AMIS is a particularly gentle, minimally invasive surgical procedure for implanting an artificial hip joint.
It is not a "keyhole" technique, which is characterized by tiny incisions and the use of endoscopic instruments. The advantage of the AMIS technique compared to open surgery lies in the smaller and tissue-sparing incision.
The incision is only around eight centimetres long. The surgeon makes the incision at the front above the hip joint.
Due to the anatomical conditions in this area, no muscles, nerves or tendons are injured during the operation. The surgeon can simply push the muscles to the side to reach the hip joint. This means that the new, artificial hip joint can be implanted without damaging the muscles.
A special leg holder is used to insert the hip endoprosthesis. It is designed to facilitate the operation and prevent stretching damage to thefemoral nerve.
Four surgical approaches have proven successful for the implantation of a hip endoprosthesis. The AMIS technique is one of them. The three other proven approaches are
- the dorsal (posterior) approach, which leads to the hip joint from behind through the gluteal muscles and the muscles of the external rotators. This means that the muscles are injured during the procedure.
- the lateral (side) approach, which leads from the side through the abductor muscles ("spreaders") to the hip joint. The muscles are also injured here.
- the anterolateral (front lateral) approach, which leads to the hip joint from the front laterally between individual muscles. Although no muscles are directly injured, damage to the middle gluteal muscle is more common due to the need to pull it to the side.
All four surgical procedures can be performed using small skin incisions. Only in the anterior approach (AMIS) and, to a limited extent, in the anterior lateral approach are the
are spared to such an extent that the procedure can be described as minimally invasive overall.
Excessive wear of the hip joint(coxarthrosis) is the reason for an artificial hip joint in around 90 percent of patients. Other reasons for surgery can include injuries and fractures as well as other diseases such as bone necrosis.
The hip joint consists of the head of the thigh bone(femur) and the cup-shaped socket of the pelvic bone. To prevent the bones from touching directly, the bones in the area of the joint are covered with cartilage. In addition, synovial fluid protects the joint from excessive friction.
If the cartilage layer wears away due to an illness or previous injury, this is known as osteoarthritis (joint wear and tear). This causes severe pain in the joint.
In most cases, the insertion of an artificial hip joint leads to a rapid improvement in pain. Without pain, the patient is also more mobile again and therefore has a better quality of life.
Due to the tissue-sparing procedure, the AMIS technique has decisive advantages over other access methods:
- There is less blood loss during the operation and less pain after the operation.
- The recovery time is shorter, allowing a quicker return to everyday life.
- The hospital stay is shortened.
- As the muscles are spared during the procedure, the stability of the hip is increased after the operation and therefore the risk of the joint "dislocating" is reduced. There are also no movement restrictions.
- Due to the reduced risk of injury to muscles and nerves, the risk of the patient limping is also reduced.
- Due to the smaller skin incision, only a smaller scar remains.
- As this area of the body has the lowest fat distribution, AMIS is also suitable for overweight people.
- The insertion of a hip prosthesis in both hip joints is possible under only one anesthetic.
Illustration of an artificial hip joint © crevis | AdobeStock
The operation can be performed under general anesthesia or spinal anesthesia. With spinal anesthesia, the patient is awake but their legs are insensitive to pain. The entire operation takes around 60 to 90 minutes.
During joint implantation using the AMIS technique, the patient is placed on their back and their leg is fixed in a special leg holder. The leg holder ensures that the leg can always be held in the optimum position during the operation.
After making a small incision in the skin above the hip joint, the surgeon follows the predetermined path between the muscles to the hip joint. The muscles themselves are not injured and bleeding can be avoided. As no important nerves cross the incision in this area, there is only a low risk of nerve injury with subsequent impairment of muscle function.
The tissue is kept open with special spreaders. This gives the surgeon a good view of the surgical area despite the small opening.
After opening the joint capsule, the femoral head damaged by osteoarthritis is first removed. The surgeon then inserts the new acetabulum into the pelvic bone. In the second step, he implants the artificial stem into the femur and attaches the prosthetic head.
The surgical wound is then sutured.
There is usually only minor pain after the operation, which is due to the muscle-sparing surgical technique. The hospital stay is only a few days.
A suitable rehabilitation program is put together individually for each patient. Outpatient physiotherapy is usually sufficient. Inpatient rehabilitation is also possible at the patient's request.
The patient can usually stand up and use walking aids on the day of the operation. The patient can be fully mobilized from the start with regard to axial ("vertical") weight-bearing. This means they can put as much weight on the hip as is comfortable for them.
After around two to three weeks, most patients no longer need walking aids. There are also no significant restrictions in terms of mobility. Sleeping on the side is possible right from the start.
Patients should refrain from building up strength in flexion and maximum load (e.g. leg press) as well as applications involving vibrations and strong axial compression during the first few months. This allows the endoprosthesis to grow well into the bone.
Overall, however, it is possible to return to everyday activities very quickly. Driving, for example, is theoretically possible again after about 2 weeks. For insurance reasons, however, the patient should wait about four to six weeks after the operation.
Depending on the occupation, it may be possible to return to work after just a few weeks. However, a longer break of several months is required for professions involving heavy physical work. This prevents the prosthesis from growing into the bone.
The result with the AMIS technique is better in terms of functional loading of the new hip joint, particularly in the first few months after the operation. Rehabilitation is significantly faster compared to alternative surgical procedures. The risk of complications is low.
As with any surgical procedure, the AMIS technique also involves a risk of
- Thromboses and embolisms as well as
- wound healing disorders and infections.
However, there is an increased risk of a small cutaneous nerve (the lateral femoral cutaneous nerve) running close to the surgical site being stretched by the surgical instruments used. This can lead to temporary slight stretching damage. This can manifest itself as a furry feeling next to or below the surgical scar, but this usually disappears again.
An artificial hip joint has an average lifespan of around 20 years.
In some cases, the prosthesis can loosen prematurely, in which case a prosthesis replacement would be necessary. Regular check-ups and, if necessary, X-ray examinations are required to detect any loosening of the prosthesis in good time.
In the case of infections, bacteria can also colonize the area of the endoprosthesis via the bloodstream and thus lead to loosening of the endoprosthesis. This can happen, for example, with a tooth infection or a sinus infection.
A reddened, swollen and warm hip joint region can indicate an infection of the endoprosthesis. It is then necessary to clarify exactly what is causing the symptoms. Patients must also inform their doctor about their artificial hip joint in the event of a trivial infection. The doctor can then initiate antibiotic therapy in good time.
Fortunately, loosened endoprostheses are relatively rare.
