scyon orthopaedics ag - kyon veterinary surgical … orthopaedics ag wear reduction in artificial...
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SCYON 1
SCYON Orthopaedics AG
Wear Reduction in Artificial Joints (THR) by Geometric Means
SCYON Fossa Cup
SCYON 2
Kyon - SCYON
SCYON 3
Hip Bearing Solutions
• Metal on Metal (The Evil) • Ceramic on Ceramic (The Mean) • Metal on Poly (The Forgiving)
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Metal on Metal Biologic reactions
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Metal on Metal Pseudotumors
• “Pseudotumour” – a cystic or solid mass
• Diagnose by MRI or ultrasound • 1% of MOM pts develop
pseudotumours by 5 years – 17pts, all female, – Pandit et al (JBJS, 2008)
• 8% at 5yrs FU indicated by preliminary study from same group (AAOS 2009)
– Prospective of 97 hips – 6 pseudotumours (5 female, 1 male)
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Ceramic - Ceramic
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Ceramic - Ceramic
• Most studies report: – 20% noise, 10% squeaking – 1% persistent/ symptomatic squeaking
• Varies from 0.7% to 40% • 2 studies report greater incidence
(8%-40%) with Ti-Mo-Zi-Fe alloy stem with V-40 taper neck profile
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Metal on UHMWPE (poly)
• Gamma or ETO sterilized (non cross linked) has been associated to high frequency of mid-term radiographic osteolysis and subsequent clinical failure and revision.
• 1st Generation: two approaches (Melting and annealing) are used to reduce the concentration of residual free radicals after irradiation. Higher incidence of fatigue fractures
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What is the solution?
• Metal/metal: Metallic debris and pseudo tumors • Ceramic/ceramic: Squeaking • Metal/highly x-linked poly: Fatigue failures
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Solution
• Use material with longest proven clinical track record – standard gamma sterilized GUR 1020,
• But address wear by geometry
• Bearing Contact
• Remove material where stress is high
Hertzian contact stress
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Solution
Standard geometry SCYON fossa
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Finite Element model
28 mm diameter CoCr head
UHMWPE layer
Ti shell support
Either standard liner geometry or SCYON fossa geometry
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Fossa Liner von Mises stresses reduced from 9.6 to 4.4 MPa
Standard geometry Point contact condition
Fossa geometry ring contact condition
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Stress distribution in Standard Geometry
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Stress distribution in Fossa Geometry
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Friction Factor
28 mm diameter CoCr head
UHMWPE layer
Either standard liner geometry or SCYON fossa geometry
N.E. Bishop, et. Al., Med. Eng. Physics., 30; 1057-1-64, 2008.
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Conventional poly liner geometry
Fluid film is quickly lost at the pole
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SCYON Fossa poly liner geometry
* N.E. Bishop, et. Al., Med. Eng. Physics., 30; 1057-1-64, 2008.
Sustained fluid flow out of the fossa maintains lubrication
Under pressure Flow restricted by seal
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SCYON Fossa Friction Factor
Zimmer * Friction factor 0.15
Conventional Geometry 0.12
SCYON Fossa 0.067
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Fossa Liner Wear testing
Endolab, Rosenheim, DE
Hip simulator according to ISO 14241-1
GUR 1020 UHMWPE liners
Vacuum packed in Nitrogen, gamma sterilized 25-37 kGy
28 mm diameter CoCr heads
liner with standard geometry as control
Compliant double shell and stiff shell backing
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Wear testing Wear distribution
Conventional Geometry Fossa Geometry
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In-vivo wear by RSA (RadioStereographic Analysis)
• Precise determination of implant/bone migration and implant/implant displacement
• RSA most precise method for determining in –vivo subsidence and wear rates
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RSA (RadioStereographic Analysis)
• Spherical Tantalum markers are placed in the implants and bone
• Tantalum is biocompatible, well tolerated by the body, and very radio-opaque
• Markers are placed in the implants, the femur and pelvis
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Control points
X-ray tubes Focus 1 Focus 2
Film cassettes
Fiducial marks
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92
x
71
111 112
41 41
72
92
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Measurements of radiographs
112.344 123.334 112.45 132.45
2D coordinates
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RSA (RadioStereographic Analysis)
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SCYON head penetration in-vivo RSA
mm
months
+/- 2 std. error
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0.00
0.05
0.20
ETO !- Poly XLPE 10 SCYON
0.10 “safe level”
0.15
(Nivbrant , Digas)
mm
Annual in-vivo wear rates, RSA
SCYON 30
Conclusion
• Fossa Liner reduces von Mises stress by factor 2
• Fossa liner improves lubrication which reduces friction factor 2 times
• Fossa Liner reduces poly wear by factor 3 in hip simulator (with CoCr heads and solid shells)
• Initial 2 year results are very promising with annual wear rate of 25 microns.