero force output
7 (14.3%) had reduced force output (<40lbf [178N])
10 (20.4%) had expected force output (≥40lbf [178N])
What are the limitations?
- We don’t have the denominator –British Spine Registry is nascent, but estimates of 140 MAGEC rod pairs implanted per year from NICE
- But we see similar ‘failure’ modes across all 7 UK clinical centres
- ,We don’t have full clinical data
- ,But we continue to collect and analyse what we have as our numbers grow –research is ongoing
Different types of MAGEC rods that we have examined at Newcastle
- 20% Mod 1, 80% Mod 2
- 92% have been 90mm rods, 8% have been 70mm
- 66% 4.5mm diameter, 34% 5.5mm diameter
What is the diameter of a drive pin and a paperclip?
0.80mm and 0.90mm
What is the definition of the strength of a material?
The strength of a material is its ability to withstand an applied load without failure or plastic deformation
Describe O-ring seals
- Most common type of seal in the world
- Effective and inexpensive
- Need to deform to work (recall earlier definition of strength!)
- Static –contained in non-moving gland
- Dynamic –contained in a moving gland
- Defined by inner diameter, cross section, hardness and material
What load can a thrust bearing take?
Only take axial, F3-8M have static load capacity of 93kgf. made from stainless steel (resist corrosion)
What loads can a radial bearing take?
Axial and radial (primarily) load . Stainless steel. SR156-Z 37kgf dynamic and 15kgf static
