To the Editor:

In a recent article entitled, " Short-term in vivo wear of crosslinked polyethylene space " by Heisel et al., several statements made by the authors prompt some questions.

The authors report " a mean linear wear rate of 0.13mm per year for conventional polyethylene and a mean linear wear rate of 0.02mm per year for the crosslinked polyethylene ". The method used to quantify the penetration of the femoral head into the polyethylene for both groups was the Martell Method. That method does not, per say, provide data on wear. It provides data only on femoral head penetration.

The authors acknowledge in the Discussion that creep is an important contributor to penetration, particularly during the first two years. Since the mean follow-up of their conventional polyethelene group was two years and two months, creep must have played an important role in the penetration reported. Still, this penetration is reported as wear.

It is not possible to quantify wear until a steady state has been established. This study does not establish when steady state penetration is reached nor the time of the cessation of creep as a major factor in penetration. In the absence of that information, it seems unlikely that the authors are justified in using the term mean linear wear rate.

In addition, reading of penetration of the femoral head into the polyethylene using the Martell Method has a substantial degree of observer sensitivity. These data were all generated by a single observer.

The standard deviations of the pedometer readings are large, approximately one million cycles per year for each group. With a two million average number of cycles per year, does this imply that some of the outliers were remarkably inactive and others might have taken as many four million steps a year, to be able to include the range of data covering two standard deviations.

These considerations lead to the single key concern. It would appear that this study is incorrect in using the term mean linear wear rates in reporting the penetration data.

Sincerely,

William H. Harris, MD Orthopaedic Biomechanics and Biomaterials Lab Massachusetts General Hospital 55 Fruit Street, GRJ 1126 Boston, MA 02114 617-726-3866 (ph), 617-726-3883 (fax)