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Reducing Bone Loss Through Femoral Stem Design

Posted on: 12/28/2006
For 30 years researchers have looked for ways to reduce bone loss after total hip replacement (THR). The stiff implant results in atrophy of the bone. This reaction occurs as the bone reacts to the mismatch in materials where the implant meets the bone. Bone resorption of this type is called stress shielding.

Many studies have tried to identify the cause of stress shielding. Stiffness of the femoral stem could be the problem. The femoral stem fits down into the femur (thigh bone). It is attached to the new ball-shaped femoral head that fits into the hip socket.

Or perhaps the cause of bone resorption is the type of material used to make the implants. Other scientists have studied the shape and angles of the stem looking for an answer. The goal is to find a way to reduce postoperative bone loss. Stress shielding can cause fractures further compounding the problem.

New femoral designs are the subject of this article. The authors ask the question, Do they influence stress shielding? Only studies using dual-energy X-ray absorptiometry (DEXA) were included. DEXA allows measurement of bone loss both before and after surgery.

The results of different types of stem design were reviewed. The following findings are reported:
  • Porous-coated stem have less bone loss
  • Large implants that fill the top portion of the femur then taper down at the bottom have greater stress shielding and bone loss. This may occur because of mismatched stiffness from top to bottom.
  • Stems made of titanium were less stiff than cobalt-chromium alloy stems. In general, there's less bone resorption with less stiffness.
  • Flat, wide implants are less stiff and more stable and may work better than round stems.
  • Splitting the stem like a clothespin or carving out flutes or slots have been tried unsuccessfully. Researchers think a solution at the bottom of the stem may not work if the problem is at the top part of the implant.

    All in all, it appears that stem stiffness, not porous coating of the stem is the major factor in bone loss from stress shielding after THR. Future studies are needed to compare results against the level of porous coating (top to bottom of the implant). With the new DEXA testing, comparisons of this type are now possible.

  • References:
    A. H. Glassman, MD, MS, et al. New Femoral Designs. In Clinical Orthopaedics and Related Research. December 2006. No. 453. Pp. 64-74.

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