The Hip Joint
How Does an Artificial Hip Compare to a Healthy Natural Hip?
An artificial hip is not identical to a healthy natural hip joint. It is much more prone to dislocation, as shown in the accompanying X-ray (Fig 8a).
This happens because the artificial hip is lacking in certain structures that normally hold the head of the femur securely in the acetabulum. For example, ligaments can be stretched and damaged in the period prior to hip replacement surgery.
After surgery, these ligaments, which lie under the repair site, will still be stretched. Also, during the operation, part of the fibrous capsule that normally surrounds and stabilizes the hip joint is removed in order to have clear access to the hip joint. Although capsular material grows back over the next 6-12 weeks, it is thinner and more scar-like than the original capsule tissue and does not necessarily function as well.
The labrum is another stabilizer that is sacrificed during surgery. This tough piece of cartilage is removed from the rim of the acetabulum to enable dislocation of the diseased femur and implantation of the prosthesis. It is not replaced.
Loss of these stabilizing structures all make it easier for an artificial hip to dislocate, particularly in the first six weeks after surgery while the capsule is regrowing. The artificial hip is especially susceptible to posterior dislocation if the leg is raised with a bent knee and rotated inward. Sudden jolting movements may also lead to dislocation. Your artificial hip will never be as stable as a normal healthy hip so you must learn to avoid certain movements.
Another difference is that the synovial fluid produced in the natural hip joint, acts as a lubricant. In the artificial joint, smooth gliding action depends on evenly honed surfaces.
Both natural components and artificial components are affected by wear and tear. While the artificial components do not show the same type of aging and deterioration that bone does, they are susceptible to corrosion and metal fatigue. Moreover, the stem component depends upon the surrounding bone tissue being sufficiently healthy to hold it in place.