Highly Crosslinked & Vitamin-E Polyethylene (Advanced Bearings)

The failure mode. Conventional ultra-high-molecular-weight polyethylene (UHMWPE) wears, and the submicron particles provoke a macrophage-driven inflammatory response. The result is periprosthetic osteolysis, then aseptic loosening and revision, and it is a major cause of long-term failure in both total hip and total knee arthroplasty.

Measuring wear. Wear is quantified as a rate, and there are two:

• Linear (penetration) wear - how far the head sinks into the cup each year, in mm per year
• Volumetric wear - the volume of polyethylene removed, in mm cubed per year

Volumetric wear is roughly the penetration multiplied by the bearing cross-section, so a larger head produces more volumetric wear for the same penetration. When a rate is interpreted, the early bedding-in (run-in) creep and penetration should be separated from the lower true steady-state wear.

How it is measured. Wear is measured radiographically. Radiostereometric analysis (RSA) is the gold standard, far more sensitive and earlier than plain-film penetration measurements, and HXLPE wear is so low that it can sit near the limit of measurement.

The osteolysis threshold. Osteolysis is rare when linear wear is below about 0.1 mm per year, and uncommon below roughly 0.05; this is the established "wear threshold". Conventional UHMWPE often exceeds it. HXLPE penetration is roughly an order of magnitude lower, frequently below the threshold, which is why osteolysis is so much less common.

Why it allows larger heads. HXLPE keeps penetration below the threshold even with a larger femoral head. Surgeons can therefore use bigger heads, with better stability and range before impingement, without the osteolysis penalty the same head would have carried on conventional polyethylene. General wear mechanisms and tribology are covered in the Wear Mechanisms and Tribology topics.

How HXLPE is made. Highly crosslinked polyethylene (HXLPE), developed in the late 1990s, is produced by high-dose irradiation, typically around 50-100 kGy of gamma or electron-beam radiation. The dose creates cross-links between the polymer chains, and the crosslinked polymer shows markedly less adhesive and abrasive wear than conventional UHMWPE.

What it was worth. The hip registry evidence is strong. In the Australian registry, HXLPE had a cumulative revision rate of 6.2% against 11.7% for conventional UHMWPE at a mean 16 years after total hip arthroplasty, about half, confirming that the in-vitro wear reduction translates into fewer revisions.

The free radicals. The same irradiation also generates free radicals. Over years on the shelf and in the body they react with oxygen, causing oxidation, embrittlement and a reduction in mechanical and fatigue properties. It is an important concern especially for the thinner, higher-stress bearings used in the knee, and it is why the radicals must be addressed.

Two stabilisation strategies are used: thermal treatment, in two versions that trade against each other, and vitamin E.

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Remelting. Heating above the melt transition eliminates the free radicals most completely and gives the best oxidation resistance. The price is reduced crystallinity, and with it lower mechanical and fatigue strength.

Annealing. Heating below the melt transition preserves the mechanical properties better but leaves residual free radicals, and so some potential for later oxidation.

Vitamin E. Alpha-tocopherol is an antioxidant that quenches the radicals chemically with no thermal step. It stabilises them and resists oxidation while preserving mechanical and fatigue properties, and it can allow higher crosslinking. It is incorporated by blending before consolidation and irradiation, which is simpler but competes with the crosslinking reaction itself, or by diffusion afterwards.

What vitamin E has shown. On wear, vitamin-E HXLPE has been shown to be similar to remelted HXLPE, on short-term data, with better-preserved mechanical properties. Registry survivorship is at least equivalent to other antioxidant liners at short-term follow-up. No long-term registry has yet shown that it reduces revision.

Toughness is the price of crosslinking. Crosslinking, and especially remelting, reduces the polyethylene's toughness, ductility and resistance to crack propagation (fracture toughness and fatigue-crack resistance) even as it reduces wear. Clinically this appears as mechanical failure of the bearing: most characteristically fracture of the acetabular liner rim or the locking mechanism, and in the knee fatigue cracking or delamination, or insert fracture.

What favours it. Failure is favoured by the following:

• A thin liner - a small cup with a large head, or a thin tibial insert
• Component malposition - a steep or retroverted cup edge-loads the rim
• Impingement - neck-on-rim levering
• Elevated or oxidised first-generation material

Mitigation. Respect a minimum liner thickness, and do not pursue a head so large that the liner is too thin. Position the components correctly to avoid edge-loading and impingement. Prefer stabilisation that preserves toughness: vitamin-E and annealed HXLPE retain more toughness than remelted HXLPE.

The hip. In total hip arthroplasty HXLPE has a clear, registry-proven wear and revision benefit over conventional UHMWPE, and it is the standard bearing.

The knee. In total knee arthroplasty the benefit is less consistent, and HXLPE has not reliably shown the same advantage. The knee wears by fatigue and delamination under high contact stress rather than by adhesion, and its tibial inserts are thinner, so the mechanical and fatigue strength that crosslinking costs matters more there. Oxidation-resistant and vitamin-E options are attractive in the knee.