Analytical Comparison of CoCrMo ImplantPassivation using Nitric Acid or Citric Acid

Vass, S A; Salvati, L.

BioInterface 2009 October 26-28th San Mateo, CA

2

Metal Ion Release• Potential Effects of Metal Ion Release

• Metalosis and Osteolysis

• Inflammatory responses

• Implant loosening• All can be aggravated by metal allergies and sensitivity

Nadim Hallab, PhD, Katharine Merritt, PhD and Joshua J. Jacobs, MD, Metal Sensitivity in Patients withOrthopaedic Implants, JBJS 83:428 (2001)

• Chromium oxide (Cr2O3) is more resistant to corrosion then mostother oxides present in CoCrMo alloys.

3

ASTM A380 Definition“The chemical treatment of stainless steel (a material) witha mild oxidant, such as a nitric acid solution, for thepurpose of enhancing the spontaneous formation of theprotective passive film”

Nitric acid passivation has greatly improved ion release by providinga chromium oxide (Cr2O3) layer.

Citric acid is now an alternative process for Nitric acid passivation.

Passivation

4

ASTM A967-05

Nitric acid passivation utilizes a solution containing 20-45 percent of nitric acid for 20-30 minutes attemperatures from 120-140oF.

Citric acid passivation utilizes a solution containing 4-10weight percent for 4-20 minutes at temperatures from70-160oF.

Passivation

5

XPS Investigation

6

XPS Investigation

Average Atomic Concentra/on (%) N=7

Co Cr Mo

Nitric Pass 45Deg 27.9 66.1 6

Citric Pass 45Deg 44.7 49.6 5.7

Control 45Deg 68.7 27.1 4.2

* Carbon and Oxygen have been ignored and the metals renormalized.

7

Citric acid passivation

Nitric acid passivation

8

Ion Release Procedure– CoCrMo Coupons – 1” diameter x 0.125” thick

– Test Conditions

• 90% Bovine Serum

• 37° C, shaker table

• 2-6 weeks total exposure

Serum Level

Teflon Fixture

9

Ion Release Results

Passivating Solution Co (ug) Cr (ug) Mo (ug) Total Ions (ug)Nitric Acid 7.6 0.8 1.3 9.7Citric Acid 16.5 1.1 1.6 19.2

Control 27.2 0.6 2.1 29.9

•Cumulative data for 2 weeks exposure•

10

Enhanced Passivation• Process

• Surface Treatment• Utilizes Plasma Treatment w/ Passivation Process

(nitric acid or citric acid)• What Does it Do?• Provides a chromium rich surface

Average Atomic Concentra/on (%) N=7

Co Cr Mo

Citric Enhanced  45Deg 15.6 76.0 8.4

Nitric Enhanced 45Deg 19.5 74.0 6.5

11

Enhanced Passivation

Co (ug) Cr (ug) Mo (ug) Total Ions (ug)Cast CoCr PC Control 27.18 0.66 2.08 29.92 Cast CoCr PC Citric UP 6.14 0.60 1.60 8.34 Cast CoCr PC Nitric UP 3.26 0.52 2.78 6.56

•Cumulative data for 2 weeks exposure•

12

Conclusion• The data suggests there is a difference between Nitric

acid and Citric acid passivation.

• Enhanced Chromium layer from Nitric acid passivation vsCitric acid passivation.

• May or may not be clinically relevant but increased metalion release can not be good.

• Enhanced passivation can resolve the differencesbetween Nitric and Citric passivation and level the playingfield.

13

Thank you!