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Total Hip Replacement - Implant Failure

STEM FAILURE

Usually develops several years after surgery - average 3.5 years.

The fracture usually begins in the middle third of the anterolateral aspect of the stem & progresses medially (due to bending & torsional forces)

The bone-cement composite holds part of the stem securely enough that sufficient stress can be applied to bend or fracture the stem

Deformation and fracture of the stem occurs in response to cyclic loading 

Increased incidence with:

  1. heavy, active patients

  2. varus position of stem

  3. stems with small cross-sectional area 

  4. stem with long neck or increased offset

  5. stainless steel - low fatigue strength 

  6. metallurgical defects in the stem

  7. poor support in the proximal third

Bending and incomplete fracture of the stem are painless

AP and lateral roentgenograms should be compared carefully with previous films to detect permanent deformation or bending of the stem. This usually is first noted in the AP view, but sometimes posterior bending can be seen in the lateral view. 

Torsion tends to deflect the stem medially and posteriorly. 

To detect whether the stem is bent, the angle made by a line drawn through the center of the head and neck and another drawn parallel to the lateral surface of the distal two thirds of the stem is measured and compared with previous roentgenograms. 

Also, superimposition of the roentgenograms may indicate whether the stem has been permanently deformed. 

If the stem is bent at all, and certainly if the bending is progressive or if an incomplete fracture is recognized, revision is indicated as soon as possible
so that the stem can be removed in one piece before it fractures completely. 




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