Background: Wear of the underside of modular tibial inserts
(backside wear) in total knee replacements has been reported by several
authors. Although, for some implant designs, this phenomenon seems to
contribute to osteolysis, the actual volume of material lost through wear of
the backside surface has not been quantified. This study describes the results
of computerized measurements of tibial inserts of one design known to be
associated with a high prevalence of backside wear in situ.
Methods: A series of retrieved total knee components of one design
were examined. The duration of implantation of the retrieved components ranged
from thirty-six to 146 months. Laser surface profilometry and computer-aided
design software were used to develop individual three-dimensional models of
each worn, retrieved tibial insert to compare with scanned unused inserts.
Volumetric subtraction of both models revealed the material lost because of
Results: Worn and unworn areas on the backside surface were easily
identified by stereomicroscopy and laser profilometry. The computer
reconstructions showed that, in all retrievals, all unworn surfaces on the
nonarticulating surface lay in one plane. The average volume (and standard
deviation) of the material lost because of backside wear was 925 ± 637
mm3 (range, 197 to 2720 mm3). On the basis of the time
in situ for each implant, the average volumetric wear rate was 138 ± 95
Conclusions: The predicted volume of material removed because of
backside wear is substantial and may be sufficient to induce osteolysis. Our
results suggest that peg-like protrusions are not generated by the extrusion
of polyethylene into screw-holes within the base-plate but by abrasion of the
underside of the bearing insert, leaving the protruding pegs as the only
remnants of the original surface.
Clinical Relevance: This study provides quantitative predictive data
supporting previous qualitative studies showing that backside wear is an
important and relevant damage mechanism in contemporary designs of knee
replacements and may produce substantial volumes of wear debris.