Vitamin E Stabilized Highly Cross-linked Polyethylene in Reversed Hybrid Total Hip

Background Reducing the rate of polyethylene (PE) wear in acetabular sockets and thereby increasing the lifespan of Total Hip Arthroplasty (THA) has been a major concern in the orthopaedic community for decades. Wear of PE and osteolysis followed by late aseptic loosening of the prosthesis is the main reason for revision surgery after THA in Norway.

During 2008 in Norway 805 out of a total of 1114 revised THA were subjugated to surgery due to aseptic loosening or osteolysis without loosening. The DRG (Diagnose Related Grouping) weight is 5.49 for secondary THA and every DRG point has a value of 33.647 NOK adding up to a direct total cost for revision after aseptic loosening of approximately 27 million USD.

Caput prosthesis with a diameter of 32mm or more has the major advantage to increase the Range of Motion (ROM) and to reduce the risk of dislocation of the hip but, the larger the caput prosthesis the more adhesive wear can be expected. This and the relative failure of metal on PE gave the incentive to develop and take into clinical practise bearings with increased resilience to wear. Today, there exists a wide range of options with a substantially lower rate of volumetric wear compared to conventional PE. The biological and immunological in vivo response to wear particles is very complicated and only partially understood. It is clear that host response to wear particles and the corrosion products play a major role in osteolysis around the prosthesis. For metal-on-metal (MoM) bearings the volumetric wear is a magnitude less but the number of particle debris is massively increased. MoM also have the problem with possible teratogen effect and allergy or hypersensitivity side effects. Ceramic on ceramic (cer-cer) has the lowest wear rate of any artificial articulations, but it is still possible to detect biologically active debris particles. Cer-cer also has other challenges including fracture of prosthesis, squeaking hip, narrow margins for placing components and cost.

Polyethylene is a thermoplastic polymer consisting of long chains of the monomer ethylene (C2H4). The mechanical properties of PE depend on variables such as the extent and type of branching and the crystal structure etc.

To improve the wear resistance of the PE an increase in the cross-linking between the chains of polyethylene can be achieved by exposing it to high-energy irradiation. The irradiation also increases the unpaired free electrons (free radicals). Free radicals can react with oxygen and start a chain reaction causing an oxidative degradation of the polyethylene and consequently cause increased wear. 1st generation HXLPE sockets have reduced the mid term incidence of particle-induced osteolysis but retrieved failed first-generation HXLPE sockets have shown surface cracking and signs of in vivo oxidative degradation.