- Discussion: Stem Failure
- stem fractures are much less common now with stronger alloys;
- cracks commonly begin on anterolateral surface of femoral component;
- frx usually begins in the middle 1/3 of anterolateral aspect of the stem and progresses medially;
- factors w/ highest correlation to stem fracture are:
- increased incidence with heavy, active patients, especially when the stem is undersized;
- stems with decreased cross sectional area and long necks
- inadequate calcar cancellous bone removal (which leads to undersizing of the femoral stem);
- poor cement mantle is a strong risk factor;
- precoating the femoral stem with methylmethacrylate (may lead to posterolateral debonding);
- poor support in proximal 1/3 (
bending cantilever fatigue);
- when stem is proximally loose and distally well fixed, which is usually found in patients with good diaphyseal cortices;
- cement debonding of the femoral stem along the proximal-lateral aspect is often a precursor to fracture;
- likewise osteolysis underneath the cement collar is precursor to stem failure;
- varus positioning of stem;
- lateral stem nicks produced by drilling for greater trochanteric wires;
-
stainless steel components;
- sintered porous coating on
cobalt chromium stems w/ a diameter less
than 11 mm, which are inserted in active heavy patients;
- Technique of Removal:
- removal of broken femoral stems poses special problem because remaining
distal portion of the stem may be held rigidly in its cement bed;
- proximal stem fragment is removed in usual manner;
- distal stem fragment is removed by creating a window in the anterior cortex
just below the level of stem fracture;
- after making window down to the metal surface, use sharp tipped punch, to
divot into the metal and push distal metal fragment proximally;
- as stem move proximally, additional divots are then made;
Fatigue fracture of a forged cobalt-chromium molybdenum femoral component inserted with cement. A report of 10 cases.
ST Woolson et al.
JBJS Vol 79-A. No 12, Dec. 1997.
The removal of fractured prosthetic components from medullary cavities: A new technique.
Contemp. Orthop 8:61, 1984.
