Problem of THA

THA is a process in which is the hip joint is replaced by a prosthetic implant, and it has a high incidence: an estimated number of more than one million operations each year worldwide. And after the sugery, both short-term and long-term problems may occur, problems may even occur during the sugery:

  • Dislocation: Dislocation is the most common complication of hip replacement surgery. At surgery the femoral head is taken out of the socket, hip implants are placed and the hip put back into proper position. It takes eight to twelve weeks for the soft tissues injured or cut during surgery to heal. During this period, the hip ball can come out of the socket. The chance of this is diminished if less tissue is cut, if the tissue cut is repaired and if large diameter head balls are used.And it also depends on the experience of the doctor.
  • Loosening and Osteolysis: After the sugery, as time goes by the bond between the bone and the implant or the cement may break down or fatigue. As a result the component moves inside the bone, causing pain. Fragments of wear debris may cause an inflammatory reaction with bone absorption which can cause loosening. This phenomenon is known as osteolysis.
  • Metal sensitivity: Concerns are being raised about the metal sensitivity and potential dangers of metal particulate debris. New publications that have demonstrated development of pseudotumors, soft tissue masses containing necrotic tissue, around the hip joint. It appears these masses are more common in women and these patients show a higher level of iron in the blood. The cause is unknown and is probably multifactorial. There may be a toxic reaction to an excess of particulate metal wear debris or a hypersensitivity reaction to a normal amount of metal debris.
  • Nerve palsy: Post operative sciatic nerve palsy is another possible complication. The incidence of this complication is low. Femoral nerve palsy is another but much more rare complication. Both of these will typically resolve over time, but the healing process is slow. Patients with pre-existing nerve injury are at greater risk of experiencing this complication and are also slower to recover.

Although literature shows the survival rate is high, there is still a large number of revisions, and most of them are because of loosening. Revision operations are more difficult to perform, carry more risk for complications and have a poorer prognosis than primary THA.

Beside the design, material composition and surface characteristics of the implant, the initial per-operative fixation of the
stem in the femoral bone has a critical influence on its long term fixation stability. This is especially the case for non cemented, press-fit fixated stems.

So this is where vibration analysis can be applied: It is used to assess the initial stability of the femoral THA component, intra-operatively.

Source: Wikipedia,

Leonard Pastrav, Monitoring of the fixation of orthopaedic implants by vibration analysis

What is a vibration test and vibration test on hip implant

Vibration testing is the shaking of a product or package to determine its ability to survive in real world conditions. These tests can be categorized into:

1) Transportation environment 
Packages and products must be assessed to ensure that they can survive transportation via truck, air or rail.

2) Operating environment 
Many products have to survive vibration in their daily working life, regardless of how they were originally transported. Examples include electronics in vehicles, construction equipment and aircraft.

3) Storage environment of sensitive products
Some sensitive products experience vibration when sitting on the floor of a building, since most buildings vibrate. External factors that cause this kind of vibration include air conditioning, other operating equipment and passing vehicles.

The assessment of hip implant using vibration techniques is an application of vibration test under operating environment. Hip joints, especially of old people, get easily damaged due to the pressure caused by body weights, impacts from the ground when walking, climbing stairs, and other internal/external factors. Ideally vibration tests can be used to identify different situations like good fixation, loose contact and half-loose contact, and thus serve to determine if  a THA ( cf. ‘ Prothesis and Total hip arthroplasty(THA) ) surgery is necessary.