Background: Wear of the polyethylene tibial bearing is a leading
cause of failure of knee replacements done prior to the current decade. The
objective of this study was to determine how patient-related factors,
implant-related factors, and limb or tibial component alignment influenced the
amount of thickness loss in polyethylene tibial bearings that were retrieved
at the time of revision surgery or after the death of the patient.
Methods: We retrieved polyethylene tibial bearings from eighty-one
unicondylar and eighty-nine total knee replacements that had been performed
because of osteoarthritis with varus deformity from 1984 to 1998. All of the
polyethylene bearings had been sterilized with gamma radiation in air.
Polyethylene loss was quantified as the change in the minimum bearing
thickness per years in vivo (the mean time in vivo [and standard deviation]
was 8 ± 4 years). Multiple linear regression was used to assess whether
polyethylene loss was associated with age, weight, gender, varus angle of the
tibial component, postoperative hip-knee-ankle angle, initial thickness of the
polyethylene, shelf age of the polyethylene, and either the type of
polyethylene (for total knee replacements, which were of one posterior
cruciate ligament-retaining design) or the manufacturer (for unicondylar knee
replacements), and to determine the magnitude by which polyethylene loss would
change if any of the significant risk factors were changed.
Results: The mean loss (and standard deviation) of polyethylene
thickness in the medial compartment of total knee replacements (0.33 ±
0.28 mm/yr) and that in medial unicompartmental knee replacements (0.49
± 0.40 mm/yr) were significantly (p < 0.05) associated with the same
three variables: patient age, postoperative hip-knee-ankle angle, and shelf
age of the polyethylene. A total knee bearing with a one-year increase in
shelf age, a unicondylar knee bearing with a six-month increase in shelf age,
a patient who was ten years younger at the time of operation, or a limb that
was aligned in 5° more varus (less valgus) had similar effects on the loss
of polyethylene thickness in the medial compartment; the coefficients of the
linear regression equations indicated that any one of these changes would
increase polyethylene loss by 0.11 to 0.14 mm/yr.
Conclusions: The wear-related loss of thickness in
gamma-irradiated-in-air polyethylene bearings from unicondylar and total knee
replacements implanted in osteoarthritic knees with varus deformity is
influenced mainly by the shelf age of the polyethylene, the age of the
patient, and the postoperative angulation of the knee in the coronal
Clinical Relevance: Although polyethylene bearings have not been
sterilized with gamma radiation in air since the end of the last decade, many
arthroplasty patients have polyethylene bearings that were sterilized with use
of this method. An understanding of the findings of this study may be of value
as these patients return for follow-up care. Whether the study findings have
relevance to bearings sterilized with other methods is unclear and will remain
so for many years.
Level of Evidence: Prognostic Level II. See Instructions
to Authors for a complete description of levels of evidence.