Copyright © 2009 by The Journal of Bone and Joint Surgery, Inc.
Commentary & Perspective
Commentary & Perspective by
John H. Dumbleton, PhD, DSc*,
Consultancy in Medical Devices, Ridgewood, New Jersey
Posted April 2009
Highly cross-linked ultra-high molecular weight
polyethylenes were introduced as hip bearing surfaces in the late 1990s after
hip-simulator studies demonstrated greatly reduced wear in highly cross-linked
ultra-high molecular weight polyethylene components as compared with
conventional polyethylene components. Various methods have been used for cross-linking,
but all have the commonality of an irradiation step to form crosslinks and a
heating step to reduce or eliminate free radicals1. Heating below
the melting temperature of polyethylene (annealing) maintains mechanical
properties, while heating above the melting temperature leads to property
reduction2. It has been learned that gamma sterilization following
cross-linking results in an increase in the level of free radicals3.
All highly cross-linked ultra-high molecular weight polyethylenes have shown
reduced wear clinically, but a corresponding reduction of osteolysis is still
somewhat unproven. Despite the lower wear rates, in vitro studies indicate that
the wear debris is more biologically active. Clinical evidence for reduced
osteolysis is beginning to emerge from computed tomography studies. I believe
that a consensus would agree that the greatly reduced wear rates outweigh the
high biological activity of the wear debris.
In this issue of The
Journal, McCalden et al. present the results of a randomized controlled
trial in which a cemented stem (VerSys; Zimmer, Warsaw,
Indiana) and a cementless acetabular
component (Trilogy, Zimmer) were used. The liners had a 28-mm inner diameter and
a 10° lip. Highly cross-linked ultra-high molecular weight polyethylene liners (Longevity;
Zimmer) were used in fifty patients, while fifty patients received conventional
polyethylene liners (Trilogy; Zimmer) that had been sterilized with gamma
radiation (25 kGy) in an inert nitrogen environment. It should be noted that
the conventional material does contain crosslinks resulting from the
sterilization process. Longevity highly cross-linked ultra-high molecular
weight polyethylene is irradiated to 10 Mrad (100 kGy) followed by remelting
with gas-plasma sterilization. The purpose of the study was to compare clinical
and radiographic results at a minimum of five years.
At five years, there was no difference in clinical outcome
between the patient groups. Penetration of the head into the polyethylene was
evaluated on anteroposterior radiographs with use of edge-detection software
(Martell Hip Analysis Suite, version 8.0.3.0; University
of Chicago, Chicago,
Illinois). So-called wear-in and deformation
of the polyethylene is largely complete at one year; thus, the continuing
penetration after one year is a measure of wear. There was a substantial number
of negative wear rates in the highly cross-linked group.
The authors noted that only 78% of the radiographs were
acceptable for analysis, with most radiographs eliminated due to software
rejection. Early versions of the Hip Analysis Suite did not correct for
elliptical distortion caused by projection of the circular components on the
radiograph from a pelvis-centered x-ray. The result was the rejection of many radiographs
and often the measurement of negative wear. The version used in the study (8.0.3.0)
does embody a correction for elliptical distortion4. However, there
may still be systematic errors due to digital processing of the films. Version
8.0.3.0 does not read DICOM images and requires conversion of PACS DICOM to
TIFF before analysis. Unfortunately, the export to TIFF causes some loss of beam-center
information and introduces errors if the acetabulum is low or high on the film.
This may be the reason for the high rejection rate and negative wear in the study
by McCalden et al. The latest version of the Hip Analysis Suite (8.0.8.0)
corrects for this shortcoming, and phantom studies indicate that wear
measurements are comparable to the accuracy attained with radiostereometric analysis5.
Radiographs were studied to determine the prevalence of osteolysis.
No osteolysis was seen at five years for either group of patients. As the
authors point out, this time period would be early for osteolysis to be
detected and one would not expect to see lesions with such low wear rates as
were reported, even with conventional polyethylene. Radiographic detection of
osteolysis is not sensitive and computed tomography appears to be the standard
both for detection and measurement of lesion volume. However, this is not a
technique to be used during routine patient follow-up. Recently it has been
shown that textural analysis of radiographs has promise in early detection of
osteolytic lesions without resorting to the use of computed tomography6.
The study made use of 28-mm heads. The use of larger head
sizes provides added protection against dislocation. Unlike conventional
polyethylene, which exhibits higher wear with increased head size, highly
cross-linked ultra-high molecular weight polyethylene exhibits wear that is
largely independent of head size7. One of the benefits of highly
cross-linked ultra-high molecular weight polyethylene is flexibility with
regard to the choice of head size that is appropriate for the patient.
Although highly cross-linked ultra-high molecular weight
polyethylene has performed well clinically, it is now recognized that there are
compromises over the choice between annealing and remelting. Next-generation highly
cross-linked ultra-high molecular weight polyethylenes are manufactured with
use of processes that seek to minimize these compromises. The result is
increased flexibility for the surgeon with regard to component size (diameter
and thickness of the insert) and the application of these materials to total
knee replacement.
Note: The author thanks Dr. John Martell, Director of the Orthopaedic Biomedical Imaging Institute, University of Chicago, for helpful discussions regarding the Hip Analysis Suite.
*The author did not receive any outside funding or grants in support of his research for or preparation of this work. The author, or a member of his immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Stryker Orthopaedics).
References
1. Kurtz SM. The UHMWPE handbook: Ultra-high molecular weight polyethylene in total joint replacement. San Diego: Elsevier Academic Press; 2004. Compendium of highly crosslinked and thermally treated UHMWPEs; p 337-61.
2. Oral E, Malhi AS, Muratoglu OK. Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE. Biomaterials. 2006;27:917-25.
3. Kurtz SM, Manley M, Wang A, Taylor S, Dumbleton J. Bull Hosp Jt Dis. 2002-2003;61:17-26.
4. Kraay MJ, Lehman RD, Martell JM, Rimnac CM. Improvements in computerized wear measurement for THA: Is it as accurate as RSA? Trans Orthop Res Soc. 2008;33:0236.
5. Martell JM, Mangla J, Chinander MR. Detection using computerized digital analysis and elliptical distortion correction. https://webshare.uchicago.edu/xythoswfs/webui/_xy-2237772_1-t_ijFJ9A9n. Accessed 2009 Jan 22.
6. Wilkie JR, Giger ML, Chinander MR, Engh CA, Hopper RH Jr, Martell JM. Temporal radiographic texture analysis in the detection of periprosthetic osteolysis. Med Phys. 2008;35:377-87.
7. Muratoglu OK, Bragdon CR, O'Connor D, Perinchief RS, Estok DM 2nd, Jasty M, Harris WH. Larger diameter femoral heads used in conjunction with a highly cross-linked ultra-high molecular weight polyethylene: a new concept. J Arthroplasty. 2001;16(8 Suppl):24-30.
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