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Backside Wear of Polyethylene Tibial Inserts: Mechanism and Magnitude of Material Loss
Michael A. Conditt, PhD1; Matthew T. Thompson, MS1; Molly M. Usrey, BS1; Sabir K. Ismaily, BS1; Philip C. Noble, PhD2
1 Institute of Orthopedic Research and Education, 6550 Fannin, Suite 2512, Houston, TX 77030. E-mail address for M.A. Conditt: mconditt@bcm.tmc.edu
2 Barnhart Department of Orthopedic Surgery, Baylor College of Medicine, 6550 Fannin, Suite 2625, Houston, TX 77030
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The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at the Institute of Orthopedic Research and Education, Houston, Texas

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2005 Feb 01;87(2):326-331. doi: 10.2106/JBJS.C.01308
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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 backside wear.

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 mm3/yr.

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.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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