Using a metal acetabular socket as well as a metal
cap over the femur head (metal-on-metal) eliminates the
polyethylene debris produced in THA. The metal wear
debris from a hip surface replacement produces smaller
particles than polyethylene wear debris. The
inflammatory response to metal debris is considerably
less than that from polyethylene debris. It is believed
that the body can partially dissolve and expel metal
since it is a naturally occurring substance in the body.
There is concern by some of the toxicity of metal, but
there is currently no definitive evidence that metal
ions cause cancer. Since a metal surface does not wear
as readily as a polyethylene lining, a larger ball
(approximately 38-51 mm) can be used that adds stability
to the joint and reduces the danger of dislocation.
The surgery time for hip surface replacement is slightly longer than that for THA. The attachment of the acetabular socket is basically the same. It is press-fitted and does not require bone cement. The attachment of the cobalt-chrome cap requires a more precise alignment, and it takes slightly longer to fit. The hole for the pin insertion must be aligned and drilled, and the dome of the femoral head must be ground and shaped to fit the cap. Some bone cement is used to affix the cap, but the interior surface of both the cap and the socket is such that bone grows into the relief surface to grip the device.
Risks involved with hip surface replacement surgery
Risks involved in the hip surface replacement surgery are the same as the risks involved in any major surgery. Risks specific to the hip surface replacement involve the potential for cracking in the neck of the femur bone due to the drilling of the guide hole through the neck for the support pin in the metal cap, and also a negative reaction of the femur head to dislocation and being reshaped to fit the metal cap leading to the development of avascular necrosis (bone death)--often referred to as AVN--due to a disruption of blood circulation to the femur head and neck. In such instances, a THA could easily be performed to correct the problem.