UCSF

Almost 200,000 total hip replacements (THRs) were implanted in 2007 in the United States. About one-third of the recipients were under the age of 65. Since THRs typically last 10-20 years, many patients will outlive their implants and require painful, costly revision surgery. Increasing the lifetime of hip replacements would reduce the need for revision surgery and greatly improve patient quality of life.

The most common cause of late-stage failure of THRs is wear-mediated osteolysis, bone loss around the implant caused by chronic joint inflammation from wear particles generated at the articulating surfaces of the hip replacement. In implants that contain an ultrahigh molecular weight polyethylene (UHMWPE) acetabular cup, the UHMWPE generates the majority of wear particles. Consequently, reducing wear at the UHMWPE surface is expected to increase the lifetime of the implant.

This research focuses on improving lubrication at the articulating surface by coating the UHWMPE with a hydrophilic layer of crosslinked poly(ethylene glycol) (PEG), a lubricious, biocompatible hydrogel. First, the plasma deposition process used to generate the PEG-like coatings was developed. Plasma deposition power, monomer flow rate, and chamber temperature all affected the coating chemistry and crosslinking, while deposition time and substrate placement determined coating thickness. The resulting coatings are 76-83% ether, indicating that they are very similar to PEG, and are much more hydrophilic and protein-resistant than uncoated UHMWPE.

The PEG-like coatings withstood pin-on-disk testing at 3 MPa contact pressure, comparable to total hip replacements in vivo. Analysis of the wear behavior at the mesoscale (tens to hundreds of microns) and microscale (less than 10 μm, similar to asperity contact) determined that the coatings fail due to a combination of adhesive, abrasive, and delamination wear mechanisms; however, delamination predominated at the mesoscale. Comparison of two sets of coatings with different degrees of crosslinking determined that, at the microscale, the more crosslinked coatings better resist wear and fail only by delamination. This suggests that the wear resistance of PEG-like coatings could be further improved by increasing the degree of covalent bonding to the UHMWPE substrate and increasing the crosslink density of the coatings.