Highly Cross-Linked Polyethylene Bearings

Background

The success of total hip arthroplasty (THA) has been well documented. Since its inception in the 1960s, the major obstacles to success in THA have been component fixation and articular bearing wear. With documented improvements in durable component fixation over the past 4 decades, wear remains the most troublesome issue in THA. Materials used for the acetabular articular bearing in THA have included Teflon, polyurethanes, metal alloys, ceramics, and polyethylenes. Over the past 4 decades, modifications to the structure and production of polyethylene have shown both positive and negative effects. Polyethylene remains the most widely used acetabular bearing surface in the world. Our knowledge of its structure and performance will strongly influence the future of THA.

Polyethylene Manufacturing

Commercial production of ultra-high-molecular-weight polyethylene (UHMWPE) began in the early 1950s. The largest current manufacturer of UHMWPE resin is Ticona (Florence, KY). All Ticona resins are labeled “GUR.” Current medical/orthopedic-grade resins include GUR 1020, GUR 1120, GUR 1050, and GUR 1150. The numeric label following GUR explains the details of the resin. The first digit (1) indicates that the resin is for orthopedic usage. The second digit represents the presence (1) or absence (0) of calcium stearate. The third digit indicates the molecular weight of the resin. The final digit is used for manufacturing corporate coding (1). Although other resins (1900 series produced by Hercules Powder, Wilmington, DE) have been used for orthopedic purposes and clinically studied, they are no longer in production. The remainder of this chapter will focus on the current GUR resins in clinical usage (GUR 1020 and GUR 1050).

Polyethylene is composed of repeating ethylene monomers, each of which contains two carbon atoms. Polyethylene molecules contain a crystalline region and an amorphous region. The polyethylene chains can be described as low or high density based on their molecular weight and chain length. Polyethylene begins as a resin or powder and is fabricated into bulk form for orthopedic implant designs. Resins are developed into bulk material by direct compression molding or are machined from ram-extruded bars or large molded sheets.

The mechanical properties of any given polyethylene implant are affected by the resin, molecular weight, and preparation of the material. The temperature, pressure, and cooling rate during the manufacture of polyethylene will influence its ultimate properties. Final component preparation is completed by direct compression molding (the polyethylene is melted and then solidified in its desired shape) or by machining the material into its desired shape.