A common feature of retrieved ceramic-on-ceramic (CoC) hips is the presence of metal transfer on the femoral head. This metal transfer represents an important change in the articulating surface and can have consequences in terms of lubrication, friction, wear, and squeaking. Given the potential impact of metal transfer on the performance of CoC bearing couples, a good understanding of the factors surrounding its occurrence is warranted. This study documents the metal transfer onto a ceramic femoral head with two subluxations onto the rim of the cup which occurred during surgery. This metal transfer is compared to that on other ceramic heads retrieved for various reported reasons, including squeaking, pain and loosening.

The first ten retrieved alumina heads of current ceramic technology (Ceramtec, Plochingen, Germany) submitted to our retrieval laboratory were assessed to document the phenomenon of metal transfer. Nine devices underwent in vivo service (mean duration 32 mo., range 13 to 84) and the tenth device was removed intra-operatively and serves as an instructive control case. It was impacted onto a trunnion and during final testing for stability subluxed anteriorly over the titanium lip of the cup. The metal transfer was immediately noted by the surgeon and the head was removed.

All ceramic heads were examined under light microscopy (Nikon Dissecting Microscope, Tokyo, Japan) and white light optical profilometry (NewView 7300, Zygo, Middlefield, CT).

The control ceramic head showed two distinct metal transfer streaks from two discrete subluxation events that were documented by the surgeon (IMT). Those streaks are aligned in a direction approximately 24o to the right (clockwise) of a line through the polar apex of the head and parallel to the axis of the femoral neck. Microscopy and profilometry indicate that they were laid down in a direction from equator-toward-pole.

Seven of the retrieved ceramic heads showed streaks of metal transfer that are very similar to those on the control ceramic head in terms of: alignment (equator-toward-pole, 20 to 45o off-axis) width (tapered point growing to approximately 1.0 to 1.5 mm), depth of metal deposition (0.25 to 0.40 μm), and depositional texture.

It is notable that the metal transfer streaks commonly observed on retrievals bear a close resemblance to that caused by a single intra-operative event wherein a hip abduction force pulled the head into contact with the titanium cup/liner rim. An important implication is that this demonstrates that metal transfer can occur with a single instance of rim contact, wherein the femoral head is forced against the metal cup rim. If metal transfer onto the head were to occur during final reduction of the hip, its presence may well be undetected and any deleterious in vivo impact of the metal transfer would be in effect from the day of surgery.

Irradiation cross-linking of UHMWPE has been shown to reduce wear while generating free radicals that oxidise in the presence of oxygen or oxidising species. Various methods have been used to minimise or eliminate the effect of these free radicals including below-melt annealing, remelting, Vitamin E infusion, or the use of other antioxidants. Each method has benefits and drawbacks with respect to wear properties, mechanical properties, and chemical properties. Accelerated aging techniques are used to evaluate the efficacy of new methods in stabilising free radicals in highly cross-linked UHMWPE.

Various procedures have been described for aging standard gamma-air sterilised UHMWPE to produce oxidation levels that represent shelf-aged bearings. An important factor in evaluating and comparing these aging techniques is validating that they reproduce the profile of oxidation (depth and magnitude) seen both in gamma-air, shelfaged polyethylene and in clinical retrievals. Moreover, the resulting oxidation level in the aged UHMWPE should predict the fatigue and/or wear damage seen in retrieved gamma-air inserts and liners.

The present study compared clinically relevant UHMWPE samples aged with ASTM 2003-00, (Method B: 70°C, 5 atm O2, 14 days) and a published lower temperature, lower oxygen-pressure environment (63° C, 3 atm O2, 28 days). Longer aging times (35 to 42 days) were also tested to examine oxidation rate and time to onset of mechanical degradation.

Both published methods result in oxidation of gamma-air and gamma-barrier sterilised polyethylene, but have little effect on remelted or antioxidant stabilised crosslinked polyethylene. These aging protocols, however, did not bring standard polyethylene to the critical oxidation level necessary for the fatigue damage that is seen in retrieved inserts and liners.

Oxidation of gamma-air and gamma-barrier sterilised UHMWPE increases exponentially with time on the shelf or in the two aging environments. Of note, longer aging times (35 to 42 days) that bring standard UHMWPE to sufficiently high oxidation levels for fatigue to occur also cause increased oxidation levels in remelted UHMWPE.

Oxidation increases were the smallest in antioxidant UHMWPE, though still detectable.

While this oxidation is not high enough in remelted material or antioxidant material to cause the fatigue damage seen in gamma-air sterilised UHMWPE, it does raise concerns about the published aging techniques and the long term stability of the new materials in vivo.

Relying on artificial aging techniques that do not adequately challenge even gamma-air polyethylene may conceal unforeseen weaknesses of new materials. Using longer aging times for existing techniques or novel aging approaches may be necessary to effectively evaluate the long term stability of new bearing materials.