Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 308

2. There is evidence that proteins participate in corrosion, increasing
metal release, without affecting externally measurable electrical
parameters (“charge-neutral” corrosion).

3. Articulation of bearing surfaces (e.g., THR femoral head) under
load is capable of disrupting, and even physically removing, por-
tions of the passive layer, even on titanium-base alloys. The con-
tinuing process of passive layer disruption and reformation may
result in metal release.

4. The role that the poly(methyl methocrylate) (PMMA) cement layer
plays in the corrosion of cemented devices is not clearly under-
stood. Loose devices clearly show signs of fretting on the stem.
There are reports of revision THAs demonstrating that the pH of
the solution present at the narrow (50-100 μm) crevice between
the stem and PMMA cement may be as low as 3, creating an envi-
ronment for crevice corrosion attack.

5. Porous structures, such as those used for fixation by biologic
ingrowth, may have as much as 10-fold-elevated corrosion rates,
compared with smooth surfaces. These rates are nearly propor-
tional to the increased specific surface area of the implant. The
effect of tissue ingrowth on corrosion rates is unknown.

6. Corrosion of chromium-containing iron-base and cobalt-base
alloys is regulated to a large degree by the presence of chromium-
oxygen compounds in the passive layer. These compounds are
highly insoluble at physiologic pH (see Figure 12.4), but the acido-
sis associated with local infection is probably sufficient to produce
pH values below 6 that will produce significant loss of the passive
layer and may interfere with normal repassivation.

We ar
depassivation

Crevice/fretting
corrosion

Porous
coating

Pitting
corrosion

Unmantling

Crevice
corrosion

Fretting

Also:
Uniform attack
Stress corrosion
Charge neutral corrosion (?)
Infection
Electric fields (?)

FIGUre 12.16 (See color insert.) types of corrosive attack on proxi-
mal femoral thr component. (adapted from semlitsch, m., Willert,
h.G., Med Bio Eng Comput 18:511-520, 1980.)