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Fatigue Fracture of a Forged Cobalt-Chromium-Molybdenum Femoral Component Inserted with Cement. A Report of Ten Cases*

WOOLSON, STEVEN T. M.D.†, STANFORD; MILBAUER, JOHN P. M.D.‡, MARSHFIELD, WISCONSIN; BOBYN, J. DENNIS PH.D.§; YUE, STEPHEN PH.D.§, MONTREAL, QUEBEC, CANADA; MALONEY, WILLIAM J. M.D.#, STANFORD, CALIFORNIA

Journal of Bone & Joint Surgery - American Volume: December 1997 - Volume 79 - Issue 12 - p 1842–8
Case Report

Ten patients who had had a total hip replacement with a forged cobalt-chromium-molybdenum femoral prosthesis (Precoat or Precoat Plus) inserted with cement were seen with a fatigue fracture of the stem an average of fifty months (range, nineteen to seventy-four months) postoperatively. The average age of the patients was sixty-one years (range, forty-three to seventy-three years), and the average weight was ninety-six kilograms (range, seventy to 130 kilograms). Eight patients had had a primary total hip replacement, and two had had a revision; all of the acetabular components had been inserted without cement. Radiographs that had been made before the fracture were available for four of the eight hips that had had a primary replacement; all four had radiographic evidence of debonding of the cement mantle from the proximal end of the stem. This probably caused exaggerated cantilever bending stresses on the proximal aspect of the stem as the distal end of the stem was well fixed. The radiographs of both hips that had had a revision demonstrated a non-union of the greater trochanter, which had resulted in separation at the cement-bone interface at the proximal portion of the femur before the fracture. Scanning electron micrographs of five of the ten fractured prostheses demonstrated a fatigue fracture that began near the anterolateral corner of the prosthesis, through characters that had been etched on the implant with a laser. Metallurgical analysis indicated subsurface voids or inclusions, or both, immediately under the region that had been etched. This finding is consistent with thermal changes to the microstructure of the alloy that probably caused a focal reduction in the material strength. A high proportion (seven) of the ten stems had a poor cement mantle. Also, of the seven small stems that were used, six had been implanted in patients who weighed more than eighty kilograms, so there was relative undersizing of the prostheses. Early debonding of the proximal end of a Precoat femoral prosthesis from the cement mantle may occur as a result of a thin cement mantle, leading to loosening and possibly to early fatigue fracture of the stem if the distal portion of the stem remains solidly fixed in the distal portion of the cement column. On the basis of our experience, we recommend that patients who have radiographic evidence of a debonded Precoat femoral component should be informed of the risk of fatigue fracture of the stem and be followed closely even though there may be no symptoms of loosening of the femoral component.

†1220 University Drive, Suite 202, Menlo Park, California 94025.

‡Marshfield Clinic, 1000 North Oak Avenue, Marshfield, Wisconsin 54449.

§Montreal General Hospital, 1650 Cedar Avenue, Montreal, Quebec HG3 1A4, Canada.

#Department of Orthopaedic Surgery, Washington University Medical Center, 1 Barnes Hospital Plaza, Suite 11300, St. Louis, Missouri 63110.

Copyright 1997 by The Journal of Bone and Joint Surgery, Incorporated
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