FATIGUE FRACTURE OF A FORGED COBALT-CHROMIUM-MOLYBDENUM FEMORAL COMPONENT INSERTED WITH CEMENT - A REPORT OF 10 CASES

Ten patients who had had a total hip replacement with a forged cobalt- chrolnium-molybdenum femoral prosthesis (Precoat or Precoat plus) inse rted with cement were seen with a fatigue fracture of the stem an aver age of fifty months (range, nineteen to seventy-four months) postopera tively. The average age of the patients was sixty-one years (range, fo rty-three to seventy-three years), and the average weight was ninety-s ix kilograms (range, seventy to 130 kilograms), Eight patients had had a primary total hip replacement, and two had had a revision; all of t he acetabular components Bad 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 radiograp hic evidence of debonding of the cement mantle from the proximal end o f the stem, This probably caused exaggerated cantilever bending stress es on the proximal aspect of the stem as the distal end of the stem wa s well fixed. The radiographs of both hips that had had a revision dem onstrated a non-union of the greater trochanter, which had resulted in separation at the cement-bone interface at the proximal portion of th e femur before the fracture, Scanning electron micrographs of five of the ten fractured prostheses demonstrated a fatigue fracture that bega n near the anterolateral corner of the prosthesis, through characters that had been etched on the implant with a laser, Metallurgical analys is indicated subsurface, voids or inclusions, or both, immediately und er the region that had been etched, This finding is consistent with th ermal 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 st eins that were used, six had been implanted in patients who weighed mo re than eighty kilograms, 60 there was relative undersizing of the pro stheses. Early debonding of the proximal end of a Precoat femoral pros thesis from the cement mantle may occur as a result of a thin cement m antle, lending to loosening and possibly to early fatigue fracture of the stem if the distal portion of the stem remains solidly fixed in th e distal portion of the cement column. On the basis of our experience, Me recommend that patients who have radiographic evidence of a debond ed 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.